Is quantum indeterminacy a result of experiment design?

[vanesch]

And just for the record IMO (in my opinion) MWI is one of the more ridiculous ideals getting ink to come out of QM.

The usual, well-argumented rebuttal against MWI

 

[RandallB]

The usual, well-argumented rebuttal against MWI

As compared to the agreements & proofs provided in favor of it your correct.

 

[vanesch]

As compared to the agreements & proofs provided in favor of it your correct.

It is hard to find out if you are ironic, but here are the main arguments in favor of MWI (as an interpretation of QM):

1) respect of unitarity (as postulated in QM) and possibility to represent the state of the universe as a ray in Hilbertspace (as postulated in QM)

2) no non-physical (because non-unitary) phenomenon in the measurement apparatus

3) the theory DOES describe a reality (although it is different from the one we perceive directly) - so no positivism which denies reality all together

4) we can save locality (any collapse is non-local) so that we can keep SR

The downside is that we have to postulate a non-trivial relationship between reality and subjective experience, but if this is done, no contradiction is derived between the postulated subjective experience and actually perceived subjective experience as we know it.

I consider the above as a rather solid ARGUMENTATION when compared to:
"one of the more ridiculous ideas"
or:
"a meaningless string of words"
or:
"naah, can't be true".

Mind you, I'm not claiming that MWI is necessarily true. I'm claiming that, when you consider the many formal advantages of this viewpoint (see argument above) that other viewpoints (as long as they don't touch upon the formalism of QM; in other words, as long as they are an INTERPRETATION of QM, and not simply a different theory) pale against it: The Copenhagen view is inconsistent with the basic axioms of QM (as Schoedinger found out rather early with his cat) and non-local, and the probabilistic view (although I can have some sympathy for it) denies any link with reality.

I note that the only "arguments" against the viewpoint are simply emotional statements and do not include a solid reasoning.

So the only way I can consider a non-MWI viewpoint, is by presenting another theory, with a clearer interpretation, which will explain all the QM successes.
There are two "candidates": local realist theories starting from classically relativistic field theories (we already KNOW that they will not be equivalent to QM thanks to Bell) and Bohmian mechanics. Bohmian mechanics is not compatible in its workings with the principle of relativity (it is non-local) and has also some of its own interpretational problems.

 

[reilly]

vanesch --

A knowledge based QM interp, as with any probabilistic theory, necessarily obeys unitarity. Further, with QM so interpreted, than all, repeat, all theories involving probability are resting on the same basis -- there's always a probability (wave)(function) collapse, by necessity in fact, and it occurs in people's brains -- that's quite close to certainty,in fact I'm fairly certain that brain scans can actually show such a collapse.QM describes at least the reality of the experimentalist, and, when, the fabric of experimental results is woven, it, metaphorically, looks strangely like normal reality -- the reality of common experience.

As usually formuted, QM is local, point interactions and all that.

I've always thought of MWI as odd. Seems to me that it's just another attempt to subvert probability. If it is such a great idea, why has it not been in place since at least Fermat's notions about games of chance? Why is this ideas absent from virtually all books on probability and statistis? (I would say all, but there are such books that I've never read.)Why not a universe in which I broke the Bank at Monte Carlo, when I found and extra million in my bank acount, why not a universe in which the Boston Red Sox won 25 World Series in a row? What good does such speculation bring?

The kindest thing I can say about MWI is that it is rather peculiar.

The knowledge approach works, and, I have yet to hear of any practical arguments against it.

I will admit that the stupendous ego of David Deutsch, which permeates his book, The Fabric of Realty, turned me away, in part, from MWI. It's one of these books that says,"Trust me, I'm right." He's a great spinner, probably could do well as a political consultant, with such catchy ideas as shadow photons. Small wonder that his views remain relatively unknown.

With all due respect, I have yet to see anything about MWI that solves any problem of QM other than with fanciful suppositions of universes we can never know. Deus ex Machina

Regards,
Reilly Atkinson

 

[leandros_p]

In case of the cat the design incorporates a known indeterminate element: the decay of an atom. We do not have a model that will predict the exact time when a given atom will decay, but we do have a statistical model, and that model is incorporated into the cat experiment. That model predicts that, if we open the box at t=Lambda, we have a 50% chance of a dead cat. Before we open the box we of course don't know the state of the cat, but that is what was designed into the experiment. How does this show any indeterminacy beyond that which was designed into the experiment?

The issue here is that an observer inside the box has a very clear knowledge about the state of the cat, but an observer outside of the box, has just an information about the state of the cat with 50% uncertainty.

This means that the same physical system (the box with the cat) provides to the inside observer scientific information which comes from constant observation that is not available to the outside observer. This difference in status produce the dramatic difference between the uncertainty of the information about the status of the system for each observer.

If you put the solar system in a box, and you have an internal observer constantly observing the planets and an external observer having a snapshot observation, they share the same certainty regarding the information about the status of the solar system in the box. The internal observer can observe constantly the orbits of the planets and he can guide a space vehicle from Earth to another planet based on his scientific information gathered by the continues observations, with the same certainty with which the external observer can guide another space vehicle from Earth to another planet based on the scientific information gathered from the system, in a snapshot. Both observers having informations from the system, the one from real time observations from inside and the other from a snapshot observation before being locked outside from the system, they share the same certainty about the status of the system during the passage of time.

In the solar system in a box example, the position of the observer is irrelevant from the degree of certainty of the scientific information for the status of the system.

With the double slit experiment we seem to have a similar situation. The indeterminacy designed into the experiment is that we don't know exactly which slit a given electron will go through. But we do know that 50% of the electrons will go through one, 50% through the other slit. The actual probability distribution turns out to be a wave function, which is perhaps a bit unusual, but no more than that. As a result the distribution of electrons on the detector screen displays an interference pattern. So far I fail to see any indeterminacy beyond what was designed into the experiment.

We can then change the experiment by removing the indeterminacy: we add some sort of apparatus that tells us through which slit each electron passed. This apparently changes the probability distribution from a wave to a classic particle yes-or-no distribution. OK, so that is maybe strange, but I still see no indeterminacy here.

This is a similar situation, like the cat in the box situation, where the information known to the observer affected the certainty about the information of the status of the cat.

The information that the observer has in hand affect the distribution of electrons on the screen.

The status of each observer, regarding the "position" of observation, affects the certainty of the information that the system is providing about the system's status.

(edit) In an analogy, the opposite thing happens in the observation of the solar system. The certainty of the scientific information about the status of the solar system is the same in both cases: when the observer makes observations from outside using information from starting and end points from the movements of the planet, or when the observer makes observations by using position traps gathering information from positions in between the starting and end points from the movements of planets. Both observers share the same certainty of the information of the status of the solar system. In the experiment with the electrons the certainty of the information of the status of the system is different for each observer, when the one knows the in between positions of the electrons and the other one does not.

In both experiments, the indeterminacy is about the diferrent degrees of certainty of the information of the system which depends on the "position" of the observer. You come to the right conclusion, that all local observers share the same uncertainty, and that all non-local observers share the same uncertainty. There is no indeterminacy, for the same class of observers. The indeterminacy is about having two classes of observers, the local and the non-local. Within each class the certainty of the scientific information of the status of the system is consistent. But the experiment is inconsistent when we compare the certainty of the scientific information of the status of the system between the two classes of observers.

This incosistency, between classes of observers, is NOT happening in systems of classic physics (like the solar system). This is not a measurement problem. It is an intrinsic paradox. The same system, in quantum physics, provides different degrees of certainty of scientific informations to the observer, depending on the "position" of the observer.

So, I think that the answer to your question "Is quantum indeterminacy a result of experiment design?" is NO. The indeterminacy is an intrinsic behaviour of the systems of quantum physics.

Leandros

 

[gstafleu]

The issue here is that an observer inside the box has a very clear knowledge about the state of the cat, but an observer outside of the box, has just an information about the state of the cat with 50% uncertainty.

Isn't that equally the case if we replace the cat+atom with a coin flipping machine? The observer inside the box knows exactly which side came up, the one outside lives in uncertainty. Until the two communicate, that is, or the outsider looks in, at which point both will agree. Which is the same situation as with the cat+atom.

Now for you solar system example, doesn't the same apply? Assuming the outside observer (1) cannot peek and (2) does not have pre-knowledge of the solar system (he just zapped over from a galaxy far far away and has never seen the solar system before), then the insider knows and the outsider doesn't.

The solar system example has the drawback that we have a pretty good deterministic model of the planetary positions, while we do not have such a model of atom decay. As a result, if both observers know the initial state, and then close the box at time T, they can then both come up with, agreeing, descriptions of the state at T+t. You cannot do that with the cat+atom (catom?), but that is because we don't have a deterministic catom model, which we knew when we started.

In other words, if you design an experiment such that all its components have well established deterministic models, then observers both inside and outside any surrounding boxes will have the same knowledge of the system's status. If you throw in a component for which you "only" have a stochastic model, well, then those on the inside will know more.

 

[leandros_p]

Isn't that equally the case if we replace the cat+atom with a coin flipping machine? ...Which is the same situation as with the cat+atom.

...You cannot do that with the cat+atom (catom?), but that is because we don't have a deterministic catom model, which we knew when we started.

If you understand that the the cat+atom model is a non-deterministic model by itself, then the experiment is not "producing" the result. It just provides non-deterministic information to the observer.

The "quantum inteterminacy" is not a product from the desigh of the experiment.

The "quantum inteterminacy" is made known, as a scientific information, by the design of the experiment. Each experiment is designed in order to acquire information. This expreriment provides the "information" of "inteterminacy", but it does not produce this "information".

Leandros

 

[vanesch]

vanesch --
A knowledge based QM interp, as with any probabilistic theory, necessarily obeys unitarity.

I don't understand what this could mean.

As usually formuted, QM is local, point interactions and all that.

The *unitary* part of QM is local, yes.

I wonder what it could possibly mean for something to interact "locally" if it is just a knowledge description. What does it mean that my "knowledge of electron A" interacts locally with "my knowledge of proton B" ?
Assuming that this is not the objective state of the electron A or the proton B, I don't see what can be "local" to it, and why my 'knowledge of electron A' cannot have any interaction with my knowledge of muon C, which is - or rather, I know that it is - 7 lightyears from here.
So how do you implement something like lorentz invariance for knowledge ?

I've always thought of MWI as odd. Seems to me that it's just another attempt to subvert probability. If it is such a great idea, why has it not been in place since at least Fermat's notions about games of chance?

The reason for MWI is of course NOT to circumvert probability or something. In fact (although many MWI proponents trick themselves IMO into the belief that they can do without it - I'm convinced that they are wrong, and that probabilistic concepts are needed also there) the only reason for MWI is to be able to take the wavefunction as an objective description of reality. You run into a lot of problems and paradoxes if you take the wavefunction as describing objective reality while accepting the probabilistic projection, but the problem is not the probabilistic aspect of it ; the problem is twofold:
1) the fact that all elementary interactions between quantum systems are described as strictly unitary operations on the quantum state (its derivative being a Hermitean operator which we call the Hamiltonian) - so there is no known mechanism to implement a non-unitary evolution, which is a projection
2) the fact that this projection cannot be formulated in a Lorentz invariant way

The problem is NOT, the probabilistic aspect.

There is a difference between the relationship between classical physics and probability, and between the quantum state and probability, and that's the following. When we use probability in classical physics, the probability distribution itself plays no physical role.
When a classical system evolves from A to A', and from B to B', then, if we assign probability p1 to A and p2 to B, we'll have an outcome B with probability p1 and an outcome B' with probability p2. If we learn that the system was finally in B', then we can "update backward" our probabilities, say that the system, after all, was in state B, and that A was just part of our ignorance. As you state, there's no reason to introduce a "parallel world" in which A was there, after all, but we happen to be in a universe where B happened.
The reason why this is superfluous is that the numbers p1 and p2 never enter into any physical consideration. They are just carried along, with the classical physics, WITHOUT INFLUENCING THE DYNAMICS.

But in quantum theory, this is not true. If the state is a |u> + b |v> , and if we now evolve this into the state a |u'> + b |v'> and we work now in the basis |x> = |u'> + |v'> and |y> = |u'> - |v'>, and measure x/y, then the probability of having x or y will depend on the numbers a and b. It is not that the coefficients a and b are somehow, a measure of our lack of knowledge, which get updated after the measurement. Because if this were true, there would be no difference between a STATISTICAL MIXTURE of states |u> and |v> and the state a|u> + b|v>
To illustrate that this is not the case, consider a = b. A statistical mixture of 50% |u> and 50% |v> will yield in an outcome which gives us 50% |x> and 50% |y>. Nevertheless, the state |u> + |v> (which has identical statistical value, right) will result in 100% state |x> and 0% state |y>.
So the values of a and b CANNOT be interpreted as describing our lack of knowledge which gets updated during the measurement. It would be hard to imagine that NOT KNOWING something (having non-zero values for a and b) would make it impossible to obtain the outcome |y>, while KNOWING something (like knowing that a = 1 and b = 0) would suddenly make the states |x> and |y> appear 50% each.
So those numbers a and b HAVE PHYSICAL MEANING. They influence what will happen later, and this cannot be seen in a purely "I didn't know, and now I learned" fashion, as probability CAN be seen in a classical context. It is the phenomenon of quantum interference which makes the "knowledge" view of the wavefunction, IMO, untenable.
The state |u> + |v> has simply DIFFERENT PHYSICAL CONSEQUENCES than the state |u>. One cannot say that |u> + |v> expresses our lack of knowledge about whether it is |u> or |v>, while the state |u> expresses our certainty of having the system in state u for sure, because if that were so, then it is strange that a lack of knowledge leads to more certainty (namely, that we will NOT have the result y) than when we know more.

Why is this ideas absent from virtually all books on probability and statistis? (I would say all, but there are such books that I've never read.)Why not a universe in which I broke the Bank at Monte Carlo, when I found and extra million in my bank acount, why not a universe in which the Boston Red Sox won 25 World Series in a row? What good does such speculation bring?

It doesn't bring any good in a classical setting, because of the fact that this "parallel possibility" has no influence what so ever on the physical dynamics. You can say, in this context, that the "parallel universe" where the initial conditions where such that the Boston Red Sox will win 25 World series in a row, has, FROM THE BEGINNING, never been there, and that we just entertained its possibility because we didn't knew all the details. When we did find out that this didn't happen, we could simply scrap this parallel universe from our list with no harm, BECAUSE IT HAS NEVER BEEN PART OF THE ONTOLOGICAL STATE OF THE UNIVERSE in the first place (only, we didn't know).

But when we know that the state is |u>, and we find |x>, we cannot go back 'scrap' somehow a state from our list. We cannot say that it actually meant that the state was actually |u> + |v>, back then. Because we MEASURED u back then, and we found u, and not v. So it is not "an imaginary parallel universe which turned out not to be the right one".

The knowledge approach works, and, I have yet to hear of any practical arguments against it.

The most important argument against it IMO, is that there is no description of reality in this view. It is hard to work with things of which you have constantly to remind yourself that "it isn't really there", and nevertheless devellop a physical intuition for.

I will admit that the stupendous ego of David Deutsch, which permeates his book, The Fabric of Realty, turned me away, in part, from MWI. It's one of these books that says,"Trust me, I'm right." He's a great spinner, probably could do well as a political consultant, with such catchy ideas as shadow photons. Small wonder that his views remain relatively unknown.

Didn't read it. My only attempt was to write a paper showing that his proof was flawed, but (as has been discussed here), it was not accepted.

With all due respect, I have yet to see anything about MWI that solves any problem of QM other than with fanciful suppositions of universes we can never know. Deus ex Machina

You are probably right that it doesn't have much practical implications. In my opinion, the most important function of MWI is to rehabilitate QM as a description of reality, and to be able to put all this positivist considerations aside. As such, it removes all ambiguity about WHEN one should apply the projection postulate, and removes the need of the distinction between a physical interaction and a non-physical measurement. In most situations, this distinction is so clear, that it doesn't need any specific treatment, but in situations such as delayed choice quantum erasers or EPR setups, one can wonder about when one should apply the projection postulate. Well, MWI solves that situation unambiguously.

I would also like to point out that "the universes we can never know" are NOT introduced or postulated. They are simply not ELIMINATED by a projection postulate.