Why is light so fast?
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 299792458 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)mv2. So
the order of magnitude of velocities we obtain when powered
by chemical energy might be given by the square root of
E/m. Actually it will be much less than that
because we are very inefficient in our use of energy,
allowing most of it to be released slowly and dissipated
as heat. Our speeds might also be related to the strength
of gravity on Earth g = 9.81m/s2 in
relation to our own size. It is no coincidence that
g takes a moderate value in conventional units, unlike
c.
It is a consequence of relativity deduced by Einstein that
the amount of energy available from a mass m is
given by E = mc2 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.
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