Expert: Courtney Seligman - 10/20/2011

Question
Hi Courtney,

I wonder if you could attempt an answer to this thought experiment I am presently struggling with.

It is said that powerful telescopic instruments can "look" into the past enabling us to see the universe as it was billions of years ago. If this is true, and given that we could muster sufficient resolving power, would it be possible to look at the part of space where the earth was at its presently estimated age of about four billion years ago and watch it being formed?
I hope that you can shed some light on this for me.
Thanks - Peter

Answer
I'm afraid the answer is no, we can't look into our past.

When we say that we can look "into the past", we mean that when we look at very distant galaxies, we see them as they were when the light by which we see them was emitted. It does not mean that we can look into our own past. That could only be seen by someone so far away that the light emitted by us at that time has just reached them.

For instance, suppose that there is a galaxy so far away that the light by which we see it took 4.5 billion years to reach us, and the light by which anyone there could see us took 4.5 billion years to reach them. Then we would see them as they were that long ago, and they would see us as we were that long ago; but neither of us would see our own pasts at all.

I suppose you could get around this, if someone if that distant galaxy beamed images of our galaxy in our direction, so that when they reached us, we could see our galaxy as they now see it -- that is, when the Solar System was forming. However, it would take many billions of years for any signal sent now to reach us, so we would have a very long wait to "see" our past, even if someone were so kind as to be beaming that sort of information at us (which, considering the effort and expense involved, and the fact that they wouldn't know we wanted to see that, would be highly unlikely).