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What happens if you would ride on top of a light stream and you would look into a mirror that is in front of you, could you actually see your own face? I am asking this because I heard that nothing can be faster than light--shouldn't this be the case here as well?


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closed as off-topic by Qmechanic Nov 17 '14 at 15:11



You won't do anything at lightspeed. Literally. Time dilation is infinite, so absolutely nothing happens. Physicists consider any internal change to indicate that a particle is traveling slower than light (there was something going on about neutrinos changing type a while back, and that would indicate that they weren't massless particles at light speed). –  David Thornley Apr 7 '11 at 2:04
What about an external observer on earth, will he sees the image of the person riding the beam of light? –  user5411 Sep 25 '11 at 21:45
@user5411 You should have asked that as a new question, but seems like you're gone... –  Tobias Kienzler Jun 11 '13 at 13:18

1 Answer 1

up vote 12 down vote accepted

This question has a great pedigree! Supposedly it's what Einstein asked himself when he was first thinking about relativity.

The answer is that you can't "ride on top of a light stream" -- that is, you can't go as fast as the speed of light.

The speed of light is invariant -- same for all observers. So at any sub-light speed you can see yourself in a mirror just fine. No matter how fast you're going, a beam of light going past you still moves at the speed of light, relative to you. No matter how hard you fire your rocket engines, you never catch up with it.

"So at any sub-light speed you can see yourself in a mirror just fine." - ignoring relativistic aberration and doppler shift, which would make a whole lot of difference near $c$ :) –  dbrane Apr 6 '11 at 20:13
@dbrane I think it is assumed the mirror is not moving relative to you (ie. you are holding the mirror, not travelling at a large relative velocity towards the mirror). So aberration and doppler shift will not cause any effect for the person holding the mirror, no matter how fast they are moving according to an arbitrary choice of coordinate system. –  Edward Apr 6 '11 at 20:21
@Edward Sure, agreed. –  dbrane Apr 6 '11 at 20:22
