When there is no experimental evidence for black holes, it makes perfect sense to be resistant/skeptical to new type of objects with solutions involving infinities (singularity). Whether that's being irrational or not is up to debate. I'd say that's a good prior when there is no data (as there wasn't at the beginning of 20th century).
On the other hand, the gold standard of scientific verification is the prediction of something not yet observed, and in that light, it seems to me to be somewhat inconsistent to reject a prediction of your own theory on the basis of it being at odds with your a priori feelings about how things should be, while still maintaining that everything else about your theory is correct.
Update: It is one thing to suppose that the singularity is a mathematical artifact, and another to reject black hole formation entirely. In this regard, the article seems vague about exactly what Einstein and Rosen were saying in their 1939 paper. It seems to me to be saying that, having concluded that collapse would not stop at the Schwartzchild radius and that would lead to a singularity, we should conclude that collapse to the Schwartzchild radius itself is not possible.
The article linked to this point is (IMHO) well worth reading, and seems to support this view of Einstein’s position.
Now it might be, but let's remember that back in the day Physics was mostly used to derive laws from observations. Only with the advancement of calculus, notation, etc. we could make predictions that later we confirmed. Granted, at the time of the black holes prediction this had been going on for a while so it should not have been too surprising, but also I don't think it'd be such a strong point as it is today.
Note that I was concurrently adding an update to my post as you posted this (though I think my update is orthogonal to your point here.)
With regard to contemporary attitudes to verification, we know that Eddington went to Principe to measure the magnitude of gravity’s effect on light specifically because it would be seen (and was) as strong confirmation of the theory.
Well yes, but they also knew stars collapsed, and that something had to happen in that case. Maybe at first they thought fine, everything becomes a white dwarf or a neutron star.
But then after Oppenheimer did the math in 1930 and showed that collapse into a black hole was within the theory of relativity would you still taken the same position?
If the theory of relativity was a person, it would not be old enough to drive or vote in 1930. :) It was an accepted theory, but it was not the wildly successful theory it is today.
Showing that a black hole was possible within the theory could just be a problem with the theory itself. Not necessarily an indication that they must exist.
This. Physics is a field of science, unlike math. They're dealing with models that keep on changing. Newton's model of F=ma is generally close enough for most things, but breaks near the speed of light. Following the model solely would give you the false idea that speed of light is just yet another number.
A person with a better astronomy background can jump in, but I'll start by quoting a couple trusted astronomy publications. In short, a white hole is an astronomical object with the property that light cannot enter it.
"Black holes are places in the Universe where matter and energy are compacted so densely together that their escape velocity is greater than the speed of light. […] Fully describing a black hole requires a lot of fancy math, but these are real objects in our Universe. […] So then what’s a white hole?
"White holes are created when astrophysicists mathematically explore the environment around black holes, but pretend there’s no mass within the event horizon. What happens when you have a black hole singularity with no mass? […]
"Now if white holes did exist, which they probably don’t, they would behave like reverse black holes – just like the math predicts. Instead of pulling material inward, a white hole would blast material out into space like some kind of white chocolate fountain. […] One of the other implications of white hole math, is that they only theoretically exist as long as there isn’t a single speck of matter within the event horizon. As soon as single atom of hydrogen drifted into the region, the whole thing would collapse."
"White holes are theoretical cosmic regions that function in the opposite way to black holes. Just as nothing can escape a black hole, nothing can enter a white hole. […]
"To a spaceship crew watching from afar, a white hole looks exactly like a black hole. It has mass. It might spin. A ring of dust and gas could gather around the event horizon — the bubble boundary separating the object from the rest of the universe. But if they kept watching, the crew might witness an event impossible for a black hole — a belch.
"Physicists describe a white hole as a black hole's "time reversal," a video of a black hole played backwards, much as a bouncing ball is the time-reversal of a falling ball. While a black hole's event horizon is a sphere of no return, a white hole's event horizon is a boundary of no admission — space-time's most exclusive club. No spacecraft will ever reach the region's edge. Objects inside a white hole can leave and interact with the outside world, but since nothing can get in, the interior is cut off from the universe's past: No outside event will ever affect the inside."
I don't know a lot of advanced physics or quantum mechanics, but black holes also confuse me.
I would imagine that black holes show the limit of modern physics, there are still things to learn, and models of physics will probably be adjusted in the future.
I guess there are physicists who are able to predict how a black hole works and appears, and how it can be explained, but I haven't read about it.
I mean, yeah there's tons of theoretical work. Predictions about lensing and structure have been done and verified. We've taken pictures of black holes. Susskind did a big fancy simulation for Interstellar that lined up great with them.
I think there's plenty of reasons to be a hard sell on relativity's take on black holes. After all, it's still a plenty hard sell today. The evidence that things that looks like black holes on the outside exist is pretty solid, but the evidence we have about the insides is zero. I don't particularly "believe" in the relativity take on black holes either, which is to say, I have very high confidence that a real and effectively-correct theory of quantum gravity will remove the singularities.
I'd love to pull off the trick of being extremely easy to convince about true things and only true things, and being very stubborn about everything else, but that's basically epistemologically begging the question. It's too much to ask of anyone.
I am halfway convinced a singularity will never actually reach the singularity form due to time dilation effects. that is, it ends up taking forever to actually reach zero size(not that time or space have much meaning inside such an environment)
You should also never see anything falling into a black hole because time is so stretched near the horizon it’d take almost literally forever to reach it. The bright light we see is the tidal force tearing matter apart as it approaches the horizon.
The general belief that singularities are “unphysical” and reflect a flaw in our models than actual reality. Hard to prove definitely right now of course.
I always thought the concept of a singularity here was just to make the math slightly simpler. That is to say that a black hole is still a spherical body with a set diameter. We just can't see it so there's no way to verify other than the fact that we can see the border of the event horizon.
As far as I know, that very much isn't the case. The singularity is a result of the math, not a trick to simplify it.
My understanding is that many researchers do think it's a "placeholder" representing our lack of insight into how physics behave at those scales, and not a real physical phenomenon, and that it will one day be possible to work past it. I believe Kerr and Penrose (both of whom did some of the most foundational work on black holes) believe it's a mathematical artifact.
- Is the math referring to the center of gravity as the singularity? Because even a sub black hole mass object would have that.
- Why is there an assumption that just after a mass becomes a black hole the matter inside it suddenly compresses further when the actual gravity of the object has only slightly increased?
- Is there a maximum density of matter in the universe and if the black hole even reaches that?
- Wouldn't you need that number to be infinite if the black hole itself is infinitely small?
- If the black hole does have a mass inside it... Do the light particles trapped inside the black hole form a blanket around the existing matter?
The math is very difficult. More difficult even than that. When ever you are trying to explore the consequences of General Relativity, it is necessary to make some simplifying assumptions. Schwarzschild started with several such assumptions. First, he assumed a completely static system, one that doesn’t change with time. Second, that the mass in the system was spherically symmetrical. He also put the center of the black hole at the origin, and so on. All of these assumptions are there to make the math easier; they’re like using spherical cows in high school.
What Schwarzschild found was that this solution had two singularities. There was a singularity at the origin, and a singularity some distance away from the origin, at a radius which is proportional to the mass of the object. In both cases, some term of Einstein’s equations become infinite, and therefore seem to stop describing reality.
A couple of years later other physicists proved that by changing the coordinate system in a certain way, that second singularity would go away. That is, this singularity was merely an artifact of one of the simplifying assumptions that Schwarzschild started with, and didn’t represent any real feature of a real black hole. But the radius at which that singularity occurred is still relevant: it tells you how big the event horizon is.
Penrose and Hawking proved that the singularity at the center of the black hole cannot merely be a mathematical artifact. Penrose got his Nobel prize for that.
> Is the math referring to the center of gravity as the singularity? Because even a sub black hole mass object would have that.
Yes, the singularity at the center of a black hole is coincident with its center of mass. But not every center of mass is a singularity. Remember, we only have a singularity if some term of Einstein’s equations for General Relativity goes infinite. In a normal object nothing is infinite at the center of mass. That only starts to happen once the object is compressed into a black hole.
> Why is there an assumption that just after a mass becomes a black hole the matter inside it suddenly compresses further when the actual gravity of the object has only slightly increased?
There isn’t. This is the most common _conclusion_, based on the math. It’s not a starting assumption.
> Is there a maximum density of matter in the universe and if the black hole even reaches that?
Unknown. A neutron star is the densest object that can exist which doesn’t have an event horizon. Get any denser, and an event horizon forms which completely hides whatever happens next.
> Wouldn't you need that number to be infinite if the black hole itself is infinitely small?
Sure, if the mass all falls into the singularity, then the density of the singularity is infinite. There’s really no problem with that.
> If the black hole does have a mass inside it... Do the light particles trapped inside the black hole form a blanket around the existing matter?
This is a non–sequitur. It’s meaningless.
What is really going on here is that mass bends spacetime. This causes our path through spacetime to bend as well. All the gee–whiz effects of relativity, like time dilation, are a result of this bending. Time passes slower for an object moving quickly because its path has been bent so much that it is passing through less of the time dimension than it would otherwise. Light is moving so fast that it doesn’t pass through time at all; from the perspective of the photon no time at all passes between when it was emitted and when it was absorbed, no matter how far it traveled through the intervening space between those events.
Inside the event horizon of a black hole, the path of every object is bent towards the singularity. They’re bent so much that time and space swap roles. The singularity is inescapable not because something is dragging you towards it, but because it is literally in your future. From the outside it looks like the singularity is a place, but if you get close enough it becomes a time in your future, beyond which there is no more time. Everything outside the black hole is twisted around into the past, where you can never go. And, unfortunately for you, a stellar mass black hole is not very big. It’s only a few dozen miles to the center, and since we move through time at the speed of light, that is a very short timeline indeed. Your future ends very abruptly, probably faster than thought.
That’s why most people conclude that the matter is crushed into the singularity. How could it resist? You’re not going to brace yourself against the walls of the universe and prevent yourself from being dragged forward into the future, no matter how strong you are.
But, quantum gravity might change that picture slightly. It may be that the distance to the singularity grows the longer the black hole exists. It may in fact grow at the speed of light, meaning that while the volume of the black hole is not infinite, you never actually reach the singularity and go splat. Your time never actually runs out. You can find some lectures by Susskind about this on Youtube if you like.
> > Is the math referring to the center of gravity as the singularity? Because even a sub black hole mass object would have that.
> Yes, the singularity at the center of a black hole is coincident with its center of mass
I don't think this is accurate. Take Schwarzschild for instance: The singularity is not a place in space you can poke with a stick. It's a spacelike singularity, and it lies in the future of any observer unlucky enough to fall into the BH.
To the external observer, the black hole is perfectly spherically symmetrical.¹ The center of mass of a spherically symmetrical object is always right in the center. Right where the singularity would be, if it weren’t for the fact that to the inside observer the singularity is now in the future at the end of time.²
These types of coordinate changes are one reason why the subject is considered so hard.
¹ Except very briefly as two black holes merge. Or in the case of a rotating black hole. Or potentially during the formation of the black hole. But those are all complications and the subject is difficult enough.
² Except for the fact that the singularity might not actually exist if the interior of the black hole is growing without bound in the timelike direction. See ER=EPR.
> There isn’t. This is the most common _conclusion_, based on the math. It’s not a starting assumption.
How? Time doesn't work inside black holes, we have no clue how that even works or if things are frozen or if it happens instantly. Math can't answer what happens when time doesn't work.
This isn't just the time paradox of near light travel, the math says that inside black holes you have an entire different dimension of time instead of our regular one, nothing can be said at all about what it looks like from our point of view since there is no concept of shared time between us and what is inside of black holes.
So either the formulas are wrong, or we can't say anything about what is inside.
> Time doesn't work inside black holes, we have no clue how that even works or if things are frozen or if it happens instantly. […] This isn't just the time paradox of near light travel, the math says that inside black holes you have an entire different dimension of time instead of our regular one
There is no such thing inside a black hole as "an entire different dimension of time." Yes, by the very definition of an event horizon, the coordinate chart of an observer at infinity cannot extend inside a black hole. Needing multiple coordinate charts is not really unusual, though. The two-dimensional sphere S² also requires at least two charts. All in all, time inside a black hole locally works the way it does outside a black hole. Yes, there might be interesting non-local effects like closed timelike curves but you can also have those outside.
> time inside a black hole locally works the way it does outside a black hole
That doesn't mean that it actually runs though, you can interpret the lack of possible shared reference of time as the black holes time either happens instantly, but also that it never ever happens and everything in it is stuck and time is frozen from our perspective. In the time frozen perspective the singularity would never form, all objects are frozen and never reach it.
Time doesn’t magically freeze inside of a black hole. That is a misunderstanding of what happens to light rays carrying the image of the infalling diver as they approach the event horizon. From the perspective of an outside observer, those light rays are slowed down more and more and light from the instant of crossing the event horizon never actually arrives; it appears to take an infinite amount of time to do so. The mistake is to think that the object is still there, hovering at the event horizon, for an infinite amount of time. The reality is that the diver notices nothing unusual as they cross the event horizon; they may not even be able to tell exactly when it happens. They certainly do not freeze in place forever.
> The reality is that the diver notices nothing unusual as they cross the event horizon
That doesn't mean that he actually crosses it from our reference frame. Relativity is all about understanding that time is relative, that applies here as well, except that there is no connection between the two reference frame unlike regular space, so we can't say anything about how time flows inside of a black hole from our perspective.
Basically what I am saying is, you can't say whether the singularity has actually formed, since your reference frame is just as connected to the instant anything fell into the black hole as it is connected to the end state of the singularity. That makes it just as valid to say that things freeze the instant they touch the black hole as to say that they merge with the singularity the instant they touch it, since both are equally valid viewpoints from our perspective.
> That doesn't mean that he actually crosses it from our reference frame.
This specifically is an incorrect understanding. All events happen in all reference frames. There are no situations where an event that happens in one reference frame fails to happen in another. At worst (or best, depending on how you view it) we can disagree about the _order_ of events, but not which events actually happened.
It’s not that the crossing fails to happen. It’s just that, from the outside it takes an infinitely long time to happen and, for the diver’s perspective, it’s the universe outside that ended really quickly.
You are correct that we just dont know what happens behind event horizont, from our perspective at least. Currently our best theory says nothing from inside can leak out, so in that spirit saying infalling matter ends up in singularity is as based in reality as it transforming into unicorns.
Regardless, its important to keep in mind that this is valid _only_ from our perspective, that from the perspective of infalling matter something _real_ happens, and that real thing that happens is probably not completely disconnected from rules we perceived in our own universe.
Throwing hands up and saying "we cant tell whats inside, so why are we even arguing about what we think happens there" is certainly a way to look at things if your only motivation is to find out whats happening inside, but that is not all we are after. We are trying to use all we currently know to reason about how the inside might look, any possibility, and try to work out backwards what unforseen implications it might have for our reality.
We are not saying that inside blackhole is sigularity, we are saying that, given our understanding of how outside looks, there might be a singularity inside. We also know the math that yields us this singularity is probably wrong (infinite), and actively are trying to figure out whats wrong with it. We are using this imaginated realoty to work backwards what we might have gotten wrong, and perhaps in this case its not even the roght way of thinking to solve this mystery; it just worked for us before, so we trying.
If you figure out whats actually happening there, and find some way to connect it to rules of our universe, everyone will cheer. If its by first discovering new effect in our universe that would elegantly fit into blackholes, or in the stroke of genius come up with a pure theory enlighting some real observations, nobody would care.
Its just when we throw a mass, it keeps going. It makes more sense to think that it goes the same inside blackhole, but maybe not. We just dont know..
You wouldn't feel going through the event horizon of a large black hole, so I see no reason why that mass would somehow just collapse into a singularity.
Also the math for a black hole states that when the black holes density reaches below the density of the universe, the black holes event horizon will expand and encompass the whole universe, meaning matter doesn't even have to fall into the black hole, the black hole could just grow into the matter and then you don't even have a velocity you just have matter entering the black hole.
We know the two above are how the math for black holes work, none of those produce likely singularities, because there is no force compressing that into a singularity. So if singularities exists in black holes, it would just be some of them, and not all parts of the black hole would have to be a part of that singularity.
I am not arguing for certainty of singularity, quite the opposite. I stated that we seriously suspect the math yielding us that singularity is wrong, we just cant figure out anything more correct yet. Personally I think the geometry of spacetime inside blackhole gets all twisted up, bubbling like a boiling pot of water, but my guess is as good (probably worse) than anyone elses.
>when blackhole density falls below the density of the universe
Which one? The outside one? Density of whole universe? Its local surroundings? Are you counting unknown pressures of stuff we, for pure lack of understanding what the hell it even is, call dark energy/mass?
The "blackhole less dense than the rest of the universe" seem like just another mathematical artefact that couldnt, for any reason, exist in real world. Not saying thinking about it is wrong, but imho more could be gained by thinking "well it surely cant exist, so what mechanism are there that stops this", instead of "this math says that, so lets subdue our whole theory to it". Very same scenario as the singularity inside.
Just because math works for something doesnt mean it is like that. In science history we had plenty of equations that worked, until we discovered some edge case where it suddenly didnt. I think its safe to bet interiors of blackholes, or blackholes encompassing whole universe, are prime candidates for such edge cases where our pressumed math models just break down. The fact that our current math models are returning infinities, we are generally accepting that to be the case.
> but imho more could be gained by thinking "well it surely cant exist, so what mechanism are there that stops this"
Well, the universe Schwarzschild radius is massively larger than the universe itself, so math says we live in a black hole. It is true this might be a mathematical artifact, but it could also be how it actually works and the observations we make in our universe and about big bang is how a black hole looks on the inside.
I don't think you should just assume the math is wrong here and throw away those observations of the inside of a black hole. Sure you should be open to that being wrong, but at least to me that is far more believable and interesting to study, then you can try to connect what we observed about the big bang with a black hole etc.
If I go through enough of these forum discussions maybe I develop this long enough, and then maybe I'll take a serious attempt at solving the math to combine the two, since nobody else will do that. I was called a genius by both my physics and math professors, so it is possible I can solve it since I have good intuition for physics and good skills at math, most people who work in physics just have one of those, even though likely I'll fail, but it would be fun to try.
What makes me believe such an attempt could work is that it could lead to a different math for gravity and black holes at small scales, and thus lead to a unified theory of QFT and GR. If I manage to do that then I could convince physicists to adopt the new interpretation of GR. Current attempts of unifying those two has started from the QFT side and tried to incorporate current GR theories, but they all failed. But I haven't seen many try to attempt that from the GR side and unify that with QFT, so maybe that would yield some results.
Anyway, I don't think that professional physicists will solve this, since they are too occupied focusing on publishable results rather than solving the core issue. What I write here isn't publishable, so no physicists would even think about working on it.
It is how GR was invented by Einstein even though he wasn't a physics researcher, sometimes you need a big jump done by an outsider. GR wasn't on anyone's radar when Einstein suggested it, likely it would never have been discovered without Einstein, or at least take centuries. The gravity formula you get from GR doesn't need space time distortions to reproduce, so theoretical physicists would just update the gravity formula to match observations without creating a new interpretation of what that means.
Newtons gravity formula doesn't come with an interpretation, updating it to match new data is exactly what they did with dark matter and dark energy since it is just a formula, so I see no reason why gravity would be any different without Einstein's GR. Black holes would just be a part of that dark matter.
Edit: Anyway, for me it is fun to argue on forums, so that is the best way for me to think and develop theories. Typically I don't really believe what I say, so part of my posts is to find evidence for it and try to convince myself that what I say is true. So I refine things quite a bit in every discussion.
I got a good idea: Nothing ever passes the event horizon, rather as an object gets close to the event horizon the combined Schwarzschild radius of the black hole and the object will at some point cover the object, meaning the event horizon would expand and engulf the object instead of the object moving through it.
This would totally change the math around the event horizons, since then space doesn't need to be connected between the interior and exterior of the hole. The outside of the hole still would work the same, but as the hole bends space to engulf objects rather than objects moving through space to enter the hole you get a very different way to look at the interior.
Not sure how the math for that would work, but it is fun to think about. Also this is obviously true, for any particle that point would happen before it enters the current radius, so the hole expands out, even if it is by an extremely small amount that is still enough to change the math around the event horizon singularity.
Edit: The main thing this changes is that now the particle doesn't have to accelerate to the speed of light as it passes the event horizon, as the event horizon expands to cover it rather than it accelerating through. As you can understand that is a massive change, it means that space inside the black hole could remain sane without breaking GR. The infinite time dilation you get at those gravity levels is why the math for space and time inside of black holes is so strange, remove that and you can get back to sanity.
Then we could assume that the interior of the black hole looks just like our universe, since our universe is smaller than its Schwarzschild radius. Basically we want to show that the interior space time of a black hole can be relatively flat.
The problem to solve then is to calculate what happens at the edge of such a universe, and try to match that with what happens at the edge of a black hole, since those two would be the two sides.
To unify that with QFT you'd have to include the particle collapse (or entanglement if you want to use the many worlds wording), where the quantum field would collapse to where the particle suddenly is inside the black hole instead of outside. I have long theorized that there is no way to unify the theories without using quantum collapses, so black holes could engulf particles via quantum field collapses only.
The math for such collapses isn't fully established though, so it would need to have more theories on the quantum side as well. But at least that means nobody has developed a theory like this yet.
The main calculation would be to calculate the probability of a particle collapsing into the black hole over time, by calculating how large part of the particle field is inside the critical area were it would be inside the new black hole radius. The particle and the black hole would get entangled for a bit where it is both inside and outside the black hole at the same time, and then the field collapses and it is fully inside.
I think that can actually work! Then the black holes event horizon is a natural quantum effect, which could potentially provide the missing link between the two.
There is no disconnection between the interior of the black hole and the exterior. They are smoothly and continuously connected, and the “normal” axis of time blends seamlessly with the “bent” axis of time inside the black hole. That’s why orbits are so weird outside the black hole.
So how long does it take for a thing to enter the singularity as seen from our reference frame? If you say there is no way to answer it, then you say that I am right, so I'd like to see a link to a formula saying how long that takes from our point of view.
I studied up to a masters in physics, I've seen the math, I don't believe there is a way to calculate that, there is no way to compare time inside with time outside.
Edit: I'd love to see such a formula if there is one btw, so if you know there is one please link!
Time and space work exactly the same inside the black hole as outside, they just swap places. One dimension that was spacelike is bent into becoming timelike. Closer to the center becomes further in the future, and outside the event horizon becomes part of the past. The singularity at the center of the black hole is now the literal end of time. Nothing exists past the end of time; you hit that singularity and you cease to exist.
From the outside, all that we can detect is the mass of the black hole. Or rather, all we can detect is some gravity pulling us in, as if there were mass inside. Einstein proved that all gravity is just the curvature of spacetime. Your body is bending the fabric of spacetime right now!¹ When you dive into a black hole, the curvature you are causing is added to that of the black hole, so that the black hole grows exactly as if your mass still existed inside of it. But really you ceased to exist when you hit the end of time at the singularity.
But of course that is how we _used_ to think of it, up until a decade or two ago. As Susskind would say, ER=EPR. Spatial connectivity (which Einstein and Rosen wrote a paper about) is exactly the same thing as quantum entanglement (which Einstein, Podolsky, and Rosen wrote a completely different paper about in the same year). I won’t try to recapitulate one of his lectures (since you can just watch one; see <https://www.youtube.com/watch?v=31fVea8_OAw> for a recent example, but there are others), but the result is that the spacetime inside the black hole grows without bound, and you never actually reach the end of time. You cannot go back, but neither do you go splat. The matter that was once a star is somewhere ahead of you in that growing region of spacetime, forever in your future.
¹ Note all possible “yo mamma” jokes for future reference.
Let me rephrase: The time we measure with our clocks from the outside doesn't apply inside of black holes. There is no way to say whether any singularity has ever formed or ever will form even according to the math, since time inside of them isn't the kind of time we can measure from the outside.
If you go inside of it the math says that now your clock inside of the black hole can still function, but that also means you are no longer connected to time outside of the black hole, so you can't say whether anything actually happens inside of them from our perspective, all the math says is what it would be like for someone inside.
You need to make extra assumptions about what it means to have a disconnected time reference frame to say anything.
> - Why is there an assumption that just after a mass becomes a black hole the matter inside it suddenly compresses further when the actual gravity of the object has only slightly increased?
Yeah, that seems like an important thing to address. Black holes don't even have to be high density, or have much matter in the center at the time of formation. If you arrange enough big chunks of metal into a spherical-shell constellation, then drop them all toward their mutual center, they can reach the threshold to become a black hole before they even start colliding. What happens around that time, and what specifically does "singularity" mean (because I see people using it in very different ways).
> - Is there a maximum density of matter in the universe and if the black hole even reaches that?
> - Wouldn't you need that number to be infinite if the black hole itself is infinitely small?
Black holes are the benchmark for maximum density. The event horizon is always exactly at the limit. By "black hole itself" do you mean something other than the event horizon?
But that maximum density depends on size, smaller things can be denser and bigger things have to be less dense.
I can imagine it's something stupidly energetic. If you don't want them turning into a hot ball of neutrons first you're either going to have to spend a lot of energy keeping them apart or they are going to be orbiting a virtual center and it will take far longer for them to reach the center singularity than you expect. This will follow the laws of the 3(+) body problem so you won't be able to calculate a perfect impact, so the birth of the singularity will be shrouded in very high energy particles escaping.
> If you arrange enough big chunks of metal into a spherical-shell constellation, then drop them all toward their mutual center, they can reach the threshold to become a black hole before they even start colliding.
When I say "black hole itself" I imagine any combination of mass at it's center that provides enough density to form a gravitational pull that will trap light.
All we know is that we’ve made a lot of crazy predictions about the structure and behavior of black holes well before we measured them and so far they’ve aligned remarkably close to the predictions. If there is a difference, it’s about the interior not how it shows up in our universe. And since we can never see inside the black hole, any alternative model would have to better explain some phenomena our existing models can’t or be even easier than relativity and explain the same things. We know string theory is hard to make consistent and it struggles to create falsifiable experiments. I would bet on relatively remaining a very very long time until we figure out how to unify quantum and relativity.
To the best of my knowledge, none of the competing theories for integration have singularities in them. It is difficult to know what a "singularity" would look like in a universe that doesn't seem to have 0-dimensional mathematical points in it.
It isn't that surprising that continuous theories have singularities in them. Reality doesn't seem to be continuous in the sense that real numbers are. It is unknown if Navier-Stokes can develop singularities in finite time, but in the real universe it won't matter because they won't lead to singularities in the real universe because it is ultimately just an approximation. "Interesting things" may happen around whatever those conditions are but nothing will be accelerated to infinite speed in the real universe. It is another theory that is a real-number-based continuous approximation made useful by the fact that real numbers are easier to do math with than physical quantities, but the real universe does not have plain real numbers in it. (This is subtly different than the controversial statement that it isn't made out of real numbers at all. But it does not have "plain" real numbers. You certainly can not expect to pull out a microscope and peer at the universe at a scale of 10^-10000 meters and see a universe that behaves exactly the same, just at a smaller scale. Our universe is definitely not scale-invariant.)
I think this is likely to be a rather accurate metaphor.
To be honest, I don't see a lot of positive reason to assume that the singularities are going to be "real". I think physicists initial reactions to the idea were correct and remain correct. I think the reasons why people think otherwise are emotional rather than logical. They've gotten attached to all the stories around black holes and all the science fiction and all the crazy ideas (parallel universes! white holes! wormholes!) and a general sense of "woo" that I for one do not find very appealing, and what people are reacting to is the loss of that, not the idea that the universe probably won't have any infinitely dense points... when the universe doesn't seem to have any points in it in the first place.
(To forstall another common rabbit trail, I don't think the universe is continuous... but that doesn't mean I automatically think it must be discrete: https://news.ycombinator.com/item?id=38433917 )
That doesn't really follow, because it seems that people were and are wanting black holes to be something else that they are both not observed to be and not described by any theory of them.
Also, I think it gets lost in physics that theories are models of reality. They are not reality themselves. General relativity is about as good as a model gets. Alongside the standard model, it's one of the most tested models around.
A singularity has a meaning in physics in terms of math, which is dividing by zero or something approaching zero in the denominator. A singularity has a physically modeled aspect in that it's the name assigned to whatever becomes of the matter that gets squished down. A black hole is effectively not a thing in itself but rather an effect of what happens when mass is squeezed beyond all known limits.
What's inside a black hole is not the only thing in the universe we can't see. The actual universe is far bigger than the visible universe, but we can't see outside of the visible universe due to how light works. So it basically doesn't even matter what's going on there. The question is if anything happening at the boundaries can tell us something about that which we can't see. It doesn't make sense to dismiss what we observe on the outside based upon what we literally cannot know of the inside.
And after all, general relativity is a classical theory. Is it really all that "weird"? It all feels somewhat mechanical and natural as you start learning it and turn off your biased intuition.
We have observed one black hole from the inside, the big bang. The reality we see today doesn't really look like what general relativity math says the inside of a black hole should look like.
The pro singularity people must then believe the black hole evaporated really quickly into a big bang to rejoin the out universe, but a more rational answer is that the big bang is what happens inside black holes rather than a singularity forming. Space inflation after the big bang would just be the black hole expanding as it ate more material in its near vicinity.
It's all science fiction, stuff I like to think about just before I go to sleep. But I like to pretend this is what happened to time, why it is only one way. see, once you pass the event horizon of a black hole it effectively removes a half dimension from reality(you can't go back towards the horizon) time is that half dimension we are missing.
It isn't just science fiction, the Schwarzschild radius of the universe is massively larger than the universe itself, we are living in a black hole according to current math.
And in theory a black hole whose density gets lower than the density of the universe will grows its event horizon to fill the entire universe. Such a large black hole doesn't have strong tidal forces, so you wouldn't even notice as you entered it, from the outside it would look like the black hole disappeared and the internals popped out rather than you entering it.
At least that is what the math says. Likely there are some more strangeness as you enter the event horizon, since at that point you effectively pass the speed of light barrier, creating infinite time dilation (a gravitational difference is equivalent to a speed difference in relativity, and the anent horizon is when that becomes the speed of light). That means we have no math to explain how time inside the black hole is related to time outside.
So while the space tidal forces wont rip you apart space wise, the time dilation difference between your different parts would be infinite, since at the event horizon one part of you will see the other part as having infinite time dilation, I'm not sure how you could get through that unscathed. The particles might still be next to each other, but you might turn to dust from infinite time passing resulting in infinite particle decay, basically resetting the state of your matter.
I did study all this math in a masters degree, I am not a working physicist but I think this is a much more reasonable interpretation and also more testable than the prevailing ones about black holes. All the math of the testable parts adds up, while the prevailing theories are wrong since the universe isn't a singularity.
Edit; Note that extremely few physicists are even researching GR seriously, barely any work has been done on it since the Einstein days, quantum field theory is where all the useful applications and hence money is at, it also has way more testable results, so almost all physicists are approaching the problem from that angle. The few who do study GR are mostly focusing on the raw math and not interpretations, since math is publishable and interpretations are not, also since GR requires so high level math that basically only those who love math over nature even wants to study it.
Case in point, Einstein himself didn't understand GR math, today he wouldn't be able to publish any GR research even though he invented the theory.
But wouldn't that mean it's black holes all the way down? If in our universe, another universe is inside every black hole, then aren't those universes quite small due to their total mass being the same as the black holes'? The mass of black holes can be quite modest. Then the universes inside those universes' black holes would be even smaller, and so on.
Also, it's my understanding that black holes shrink over the time, so they aren't expanding and certainly not at increasing rates like our universe is. The ones in the center of galaxies "gobble" up gas, but most black holes aren't at the centers of galaxies.
> The ones in the center of galaxies "gobble" up gas, but most black holes aren't at the centers of galaxies.
Most black holes aren't as large as our universe, although if that theory is true we can't say much about what a typical black hole would be like in the larger universe.
It is also possible our universe ate the entire outer universe. At a certain size the black holes density is less than the universe density, at that point its event horizon will expand forever and eat the whole universe. That could happen to our universe as well. That probably wouldn't destroy the outer universe though, since entering such a large black hole doesn't rip you apart, so in a way that is a theoretical way to "exit" a black hole, although technically you make the outer universe enter it.
Edit: Back in college I intend to go into theoretical physics, but once I got to string theory etc I no longer believed in it, and continuing from there would mean I'd have to study that sort of nonsense for half a decade just to enter the field. That makes theoretical automatically turn away anyone who have alternate interpretations, so you wont find them form working physicists. I know all the math from the tested parts of GR, the rest are just untested theories, mine are as good as any there.
So here is the evidence: We know the big bang fulfilled the math criteria of being a black hole. We also know how black holes work from the outside from observations today. The simplest theory explaining both of those is that the math for the inside of the black hole is wrong, while it is right for the outside, and that we observe in our own universe/black hole is what really happens inside. Any other explanation simply has less evidence behind it.
Speculating that there is some strange singularity there is just as much sci fi as speculating that there isn't and that what we observe from the big bang is the normal state. Any theory saying that the black hole has a singularity inside would need to explain why the universe isn't a singularity, otherwise its provably wrong, since we know the universe isn't a singularity.
The only evidence against the universe and the big bang being a black hole is that it lacks a singularity, everything else adds up to it being a black hole. Ignoring that evidence is more unscientific imo. Also saying that the universe expands due to "dark eneregy" is also baseless sci-fi in that case.
Anyway, if black holes do become big bangs like that then it could be testable by looking at distributions after a big bang etc and compare that with some different ways of calculating black holes sucking up mass in different scenarios to see if it is reasonable. Science starts with speculating about stuff, speculation is not non-science. At least this is more testable than stuff like dark energy.
Edit: I've also studied all the math and physics in college for the testable parts of GR, I haven't studied string theory, at that point I no longer felt the physics made sense, I don't think it is wrong for me to hold alternative theories to the untested ones.
I see no epistemic problem with a singularity existing. After all electron are singularities in the EM field, of a sort. Why shouldn't a lot of electrons together form a giant singularity?
Truth be told, we don't really know that electrons are singularities in the EM field. Classically that's the case, of course. But in QFT, as you start to probe to shorter distances (and higher energies), our theories eventually break down because gravitational effects cannot be ignored. (For instance, how do you describe the EM field at distances shorter than a Planck length? The energies needed to probe to that limit would collapse the system into a black hole.)
> Truth be told, we don't really know that electrons are singularities in the EM field
We do know they aren't such singularities. QFT says they aren't, the electron is a quantum field spread out over an area it isn't a point particle. QFT is the most well tested theory in existence, so if we can't call that "know" then we don't know anything at all in physics.
What we don't know if whether there is limit to how small that quantum field can get, but we do know it can't become a point, due to the Heisenberg uncertainty principle that states that for a particle to be a point it would have to have infinite momentum.
Note: Heisenberg uncertainty principle was named at the time when people still believed quantum fields were probability distributions, but since then we have established that the quantum field is the particle and not just a probability of where the particle is. That means the Heisenberg uncertainty principle establishes a relationship between momentum and size instead of just knowledge.
I'm not at all an expert in this, but my limited understanding is that electrons are only "singularities" in classical EM; in quantum field theory they're just solitons of the underlying quantum field and can be analyzed with no infinities present. We haven't found a QFT equivalent for gravity yet, but I think the broad consensus is that the gravitational singularities will disappear once we do.
In qft there are no singularities, everything are fields. Particles are just such fields, a singularity in qft would mean you have a Dirac delta shaped field, I don't think such a field exists in reality and I don't think many working physicists would believe in them either. The big reason they can't even come up with math for the black hole singularities is just because the theory doesn't support working with such Dirac shaped fields.
Edit: When they say electrons doesn't have a size, what they mean is that there is no limit to how small you can make an electron, not that it is always compressed into a point.
Quantum theories arise because the singularities in black holes create a topological regime within their vicinity — and for SMBs, that’s a whole galaxy.
I think it’s interesting that different people feel different things are “natural”.
In layman's terms, isn't the take that the area of a black hole is devoid of space? Therefore you can never see or measure it except for forces generated by it within our space.
The claim of general relativity is that a black hole is a singularity in spacetime, meaning that space effectively "ends" there. A very crude model would be cutting a hole out of fabric and then sewing the circle up into a point. The area of the hole is "gone" --- it's not like it's there and you can't see it, it's just not there. I guess that it is what you mean by "devoid of space".
Then the rest of the fabric is bent in weird ways around the fact that there's a chunk missing, which in physics manifests as as everything being pulled towards it (and there is a point, the event horizon, at which things are no longer possible to interact with, like you're too deep in the hole to get out again.
There was one insulting dismissal of one physicists grasp of physics at one point, but Einstein didn't end the conversation with LeMaitre then and eventually conceded that he was wrong, so it wasn't a 'conclusion', either.
I wonder if any mass could even colapse beyond Planck length?
Maybe theory unifying quantum mechanics with Einstein's general relativity would have more definite answers.
For any given radical-but-rational concept, people must instantaneously accept the proposition (instantaneously being less than 0.1 seconds after becoming aware of the concept). Anything else constitutes science denialism.
Plenty of physicists are still "resistant" to the idea of black holes having a real singularity. The recent Kerr paper [0] only adds fuel to this resistance with its hilarious statement "Faith, not science!" about singularity proponents.
> Plenty of physicists are still "resistant" to the idea of black holes having a real singularity.
That's because most physicists expect that new physics, probably some form of quantum gravity, will come into play before the singularity is reached, so that there won't actually be one. Which is a perfectly reasonable expectation.
> The recent Kerr paper [0] only adds fuel to this resistance with its hilarious statement "Faith, not science!" about singularity proponents.
Kerr misrepresents the actual opinion of most physicists in that paper. The simplest way to see this is to realize that Kerr's statement that if a theory predicts singularities, the theory is wrong, is something that virtually all physicists who work in GR agree with! Practically nobody believes that there are actual curvature singularities; they believe that GR is not correct in that regime, and some kind of new physics is involved, as I noted above. So in the sense Kerr uses the term, there are practically no "singularity proponents" at all. He is attacking a straw man.
>The simplest way to see this is to realize that Kerr's statement that if a theory predicts singularities, the theory is wrong, is something that virtually all physicists who work in GR agree with!
Maybe it was my misfortune, but the university professor who taught me GR thought that singularities are "real", so this anecdote (sadly) disproves your absolutist opinion. And there are heaps of real papers from real scientists which seriously discuss potential consequences of having "real" singularities. And if we'll take a look at the pop-sci coverage, it's orders of magnitude worse... Just see the Veritasium video linked in the other comment.
> the university professor who taught me GR thought that singularities are "real"
Did he teach it from Wald's classic textbook? Wald, Chapter 9, explicitly disclaims any belief that singularities are real and explicitly says the classical GR prediction of singularities indicates a breakdown in the theory in that regime. Other classic textbooks like Misner, Thorne, & Wheeler say similar things. So do many peer-reviewed papers published in the field.
Sadly, I am not surprised, though I am disappointed, that many university professors do not know the subject they are teaching well enough to be aware of what I have just said. But the fact remains that the statements I have described are the ones Kerr (who certainly cannot claim ignorance on the part of whoever taught him GR as an excuse) should have been looking at to gauge the prevailing opinion of actual researchers in the field. But he didn't.
> if we'll take a look at the pop-sci coverage
Nobody should be relying on pop science to learn actual science. Nor should anyone claim that pop science coverage is an accurate gauge of the opinions of actual researchers in the field.
Haven’t we basically proven that even horizon is real? At which point, isn’t anything inside sealed off by gravity, the fact that time slows down infinitely, the hypothesised firewall, etc?
I.e. if a black hole has a singularity or a super heavy pokemon, there is no distinguishable difference in our universe?
> Haven’t we basically proven that even horizon is real?
No. All we have shown is that there are compact regions in our universe that look like event horizons--but could also be just apparent horizons. To know that they are actual, true event horizons, we would have to know the entire future of the universe, since an event horizon is a globally defined surface in spacetime and its presence and location cannot be known unless you know the entire spacetime, i.e., the entire future. If you don't know that--and of course we don't--you can't tell an actual event horizon from an apparent horizon.
> isn’t anything inside sealed off by gravity, the fact that time slows down infinitely, the hypothesised firewall, etc?
None of these are actual features of actual event horizons (or apparent horizons, for that matter). The first is a missatement--the gravity of the object is not "sealed off" and can still be detected outside. The second is an illusion caused by an unfortunate choice of coordinates. The third is pure speculation, and speculation which does not appear to have gained much traction.
> if a black hole has a singularity or a super heavy pokemon, there is no distinguishable difference in our universe?
Most physicists do not believe that either of these (if I take "super heavy pokemon" to mean "some kind of ordinary compact object like a neutron star") is what actually happens.
> Haven’t we basically proven that even horizon is real?
We haven’t observed event horizons except as fuzzy highly processed images. They look a lot like what you’d expect from a black hole, but we have hardly ruled out all possibilities. And there are theoretical effects near an event horizon for charged and spinning black holes, that as far as I know we’ve never been able to observe.
It doesn’t seem out of the question that a black hole without a singularity might exhibit behaviour near the event horizon that is measurably different from the theories we have now.
> the Nobel prize was awarded for the Singularity Theorem
Sure, because those theorems played a very important role in clarifying exactly what classical GR predicts in these regimes, and therefore in focusing the efforts of physicists in fruitful directions. That remains the case even if it turns out that in our actual universe, at least some of the assumptions of those theorems are violated so that there aren't any actual singularities. One of the key contributions of the singularity theorems was in making those assumptions clear, so that physicists who did not believe singularities were physically realistic knew exactly where to look for other possibilities.
Infinite density is pretty crazy though. Would it change much from the outside perspective if there was something more like an ultra compact neutron star at the center?
> Would it change much from the outside perspective if there was something more like an ultra compact neutron star at the center?
There can't be anything like that at the center; that kind of solution doesn't work. At least, not if there is an actual event horizon.
The most likely solution, IMO, is that there is no actual event horizon, and the horizons we are seeing in our observations are only apparent horizons. There are theoretical possibilities such as the "Bardeen black hole" (which is misnamed since it has no actual event horizon) that look like standard classical GR black holes from the outside for a time on the order of the Hawking evaporation time, which means we would not be able to observe the difference now or for the foreseeable future. But what is deep inside those solutions is not any kind of ordinary compact object like a neutron star.
Pardon my ignorance, but why not? This is what I’ve long assumed is the true nature of black holes — not some kind of divide-by-zero error, but merely a very very heavy object.
Because no ordinary object, i.e., object made of ordinary matter obeying an ordinary matter equation of state, can be that compact. It's not possible.
Solutions like the "Bardeen black hole" adopt a different equation of state in their deep interior, something more like dark energy, which is not any kind of "ordinary object". That is the only way to have an object that compact without an event horizon. The missing piece in such solutions is how that kind of equation of state could arise as a result of something like stellar collapse: that is where some kind of new physics, possibly some form of quantum gravity, might be needed.
Kinda, you can't have the event horizon without a singularity.
The one thing I'm not 100% sure on is whether an event horizon can form. I have never been able to shake the feeling that with Hawking radiation giving each black hole a large but finite lifetime and with matter falling in beyond the infinite future (to outside observers) you'd have the incoming matter encounter the death of the black hole before it can fall in. This would make a black hole a kind of explosion powered by Hawking radiation that looks frozen in time to outside observers.
Lacking a complete theory that can adequately explain both Hawking radiation and general relativity it's quite hard to know if this makes any sense. Though I can't rule out an answer already exists but is simply not known to me.
A very simplified idea is that the denser you get, the slower time flows (for an "outside" observer). So while for an "inner" observer density rises "fast" to the point of singularity, for an "outside" observer time just freezes inside the forming black hole. Finally, if there is a way for black holes to "evaporate" (e.g. Hawking radiation), the "inner" observer will be able to observe shrinking black hole.
No, this doesn't work. The "slower time flows" you is an optical illusion caused by the curvature of spacetime. It is not a way to avoid the formation of a black hole.
Can you please give some reference for this claim? Time stopping at the event horizon is quite the standard view in "science communication" of black holes.
> Can you please give some reference for this claim?
Any GR textbook. I referenced two classic ones elsewhere in this discussion.
Or, for that matter, you could read the classic paper by Oppenheimer and Snyder which was the first published model of actual gravitational collapse to form a black hole. And was published in...1939. What I'm saying is not news.
> Time stopping at the event horizon is quite the standard view in "science communication" of black holes.
That's because "science communication" sucks at accurately portraying the actual science.
To this day I believe black holes remain a mostly theoretical phenomena. Which also happens to deal with infinity-mathematics in the singularity, something we don't see in reality otherwise.
Yes, observations have been made to their advantage, through LIGO and more recently the image from 2022 of a black hole made by the Event Horizon Telescope. But I would stress that the ""image"" is much more complex than simply a photo from really good ""telescope"", it's a image made of combined data from radio telescopes all around the world. There is enough weird hoops that they need jump through that the observation can reasonably be doubted in my opinion, which isn't to say that what they've attempted is awesome and inspiring.
> black holes remain a mostly theoretical phenomena
The fact that there are compact objects in our universe that, from the outside, look like the black holes that GR predicts, is not "theoretical", it's observation.
The theoretical question is whether these objects really are classical GR black holes "all the way down", so to speak, or whether there is some new physics that comes into play deep inside them (which many physicists think will involve some form of quantum gravity) that changes things. Even if the latter is the case, the objects might still look like classical GR black holes from the outside for a very, very long time, on the order of the Hawking evaporation time (10^67 years for a one solar mass hole, and the time goes up as the cube of the mass). So for all practical purposes, treating them as classical GR black holes works fine now and for the foreseeable future.
The black hole “photo” was constructed using machine learning. I don’t know how it can reasonably be called a “photo.” And although I’m sure their work was reviewed, knowing the nature of computers and data, I wouldn’t be surprised if there were all sorts of errors in the construction of the model that produced that output. What makes the output correct anyway, the fact that it looks like what we expected?
> The black hole “photo” was constructed using machine learning. I don’t know how it can reasonably be called a “photo.” And although I’m sure their work was reviewed, knowing the nature of computers and data, I wouldn’t be surprised if there were all sorts of errors in the construction of the model that produced that output. What makes the output correct anyway, the fact that it looks like what we expected?
The images of the EHT data were made using several independent techniques performed by different sub-teams who largely worked independently (e.g., [0] for M87). Most of the imaging techniques used are not machine learning approaches, but are rather standard imaging approaches, some of which have have been used for decades in radio astronomy (e.g., CLEAN). The fact that the images made from these different techniques look similar is what lends support to the idea that the "photo" is a reasonable representation. Sure, some images have been created using machine learning approaches, but other images have been done using well vetted approaches that are not machine learning.
But if one doesn't want to trust the imaging techniques, the size of the black hole shadow can be estimated directly from the radio telescope measurements. These are the "visibilities" (essentially measurements of components of the Fourier Transform of the distribution of radio emission on the sky) and the amplitudes of the visibilities (e.g., figure 2 of [1]) indicates that the brightness distribution of the source is approximately a thin ring, with the ring diameter setting the baseline length at which the amplitudes have their minimum.
So the basic result about the black hole shadow can be made from the visibilities themselves, without relying on the imaging. But the images of course make it easier to see some of the complexity (e.g., that the ring around M87 is not axisymmetric).
The objects we have detected look like classical GR black holes, yes.
The problem is that, theoretically, there are other possible objects that look like classical GR black holes from the outside for a very, very long time, but still aren't classical GR black holes. We have no direct way of telling from observation whether the objects we have detected really are classical GR black holes, or whether they are one of the other possibilities.
TBH, "irrationally" is a value judgment that I can't agree with.
Einstein's approach to physics was to reason out the consequences of observable phenomena. He approached understanding of the nature of the speed of light by realizing that a universe where you could travel as fast as (or faster than) a light ray would have consequences that we didn't see. So when the math tells him "gravity causes some regions of space to blow up to become inescapable," and we hadn't seen black holes yet, I think his first intuition being that we'd missed a trick that made the math fail to match reality was a fine intuition.
Math is only useful to physics to the extent that it actually models reality, and not all the extrapolations it makes turn out to have practical grounding; sometimes chasing the extrapolation reaches a contradiction that demands adjustments to math to fix the model. Chasing such adjustments in light of evidence is how we reached relativity in the first place.
> He approached understanding of the nature of the speed of light by realizing that a universe where you could travel as fast as (or faster than) a light ray would have consequences that we didn't see.
A sound wave could be outrun though. I don't see any step in why a such an philosofical approach would apply to light but not sound?
The relevant quote from his Autobiographical Notes is
"...a paradox upon which I had already hit at the age of sixteen: If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as an electromagnetic field at rest though spatially oscillating. There seems to be no such thing, however, neither on the basis of experience nor according to Maxwell's equations. From the very beginning it appeared to me intuitively clear that, judged from the standpoint of such an observer, everything would have to happen according to the same laws as for an observer who, relative to the earth, was at rest. For how should the first observer know or be able to determine, that he is in a state of fast uniform motion? One sees in this paradox the germ of the special relativity theory is already contained."
In short, he realized that if things could travel at lightspeed, we should be moving relative to something out there at lightspeed, so we should be experiencing its light as this weird "electromagnetic field at rest though spatially oscillating" (i.e. a field coming from apparently nowhere that changes in intensity as you move around in space but not in time)... but nobody had ever seen that phenomenon.
Einstein seems to have put a lot of stock in "If physics allows it, we should be seeing it" as a razor of sorts, so it makes sense that when the math said black holes should exist but they hadn't been seen, his default position was to be skeptical of the math and the intuition. Such skepticism had served well in the past for him.
(Worth noting for completeness: The "variable speeds of light predict stationary waves, but we never observe such things" objection is just one objection that comes up in Einstein's thought experiments. A more complete treatment of the subject is given in at https://sites.pitt.edu/~jdnorton/Goodies/Chasing_the_light/).
Maybe enforcing the second law of thermodynamics? Or they're a consequence of the same. It's a region of space where the conditions that existed before the big bang have ostensibly reestablished themselves, which is why it appears as a 2d boundary phenomenon to us.
An irrational belief is one you did not come to through reason.
Irrational doesn't mean bad or wrong. It just means it did not come from a rationale. Many beliefs we have are presuppositions which we demand strong rationale for changing, despite not needing similar rationale to arrive at the presupposed belief.
Believing the earth is flat because it looks flat is an example of an irrational belief practically everyone holds until taught otherwise. Some people hold onto the belief even after being given good reason to abandon it. We call that irrational behavior.
Having rationale to support your belief is inconsequential to whether it is rational or not. The important distinction is whether you arrived at the belief through rationale or not. Everyone believes the earth is flat by default because it appears flat. That's what makes it irrational.
If you later revert to believing in the globe because of reason and then convert back for reasons none of that matters. The initial belief was always irrational.
However, our access to facts and information is vastly different between two different people. That's because we grow in different spaces, meet different people, and have vastly different experiences.
Then you fall into this trap where you think you are right. You ARE right, but only in the context of your experience. Since it's rarely possible to ever know if you are universally right, you have to keep an open mind, which can be difficult.
This stuff was cosmic horror to all these folk. It caused existential crisis when you try imagine what is happening in and around a black hole. I'm not surprised they did everything they could to deny and disprove it.
Most would have had religious convictions that were really challenged by the physical reality of the universe.
When there is no experimental evidence for black holes, it makes perfect sense to be resistant/skeptical to new type of objects with solutions involving infinities (singularity). Whether that's being irrational or not is up to debate. I'd say that's a good prior when there is no data (as there wasn't at the beginning of 20th century).
On the other hand, the gold standard of scientific verification is the prediction of something not yet observed, and in that light, it seems to me to be somewhat inconsistent to reject a prediction of your own theory on the basis of it being at odds with your a priori feelings about how things should be, while still maintaining that everything else about your theory is correct.
Update: It is one thing to suppose that the singularity is a mathematical artifact, and another to reject black hole formation entirely. In this regard, the article seems vague about exactly what Einstein and Rosen were saying in their 1939 paper. It seems to me to be saying that, having concluded that collapse would not stop at the Schwartzchild radius and that would lead to a singularity, we should conclude that collapse to the Schwartzchild radius itself is not possible.
The article linked to this point is (IMHO) well worth reading, and seems to support this view of Einstein’s position.
Now it might be, but let's remember that back in the day Physics was mostly used to derive laws from observations. Only with the advancement of calculus, notation, etc. we could make predictions that later we confirmed. Granted, at the time of the black holes prediction this had been going on for a while so it should not have been too surprising, but also I don't think it'd be such a strong point as it is today.
Note that I was concurrently adding an update to my post as you posted this (though I think my update is orthogonal to your point here.)
With regard to contemporary attitudes to verification, we know that Eddington went to Principe to measure the magnitude of gravity’s effect on light specifically because it would be seen (and was) as strong confirmation of the theory.
Well yes, but they also knew stars collapsed, and that something had to happen in that case. Maybe at first they thought fine, everything becomes a white dwarf or a neutron star.
But then after Oppenheimer did the math in 1930 and showed that collapse into a black hole was within the theory of relativity would you still taken the same position?
If the theory of relativity was a person, it would not be old enough to drive or vote in 1930. :) It was an accepted theory, but it was not the wildly successful theory it is today.
Showing that a black hole was possible within the theory could just be a problem with the theory itself. Not necessarily an indication that they must exist.
This. Physics is a field of science, unlike math. They're dealing with models that keep on changing. Newton's model of F=ma is generally close enough for most things, but breaks near the speed of light. Following the model solely would give you the false idea that speed of light is just yet another number.
Case in point: the same equations predict white holes, which today are widely believed to not exist.
I’ve never seen one before — no one has — but I’m guessing it’s a white hole.
A white hole?
A door to past.
What is white hole ?
A person with a better astronomy background can jump in, but I'll start by quoting a couple trusted astronomy publications. In short, a white hole is an astronomical object with the property that light cannot enter it.
From the Universe Today (at https://www.universetoday.com/122715/what-are-white-holes/ ):
"Black holes are places in the Universe where matter and energy are compacted so densely together that their escape velocity is greater than the speed of light. […] Fully describing a black hole requires a lot of fancy math, but these are real objects in our Universe. […] So then what’s a white hole?
"White holes are created when astrophysicists mathematically explore the environment around black holes, but pretend there’s no mass within the event horizon. What happens when you have a black hole singularity with no mass? […]
"Now if white holes did exist, which they probably don’t, they would behave like reverse black holes – just like the math predicts. Instead of pulling material inward, a white hole would blast material out into space like some kind of white chocolate fountain. […] One of the other implications of white hole math, is that they only theoretically exist as long as there isn’t a single speck of matter within the event horizon. As soon as single atom of hydrogen drifted into the region, the whole thing would collapse."
And from Space.com (at https://www.space.com/white-holes.html ):
"White holes are theoretical cosmic regions that function in the opposite way to black holes. Just as nothing can escape a black hole, nothing can enter a white hole. […]
"To a spaceship crew watching from afar, a white hole looks exactly like a black hole. It has mass. It might spin. A ring of dust and gas could gather around the event horizon — the bubble boundary separating the object from the rest of the universe. But if they kept watching, the crew might witness an event impossible for a black hole — a belch.
"Physicists describe a white hole as a black hole's "time reversal," a video of a black hole played backwards, much as a bouncing ball is the time-reversal of a falling ball. While a black hole's event horizon is a sphere of no return, a white hole's event horizon is a boundary of no admission — space-time's most exclusive club. No spacecraft will ever reach the region's edge. Objects inside a white hole can leave and interact with the outside world, but since nothing can get in, the interior is cut off from the universe's past: No outside event will ever affect the inside."
A region of space time matter/light/information can exit but you can’t send anything to.
https://en.wikipedia.org/wiki/White_hole
I don't know a lot of advanced physics or quantum mechanics, but black holes also confuse me.
I would imagine that black holes show the limit of modern physics, there are still things to learn, and models of physics will probably be adjusted in the future.
I guess there are physicists who are able to predict how a black hole works and appears, and how it can be explained, but I haven't read about it.
I mean, yeah there's tons of theoretical work. Predictions about lensing and structure have been done and verified. We've taken pictures of black holes. Susskind did a big fancy simulation for Interstellar that lined up great with them.
my guess, in short, is that black holes are equivalent to infinite loops, and white holes solve the halting problem
What does that even mean?
That's quite a wild guess.
Given he was clearly convinced in the end by sound arguments, was it really irrational or was he just a hard sell for good reasons?
I think there's plenty of reasons to be a hard sell on relativity's take on black holes. After all, it's still a plenty hard sell today. The evidence that things that looks like black holes on the outside exist is pretty solid, but the evidence we have about the insides is zero. I don't particularly "believe" in the relativity take on black holes either, which is to say, I have very high confidence that a real and effectively-correct theory of quantum gravity will remove the singularities.
I'd love to pull off the trick of being extremely easy to convince about true things and only true things, and being very stubborn about everything else, but that's basically epistemologically begging the question. It's too much to ask of anyone.
I am halfway convinced a singularity will never actually reach the singularity form due to time dilation effects. that is, it ends up taking forever to actually reach zero size(not that time or space have much meaning inside such an environment)
You should also never see anything falling into a black hole because time is so stretched near the horizon it’d take almost literally forever to reach it. The bright light we see is the tidal force tearing matter apart as it approaches the horizon.
> I have very high confidence that a real and effectively-correct theory of quantum gravity will remove the singularities.
What gives you this confidence?
The general belief that singularities are “unphysical” and reflect a flaw in our models than actual reality. Hard to prove definitely right now of course.
I always thought the concept of a singularity here was just to make the math slightly simpler. That is to say that a black hole is still a spherical body with a set diameter. We just can't see it so there's no way to verify other than the fact that we can see the border of the event horizon.
As far as I know, that very much isn't the case. The singularity is a result of the math, not a trick to simplify it.
My understanding is that many researchers do think it's a "placeholder" representing our lack of insight into how physics behave at those scales, and not a real physical phenomenon, and that it will one day be possible to work past it. I believe Kerr and Penrose (both of whom did some of the most foundational work on black holes) believe it's a mathematical artifact.
My random thoughts on this:
- Is the math referring to the center of gravity as the singularity? Because even a sub black hole mass object would have that.
- Why is there an assumption that just after a mass becomes a black hole the matter inside it suddenly compresses further when the actual gravity of the object has only slightly increased?
- Is there a maximum density of matter in the universe and if the black hole even reaches that?
- Wouldn't you need that number to be infinite if the black hole itself is infinitely small?
- If the black hole does have a mass inside it... Do the light particles trapped inside the black hole form a blanket around the existing matter?
The math is very difficult. More difficult even than that. When ever you are trying to explore the consequences of General Relativity, it is necessary to make some simplifying assumptions. Schwarzschild started with several such assumptions. First, he assumed a completely static system, one that doesn’t change with time. Second, that the mass in the system was spherically symmetrical. He also put the center of the black hole at the origin, and so on. All of these assumptions are there to make the math easier; they’re like using spherical cows in high school.
What Schwarzschild found was that this solution had two singularities. There was a singularity at the origin, and a singularity some distance away from the origin, at a radius which is proportional to the mass of the object. In both cases, some term of Einstein’s equations become infinite, and therefore seem to stop describing reality.
A couple of years later other physicists proved that by changing the coordinate system in a certain way, that second singularity would go away. That is, this singularity was merely an artifact of one of the simplifying assumptions that Schwarzschild started with, and didn’t represent any real feature of a real black hole. But the radius at which that singularity occurred is still relevant: it tells you how big the event horizon is.
Penrose and Hawking proved that the singularity at the center of the black hole cannot merely be a mathematical artifact. Penrose got his Nobel prize for that.
> Is the math referring to the center of gravity as the singularity? Because even a sub black hole mass object would have that.
Yes, the singularity at the center of a black hole is coincident with its center of mass. But not every center of mass is a singularity. Remember, we only have a singularity if some term of Einstein’s equations for General Relativity goes infinite. In a normal object nothing is infinite at the center of mass. That only starts to happen once the object is compressed into a black hole.
> Why is there an assumption that just after a mass becomes a black hole the matter inside it suddenly compresses further when the actual gravity of the object has only slightly increased?
There isn’t. This is the most common _conclusion_, based on the math. It’s not a starting assumption.
> Is there a maximum density of matter in the universe and if the black hole even reaches that?
Unknown. A neutron star is the densest object that can exist which doesn’t have an event horizon. Get any denser, and an event horizon forms which completely hides whatever happens next.
> Wouldn't you need that number to be infinite if the black hole itself is infinitely small?
Sure, if the mass all falls into the singularity, then the density of the singularity is infinite. There’s really no problem with that.
> If the black hole does have a mass inside it... Do the light particles trapped inside the black hole form a blanket around the existing matter?
This is a non–sequitur. It’s meaningless.
What is really going on here is that mass bends spacetime. This causes our path through spacetime to bend as well. All the gee–whiz effects of relativity, like time dilation, are a result of this bending. Time passes slower for an object moving quickly because its path has been bent so much that it is passing through less of the time dimension than it would otherwise. Light is moving so fast that it doesn’t pass through time at all; from the perspective of the photon no time at all passes between when it was emitted and when it was absorbed, no matter how far it traveled through the intervening space between those events.
Inside the event horizon of a black hole, the path of every object is bent towards the singularity. They’re bent so much that time and space swap roles. The singularity is inescapable not because something is dragging you towards it, but because it is literally in your future. From the outside it looks like the singularity is a place, but if you get close enough it becomes a time in your future, beyond which there is no more time. Everything outside the black hole is twisted around into the past, where you can never go. And, unfortunately for you, a stellar mass black hole is not very big. It’s only a few dozen miles to the center, and since we move through time at the speed of light, that is a very short timeline indeed. Your future ends very abruptly, probably faster than thought.
That’s why most people conclude that the matter is crushed into the singularity. How could it resist? You’re not going to brace yourself against the walls of the universe and prevent yourself from being dragged forward into the future, no matter how strong you are.
But, quantum gravity might change that picture slightly. It may be that the distance to the singularity grows the longer the black hole exists. It may in fact grow at the speed of light, meaning that while the volume of the black hole is not infinite, you never actually reach the singularity and go splat. Your time never actually runs out. You can find some lectures by Susskind about this on Youtube if you like.
> > Is the math referring to the center of gravity as the singularity? Because even a sub black hole mass object would have that.
> Yes, the singularity at the center of a black hole is coincident with its center of mass
I don't think this is accurate. Take Schwarzschild for instance: The singularity is not a place in space you can poke with a stick. It's a spacelike singularity, and it lies in the future of any observer unlucky enough to fall into the BH.
To the external observer, the black hole is perfectly spherically symmetrical.¹ The center of mass of a spherically symmetrical object is always right in the center. Right where the singularity would be, if it weren’t for the fact that to the inside observer the singularity is now in the future at the end of time.²
These types of coordinate changes are one reason why the subject is considered so hard.
¹ Except very briefly as two black holes merge. Or in the case of a rotating black hole. Or potentially during the formation of the black hole. But those are all complications and the subject is difficult enough.
² Except for the fact that the singularity might not actually exist if the interior of the black hole is growing without bound in the timelike direction. See ER=EPR.
Ah, but it's not! It's symmetrical in 3D, but not in 4D. So if you move close to the event horizon, the symmetry breaks.
> There isn’t. This is the most common _conclusion_, based on the math. It’s not a starting assumption.
How? Time doesn't work inside black holes, we have no clue how that even works or if things are frozen or if it happens instantly. Math can't answer what happens when time doesn't work.
This isn't just the time paradox of near light travel, the math says that inside black holes you have an entire different dimension of time instead of our regular one, nothing can be said at all about what it looks like from our point of view since there is no concept of shared time between us and what is inside of black holes.
So either the formulas are wrong, or we can't say anything about what is inside.
> Time doesn't work inside black holes, we have no clue how that even works or if things are frozen or if it happens instantly. […] This isn't just the time paradox of near light travel, the math says that inside black holes you have an entire different dimension of time instead of our regular one
There is no such thing inside a black hole as "an entire different dimension of time." Yes, by the very definition of an event horizon, the coordinate chart of an observer at infinity cannot extend inside a black hole. Needing multiple coordinate charts is not really unusual, though. The two-dimensional sphere S² also requires at least two charts. All in all, time inside a black hole locally works the way it does outside a black hole. Yes, there might be interesting non-local effects like closed timelike curves but you can also have those outside.
> time inside a black hole locally works the way it does outside a black hole
That doesn't mean that it actually runs though, you can interpret the lack of possible shared reference of time as the black holes time either happens instantly, but also that it never ever happens and everything in it is stuck and time is frozen from our perspective. In the time frozen perspective the singularity would never form, all objects are frozen and never reach it.
Time doesn’t magically freeze inside of a black hole. That is a misunderstanding of what happens to light rays carrying the image of the infalling diver as they approach the event horizon. From the perspective of an outside observer, those light rays are slowed down more and more and light from the instant of crossing the event horizon never actually arrives; it appears to take an infinite amount of time to do so. The mistake is to think that the object is still there, hovering at the event horizon, for an infinite amount of time. The reality is that the diver notices nothing unusual as they cross the event horizon; they may not even be able to tell exactly when it happens. They certainly do not freeze in place forever.
> The reality is that the diver notices nothing unusual as they cross the event horizon
That doesn't mean that he actually crosses it from our reference frame. Relativity is all about understanding that time is relative, that applies here as well, except that there is no connection between the two reference frame unlike regular space, so we can't say anything about how time flows inside of a black hole from our perspective.
Basically what I am saying is, you can't say whether the singularity has actually formed, since your reference frame is just as connected to the instant anything fell into the black hole as it is connected to the end state of the singularity. That makes it just as valid to say that things freeze the instant they touch the black hole as to say that they merge with the singularity the instant they touch it, since both are equally valid viewpoints from our perspective.
> That doesn't mean that he actually crosses it from our reference frame.
This specifically is an incorrect understanding. All events happen in all reference frames. There are no situations where an event that happens in one reference frame fails to happen in another. At worst (or best, depending on how you view it) we can disagree about the _order_ of events, but not which events actually happened.
It’s not that the crossing fails to happen. It’s just that, from the outside it takes an infinitely long time to happen and, for the diver’s perspective, it’s the universe outside that ended really quickly.
You are correct that we just dont know what happens behind event horizont, from our perspective at least. Currently our best theory says nothing from inside can leak out, so in that spirit saying infalling matter ends up in singularity is as based in reality as it transforming into unicorns.
Regardless, its important to keep in mind that this is valid _only_ from our perspective, that from the perspective of infalling matter something _real_ happens, and that real thing that happens is probably not completely disconnected from rules we perceived in our own universe.
Throwing hands up and saying "we cant tell whats inside, so why are we even arguing about what we think happens there" is certainly a way to look at things if your only motivation is to find out whats happening inside, but that is not all we are after. We are trying to use all we currently know to reason about how the inside might look, any possibility, and try to work out backwards what unforseen implications it might have for our reality.
We are not saying that inside blackhole is sigularity, we are saying that, given our understanding of how outside looks, there might be a singularity inside. We also know the math that yields us this singularity is probably wrong (infinite), and actively are trying to figure out whats wrong with it. We are using this imaginated realoty to work backwards what we might have gotten wrong, and perhaps in this case its not even the roght way of thinking to solve this mystery; it just worked for us before, so we trying.
If you figure out whats actually happening there, and find some way to connect it to rules of our universe, everyone will cheer. If its by first discovering new effect in our universe that would elegantly fit into blackholes, or in the stroke of genius come up with a pure theory enlighting some real observations, nobody would care.
Its just when we throw a mass, it keeps going. It makes more sense to think that it goes the same inside blackhole, but maybe not. We just dont know..
> Its just when we throw a mass, it keeps going
You wouldn't feel going through the event horizon of a large black hole, so I see no reason why that mass would somehow just collapse into a singularity.
Also the math for a black hole states that when the black holes density reaches below the density of the universe, the black holes event horizon will expand and encompass the whole universe, meaning matter doesn't even have to fall into the black hole, the black hole could just grow into the matter and then you don't even have a velocity you just have matter entering the black hole.
We know the two above are how the math for black holes work, none of those produce likely singularities, because there is no force compressing that into a singularity. So if singularities exists in black holes, it would just be some of them, and not all parts of the black hole would have to be a part of that singularity.
I am not arguing for certainty of singularity, quite the opposite. I stated that we seriously suspect the math yielding us that singularity is wrong, we just cant figure out anything more correct yet. Personally I think the geometry of spacetime inside blackhole gets all twisted up, bubbling like a boiling pot of water, but my guess is as good (probably worse) than anyone elses.
>when blackhole density falls below the density of the universe
Which one? The outside one? Density of whole universe? Its local surroundings? Are you counting unknown pressures of stuff we, for pure lack of understanding what the hell it even is, call dark energy/mass?
The "blackhole less dense than the rest of the universe" seem like just another mathematical artefact that couldnt, for any reason, exist in real world. Not saying thinking about it is wrong, but imho more could be gained by thinking "well it surely cant exist, so what mechanism are there that stops this", instead of "this math says that, so lets subdue our whole theory to it". Very same scenario as the singularity inside.
Just because math works for something doesnt mean it is like that. In science history we had plenty of equations that worked, until we discovered some edge case where it suddenly didnt. I think its safe to bet interiors of blackholes, or blackholes encompassing whole universe, are prime candidates for such edge cases where our pressumed math models just break down. The fact that our current math models are returning infinities, we are generally accepting that to be the case.
> but imho more could be gained by thinking "well it surely cant exist, so what mechanism are there that stops this"
Well, the universe Schwarzschild radius is massively larger than the universe itself, so math says we live in a black hole. It is true this might be a mathematical artifact, but it could also be how it actually works and the observations we make in our universe and about big bang is how a black hole looks on the inside.
I don't think you should just assume the math is wrong here and throw away those observations of the inside of a black hole. Sure you should be open to that being wrong, but at least to me that is far more believable and interesting to study, then you can try to connect what we observed about the big bang with a black hole etc.
If I go through enough of these forum discussions maybe I develop this long enough, and then maybe I'll take a serious attempt at solving the math to combine the two, since nobody else will do that. I was called a genius by both my physics and math professors, so it is possible I can solve it since I have good intuition for physics and good skills at math, most people who work in physics just have one of those, even though likely I'll fail, but it would be fun to try.
What makes me believe such an attempt could work is that it could lead to a different math for gravity and black holes at small scales, and thus lead to a unified theory of QFT and GR. If I manage to do that then I could convince physicists to adopt the new interpretation of GR. Current attempts of unifying those two has started from the QFT side and tried to incorporate current GR theories, but they all failed. But I haven't seen many try to attempt that from the GR side and unify that with QFT, so maybe that would yield some results.
Anyway, I don't think that professional physicists will solve this, since they are too occupied focusing on publishable results rather than solving the core issue. What I write here isn't publishable, so no physicists would even think about working on it.
It is how GR was invented by Einstein even though he wasn't a physics researcher, sometimes you need a big jump done by an outsider. GR wasn't on anyone's radar when Einstein suggested it, likely it would never have been discovered without Einstein, or at least take centuries. The gravity formula you get from GR doesn't need space time distortions to reproduce, so theoretical physicists would just update the gravity formula to match observations without creating a new interpretation of what that means.
Newtons gravity formula doesn't come with an interpretation, updating it to match new data is exactly what they did with dark matter and dark energy since it is just a formula, so I see no reason why gravity would be any different without Einstein's GR. Black holes would just be a part of that dark matter.
Edit: Anyway, for me it is fun to argue on forums, so that is the best way for me to think and develop theories. Typically I don't really believe what I say, so part of my posts is to find evidence for it and try to convince myself that what I say is true. So I refine things quite a bit in every discussion.
I got a good idea: Nothing ever passes the event horizon, rather as an object gets close to the event horizon the combined Schwarzschild radius of the black hole and the object will at some point cover the object, meaning the event horizon would expand and engulf the object instead of the object moving through it.
This would totally change the math around the event horizons, since then space doesn't need to be connected between the interior and exterior of the hole. The outside of the hole still would work the same, but as the hole bends space to engulf objects rather than objects moving through space to enter the hole you get a very different way to look at the interior.
Not sure how the math for that would work, but it is fun to think about. Also this is obviously true, for any particle that point would happen before it enters the current radius, so the hole expands out, even if it is by an extremely small amount that is still enough to change the math around the event horizon singularity.
Edit: The main thing this changes is that now the particle doesn't have to accelerate to the speed of light as it passes the event horizon, as the event horizon expands to cover it rather than it accelerating through. As you can understand that is a massive change, it means that space inside the black hole could remain sane without breaking GR. The infinite time dilation you get at those gravity levels is why the math for space and time inside of black holes is so strange, remove that and you can get back to sanity.
Then we could assume that the interior of the black hole looks just like our universe, since our universe is smaller than its Schwarzschild radius. Basically we want to show that the interior space time of a black hole can be relatively flat.
The problem to solve then is to calculate what happens at the edge of such a universe, and try to match that with what happens at the edge of a black hole, since those two would be the two sides.
To unify that with QFT you'd have to include the particle collapse (or entanglement if you want to use the many worlds wording), where the quantum field would collapse to where the particle suddenly is inside the black hole instead of outside. I have long theorized that there is no way to unify the theories without using quantum collapses, so black holes could engulf particles via quantum field collapses only.
The math for such collapses isn't fully established though, so it would need to have more theories on the quantum side as well. But at least that means nobody has developed a theory like this yet.
The main calculation would be to calculate the probability of a particle collapsing into the black hole over time, by calculating how large part of the particle field is inside the critical area were it would be inside the new black hole radius. The particle and the black hole would get entangled for a bit where it is both inside and outside the black hole at the same time, and then the field collapses and it is fully inside.
I think that can actually work! Then the black holes event horizon is a natural quantum effect, which could potentially provide the missing link between the two.
There is no disconnection between the interior of the black hole and the exterior. They are smoothly and continuously connected, and the “normal” axis of time blends seamlessly with the “bent” axis of time inside the black hole. That’s why orbits are so weird outside the black hole.
You’re simply wrong about this.
So how long does it take for a thing to enter the singularity as seen from our reference frame? If you say there is no way to answer it, then you say that I am right, so I'd like to see a link to a formula saying how long that takes from our point of view.
I studied up to a masters in physics, I've seen the math, I don't believe there is a way to calculate that, there is no way to compare time inside with time outside.
Edit: I'd love to see such a formula if there is one btw, so if you know there is one please link!
> Time doesn't work inside black holes…
Time and space work exactly the same inside the black hole as outside, they just swap places. One dimension that was spacelike is bent into becoming timelike. Closer to the center becomes further in the future, and outside the event horizon becomes part of the past. The singularity at the center of the black hole is now the literal end of time. Nothing exists past the end of time; you hit that singularity and you cease to exist.
From the outside, all that we can detect is the mass of the black hole. Or rather, all we can detect is some gravity pulling us in, as if there were mass inside. Einstein proved that all gravity is just the curvature of spacetime. Your body is bending the fabric of spacetime right now!¹ When you dive into a black hole, the curvature you are causing is added to that of the black hole, so that the black hole grows exactly as if your mass still existed inside of it. But really you ceased to exist when you hit the end of time at the singularity.
But of course that is how we _used_ to think of it, up until a decade or two ago. As Susskind would say, ER=EPR. Spatial connectivity (which Einstein and Rosen wrote a paper about) is exactly the same thing as quantum entanglement (which Einstein, Podolsky, and Rosen wrote a completely different paper about in the same year). I won’t try to recapitulate one of his lectures (since you can just watch one; see <https://www.youtube.com/watch?v=31fVea8_OAw> for a recent example, but there are others), but the result is that the spacetime inside the black hole grows without bound, and you never actually reach the end of time. You cannot go back, but neither do you go splat. The matter that was once a star is somewhere ahead of you in that growing region of spacetime, forever in your future.
¹ Note all possible “yo mamma” jokes for future reference.
Let me rephrase: The time we measure with our clocks from the outside doesn't apply inside of black holes. There is no way to say whether any singularity has ever formed or ever will form even according to the math, since time inside of them isn't the kind of time we can measure from the outside.
If you go inside of it the math says that now your clock inside of the black hole can still function, but that also means you are no longer connected to time outside of the black hole, so you can't say whether anything actually happens inside of them from our perspective, all the math says is what it would be like for someone inside.
You need to make extra assumptions about what it means to have a disconnected time reference frame to say anything.
> - Why is there an assumption that just after a mass becomes a black hole the matter inside it suddenly compresses further when the actual gravity of the object has only slightly increased?
Yeah, that seems like an important thing to address. Black holes don't even have to be high density, or have much matter in the center at the time of formation. If you arrange enough big chunks of metal into a spherical-shell constellation, then drop them all toward their mutual center, they can reach the threshold to become a black hole before they even start colliding. What happens around that time, and what specifically does "singularity" mean (because I see people using it in very different ways).
> - Is there a maximum density of matter in the universe and if the black hole even reaches that?
> - Wouldn't you need that number to be infinite if the black hole itself is infinitely small?
Black holes are the benchmark for maximum density. The event horizon is always exactly at the limit. By "black hole itself" do you mean something other than the event horizon?
But that maximum density depends on size, smaller things can be denser and bigger things have to be less dense.
> What happens around that time
I can imagine it's something stupidly energetic. If you don't want them turning into a hot ball of neutrons first you're either going to have to spend a lot of energy keeping them apart or they are going to be orbiting a virtual center and it will take far longer for them to reach the center singularity than you expect. This will follow the laws of the 3(+) body problem so you won't be able to calculate a perfect impact, so the birth of the singularity will be shrouded in very high energy particles escaping.
Nature abhors a naked singularity.
It wouldn't be naked, there would be an event horizon.
Even approaching the critical density, barely anything would be escaping.
But if it's really really big, there wouldn't need to be any notable spaghettification.
> If you arrange enough big chunks of metal into a spherical-shell constellation, then drop them all toward their mutual center, they can reach the threshold to become a black hole before they even start colliding.
When I say "black hole itself" I imagine any combination of mass at it's center that provides enough density to form a gravitational pull that will trap light.
All we know is that we’ve made a lot of crazy predictions about the structure and behavior of black holes well before we measured them and so far they’ve aligned remarkably close to the predictions. If there is a difference, it’s about the interior not how it shows up in our universe. And since we can never see inside the black hole, any alternative model would have to better explain some phenomena our existing models can’t or be even easier than relativity and explain the same things. We know string theory is hard to make consistent and it struggles to create falsifiable experiments. I would bet on relatively remaining a very very long time until we figure out how to unify quantum and relativity.
To the best of my knowledge, none of the competing theories for integration have singularities in them. It is difficult to know what a "singularity" would look like in a universe that doesn't seem to have 0-dimensional mathematical points in it.
It isn't that surprising that continuous theories have singularities in them. Reality doesn't seem to be continuous in the sense that real numbers are. It is unknown if Navier-Stokes can develop singularities in finite time, but in the real universe it won't matter because they won't lead to singularities in the real universe because it is ultimately just an approximation. "Interesting things" may happen around whatever those conditions are but nothing will be accelerated to infinite speed in the real universe. It is another theory that is a real-number-based continuous approximation made useful by the fact that real numbers are easier to do math with than physical quantities, but the real universe does not have plain real numbers in it. (This is subtly different than the controversial statement that it isn't made out of real numbers at all. But it does not have "plain" real numbers. You certainly can not expect to pull out a microscope and peer at the universe at a scale of 10^-10000 meters and see a universe that behaves exactly the same, just at a smaller scale. Our universe is definitely not scale-invariant.)
I think this is likely to be a rather accurate metaphor.
To be honest, I don't see a lot of positive reason to assume that the singularities are going to be "real". I think physicists initial reactions to the idea were correct and remain correct. I think the reasons why people think otherwise are emotional rather than logical. They've gotten attached to all the stories around black holes and all the science fiction and all the crazy ideas (parallel universes! white holes! wormholes!) and a general sense of "woo" that I for one do not find very appealing, and what people are reacting to is the loss of that, not the idea that the universe probably won't have any infinitely dense points... when the universe doesn't seem to have any points in it in the first place.
(To forstall another common rabbit trail, I don't think the universe is continuous... but that doesn't mean I automatically think it must be discrete: https://news.ycombinator.com/item?id=38433917 )
the dunning kruger effect
That doesn't really follow, because it seems that people were and are wanting black holes to be something else that they are both not observed to be and not described by any theory of them.
Also, I think it gets lost in physics that theories are models of reality. They are not reality themselves. General relativity is about as good as a model gets. Alongside the standard model, it's one of the most tested models around.
A singularity has a meaning in physics in terms of math, which is dividing by zero or something approaching zero in the denominator. A singularity has a physically modeled aspect in that it's the name assigned to whatever becomes of the matter that gets squished down. A black hole is effectively not a thing in itself but rather an effect of what happens when mass is squeezed beyond all known limits.
What's inside a black hole is not the only thing in the universe we can't see. The actual universe is far bigger than the visible universe, but we can't see outside of the visible universe due to how light works. So it basically doesn't even matter what's going on there. The question is if anything happening at the boundaries can tell us something about that which we can't see. It doesn't make sense to dismiss what we observe on the outside based upon what we literally cannot know of the inside.
And after all, general relativity is a classical theory. Is it really all that "weird"? It all feels somewhat mechanical and natural as you start learning it and turn off your biased intuition.
We have observed one black hole from the inside, the big bang. The reality we see today doesn't really look like what general relativity math says the inside of a black hole should look like.
The pro singularity people must then believe the black hole evaporated really quickly into a big bang to rejoin the out universe, but a more rational answer is that the big bang is what happens inside black holes rather than a singularity forming. Space inflation after the big bang would just be the black hole expanding as it ate more material in its near vicinity.
It's all science fiction, stuff I like to think about just before I go to sleep. But I like to pretend this is what happened to time, why it is only one way. see, once you pass the event horizon of a black hole it effectively removes a half dimension from reality(you can't go back towards the horizon) time is that half dimension we are missing.
It isn't just science fiction, the Schwarzschild radius of the universe is massively larger than the universe itself, we are living in a black hole according to current math.
And in theory a black hole whose density gets lower than the density of the universe will grows its event horizon to fill the entire universe. Such a large black hole doesn't have strong tidal forces, so you wouldn't even notice as you entered it, from the outside it would look like the black hole disappeared and the internals popped out rather than you entering it.
At least that is what the math says. Likely there are some more strangeness as you enter the event horizon, since at that point you effectively pass the speed of light barrier, creating infinite time dilation (a gravitational difference is equivalent to a speed difference in relativity, and the anent horizon is when that becomes the speed of light). That means we have no math to explain how time inside the black hole is related to time outside.
So while the space tidal forces wont rip you apart space wise, the time dilation difference between your different parts would be infinite, since at the event horizon one part of you will see the other part as having infinite time dilation, I'm not sure how you could get through that unscathed. The particles might still be next to each other, but you might turn to dust from infinite time passing resulting in infinite particle decay, basically resetting the state of your matter.
I did study all this math in a masters degree, I am not a working physicist but I think this is a much more reasonable interpretation and also more testable than the prevailing ones about black holes. All the math of the testable parts adds up, while the prevailing theories are wrong since the universe isn't a singularity.
Edit; Note that extremely few physicists are even researching GR seriously, barely any work has been done on it since the Einstein days, quantum field theory is where all the useful applications and hence money is at, it also has way more testable results, so almost all physicists are approaching the problem from that angle. The few who do study GR are mostly focusing on the raw math and not interpretations, since math is publishable and interpretations are not, also since GR requires so high level math that basically only those who love math over nature even wants to study it.
Case in point, Einstein himself didn't understand GR math, today he wouldn't be able to publish any GR research even though he invented the theory.
But wouldn't that mean it's black holes all the way down? If in our universe, another universe is inside every black hole, then aren't those universes quite small due to their total mass being the same as the black holes'? The mass of black holes can be quite modest. Then the universes inside those universes' black holes would be even smaller, and so on.
Also, it's my understanding that black holes shrink over the time, so they aren't expanding and certainly not at increasing rates like our universe is. The ones in the center of galaxies "gobble" up gas, but most black holes aren't at the centers of galaxies.
> The ones in the center of galaxies "gobble" up gas, but most black holes aren't at the centers of galaxies.
Most black holes aren't as large as our universe, although if that theory is true we can't say much about what a typical black hole would be like in the larger universe.
It is also possible our universe ate the entire outer universe. At a certain size the black holes density is less than the universe density, at that point its event horizon will expand forever and eat the whole universe. That could happen to our universe as well. That probably wouldn't destroy the outer universe though, since entering such a large black hole doesn't rip you apart, so in a way that is a theoretical way to "exit" a black hole, although technically you make the outer universe enter it.
Edit: Back in college I intend to go into theoretical physics, but once I got to string theory etc I no longer believed in it, and continuing from there would mean I'd have to study that sort of nonsense for half a decade just to enter the field. That makes theoretical automatically turn away anyone who have alternate interpretations, so you wont find them form working physicists. I know all the math from the tested parts of GR, the rest are just untested theories, mine are as good as any there.
That seems more like a fun sci-fi speculation
So here is the evidence: We know the big bang fulfilled the math criteria of being a black hole. We also know how black holes work from the outside from observations today. The simplest theory explaining both of those is that the math for the inside of the black hole is wrong, while it is right for the outside, and that we observe in our own universe/black hole is what really happens inside. Any other explanation simply has less evidence behind it.
Speculating that there is some strange singularity there is just as much sci fi as speculating that there isn't and that what we observe from the big bang is the normal state. Any theory saying that the black hole has a singularity inside would need to explain why the universe isn't a singularity, otherwise its provably wrong, since we know the universe isn't a singularity.
The only evidence against the universe and the big bang being a black hole is that it lacks a singularity, everything else adds up to it being a black hole. Ignoring that evidence is more unscientific imo. Also saying that the universe expands due to "dark eneregy" is also baseless sci-fi in that case.
Anyway, if black holes do become big bangs like that then it could be testable by looking at distributions after a big bang etc and compare that with some different ways of calculating black holes sucking up mass in different scenarios to see if it is reasonable. Science starts with speculating about stuff, speculation is not non-science. At least this is more testable than stuff like dark energy.
Edit: I've also studied all the math and physics in college for the testable parts of GR, I haven't studied string theory, at that point I no longer felt the physics made sense, I don't think it is wrong for me to hold alternative theories to the untested ones.
I see no epistemic problem with a singularity existing. After all electron are singularities in the EM field, of a sort. Why shouldn't a lot of electrons together form a giant singularity?
Truth be told, we don't really know that electrons are singularities in the EM field. Classically that's the case, of course. But in QFT, as you start to probe to shorter distances (and higher energies), our theories eventually break down because gravitational effects cannot be ignored. (For instance, how do you describe the EM field at distances shorter than a Planck length? The energies needed to probe to that limit would collapse the system into a black hole.)
> Truth be told, we don't really know that electrons are singularities in the EM field
We do know they aren't such singularities. QFT says they aren't, the electron is a quantum field spread out over an area it isn't a point particle. QFT is the most well tested theory in existence, so if we can't call that "know" then we don't know anything at all in physics.
What we don't know if whether there is limit to how small that quantum field can get, but we do know it can't become a point, due to the Heisenberg uncertainty principle that states that for a particle to be a point it would have to have infinite momentum.
Note: Heisenberg uncertainty principle was named at the time when people still believed quantum fields were probability distributions, but since then we have established that the quantum field is the particle and not just a probability of where the particle is. That means the Heisenberg uncertainty principle establishes a relationship between momentum and size instead of just knowledge.
I'm not at all an expert in this, but my limited understanding is that electrons are only "singularities" in classical EM; in quantum field theory they're just solitons of the underlying quantum field and can be analyzed with no infinities present. We haven't found a QFT equivalent for gravity yet, but I think the broad consensus is that the gravitational singularities will disappear once we do.
Afaik even in qft they are effectively singularities, but I'm not an expert in that yet.
It's not that they are infinities, it's that they're point discontinuities in the A field.
In qft there are no singularities, everything are fields. Particles are just such fields, a singularity in qft would mean you have a Dirac delta shaped field, I don't think such a field exists in reality and I don't think many working physicists would believe in them either. The big reason they can't even come up with math for the black hole singularities is just because the theory doesn't support working with such Dirac shaped fields.
Edit: When they say electrons doesn't have a size, what they mean is that there is no limit to how small you can make an electron, not that it is always compressed into a point.
Yeah about that. Turns out position and momentum are a bit more complicated than we thought when electromagnetism was first described.
Suffice it to say an electron that has a singular position is about as realistic as one that has a single momentum but exists everywhere in space.
Alternative:
Quantum theories arise because the singularities in black holes create a topological regime within their vicinity — and for SMBs, that’s a whole galaxy.
I think it’s interesting that different people feel different things are “natural”.
> It's too much to ask of anyone.
Ternary logic is available (physically at least).
In layman's terms, isn't the take that the area of a black hole is devoid of space? Therefore you can never see or measure it except for forces generated by it within our space.
The claim of general relativity is that a black hole is a singularity in spacetime, meaning that space effectively "ends" there. A very crude model would be cutting a hole out of fabric and then sewing the circle up into a point. The area of the hole is "gone" --- it's not like it's there and you can't see it, it's just not there. I guess that it is what you mean by "devoid of space".
Then the rest of the fabric is bent in weird ways around the fact that there's a chunk missing, which in physics manifests as as everything being pulled towards it (and there is a point, the event horizon, at which things are no longer possible to interact with, like you're too deep in the hole to get out again.
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Read the article, resistance came from a sense of personal repugnance and concluded in the form of personal attacks. It was irrational.
There was one insulting dismissal of one physicists grasp of physics at one point, but Einstein didn't end the conversation with LeMaitre then and eventually conceded that he was wrong, so it wasn't a 'conclusion', either.
This occurred over decades. It was not a simple insult mid conversation.
How about we finesse that by taking out the baity bit from the title above.
I wonder if any mass could even colapse beyond Planck length? Maybe theory unifying quantum mechanics with Einstein's general relativity would have more definite answers.
For any given radical-but-rational concept, people must instantaneously accept the proposition (instantaneously being less than 0.1 seconds after becoming aware of the concept). Anything else constitutes science denialism.
The Scientismic Method is powerful, and broadly distributed!
Plenty of physicists are still "resistant" to the idea of black holes having a real singularity. The recent Kerr paper [0] only adds fuel to this resistance with its hilarious statement "Faith, not science!" about singularity proponents.
[0]: https://arxiv.org/abs/2312.00841
> Plenty of physicists are still "resistant" to the idea of black holes having a real singularity.
That's because most physicists expect that new physics, probably some form of quantum gravity, will come into play before the singularity is reached, so that there won't actually be one. Which is a perfectly reasonable expectation.
> The recent Kerr paper [0] only adds fuel to this resistance with its hilarious statement "Faith, not science!" about singularity proponents.
Kerr misrepresents the actual opinion of most physicists in that paper. The simplest way to see this is to realize that Kerr's statement that if a theory predicts singularities, the theory is wrong, is something that virtually all physicists who work in GR agree with! Practically nobody believes that there are actual curvature singularities; they believe that GR is not correct in that regime, and some kind of new physics is involved, as I noted above. So in the sense Kerr uses the term, there are practically no "singularity proponents" at all. He is attacking a straw man.
>The simplest way to see this is to realize that Kerr's statement that if a theory predicts singularities, the theory is wrong, is something that virtually all physicists who work in GR agree with!
Maybe it was my misfortune, but the university professor who taught me GR thought that singularities are "real", so this anecdote (sadly) disproves your absolutist opinion. And there are heaps of real papers from real scientists which seriously discuss potential consequences of having "real" singularities. And if we'll take a look at the pop-sci coverage, it's orders of magnitude worse... Just see the Veritasium video linked in the other comment.
> the university professor who taught me GR thought that singularities are "real"
Did he teach it from Wald's classic textbook? Wald, Chapter 9, explicitly disclaims any belief that singularities are real and explicitly says the classical GR prediction of singularities indicates a breakdown in the theory in that regime. Other classic textbooks like Misner, Thorne, & Wheeler say similar things. So do many peer-reviewed papers published in the field.
Sadly, I am not surprised, though I am disappointed, that many university professors do not know the subject they are teaching well enough to be aware of what I have just said. But the fact remains that the statements I have described are the ones Kerr (who certainly cannot claim ignorance on the part of whoever taught him GR as an excuse) should have been looking at to gauge the prevailing opinion of actual researchers in the field. But he didn't.
> if we'll take a look at the pop-sci coverage
Nobody should be relying on pop science to learn actual science. Nor should anyone claim that pop science coverage is an accurate gauge of the opinions of actual researchers in the field.
Haven’t we basically proven that even horizon is real? At which point, isn’t anything inside sealed off by gravity, the fact that time slows down infinitely, the hypothesised firewall, etc?
I.e. if a black hole has a singularity or a super heavy pokemon, there is no distinguishable difference in our universe?
> Haven’t we basically proven that even horizon is real?
No. All we have shown is that there are compact regions in our universe that look like event horizons--but could also be just apparent horizons. To know that they are actual, true event horizons, we would have to know the entire future of the universe, since an event horizon is a globally defined surface in spacetime and its presence and location cannot be known unless you know the entire spacetime, i.e., the entire future. If you don't know that--and of course we don't--you can't tell an actual event horizon from an apparent horizon.
> isn’t anything inside sealed off by gravity, the fact that time slows down infinitely, the hypothesised firewall, etc?
None of these are actual features of actual event horizons (or apparent horizons, for that matter). The first is a missatement--the gravity of the object is not "sealed off" and can still be detected outside. The second is an illusion caused by an unfortunate choice of coordinates. The third is pure speculation, and speculation which does not appear to have gained much traction.
> if a black hole has a singularity or a super heavy pokemon, there is no distinguishable difference in our universe?
Most physicists do not believe that either of these (if I take "super heavy pokemon" to mean "some kind of ordinary compact object like a neutron star") is what actually happens.
> Haven’t we basically proven that even horizon is real?
We haven’t observed event horizons except as fuzzy highly processed images. They look a lot like what you’d expect from a black hole, but we have hardly ruled out all possibilities. And there are theoretical effects near an event horizon for charged and spinning black holes, that as far as I know we’ve never been able to observe.
It doesn’t seem out of the question that a black hole without a singularity might exhibit behaviour near the event horizon that is measurably different from the theories we have now.
>He is attacking a straw man.
But the Nobel prize was awarded for the Singularity Theorem(?). Maybe Kerr is attacking something in between a straw-man and a steel-man? Clay-man?
> the Nobel prize was awarded for the Singularity Theorem
Sure, because those theorems played a very important role in clarifying exactly what classical GR predicts in these regimes, and therefore in focusing the efforts of physicists in fruitful directions. That remains the case even if it turns out that in our actual universe, at least some of the assumptions of those theorems are violated so that there aren't any actual singularities. One of the key contributions of the singularity theorems was in making those assumptions clear, so that physicists who did not believe singularities were physically realistic knew exactly where to look for other possibilities.
Infinite density is pretty crazy though. Would it change much from the outside perspective if there was something more like an ultra compact neutron star at the center?
> Would it change much from the outside perspective if there was something more like an ultra compact neutron star at the center?
There can't be anything like that at the center; that kind of solution doesn't work. At least, not if there is an actual event horizon.
The most likely solution, IMO, is that there is no actual event horizon, and the horizons we are seeing in our observations are only apparent horizons. There are theoretical possibilities such as the "Bardeen black hole" (which is misnamed since it has no actual event horizon) that look like standard classical GR black holes from the outside for a time on the order of the Hawking evaporation time, which means we would not be able to observe the difference now or for the foreseeable future. But what is deep inside those solutions is not any kind of ordinary compact object like a neutron star.
Pardon my ignorance, but why not? This is what I’ve long assumed is the true nature of black holes — not some kind of divide-by-zero error, but merely a very very heavy object.
> why not?
Because no ordinary object, i.e., object made of ordinary matter obeying an ordinary matter equation of state, can be that compact. It's not possible.
Solutions like the "Bardeen black hole" adopt a different equation of state in their deep interior, something more like dark energy, which is not any kind of "ordinary object". That is the only way to have an object that compact without an event horizon. The missing piece in such solutions is how that kind of equation of state could arise as a result of something like stellar collapse: that is where some kind of new physics, possibly some form of quantum gravity, might be needed.
Kinda, you can't have the event horizon without a singularity.
The one thing I'm not 100% sure on is whether an event horizon can form. I have never been able to shake the feeling that with Hawking radiation giving each black hole a large but finite lifetime and with matter falling in beyond the infinite future (to outside observers) you'd have the incoming matter encounter the death of the black hole before it can fall in. This would make a black hole a kind of explosion powered by Hawking radiation that looks frozen in time to outside observers.
Lacking a complete theory that can adequately explain both Hawking radiation and general relativity it's quite hard to know if this makes any sense. Though I can't rule out an answer already exists but is simply not known to me.
A very simplified idea is that the denser you get, the slower time flows (for an "outside" observer). So while for an "inner" observer density rises "fast" to the point of singularity, for an "outside" observer time just freezes inside the forming black hole. Finally, if there is a way for black holes to "evaporate" (e.g. Hawking radiation), the "inner" observer will be able to observe shrinking black hole.
No, this doesn't work. The "slower time flows" you is an optical illusion caused by the curvature of spacetime. It is not a way to avoid the formation of a black hole.
Are you talking about the Hawking-Penrose theorems? Have you read the Kerr paper?
> Are you talking about the Hawking-Penrose theorems?
No. Your wrong claim about the slowing of time flow has nothing to do with those.
> Have you read the Kerr paper?
Yes. It doesn't say anything like what you were saying. I have commented on it elsewhere in this discussion.
Can you please give some reference for this claim? Time stopping at the event horizon is quite the standard view in "science communication" of black holes.
> Can you please give some reference for this claim?
Any GR textbook. I referenced two classic ones elsewhere in this discussion.
Or, for that matter, you could read the classic paper by Oppenheimer and Snyder which was the first published model of actual gravitational collapse to form a black hole. And was published in...1939. What I'm saying is not news.
> Time stopping at the event horizon is quite the standard view in "science communication" of black holes.
That's because "science communication" sucks at accurately portraying the actual science.
> "science communication" sucks at accurately portraying the actual science.
Completely agree.
To this day I believe black holes remain a mostly theoretical phenomena. Which also happens to deal with infinity-mathematics in the singularity, something we don't see in reality otherwise.
Yes, observations have been made to their advantage, through LIGO and more recently the image from 2022 of a black hole made by the Event Horizon Telescope. But I would stress that the ""image"" is much more complex than simply a photo from really good ""telescope"", it's a image made of combined data from radio telescopes all around the world. There is enough weird hoops that they need jump through that the observation can reasonably be doubted in my opinion, which isn't to say that what they've attempted is awesome and inspiring.
Please correct me if you have other information.
> black holes remain a mostly theoretical phenomena
The fact that there are compact objects in our universe that, from the outside, look like the black holes that GR predicts, is not "theoretical", it's observation.
The theoretical question is whether these objects really are classical GR black holes "all the way down", so to speak, or whether there is some new physics that comes into play deep inside them (which many physicists think will involve some form of quantum gravity) that changes things. Even if the latter is the case, the objects might still look like classical GR black holes from the outside for a very, very long time, on the order of the Hawking evaporation time (10^67 years for a one solar mass hole, and the time goes up as the cube of the mass). So for all practical purposes, treating them as classical GR black holes works fine now and for the foreseeable future.
The black hole “photo” was constructed using machine learning. I don’t know how it can reasonably be called a “photo.” And although I’m sure their work was reviewed, knowing the nature of computers and data, I wouldn’t be surprised if there were all sorts of errors in the construction of the model that produced that output. What makes the output correct anyway, the fact that it looks like what we expected?
> The black hole “photo” was constructed using machine learning. I don’t know how it can reasonably be called a “photo.” And although I’m sure their work was reviewed, knowing the nature of computers and data, I wouldn’t be surprised if there were all sorts of errors in the construction of the model that produced that output. What makes the output correct anyway, the fact that it looks like what we expected?
The images of the EHT data were made using several independent techniques performed by different sub-teams who largely worked independently (e.g., [0] for M87). Most of the imaging techniques used are not machine learning approaches, but are rather standard imaging approaches, some of which have have been used for decades in radio astronomy (e.g., CLEAN). The fact that the images made from these different techniques look similar is what lends support to the idea that the "photo" is a reasonable representation. Sure, some images have been created using machine learning approaches, but other images have been done using well vetted approaches that are not machine learning.
But if one doesn't want to trust the imaging techniques, the size of the black hole shadow can be estimated directly from the radio telescope measurements. These are the "visibilities" (essentially measurements of components of the Fourier Transform of the distribution of radio emission on the sky) and the amplitudes of the visibilities (e.g., figure 2 of [1]) indicates that the brightness distribution of the source is approximately a thin ring, with the ring diameter setting the baseline length at which the amplitudes have their minimum.
So the basic result about the black hole shadow can be made from the visibilities themselves, without relying on the imaging. But the images of course make it easier to see some of the complexity (e.g., that the ring around M87 is not axisymmetric).
[0] https://ui.adsabs.harvard.edu/abs/2019ApJ...875L...4E/abstra...
[1] https://ui.adsabs.harvard.edu/abs/2019ApJ...875L...1E/abstra...
They've been detected with gravity waves, radio telescopes and optical telescopes and explain a wide variety of phenomena in the center of galaxies.
What evidence would you require to confirm that they exist?
The objects we have detected look like classical GR black holes, yes.
The problem is that, theoretically, there are other possible objects that look like classical GR black holes from the outside for a very, very long time, but still aren't classical GR black holes. We have no direct way of telling from observation whether the objects we have detected really are classical GR black holes, or whether they are one of the other possibilities.
This article seems like a rehash of the recent Veritasium video: https://www.youtube.com/watch?v=6akmv1bsz1M
For example, compare the picture of Karl Schwarzschild in the article to 7:54 in the Veritasium video. Just a coincidence?
https://today-in-wwi.tumblr.com/post/144234528063/karl-schwa...
Looks like the BBC used this photo as reference for the drawing.
If you prefer the video version of the theory....
https://www.youtube.com/watch?v=6akmv1bsz1M
The edited headline we have now makes it sound like twentieth-century physicists had some sort of anti-gravity thing going on.
Why was "irrationally" edited out of the submission title?
TBH, "irrationally" is a value judgment that I can't agree with.
Einstein's approach to physics was to reason out the consequences of observable phenomena. He approached understanding of the nature of the speed of light by realizing that a universe where you could travel as fast as (or faster than) a light ray would have consequences that we didn't see. So when the math tells him "gravity causes some regions of space to blow up to become inescapable," and we hadn't seen black holes yet, I think his first intuition being that we'd missed a trick that made the math fail to match reality was a fine intuition.
Math is only useful to physics to the extent that it actually models reality, and not all the extrapolations it makes turn out to have practical grounding; sometimes chasing the extrapolation reaches a contradiction that demands adjustments to math to fix the model. Chasing such adjustments in light of evidence is how we reached relativity in the first place.
> He approached understanding of the nature of the speed of light by realizing that a universe where you could travel as fast as (or faster than) a light ray would have consequences that we didn't see.
A sound wave could be outrun though. I don't see any step in why a such an philosofical approach would apply to light but not sound?
Because photons behave in a fundamentally different way than sound. See the Michelson-Morley experiment.
The relevant quote from his Autobiographical Notes is
"...a paradox upon which I had already hit at the age of sixteen: If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as an electromagnetic field at rest though spatially oscillating. There seems to be no such thing, however, neither on the basis of experience nor according to Maxwell's equations. From the very beginning it appeared to me intuitively clear that, judged from the standpoint of such an observer, everything would have to happen according to the same laws as for an observer who, relative to the earth, was at rest. For how should the first observer know or be able to determine, that he is in a state of fast uniform motion? One sees in this paradox the germ of the special relativity theory is already contained."
In short, he realized that if things could travel at lightspeed, we should be moving relative to something out there at lightspeed, so we should be experiencing its light as this weird "electromagnetic field at rest though spatially oscillating" (i.e. a field coming from apparently nowhere that changes in intensity as you move around in space but not in time)... but nobody had ever seen that phenomenon.
Einstein seems to have put a lot of stock in "If physics allows it, we should be seeing it" as a razor of sorts, so it makes sense that when the math said black holes should exist but they hadn't been seen, his default position was to be skeptical of the math and the intuition. Such skepticism had served well in the past for him.
(Worth noting for completeness: The "variable speeds of light predict stationary waves, but we never observe such things" objection is just one objection that comes up in Einstein's thought experiments. A more complete treatment of the subject is given in at https://sites.pitt.edu/~jdnorton/Goodies/Chasing_the_light/).
What is the function of the blackholes in a rational universe?
Maybe enforcing the second law of thermodynamics? Or they're a consequence of the same. It's a region of space where the conditions that existed before the big bang have ostensibly reestablished themselves, which is why it appears as a 2d boundary phenomenon to us.
I am a total crank.
1/1,000,000 cranks will be right.
There are theories that our universe is the inside of a black hole.
>https://en.wikipedia.org/wiki/Black_hole_cosmology
If that is the case, then black holes are giant entropy recycling machines.
I don’t want to open another box, but how exactly do people distinguish between rational vs irrational behavior?
You might define it as quick system 1 vs thoughtful system 2 reaction.
But I’m pretty sure Einstein had thought this through and had good, rational reasons at that time to “resist” the idea of black holes.
An irrational belief is one you did not come to through reason.
Irrational doesn't mean bad or wrong. It just means it did not come from a rationale. Many beliefs we have are presuppositions which we demand strong rationale for changing, despite not needing similar rationale to arrive at the presupposed belief.
Believing the earth is flat because it looks flat is an example of an irrational belief practically everyone holds until taught otherwise. Some people hold onto the belief even after being given good reason to abandon it. We call that irrational behavior.
I like the flat earth example.
However, it seems to me that your definition is circular.
You define irrational as the opposite of being rational.
I’m pretty sure that flat earthers have their own “rationale” why the earth must be flat and we are the irrational ones to them.
The circular definition is not good enough.
Having rationale to support your belief is inconsequential to whether it is rational or not. The important distinction is whether you arrived at the belief through rationale or not. Everyone believes the earth is flat by default because it appears flat. That's what makes it irrational.
If you later revert to believing in the globe because of reason and then convert back for reasons none of that matters. The initial belief was always irrational.
So basically you’re saying that being irrational means ignoring facts that the majority has established to be true?
I think pretty much everyone is rational.
However, our access to facts and information is vastly different between two different people. That's because we grow in different spaces, meet different people, and have vastly different experiences.
Then you fall into this trap where you think you are right. You ARE right, but only in the context of your experience. Since it's rarely possible to ever know if you are universally right, you have to keep an open mind, which can be difficult.
Rationality is apparently relative.
There may be exceptions, but most of the time I find people call "irrational" any reasoning they disagree with.
It’s still possible to have biases when thinking something through deeply
Absolutely. But then what exactly is being rational
Logically inconsistent?
I would expect a BBC title to be designed to appeal to emotion way more than to carry any sensible truth though.
This stuff was cosmic horror to all these folk. It caused existential crisis when you try imagine what is happening in and around a black hole. I'm not surprised they did everything they could to deny and disprove it.
Most would have had religious convictions that were really challenged by the physical reality of the universe.