# Special & General Relativity Questions and Answers

## What is light; how does it travel; and how the ding dang do you measure its speed?

Physicists tend to be very pragmatic folks when it comes to certain things. If you have a theory such as quantum mechanics that works, you don't spend much time asking what it all means, you just follow what the experiments and mathematics are telling you is 'out there' and you leave such big questions for the 21st Century philosophers to ponder.

The speed of light was measured by Danish astronomer Ole Roemer back in 1675. He noticed that the predicted transits of Jupiter by its satellites would come over an hour after they were scheduled, and concluded that this was because light took time to get from Jupiter to earth. SInce the distance to Jupiter was known, it was a simple matter to divide this by the delay time to get the velocity of light pretty accurately. Today, thanks to high speed electronics, you can measure time intervals to nanoseconds, during which time light travels about a foot or so.

As for what light is of itself, the answer depends on whether you which to speak the language ot particles or waves. Both are equivalent and self consistent, but I prefer the particle description to guide my feeble intuition.

Light is composed of particles called photons. Each photon is a discrete packet of electromagnetic energy which travels at, what else, the speed of light. The packets carry no mass, but they do have an effective mass that is determined by the energy they carry compliments of Einstein's famous E = mc2. The amount of energy that each photon carries is determined by its size, which in the wave description, is just the wavelength of the light wave. Each quantum can be thought of as one complete oscillation of the electromagnetic wave. Specifically E = hc/wavelength where h is Planck's constant and c is the velocity of light.

Exactly what is light, we don't really know other than in the quantitative details of the above description. The above description IS what light is, just as the detailed description of a dog is what a dog is.

How does light move? During the last century, it was pretty well believed that light was a wave phenomenon that needed a medium... the ether...in order to propagate from one place to another. It was thought of as some odd kind of water wave. Then a series of famous experiments proved that the ether didn't exist, so physicists had to re-think what the essential nature of light was. The best understanding we have is that it is a disturbance in the electromagnetic fields of charged bodies. When you look at the electric field of a stationary charge, its lines of electrostatic force are directed radially away from the charge into space. These fields are dynamic things which travel at the speed of light. Now, if you accelerate this charge, the geometry of the field changes, and the information about where the new field is located in space travels outwards as a kink in the electromagnetic field. This happens because a change in the electric field generates a changing magnetic field which then generates a changing electric field and so on out into space at the velocity of light. This is what Maxwell discovered in his famous wave equation for the electromagnetic field. Light is, essentially, a self-propagating pulse of information that tells us that the state of some electromagnetic field has been altered somewhere in space.

If you prefer the quantum description of photon 'bullets' being ejected into space at light speed, that's fine too. Isn't physics grand?