4 ms·

Theoretical question: if spectrography advanced to the point that we could see individual organisms and their behavior from lightyears away, and then we placed

Theoretical question: if spectrography advanced to the point that we could see individual organisms and their behavior from lightyears away, and then we placed a very large mirror very far away from our planet, could we aim a telescope at that mirror and watch ourselves going about our lives in the past? Could we use this technology to solve crimes, answer historical questions, see dinosaurs, etc?

pixl97 4y ago
I don't think it's possible. R squared pretty much means that no light from an individual organism will ever reach you. The number of photons reflected from a planet is a staggeringly large number, but the distances involved mean that the faction that actually hit your spectrograph are so incredibly tiny that its impossible to tell what they are from. Now, that's just in one way travel. In two way reflected travel it gets even worse. Only some tiny fraction of our light will hit the mirror due to R2. There is no such thing as a perfect mirror so only a tiny percent of the tiny percent will be reflected back. Distances in space are stupid. Our minds cannot comprehend them.
- moehm 4y ago
  > Distances in space are stupid. Our minds cannot comprehend them. It's not even an Euclidean space.
- layer8 4y ago
  The mirror just needs to be large enough, concave and spherical. ;)
sockaddr 4y ago
> see dinosaurs Only if you had the ability to outrun a photon that has been traveling for 65,000,000 years, park, set up camp, and build a mirror. But then, if you can do that why are you building a mirror?
- scrumper 4y ago
  What if you could avoid the travel? Perhaps some chance alignment of distant black holes creating something like a gravitational mirror? Bend the light, bend it some more, bend it so it reaches earth today. A 65 million light year round trip.
  - ChainOfFools 4y ago
    Or we have discovered aliens have already done this for us, perhaps to amuse themselves at the chaos that will ensue once we figure out what it is we're looking at and what it means for all of our civil, social, and religious institutions.
    - mickael-kerjean 4y ago
      I'd buy that book if it existed
      - brian_cloutier 4y ago
        Three Body Problem is not quite this but is pretty close to this, you might like it.
alanbernstein 4y ago
Practical answer: with this method, the absolute bound on how far back in time you can see is today.
klyrs 4y ago
Yes! Problem being that if we launched the mirror a light-year away, it would take more than a year to get there, and once we got the telescope set up, it would only show us a year in the (relatively speaking) past. Send it a million light years away, it would take more than a million years to get there, and our million-year-distant descendents would only see a million years in their past. So we wouldn't see into our past... it would be cheaper to just launch a camera into space with a long buffer of tape. However, black holes are known to curve space-time and I've often wondered if we could find a sequence of black holes in a configuration that would bend the earth's light back on itself... theory being that if we had good enough resolution we might actually be able to see our deep past. I have no optimism that we could resolve organisms though -- just finding the sun, and seeing what it looked like in the past would be an incredible feat.
- yreg 4y ago
  We should pop up a mirror facing away from Earth as a service for anyone who wants to look at their past. Hopefully someone else did that for us, just need to find it. It's a common courtesy.
  - klyrs 4y ago
    Be the change you want to see in the galaxy...
  - gfodor 4y ago
    This is kind of a fascinating idea - wouldn’t it be kind of an amazing “public service” to just deploy a large reflective perfect sphere into space at the galactic restful reference frame?
  - macintux 4y ago
    Update: April Fools joke that won’t die. Mea culpa. Reminds me of the Dr. Who episodes recovered via Arecibo. https://dailypop.wordpress.com/2011/11/01/lost-doctor-who-episodes-found-in-deep-space/ https://dailypop.wordpress.com/2011/11/01/lost-doctor-who-ep...
nuccy 4y ago
Since this is a theoretical question, I can theorize an actual possibility for such effect - bending of light by massive objects which results in actually observed gravitational lensing (focusing of light from distant object by front objects). If just by pure coincident galaxy clusters (the most massive objects we know) happen to be located just right, light from Earth can bend enough to point back on us. Those clusters are far enough though that we can then "see" on the time scales of Earth forming, so getting to any historical questions or even dinosaurs would actually take some waiting.
nmeagent 4y ago
Theoretically yes, but you would need a very large telescope to resolve individual organisms at the distance of one or more light years. The Rayleigh criterion gives us a minimum diameter of 1.22 * wavelength / angle, and the angle to resolve 1 meter at 1 ly is approximately 1.06e-16 radians. So for 508 nm visible (sun)light the minimum diameter would be ~5.86e9 meters (or a little more than 919 Earth radii) per light year. https://en.wikipedia.org/wiki/Angular_resolution#Explanation https://en.wikipedia.org/wiki/Angular_resolution#Explanation

pixl97 4y ago

I don't think it's possible. R squared pretty much means that no light from an individual organism will ever reach you. The number of photons reflected from a planet is a staggeringly large number, but the distances involved mean that the faction that actually hit your spectrograph are so incredibly tiny that its impossible to tell what they are from. Now, that's just in one way travel. In two way reflected travel it gets even worse. Only some tiny fraction of our light will hit the mirror due to R2. There is no such thing as a perfect mirror so only a tiny percent of the tiny percent will be reflected back. Distances in space are stupid. Our minds cannot comprehend them.

moehm 4y ago
> Distances in space are stupid. Our minds cannot comprehend them. It's not even an Euclidean space.
layer8 4y ago
The mirror just needs to be large enough, concave and spherical. ;)

sockaddr 4y ago

> see dinosaurs Only if you had the ability to outrun a photon that has been traveling for 65,000,000 years, park, set up camp, and build a mirror. But then, if you can do that why are you building a mirror?

scrumper 4y ago
What if you could avoid the travel? Perhaps some chance alignment of distant black holes creating something like a gravitational mirror? Bend the light, bend it some more, bend it so it reaches earth today. A 65 million light year round trip.
- ChainOfFools 4y ago
  Or we have discovered aliens have already done this for us, perhaps to amuse themselves at the chaos that will ensue once we figure out what it is we're looking at and what it means for all of our civil, social, and religious institutions.
  - mickael-kerjean 4y ago
    I'd buy that book if it existed
    - brian_cloutier 4y ago
      Three Body Problem is not quite this but is pretty close to this, you might like it.

alanbernstein 4y ago

Practical answer: with this method, the absolute bound on how far back in time you can see is today.

klyrs 4y ago

Yes! Problem being that if we launched the mirror a light-year away, it would take more than a year to get there, and once we got the telescope set up, it would only show us a year in the (relatively speaking) past. Send it a million light years away, it would take more than a million years to get there, and our million-year-distant descendents would only see a million years in their past. So we wouldn't see into our past... it would be cheaper to just launch a camera into space with a long buffer of tape. However, black holes are known to curve space-time and I've often wondered if we could find a sequence of black holes in a configuration that would bend the earth's light back on itself... theory being that if we had good enough resolution we might actually be able to see our deep past. I have no optimism that we could resolve organisms though -- just finding the sun, and seeing what it looked like in the past would be an incredible feat.

yreg 4y ago
We should pop up a mirror facing away from Earth as a service for anyone who wants to look at their past. Hopefully someone else did that for us, just need to find it. It's a common courtesy.
- klyrs 4y ago
  Be the change you want to see in the galaxy...
- gfodor 4y ago
  This is kind of a fascinating idea - wouldn’t it be kind of an amazing “public service” to just deploy a large reflective perfect sphere into space at the galactic restful reference frame?
- macintux 4y ago
  Update: April Fools joke that won’t die. Mea culpa. Reminds me of the Dr. Who episodes recovered via Arecibo. https://dailypop.wordpress.com/2011/11/01/lost-doctor-who-episodes-found-in-deep-space/ https://dailypop.wordpress.com/2011/11/01/lost-doctor-who-ep...

nuccy 4y ago

Since this is a theoretical question, I can theorize an actual possibility for such effect - bending of light by massive objects which results in actually observed gravitational lensing (focusing of light from distant object by front objects). If just by pure coincident galaxy clusters (the most massive objects we know) happen to be located just right, light from Earth can bend enough to point back on us. Those clusters are far enough though that we can then "see" on the time scales of Earth forming, so getting to any historical questions or even dinosaurs would actually take some waiting.

nmeagent 4y ago

Theoretically yes, but you would need a very large telescope to resolve individual organisms at the distance of one or more light years. The Rayleigh criterion gives us a minimum diameter of 1.22 * wavelength / angle, and the angle to resolve 1 meter at 1 ly is approximately 1.06e-16 radians. So for 508 nm visible (sun)light the minimum diameter would be ~5.86e9 meters (or a little more than 919 Earth radii) per light year. https://en.wikipedia.org/wiki/Angular_resolution#Explanation https://en.wikipedia.org/wiki/Angular_resolution#Explanation