# The size of a photon

1. Oct 28, 2006

### pej.dgr

A good case can be made for the hypothesis that all photons are the same size and have the same spatial distribution (energy density/energy) of mass-energy as the electron.

It runs like this: We all accept that an electron in motion can have a De Broglie wavelength ranging from, say, 10^-15 m to 10^50 m, and that throughout this range the size of the electron is unchanged. I can find no experimental results that conflict with the assumption that this is true for the photon as well. The photons produced in electron-positron annihilation must, at least momentarily, have the same mass-energy density as their parent particles. There is no evidence that it changes as they move apart. At any speed the kinetic energy density of an electron is proportional to its rest mass energy density. In any emission of part of this energy as a photon this will at least initially have the same spatial distribution.

Can anyone cite any experimental results that conflict with this hypothesis?

Phil Gardner

2. Oct 29, 2006

### ZapperZ

Staff Emeritus
Just think: you are saying a radio wave, that can have wavelength of the order of miles, has the same "spatial distribution" (which you haven't defined to what that means) as a gamma ray that has a wavelength in angstroms. Would a diffraction experiment change your mind?

Note your bastardization of "de Broglie" wavelength". It is NOT defined to any extent as the SIZE of the object. If it is, then you have something mysterious here, because the size of an electron can somehow depends on how fast it is moving - and no, this is not the relativistic effect either! A QM wavefunction that you use does not give you a "size".

Zz.

Last edited: Oct 29, 2006
3. Oct 29, 2006

### cesiumfrog

Does an electron have a size?

4. Oct 29, 2006

### Staff: Mentor

No indication that the electron has a finite size has been found in high-energy scattering experiments. A quick Google search gave me an upper limit of about $10^{-19}$ m for the radius of the electron, from an experiment at CERN. I don't know if this is the most recent figure.

(That is, IF the electron has a finite radius, it has to be smaller than this.)

5. Oct 29, 2006

### yogi

The classical radius is about (10^-15)m - but electrons are not clumps of matter - so looking for a size is an abstraction. They behave differently in different experiments

6. Oct 30, 2006

### lightarrow

Which physics principles would this violate?

7. Nov 2, 2006

### jnorman

the electron is a point particle - it has no volume. to discuss the "size" of a photon is meaningless. you cannot state with certainty anything about a photon in between the time it is emitted and the time it is absorbed.

btw, an earlier post talked about slowing down the speed of light in lab experiments - IMO, that is a hogwash. C is a constant in a vacuum - and all of spacetime is a vacuum in between any two given particles. any artifact of measurement which indicates a variance in C is being misled by absorption/re-emission actions, and is representative of merely an apparent slowing of C.

8. Nov 2, 2006

### ZapperZ

Staff Emeritus
It violates the physics principle of understanding the principle FIRST before using it.

Zz.

9. Nov 2, 2006

### Chaos' lil bro Order

Lol. Do you ever have a good day zapper?

10. Nov 3, 2006

### lightarrow

If this were true for all photons, then what creates the track in a bubble chamber at the passage of an highly energetic gamma ray?

Last edited: Nov 3, 2006
11. Nov 3, 2006

### lightarrow

This principle states that an electron has no size, or that is point-like? Explain this principle, please.

12. Nov 3, 2006

### ZapperZ

Staff Emeritus
No. If you look at what you underlined from my quote, it is the "deBroglie" wavelength, which was the issue in that quote. Check the deBroglie wavelength and see how it changes with the velocity of an electron.

Zz.

13. Nov 3, 2006

### Staff: Mentor

The gamma-ray photon itself does not create a track in a bubble chamber. What does create a track (or rather, tracks) is the $e^{+}e^{-}$ pair that the photon produces.

I saw lots of these while working on a bubble chamber experiment as a graduate student many years ago.

14. Nov 3, 2006

### lightarrow

I know how it changes with electron's velocity. I intended that if the "size" of an electron would be related with its wavelenght, and so, would depend upon its velocity, I don't understand why this should be so strange, or impossible. I think there already are many strange things in QM and this would't certainly be the most strange one.

15. Nov 3, 2006

### lightarrow

Ok, but you mean that an highly energetic photon is completely "invisible" until it hits a nucleus (creating a pair), and is only possible to determine its direction? Or (if it has enough energy) it can interact in a point (Compton or pair production) and in another one and so on, so that we can trace its path?

16. Nov 3, 2006

### ZapperZ

Staff Emeritus
It isn't strange for QM, it is strange for someone wanting to find the SIZE of something based on our pre-defined understanding of what a "size" is. If you tell me how you'd measure the size of something, then that's another issue. However, to say that "Oh, let's use the deBroglie wavelength" as the size, then you've defined something entirely new without bothering to define and justify it FIRST.

This is ignoring still the fact that the wavefunction defined by the deBroglie wavelength has nothing to do with a "size". What operator would you associate this with? The position operator is just that, a position, not a "width" of the object. That is what I meant when I said that it is a bastardization of the deBroglie wavelength - concluding something in ways it wasn't meant to be used.

Zz.

17. Nov 3, 2006

### Staff: Mentor

We were dealing with energies above about 1 GeV, up to several tens of GeV, and the bubble chamber's working substance was a liquid helium-neon mixture, so pair production was by far the dominant photon interaction mechanism. We could get the momentum and energy by measuring the electron and positron tracks (in a magnetic field). At those energies, the energy and momentum taken up by the recoiling nucleus are negligible.

At much lower energies Compton scattering would become important, but all you'd see would be the recoiling electron. I don't know if you could see enough successive Compton-scattering events from the "same" photon to get a usable track.

18. Nov 4, 2006

### lightarrow

If you assume that QM is a complete descrition of reality, I don't undesrstand why you work as experimentalist; I expected from you to talk about experimental limits of a particle size, or experimental limits in measuring a particle size, as jtbell did. Why does an electron leave a track in a bubble chamber? Why does high momentum (little wavelenght) electrons go straight after a slit in a screen, while low momentum ones form a spread diffraction pattern? I mean, I know how QM describes all this, but does QM, for example, gives the value of a nucleus size? Or we need something more, for example nuclear force theory, (assumed those values could be determined theorically)?

Last edited: Nov 4, 2006
19. Nov 4, 2006

### lightarrow

Since the least energy required to form a couple is 0.511*2 MeV, it's strange that a several tens of GeV photon produces only one pair in a chamber. How is this explained? However, with higher energy, do you think it would be possible to have more than one interaction from a single photon?

20. Nov 4, 2006

### ZapperZ

Staff Emeritus
You are making an incoherent argument here. What does what you said have anything to do with my response so far? I was addressing YOUR question to me regarding the deBroglie wavelength and how it was misused. You didn't ask me for a treatise on what an electron is. I didn't know that this is what this thread has become, because it certainly is NOT part of the OP. So if you wish to hijack this thread to be on the "size of an electron", then make one!

BTW, ALL electrons, be it "high momentum" or "low momentum" can make diffraction patterns. Whoever told you that the high momentum electrons "go straight" through?

There's a lot of very confusing disconnected statements in your questions. Why don't you create a thread and ask ONE question at a time?

Zz.