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This question already has an answer here:

How can we assume that gaining the speed of light slows down time?

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marked as duplicate by John Rennie, JamalS, Danu, Emilio Pisanty, Neuneck Oct 15 '14 at 10:19

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    $\begingroup$ Though the question is phrased in an unclear way I'm guessing it's asking about the physical reality of time dilation. In that case there are several questions that address this issue. $\endgroup$ – John Rennie Oct 15 '14 at 9:02
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    $\begingroup$ It is probably also worth pointing out that we don't assume this, we deduce it from more basic assumptions ie. relativity and the universal speed of light. $\endgroup$ – m4r35n357 Oct 15 '14 at 9:06
  • $\begingroup$ We don't assume this, we measure it's effects. Every day. $\endgroup$ – dmckee Oct 15 '14 at 14:39
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A particle moving at the speed of of light does not experience time, as it has no rest frame. Furthermore, a particle cannot continuously accelerate and eventually reach the speed of light, since massless particles can only move as fast as light. They either move at the speed of light or do not exist at all.

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