Quantum interpretations and indistinguishable elementary particles

[Ben vdP]

TL;DR

Elementary particles are indistinguishable and this has consequences.

How do different interpretations take this into account or deal with situations where one cannot identify a particular quantum?

Does the way forces work (the exchange of virtual particles) provide a mechanism for being indistinguishable?

On one hand, a minimal view of quantum mechanics is to see the Schrödinger equation and wavefunction as part of a description for a "generic" quantum system or ensemble of identical prepared systems ala Ballentine ( The Statistical Interpretation of Quantum Mechanics ).
In the act of measurement one moves from a "generic cat" to a "more specific cat" or subensemble.

On the other hand there are interpretations that view the wavefunction as representing a complete state description or representing an individual physical system.
With Copenhagen you then get the collapse of the wave function or the many world interpretation with the splitting up into many worlds.

It looks there is nothing in the middle.


A simple example:

Consider light going through a window.
On macro level it makes sense to say that a photon arrives at the window, travels through the glass and leaves the window.
On micro level the story could be very different. There could be all kinds of interactions taking place, the photon could get absorbed, there could be a re-emit.
And also, elementary particles are indistinguishable, if a quantum is detected at a certain position then at a later time or later detection you cannot say that it is the same or a different one at the same position.


As a consequence, it is impossible to tell if the photon leaving the window is still the same one that entered the window.

So at a glance, it is prohibited to use a wavefunction for a single photon in the large i.e. across the window.
Or more generally across a measurement or interaction at macro level.

What could be a justification to still try to do so or do you just pretend it as an approximation?

How do different interpretations deal with such issues?

 

[.Scott]

On micro level the story could be very different. There could be all kinds of interactions taking place, the photon could get absorbed, there could be a re-emit.

A photon that is "absorbed and reemitted" will likely leave behind energy and end up with a higher wavelength. Or, in the case of a laser, stimulate the emission of light at a lower wavelength.
At a QM entity, exactly what happens when a photon is travelling is not simple.
The landing spot of a photon is a function of what it would have encountered in all of its potential trajectories, not just the one that it appeared to follow.

And also, elementary particles are indistinguishable, if a quantum is detected at a certain position then at a later time or later detection you cannot say that it is the same or a different one at the same position.

Certainly. If I form a negative ion by adding an electron to a molecule and then take that electron back, it is meaningless to ask if I got the same one back that I put in.

As a consequence, it is impossible to tell if the photon leaving the window is still the same one that entered the window.

Even the meaningful of the question is unclear. Think of the detection of a particle that just landed on the screen in a double slit experiment. Would you be asking if that photon was the same one that existed before it entered the slits? Which slit?

How do different interpretations deal with such issues?

I think that you are holding onto an model that says that photons follow a specific trajectory. Any "interpretation" that relies on that would be the approximation. The wave function is the precise answer.