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Name: Mohammed
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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.

Vince Calder


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.

Matt Voss

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.

Richard Barrans
Department of Physics and Astronomy
University of Wyoming

Dear Mohammed:

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.

R. Avakian
Oklahoma State University - Okmulgee

Hello Mohammed,

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 truly fascinating.

So, it is good to ask this question every once in a while.

best wishes,

Michael S. Pierce
Materials Science Division
Argonne National Laboratory

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