Original by Philip Gibbs, 1997.

One answer is that it isn't. When physicists work out equations in relativity
they often set the speed of light to one: *c = 1*. This makes the equations
more tidy. It amounts to defining natural units of measurement in which the speed of
light is exactly one unit. For example, if the second is kept as the basic unit of
time, then the unit of length must be equal to exactly 299,792,458 metres. This unit
is called the light-second because it is the distance travelled by light in one
second. The speed of light is then one light-second per second.

This is not a complete answer. The speed of light is high when measured in our standard units such as metres per second or miles per hour. Those units are defined by arbitrary conventions which have their roots in ancient ways of keeping time and measuring distances. It is probably no accident that the second is about the average duration of a heart beat and the metre or yard is the distance of one human step. So the real question is "Why is the speed of light so high in terms of familiar every day measurements?" or "Why are the speeds at which we normally move so slow compared to the natural units in which the equations of physics take the most tidy form?"

These are very meaningful questions but ones to which we have only partial
answers. The speeds at which we walk and live are limited by the amounts of energy
*E* available to us from the chemical processes which drive our muscles compared to
the amount of mass *m* which is to be moved. Kinetic energy at low speed is
given by the formula *E=(1/2)mv ^{2}*. So the order of magnitude of
velocities we obtain when powered by chemical energy might be given by the square root of

It is a consequence of relativity deduced by Einstein that the amount of energy
available from a mass *m* is given by *E = mc ^{2}* so the question
now becomes (in part at least) "Why is so little of the energy of matter available in
the form of chemical energy?" If our metabolisms worked using nuclear reactions
instead of chemical reactions we might move much faster (other factors permitting) and
then our units of length and time would be different, and the speed of light would not
seem so high. These relative scales of energy are determined by such parameters as
the coupling constants of the natural forces and the masses of particles. We know
from observation that these take values which vary widely in scale over many orders of
magnitude. We do not yet know why this is. It may be that the values are
arbitrary and their differing values have to be put down to something ontological such as
the anthropic principle, or it may be that they are determined without ambiguity from a
unified theory of forces which split naturally at different scales. The strength of
gravity on Earth comes from similar parameters in cosmology and similar principles may
apply to the question of why hospitable planets have moderate gravitational fields.
Until more is known about the fundamental parameters and how they derive from deeper
principles, a complete answer cannot be given.