Is the speed of light absolute

The measure of all things

It is no coincidence that 2015 was chosen for the year of light: Exactly a hundred years ago, in 1915, Albert Einstein published his theory of general relativity, ten years earlier his theory of special relativity - and with both of them presented the complete worldview of physics on the head. Because with them the light got a whole new meaning: It became the measure of all things.


Time is relative, the speed of light is not

Before Einstein, light was just one of many phenomena in the cosmos. Aristotle had already established that light spreads in a straight line, but can radiate in all directions from one point. At the time, it was thought to be variable how fast the light moves. Only time was considered absolute. Theoretically, according to what was thought at the time, it would therefore be quite possible to race after a beam of light and catch up with it.

But Einstein put an end to it. For the first time he postulated that it is not time but the speed of light that is absolute. It is a natural constant and therefore a fundamental part of the cosmos. There is nothing in the universe that can move faster than light. And at the same time, light always spreads at the same speed, regardless of whether I move while measuring or not. This does not apply to time, however: it seems to pass more slowly for an observer, the faster he himself moves through space.

Slowed down light

If the speed of light is a natural constant, then it should actually always remain unchanged. The light should therefore not be able to be slowed down or even stopped. Usually this is also true. In recent years, however, physicists have succeeded in tricking this constant with special crystals and ingenious experiments: In this way, the researchers "freeze" light beams for a few seconds and even up to a minute.

View of the experiment for stopping and storing light pulses. © Katrin Binner / TU Darmstadt

The physicists usually use a crystal of the rare earth element praseodymium that has been cooled down to a few degrees above absolute zero as a “brake block”. This is bombarded with a split laser beam, the two partial beams of which hit each other perpendicularly. The first partial beam acts as a brake, it excites the ions in the crystal lattice and thereby changes the crystal properties. The second partial beam now hits this new medium made of crystal and laser light and is greatly slowed down in it.

"Light entering the crystal is slowed down enough to stop it and stay there until we let it go again," explains Morgan Hedges of the Australian National University. "When we let it go again, we'll get everything out the way it came in, accurate down to the last photon." These experiments are far more than just physical gimmicks. In future, such light traps could be used to store optical information directly without previously having to convert it into electronic or magnetic data.

Nadja Podbregar
Status: January 16, 2015

January 16, 2015