Light as a Wave

Question

We know the light is a form of a wave. We also know wave requires a medium for its propagation. If the light is a form of a wave, then how does it travel in a vacuum, which is not a medium?

Answer 1

It’s a great question, one that puzzled virtually every physicist during the last quarter of the nineteenth century, including some of the greatest minds of the time. It was not until 1905 that the puzzle was resolved, when 26-year old Albert Einstein, completely unknown at the time, proposed a radical solution.

Einstein postulated that light propagates in empty space requiring absolutely no medium whatsoever. He then developed a completely self consistent theory, called the Special Theory of Relativity, that revolutionized virtually everything that physicists thought they knew about physics at the time. In the 115 years since Einstein proposed his new theory, the predictions of Special Relativity have been extensively tested, and they have always been shown to be correct to very high levels of precision and accuracy. In fact, no experiment has ever contradicted the principles of Special Relativity.

Einstein came to the conclusion that light waves do not require a medium to propagate. He based this conclusion on two purely empirical and well-tested observations:

1. Light propagates as a wave.
2. Light propagates through empty space.

If you consider these two facts, you have to conclude that one of two things must be true: either, empty space contains a mysterious and as-of-yet undetected medium through which light waves propagate, or light waves are able to propagate through empty space without the need for a medium.

Before Einstein, the overwhelming consensus among practicing physicists was that there must be a medium that permeates all of empty space. They dubbed this mysterious medium the "Luminiferous Aether," or simply "Aether" for short, and set out to see if they could predict and/or measure its properties. Right from the start, however, there were many theoretical and empirical problems with the Aether hypothesis, not least of which was how something that is so diffuse that it could not even be detected could at the same time provide enough tension to propagate light at such an enormous speed.

Einstein rejected the conventional wisdom and simply did away with the entire notion of the Aether as completely unnecessary. In its place, Einstein asserted two very simple postulates:

1. The laws of physics, including electromagnetism, are the same for all observers moving with respect to one another in inertial frames of reference (constant velocity and zero or negligible gravity).
2. Light propagates at a constant rate of speed for all observers regardless of their relative motions with respect to each other and with respect to the source of the light.

The second of these two postulates, the so-called constancy of the speed of light, was quite new and radical at the time, and its implications for physics were nothing short of revolutionary.

So, it is simply not true that all waves require a medium to propagate. It turns out that only some kinds of waves require a medium, notably sound waves and mechanical waves, whereas other kinds of waves, specifically electromagnetic waves, require no medium whatsoever.

Ten years later, in 1915, Einstein would extend his Special Theory of Relativity to include accelerated motion and gravitational fields. This new theory, the General Theory of Relativity, predicted the existence of an entirely new type of wave that had never previously been observed or even predicted to exist -- that is, gravitational waves. Just like electromagnetic waves, gravitational waves would propagate through empty space, without a medium, and remarkably, the speed of propagation for gravitational waves would be exactly the same as the speed of light.

In 2015, exactly 100 years after Einstein first predicted their existence, gravitational waves were observed, measured, and recorded for the first time. The specific event that generated these gravitational waves was the collision and merger of two massive black holes. Incidentally, the entire notion of black holes is another fascinating consequence of General Relativity, and the possibility of black holes was first predicted by a German physicist, Karl Schwarzschild, shortly after Einstein first published his theory in 1915.

For more information, please search the web for

1. “Michelson-Morley Experiment”
2. “Luminiferous Aether”
3. “constancy of the speed of light”
4. “Postulates of Special Relativity”
5. "Gravitational Waves"
6. "Black Holes"