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

(phys.org)
271 points wglb | 3 comments | 16 Nov 24 16:48 UTC | HN request time: 0s | source
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waltbosz ◴[16 Nov 24 19:38 UTC] No.42158640[source]▶
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.

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yreg ◴[16 Nov 24 20:57 UTC] No.42159264[source]▶
Is it only one direction or does it work the same from the other side?
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M_bara ◴[16 Nov 24 21:00 UTC] No.42159288[source]▶
Should work the other way too. Physics and symmetry:)
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ted_dunning ◴[16 Nov 24 21:49 UTC] No.42159648[source]▶
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.

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1. quantadev ◴[16 Nov 24 22:04 UTC] No.42159750{3}[source]▶
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.

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2. ben_w ◴[17 Nov 24 00:24 UTC] No.42160840[source]▶
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?

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3. quantadev ◴[17 Nov 24 01:05 UTC] No.42161066[source]▶
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.