Why is the speed of light a constant?

The idea that the speed of light (in a vacuum) is a constant and the same for all observers independently of their relative speeds is one of the two fundamental postulates of Einstein's theory of relativity. The other is that the laws of physics are the same for all inertial observers. If they are accelerating with respect to each other the laws are not the same.

The theory of relativity was developed to explain various experimental observations that could not be explained by classical mechanics. For example under classical mechanics the concept of cause and effect breaks down at high speeds. At low speeds classical mechanics is a good approximation to relativistic mechanics. The theory of relativity has been very successful and many of its predictions have been experimentally verified. Examples include the bending of light due to gravity and the change in mass and perceived time with speed and acceleration. Scientists are therefore almost unanimously convinced that the speed of light is in fact constant. Why this should be so and why the speed of light is the particular value observed is a hotly debated question. No one has yet been able to derive this from first principles and explanations tend to be more philosophical than scientific.

A theory that could explain this would revolutionise modern physics. Attempts to unify the two fundamental theories of physics - general relativity, which deals with the universe on a large scale and quantum mechanics, which is applied on a very small scale - into a theory of quantum gravity may provide the answer. Physicists are beginning to work on this but such a theory is a long way off - the theory must be able to account for phenomena as diverse as black holes and the motion of the particles within an atom.