MadSci Network: Astronomy

Re: with a mirror in the distant space could we see our past in the future?

Date: Mon Apr 16 11:08:50 2001
Posted By: Juan Cabanela, Faculty, Physics, Astronomy, & Engineering Science, Saint Cloud State University
Area of science: Astronomy
ID: 987373015.As

It's an interesting idea to do this. The major problem is how fast you can move the mirror out. If a mirror magically appeared 500 light years from Earth and we had a telescope powerful enough and the alignment of the mirror was just right, we could in theory see into our past, 1000 years back in fact.

However, as with many theoretical ideas, the actual implementation of such a system would be well beyond our foreseeable capabilities. The two major problems I can see with this are that:

  1. Your mirror would have to be perfectly aligned back to Earth, since an misalignment of (my estimate) about 1 parts in a 3 trillion in angle would be enough for the reflection to miss the Earth. And even if you can see the Earth, you will only see a tiny portion of the Earth...much like how looking out a distant window only gives you a limited view of the outside world.
  2. The other, more important problem is the speed of light. Let's say instead of the mirror magically appearing out there, you have to get it there from Earth. Due to the constraints of special relativity, you can not move the mirror faster than the speed of light, so your mirror will take at least 500 years to get out 500 light years from Earth, and most likely a lot longer. The fastest spacecraft we have built is only travelling 0.02% the speed of light so it would take 25000 years to get out this far. In essence, you will never be able to see farther into the past than the day you launched the mirror, and in fact, it is most likely you would not even get to see anything near the launch date in your mirror. Realistically, you'd probably be only able to see events that occur after the mirror reaches its final location.

Frankly, it would be a lot less expensive to put a mirror into space than it would be to archive as much of today's media (written media, video, audio, fiction, etc) and use that in the future to study the past. You'd get a much better view too.

[Moderator's Note: There's a neat little short story by Italo Calvino on a subject very close to this one. It is called "The Light-Years," and is published in the collection Cosmicomics. Be careful, though, because Calvino doesn't quite understand all the science....]

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