# Photon Size

1. Feb 2, 2004

### jimmy p

I doubt that my physics teacher (who is coincidentally Mark2711 on this forum) has found out the answer for this question that he was wondering about , so i have decided to take the initative and ask.

We know that photons are the quantas of energy in the electromagnet spectrum. Well we know that they are small for visible light yeah? Well can anyone tell what length a photon of a radio wave would be, seeing as radio waves can stretch between 1cm and 10km!

2. Feb 2, 2004

### rocketcity

Jimmy P,

The energy of a photon is equal to h times its frequency, or h times c divided by its vacuum wavelength. But I understand that you're asking about the size of a photon.

To my knowledge, there is no consistent way to describe the radius of a photon. In that photons are a way of explaining the 'particle' nature of light, they are point particles, with neither mass nor dimensions.

The experiments we've done that led us to the particle model for light tend to force light to have a definite location at a certain time, e.g., a photon coming through a narrow slit strikes a piece of photographic film at a certain location. So I suppose that if photons do have a length, width, or height, all of these would have to be less than the length of the atomic spacing in crystals, since a photon interacts with one atom at a time, and not a whole region of atoms.

Also, if visible light photons do have a size, and its tiny, then I would expect radio photons to be even smaller: they have much less energy per photon. You'd be better off seeing if high-energy ultraviolet photons have a measurable size.

Is anyone aware of experiments (or theory) suggesting that they do? I mean analogous to the Bohr radius, where the pdf of the photon is 99% or 80% or whatever within a certain radius.

P

3. Feb 2, 2004

### turin

I agree with rocket city in my own ignorance. From what I've seen and heard about it, the model suggests that the photon is a point particle.

4. Feb 2, 2004

### jimmy p

ok so basically a photon is a point source of energy and not a portion of a wave, though it has a wavelength. So the size of the wave it is from has no effect on the radius where it is likely to be found?

5. Feb 2, 2004

### rocketcity

One thing's for sure: a photon is not a 'portion of a wave.' The particle theory of light is an alternative to the wave theory of light, so the two pictures don't go together. We do talk about the 'wavelength' of a photon, but you'll notice I called it the 'vacuum wavelength' Vacuum wavelength is just a short way of saying 'divide the speed of light in vacuum by this photon's frequency'. It's more proper, and much more common, to talk about the frequency of a photon.

P

6. Feb 2, 2004

### jimmy p

soz, that 'portion of a wave' was used for want of a better term. So does the length of a wave effect the photons radius?

7. Feb 2, 2004

### NateTG

Sooner or later this will get to the question:
"How do you define photon radius?"

If you think of a photon as a wave, then it doesn't necessarily have any usable notion of radius.

That said, there are certainly sane notions of 'interaction area' that a photon might have. One may, for example, use the notion of atomic radius and a gas laser amplifier to determine an approximate interaction volume for a particular photon wavelength although it may require a large number of trials.

An alterative notion is to see what size holes or slots a photon might fit through. You will get different numbers for using something like a sive, or a pinhole.

8. Feb 2, 2004

### ranyart

The question has a number of paramaters, IE: What if any?

9. Feb 3, 2004

### jimmy p

GAR!!! now im confused !!!! .....ok maybe i dont mean radius, 'interaction area' or area in which the photon will probably be. you guys dont make it very easy.

10. Feb 3, 2004

### NateTG

I hate to break the one to break this to you, but in many ways science is a bunch of usefull lies. Since high school physics is typically a further simplification, there is also a layer of misdirection.

There are certain effects that are explained well by the notion of photons as particles - I believe the photo-electric effect is the big one - and effects that are explained by the notion of photons as waves - classically the two-slot experiment.

This means that photons are not like rocks which have reasonably well-defined physical dimensions, but something different which we're not accustomed to dealing with.

The interpretation of what a photon (or any other particle on that scale) is and how it interacts with other things is currently quite unclear. There are many different notions - Many Worlds, the Copenhagen Interpretation, and 'shut up and calculate' - are all currently viable theories for the interactions of particles at that scale.

Now, if you're trying to predict the behavior of light in a certain situation, that's a nice concrete question that can be answered well.

11. Feb 4, 2004

### gmorgan

If you were to isolate a single photon you would not be able to tell its size. By QM the photon would take up all possible positions for itself in some degree or another. This is QM so its not proper to talk about size or position or motion. You must either consider a range of positions the photon could be at or its energy levels. Bar a few other quantities anything else is meaningless at this small scale.