Two galaxies aligned in a way where their gravity acts as a compound lens

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.

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.

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.

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

Is it only one direction or does it work the same from the other side?

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.

Should work the other way too. Physics and symmetry:)

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.

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.

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.

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.

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.

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.

Anywhere in the galaxy within the super narrow beam that the Arecibo antenna happened to cover at the time, at least.

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 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.

> 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?

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.

> 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).

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.

This Wikipedia page suggests otherwise - https://en.wikipedia.org/wiki/Gravitational_lens

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.

>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?

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."

This time scale is nuts to me. I cannot fathom it. Just…wow. None of this (humanity) matters in the grand scale.

sheesh, everyone knows we'd just use the sun as an RF amplifier first

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.

Isn't the universe (near) chaotic on those timescales and you can only predict the general flow? Or is this me mixing things up?

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.

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

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..

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.

What's the advantage of sending a signal through the lens relative to sending it in some other arbitrary direction?

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.

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)

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.

Regular EM Radio waves are not photons. Photons have special configuration which prevents leaks into surrounding space, while regular radio waves are just waves.

> 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.

Nope. Radio waves are made of photons. All EM waves are made of photons.

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.

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».

Photons are EM-waves. Are photons made of photons?

That’s some sci-fi gold

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.

Great, now add to that the fact that radio waves are an EM-wave too, and that answers your original confusion.

Maybe you should contribute something useful to discussion.

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.

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.

I did. I told you Radio waves are made of photons.

Saying that Radio waves are a particular frequency range of photons is not a tautology. The only one making up tautologies is you.