Evidence that electron is a point particle?

[jewbinson]

Most theories (/ when doing experiments we) assume that the electron, quark, etc are really point particles.

What is the evidence for this? I don't think, for example, that experiments at cern convey that they are. They could have an internal structure with a stronger force than the strong force keeping it all intact, for example. Is the point that it does not matter if they have internal structure or not? Or (similarly) that we will never be able to find out if it has internal structure or not due to energy limitations in physical experiments?

 

[jtbell]

There is no experimental evidence as yet that they have internal structure, nor is there any compelling theoretical reason to assume that they do (for example, like most physicists assuming for 25 years that neutrinos existed, before they were actually detected, in order to preserve conservation of energy and momentum in beta decay).

Therefore most physicists assume provisionally that they have no internal structure, except of course for those theorists who are working on string theory and other speculative approaches to deeper unification of particle physics.

All physical theories are like this, by the way. We just don't say so explicitly very often, because it would be tiresome to qualify every statement with something like "to the best of our knowledge, subject to future experimental evidence and/or theoretical developments."

 

[fellupahill]

Can someone provide examples of experiments that show electrons, photons, etc as point particles.

 

[Matterwave]

No experiment can really show they are "points" since points have no physical extent. There are experiments which limit the upper bound of the electron radius though. I'm sure you can find those.

 

[tom.stoer]

1) All scattering cross sections of electrons with some other stuff (positrons, protons, ...) can be explained using theoretical models (QED, ...) where electrons do not have any internal structure. Of course this may be due to the limited energy range and could be proven wrong at (much) higher energies, but ...

2) ... if electrons have an internal structure then they must be bound VERY tightly (b/c otherwise it would have been possible to break them in constituents; b/c of the tininess
of the electrons one would expect that an electron as bound state would have VERY large mass, so in addition to the problem of the electron as a bound state one has to face to problem to explain its small mass; that could e.g. be due to an unknown 'Goldstone-like" mechanism for a spontaneously broken symmetry which creates massles fermions instead of bosons.

I think that all this is highly speculative; as long as there are no indications for a substructure in the sense of 1) we should not care about 2)