Re: What is light made of? Energy, or matter?

Dear `Frodo'!

These are very important questions you ask, and they have been asked by scientists for hundreds of years. Unfortunately, it is not the task of modern physics to explain nature, but just to describe it in the most simple terms. Nevertheless, I will try to answer your questions one by one:

1. Does light have mass?
   No, at least not in the usual sense. The particles that light is made of (the photons) do not have rest mass, which means that you cannot `stop' a photon to put it on a scale --- it must travel at the speed of light!. As you might know, Special Relativity predicts that a body having rest mass cannot be accelerated to a velocity equal to or greater than that of light. So one could also see it the other way round: Photons travel at the speed of light, so they cannot have rest mass.

   Nevertheless, this does not mean that you can't attribute a mass to a photon with a certain frequency. According to Einstein's famous E=mc², a photon with frequency f has a mass of hf/c², where h is Planck's constant. But this mass is only a number which is related to the photon's energy content and has nothing to do with the concept of rest mass.

2. What is used to create it?
   The answer to this question is not easy to give, as it depends on the theoretical picture you use to describe light. Quantum field theory predicts several mechanisms by which photons are produced. Pair annihilation (the collision of an electron with a positron can produce two photons), bremsstrahlung (an electron which gets accelerated will necessarily radiate off photons), and decay of an excited state (like in a nucleus emitting gamma radiation or an excited atom making a transition to its ground state) are the most prominent ones. As to how this comes about exactly, nobody has a clue. The processes described above are allowed by the laws of field theory which have proved to be very reliable, and the theoretical predicitions agree well with experiments. The question `where does this photon come from' can only be answered within the framework of Field Theory, and the answer is `It is an excitation of the vacuum (like any other particle), and theory predicts its production in several processes'.

3. Is it a form of matter turned into energy, or energy into matter?
   The distinction between energy and matter is sort of blurred, because you can actually `weigh' energy (think of a nucleus in which the nucleons are bound via the nuclear force; the nucleus is somewhat lighter than the sum of the masses of its components, so we have to assume that the binding energy which was set free in its creation accounts for the difference). Think of photons as the result of one form of energy converted into another. Example: If an electron gets decelerated, the photons it emits (bremsstrahlung!) get their energy from the kinetic energy of the electron.

4. Is there a fundamental principle common to all methods of its creation?
   Well, I could go into the details of Quantum Field Theory now, but let me just tell you that all the processes by which light is produced can be traced down to certain mathematical constructions called `vertices'. They involve creation and destruction of particles on a very elementary level, and they provide the framework for many calculations. But let me stress again that these are only models which we think are appropriate to describe nature. We can by no means be sure whether these models are correct, but they have proved to be reliable, and that is what really counts.

I somehow can't believe that my comments answered all your questions, but I hope I could help to give you some new insights. I suggest reading Feynman's excellent book `QED - The Strange Theory of Light and Matter'. After that, feel free to contact me via email if you have further questions.

Bye,
Georg.