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Re: electron, proton size



Hi all-
John Denker writes, apropos the electron size:
*********************************************************
b) If you are looking for a short-range force that tells you something
about the internal structure of the electron, then the size is zero.
Nobody AFAIK has ever managed to whack an electron hard enough to see any
internal structure, or any force other than the long-range electromagnetic
force.

c) There is AFAIK no clear theoretical reason to expect electrons to have
internal structure.

Superstring theory may change this, but that theory is in a pretty murky
state right now, and very far from any experimental test.

d) There is the Compton wavelength, but that has more to do with virtual
electron/positron pairs than with real electrons.

e) There is the classical electron radius as discussed in Feynman II-28,
but experimental evidence shows that nothing special happens at this
radius; it is certainly not like an asteroid hitting the surface of the moon.

BOTTOM LINE: for all practical purposes I can think of, you can think of
the electron as having *zero* core size, surrounded by the long-range EM field.

================
I disagree, in part.
I agree with John's definition that the size is determined by the
distance at which the scattering is no longer due only to the long range
force. There are two regimes where the electrons begin to show structure.
First, we replace the notion of "distance" with its Fourier transform
(essentially the inverse of the distance), the momentum transfer. When the
momentum transfer becomes large enough, we begin to see new phenomena
ocurring such as the production of electron pairs, hard photons, and hadrons.
This regime is not customarily related to a notion of "electron size".
The second regime, however, is where a truly short range force
changes the elastic scattering of two electrons. This is the so-called
"weak force" and corresponds to the exchange of Z-particles, which behave
much like massive photons. This happens at a distance of around 10^(-16)
cm. In quasi-poetic language, one can say that this is the distance at
whci the weak force becomes strong and the elctron behaves as though it had
a hard core.
Regards,
Jack

"I scored the next great triumph for science myself,
to wit, how the milk gets into the cow. Both of us
had marveled over that mystery a long time. We had
followed the cows around for years - that is, in the
daytime - but had never caught them drinking fluid of
that color."
Mark Twain, Extract from Eve's
Autobiography