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

(phys.org)
268 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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kcmastrpc ◴[16 Nov 24 19:47 UTC] No.42158706[source]▶
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.
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Voultapher ◴[16 Nov 24 20:35 UTC] No.42159095[source]▶
But wouldn't the size and age of the universe also imply that someone has looked at just the right moment somewhere somewhen.
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drexlspivey ◴[16 Nov 24 20:59 UTC] No.42159276[source]▶
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.
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quantadev ◴[16 Nov 24 22:33 UTC] No.42159991[source]▶
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.
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WJW ◴[17 Nov 24 00:05 UTC] No.42160730[source]▶
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.
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1. quantadev ◴[17 Nov 24 00:17 UTC] No.42160796[source]▶
> 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?

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2. WJW ◴[17 Nov 24 21:25 UTC] No.42167438[source]▶
> 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.

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3. quantadev ◴[17 Nov 24 22:47 UTC] No.42168068[source]▶
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.