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In the theoretical system constructed by science, we do not belong to this material world. We are not in this world, outside the world, we are just observers of the world. The reason for thinking that we are in this world is that our bodies are in this world, not only my body, but also my friends, cats, dogs, houses and all creatures around me in this world, which is the only way for me to communicate with them.

-- Erwin Schrodinger (1887-1961)

Fig. 1 there are many long-standing thought experiments in Schrodinger equation quantum mechanics, most of which are used to point out flaws in quantum mechanics. Erwin Schrodinger, a theoretical physicist and one of the pioneers of quantum mechanics, envisioned such a thought experiment:

Keep a cat in a closed container containing radioactive sources and poisonous gases. The radioactive source has a certain probability of decay per unit time. When the decay of the radioactive source is detected, the toxic gas will be released and the cat will die; if the radioactive source does not decay, the cat will survive.

The Copenhagen interpretation (Copenhagen interpretation) in quantum mechanics points out that the properties of physical systems are not definite and can only be measured by the probability terms of quantum mechanics, and the measured behavior will affect the system, causing the probability set to be reduced to one of many possible values, which is called wave function collapse (wave-function collapse).

For example, before you look at the moon, the moon is in any possible state, such as a full moon, a half moon, a new moon, etc. But as soon as you look at it, the moon will collapse to a possible state. Therefore, observation plays an important role in quantum mechanics. Consider the thought experiment above, which means that after a period of time, the cat lives and dies at the same time. When you look into the box, you will see at this moment whether the cat is alive or dead, not both dead and alive.

There is a question here, that is, when does the quantum superposition state end and when does it collapse to one of the possible states? Quantum superposition does not apply to large objects, such as cats, because living things cannot live and die at the same time. Therefore, Schrodinger decided that there must be inherent defects in the interpretation of Copenhagen. Schrodinger's cat tries to point out how strange this interpretation really is by expanding the scale of the Copenhagen interpretation.

You have a box containing cats, and there is a superimposed particle that satisfies this setting: when the particle is in one state, the cat will be poisoned to death; when the particle is in another state, the cat will be safe.

Since the survival state of the cat is determined by the state of the particles, if the particles are in the superposition state, the cat must also be in the superposition state.

According to the Copenhagen interpretation, the cat is actually both alive and dead. When you open the box, the cat will be in a state. Here is an important question about observe: are cats observers?

In order to understand this, you need to understand the concept of "observer". The observer in quantum mechanics is closely related to the observer effect-in which the observed behavior is bound to interact with the observed object and affect the characteristics of the object in the interaction. The cat has the same right to be called an observer as the person who opens the box, because it must be able to tell whether the particles decay based on the presence of toxic gases. The cat will constantly observe the air in the box and permanently collapse the vial containing the poisonous gas to a "broken" or "intact" state.

It's not as understandable as it seems.

Here, the person who opens the box and the cat inside the box can be regarded as either an observer or a non-observer. According to this logic, the bottle that releases the gas can be counted as measuring equipment to detect decay, so can it also be regarded as an observer or a detector? Isn't it constantly collapsing the wave functions of radioisotopes? At which point in the chain reaction is the superposition destroyed?

This leads to another question: what exactly is an observer? Can any object be regarded as an observer? If everything is an observer, why do we see quantum effects?

In order to solve this problem, American physicist Hugh Everett (Hugh Everett) proposed a beautiful interpretation called the Multi-World interpretation of Quantum Mechanics (Many-Worlds Interpretation), which was first published in his doctoral thesis in 1957.

Fig. 3 Hugh Everett (second from right) (from left to right: Charles Missner, Hale Trott, Niels Bohr, David Harrison) interprets that the general wave function is objective and real, and there is no wave function collapse. This means that all possible results of quantum measurement are physically realized in some "world" or universe. Therefore, this strange idea looks at the whole thought experiment from another angle, which shows that the "Schrodinger's cat" paradox in quantum mechanics is a series of events, and each quantum event is a bifurcation point. Cats live or die even before the box is opened, but these "living" or "dead" cats are real in different branches of the universe, but they are not related to each other.

Figure 4: the separation of the universe caused by two superimposed and entangled quantum mechanical states in 1935, this example presented by Erwin Schrodinger, an Austrian physicist and Nobel laureate in physics, points out that there is an illogical contradiction between what quantum theorists believe to be the properties and behavior of matter at the micro level, there is an illogical contradiction between the properties and behavior of matter that ordinary people think is visible to the naked eye.

Figure 5 Erwin Schrodinger over the years, Schrodinger's cat has been used as an analogy to explain the differences between emerging theories in quantum mechanics. For example, in the theory of multi-world interpretation, cats are both dead and alive, because the observer and the cat represent two realities: one cat is dead and the other is alive. Schrodinger himself is said to have said in his later years that he wished he had never seen the cat.

When observing a very microscopic world, particles and waves are equally important. On this scale, we cannot use known experience to judge the laws of motion of things. All experience is wrong, and there is no similar physical example to compare what is going on inside the atom. At this time, atoms are like atoms, and there are no other metaphors.

-John Grippin (John Gribbin)

Original text link:

Https://www.cantorsparadise.com/schr%C3%B6dingers-cat-the-most-bizarre-thought-experiment-in-physics-94fb3b0e962b

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop), author: Physics History, translator: Nuor, revision: zhenni

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