Was Einstein wrong? Einstein vs. Bohr: The EPR experiment

In order to explain the strange properties of quantum, Bohr pioneered the "uncertainty principle", but Einstein disliked this theory very much. In order to refute Bohr's uncertainty principle, Einstein pondered for many years, and finally in May 1935, together with two other scientists, he came up with a thought experiment that could refute Bohr, which is the famous "EPR experiment".

What kind of experiment is this? To understand it, we need to know some prerequisite knowledge. First of all, scientists have discovered through experiments that electrons have the property of "spin" (but please note that the so-called "spin" does not mean that electrons are really spinning like a gyroscope, it is just a property in quantum mechanics), and there is a particularly strange thing about the spin of electrons: there are only two degrees of freedom. For example, if we observe the electron from above, the electron is either spin-up or spin-down. However, if we observe the electron from the side instead, the electron becomes either spin-left or spin-right.

Next, physicists invented a device called a polarizer, which can screen electrons, for example, only allow electrons with spins pointing up to pass through, or only allow electrons with spins pointing left to pass through. When scientists used polarizers to experiment with electrons, an extremely surprising result appeared:

First, we let an electron fly towards this polarizer. If it passes through, it means that the electron is spin-up. Then, put another polarizer behind this polarizer. At this time, the electron passes through the next polarizer 100%, which is exactly in line with people's expectations. If we replace the second polarizer with a right-pointing polarizer, the electron has a 50% chance of passing through polarizer No. 2 (because half of the electrons with spin-up have spins to the left and half have spins to the right). Do the experiment 100 times, and about 50 of them will fly through. The more times you do it, the more accurate it will be.

Next, we will witness a crucial experiment that is extremely strange. We put another polarizer No. 3 facing upwards at the back. According to normal logic, this electron should pass through polarizer No. 3 100%, but what surprised physicists was that the result of the experiment was that this electron still only had a 50% probability of passing through polarizer No. 3, even though both No. 3 and No. 1 were upward polarizers.

This means: it is impossible to accurately measure the spin state of an electron in two different directions at the same time!

Seeing such experimental results, the Copenhagen School headed by Bohr was very happy. Bohr believed that this was the best evidence of the uncertainty principle of electrons, and that electrons themselves do not have a definite spin state. Before measurement, electrons are in a superposition state of all spin states. It is meaningless to ask which state it is in!

It was in this context that Einstein proposed the EPR experiment:

First, we prepared a pair of electrons with a total spin of zero in the laboratory (this was theoretically possible, but it was not technically possible at the time).

Then, we let this pair of electrons separate, the blue electron flies to the left, and the red electron flies to the right, and let them separate far enough, for example, one flies to Shanghai and the other flies to Beijing. We put a polarizer in Beijing and Shanghai respectively. Now, suppose both electrons pass through the polarizer, and suppose the red electron spins up, because the total spin is zero, so it can be seen that the blue electron must spin down. Suppose the blue electron passes through the right polarizer, that is, the blue electron spins to the right, then the red electron must spin to the left. In this way, we have determined the spin states of the red and blue electrons in two directions.

It can be seen that it is not that electrons have some magical superposition state. The uncertainty principle is essentially because the measurement behavior interferes with the spin state of electrons. As long as we do not measure it, their spin state remains certain!

This trick is so powerful! It makes people feel impeccable. In 1935, the entire physics community was paying attention to this EPR experiment. Of course, Bohr would not sit idly by and wait for death. He later fought back: There is a key assumption in the EPR experiment that is wrong, that is - measuring the behavior of red electrons will not affect blue electrons, and measuring blue electrons will not affect red electrons. But in fact, red and blue electrons are in a magical quantum entanglement state. No matter how far apart they are, as long as one is measured, it will immediately interfere with the other.

This is the famous EPR experiment dispute.

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Science Voice

Reviewer: Zhou Xiaoliang, Senior Engineer of Beijing Jiaotong University Physics Laboratory

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.