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## Main Question or Discussion Point

**time "stopped" for a photon travelling at the speed of light?**

Is time "stopped" for a photon travelling at the speed of light? Or is this a nonsensical question?

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Is time "stopped" for a photon travelling at the speed of light? Or is this a nonsensical question?

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From the photon's point of view, it arrives at its destination immediately after leaving, in zero time. It can travel through the entire universe in no time at all, literally, which is logical since in its frame of reference it also travels zero distance.

From our point of view, obviously, it's just traveling at the speed of light so it may take millions of years to reach its destination.

It's a bit like the space traveler who leaves on a long space trip. As he accelerates to nearly the speed of light, space around him shrinks (in the direction of movement) so that distant galaxies suddenly become so close that he can reach them in a few years. Same thing for the return trip. However, as seen from the earth, he will have been away for millions of years. In fact, the earth may not even exist anymore when he tries to come back.

The photon is just the same, only it travels AT the speed of light, so lengths are shrunk to zero and travel takes no time at all, in its frame of reference.

If I'm not mistaken, this also means a photon, or any other particle traveling at the speed of light, will never decay since it doesn't have time to do so.

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On the other hand, time does not really make sense for a photon so the question is indeed nonsensical, as explained on this site:

http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html

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This is really a nonsensical question, since the rest frame of a photon is not a valid reference frame.

As the article posted by michelcolman points out, we can ask what happens as we get closer and closer to the speed of light. This question makes sense, since all the reference frames we would be in would be valid.