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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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    What is the difference between a galaxy "as it is 1 billion years ago" and the same galaxy "as it was 1 billion years ago?" – WillO Feb 28 '16 at 22:11
  • The speed of light is not infinite. Photons from a far-away galaxy do not instantly reach your eye. They reach your eye a billion years after they are emitted. In what sense have they not "moved across time"? – WillO Feb 28 '16 at 22:12
  • I am in no way implying that photons from a far away galaxy do instantly reach your eye in an instant. That is a different kettle of fish. – Sean Feb 28 '16 at 22:14
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    @Sean: are you suggesting that photons do not experience time? This is sometimes suggested, based on time dilation. Perhaps a photon doesn't have a point of view! This is because it does not have an inertial reference frame in which it is at rest. So any conclusions you may make must be carefully analyzed with more material objects. – Peter Diehr Feb 28 '16 at 22:21
  • Peter: I read this kind of "automatic answer" very often. In practice you could attach a frame to an object moving arbitrarily close to c speed, and thus arbitrary close to a photon, and speak for this guy instead. For most of the questions involving "photon referential", this asymptotism allows to perfectly handle the point, which should thus not be raised as an objection. – Fabrice NEYRET Feb 28 '16 at 22:47
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    @FabriceNEYRET: Everything in the Universe, including you, is currently moving at a speed arbitrarily close to c, relative to something or other. This does not imply that your experience of the world is arbitrarily close to that of a photon. – WillO Feb 28 '16 at 23:42
  • Sean: your new comment in italics at the end of your question is a reasoning bias named "Galilean syndrom". It's not because once somebody (or you) said being wrong was indeed right that it proves that you now said wrong is right. Beside, the fact that you answered your own question that your teacher though was rubish probably means that at that time too, your question was so badly formulated that only you understood what you meant or wanted. I'm disapointed that you did no effort to reformulate your question that obviously nobody understand. Thinking it means you are genious is very bad sign. – Fabrice NEYRET Feb 29 '16 at 06:49
  • I never thought of myself as a genious. In fact I had never even seen the word until I just now found it in "urbandictionary" via Google. – Sean Feb 29 '16 at 08:38

2 Answers2

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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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  • What you are saying is that our role of crossing time plays a part in the big picture, but the light seeing no distance in time nor space between that galaxy and my eye, plays no part at all in the big picture. In other words, if light did move across time as do we, the outcome would be no different ? – Sean Feb 28 '16 at 22:23
  • the messenger does not need to know how to read to transmit letters :-) – Fabrice NEYRET Feb 28 '16 at 22:25
  • In either case, the image of the galaxy would be the same, but that is not the point being made here. – Sean Feb 28 '16 at 22:30
  • Then make you point clear, since it seems to be obscure to everyone ;-) – Fabrice NEYRET Feb 28 '16 at 22:42
  • So obscure, but clearly negative despite its nothingness ! – Sean Feb 28 '16 at 22:46
  • It's not too late to reformulate your question to make your point more clear (you can edit it, see button below). – Fabrice NEYRET Feb 28 '16 at 22:49
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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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  • It's interesting though that if some traveler who was from that distant galaxy that is located a billion light years away from us, was able to travel at just a smidgen below the speed of light, and his travel led him to our place of the here and now, from his point of view he has traveled across almost no time at all. From his point of view, his galaxy and ours are separated by very little time. – Sean Feb 29 '16 at 08:31