Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 04/06/2013 11:30:01
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Ali Rhayem asked the Naked Scientists:
We see objects in the universe because their light reaches us, so every time we look at the night sky, we see a snapshot of how objects looked like at various times in the past depending on when their light was emitted. The further and deeper we look the older the object is.
But is it at all possible to see the same object at two different stages of its evolution, i.e. at two different points in time, in a single snapshot?
If we think of this in discrete terms, as if the object (any form of matter or else) emits light at point A, moves away faster than light then emits light again at point B. Provided that the object emits the light at both A and B within the cosmological event horizon where light can still eventually reach us, the light from point A reaches us first then the light from point B, thus we see two images of the same object in a single snapshot.
What do you think?
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The trouble with that is the part where the object moves faster than the speed of light. That isn't going to happen. But, there is gravitational lensing, which allows multiple images of a galaxy to be seen round the edges of a nearer galaxy which is directly in front of the other. Even then, all the distorted images of the further-away galaxy will be following roughly the same length of path. What we really need is a series of gravitational lenses taking light on a much longer path, but it would take a lot of luck to get enough galaxies in the right places to do this, and even if it happened the image would be so degraded (distorted and dimmed) that it probably wouldn't be identifiable.
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To see two distinct images of an object traveling at greater than the speed of light (or the universe stretching), then you would need it to have a variable speed with respect to us. Faster & slower (a very rapid orbit?) Otherwise, the best one could hope for would be to see a smudge.
The other thing that will occur is that the faster an object is moving, the more red-shifted at least the light generated on it will be. So, if you are looking at a star that is receding from us at close to the speed of light, you won't be "seeing" it at all. Rather, it will be picked up in extremely long wave radio waves. At the speed of light, the wavelength would essentially become infinite.