copenhagen interpretation
Since the 1920s, many physicists believe that in quantum mechanics, the state of microscopic particles is described by wave functions. When microscopic particles are in a certain state, their mechanical quantities (such as coordinates, momentum, angular momentum, energy, etc. Generally, there are a series of possible values, and each possible value appears with a certain probability (when a macroscopic object is in a certain state, its mechanical quantity has a certain value). In other words, the motion of microscopic particles is uncertain and probabilistic. Wave function can describe the probability of microscopic particles in space.
The famous "single electron double slit interference" experiment in physics is the embodiment of the uncertainty and randomness of microscopic particle motion. In this experiment, a single electron actually interferes after passing through a double slit. According to classical mechanics, electrons can only pass through one slit at the same time, and it is impossible to pass through two slits at the same time and interfere. According to quantum mechanics, the motion state of electrons exists in the form of wave function, and electrons may pass through this slit and that slit at the same time and interfere. However, when scientists try to determine which slit the electron has passed through through through the instrument, they can always find the electron only in one of the slits. Neither instrument can detect electrons at the same time, and electrons can only pass through one slit at a time. It seems that the observation behavior of the measurer has changed the motion state of electrons. What is the explanation for this abnormal phenomenon? Physicist Bohr put forward the famous "Copenhagen explanation": when people are not observing, electrons have the probability of existence in both slit positions; But once the electron is detected, for example, at the left seam position, the electron has an accurate position, and its probability at this point is 1, and the probability at other points is 0. In other words, the wave function of the electron "collapses" to this point at the moment it is measured.
Bohr introduced the observer and his consciousness into quantum mechanics, linking it with the motion state of microscopic particles. However, the explanation of observers and "collapse" is not very clear and convincing, and it has also been questioned by many scientists. For example, how did the collapse happen, was it instantaneous, or did it wait until photons entered people's eyes and excited electric pulse signals on the retina?
many worlds interpretation
Then, is there any way to bypass the so-called "collapse" and "observer" and eliminate the subjective component of the observer from the physics that should study objective laws?
Everett put forward a bold idea: if the wave function does not "collapse", it must keep increasing linearly. That is to say, the electrons in the above experiment are still in the superposition state of left/right slits even after re-observation. Everett further pointed out that the human world is also superimposed. When an electron passes through a double slit, not only the electron but also the whole world is in a superposition state. In other words, when electrons pass through the double slit, two superimposed worlds appear. In one world, electrons pass through the slit on the left, while in another world, electrons pass through the slit on the right. In this way, the wave function does not need to "collapse" to randomly select left or right, because it shows the superposition of two worlds: people living in one world find that electrons pass through the slit on the left, while in the other world, the electrons observed by people are on the right. In the case of Schrodinger's cat, Everett points out that both cats are real. There is a live cat and a dead cat, but they are in different worlds. The question is not whether the radioactive atom in the box decays, but whether it decays. When the observer looks into the box, the whole world splits into two versions. The two versions are the same in all other respects. The only difference is that in one version, atoms decay and cats die; In another version, the atom has not decayed, and the cat is still alive. As mentioned earlier, "the atom decays and the cat dies;" The atom has not decayed, and the cat is still alive. " These two worlds will completely evolve independently and in parallel, just like two parallel worlds. Quantum process creates "two worlds", which is Everett's avant-garde "multi-world explanation".
The advantage of this explanation is that the Schrodinger equation will always hold and the wave function will never collapse, which simplifies the basic theory. Its problem is that the idea is too bizarre, and the price paid is that these parallel worlds are equally real. No wonder some people say, "In the history of science, multi-world interpretation is undoubtedly the most daring and ambitious theory put forward at present."