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The Copenhagen interpretation of quantum mechanics contains the view that there are no deterministic hidden variables that lead to the probability of the quantum world. This means that it is usually impossible to predict the results of any measurement with certainty, and there is no deeper reality hidden under quantum mechanics. But there are other theories that embrace determinism and have been looking for these hidden variables.

In 1932, John von Neumante claimed to prove that there were no hidden variables. However, the validity of his proof was questioned by the philosopher Reichenbach. Albert Einstein insisted that quantum mechanics could not be a complete theory and that his preferred argument depended on the principle of locality. Together with Boris Podolski and Nathan Rosen, he proposed a thought experiment, now known as the EPR paradox.

The thought experiment involves a pair of particles in an entangled state. If you measure the position of the first particle, you can predict the position of the second particle. Similarly, if the momentum of the first particle is measured, the momentum of the second particle can also be predicted. They believe that any action taken on the first particle cannot immediately affect the other, because this will cause information to travel faster than the speed of light, which is not allowed by the theory of relativity.

They believe that if there is no way to distribute the system, we can definitely predict the value of a physical quantity, then there must be a realistic element corresponding to that quantity. From this, they inferred that the second particle must have a definite position and momentum value before it can be measured. This contradicts quantum mechanics because Heisenberg's uncertainty principle requires that position and momentum cannot be determined at the same time. They assume that these elements in reality are localized because each element belongs to a point in space-time.

Bohr responded that he thought their reasoning was wrong because the measurements of position and momentum were complementary and choosing to measure one ruled out the possibility of measuring the other. For Einstein, the most important problem he saw was nonlocality, and the measurement of particle A resulted in two different quantum states of particle B. He believes that because of the localization, the real state of particle B cannot depend on which measurement of An is made, so the quantum state cannot be related to the real state.

In 1951, David Bohm proposed a variant of the EPR thought experiment. If we use the source that emits electron-positron pairs, where electrons go to point An and positrons go to point B. Alice is our observer at point An and Bob is our observer at point B. If we assume that these pairs are entangled, there are two cases: the spin of the electron is up and the positron is down; the electron spin is down and the positron is up. Because the particles are in a superposition state, it is impossible to know the spin state of any particle without measurement.

Suppose Alice measures the spin now, and if she measures the spin up, this means that if Bob is going to measure the spin now, we will be 100% sure that he will measure a downward spin. In fact, the spin may not go up and down but along the X or Y axis, and if Alice measures a positive X spin, Bob then measures a negative X spin. No matter what axis their spins are measured, they are always opposite.

In quantum mechanics, X spin and Z spin are incompatible observables. So if Alice measures Z spin and gets a positive Z result, and now Bob violates the rules to measure X spin, in this case, Bob has a 50% chance of measuring positive X. When measured along the same axis as Alice's electron, Bob's positron will have a definite spin, but when measured on different axes, its spin will be uniform and random. This seems to mean that the information is transmitted instantly from Alice's position, giving Bob's positron a definite spin.

In 1964, Bell began to study whether hidden variables could be used to solve non-localized problems. He showed that when Alice and Bob were both measuring on the same axis, hidden variables could explain this. However, when any angle measurement is allowed, the theory of locally hidden variables becomes unable to reproduce the quantum mechanical correlation, which leads to the so-called Bell inequality. Bell then showed that quantum physics predicted correlations that violate this inequality. The only way hidden variables can explain the prediction is that they are non-localized, which means that no matter how far apart the two particles are, they can interact immediately.

Is there only one way to look at Bell Theorem? In the Copenhagen interpretation, the violation of Bell inequality is regarded as a reason for rejecting the hypothesis commonly referred to as "counterfactual certainty". This means that it is impossible to meaningfully talk about the certainty of the results that have not yet been carried out, in other words, that the unperformed experiments have no results.

There are other theories that explain this in very different ways. Multi-world interpretation, also known as Everett interpretation. Hugh Everett, an American physicist, first proposed a multi-world interpretation of quantum physics. Unlike the Copenhagen interpretation, the wave function never collapses, and it believes that all the possibilities of quantum superposition are objectively real. It can produce a correlation that violates Bell's inequality because it violates Bell's implicit hypothesis.

In Everett's view, there is something wrong with our concept of reality. We think that there is only one measurement result, but in fact all of these results have occurred, and we can only see one of these realities, and the other reality has a separate physical existence. Therefore, the whole universe can be thought of as a huge wave function that contains all possible realities. It begins with the superposition of all possible states of all particles, and as it evolves, some of them superimpose and decompose, making certain realities different and isolated from each other. So measurement behavior doesn't really create a new reality, it just separates it from other realities.

When a concept called decoherence was developed in the 1970s and 1980s, the theory was given new life, providing a clear reason for the division of the quantum world. Using the whole concept, the division of the world is not a sudden event, but through decoherence evolution. The main scientific attraction of this concept is that it does not require any changes or additions to the standard mathematical representation of quantum mechanics, that the wave function does not mysteriously collapse, and that its predicted results are exactly the same as what we have observed.

Although multi-world interpretation eliminates the most annoying aspect of nonlocality: long-distance interaction, other aspects of quantum nonlocality: the inseparability of remote objects in the form of entanglement still exists. There are other explanations such as stochastic mechanics, guided wave theory and so on, which we will talk about later.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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