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

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.

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.

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.

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?

That’s some sci-fi gold

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

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?

Cool! Was hoping to see a magnification amount like 100x etc

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.

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".

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.

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.

  > It's Mega-Jansky...  Why didn't they use Milli-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.

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.