# I Is collapse indispensable?

Tags:
1. Jan 28, 2016

### A. Neumaier

I studied lots of points of view, and lots of how physicists actually use quantum mechanics in the applications. I came to the conclusion that there is an objective and a subjective side to quantum mechanics.

The collapse belongs to the subjective side, since it is associated with ''knowledge'' of which nature is ignorant.

''shut up and calculate'' belongs to the objective side. it couldn't work if the collapse were indispensable.

Properly distinguishing between an objective and a subjective side clears up a lot of the confusion prevailing in the foundations of QM.

Last edited: Jan 28, 2016
2. Jan 28, 2016

### A. Neumaier

Yes, there is no consensus. How could it be otherwise in the foundations of QM, where conflicting opinions abound for nearly a century, and there is no consensus about anything! The reason why there is no consensus is that the whole context is a bit vague, and since a lot depends on what people are looking for.

Can you please give a precise reference for Nielsen and Chuang?

3. Jan 28, 2016

### ddd123

How do those that believe in collapse frame it within special relativity?

4. Jan 28, 2016

### A. Neumaier

It happens of course in the frame of the observer only. This is the way things are reconciled with causality.

Measurement in the context of relativity is discussed in a survey article by Terno and Peres. They write on p.6:
''Dirac (1947) wrote “a measurement always causes the system to jump into an eigenstate of the dynamical variable being measured.” Here, we must be careful: a quantum jump (also called collapse) is something that happens in our description of the system, not to the system itself.''

Last edited: Jan 28, 2016
5. Jan 28, 2016

### ddd123

Haven't we already ruled out collapse as objective? That was short.

Half-joking of course, what I wonder is how its proponents justify it.

6. Jan 28, 2016

### bcrowell

Staff Emeritus
Wavefunction collapse is a feature of the Copenhagen interpretation (CI). It's not present in the many-worlds interpretation (MWI). Anything that appears in one interpretation of quantum mechanics but not in another is purely a matter of philosophy, and can never be tested by any experiment.

Isn't this all well known and not controversial?

7. Jan 28, 2016

### A. Neumaier

I think it is not that easy. The MWI is, in my opinion, an interpretation that explains nothing, and hence doesn't qualify in the same way as the Copenhagen interpretation. The MWI doesn't explain why casting a classical die, considered as a quantum system, produces a definite outcome
with a probability of 1/6 for each result. Instead, the MWI says that all worlds coexist, even those which produce sixes upon every cast of the die. If we were in that world, we could still uphold the MWI although our probabilities are very different from those predicted by quantum mechanics. Thus the MWI explains equally well everything, including all things that happen extremely rarely in our world, and hence has no scientific power at all.

Thus saying that anything that does not appear in the MWI can never be tested by experiment is simply not correct.

8. Jan 28, 2016

### wle

I think there are so-called "stochastic collapse" interpretations where the idea is that wavefunction collapse is an objective process that occurs at random time intervals (independent of the presence of observers). Otherwise, wavefunction collapse is considered subjective in the interpretations of quantum physics that I'm aware of.

9. Jan 28, 2016

### bcrowell

Staff Emeritus
No interpretation of quantum mechanics explains anything. They're just little fables we tell ourselves.

As you say yourself, your argument is an argument about classical physics, not quantum mechanics, and therefore it tells us nothing about the advantages of one interpretation of quantum mechanics over the other.

10. Jan 28, 2016

### A. Neumaier

The stochastic collapse theories are not interpretations of the standard quantum mechanics but variations of it that make testable predictions different from the main stream quantum mechanics.

On the other hand, there are similar quantum jump models that do not claim to be fundamental or have interpretational value but are thought to describe approximations to a more fundamental unitary dynamics.

11. Jan 28, 2016

### A. Neumaier

You misunderstood my argument.

Standard quantum mechanics predicts that a classical die gives probabilities 1/6 for each particular result (since rigid body theory can be deduced via statistical mechanics, and the classical behavior of a die follows from it). A world in which we only throw sizes is incompatible with these predictions with probability arbitrarily close to 1. Whereas such a world is fully compatible with MWI.

12. Jan 28, 2016

### bcrowell

Staff Emeritus
You're just repeating your argument. I understand your argument, but I don't agree with it for the reason given in #9. It's a thought experiment about randomness in general, not about quantum randomness.

13. Jan 28, 2016

### A. Neumaier

Whether quantum randomness is or isn't different from randomness in general is a matter of interpretation, and hence wouldn't make a difference, according to your dictum above.

14. Jan 28, 2016

### wle

I'm not sure there isn't wriggle room here. What you say may be true of stochastic collapse models but I think in general the standard textbook account of quantum physics is vague enough that different interpretations could in principle subtly contradict one another. One example I can think of: textbook QM says that the wavefunction collapses when a quantum system is measured but (among other things) is vague about exactly at what time this occurs or even that it's an instantaneous process (as opposed to just very rapid). This could make a difference since textbook QM also says that the wavefunction will continue to evolve according to the Schrödinger equation after this collapse, so different interpretations attempting to model the measurement process more precisely could slightly disagree on how long a quantum state undergoes Schrödinger evolution (or whatever the equivalent of this is in a given interpretation) between measurements.

15. Jan 28, 2016

### bcrowell

Staff Emeritus
We've been using "interpretation" in a specific technical sense relating to interpretations of quantum mechanics. Maintaining that restriction to that specific meaning of the term, your statement is false.

16. Jan 28, 2016

### wle

I don't see how this, in itself, is different from standard probability theory. In case it's not clear, in MWI there's roughly one branching per measurement, so if an observer observes a six-sided quantum die ten times, the end result is $10^{6}$ observer-correlated-with-die branches, only one, or a fraction $1 / 6^{10}$, of which corresponds to the sequence "all sixes".

From what I know of MWI there are unresolved issues with it, and this might include explaining why we in general perceive the quantum mechanical weight associated with a branch as a probability, but I don't think the problem is quite as simplistic as your summary would imply.

17. Jan 28, 2016

### atyy

That is well known and not controversial. What is controversial is whether MWI works (the debate is technical and not a matter of taste).

What I assert is that some version of CI is the only consensus interpretation. So textbook quantum mechanics that is correct is CI, and no other interpretation. All other interpretations are BTSM.

18. Jan 28, 2016

### atyy

Just after Eq 2.98, p87:
"According to Postulate 2, the evolution of this larger isolated system can be described by a unitary evolution. Might it be possible to derive Postulate 3 as a consequence of this picture? Despite considerable investigation along these lines there is still disagreement between physicists about whether or not this is possible. We, however, are going to take the very pragmatic approach that in practice it is clear when to apply Postulate 2 and when to apply Postulate 3, and not worry about deriving one postulate from the other."

Last edited: Jan 28, 2016
19. Jan 28, 2016

### ddd123

Could you expand on this? Thanks.

20. Jan 28, 2016

### atyy

There is a way to "avoid" collapse, but one needs a new postulate - the generalized Born rule. The generalized Born rule is rarely stated in full generality, but an example of the the generalized Born rule is Eq 37 of http://arxiv.org/abs/quant-ph/0209123.

The usual Born rule plus collapse is equivalent to the generalized Born rule. If there is no collapse, that is equivalent to claiming that the axioms of QM with the usual Born rule but without collapse are sufficient to derive the generalized Born rule. As far as I know, that has not be done.

Last edited: Jan 28, 2016