Very simply.
When Hubble looks at a star, it is literally looking into the past. Because light travels at a finite (and constant) speed, we observe things long after they actually happen.
The earth has been on its journey for 4.6 billion years.
So, if I point Hubble to a point where the earth would have been say 1 million years ago why can't I see it?
I know it has something to do with the relative position of the observer. And I know it something to do with causality... but can someone explain the physical law that governs WHY Hubble could not target earth sometime in the past?
2007-12-31 04:08:00 · 6 answers · asked by acyberwin 5 in Science & Mathematics ➔ Physics
eyeonthescreen:
Yes, I realize that it would have something to do with the fact that the escaping light is moving much faster than then object. But it seems to me that when I found this point, what I would be observing is the object in the future...
I think the problem has something to do with either time dilation or the relativity of simultaneity... or both?
Anyone have any other insight?
2007-12-31 05:40:54 · update #1
This is easy...because the Earth is not there 4.6 B years ago, it is here and now. That light that existed on Earth back then has long since gone out into space...away from our blue marble. So, when you point the Hubble to scan a bit of the universe where Earth existed 1 M years ago, that light back then has passed us up in the here and now.
It's no longer observable to us. Even the Alpha Centaurians, living near the star Alpha Centauri 4.3 light years away from our solar system would not be able to see Earth where it was 1 M years ago. For the same reason, the Earth light from then has long passed them by. To observe Earth 1 M years ago, the observe has to be 1 M light years or more away from where Earth existed back then.
A set of velocity equations will show this. Assume L = ct and l = vt; where L and l are the distance light and the Earth travel in time t at velocities c and v respectively. v <<< c; so that l <<< L for any given time elapsed. Assume t = 0 when the light escapes from Earth 1 M years ago. Here and now, the light will have traveled L >>> l while Earth limps along to cover l. That light from old Earth has long since passed us up in the here and now.
PS: There is nothing relativistic about this. L and l are the same old Euclidian distances we are used to without any Lorentz transformations. And time is not dilated etc. No, the answer is just that light from then has come and gone; leaving us in the here and now with nothing to see.
2007-12-31 04:27:19 · answer #1 · answered by oldprof 7 · 0⤊ 0⤋
It's because the light that left the earth 1 million years ago--that is, the light that shows what the earth looked like back then--has already left the building.
Think of those light beams as a bunch of fast rocket ships, each one carrying a photograph of the earth. These rocket ships are constantly "blasting off" and heading directly away from the earth. The ones that blasted off a million years ago--that is, the ones that contain the "photos" of the earth of 1 million B.C. -- are already a million light years away from us. They are outside our solar system, and have even left the galaxy by now. The only way we can see those old "photos" is to somehow overtake those "rocket ships." That would require a rocket ship (a real one) of our own, that could travel faster than light, and could travel far outside our galaxy.
2007-12-31 12:44:22 · answer #2 · answered by RickB 7 · 0⤊ 0⤋
Lets say that 4.6 billion years ago the earth was at position 1 and right now it is at position 2.
The only way that we would be able to to see the light from position 1 now would be if the earth was moving faster than the speed of light from position 1 to position 2. And that just can't happen.
The only reason we can do this with stars is that they are already billions of light years away.
But I always thought it would be cool if we could point Hubble at a giant mirror in space that is far far away and see the earth in the past.
2007-12-31 12:20:49 · answer #3 · answered by ? 3 · 2⤊ 0⤋
For all intents and purposes, Earth has been pretty much where it is right now, not so far away that light travelling from it would have taken a million years to reach the place where Hubble is. Light years is a distance measurement, not a way of looking into the past. And reflected light (which is the way we see the Earth) is not likely to be detectable at a distance where time-difference would be a factor. The light-year factor comes into play when Hubble looks at stars, that emit light.
2007-12-31 12:16:13 · answer #4 · answered by Anonymous · 1⤊ 0⤋
Earth is a very tiny object to the HST, so even if it were possible to 'look back' at the old Earth, nothing useful would be revealed. Also, there are conditions and physics that are quite beyond me, so let's wait for more answers...
2007-12-31 12:16:01 · answer #5 · answered by Thomas E 7 · 0⤊ 1⤋
I don't know the answer to that, but it's a GREAT friggin question! I never actually thought of that. I wonder!
2007-12-31 12:13:38 · answer #6 · answered by Anonymous · 1⤊ 2⤋