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Comment by jcims | Hacker News Reader

Somewhere outside the Schwarzschild radius of every black hole is a region in which the path of incoming photons is 'bent' 180 degrees and they are sent back roughly in the direction from which they came.

In my mind this means that some infinitesimally small portion of the photons that fall on the earth every day were reflected from the earth ~50K years ago and sent in the direction of the black hole at the center of the Milky Way.

I sometimes wonder if we could build a telescope large enough to resolve these photons into an image of the earth from that time. This would likely require 5,000 years of technological development and construction, but it does seem like it's physically possible. (Maybe there's too much dust or distortion or background noise to actually pick out the signal, dunno)

We couldn't go back XXX million years, but maybe XX thousand?

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Most black holes, especially ones at the heart of galaxies, are surrounded by huge clouds of dust and gas, and their own accretion disks. Also even for a bare black hole, it's gravity field won't be perfectly uniform to any image would be hugely distorted (like a lens made of poor quality uneven glass). On top of that as photons transit through the gravity field, even if that was perfectly uniform photons on slightly different paths would have their wavelengths stretched different amounts causing diffraction smearing the image. Finally the light path would have to be pretty close to the black hole to slingshot all the way round it, so the gravitational gradient is going to be very steep, meaning all of these effects are going to be very severe. In reality, even for a bare nonrotating black hole (much worse effects if it's rotating), which isn't really a thing as far as we know, the resulting signal is unlikely to be distinguishable from random noise.

The problem is the wavelength of light puts a hard limit on angular resolution, its the same reason why a spy satellite cannot read your newspaper, or microscopes can not see atoms, its physically impossible.

https://en.wikipedia.org/wiki/Angular_resolution

It is my understanding that you can mitigate this to an extent by making larger scopes or employing techniques like interferometry. I haven't found an absolute 'hard limit', only a limit based on the size of the resolving device.

I'm generally a moron in this area though so I'm certainly missing large swaths of information.

On the other hand, earth is a finite sized object, and so are photons, so there are only so many photons to work with no matter how large your detector.

[I agree.]

I'm too lazy to try the calculations now, but if the nearest black hole is 2800 ly away, I guess the size of lens to resolve a dinosaur have to be much bigger than the solar system. Probably a few light-year [calculation needed]. Does the lens reach Proxima Centaury? Does the lens collapse and form another black hole?

Also, the black holes are big, but they are more far away than big, so the angular size of the area that does some interesting lens effect is very small. Then even if you have a giant lens and collect all the light that the black hole has blended (more than 90° or something), you will get a extremely dim image. But photons are discrete, and visible photons have a minimal energy, so the image will be composed of a very small amount of dots, and not something smooth where you can see anything.

You have to wonder what is possible when you see articles like this one when you collect enough data: https://news.ycombinator.com/item?id=17692447

The earth will have moved from its original position, so you'd probably need to catch photons that were shot off on 'paths' that will intersect your future position, which may not necessarily be photons that were 180-degree reversed.

I have followed the same idea regarding radio waves. Would it be possible to travel far enough out to receive radio or TV broadcasts from WWII? Presumably all that information in the form of waves is still out there (assuming it hasn't been too distorted by inference, or far too small of an amplitude).

No, because you can't catch up with radio waves since they are going the speed of light.

But using OP's idea, they could be reflected back at the earth, such that 2018 - 1942 = 2 x numLightYears away from the scattering object. I agree, you wouldn't be able to go "in front of the waves" but you might be able to go to a point in space where they'd eventually arrive through scattering. I think it's very unlikely anyways since the solid angle for them to be reflected right back at a certain point is super small.

Very unlikely indeed. I wish I was smart enough to do the math but I'm sure it's more difficult than resolving a planet 50k light years away. Probably more like resolving a planet in another galaxy.

I fixed the typo, the factor of a half should have been on the other side of the equation. To actually calculate a realistic probability would require more work; farther objects would imply higher attenuation, etc.

It's late so I'm a bit too lazy to find the paper I remember, but this is the generalized form of that concept: https://thetechreader.com/tech/it-is-scientifically-possible...

I think that signal strength would decrease along the lines of the inverse square, so after you're some distance out, depending on the initial signal strength you'd find it had decayed to a level indistinguishable from background noise.

I think.

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