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When Einstein defined the speed of light he stated that  “. . . [light] is always propagated in empty space with a definite velocity c. . . “; in other words, the speed of light (c) is always a constant; Einstein did not specify that the measurement of that speed is a constant, probably because he didn’t recognize the that the two concepts were not synonymous and that the measured speed of a photon through space is not necessarily constant! In 1905, long before the development of GPS systems, it didn’t matter if the two concepts were not synonymous but that is no longer the case; in today’s world, some of the uses of GPS systems demand maximum accuracy, and how we measure the speed of light affects that accuracy. Max Planck defined the speed of light as equal to a Planck length divided by a Planck time. A Planck length is defined in meters. The length of a meter is not as easy to define as it once was; it used to consist of the distance between two marks on a platinum-iridium stick located at the International Bureau of Weights and Measures in Sevres, France. It now consists of the “length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second”. (Note that this definition still treats the meter’s length as a physical definition although it does not define the duration of a second)! A Planck time is measured in seconds. Defining the duration of the second is not as simple as it used to be; the second used to be 1/86,400 of the length of a day (24 hours). When it was discovered that the length of a day was gradually increasing (as the result of the rotational speed of the earth gradually decreasing), it was decided to use a different standard; namely “the duration of 919,263,1770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.” So, to measure the speed of light we establish a measured distance and, using a measuring device, such as a caesium clock, we determine the time that it takes for a photon to travel that distance. Not only are the techniques of such a measurement very complex but the concept itself is complicated  we don’t really measure time, what we measure is the effect that the passage of time has on the instruments we use for such measurements. One of these ‘instruments’ is the human body: the passage of time results in changes to the human body  we get older, our whole body grows, including our hair (my beard grows about 1/4th of an inch a week); women become pregnant and the duration of their pregnancy is can be termed a measurement of time  in other words, as living creatures we experience time and we use our experience as a base (but not the only base) for measuring the duration of time. We also use tools to measure time, but these tools are very different from the tools we use to measure matter because the tools do not directly measure the duration of time, instead the measurements shown by the tools require interpretation. For example, it is probable that the first measurement of time was the sundial. The sundial is composed of a vertical stick which casts a shadow on a flat surface. A clever person conceived the idea of drawing a scale on the surface with markings which could be used to represent the time of day and its relation to the length of the year. The shadow itself was not a characteristic of time, it had no meaning until it was interpreted by using the markings on the surface. These markings became what could be considered to be the first clock. Physically, the markings remained as a construct which enabled people to interpret the shadow as a tool to measure time; the shadow, by itself, did not measure time. Another, more recent, way of measuring time is based on an observable physical characteristic of the caesium atom which occurs at very regular predictable intervals. Scientists count these intervals and have determined that a specified number of these intervals provide a very accurate measurement of time. Again, we make use of a physical characteristic of matter to provide us with a meaningful way of measuring time; we are not actually measuring time itself. So what is the basis for the suggestion above that the measurement of the speed of light is not a constant? The answer can be given in two words  Time Dilation. Descriptions of the nature of time dilation (and how it can be calculated) can be found in a number of places including Wikipedia. How can time dilation affect the measurement of the speed of light? Basically it’s very simple. Every object that moves through space is subject to time dilation, including every device that measures time. The faster that the object moves through space, the greater the time dilation; the greater the time dilation, the more slowly the object ages. Nothing can move through space faster than the photons of which light consists; photons experience 100% time dilation and photons do not experience aging. In 1971 Hafele and Keating flew caesium clocks around the earth in commercial airliners to compare the elapsed time against that of a clock that remained at the U.S. Naval Observatory. One of the clocks was flown east, the other was flown west. Because of time dilation, the flying clock that flew east was expected to age less than the clock flying west because it was moving faster through space. Also, both flying clocks were expected to age more quickly than the stationary clock because the force of gravity on them was less. The experiment proved that the expectations were correct  the clocks experienced time at a rate that was inversely related to the speed of the object, (defined as a percentage of the speed of light). Extending the predictions, clocks at the equator of the earth experience time more slowly than clocks at the north pole because their speed through space is greater, clocks in the summer experience time at a different rate than clocks in the winter because the earth travels around the sun in an ellipse, not in a circle, clocks on the satellite and on the various GPS satellites experience time more slowly than the clocks on earth and clocks on Mars experience time at a different rate than clocks on Earth. In other words, we are unable to measure the absolute speed of anything, including light, because we don’t know the absolute speed of the earth through our solar system; we don’t know the absolute speed of our solar system through our galaxy and we don’t know the absolute speed of our galaxy through the universe. Although the difference between these different rates is very small because the difference between the relative speeds is so small, when one is depending on a definition of the speed of light to make very exact calculations, such as in GPS calculations, the difference can be important. Since the speed of light is now used to determine other standards, such as the standard unit of measure, again the difference can be important. None of the above affects the actual speed of a photon through space, it is only the measurement of that speed that is affected.

On the Electrodynamics of Moving Bodies, translated by W.Perrett and G.r. Jeffery, in The Principles of Relativity, New York; Diver and Methuen, 1952 Hafele, J.C.; Keating, R.E. (July 14, 1972). “Around-the-World Atomic Clocks:Predicted Relativistic Time Gains”. Science 177 (4044): 166-168