By putting a mirror in space, would we be able to see into the past?
I only ask this because of how fast light travels. The question remains in the title. Why, or why not, would this work?
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.
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.
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.
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.
@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.
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?
@called2voyage that's a nice idea, but we'd have to wait 4.5 billion years for the first images to reach us. To be receiving images *now* from our beginning the mirror would need to be half the age of the earth in light-years. In other words, 2.25 billion light-years away. It's a two-way trip, after all.
@Cyberherbalist Good point, the actual calculations are slightly more complex than I let on, but the general idea is--yes--you could look into the past of Earth in this theoretical scenario.
@ilarsona No, the camera would have to already be pointed at us. To send a camera out takes time, and it takes time (the speed of light) for the signal to get back to us so by the time the data reaches us we don't have any information more useful than what we could record here.