Photon question -- is it a particle?

[f95toli]

The TED talk does not say "Photon is a particle and a wave structure phenomenon at the same time". It says that light sometimes behaves like a particle and sometimes like a wave. It can't do both at the same time! I am saying that it behave like a particle when the wave function has collapsed, and it behaves like a probability wave of direction until it collapses, by being detected, or hitting an object. You don't know its exact path until it is detected.

That is not quite correct either. First of all. there is no wavefunction for the photon, at least not in the usual sense (you can define quantities that are a bit similar to a wavefunction). Secondly, it is possible to create states of light where it behaves very much like a particle (so-called Fock states), but these are unusual and not created by any conventional source of light (including lasers); most of the time classical (wave) EM works fine.

However, in general all we can say is that light behaves as if it sometimes has particle-like or/or wave-like properties; but this does not means that it "is" (whatever that means) either a wave or a particle (or even a mixture of the two). Note also that the "nature" of light is very well defined in the (complete) theory of light (quantum electrodynamics) so there is no unsolved mystery here. The issue in this context is that you need to know a fair bit of math to understand QED: there is no easy way to explain it and you can certainly not use classical concepts to do so if you want to be accurate.

The best popular description of what light really "is" I've come across is Feynman's book on QED.

 

[Alastair McD]

I don't think my description differs from that of Feynman.

 

[my2cts]

there is no wavefunction for the photon

The potential is the wave function. It obeys a massless Klein-Gordon equation.

 

[Nugatory]

I am saying that it behave like a particle when the wave function has collapsed, and it behaves like a probability wave of direction until it collapses, by being detected, or hitting an object. You don't know its exact path until it is detected.

That picture doesn't quite work either. Even if we gloss over f95toli's point about the problem with attaching a wave function to the photon at all, and even if we note that a state that is collapsed in one basis is uncollapsed in another (so we can't tie the particle/wave distinction to a collapsed/uncollapsed distinction because that latter distinction doesn't exist)... We still don't know the exact path. All we know is that a photon was detected at a particular point.

 

[ontheroad]

At our size scale, and in our environment, quantum phenomena do not exist: I mean that bullets and baseballs, ocean waves and sound waves do not behave identically to photons. Therefore we have no innate understanding of quantum phenomena. However, photons do display some of the same phenomena as the above mentioned, and this is useful.

 

[Alastair McD]

Nugatory wrote: "so we can't tie the particle/wave distinction to a collapsed/uncollapsed distinction because that latter distinction doesn't exist."

Before we open the box Schrodiger's cat is neither dead nor alive. After we open the box it is dead or alive. The cat's state has changed from an unknown probability state (wave function) to a physical cat's body just as real as a photon.

 

[Nugatory]

Nugatory wrote: "so we can't tie the particle/wave distinction to a collapsed/uncollapsed distinction because that latter distinction doesn't exist."

Before we open the box Schrodinger's cat is neither dead nor alive. After we open the box it is dead or alive. The cat's state has changed from an unknown probability state (wave function) to a physical cat's body just as real as a photon.

That doesn't affect the point about there not being a collapsed/uncollapsed distinction. Consider the common example of a photon approaching a vertically oriented polarizing filter. The photon has been prepared in the state $(|V\rangle+|H\rangle)/\sqrt{2}$; this is the uncollapsed superposition of the states $|V\rangle$ and $|H\rangle$, vertical and horizontal polarization respectively. When the photon encounters the filter, its state collapses to either $|V\rangle$ and it passes the filter or to $|H\rangle$ and it is absorbed by the filter. It's tempting to say that before the interaction we had an uncollapsed state and after the interaction we had a collapsed state... But we don't. The apparently collapsed and unsuperposed post-interaction state $|V\rangle$ is the exact same state as the uncollapsed and superposed state $(|L\rangle+|R\rangle)/\sqrt{2}$ where $|L\rangle$ and $|R\rangle$ are polarization along the +45 and -45 degree axes. For that matter, the initial and apparently uncollapsed state $(|V\rangle+|H\rangle)/\sqrt{2}$ is also exactly the state $|L\rangle$.
Thus there's no way of distinguishing between collapsed and uncollapsed states; it's just a matter of how we look at them. However, as I said above, Schrodinger's cat is unrelated to this question - neither "live cat" nor "dead cat" are pure states to which the notion of superposition and collapse applies. Instead, they're both mixed states.

You may also be misunderstanding the point of Schrodinger's thought experiment. He was not seriously suggesting that the cat would be neither dead nor alive. He was pointing out a problem with the then current (eighty-plus years ago) understanding of quantum theory, namely that there was nothing in the theory that explained why the cat wouldn't end up in such a superimposed state even though everyone knew perfectly well that it didn't. It took almost a half-century to find the answer to that problem - you may want to google for "quantum decoherence" or give this relatively gentle book a try.