Posted: Oct 08, 2011 11:44 am
by twistor59
zaybu wrote:
twistor59 wrote:
zaybu wrote:

And those measurable quantities are mass, charge, spin, parity. Never mind that the initial assumption is that we have particles in the model.

This reminds me of Boltzman who postulated that a gas was made of atoms and proceeded to change the whole field of thermodynamics. He never saw atoms, and atoms were only confirmed years after his death. Yet, during his lifetime, he was ridiculed for believing that atoms were real. I guess you would have been one of these people laughing at him.

I really don't understand where you get this impression that I don't believe in particles. Could you quote a post from me which states or implies this ...

Well, you started this thread in a debate that started on another thread, in which you wrote: "I would prefer to refer to photons as "field quanta", and reserve the term "particle" for entities that had position operators." After which I pointed out in QFT, the position operators are fields,

But fields and position operators are two completely different things.

zaybu wrote:
and from there we were arguing about fields being "particle exchanges", which you disagree that we only see in and out states, which I questioned why we should even pursue the Higg boson, a so-called virtual particles not to be found in any in or out states, but then you claimed we should do our outmost to see a free Higgs boson, which would be an out state. So I really don't know where you stand. You've been skirting around the issue.

For me, the Standard Model is very clear: there are two types of "particles": 1) fermions with half-integral spins, which make up what we consider as bulk matter; 2) bosons, with integral spin that mediate the known four basic forces of nature (fields). My claim from the very beginning has been that when we explore nature at smaller and smaller scales, what we see is particles. You disagree, what exactly, I really don't know.

OK, if I haven't made myself clear, I'll try again:

1) This started with a discussion of photons. In the sentence you quote, I was trying to make the point that photons often display behaviour which is not particle-like at all. You can discuss localization of an electron, by which I mean that you can define a position operator, and talk about position eigenstates (Newton Wigner). You cannot do the same for photons. "Particle" is not an ideal term for either really, but it's less ideal for massless spin 1 bosons than for the other cases.

2) You objected that discussion of single photons in terms of wavefunctions has been superseded by the treatment of photons in QED. I counter objected that the usual QED treatments do not tell the whole story - they are fine for computing scattering cross sections in particle physics, but not so useful for quantum optics - in these scenarios it still helps to think in terms of wavefunctions.

3) At some point, to tro...err, to liven up the debate a bit, I threw in the fact that I didn't believe that VIRTUAL photons necessarily represented elements of reality.

So, this is a summary of my viewpoint:

* "Particle" is a bit of a crappy term because it suggests little bullets. "Quantum" is better because it sounds mysterious, and these critters are mysterious. However, we're stuck with "particle". So be it.
* Photons, electrons, W, Z, quarks etc are real entities. Their properties can be measured.
* Photons aren't as easy to localize as electrons.
* VIRTUAL (meaning internal lines in Feynman diagrams) photons, electrons, W, Z, quarks etc are not "elements of reality". At least I think they're not - they're only present because we do perturbation theory, which is just an approximation technique.