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A photon moves across space, but it does not move across time.

If we look at a galaxy that is 1 billion light years away, then we see that galaxy as it appeared 1 billion years ago.

Now the photons which constructed the image of the galaxy, did not move across time, however, we, did.

Thus the question arises... Since the light itself has not traveled across time, are we literally looking at the galaxy as it "is" 1 billion years ago, rather than be looking at it as it "was", 1 billion years ago.

( If the question seems like rubbish, that can be a good sign. I asked my science teacher a question back in grade 9 and was told that the question was just a load of rubbish. Years later, with a lot of work I answered my question, and did so without even knowing that I had independently discovered SR, and I had also independently derived all of the SR equations. So if you hit the negative vote, that's ok. It will probably get me going once again.)

Sean
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In the frozen "referential of the photon"(*) time has not changed. In your's and in the one of the emitting galaxy, it has.

(*): NB: strictly speaking you can't attach a referential to a photon, only to an object moving arbitrarily close to c speed.

Fabrice NEYRET
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In relativity time is viewed rather differently to the way the non-physicist masses view it. You might want to have a look at What is time, does it flow, and if so what defines its direction? for some non-essential but possibly interesting background information.

In this case the observer on Earth is choosing a coordinate system with which to identify spacetime points. In this coordinate system the light rays we receive now were emitted at the points $(-T, cT)$. So the galaxy we see now is the image of the galaxy when it was at the spacetime point $(-T, cT)$. We are seeing the galaxy as it was a time $T$ ago.

There are already lots of questions on this site about the fact that time doesn't pass for light, but they are all misconceived. There is no coordinate tranformation between our coordinate system and the coordinate system of anything moving at velocity $c$ relative to us, so the question of how time moves for light is a meaningless one.

In any case the coordinate system used by the light need not concern us. We are concerned only with events in our coordinates, and these do not depend on what the light does or doesn't experience as it propagates between spacetime points in our coordinates.

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John Rennie
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