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I only ask this because of how fast light travels. The question remains in the title. Why, or why not, would this work?

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Mirror or not, we are always seeing into the past. When you read a book you are seeing the book as it was a very small fraction of a second ago. –  astromax Jan 15 at 0:47
    
You couldn't use this method to see times before the mirror was put in place. For example, a mirror 1 light-year away would (in principle) let us see Earth as it was 2 years ago -- but it would take more than a year to get it there. –  Keith Thompson Jan 15 at 16:55
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up vote 3 down vote accepted

I think the question is referring to situating a very large mirror in space facing earth. If we were to put it several light minutes away, then events occurring opposite the mirror could be reviewed de novo with more preparation upon the warning we received upon the first light of the event arriving at earth.

For example, a supernova going off in M31 might not be under observation at the moment its light first arrives, and so the initial observations might be lost. However, with a mirror facing M31, we would be able to observe that mirror as the event unfolded, having been warned that there was something worth watching, in advance.

Nice idea! But it would likely be much less expensive to simply have multiple telescopes always watching "prime" starscape for unexpected events.

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That's an interesting interpretation of the question. –  Stan Liou Jan 15 at 1:07
    
My question exactly. So it would work? –  ilarsona Jan 15 at 4:17
    
Of course it would work, @ilarsona. It would be prohibitively expensive, though. And a mirror for one direction would block the view to the other. –  Cyberherbalist Jan 15 at 17:37
    
@ilarsona Suppose an advanced extraterrestrial race living 4.5 billion years ago constructed a perfectly flawless mirror of sufficient size 4.5 billion light years from Earth, pointed precisely at Earth. We would today be able to watch the entire history of the Earth play out by observing the Earth in that mirror through our telescopes. Whether such a mirror could be constructed is another matter. –  called2voyage Jan 15 at 18:43
    
So what about just a couple cameras then? Sure... hubble and all that, but those work with in the field of light as well... can we record the past? –  ilarsona Jan 15 at 20:03
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Yes, we always look into the past, when looking somewhere. There is for instance a mirror on the moon. When sending a laser beam to that mirror, we can detect the reflected light about 2.5 seconds later. This could be interpreted as looking 2.5 seconds into the past, when the laser has been fired. Details here.

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Some quantum tunneling effects (en.wikipedia.org/wiki/Quantum_tunnelling) have been discussed to travel faster than light, although weakening the signal; I can't rule out, that this is possible. –  Gerald Jan 14 at 20:59
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More about that discussion here: en.wikipedia.org/wiki/G%C3%BCnter_Nimtz –  Gerald Jan 14 at 21:08
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