Light Matters

22 November 2014

An airman conducts a laser experiment. United States Air Force
An airman conducts a laser experiment.

Yesterday I talked about time dilation due to both gravity and relative motion. The reason for both of these effects is the fact that light in a vacuum always has a constant speed. The first experimental evidence for the constancy of light was performed in 1887 by Michelson and Morley, and by 1984 measurements of light speed had become so precise that it was defined as an absolute physical constant. But why is that? What makes light so special?

The short answer is that it just is. Experiments have found the speed of light is constant to within a few parts per quintillion, which is astoundingly accurate. Experimentally that’s just how light works. But on some level that seems an unsatisfactory answer, and there are underlying reasons why we would expect light to have a constant speed.

On the face of things it would seem that relativity should imply that all speeds are relative. If I’m on a train traveling at 10 km/hr (relative to the ground), and I throw a ball at 5 km/hr relative to the ground, then the ball should be moving 15 km/hr relative to the ground. Likewise, if I see water wave in a canal traveling at 4 km/hr, and I run with the waves along the canal at 3 km/hr, I would see the waves moving at 1 km/hr relative to me. In fact, this idea of “everything is relative” is known as Galilean relativity, and pretty well summarizes Galileo’s view of relative motion.

What makes light different is that it isn’t made of chunks of matter like little baseballs, nor is it a wave in the way water waves or sound waves are. Yes, light can be described as particle-like quanta we call photons, and yes, light can be described as a wave, but unlike other waves light doesn’t wave through a medium. Sound waves are vibrations that travel through air (or other materials). Without the medium of air, there would be no sound, hence the old saying that in space no one can hear you scream. Likewise, water waves can only travel through water.

But light waves occur because of a fundamental connection between electricity and magnetism. If you’ve ever played with battery-powered toys, you know the battery must be in place (and it must be charged) for things to work. When you connect the battery, it uses electrochemical energy to create an electric field in the circuit of the toy. If you’ve ever wrapped wire around a nail and connected it to a battery, you know that the nail becomes magnetized when the battery is connected. Thus, the electric field creating the current in the wire can induce a magnetic field. Likewise, if you take a magnet and shake it back and forth near a coil of wire, you can induce an electric field (and thus a current) in the wire.

The ability to create magnetic fields from electric fields and vice versa is central to our modern generation of electric power. But it also means that electromagnetic waves don’t travel through a medium. A changing electric field induces a magnetic field, and a changing magnetic field induces an electric field. So in essence, the electric and magnetic fields induce waves in each other. Basically, light, like all electromagnetic waves, simply waves itself. As a result, the speed of light can’t be relative to something else, because it doesn’t wave relative to anything else.

This inherent speed of light is built into the very nature of what light is. Since the fundamental leptons and quarks that comprise matter have electric charge, they are also subject to the fundamental nature of light speed. All the strange aspects of time dilation and warped gravity work in such a way that the speed of light is always preserved.

This is why light matters.