Speed of Light and Time
Does the speed of light change with time? Thanks.
Some theoretical cosmologists have posed the question, "What are the
implications if the speed of light has changed over the lifetime of
the Universe?" Obviously, the implications would be profound, but to
my knowledge there is no credible experimental evidence that the
speed of light has changed, in fact the weight of the existing
evidence is just the opposite. In any case any change would be extremely small.
The speed of light is a constant, in a given medium. Time is
actually one part of speed, the other part being distance. Look at
miles per hour. Miles is the distance and hour is the time
factor. Since the speed of light is a constant, if the amount of
time it takes light to travel increases, then the distance traveled
must also increase and if the distance decreases, then the time it
takes to travel that distance must decrease as well. While the
speed is a constant, remember the constant is only for a given
medium. The speed of light in a vacuum is different than it is in
water, but the speed in water will always be the same no matter
where that water is and the speed of light in a vacuum will always
be the same no matter where that vacuum is.
Not as far as we can tell. In fact, as far as we can tell, if a
fundamental constant such as the speed of light were to change with
time, conservation of energy would no longer hold. THAT would be interesting.
Department of Physics and Astronomy
University of Wyoming
According to present theory, first proposed by Einstein in his
Relativity studies, the speed of light in a vacuum is the same no matter
where it is measured or by whom. So far, astrophysicists, in their
studies of deep space objects and galaxies, have not found any hard
evidence that the speed of light has ever changed over time.
Having said that, the speed of light is different in different
materials. For example, light travels more slowly in glass than in air.
That is why lenses can focus light and magnify images.
Some scientists are looking into what would happen had the speed
of light changed and, I remember an article and interview on just that
topic in New Scientist magazine several years ago.
Oklahoma State University - Okmulgee
That is a great question.
The answer is, as far as we know, that the speed of light in vacuum
appears constant. That statement is true on both fronts of scientific
inquiry. All experimental, observable evidence, both in the lab and
from astronomers, says that the speed of light does not change over
time. Also there are no compelling reasons to expect the speed of light
to change over time (i.e., having c vary with time does not seem to solve
any other problems). So both theory and experiment tell us that it is
constant to the best of our abilities.
Now, that still leaves open a few possibilities. It could be changing
very, very, VERY slowly. If it were changing slowly enough, then we
might not be able to have observed it yet. Human time scales are quite
short compared to many things in nature. It is also possible that it
was changing more quickly at some point in the past and then slowed (or
became a constant). However astronomical observations tell us that, at
least since the first few minutes of the cosmos, the speed of light has
been fairly constant. Both possibilities still come up from time to
time in discussion and even every so often in papers. But they are pure
speculation at this point.
Nonetheless, it is a great question and one that people should ask every
so often. Not all things that are defined as "constants of nature" are
truly constants. To our credit they are no longer called constants after
that, but people often still think of them that way. The most famous
example I can think of is that the measured values of both the mass and
charge (or more precisely the strength of the electromagnetic
interaction) appear to change at very high energies. Most people tend
to think of the charge and mass of an electron to both be constants.
And in all "everyday" circumstances, and even most "laboratory"
circumstances this is true enough. However, at very high energies these
values appear to change. That they change is to me one of the most
beautiful and confusing aspects of nature. The fact that this behavior
is actually described (and required of!) by quantum electrodynamics is
So, it is good to ask this question every once in a while.
Michael S. Pierce
Materials Science Division
Argonne National Laboratory
Click here to return to the Physics Archives
Update: June 2012