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This question already has an answer here:

I’m travelling at the speed of light. If I hold a mirror in front of me could I see my face?

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marked as duplicate by PM 2Ring, Thomas Fritsch, Qmechanic Jun 11 at 15:27

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    $\begingroup$ Possible duplicate of Reflection At Speed of Light $\endgroup$ – PM 2Ring Jun 11 at 14:51
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    $\begingroup$ You can't travel at the speed of light. But if you were moving at 99.999999% of the speed of light in some frame, the mirror would work as usual. $\endgroup$ – PM 2Ring Jun 11 at 14:53
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    $\begingroup$ @PM2Ring said, "99.99...%", but that doesn't fix your question because you did not say, travelling with respect to what? "Travelling," by itself, means nothing physics. Physics answers questions about the relative motion between different objects. Since there's nothing in your question about the landscape or anything else that you are travelling past, it's really just a question about you and the mirror that you are holding. All you really are asking is, "If I hold a mirror in front of me, can I see my face?" And, as PM 2Ring said, the answer to that is always, "Yes..." $\endgroup$ – Solomon Slow Jun 11 at 21:20
  • $\begingroup$ ...Assuming that some source of light is travelling with you, and that your eyes are open, and that the mirror is facing the right way, and... $\endgroup$ – Solomon Slow Jun 11 at 21:21
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If you're traveling at precisely the speed of light, you can't "hold" anything. In fact, you can't think either, much less see anything. This is because all of the above require that interactions happen between different parts of your body, or between your body and the environment. In order for two objects to interact at a particular moment in time, they must be lightlike or timelike separated at that moment (in other words, one must be reachable from the other using the fastest possible object). If you and a mirror are both traveling in the same direction at the speed of light, you could never interact with the mirror if it was in front of you, and the mirror could never interact with you if it were behind you. This is because nothing propagating from the object in back could ever "catch up with" the object in front.

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