# Thread: If something was traveling faster than the speed of light what would you see?

1. Newbie
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## If something was traveling faster than the speed of light what would you see?

Earlier today I was reading about how Adam Reiss got a Nobel prize for determining that the universe is expanding at 74.2 kilometres per second per mega parsec.

If that's true, and I have not reason to doubt it, then that means some parts of our universe are indeed moving faster than the speed of light.
With the speed of light at 3x10^8 m/s, and radius of the universe of about 46 billion light years (14103.6 mega parsecs) the outer edge of the universe would be expanding at 14103.6 x 74.2 km = 1,046,487 km/s which would be roughly 3 times the speed of light

If something was travleling faster than the speed of light what would you see?

To put this in context imagine we are looking at, or trying to look at, a galaxy near the outer edge of the universe that has a galaxy behind it.
Would we see either galaxy at all, would we see absolutely nothing or would we see what was in that position before the galaxy moved to it?
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2. Hi clarksleap, welcome to CQ.

3. Originally Posted by clarksleap
...

If something was travleling faster than the speed of light what would you see?

To put this in context imagine we are looking at, or trying to look at, a galaxy near the outer edge of the universe that has a galaxy behind it.
Would we see either galaxy at all, would we see absolutely nothing or would we see what was in that position before the galaxy moved to it?
...
All these rules about not travelling faster than light apply to a local (non-expanding) space. Once space is expanding, it gets a little more complicated, but not impossible, to give Earthly analogies to explain what is happening.
Most people discussing this issue start by pointing out that a photon from an ion in a star in a galaxy emitted 12 billion years ago heading towards where we will eventually be keeps getting closer to us, and eventually crosses a line where it is not moving away from us faster than light, and will therefore arrive here. The math and details are probably better seen by looking at a website that explains it. In the old days people used to go to Ned Wright's site. It is still a good place to start.

4. Order of Kilopi
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To add to that, the answer to your question is "nothing too strange" is what you'd see-- the galaxy would just look redshifted because the wavelengths of light would be stretched by the same factor that "space itself" has expanded since the light was emitted. As antoniseb said, the light takes a finite time to get to you even if it starts out in a region moving away from you faster than light, and indeed the same thing would happen for ants crawling on a rubber tablecloth that is stretching faster than ants can walk. This is why it is better to think of the cosmological redshift as being due to a stretching of space, rather than from sources moving at high speed through a stationary space.

5. Originally Posted by clarksleap
Earlier today I was reading about how Adam Reiss got a Nobel prize for determining that the universe is expanding at 74.2 kilometres per second per mega parsec.
Just a detail, but Riess got the Nobel Prize (along with Perlmutter and Schmidt) for showing that the rate of expansion is (now) increasing; i.e. that expansion is accelerating.

6. And, in fact, according to the current cosmological model, a lot of the strongly red-shifted galaxies that we can see were moving away from us faster than light at the time they emitted the light we're seeing, and they're still doing so "now".
Lineweaver and Davis's Scientific American article (440KB pdf) is a good place to find an explanation.

Grant Hutchison

7. Originally Posted by Ken G
To add to that, the answer to your question is "nothing too strange" is what you'd see-- the galaxy would just look redshifted because the wavelengths of light would be stretched by the same factor that "space itself" has expanded since the light was emitted.
And even in a "Big Rip" scenario, or other similar possibilities, where a galaxy can be within our cosmic horizon (and hence visible) and then end up beyond the cosmic horizon (and thus no longer visible), this is still what you'd see. The galaxy would steadily fade and redshift more, until it could no longer be seen.

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Originally Posted by grant hutchison
And, in fact, according to the current cosmological model, a lot of the strongly red-shifted galaxies that we can see were moving away from us faster than light at the time they emitted the light we're seeing, and they're still doing so "now". Lineweaver and Davis's Scientific American article (440KB pdf) is a good place to find an explanation.
Also see their paper Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe: "We show that we can observe galaxies that have, and always have had, recession velocities greater than the speed of light". Along with Ant on a rubber rope on Wikipedia.

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