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Does quantum theory really undermine determinism?

John Salerno

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Ok, be gentle with me please, my physics is poor and English even worse! What I can't follow with all of this is the following. Schrodinger developed wave equations that model particle behaviour at <macroscopic scale, and these behaviours also tend to be lost at >macroscopic scale. As far as I know or understand, these equations are built on the principles of standing waves. This would suggest to me that there is a defined position. But now HUP comes along and says that defining the areas that a wave-particle will occupy can only be accomplished statistically and through probabilities, which to me contradicts the proposition that such elements could therefor be standing-waves. I have Messiah's book, translated into English (I might like to get the original French version someday) but I can only manage the first few chapters as it stands.


A few points.


1. The Schrodinger equation really describes the evolution of the state function over time. It is not a wave equation in the strict sense, and it has nothing to do with standing waves. The Schrodinger equation is completely deterministic and the stochastic nature of QM is not reflected directly in it. However, in order to obtain a measurement of a physical quantity what one does is apply the Hermitian operator correcponding to the desired observable to the state function, which will produce the probability of the measurable being one of the eigenvalues of the operator. It is the interpretation of the Schrodinger equation that results in the stochastic nature of quantum mechanics.


2. Contrary to much that you read the Schrodiinger equation is not the entire story, not by a long shot. The Schrodinger applies only to elementary non-relativistic quantum mechanics. It does not apply to the more accurate and sophisticated quantum field theories, which are relativistic.


3. QM is inherently stochastic. See my earlier post. And this is not a result of the HUP. In fact the real effect of the Schrodinger equation, combined with the operators that represent observables, is simply to describe the time evolution of probability density functions. The HUP simple reflects the fact that complimentary operators do not commute (the order in which you apply them makes a difference).


4. Messiah is a good book. But you might find reading the third voluem of the Feynman Lectures on Physics enlightening. It is accessible and lets you see quantum mechanics through the eyes of a true master of the subject.


5. I tend to like Feynman's perspective: There is no such thing as wave particle duality. Elementary particles are particles. But they are not Newtonian marbles.

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you might find reading the third voluem of the Feynman Lectures on Physics enlightening. It is accessible and lets you see quantum mechanics through the eyes of a true master of the subject.


I'll definitely give this a try, I often find I pick too difficult a text for a first read and I'm left baffled. Having an overview that allows me to reach the concepts first is probably best. I've been looking to Phys Chem texts to fill in the gap. I think however that these texts have been supporting the very views that you have just pointed out as being maybe too strict in their adherence to ideas that might be better stated in other ways. They also tend to put a good deal of emphasis on the Bohr model as it applies to ideas related to equipartition theorem and--as Swansont has mentioned before--this isn't a proper way to address QM.


Appreciated! :D

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  • 3 weeks later...

Gah, I really want to wrap my head around this but it just doesn't make sense! I've read these replies, I've read Wikipedia, I'm currently reading "The Grand Design," but I still don't understand this concept (from Wikipedia):


The uncertainty principle states a fundamental property of quantum systems, and is not a statement about the observational success of current technology.


I just don't see how the uncertainty principle is anything other than a description of the problems with measuring techniques, rather than an inherent property of the system being measured. I fully understand how measuring a particle's position, for example, will affect it's momentum, but that still seems like a problem with current measurement technology rather than with the properties of the particle itself. Why isn't it the case the particle still really does have a specific position and momentum, and the problem is simply that we cannot measure them both simultaneously? I don't understand why the uncertainty principle HAS to mean that a particle never has a specific position and momentum at any given time.


Finally, in reading "The Grand Design," a distinction seems to be made between the uncertainty principle and the idea that measuring something will necessarily change it. Are these two different ideas, or are they the same thing? I thought that the uncertainty principle was basically saying if you measure one thing, it changes something else. But the book seems to be suggesting these are two distinct ideas in quantum physics.



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Uncertainty is an inherent property of quantum reality. The measurement problem is used to explain the situation because most people find it very hard to believe reality is so strange. Go back and re-read some of the excellent guidance you've been given in previous posts. Oh, and a very good authority says 'The Grand Design' is a sell-out and not up to the standard of his previous books ( I haven't read it yet myself ), and you'd be better served by the Feynman lecture series ( which I have read and are excellent ).

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Oh, and a very good authority says 'The Grand Design' is a sell-out and not up to the standard of his previous books ( I haven't read it yet myself ), and you'd be better served by the Feynman lecture series ( which I have read and are excellent ).


I don't know who that "authority" is, but thus far I'm finding the book to be very similar to A Briefer History of Time. It covers a lot of the same material, but it also discusses M-theory in more depth (which I think is the topic of the next chapter I have to read). I guess it depends on what is meant by "sell-out."

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  • 1 month later...

I'm currently reading Stephen Hawking's book "A Briefer History of Time" and I was a little confused by the suggestion that the uncertainty principle undermines determinism, as stated in this sentence from the book:




As explained in the book, the uncertainty principle states that the more accurately the position of a particle is measured, the less accurate the measurement will be of the particle's velocity. Therefore, the "initial conditions" of the system can never be accurately known in order to determine the past or future state of the universe.


This much is easy to understand, but it seems that the uncertainty principle really only undermines our ability to calculate these past or future states of the universe, not that it actually undermines the fact that the universe still is deterministic (not that it necessarily is, but the uncertainty principle as stated above doesn't seem to suggest otherwise), even if we can never calculate any given state of it because of this uncertainty.


However, I have seen elsewhere that perhaps the uncertainty principle suggests more than the simple explanation given in the book. That, in fact, it explicitly says that the position or velocity of a particle is not actually determined at all until it is measured. This seems to really hurt determinism, but is that really what the uncertainty principle says? That a particle's position or velocity essentially doesn't exist at all until we measure it, or is it simply that we can never know a particle's position or velocity for sure until the time of its measurement?


Basically, my question comes down to this: even given the uncertainty principle and the probabilistic nature of quantum mechanics, couldn't it still be the case that the universe is perfectly deterministic, even if we can't accurately make the measurements to determine these past or future states ourselves? Doesn't a particle still have a certain position and velocity at any given time, even if measuring one of these will then change the other?


Hawking hints at this idea:




Frankly, I think the mention of a "supernatural being" unnecessarily muddies the discussion and makes Hawking dismiss the idea too easily. It's not necessary to postulate anything supernatural in order to retain determinism.


The above quote could easily have been stated as something like this: "We could still imagine that there is a set of laws that determine events completely, despite our inability to measure the present state of the universe without disturbing it." Then the second sentence would be irrelevant, because it would be of interest to us to know that it is possible for the universe still to be perfectly deterministic, even if we can't (yet) discover all the laws.


Yes quantum theory undermines determinism, but it has nothing to see with the uncertainty principle, but with the non-unitary evolution of the quantum state (vonNeuman postulate).

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