# What makes the speed of light constant

## How constant is the speed of light?

The special theory of relativity is, at least by name, widely known and today forms one of the cornerstones of modern physics. The theory assumes, among other things, that the speed of light in a vacuum is the same for all observers - regardless of whether they are at rest or moving at a certain speed. From this assumption all more or less clear conclusions of the theory follow, such as time dilation and length contraction.

The idea that the speed of light is constant took some getting used to. And scientists carried out quite a few experiments to show the opposite - unsuccessfully, at least within the scope of their measurement accuracy. But in principle nothing speaks against the fact that there may be tiny variations of the constant that could not be measured until now, which is why it still makes sense today to carry out experiments with increased accuracy.

First of all, the Michelson-Morley experiment should be mentioned here, which primarily serves to demonstrate the independence of the speed of light from the direction. Here, two light beams, which cover part of their path perpendicular to each other, are superimposed on each other and caused to interfere. The experimental setup can be rotated, whereby any directional dependencies of the speed of light should be shown by changes in the interference pattern.

In the Kennedy Thorndike experiment, on the other hand, the experimental set-up is fixed in the laboratory and therefore only follows the movement of the earth, so that only the daily and annual changes can be observed that result from the rotation of the earth and its movement around the sun. Since the earth moves towards an outer fixed point in the course of the year at a speed of up to 30 kilometers per second or moves away from it, this experiment can be used to determine any changes due to this speed.

But while the experiment according to Michelson and Morley provides quite precise results, the results of the Kennedy-Thorndike experiment have been far less precise - so far, because Claus Braxmaier from the University of Konstanz and his colleagues carried out a much more precise experiment for 190 days from October 1997.

To do this, they used a sapphire crystal frozen at four Kelvin as an optical resonator. At this temperature the material shows almost no changes in length, so that light forms a standing wave with a fixed frequency in it. If a frequency change occurs, then this can only be the result of a change in the speed of light. In order to detect such frequency changes as precisely as possible, the scientists compared the resonator frequency with an independent standard - a kind of atomic clock.

In this way, the researchers were able to redefine the range of possible deviations in the natural constant and increase the overall accuracy with which the special theory of relativity can be verified by a factor of three. In principle, even a hundredfold improvement would be possible with this experiment.