[danbruc]

This might sound dumb, but how does a point have a wavelength?

This turns out to be possibly surprisingly complicated. I thought I knew the answer, that all photons are the same and have no wavelength by themselves and that the wavelength is in the wave function. Now I just wanted to check that I am not mistaken in order to not spread false information and of course failed to verify what I thought. It may be correct, I may be incorrect, it may be an approximation, I can not tell, that will probably require a day of reading to understand.

Because photons are massless you have to use quantum field theory, simple quantum mechanics does not apply. This means there is no wave function as in quantum mechanics. The classical electromagnetic field seem not to be well-defined for single photons due to the uncertainty principle. It matters whether or not you take absorption and emission into account. Just google photon wavelength, there is a lot to read. All of this may obviously be wrong, mostly just bits and piece I just picked up while skimming articles.

I will certainly try to figure this one out, such an obvious question and something I thought to understand at least in broad strokes. But not today, its late enough. This paper [1] might be useful nut I did not yet read it.

I figured you might refer to a sphere but I think a ball would be the more likely thing if elementary particles were not points. Was I correct thinking you can untangle world lines of balls?

[1] http://www.cft.edu.pl/~birula/publ/APPPwf.pdf

[zmgehlke]

I appreciate the reply!

Yes, if you put little R^4 balls around points of the line, you can still untangle it. It's basically still a line. (They might have to be really little balls, but the definitions all use neighborhoods for defining the legality of knot moves, so you can't "shrink" the knot out of existence.)

My actual thought was slightly more complicated, in that particles dont need to necessarily not be a point all the time, since a sphere you're slicing with a plane can appear as a point -- see sibling comment for what inspired the idea.

[danbruc]

Final thought for tonight. It is certainly an extremely weak argument because it is root in intuition from our macroscopic world, but based on this it makes sense that elementary particles have to be points. At least for me it is somewhat hard to imagine how something could be spatially extended and elementary, i.e. not subdividable. If it has a length, an area, or a volume, I should be able to break it into pieces. This of course immediately conflicts with string theory with which I am sympathetic, sometimes more, sometimes less. But as said, it is an extremely weak argument rooted in a world many orders of magnitudes from where I want to apply it.

[raattgift]

> Because photons are massless you have to use quantum field theory, simple quantum mechanics does not apply

Photons themselves are quantum objects; they do not appear in classical theories of light.

They have several physical attributes which are quantized: spin (+/- 1), helicity (+/- \hbar), chirality (+/- \hbar), charge (0). The masslessness is what makes chirality == helicity as massless particles spin in the same direction along the axis of motion for any observer seeing such an axis.

Frequency is not quantized. A monochromatic light source at frequency f will emit individual photons with energy hf, but the frequency is observer dependent (e.g. one observer may decide that the photons are all hf_{lower} and another may decide that the photons are all hf_{higher}, and an accelerated observer may see the photons arriving from the source with different frequencies at different times; and in all cases each photon will have some hf_{observer} energy).

Photon number is also not conserved -- they can be freely emitted or absorbed. They also always move relativistically (even "slow light" in media does). These features conspire such that the wave-function of a photon only makes sense as long as the photon is neither emitted nor absorbed, and only in a fully relativistic quantum theory. That's what drives your "... you have to use [relativistic] quantum field theory". [0]

You can however devise an (effective) single-photon wave function, and this is done for single-photon experiments, e.g. in studies of single-photon spatial structure, as in the Bialynicki-Birula paper you link to (see eqn 42, and note the point about non-renormalizability).

World-lines are attached to all objects in a relativistic theory (i.e., one where there is Poincaré invariance in the infinitesimal neighbourhood around every point in spacetime), whether those objects are classical or quantum.

Extended objects technically have worldlines attached to each of their microscopic components, but one can usually consider such objects as having a single worldline in some limit; as in fully classical Newtonian mechanics, this would usually be attached to the centre of mass. (In special relativity, which is the theory of flat spacetime, you just use the Schrödinger equation. In very weakly curved spacetime you'd use the Schrödinger-Newton equation, and progress from there as necessary).

[0] It's not strictly the masslessness. Neutrinos have a tiny positive mass and almost always move relativistically. (The exceptions form the cosmic neutrino background, the elements of which have lost momentum in the expansion of space).