One fun thing think about is that these two galaxies are only aligned from our perspective in the universe. Viewed from a different location, and they're just two normal galaxies.
Also, imagine having the technology to send signals through the lens and get the attention of intelligent life on the other side.
>In order to use them as a signaling platform (how?) the signal would have needed to have been sent several billion years ago.
Conceivably, a civilization could predict in advance that two galaxies would form a lens configuration, and send a signal that arrived just as the lens formed, correct?
I mostly agree with you. The three body problem (3+) is chaotic at those timescales. But I suppose all thats needed for predicting this lensing is a two body problem if they're isolated enough, which is much more predictable
Which just means there’s no closed form solution. You can simulate these bodies fine provided you have sufficient numerical accuracy and very accurate measurements of initial conditions (this is the part that’s practically impossible)
Take a Monte Carlo approach. Run a bunch of simulations to generate a probabilistic point cloud in spacetime of where the lenses end up. Fire a signal through every point in the cloud.
Even assuming a civilization can predict the alignment of the lenses (galaxies), they'd still need quite a powerful signal just to reach the first lens, let alone the second, and then a potential civilization who may be listening at just the right time on the other side. Hard to beat background noise even at distances of a few light years.
That's assuming the development of the two civilizations starts simultaneously and is predictable to the point the signal reaches the other side. That side of the lenses may never see a civilization developing at all, or at least not one surviving long enough to receive that obscure signal.
These distances and time periods are unfathomably long. I can see predicting the alignment of galaxies but predicting a civilization with an adequate evolution stage will exist at the right spot, at the right time is very different. Any civilization with this power of prediction probably has a level of advancement that makes the difference between humans and amoeba look positively non-existent, and probably wouldn't bother with broadcasting lowly radio waves into the universe.
I can't imagine the universe and evolution of life being so deterministic and predictable especially over this time scale, no matter what tech you have.
Over such timescales since you'd aim at another galaxy wholesale you coud bet on Drake equation plus hope a civilisation has survived long enough for a wide enough window to be able to receive the transmission.
> probably wouldn't bother with broadcasting lowly radio waves into the universe.
I bet we would be very glad to receive such a transmission, even when knowing full well "replying" isn't a realistic option (both due to technology limitations and the RTT meaning that even if the reply receives were descendants, they'd be so far removed as to be entirely another ship-of-theseus civilisation)
A gift in a cosmic dying sigh could be motivation enough.
"Should anyone receive this, know that, as far as life forms go, you were not quite alone and life existed beyond yours. We're sending this knowing full well we'll be long gone, but during all of our civilisation history we could only hypothesise that we were not. We hoped but never knew, may this transmission relieve you of the doubts we had; you now unambiguously know."
At that point you might just as well send out a high power broadcast of Never Gonna Give You Up and congratulate yourself on a job well done, indulging in imaginations of fantastic ways of how it might get received somewhere half a universe lifespan later.
So conceiably someone could have sent a signal from the other part of the lense some billion years ago and we "just" need to figure out what to listen for.
I don't think so, that would mean these "someones" would have to be developed enough to send interstellar messages through gravitational lenses when the very first solar systems and rocky planets were being formed, around 10 billion years ago. It seems too early for technology at that level.
It's kind of interesting in terms of analytics... can we predict when lenses will appear and disappear, from our perspective? What might we do with that information once we are more advanced?
1. Yes it would be somewhat predictable to find these lenses for a civilization more advanced than ours.
2. Unless we find faster than light communication (which, with our current understanding of physics is about as likely as humans jumping to the moon) there is nothing we could use it for other than definite proof that other life has evolved in the universe. Interesting data, but they're most likely extinct for billions of years already and even if they're not, the compound gravity lens will have moved out of alignment by then so we have no means to send a message back.
And technically they are only temporarily so, given enough millions of years they will drift apart and lose the alignment.
Also, other stars can come to align in the future. Makes me wonder if we can antecipate other cases like this and create a future schedule of "To Observe" so future generations can look at them. Although, these generations might be so distant from ours that might not even be considered of the same species
> ... This search would have revealed optical laser light from the directions of Alpha Cen B if the laser had a power of at least 1.4–5.4 MW (depending on wavelength) and was positioned within the 1 arcsec field of view (projecting to 1.3 au), for a benchmark 10-m laser launcher
For comparison, with our measly human technology...
> The Vulcan 20-20 laser is so named because it will generate a main laser beam with an energy output of 20 Petawatts (PW) alongside eight high energy beams with an output of up to 20 Kilojoules (KJ). This is a 20-fold increase in power which is expected to make it the most powerful laser in the world.
> The entire message consisted of 1,679 binary digits, approximately 210 bytes, transmitted at a frequency of 2,380 MHz and modulated by shifting the frequency by 10 Hz, with a power of 450 kW.
> The broadcast was particularly powerful because it used Arecibo's megawatt transmitter attached to its 305 meter antenna. The latter concentrates the transmitter energy by beaming it into a very small patch of sky. The emission was equivalent to a 20 trillion watt omnidirectional broadcast, and would be detectable by a SETI experiment just about anywhere in the galaxy, assuming a receiving antenna similar in size to Arecibo's.
A perfectly parallel source wouldn't fall off with inverse square, but all real sources are not — and cannot be — perfectly parallel.
What you get from lasers is very high gain in the direction it is pointed in, but it's still subject to the inverse square law.
It's capable of being enough gain to be interesting, to be seen from a great distance.
If you engineer it so the gain is enough to outshine the rest of the parent galaxy in the direction it is pointed, then that's effectively good enough because the galaxy is also following inverse-square and you'll continue to outshine the parent galaxy even as you and it both get weaker, but it's still falling off inverse-square.
The energy density drops off as inverse square law, but the photons go forever. Radio is just photons (light) so it goes forever until it interacts with something it hits. The expanding universe will stretch it's wavelength slightly however.
Sure, but the amount of photons as a percentage of the background radiation drops as a function of the distance. It's not all that far away in cosmic distances when any signal from Earth is millions of times less powerful than the noise level.
A single photon is not a viable communication signal, certainly not at interstellar distances. In practice you need to send out some sort of modulated beam. Even very narrow beams have nonzero dispersion, so the further you get the lower the signal energy will be at an antenna of a given size. So to get more energy you'd need a bigger antenna, but that in turn means receiving more of the background noise as well. In practice there is a minimal signal strength level at which it is still practical to receive the signal.
Long story short: A photon will go on forever (unless it hits something), but a radio signal rapidly spreads out so much that no realistic receiver will be able to recover it from out of the cosmic background noise.
I didn't say sending single photons at a time is a viable communications mechanism. I said a photon will travel indefinitely, without losing any energy, until it interacts with something.
Interestingly, if you send out a single photon from a radio antenna not even the universe itself will have 'determined' which direction it even went until it DOES interact, because there would be a Quantum Mechanical superposition/indeterminacy similar to the famous slit-experiment, if you were dealing with one photon at a time.
So even the thought experiment itself is complex due to wave/particle duality.
Regular EM Radio waves are not photons. Photons have special configuration which prevents leaks into surrounding space, while regular radio waves are just waves.
The experiment (one of them, that I'm aware of) that cements wave-particle duality is that you can dial the energy of an emitter down until it's emitting one photon at a time and still detect interference in a double-slit experiment. This is impossible if the photons and waves are distinguishable phenomena.
Radio waves are photons; photons are quantum entities that have particle- and wavelike behavior simultaneously.
Maybe check Wikipedia? Because it refutes you in the first sentence on the articles for "radio", "photon", and "light". You're just being pedantic about word definitions to play games with people.
Similarly, eclipses are pretty much arbitrary. You stand somewhere else in the solar system, nada. Or go fly over into the shadow of whatever, eclipse any time you like!
And why do we ignore the most common eclipse, the 'terrestrial eclipse'? Happens literally all the time. Also called 'night'.
I love that my pondering comment generated so much discussion. Much more technical than I can fathom.
Another thought that occurred to me, we humans are short lived and trying to think about the length of time such a message would take to travel far exceeds out lifetime. Even the thought of humanity lasting that long is difficult. But imagine if there were intelligent life forms that lived a single life on galactic timescales. To them, this discussion of sending a message that reached someone wouldn't be so pessimistic.
But elsewhere in the universe it would just be two other galaxies forming the lens. The Copernican principle would suggest that any phenomena we observe are likely to be common in the universe.
Not really, its premise is using our Sun, not some lens composed of 2 galaxies (that would probably misalign well before our signal would reach them), not sure how you came up with such an idea.
Yes in a vague sense. And No in a strong practical sense.
Lensing works in reverse except for time delays which make the idea much more complex. The object's past is projected to us now, but our past would be projected to somewhere that the far object no longer occupies. Double lensing makes this even less reversible.
When the light we are now seeing was emitted, the lensing wasn't in place. In fact, the galaxies doing the lensing hadn't even evolved to the state that we see them in.
So if we sent a response to what we see now, it wouldn't make it back to the lensed objects.
That's just for single lensing. Double lenses are a massive coincidence of events at 4 points in time and space (emission, first deflection, second deflection and observation). That means that light going the other way wouldn't have the two intermediate points in the right place at the right times so it all breaks down for us and the object we see. There are some points that would be double lensed in the reverse direction but the locations and times for the source and observer have only very vague correlation to our location and the location of the object we see.
A simpler answer is just what happens if you look thru a telescope or binoculars "the wrong way" (backwards). The correct way shows a "zoomed in" view of that you're viewing, but looking the wrong way shows a "zoomed out" view.
So lifeforms on the other end of this cosmic "lens[es]" cannot use it to see us better, because in fact it makes us look further away from them than we are, from their perspective.
If I understand right, objects further than a redshift of z ~= 1.8 can't be reached by any signal we emit, and the second galaxy is at a redshift of z = 1.885. But I don't know how precisely (standard deviations rather than decimal places) the distance to the outbound cosmological horizon is being approximated, so it might be reachable by a signal sent by us:
The question I addressed is "does the lensing work the same from the other end". It's a very specific and clear question, and the answer is "no it does not", because if you reverse a telescope lens you get the opposite effect (from zoom-in to zoom-out)
The question of at what distance and relative velocity are the two locations so far apart that light can never make it from one to the other (due to expanding universe) is a completely separate issue.
It would be cool if we some day had special days of astronomy where every telescope is turned to galactic eclipses the way they once did for solar eclipses.
The sky is huge and we are moving, so surely some would happen in our lifetimes?
The farther away they are the slower they would scan the sky behind them or go out of conjunction. So I guess one of the objects would have to be something relatively close.
Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is.
It takes light, the fastest thing that can be, 100,000 years to cross the Milky Way.
The Sagittarius Dwarf Spheroidal Galaxy is currently in the process of being consumed by the Milky Way and is expected to pass through it within the next 100 million years.
So, unless you're even more optimistic about life extension technology than I am, not in our lifetimes, no.
I made this comment before but someone on HN made a good argument is way harder than it sounds and given it's size/cost/function it'd basically have to point in one direction, it's not like an easily moveable telescope you can scan around with.
This is true, but also, keep in mind that the JWST was insanely hard and almost cancelled a few times :)
The SGL would be much, much harder than the JWST would be, and the JWST was already stretching our capabilities.
The SGL needs to be 650AU away from us.
Voyager 1 and 2 are currently 165AU and 120AU away.
The JWST is 0.01 AU from us.
And you can only look in one direction after the probe finally gets into position. Once you're 650AU away, it's not really feasible to move "sideways" far enough to look at something else.
The ratios between 650AU, 165AU, and 0.01AU are somewhat moot.
In 1957 Sputnik 1 had an apogee of ~900km from the Earth.
By 1969 NASA was sending rockets ~385000km to the moon.
By 1979 Voyager 1 & 2 were reaching Jupiter ~5AU from Earth.
We went from 900km to 5AU in 22 years.
If SpaceX achieves their stated goals of lowering $/kg to orbit and rapid re-usability with Starship it will unlock things like asteroid/lunar mining and space based manufacturing which will allow the construction of the kind of infrastructure needed to make distances like 650AU achievable in reasonable time frames.
Low hanging fruit is easier to pluck. It might be that we cannot progress much further without consuming so many resources that Earth is left an uninhabitable husk.
That is precisely why we must transition to space based resource extraction and manufacturing.
There are practically infinite resources at our fingertips on the moon, the asteroid belts and eventually the gas giants.
What we need to unlock this are the means to economically launch a minimum viable self replicating infrastructure into space to take advantage of this.
The feedback loops that will ensue should we succeed will allow us to save Earth ecology, radically transform the human condition, and unlock the ability to explore the universe in ways that we can only imagine.
I'd think to make it practical you'd have to have kind of (semi-) automatic space based assembly infrastructure that builds them and launches them. Launching these probes individually seems like it would be impractical. Building that infrastructure wouldn't be easy at all and I don't see that happening in the next 50 years.
The neat thing about a how an Einstein ring works is that you don't need to stop, because rather than there being a focal point, there is a focal line, moving directly outward from the Sun. This means a probe could spend potentially decades imaging the same target on its outward path, if it had sufficient power.
No, because the light requires twice the time to travel there then back. If Earth did not move relative to the lens, it would work. Sadly we move, a lot, so what was here 2x ago was something not-earth.
To see earth, the lensing would been to be focused on where Earth was 2x ago. Still possible in theory, and you might even argue just as likely as a fully reflecting curve. But you'd not call it "back towards us". It would need to be "curved to where earth was".
Seems like if you could retrodict the position of past lenses, and predict their effects, perhaps it would somehow be possible to send a spacecraft to a specific location in order to observe Earth's past.
The idea being that a spacecraft traveling at 99% of light speed can't ordinarily catch up with light reflected by Earth. But if the light curves, and the spacecraft can travel directly towards where the light will end up (spacecraft traveling "as the crow flies"), it might be possible to catch up.
Same way I might be able to catch up with Usain Bolt at a track event if he's forced to run on the track, and I'm allowed to run across the turf in the middle.
Would this be the case even if you were moving toward or along-side the 'reflector' (black hole/other body)? For the sake of discussion assume we are at or beyond the focal point.
Technically? But the image would be very very very small, so we'd need a detector bigger than the solar system (guesstimate) to see it. That's to see it: I can't imagine what it would take to resolve the image. The tricks in this paper are a start.
> The first lens is relatively close to the source, with a distance estimated at 10.2 billion light-years. What happens is that the quasar’s light is magnified and multiplied by this massive galaxy. Two of the images are deflected in the opposite direction as they reach the second lens, another massive galaxy. The path of the light is a zig-zag between the quasar, the first lens, and then the second one, which is just 2.3 billion light-years away
So, given a simplistic model with no relative motion between earth and the presumed constant location lens:
Earth formation: 4.54b years ago
2.3b * 2 = 4.6b years ago
10.2b * 2 = 20.4b years ago
Does it matter that our models of the solar systems typically omit that the sun is traveling through the universe (with the planets swirling now coplanarly and trailing behind), and would the relative motion of a black hole at the edge of our solar system change the paths between here and a distant reflector over time?
I believe this could only happen around a black hole. In that case, yes: light that we emitted umpty-million years ago could be shot back at us. The problem is that there would be no focussing. At best it would be like looking at an Earth 2 x umpty-million light years away. I'm guessing that it would actually be worse, with the black hole dispersing light.
(IANAastronomer, but I have opinions on any given topic...)
In case anyone was wondering, like me, what the MJy unit in the lower diagram is: It's Mega-Jansky. Just funny that it's then being rescaled by 10^-9. Why didn't they use Milli-Jansky in the first place?
> It's Mega-Jansky... Why didn't they use Milli-Jansky
*megajansky, millijansky
"All prefix names are printed in lowercase letters, except at the beginning of a sentence.
Unit names are normally printed in upright type and they are treated like ordinary nouns. In English, the names of units start with a lower-case letter (even when the symbol for the unit begins with a capital letter), except at the beginning of a sentence or in capitalized material such as a title...
When the name of a unit is combined with the name of a multiple or sub-multiple prefix, no space or hyphen is used between the prefix name and the unit name...
However yes, you raise a good question. I was surprised to learn that astronomy and astrophysics prefer CGS, so by convention the diameter of the Sun is given in centimeters, and the mass of the Sun in grams! For janskys there's no excuse though.
Not really -- not the sort of lens we're familiar with, one that concentrates light at a single focus. Technical methods can exploit these chance alignments to detect objects otherwise inaccessible, but not as coherent images.
Complete newbie on the topic of GL but I find it very interesting.
Q: We seem to want to use our sun for GL experiments. And that requires sending something out 500 AU out for focal length purposes. My question is somewhat theoretical so assume there are no alignment issues for the following thought. Are we not already at the "focal point/line" of some other 'entities' (sun/heavy body/galaxy/etc) gravitational lens?
Hmm... is that what this paper is talking about - the fact that the GL observation was made from Earth on some infinite focal length. Listening to other YT videos on the topic seemed to imply the resolution would be much higher. Or is the resolution location/technology related?
With all the galaxies out there, it seems likely that Earth should be in the focus of lots of systems of this kind. Hopefully in the future we'll be able to find and use these galactic telescopes!
Disclaimer: I am a layman, not trained at all. But I am interested in this stuff.
Our most powerful telescopes can see "back in time", by looking at stuff far enough away that it took nearly the entire age of the universe for the light to reach us.
I would guess that we can use this natural compound lens to "see farther" with our current telescopes than we might otherwise be able to see.
Our current best telescope, the JWST, can almost see to the very beginning of when it was possible to see, somewhere between 300k and 200M years after the big bang [0].
Somewhere in this time period, the universe cooled enough for normal matter to form.
The JWST still cannot see the actual 'edge' of when this occurred.
Maybe with this natural compound lens, we can see all the way to the edge.
And if we could see where the edge actually is, then maybe we can refine the estimate to a tighter range than [300k,200M], which would give us a better estimate of the expansion rate of the earlier universe.
> This unique configuration offers the opportunity to combine two major lensing cosmological probes: time-delay cosmography and dual source-plane lensing since J1721+8842 features multiple lensed sources forming two distinct Einstein radii of different sizes, one of which being a variable quasar. We expect tight constraints on the Hubble constant and the equation of state of dark energy by combining these two probes on the same system. The z2=1.885 deflector, a quiescent galaxy, is also the highest-redshift strong galaxy-scale lens with a spectroscopic redshift measurement.
Not an expert, just trying to add some more context.
With time-delay cosmography[1] one exploits that unless the source is perfectly in the center of the line of sight, then the photons that make up one lensed copy have traveled a different distance from the source than photons that make up a different lensed copy. This effect can be used to measure absolute distance and give an accurate measure of the Hubble constant.
With dual source-plane lensing[2] one exploits that if two different sources lensed by the same lens, one can take the ratio of the measurements between the two sources and get results that are significantly less affected by the lens itself and is completely independent of the Hubble constant.
One fun thing think about is that these two galaxies are only aligned from our perspective in the universe. Viewed from a different location, and they're just two normal galaxies.
Also, imagine having the technology to send signals through the lens and get the attention of intelligent life on the other side.
In order to use them as a signaling platform (how?) the signal would have needed to have been sent several billion years ago.
At 10 billion light years away from the most distant lens it is 100% certain that they are no longer in a gravitational lensing configuration.
For a frame of reference, the Milky Way will be in the middle of its epic merger with Andromeda in about 5 billion years.
>In order to use them as a signaling platform (how?) the signal would have needed to have been sent several billion years ago.
Conceivably, a civilization could predict in advance that two galaxies would form a lens configuration, and send a signal that arrived just as the lens formed, correct?
Isn't the universe (near) chaotic on those timescales and you can only predict the general flow? Or is this me mixing things up?
I mostly agree with you. The three body problem (3+) is chaotic at those timescales. But I suppose all thats needed for predicting this lensing is a two body problem if they're isolated enough, which is much more predictable
Which just means there’s no closed form solution. You can simulate these bodies fine provided you have sufficient numerical accuracy and very accurate measurements of initial conditions (this is the part that’s practically impossible)
Take a Monte Carlo approach. Run a bunch of simulations to generate a probabilistic point cloud in spacetime of where the lenses end up. Fire a signal through every point in the cloud.
Even assuming a civilization can predict the alignment of the lenses (galaxies), they'd still need quite a powerful signal just to reach the first lens, let alone the second, and then a potential civilization who may be listening at just the right time on the other side. Hard to beat background noise even at distances of a few light years.
But if you can do that, you know you have plenty of time for a civilization to develop on the other end to listen.
Might just not be us.
That's assuming the development of the two civilizations starts simultaneously and is predictable to the point the signal reaches the other side. That side of the lenses may never see a civilization developing at all, or at least not one surviving long enough to receive that obscure signal.
These distances and time periods are unfathomably long. I can see predicting the alignment of galaxies but predicting a civilization with an adequate evolution stage will exist at the right spot, at the right time is very different. Any civilization with this power of prediction probably has a level of advancement that makes the difference between humans and amoeba look positively non-existent, and probably wouldn't bother with broadcasting lowly radio waves into the universe.
I can't imagine the universe and evolution of life being so deterministic and predictable especially over this time scale, no matter what tech you have.
Over such timescales since you'd aim at another galaxy wholesale you coud bet on Drake equation plus hope a civilisation has survived long enough for a wide enough window to be able to receive the transmission.
> probably wouldn't bother with broadcasting lowly radio waves into the universe.
I bet we would be very glad to receive such a transmission, even when knowing full well "replying" isn't a realistic option (both due to technology limitations and the RTT meaning that even if the reply receives were descendants, they'd be so far removed as to be entirely another ship-of-theseus civilisation)
A gift in a cosmic dying sigh could be motivation enough.
"Should anyone receive this, know that, as far as life forms go, you were not quite alone and life existed beyond yours. We're sending this knowing full well we'll be long gone, but during all of our civilisation history we could only hypothesise that we were not. We hoped but never knew, may this transmission relieve you of the doubts we had; you now unambiguously know."
At that point you might just as well send out a high power broadcast of Never Gonna Give You Up and congratulate yourself on a job well done, indulging in imaginations of fantastic ways of how it might get received somewhere half a universe lifespan later.
So conceiably someone could have sent a signal from the other part of the lense some billion years ago and we "just" need to figure out what to listen for.
I don't think so, that would mean these "someones" would have to be developed enough to send interstellar messages through gravitational lenses when the very first solar systems and rocky planets were being formed, around 10 billion years ago. It seems too early for technology at that level.
It's kind of interesting in terms of analytics... can we predict when lenses will appear and disappear, from our perspective? What might we do with that information once we are more advanced?
1. Yes it would be somewhat predictable to find these lenses for a civilization more advanced than ours.
2. Unless we find faster than light communication (which, with our current understanding of physics is about as likely as humans jumping to the moon) there is nothing we could use it for other than definite proof that other life has evolved in the universe. Interesting data, but they're most likely extinct for billions of years already and even if they're not, the compound gravity lens will have moved out of alignment by then so we have no means to send a message back.
This time scale is nuts to me. I cannot fathom it. Just…wow. None of this (humanity) matters in the grand scale.
And technically they are only temporarily so, given enough millions of years they will drift apart and lose the alignment.
Also, other stars can come to align in the future. Makes me wonder if we can antecipate other cases like this and create a future schedule of "To Observe" so future generations can look at them. Although, these generations might be so distant from ours that might not even be considered of the same species
I’m sure there are plenty of civilizations that have done this, but on the time scale of the universe no one happens to look at just the right moment.
But wouldn't the size and age of the universe also imply that someone has looked at just the right moment somewhere somewhen.
Don’t radio waves weaken proportionally to the square of the distance? No one would be able to detect them past a (relatively) small distance.
Omnidirectional source, yes.
However, beamed sources don't fall off that way.
A search for optical laser emission from Alpha Centauri AB - https://academic.oup.com/mnras/article/516/2/2938/6668809
> ... This search would have revealed optical laser light from the directions of Alpha Cen B if the laser had a power of at least 1.4–5.4 MW (depending on wavelength) and was positioned within the 1 arcsec field of view (projecting to 1.3 au), for a benchmark 10-m laser launcher
For comparison, with our measly human technology...
https://www.ukri.org/news/uk-science-facility-receives-85m-f...
> The Vulcan 20-20 laser is so named because it will generate a main laser beam with an energy output of 20 Petawatts (PW) alongside eight high energy beams with an output of up to 20 Kilojoules (KJ). This is a 20-fold increase in power which is expected to make it the most powerful laser in the world.
Or even five decades ago (TODAY!) ... https://en.wikipedia.org/wiki/Arecibo_message
> The entire message consisted of 1,679 binary digits, approximately 210 bytes, transmitted at a frequency of 2,380 MHz and modulated by shifting the frequency by 10 Hz, with a power of 450 kW.
https://www.seti.org/seti-institute/project/details/arecibo-...
> The broadcast was particularly powerful because it used Arecibo's megawatt transmitter attached to its 305 meter antenna. The latter concentrates the transmitter energy by beaming it into a very small patch of sky. The emission was equivalent to a 20 trillion watt omnidirectional broadcast, and would be detectable by a SETI experiment just about anywhere in the galaxy, assuming a receiving antenna similar in size to Arecibo's.
A perfectly parallel source wouldn't fall off with inverse square, but all real sources are not — and cannot be — perfectly parallel.
What you get from lasers is very high gain in the direction it is pointed in, but it's still subject to the inverse square law.
It's capable of being enough gain to be interesting, to be seen from a great distance.
If you engineer it so the gain is enough to outshine the rest of the parent galaxy in the direction it is pointed, then that's effectively good enough because the galaxy is also following inverse-square and you'll continue to outshine the parent galaxy even as you and it both get weaker, but it's still falling off inverse-square.
I stand corrected on the inverse square.
I still hold that it would be possible to send and detect signals set with intention with not too much more advanced technology than what we have.
Anywhere in the galaxy within the super narrow beam that the Arecibo antenna happened to cover at the time, at least.
The energy density drops off as inverse square law, but the photons go forever. Radio is just photons (light) so it goes forever until it interacts with something it hits. The expanding universe will stretch it's wavelength slightly however.
Sure, but the amount of photons as a percentage of the background radiation drops as a function of the distance. It's not all that far away in cosmic distances when any signal from Earth is millions of times less powerful than the noise level.
> amount of photons as a percentage of the background
That's what "density" means. (i.e. the amount of something per unit volume)
> noise level
A photon will travel thru space forever without losing energy, unless it hits something. What noise are you talking about?
> A photon will travel thru space forever without losing energy, unless it hits something. What noise are you talking about?
I'm talking about the https://en.wikipedia.org/wiki/Noise_floor, in particular the unavoidable receiver noise caused by the cosmic background radiation.
A single photon is not a viable communication signal, certainly not at interstellar distances. In practice you need to send out some sort of modulated beam. Even very narrow beams have nonzero dispersion, so the further you get the lower the signal energy will be at an antenna of a given size. So to get more energy you'd need a bigger antenna, but that in turn means receiving more of the background noise as well. In practice there is a minimal signal strength level at which it is still practical to receive the signal.
Long story short: A photon will go on forever (unless it hits something), but a radio signal rapidly spreads out so much that no realistic receiver will be able to recover it from out of the cosmic background noise.
I didn't say sending single photons at a time is a viable communications mechanism. I said a photon will travel indefinitely, without losing any energy, until it interacts with something.
Interestingly, if you send out a single photon from a radio antenna not even the universe itself will have 'determined' which direction it even went until it DOES interact, because there would be a Quantum Mechanical superposition/indeterminacy similar to the famous slit-experiment, if you were dealing with one photon at a time.
So even the thought experiment itself is complex due to wave/particle duality.
Regular EM Radio waves are not photons. Photons have special configuration which prevents leaks into surrounding space, while regular radio waves are just waves.
Nope. Radio waves are made of photons. All EM waves are made of photons.
Radio waves are not photons. Light beams are not photons.
Light beams (or similar sources of EM waves generated by individual electrons or nucleus) are made by photons. We can record individual photons.
Maybe, radio waves are made of photons, but nobody confirmed that yet, so I can safely say «no». If you can confirm that, Nobel prize is yours.
Are radio waves quantized? Of course, at Planck scale.
Is it possible to form a single 100kHz photon using a macro antenna? I hope for «yes», but I have no idea about «how».
The experiment (one of them, that I'm aware of) that cements wave-particle duality is that you can dial the energy of an emitter down until it's emitting one photon at a time and still detect interference in a double-slit experiment. This is impossible if the photons and waves are distinguishable phenomena.
Radio waves are photons; photons are quantum entities that have particle- and wavelike behavior simultaneously.
Maybe check Wikipedia? Because it refutes you in the first sentence on the articles for "radio", "photon", and "light". You're just being pedantic about word definitions to play games with people.
Maybe you should contribute something useful to discussion.
I did. I told you Radio waves are made of photons.
Photons are EM-waves. Are photons made of photons?
Great, now add to that the fact that radio waves are an EM-wave too, and that answers your original confusion.
So, in your opinion, photons are EM-waves, which are made of photons, which are EM-waves, ad infinitum? Or you oppose this?
Please, say something useful.
Saying that Radio waves are a particular frequency range of photons is not a tautology. The only one making up tautologies is you.
Similarly, eclipses are pretty much arbitrary. You stand somewhere else in the solar system, nada. Or go fly over into the shadow of whatever, eclipse any time you like!
And why do we ignore the most common eclipse, the 'terrestrial eclipse'? Happens literally all the time. Also called 'night'.
When we do start getting anywhere else in the solar system in reasonable time, then and only then will eclipses be "not special events". Until then..
I love that my pondering comment generated so much discussion. Much more technical than I can fathom.
Another thought that occurred to me, we humans are short lived and trying to think about the length of time such a message would take to travel far exceeds out lifetime. Even the thought of humanity lasting that long is difficult. But imagine if there were intelligent life forms that lived a single life on galactic timescales. To them, this discussion of sending a message that reached someone wouldn't be so pessimistic.
But elsewhere in the universe it would just be two other galaxies forming the lens. The Copernican principle would suggest that any phenomena we observe are likely to be common in the universe.
Thats probably not happening at that scale. I know this is the premise of interstellar communication in the three body problem. It's not real.
Not really, its premise is using our Sun, not some lens composed of 2 galaxies (that would probably misalign well before our signal would reach them), not sure how you came up with such an idea.
Using things at that scale to talk? It's not a thing in either case.
This Wikipedia page suggests otherwise - https://en.wikipedia.org/wiki/Gravitational_lens
What's the advantage of sending a signal through the lens relative to sending it in some other arbitrary direction?
Is it only one direction or does it work the same from the other side?
> Is it only one direction or does it work the same from the other side?
The relationship is (must be) symmetrical. Were this not so, it would violate a principle called "Maxwell's Daemon" (https://en.wikipedia.org/wiki/Maxwell%27s_demon).
Should work the other way too. Physics and symmetry:)
Yes in a vague sense. And No in a strong practical sense.
Lensing works in reverse except for time delays which make the idea much more complex. The object's past is projected to us now, but our past would be projected to somewhere that the far object no longer occupies. Double lensing makes this even less reversible.
When the light we are now seeing was emitted, the lensing wasn't in place. In fact, the galaxies doing the lensing hadn't even evolved to the state that we see them in.
So if we sent a response to what we see now, it wouldn't make it back to the lensed objects.
That's just for single lensing. Double lenses are a massive coincidence of events at 4 points in time and space (emission, first deflection, second deflection and observation). That means that light going the other way wouldn't have the two intermediate points in the right place at the right times so it all breaks down for us and the object we see. There are some points that would be double lensed in the reverse direction but the locations and times for the source and observer have only very vague correlation to our location and the location of the object we see.
A simpler answer is just what happens if you look thru a telescope or binoculars "the wrong way" (backwards). The correct way shows a "zoomed in" view of that you're viewing, but looking the wrong way shows a "zoomed out" view.
So lifeforms on the other end of this cosmic "lens[es]" cannot use it to see us better, because in fact it makes us look further away from them than we are, from their perspective.
If only relativity were so simple :)
If I understand right, objects further than a redshift of z ~= 1.8 can't be reached by any signal we emit, and the second galaxy is at a redshift of z = 1.885. But I don't know how precisely (standard deviations rather than decimal places) the distance to the outbound cosmological horizon is being approximated, so it might be reachable by a signal sent by us:
https://upload.wikimedia.org/wikipedia/commons/8/88/Home_in_...
Not sure what the practical analogy would be. You can't use an exploding telescope?
The question I addressed is "does the lensing work the same from the other end". It's a very specific and clear question, and the answer is "no it does not", because if you reverse a telescope lens you get the opposite effect (from zoom-in to zoom-out)
The question of at what distance and relative velocity are the two locations so far apart that light can never make it from one to the other (due to expanding universe) is a completely separate issue.
That’s some sci-fi gold
sheesh, everyone knows we'd just use the sun as an RF amplifier first
This is seriously cool. One lens galaxy is amazing, but two! (Too bad that this is not steerable.)
Underlying paper: https://arxiv.org/abs/2411.04177
It would be cool if we some day had special days of astronomy where every telescope is turned to galactic eclipses the way they once did for solar eclipses.
The sky is huge and we are moving, so surely some would happen in our lifetimes?
Surely any such eclipse lasts a long time. From the perspective of our lifetimes it is static.
The farther away they are the slower they would scan the sky behind them or go out of conjunction. So I guess one of the objects would have to be something relatively close.
Are we not talking about galaxies? No galaxy is "close".
Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is.
It takes light, the fastest thing that can be, 100,000 years to cross the Milky Way.
The Sagittarius Dwarf Spheroidal Galaxy is currently in the process of being consumed by the Milky Way and is expected to pass through it within the next 100 million years.
So, unless you're even more optimistic about life extension technology than I am, not in our lifetimes, no.
> It takes light, the fastest thing that can be, 100,000 years to cross the Milky Way.
Relative to us. For the light itself it takes no time.
Our own frame of reference is the only one that matters, given the comment I'm replacing to.
Isn't the idea to use the sun as a lense already enough? The main problem being the focal point at 500+ au.
Cool! Was hoping to see a magnification amount like 100x etc
Fund the SGL Telescope!
https://www.universetoday.com/149214/if-we-used-the-sun-as-a...
Seriously, we could build that, it's at the limit of our tech but if it was either we walk on the moon again or build SGL, I'd pick SGL
I made this comment before but someone on HN made a good argument is way harder than it sounds and given it's size/cost/function it'd basically have to point in one direction, it's not like an easily moveable telescope you can scan around with.
"way harder than it sounds" is how we move forward
walking on the moon was beyond our limits when it was announced
JWST was insanely hard and almost cancelled a few times, look at it now
This is true, but also, keep in mind that the JWST was insanely hard and almost cancelled a few times :)
The SGL would be much, much harder than the JWST would be, and the JWST was already stretching our capabilities.
The SGL needs to be 650AU away from us. Voyager 1 and 2 are currently 165AU and 120AU away.
The JWST is 0.01 AU from us.
And you can only look in one direction after the probe finally gets into position. Once you're 650AU away, it's not really feasible to move "sideways" far enough to look at something else.
The ratios between 650AU, 165AU, and 0.01AU are somewhat moot.
In 1957 Sputnik 1 had an apogee of ~900km from the Earth.
By 1969 NASA was sending rockets ~385000km to the moon.
By 1979 Voyager 1 & 2 were reaching Jupiter ~5AU from Earth.
We went from 900km to 5AU in 22 years.
If SpaceX achieves their stated goals of lowering $/kg to orbit and rapid re-usability with Starship it will unlock things like asteroid/lunar mining and space based manufacturing which will allow the construction of the kind of infrastructure needed to make distances like 650AU achievable in reasonable time frames.
Low hanging fruit is easier to pluck. It might be that we cannot progress much further without consuming so many resources that Earth is left an uninhabitable husk.
I agree totally.
That is precisely why we must transition to space based resource extraction and manufacturing.
There are practically infinite resources at our fingertips on the moon, the asteroid belts and eventually the gas giants.
What we need to unlock this are the means to economically launch a minimum viable self replicating infrastructure into space to take advantage of this.
The feedback loops that will ensue should we succeed will allow us to save Earth ecology, radically transform the human condition, and unlock the ability to explore the universe in ways that we can only imagine.
True but I'd be more convinced by an argument based on tech and engineering constraints than extrapolating a progress line.
>we move forward
Do you work in something related to Astro?
Yeah, you basically need to launch a new one for every target you want to image.
I'd think to make it practical you'd have to have kind of (semi-) automatic space based assembly infrastructure that builds them and launches them. Launching these probes individually seems like it would be impractical. Building that infrastructure wouldn't be easy at all and I don't see that happening in the next 50 years.
Probably even many, because it‘s energetically impractical to stop at the focal point.
The neat thing about a how an Einstein ring works is that you don't need to stop, because rather than there being a focal point, there is a focal line, moving directly outward from the Sun. This means a probe could spend potentially decades imaging the same target on its outward path, if it had sufficient power.
If the lens curved light back toward us, could we see earth several million years ago?
No, because the light requires twice the time to travel there then back. If Earth did not move relative to the lens, it would work. Sadly we move, a lot, so what was here 2x ago was something not-earth.
To see earth, the lensing would been to be focused on where Earth was 2x ago. Still possible in theory, and you might even argue just as likely as a fully reflecting curve. But you'd not call it "back towards us". It would need to be "curved to where earth was".
Seems like if you could retrodict the position of past lenses, and predict their effects, perhaps it would somehow be possible to send a spacecraft to a specific location in order to observe Earth's past.
The idea being that a spacecraft traveling at 99% of light speed can't ordinarily catch up with light reflected by Earth. But if the light curves, and the spacecraft can travel directly towards where the light will end up (spacecraft traveling "as the crow flies"), it might be possible to catch up.
Same way I might be able to catch up with Usain Bolt at a track event if he's forced to run on the track, and I'm allowed to run across the turf in the middle.
https://en.wikipedia.org/wiki/Closed_timelike_curve
Would this be the case even if you were moving toward or along-side the 'reflector' (black hole/other body)? For the sake of discussion assume we are at or beyond the focal point.
Technically? But the image would be very very very small, so we'd need a detector bigger than the solar system (guesstimate) to see it. That's to see it: I can't imagine what it would take to resolve the image. The tricks in this paper are a start.
To zoom into a reflection on a lens or a water droplet?
From "Hear the sounds of Earth's magnetic field from 41,000 years ago" (2024) https://news.ycombinator.com/item?id=42010159 :
> [ Redshift, Doppler effect, ]
> to recall Earth's magnetic field from 41,000 years ago with such a method would presumably require a reflection (41,000/2 = 20,500) light years away
To see Earth in a reflection, though
Age of the Earth: https://en.wikipedia.org/wiki/Age_of_Earth :
> 4.54 × 10^9 years ± 1%
"J1721+8842: The first Einstein zig-zag lens" (2024) https://arxiv.org/abs/2411.04177v1
What is the distance to the centroid of the (possibly vortical ?) lens effect from Earth in light years?
/? J1721+8842 distance from Earth in light years
- https://www.iflscience.com/first-known-double-gravitational-... :
> The first lens is relatively close to the source, with a distance estimated at 10.2 billion light-years. What happens is that the quasar’s light is magnified and multiplied by this massive galaxy. Two of the images are deflected in the opposite direction as they reach the second lens, another massive galaxy. The path of the light is a zig-zag between the quasar, the first lens, and then the second one, which is just 2.3 billion light-years away
So, given a simplistic model with no relative motion between earth and the presumed constant location lens:
Does it matter that our models of the solar systems typically omit that the sun is traveling through the universe (with the planets swirling now coplanarly and trailing behind), and would the relative motion of a black hole at the edge of our solar system change the paths between here and a distant reflector over time?"The helical model - our solar system is a vortex" https://youtube.com/watch?v=0jHsq36_NTU
I believe this could only happen around a black hole. In that case, yes: light that we emitted umpty-million years ago could be shot back at us. The problem is that there would be no focussing. At best it would be like looking at an Earth 2 x umpty-million light years away. I'm guessing that it would actually be worse, with the black hole dispersing light.
(IANAastronomer, but I have opinions on any given topic...)
@Dang is there a version of /best but for comments? The thought experiment in this comment broke my mind.
https://news.ycombinator.com/highlights
In case anyone was wondering, like me, what the MJy unit in the lower diagram is: It's Mega-Jansky. Just funny that it's then being rescaled by 10^-9. Why didn't they use Milli-Jansky in the first place?
https://en.wikipedia.org/wiki/Jansky
"All prefix names are printed in lowercase letters, except at the beginning of a sentence.
Unit names are normally printed in upright type and they are treated like ordinary nouns. In English, the names of units start with a lower-case letter (even when the symbol for the unit begins with a capital letter), except at the beginning of a sentence or in capitalized material such as a title...
When the name of a unit is combined with the name of a multiple or sub-multiple prefix, no space or hyphen is used between the prefix name and the unit name...
Examples: pm (picometre), mmol (millimole), GΩ (gigaohm), THz (terahertz)"
Source: https://www.bipm.org/en/publications/si-brochure
However yes, you raise a good question. I was surprised to learn that astronomy and astrophysics prefer CGS, so by convention the diameter of the Sun is given in centimeters, and the mass of the Sun in grams! For janskys there's no excuse though.
So they were looking in the neighborhood, basically found light sources that looked like they might be duplicates and they were, therefore lensing.
Can we then find more lensing with even more compounding on purpose instead of accidentally if we sift existing data for such dupes?
> ... "in a way where" ...
The elements of Style (https://en.wikipedia.org/wiki/The_Elements_of_Style) : "Make every word count."> ... "acts as a compound lens" ...
Not really -- not the sort of lens we're familiar with, one that concentrates light at a single focus. Technical methods can exploit these chance alignments to detect objects otherwise inaccessible, but not as coherent images.
I often see remarks like this one -- "Acts as a compound lens!" -- but that's not correct. It's more like this: https://arachnoid.com/relativity/graphics/curvature_diagram....
Such alignments are more likely to produce what's called an "Einstein ring" (https://en.wikipedia.org/wiki/Einstein_ring). Very useful, but not remotely a "compound lens".
See Figure 7 in (https://arachnoid.com/relativity/index.html#General_Relativi...) for an interactive gravitational lens simulator.
Complete newbie on the topic of GL but I find it very interesting.
Q: We seem to want to use our sun for GL experiments. And that requires sending something out 500 AU out for focal length purposes. My question is somewhat theoretical so assume there are no alignment issues for the following thought. Are we not already at the "focal point/line" of some other 'entities' (sun/heavy body/galaxy/etc) gravitational lens?
Hmm... is that what this paper is talking about - the fact that the GL observation was made from Earth on some infinite focal length. Listening to other YT videos on the topic seemed to imply the resolution would be much higher. Or is the resolution location/technology related?
With all the galaxies out there, it seems likely that Earth should be in the focus of lots of systems of this kind. Hopefully in the future we'll be able to find and use these galactic telescopes!
> the finding will allow other researchers to more precisely calculate the Hubble constant
How would a compound lens lead to a better estimate of the expansion rate of the universe?
Disclaimer: I am a layman, not trained at all. But I am interested in this stuff.
Our most powerful telescopes can see "back in time", by looking at stuff far enough away that it took nearly the entire age of the universe for the light to reach us.
I would guess that we can use this natural compound lens to "see farther" with our current telescopes than we might otherwise be able to see.
Our current best telescope, the JWST, can almost see to the very beginning of when it was possible to see, somewhere between 300k and 200M years after the big bang [0].
Somewhere in this time period, the universe cooled enough for normal matter to form.
The JWST still cannot see the actual 'edge' of when this occurred.
Maybe with this natural compound lens, we can see all the way to the edge.
And if we could see where the edge actually is, then maybe we can refine the estimate to a tighter range than [300k,200M], which would give us a better estimate of the expansion rate of the earlier universe.
[0] https://www.universetoday.com/168872/webb-observations-shed-...
From the abstract:
> This unique configuration offers the opportunity to combine two major lensing cosmological probes: time-delay cosmography and dual source-plane lensing since J1721+8842 features multiple lensed sources forming two distinct Einstein radii of different sizes, one of which being a variable quasar. We expect tight constraints on the Hubble constant and the equation of state of dark energy by combining these two probes on the same system. The z2=1.885 deflector, a quiescent galaxy, is also the highest-redshift strong galaxy-scale lens with a spectroscopic redshift measurement.
Not an expert, just trying to add some more context.
With time-delay cosmography[1] one exploits that unless the source is perfectly in the center of the line of sight, then the photons that make up one lensed copy have traveled a different distance from the source than photons that make up a different lensed copy. This effect can be used to measure absolute distance and give an accurate measure of the Hubble constant.
With dual source-plane lensing[2] one exploits that if two different sources lensed by the same lens, one can take the ratio of the measurements between the two sources and get results that are significantly less affected by the lens itself and is completely independent of the Hubble constant.
[1]: https://arxiv.org/abs/2210.10833
[2]: https://arxiv.org/abs/2204.03020
How interconnected everything is, even on a cosmic scale