# B Why did Bohr & Company hold quantum reality to be indeterminate?

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1. Nov 9, 2016

### jeremyfiennes

QM declares reality to be inherently indeterminate. Another hypothesis is that it is in fact determinate, but due to measurement uncertainty appears to be indeterminate. Why did Bohr, Heisenberg & Co adopt the first hypothesis and reject the equally valid second, when the two are experimentally indistinguishable? Bell's theorem, etc, did not exist at that time.

2. Nov 9, 2016

### PeroK

Is this something you have tried to find online?

3. Nov 9, 2016

### jeremyfiennes

Yes. But the answers are normally in Bell terms which did not exist at the time. I'm interested in Bohr & Co's original reasoning, and how they answered Einstein's objections.

4. Nov 9, 2016

### Demystifier

The Bohr's philosophy can be summarized by the following quote:
"There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature."

By contrast, Einstein and Bell believed that there is quantum world, and they believed that they can say something about what nature is.

5. Nov 9, 2016

### jeremyfiennes

Then why do quantum physicists say it is? Capra, for instance: " Heisenberg's Uncertainty Principle says that one can never measure with accuracy both the position and the velocity of a particle. This has nothing to do with our measuring techniques. It is inherent in reality." Hawking: "Indeterminacy is a fundamental inescapable property of the world." And so on.

6. Nov 9, 2016

### vela

Staff Emeritus
The uncertainty principle can be derived mathematically. The fact that $\hat{x}$ and $\hat{p}$ don't commute, for instance, leads to $\Delta x \Delta p \ge \hbar/2$. What this means is if a state is described by the wave function $\psi(x)$ in the position basis and the same state is described by $\phi(p)$ in the momentum basis, $\psi$ and $\phi$ have the property that $\Delta x \Delta p \ge \hbar/2$. This has nothing to do with measurement; it's just referring to the wave functions that describe the state of the system.

7. Nov 9, 2016

### jeremyfiennes

What lead me into this query was that I read in the afterword to Griffiths' book on QM that the reason for the "indeterminate reality" thesis was that if one makes a second quantum measurement "immediately" after the first, one gets "exactly" the same result. My reaction was 1) I never anyone say that before; 2) it doesn't make much sense to me; 3) Griffiths doesn't defines either what he means by "immediately", nor by "exactly". I am somewhat confused, since he is evidently an authority on the subject. Any comments? Thanks.

8. Nov 9, 2016

### PeroK

If you look at section 1.5 Momentum of Griffiths (I have the 2nd edition) you will see an explanation. In short, one accurate measurement of position will collapse the wave-function into a spike and further measurements will yield answers based on that wave-function, not the original. So, for a given state you cannot get the expected value of position by repeated measurements on one particle. Instead, you must prepare an ensemble of particles in the identical state and perfom one measurement of position on each. The average of those position measurements will give you the expected value for the original state.

9. Nov 9, 2016

### ZapperZ

Staff Emeritus
Actually, there is a subtle mixing of two different concepts here, and it will cause this to be a jumbled mess of confusion.

The "indeterminate" state here refers to the state of a system BEFORE measurement. The modern phrase for this is called "realism". In classical realism, the system has definite state even before a measurement. Say you've throw a dice and then you covered it with your hand. Even before you look at it, the dice already has a definite value facing up. It is just you haven't measured it, and so you say that the probability of such-and-such a number facing up is 1/6.

This is not true for QM system. Before a measurement, ALL the possible values are there and the system is a superposition of all these values. It violates classical realism. So it has NOTHING to do with the HUP and the act of measurement (that is a different issue entirely). It has everything to do with the Schrodinger Cat being dead+alive within the Copenhagen Interpretation, i.e. it is not in one definite state before a measurement.

I don't know what this "another hypothesis" that you are referring to, but if you look around, there the phase space for the validity of classical realism seems to be getting smaller and smaller as more and more experimental evidence supporting the QM picture has come in. See, for example, this and this.

Zz.

10. Nov 9, 2016

### jeremyfiennes

Ok. Thanks you two. I would be exaggerating to say that it is totally clear, but it is at least considerably less murky.

11. Nov 9, 2016

### Staff: Mentor

Assuming that this is a reference to The Tao of Physics, please bear in mind that this is a pop science book. Pop science books are not good sources if you want to learn about the actual science.

12. Nov 9, 2016

### Staff: Mentor

Without commenting on how legit your statements are (QM does NOT say reality is inherently indeterminate) Einstein and Bohr did the best they could at the time.

Read the words of Weinberg - they were both wrong:
http://scitation.aip.org/content/aip/magazine/physicstoday/article/58/11/10.1063/1.2155755

Also Einsteins actual contributions are often misunderstood:
https://www.amazon.com/Einstein-Quantum-Quest-Valiant-Swabian/dp/1491531045

His view changed during his lifetime and he became a proponent of the ensemble interpretation. With a minor update Einstein ran afoul of the Kochen Sprecker theorem and needs decoherence to get around it. It is now one if the main interpretations espoused in what I consider the best book on QM - Ballentinne:
https://www.amazon.com/Quantum-Mechanics-Development-Leslie-Ballentine/dp/9810241054

Thankd
Bill

Last edited by a moderator: May 8, 2017
13. Nov 9, 2016

### Staff: Mentor

@jeremyfiennes Please note that this subject is probably beyond the scope of a "B" level thread. If you have sufficient background for an "I" level thread I can change this thread to that level so you can ask further questions if you have them. Otherwise this thread has probably gone as far as it can go.

14. Nov 10, 2016

### Demystifier

I wouldn't count Capra as physicist. Hawking, on the other hand, at another place said that he likes many-world interpretation, so he is rather inconsistent because indeterminacy is not a fundamental inescapable property according to the many-world interpretation.

15. Nov 10, 2016

### jeremyfiennes

Thanks all. Bhobba: the Weinberg article was interesting. Demystifier: my understanding is that the realist MWI interpretation was developed specifically to avoid the "inherently indeterminate" thesis of the Copenhagen interpretation. Peter: if an upgrade would help, I am interested. My basic query remains: Bohr and Heisenberg had two options. 1) a simple one: that reality is determinate, but due to Heisenbergian uncertainty our knowledge of it is inherently uncertain (either an accurate velocity or an accurate position, but not both). 2) a contentious "esoteric" option: that reality itself is inherently indeterminate, and that measurement reduces it to a determined value via wave-function collapse. So why did B&H go for the contentious esoteric option? What was their reasoning? This also apparently contradicts Occam's razor principle that, given a choice, the simpler is to be preferred.

16. Nov 10, 2016

### PeroK

Or:

Bohr and Heisenberg had two options. 1) A simple one: that reality is inherently uncertain, as evidenced by experiment; 2) A contentious, esoteric option: that reality is inherently certain and a new fundamental theory is needed to explain the uncertainty of experiment.

As far as Occam's razor is concerned, a far more complicated set of assumptions is required to sustain 2), so 1) may be preferred by this measure.

17. Nov 10, 2016

### Demystifier

I think that's not the central dilemma. Even if one accepts uncertainty, one can still consider two possibilities: 1a) reality is random but objective (some random observables have values even when they are not measured), or 1b) reality is random and non-objective. It is this latter dilemma, objectivity vs non-objectivity, that is central to the EPR argument, Bell theorem, etc.

18. Nov 10, 2016

### PeroK

That's a good point. I suppose my question comes down to why should we favour or be prejudiced towards an ultimately objective reality? It has always seemed to me (even before I learned QM) that to assume the everyday notions of an objective reality map down to ultimately the smallest scale is fraught with contradiction.

19. Nov 10, 2016

### Staff: Mentor

No, the point of MWI is to avoid the non-unitary reduction of the wave function at measurement time. Copenhagen basically says the wave function evolves according to Schrodinger's equation except when it doesn't; MWI gets rid of the "except when it doesn't" part.

20. Nov 10, 2016

### jeremyfiennes

PeroK (1): Permit me to disagree. Due to uncertainty, reality will always appear to be indeterminate, even if it isn't "really". So experiment does not prove its indeterminancy. The "determinate" and "indeterminate" hypotheses are experimentally indistinguishable. That is why B&H had two equally valid options. And for some strange reason - the object of my query - apparently chose the more counter-intuitive one.
Demystifier: I'm somewhat confused. For me objective = determinate (determinable), and not-objective = indeterminate.
PeroK (2): I would again say apparent contradiction, due to uncertainty.
Nugatory: I'm not with you on the "except when it doesn't" bit. I thought the wave function evolved according to Schrodinger's equation until a measurement is made.
Thanks to all.