They say that the speed of light is the same for all inertial observers, but what if the observer is a particular photon within a laser beam? Each photon in a laser beam would see all the other photons passing it at speed c, therefore the laser beam would instantly break up. How do you solve this conundrum?

The answer is that a photon can't count as an observer. Time slows down for objects that move at close to the speed of light. For an object moving at the speed of light, time would grind to a halt. So how fast you clock something moving past you if you can't measure time turns out to be a nonsensical question.

Chris studies the large scale structure of the universe using the peculiar velocities of galaxies. He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.