# Simple relativity question

I have a simple question in foundation SR that I hope someone can clarify.

Given a mirror of length l travelling at a relativistic speed wrt a 'stationary' frame of reference containing two laser lights spaced l apart as well. If the mirror were not moving at all, the laser lights can be fired simultaneously such that each signal hits the edge of the mirror precisely.

Suppose that this mirror is travelling relativistically now. In our 'stationery' frame of reference, it undergoes length contraction. Therefore we can time the firing of our two laser lights such that the beams contain this mirror completely between them, since the mirror is now shorter.

But from the frame of the travelling mirror, it views the space between the two laser sources as contracted as well, so as it passes them, any firing of the lasers attempting to 'enclose' the mirror will result in either beam striking the mirror, since the spacing between the sources is < l .

How do we resolve this?

• I'm positive this must be a duplicate of something, but I'm not sure what our canonical question on this topic is. – David Z Feb 20 '15 at 5:57
• You mean something like yukterez.ist.org/laterne.gif ? – Yukterez Feb 20 '15 at 6:26
• Fire one beam really early, fire other beam really late, obviously the moving mirror will be enclosed by the beams. I guess you mean enclosing the mirror should become easier as mirror contracts, but also enclosing the mirror should become harder as distance of laser sources contracts. – stuffu Feb 20 '15 at 7:36
• In the mirror's frame of reference, the lasers fire simultaneously. In the laser's frame, they do not. In both cases, the lasers strike the edges of the mirror. – mmesser314 Feb 20 '15 at 11:57
• Sounds like the standard ladder paradox – By Symmetry Feb 20 '15 at 12:02