A reader asks me to explain
my view of the EPR Bohm paradox
, since I seem to reject what everyone else says.
Actually, my view is just textbook quantum mechanics. I do not subscribe to hidden variables, superdeterminism, action-at-a-distance, or anything exotic.
I am also a logical positivist
, so I stick to what we know, and try not to jump to unsupported conclusions.
Experiments show that electrons and photons have both particle and wave properties. Quantum mechanics teaches that an electron is a mysterious quantum
that is not exactly a particle or a wave, but something else. It obeys various physical laws, like conservation of energy and momentum, and Heisenberg uncertainty.
The EPR paper looks at a physical process that emits two equal and opposite electrons. Only I prefer to call them quanta, because they are not really particles with definite position and momentum at the same time.
Mathematically, the two quanta are represented as a single quantum state. A measurement of one collapses the state, according to the rules of quantum mechanics. Quantitative predictions are in excellent agreement with experiment.
In particular, you can measure the momentum of one quantum, and know that the other must be equal and opposite. Physically there is nothing strange about this, as it is a consequence of momentum being conserved.
But it is a little strange if you combine this with Heisenberg uncertainty, which normally prevents us from making a precise statement about momentum until it is measured. Measuring one quantum allows us to say something about a distant quantum.
Bohm pointed out that you get the same paradox when measuring spin, and tried to argue for hidden variables.
One way people have tried to explain this is with action-at-a-distance, with measuring one quantum having an instantaneous physical effect on the other. But that is so contrary to everything else we know about physics, then such an explanation would only be a last resort.
Another is to model the quantum with hidden variables. All such attempts have failed. In particular, if you think of the quantum as having a definite position, momentum, and spin before the measurement, you get contradictions with experiment.
So what is left? There is still the original Bohr logical positivist view. Things are clearer if you avoid trying to answer unanswerable questions. Physics is about observables. Concepts like position, momentum, and spin are not really properties of quanta. They are properties of observations.
We have no true mathematical representation of the quantum. We have a mechanics of observables. We predict observations, and avoid trying to say what is not observed.
When people try to visualize the quantum, they inevitably form some classical (pre-quantum) picture that we know is wrong. So they get paradoxes and say that quantum mechanics is incomprehensible.
Or they try some mathematical model that ends up being a hidden variable model, and it does not work.
So again, here is what happens. A physical process emits two equal and opposite quanta. They seem particle-like and wave-like, but they are physical objects that lack a true mathematical formulation. From the physics, we know that they are equal and opposite, and from quantum mechanics formulas, we can make certain predictions about observables. In particular, observations about the two quanta are correlated. Correlation is not necessarily causation.
Does some physical process like radioactive decay determine the state of an emitted quantum? I have an open mind on that, because I don't see how the question makes any sense. How can any experiment tell us one way or the other? You can believe it or not believe it; it is all the same to me.
Physicists make arguments that EPR-like experiments prove true randomness. I have posted denials of that, and I do not even believe that there is any such thing as true randomness. Randomness is a mathematical formalism that is actually deterministic, or a figure of speech for how certain theories fail to predict certain outcomes. That's all. When physicists talk of true randomness, they are usually talking nonsense.
What about the quanta being separable? Action-at-a-distance seems like hokum to me. It is as implausible as a perpetual motion machine. There is no evidence for it, and a lot of reasons for thinking it impossible.
I say that the physical quanta are separate and cannot influence each other. At the same time, our knowledge of the two quanta are linked, and info about one tells us something about the other.
In the terminology of Bell's theorem
, I am rejecting counterfactual definiteness
, just as all mainstream physicists have for decades.
Counterfactual definiteness says that the photons in the double-slit experiment
must entirely go thru one slit or the other, just as if they were measured as particles at the slits. But mainstream quantum mechanics teaches that this is completely false, and such measurement would destroy the interference pattern. The light goes thru both slits at once.
You cannot do a completely passive observation of a quantum giving it a particular position, momentum, or spin. Any such measurement changes it, because quanta do not have such properties.
Rejection of counterfactual definiteness is essential to XX century physics, and is embodied by these slogans:
Another thing that people have emphasized since quantum mechanics was developed is the idea that we should not speak about those things which we cannot measure. (Actually relativity theory also said this.) [Feynman]
Unperformed experiments have no results. [Peres]
Somehow the 21st century has brought us more and more physicists who would rather believe in spookiness or parallel universes. A serious disease has infected Physics.
You will probably say I am cheating because I am not seeking a complete mathematical description of an electron, or that it is a cop-out to say that the wave function is just a representation of our knowledge.
My answer is that this issue goes to the heart of what science is all about. The job of physics is to use mathematics to predict outcomes for experiments. It is not to provide mathematical representations for things you cannot observe. All of the EPR paradoxes are based on naive expectations for counterfactuals, and not observations. Stick to observables, and the experiments are not so strange.