You've caught my attention. Explain.
The speed of light in a vacuum is constant at a little under 300000km/s. In order for it to move in the exact opposite direction, (i.e. with velocity of 300000km/s to -300000km/s), there would be a very short moment (probably 1 Planck second) when it would have to be stationary. It would also then have infinite acceleration because it changed from 0 to 300000km/s in 0 seconds.
But if it came in at a right angle it wouldn't go in the opposite direction. Also, although short, 1 Planck second is still a unit of time and you can divide. It wouldn't be infinite.
It would. A light ray will reflect back on itself when it hits a mirror perpendicularly. I'm not sure about the Planck second thing. I always thought it was like the frame rate of reality or something like that. You can't exist between frames in a movie, so time is like one frame per planck second. I'll look it up once I wake up.
A Planck second is the shortest measurable unit of time. It's theoretically the length a photon takes to travel the length of the quantum strings in String Theory.
And for the 90 degrees I pictured it as its reflection equalling 90. Not 90 from the mirror.
So yeah. The shortest measurable unit of time would be the frame rate of the universe, would it not?