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

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.

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.

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.

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.

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

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.

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)