I The quantum wave is not real?

[weirdoguy]

Discussions of interpretational issues should, IMHO, focus much more on QFT than QM.

This! I don't quite understand why people focus so much on non-relativistic theory. Relativity can turn all of this efforts on its head.

 

[pines-demon]

Thanx, this makes sense. Its interesting that it is so hard to define whats "real", I do understand that something that has energy is real.

Energy makes EM real does not seem to be a good argument, I can also write the Lagrangian of particle with the wavefunction in it, does the wave function carry energy?

 

[gentzen]

This! I don't quite understand why people focus so much on non-relativistic theory. Relativity can turn all of this efforts on its head.

Maybe because QFT somehow invites "everybody" to make overarching statements, which are not defensible in their stated generality?

Fine, you don't need to say much. Just clearly replace the statement

Schrodinger's equation is simply the low energy approximation of the Lorentz-invariant Klein-Gordon equation of quantum field theory

by something which is less disparaging towards nonrelativistic QM. As I said, the Klein-Gordon equation only applies to bosons, and this is not a limitation of nonrelativistic QM. It is true that nonrelativistic QM has trouble creating or destroying particles, and hence has a hard time with photons, because they are massless.

Please also see my point. I have invested huge efforts to understand nonrelativistic QM good enough to be able to somehow use it in my daily job. I have some knowledge of QFT, but far less than of QM. It is fine for me to read your references to Zee, or Demystifier's paper. But the way this discussion here went feels very bad to me.

In this discussion here, we have for example the following statement:

The quantum field, having energy, etc., is real. For example, E=MC2 says mass is a form of energy, and I do not know anyone who thinks mass is not 'real'.

So you just mention the words "quantum field", and somehow this makes our trouble with explaining what we mean by "real" suddenly go away?

 

[martinbn]

In this discussion here, we have for example the following statement:

Now, the deeper theory, QFT, is a different matter. The quantum field, having energy, etc., is real. For example, E=MC2 says mass is a form of energy, and I do not know anyone who thinks mass is not 'real'.

So you just mention the words "quantum field", and somehow this makes our trouble with explaining what we mean by "real" suddenly go away?

I think, by real he means having an objective existence, that it is part of the territory and not the map. I think those are constantly being confused or people are not trying to write more clearly.

 

[bhobba]

Thanks; this makes sense. It's interesting that it is so hard to define what's "real." I do understand that something that has energy is real.

It is a philosophical problem that, by forum rules, can not be discussed here. It is recognised, however, that in the quantum interpretations section, it will occasionally be brushed up against, so some leeway is given.

For most applied math/science, the external reality we all experience is objective. As one physicist (Victor Stenger) said, it is what kicks back. Indeed, when we kick a rock, it kicks back, so generally, it is considered real. It has other properties like mass, which you feel when you lift or move it (eg kicking back). From E=MC^2, energy is then real. EM Fields (classical or quantum) are real; due to Noether's theorem, they have energy.

Other things are not real, e.g., probability, which depends on your interpretation (Bayesian or Frequentist). That is not real.

Observations, ie things that occur and can be measured, are real in QM. They are included in the observations observable. This is the starting point of QM and can be justified from a simple model. I will not provide details unless people want to see them. This is the first axiom of QM as found in Ballentine. The second axiom (the state and the Born Rule ) follows (with some caveats) from the first. Ballentine develops QM just from those axioms. The reality of the state is interpretation-dependent. I believe it is just a calculational device to help calculate probabilities, which are not real. The following paper explains that the state is not the limiting case of QFT (antiparticles are the issue):

https://arxiv.org/abs/1712.06605

Obviously, for ordinary QM, the fields still exist, but they are not the quantum state of ordinary QM. For most practical purposes, they are the same; as I said, antiparticles are the issue, but they are usually neglected.

Thanks
Bill

 

[bhobba]

I think, by real he means having an objective existence, that it is part of the territory and not the map. I think those are constantly being confused or people are not trying to write more clearly.

Exactly, and well said.

Thanks
Bill

 

[bhobba]

Energy makes EM real does not seem to be a good argument, I can also write the Lagrangian of particle with the wavefunction in it, does the wave function carry energy?

Interesting point. Since I think the wave-function is not real, when using the path integral approach, one gets the Lagrangian of an actual particle; it must be related to the quantum field. As the paper I linked to explains, they are very close in ordinary QM, The problem is that they must also include antiparticles, which they usually do not. The quantum state is an abstract approximation, but it is enough to (via Noether) give energy, momentum, etc. What determines the Lagrangian must be valid for the conclusion, e.g., having energy to be valid.

Thanks
Bill

 

[WandaS]

2. The photon behaves as if it goes through both slits and gives rise to an interference pattern, this is because it can be seen as electromagnetic waves, but when you measure/detect the photon, the pattern disappears, does this mean the photon looses its electromagnetic wave-properties? [...]

The interference pattern has nothing to do with electromagnetic waves, but with indeterminism and superposition, I think here's the misunderstanding.

I would completely forget the EM waves here, you can do the same experiment with electrons too. The position of an electron is given by its wave function, at a wave's peak the probability to find it is highest, and least at its trough. The wave function is so a "probability wave".

Prior to a measurement, an electron has no definite position, but is in all positions allowed by the wave packet at once, it is in superposition of all possible states or positions. When we measure its position, we'll find it within the wave packet, but what position exactly is totally random; with most probability at its peaks, but could also be at some other place within the wave. But we can't say that it was at this place already before the measurement, as prior to the measurement its position is not determined.

It's as if the measurement "chooses" a random position out of the possible ones, without reason and therefore in an impredictable manner. That's why it's called "collapse" of the wave function.

But how can we be sure that an electron wasn't already at a definite position (but was in superposition) before the measurement?
Because of the interference pattern we see at the double slit experiment:

If the electrons had at each moment a definite position (we just don't know), we would see two stripes at the detection screen, like when we throw balls, or like when just one of the slits is open.
The interference pattern is the result of the "probability waves": two peaks add to a higher peak (highest probability), a peak and a trough cancel each other out, resulting in the bright and dark stripes of the pattern.

(If we shoot one electron at a time, we also see an interference pattern: it's as if the electron goes through both slits at once (superposition), creating the same pattern at the screen.)

As to the reality of the wave, the Copenhagen Interpretation says something like:
Only what can be measure can be seen as real, and therefore the wave is not an element of reality in its strict sense (but a mathematical tool for making probabilistic predictions).

 

[anuttarasammyak]

What are examples of real and not real subjects in physics ? Real seems category error in physics to me.

That said, is information including wave function, its entanglement, entropy ( and temperature ? ) widely regarded to be not real ones in physics ?

[EDIT]
Wave function, which is an infromation but not real entity, follows physical equation, i.e. Shoredinger equation. Distribution function which is an information but not real entity in physics follows Liouville equation. Even they are not real, the information develop according to physical equation, which is amazing for me.

 

[bhobba]

That said, is information including wave function, its entanglement, entropy ( and temperature) widely regarded as not real in physics?

Wave function - reality is interpretation-dependent
Entanglement - very real, as Bell showed
Temperature - real

Probability, based on the so-called Cox axioms, is quantified plausibility, so it is not real (it does not 'kick back'):

https://en.wikipedia.org/wiki/Cox's_theorem

There is a. branch of math known as rigorous probability theory where everything is proven from the so-called Kolmogorov Axioms. Books like A First Look at Rigorous Probability Theory by J Rosenthal take this approach. Using just the Kolmogorov axioms, no position is taken on reality - it is just a mathematical system like group theory.

Ross - Introduction to Probability Models takes an intuitive approach in moving from the Kolmogorov axioms to applications

The classic Feller - An Introduction to Probability Theory and Its Applications takes a middle ground.

Thanks
Bill