Exploring the EPR Paper: Unpacking the Bell Papers and Einstein's Views

In summary, the conversation discusses the EPR paper and the concept of physical reality. The statement from the paper states that if a system can be predicted with certainty without disturbing it, then there exists an element of physical reality corresponding to it. However, the speaker questions this logic and how it applies to predicting the state of a system. They also mention the two distinct sections of the paper, with the first discussing non-commuting operators and the second dealing with a gedanken conjecture. The speaker also brings up the issue of classical conjugate variables and how it relates to the concept of physical reality. They also note some issues with the first half of the paper and the assumption of a constant momentum in the wave function.
  • #1
LAncienne
20
0
I'm slowly working my way through what I need to really understand the Bell papers. Everyone says that a good start is the EPR paper. I understand that Einstein may not have read the final version before publication and may not even have wanted his name appended (sort of convoluted logic and clumsy wording, and Einstein was actually really good at this stuff!) There is a statement direct quote: . If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity. .Somehow, to me this doesn't seem to hold water. Maybe someone can clarify it. How can I predict the state of a system, much less without disturbing it? If I'm driving a car at 60 miles an hour, I can look at the speedometer and know I am going that speed, but even there, there is no prediction. The world doesn't seem to have dials and meters attached to it in any reasonable way, so, what the heck is meant by this statement?

I know they use this definition later in the two particle wave function argument, but that seems to be using what they considered a result to really come up with the definition. Fairly circularly transparent, it seems to me. Am I missing something?
 
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  • #2
LAncienne said:
I'm slowly working my way through what I need to really understand the Bell papers. Everyone says that a good start is the EPR paper. I understand that Einstein may not have read the final version before publication and may not even have wanted his name appended (sort of convoluted logic and clumsy wording, and Einstein was actually really good at this stuff!) There is a statement direct quote: . If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity. .Somehow, to me this doesn't seem to hold water. Maybe someone can clarify it. How can I predict the state of a system, much less without disturbing it? If I'm driving a car at 60 miles an hour, I can look at the speedometer and know I am going that speed, but even there, there is no prediction. The world doesn't seem to have dials and meters attached to it in any reasonable way, so, what the heck is meant by this statement?

I know they use this definition later in the two particle wave function argument, but that seems to be using what they considered a result to really come up with the definition. Fairly circularly transparent, it seems to me. Am I missing something?

Yes, the issue that he is referring to is as follows: Say I have two pairs of socks of different colors. I pull out one member of a pair and give to Alice. I give the other sock to Bob, and tell both of them not to look at the color. Alice flies to Atlanta and then looks at the sock, and it is yellow. She immediately knows that Bob, who flew to Boston, has a yellow sock too. (Yellow is my favorite color, by the way.)

So according to EPR, Alice was able to predict, with certainty, the results of an investigation into the color of the sock given to Bob. And she is able to do this without disturbing Bob or Boston in any way. Therefore, according to EPR, there is an element of reality to the color of Bob's sock and that is a completely objective statement. It is not observer dependent.

BTW, I wouldn't worry too much about Einstein's objections to the EPR paper itself. He definitely believed the conclusion, as he restated it famously: "I think that a particle must have a separate reality independent of the measurements. That is: an electron has spin, location and so forth even when it is not being measured. I like to think that the moon is there even if I am not looking at it." That was an opinion that was held by many for years. Of course, now we know about Bell but Einstein didn't live that long.
 
  • #3
Thanks for your comments and explanation Dr. Chinese (interesting moniker ;-{) ) I probably should have included the rest of the paragraph after the excerpt that I quoted, vis:It seems to us that this criterion, while far from exhausting all possible ways of recognizing a physical reality, at least provides us with one such way, whenever the conditions set down in it occur. Regarded not as a necessary, but merely as a sufficient, condition of reality, this criterion is in agreement with classical as well as quantum-mechanical ideas of reality.. This still seems to be a pretty self-serving (regarding the argument in the second half of the paper) definition, and there may be others definitions that obviate the arguments in the first half.

The paper seems to be in two quite distinct sections, with the first dealing with non-commuting operators and the HUP and the second with their gedanken conjecture. I keep harking back to classical conjugate variables which are Fourier transforms of each other. Classically, one can not say that, because I do not know (and may not even be able to determine) the value of some physical parameter that it does not exist and hence is not "real" in some sense.

There seems to also be a bit of fancy footwork in the first half in making the momentum in the wave function a constant (hence surprize , surprize, it is measured as a delta function fixed value). No need for a reply, just thinking out loud (okay, in writing!)
 

What is the EPR paper?

The EPR paper refers to a scientific paper titled "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?" published in 1935 by Albert Einstein, Boris Podolsky, and Nathan Rosen. It is a highly influential paper in the field of quantum mechanics and has sparked many debates and discussions among scientists.

Who were the authors of the EPR paper?

The authors of the EPR paper are Albert Einstein, Boris Podolsky, and Nathan Rosen. All three were renowned physicists of their time and made significant contributions to the field of theoretical physics.

What is the significance of the EPR paper?

The EPR paper is significant because it introduced the concept of "entanglement" in quantum mechanics and raised questions about the completeness of quantum mechanics as a theory. It also sparked debates on the interpretation of quantum mechanics and the role of hidden variables in the theory.

What is the Bell inequality?

The Bell inequality is a mathematical inequality proposed by John Stewart Bell in 1964 as a way to test the validity of Einstein's views on quantum mechanics. It states that certain correlations between quantum particles cannot be explained by local hidden variables, thus challenging Einstein's belief in a deterministic universe.

How has the EPR paper influenced modern physics?

The EPR paper has had a significant impact on modern physics, particularly in the field of quantum mechanics. It has led to further research and experiments on entanglement, hidden variables, and the foundations of quantum mechanics. It has also influenced the development of new theories and interpretations of quantum mechanics, such as the Many-Worlds interpretation and the Copenhagen interpretation.

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