Re: What is the width of a light ray

This is a tricky question, and I'm not sure what answer your teacher had in mind! (also, I'm an astrophysicist, and a particle physicist might have a different way of answering too...)

Conventionally, fundamental particles like the photon, electron, quarks etc. are taken to be point-like particles with no size at all. So what might we mean by the width of a photon?

I can think of 3 possible answers:

1) quantum theory tells us that "particles" also behave as waves. Waves, of course, can spread out, and hence the "width" of the wave could be any size. The bizare thing about quantum theory (and the world really seems to work like this) is that when we try to pin down the particle in an experiment, the wave suddenly "collapses" and we're back at the point-particle again. (if you can't imagine why physicists would invent the complicated business with waves, if they keep collapsing, recall that particles like photons produce optical interference effects, which only seem to make sense if they can also be treated as waves!)

2) according to "string theory" (which for the past 20 years or so has been the most talked about new theory of fundamental physics), even point particles are not really points at all, but instead are like ultra-tiny pieces of string - where the length is only about a million, billion, billionth the size of an atom. So this could be another answer to the width of a photon, although string-theory remains quite speculative.

3) another way of thinking about the sizes of particles is to ask whether they can collide. One would imagine that truly point-like particles could never "hit" each other. However, in practice particles do exert forces on each other (which in quantum field theory consists of so-called "virtual particles" being exchanged between the "real particles"). So for example, photons can collide with electrons, and the "size" this corresponds to is about a million, billionth of a meter. Indeed, pairs of photons can even collide with each other, although that's observed much less often, so this could again be considered as a "width" of a photon.