Does the Speed of Light Slow Down Over Time?

By Ronald Ebert

(Published in the Sept/Oct 1997 (vol.17,No.5) issue of
Reports of the National Center for Science Education.)


It seems that old creationist ideas never die. That has been true of the claim that dinosaur tracks and human tracks are found together at Paluxy River in Texas. Even though the Institute for Creation Research has disavowed this, it is constantly revived as justification for a young earth (see, for example, Kuban, 1989). It is also true of the claim that the speed of light slows over time. Scientists have compelling evidence that the universe is some 10-15 billion years old. In order to reconcile this evidence with the creationist belief that the universe is only 6-7000 years old, creationists have claimed that the speed of light was extremely high, perhaps infinite, in the distant past and has only recently settled down to its currently accepted value (Setterfield, 1981). Even though this claim has been rejected by the ICR (Aardsma, 1988), it continues to resurface. In one instance in 1996, a creationist on the Internet’s Astro listserv mailing list brought this up. But not only is there not a shred of evidence for this idea, scientists are so confident that the speed of light is invariant that today it is used as the standard of length measurement.

What is meant by the term, speed of light?

Unless stated otherwise, the term "speed of light" is understood to mean the speed of light in a vacuum. Light travels slower through other mediums, such as air, glass and water. This happens because light is absorbed and re-radiated by the atoms that make up the medium. And it’s not just light that we’re concerned with. All electromagnetic radiation, which includes radio, microwave, infrared, light and X-rays travel at the speed of light.

The history of speed of light measurements

To see why the confidence in the invariance of the speed of light is so high, we need to look at the history of its measurement, and some of the foundations that lead to the development of Albert Einstein’s theory of special relativity. The first attempt to measure the speed of light that was successful was made by Olaus Roemer in the late 1600s. He timed the differences in the orbital motions of the moons of Jupiter from when Jupiter and the Earth are relatively close compared to when they are far apart. Based on his measurements, the speed of light was calculated to be 2.3 x 108 meters/second (m/s). (Jones, Childers 1990a:613). Not bad given the uncertainties of the size of earth’s orbit at that time.

In 1849, Louis Fizeau performed the first experiment on the Earth to measure the speed of light. His apparatus consisted of a toothed wheel, a source of light, and an arrangement of lenses and mirrors that allowed light to move along a path, be reflected from a mirror and through the toothed wheel and back. The toothed wheel was set to rotating, and light’s passage through the teeth could be matched to the wheel’s speed. Fizeau’s calculations yielded a value of 3.15 x 108 m/s for the speed of light. (Jones, Childers 1990b:613).

After Fizeau established this method, others have used it or a similar method involving light reflecting off a rotating mirror to make ever more accurate measurements. Other methods have also been devised. They include using Kerr Cells, which chop light up similar to the toothed wheel but are controlled electronically, geodimeters which used modulated light for measuring distances and were mainly used for geologic work, and microwaves and lasers, where measurements based on their frequency, wavelength and phase relationships were made. Below is a table of some selected measurements made over the years. (Halliday, Resnick 1978:925; Halliday, Resnick 1988:543; Monchalin, 1977)

Date Experimenter Method Speed (m/s) Uncertainty

( m/s)

1862 Foucault Rotating mirror 298,000,000 500,000
1876 Cornu Toothed wheel 299,990,000 200,000
1880 Michelson Rotating mirror 299,910,000 50,000
1883 Newcomb Rotating mirror 299,860,000 30,000
1883 Michelson Rotating mirror 299,853,000 60,000
1926 Michelson Rotating mirror 299,796,000 4000
1928 Karolus and Mittelstaedt Kerr Cell 299,778,000 10,000
1932 Michelson and others Rotating mirror 299,774,000 11,000
1941 Anderson Kerr Cell 299,776,000 14,000
1950 Bergstrand Geodimeter 299,792,700 250
1950 Essen Microwave cavity 299,792,500 3000
1951 Aslakson Shoran radar 299,794,200 1900
1952 Froome Microwave interferometer 299,792,600 700
1954 Florman Microwave interferometer 299,795,100 1900
1957 Bergstrand Geodimeter 299,792,850 160
1958 Froome Microwave interferometer 299,792,500 100
1965 Kolibayev Geodimeter 299,792,600 60
1967 Grosse Geodimeter 299,792,500 50
1972 Evenson and others Laser 299,792,457.4 1.1
1974 Blaney and others Laser 299,792,459.0 0.6
1976 Woods and others Laser 299,792,458.8 0.2
1977 Monchalin and others Laser 299,792,457.6 .73

As can be seen, as time went on, the measurements became more accurate and precise. By the early 1980s the primary limitation of the measurement was the precision with which the length of the meter could be established. The standard of length was determined for many years by a standard meter bar that was kept at the International Bureau of Weights and Measures near Paris. In order to eliminate the need to make secondary standards based on measurement to the standard bar, an atomic standard for the meter, based on the wavelength of light, was adopted in 1960. The meter was redefined to be 1,650,763.73 wavelengths of a particular orange-red light emitted by atoms of krypton-86, something that could be reproduced in any well-equipped laboratory. But this too, was inadequate. The highest international authority on units of measurement is the General Conference on Weights and Measures, which is an intergovernmental body.

In 1983 the 17th General Conference on Weights and Measures adopted a new definition of the meter, based on the best value for the speed of light:

"The meter is the length of path traveled by light in vacuum during a time interval of 1/299,792,458 of a second."


Why base length on the speed of light?

Why were the conference participants so confident in the invariance of the speed of light as to base the standard of length upon it? There was more involved than just the quality of the measurements. There was also our modern understanding of light based on modern theories of physics. In 1865, James Clerk Maxwell provided a unified picture of electricity and magnetism with only four straightforward equations. By using these equations, Maxwell showed that an impressive array of electric and magnetic phenomena could be interpreted and explained - indeed the equations demonstrated that these two are really one - electromagnetic phenomena. The equations correlated experiments in a vast area and made predictions of new results that were later confirmed. They account for such facts as a compass needle points north, you can see your image reflected from a quite lake, you draw sparks when you touch a metal object after walking across a carpet on a dry day, and you can pick up your telephone and talk to anyone in the world. They are the basis for the operation of all electromagnetic and optical devices including eyeglasses, microwave ovens, motors, generators, cellular telephones and telescopes.

Maxwell’s four equations contain two constants, the permittivity of free space and the permeability of free space. They are derived from static electrical and magnetic phenomena, not waves. When the two are combined, a speed emerges, one which is identical to the speed of light. But this is remarkable! It says that disturbances in electric and magnetic fields travel at the speed of light in free space, and strongly implies (as was later shown to be true) that light itself is an electromagnetic phenomena. But what is this speed from these two constants in reference to? The surface of the earth? The center of the sun? The center of the milky way galaxy? All of these possible reference frames move with respect to each other.

Maxwell and his contemporaries didn’t really know, but they revived an idea first proposed by Christian Huygens in 1687, that the speed of light was in reference to an ether - a massless material that pervaded the entire universe. The ether would not only be the medium that underlies electromagnetic waves in the same way that water underlies water waves, but it would also provide an absolute frame of reference by which any motion could be referred.

This was a great idea, but there was a big problem with it. Numerous experiments had been devised to detect and measure some properties of the ether, but all had failed to find any trace of it. The most famous one was the Michelson-Morley experiment in 1887, where a very sensitive apparatus was designed to compare the speed of light in one direction with the speed of light in a perpendicular direction. It was thought that since the earth orbits the sun, it should be moving through the ether and causing an apparent ether wind on the surface of the earth, and that in turn would cause a difference in the speed of light in the two directions. But the Michelson-Morley experiment found no trace of an ether wind, nor has any experiment done since been able to detect the ether. And the speed of light has never been shown to vary based on the direction from which it was measured, let alone in any other way.

The solution to this dilemma was provided by Albert Einstein, who had been thinking about these problems for a number of years and then devised a theory to account for the behavior of light, along with related problems. He published his theory in 1905 and it has since become known as the special theory of relativity. The theory is founded on two principles:

The laws of physics are invariant in all inertial reference frames.

This means that the laws of physics apply the same way in all non-accelerating reference frames. It is simply an extension of a previously well-established common sense principle, that Newton’s laws of motion apply equally in all inertial frames, to the principle that all laws of physics so apply. Einstein recognized the truth of this from Maxwell’s formulation of the laws of electricity and magnetism, laws which have held up under many tests of extraordinary precision.

It is a law of physics that the speed of light in empty space is the same in all inertial reference frames, independent of the speed of the source or detector of light.

He pointed out that this meant that the ether could not be detected by any experimental means, and therefore it was a useless concept which should be discarded.

From Einstein’s time up to today, special relativity has been subjected to numerous tests and has passed them all with flying colors. It is one of the strongest theories in all of science. It is the confidence in special relativity, backed by all the experimental evidence, that was responsible for the 17th General Conference on Weights and Measures participants’ confidence in the invariance of the speed of light.

What would happen if the creationist claim was true?

The speed of light is a fundamental constant of nature. The word constant, of course, means something that is unchanging. In a sense, the speed of light can be considered to be derived from other constants - most directly from the permittivity and permeability of free space, but these constants, too, are related to other constants such as the charge on an electron, Planck's constant, and the fine structure constant. So if the speed of light were changing, these other constants wouldn’t be constant anymore and this would show up in many kinds of experiments and observations in physics, astronomy, and chemistry - indeed, everything, for atoms and molecules would not be as we know them today. If the speed of light was faster in the past, then we should see this in the ratios of spectral lines of distant quasars coming from different types of atomic transitions. The resulting frequencies have different dependencies on the electron charge and mass, the speed of light, and Planck's constant, and we can compare these to their present values on earth. Needless to say, the spectral signature of elements found in the quasars agrees perfectly with those here on the earth today.


Author’s Background

Ron Ebert is a staff member of the University of California at Riverside Physics Department. He is an amateur astronomer and gives talks on astronomy and physics to school groups, astronomy clubs, and audiences of the general public. He also puts on physics demonstration shows for these groups. He is active on the Internet’s Astro and Skeptic listserv mailing groups.

I want to thank Gordon VanDalen, professor of physics at UCR and Hartland Schmidt, professor of chemistry at UCR for their ideas that have contributed to this paper.


Aardsma, G. Acts and Facts, June 1988.

Halliday, D., Resnick, R. Physics, Third edition, part 2. John Wiley & Sons, Inc., 1978:925

Halliday, D., Resnick, R. Fundamentals of Physics, Extended third edition. John Wiley & Sons, Inc. 1988:543.

Jones, E., Childers,R. Contemporary College Physics. Addison-Wesley Publishing Company, 1990:612-613.

Kuban, Glen J. 1989. Retracking Those Incredible Man Tracks. NCSE Reports 9(4): special insert

Monchalin. Optics Letters. July 1977, vol. 1 (no.1):5-7

Setterfield, B. 1981. The Velocity of Light and the Age of the Universe, Part 1. Ex Nihilo, vol. 4, no. 1. This is an Australian creationist publication.

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