Expert: Tom Whiting - 10/28/2010

Question
As we know, when we view a night sky we see the light from distant stars or distant galaxies as they appeared in the past some light years ago. Essentially, as we observe, we are looking at the past. As this light continues past Earth and beyond, some of it gets captured by a black hole, from which nothing (not even light) can escape.

Suppose we could stop time and rewind the universe and we were able to view it running backwards as it did.

What would we see?

All light it running backwards toward its source- no light is approaching Earth. Would the universe be completely black? Or might it be that the only visible things in space are black holes from which all light that was lost is now escaping?  Or would it look like a photo negative where everything that is black becomes white and everything that is light becomes black since everything that emitted light now absorbs it and everything that absorbs light now emits it?

Finally, when we see the night sky normally and realize that all light approaching us is fronm the past, if we rewind could the universe wouldn't we then actually be looking into the future?

Answer
Hi David,
First, we need to straighten out your question, like the first sentence.  A light year is a unit of distance (about 6 trillion miles), not time, so the first statement should read "...as they appear in the past some (millions of) years ago".  Yes, as we observe outward, we are seeing light from the past from our perspective.  
Your hypothetical question isn't "stopping" time, it's running it backwards, which of course is impossible (because we can't take the square root of a negative number in the Lorentz transformation equations). But if it were to hypothetically happen, we would see a contracting Universe instead of an accelerating, expanding Universe.
But light would not go 'backwards' because to a light photon, time passage equals zero (infinite dilation)... as given by the Lorentz transformation equation, time at velocity v = time at rest divided by the square root of 1 - v squared/c squared. For light, and any electromagnetic radiation, v = c, thus a zero in the denominator, so the time dilation is infinite, and time passage = zero,from a photon's point of view, irrespective of what the source and receiver, are doing... approaching or departing - expanding or contracting.
So we would see nothing out of the ordinary, except a contracting Universe (shrinking) but the light would look the same. Instead of seeing a red shift, all galaxies (except for some of the Local Group which gravity can over-ride the spacial contraction) would all exhibit
a blue shift... and the more distant galaxies would show a greater blue shift... just as with expanding Universe, the more distant galaxies exhibit a higher red shift. So no, we would still see light blue-shifted instead of red shifted... light photons would not go "backwards".
{IN your hypothetical scenario, you're just running the 'movie film'
backwards, not the real light photons.}
So no, we could not see the future... no one knows the future, not even your best... stock broker on Wall street. They want you to think that they can predict the future, but they really can't.  And neither can astrologers, fortune tellers, palm readers, and the like.
Clear Skies,
Tom Whiting
Erie, PA