What is a Schrodinger cat? This has to start from the beginning. E. Schrodinger (1887- 196 1) is a famous Austrian physicist and one of the founders of quantum mechanics. He won the Nobel Prize in Physics with 1933. Quantum mechanics is a theory that describes atoms, electrons and other microscopic particles. The microscopic laws it reveals are similar to those seen in daily life. The state of microscopic particles in so-called "superposition state" is uncertain. For example, electrons can be located in several different places at the same time and will not appear in a certain place until they are observed and measured (observed). If this happens in the daily life of the macro world, it is like: where I am at home is uncertain. When you look at me, I suddenly appear somewhere-living room, dining room, kitchen, study or bedroom. Before you looked at me, I was invisible at home like a cloud, wandering between walls. This kind of "magic" is ridiculous to ordinary people, and physicists such as Schrodinger can't figure it out. So Schrodinger invented this paradox to attract attention. Sure enough! Physicists have been arguing endlessly.

Experimental content

Put a cat in a closed box, and then connect the box to an experimental device with radioactive nuclei and a container full of toxic gases. Imagine that this radioactive nucleus has a 50% probability of decay within an hour. If it decays, it will emit a particle, which will trigger the experimental device to open the container containing toxic gas, thus killing the cat. According to quantum mechanics, in the absence of observation, the nucleus is in the superposition state of decay and undecomposed. But if the box is opened an hour later, the experimenter can only see two situations: "decaying core and dead cat" or "undecomposed core and live cat". Schrodinger published a paper entitled "The Present Situation of Quantum Mechanics" in 1935. In the fifth part of the paper, Schrodinger describes the cat experiment, which is often regarded as a nightmare: the Copenhagen school says that before measurement, the state of a particle is fuzzy, and it is in a mixed superposition state of various possibilities. For example, when radioactive atoms decay is completely random. As long as there is no observation, it will be in the superposition state of decay/non-decay, and only when it is actually measured will a state be randomly selected. So let's put this atom in an opaque box and keep it in this superposition state. Now Schrodinger imagines a precise device with ingenious structure. Every time an atom decays and releases a neutron, it will trigger a series of chain reactions. The final result is to break a poison gas bottle in the box, and there is also a poor cat in the box. The thing is obvious: if the atom decays, the poison gas bottle will be broken and the cat will be poisoned. If atoms don't decay, then cats live well. The ingenuity of this ideal experiment lies in that through the causal chain of "detector-hammer-poison bottle", it seems that the "decayed-undecomposed superposition state" of uranium atoms is linked with the "dead-alive superposition state" of cats, so that the micro uncertainty of quantum mechanics becomes macro uncertainty; Micro-chaos becomes macro-absurdity-cats are either dead or alive, and it is impossible to be dead and alive at the same time! No wonder Hawking, a famous British scientist, heard Schrodinger's cat paradox and said, "I'll take a gun and kill the cat!" " "

Schrodinger's Cat's Doubt

If we don't open the lid of the secret room, according to our daily experience, we can conclude that the cat is either dead or alive. These are her two eigenstates. However, if Schrodinger equation is used to describe Schrodinger cat, it can only be said that she is in a superposition state of life and death. Only by lifting the lid can we know for sure whether the cat is dead or alive. At this point, the cat's wave function immediately shrinks from the superposition state to an eigenstate. Quantum theory holds that the cat will never know whether it is dead or alive without opening the lid, and she will always be in a state of superposition of half-dead and half-dead. This seriously violates our daily experience. We either die or live. How can we achieve immortality? Schrodinger sarcastically said: According to the explanation of quantum mechanics, the cat in the box is in a "dead-alive superposition state"-both dead and alive! Wait until you open the box and take a look at the cat before deciding its life and death. (Please pay attention! This is not a discovery, but a decision. One look is fatal! As Prince Hamlet said, "To be or not to be, that is a question." Only when you open the box, the superposition state suddenly ends (in mathematical terms, "collapse"), and Prince Hamlet's hesitation finally ends, do we know the definite state of the cat: dead or alive. The advantage of Copenhagen's probability explanation is that there is only one result, which is consistent with what we have observed. But there is a big problem: it requires the wave function to collapse suddenly. But there is no formula in physics to describe this collapse. Nevertheless, for the sake of pragmatism, physicists have long accepted Copenhagen's explanation. The price paid is: violation of Schrodinger equation. No wonder Schrodinger has been bitter.

Natural reasoning

When they are all locked in the box, the atom is in the superposition state of decay/non-decay because we have not observed it. Because the state of atoms is uncertain, so is the state of cats. Only when we open the box and have a look, the matter is finally decided: either the cat dies in the box or it is alive and kicking. The question is, what was the state of the cat before we opened the box? It seems that the only possibility is that it is in a superposition state like our atom, and the cat is caught in a dead/alive mixture. A cat died and lived at the same time? In the superposition of neither dead nor alive? This conflicts with common sense too much, and at the same time, it is also a strange talk from a biological point of view. If a live cat comes out of the box, if it can talk, will it describe the strange feeling of dead/alive superposition? I'm afraid it's unlikely. In other words, the concept of "Schrodinger cat" was put forward to solve the grandmother paradox brought by Einstein's theory of relativity, that is, parallel universe theory. Schrodinger's cat paradox actually raises a very important question: what is the observation of quantum mechanics? Observation or measurement is related to human subjectivity. People are outside the box and need to open the box to decide whether the cat lives or dies. As we all know, the life and death of the cat in the box is determined by the decay of uranium-the cat was alive before the decay and died after the decay, regardless of whether someone opened the box for observation. So the problem lies in the subjectivity of observation, and we should go deeper in this direction. Microscopic observation is different from macroscopic observation. Macroscopic observation has no effect on the observed object. As the saying goes, "Take a look at the head office." It means it has no effect on what you see, so don't worry. Microscopic observation has an influence on the observed object and will cause changes. Take the observed electrons as an example, only lighting can see them. The minimum unit photon energy of light is small, but it is not zero. Photons shine on the observed electrons, which has a great influence on them. So a look at the micro world will also cause trouble! According to quantum mechanics, the observation results change the state of the observed object: a state jumps out of the original uncertain superposition state. Further investigation, observation is nothing more than the interaction between observation means (such as photons) and observed objects (such as electrons), which is not necessarily related to observers, and observers can use detectors and other instruments instead. After decades of exploration, physicists finally realized that in the transition from superposition state to deterministic state, the role once played by observation should be replaced by interaction, which is not only more common but also more objective. Speaking of Schrodinger's cat paradox, we can completely rule out people's subjective factors-the life and death of cats can't be decided by people at a glance. The reader will say, "Isn't it just an imaginary cat? Let Hawking shoot him. " It's not that simple, otherwise many physicists wouldn't be so diligent. Schrodinger's cat paradox leads to a deeper question: what is the relationship between a large number of organisms composed of atoms and molecules and the quantum mechanical laws followed by these microscopic particles? This is not only an important theoretical issue, but also of practical significance. For example, the mechanism of self-consciousness is still an unsolved mystery, and some people think it may be related to quantum mechanics or deeper microscopic laws. Another example is "epiphany" in the process of thinking. Will it be related to the above-mentioned "a state jumps out of the original uncertain superposition state"? It may also be related to the origin of life, the variation of species, the mechanism of photosynthesis and so on. In a word, the secret of life and the mystery of thinking cannot be unrelated to the laws of quantum mechanics. No wonder Schrodinger later became interested in life science. From 65438 to 0946, he wrote the famous book What is Life and put forward some original opinions. Unfortunately, in his lifetime, the cat in this poor box is still unknown.

Subsequent research

Physics is an experimental science, and everything must be judged by experiments. An earlier batch of experiments about Schrodinger's cat was to isolate a single atom or molecule in a superposition state from the surrounding environment, and then interact with it in a controllable way to observe its changes. It is found that the key lies in the interaction of environment, which leads to the transformation of the original quantum superposition state into a classical deterministic state. But it is too simplistic to regard these subjects as Schrodinger cats. A single atom or molecule is far from Schrodinger's cat. 1996 in may, Monroe and others of the national institute of standards and technology (NIST) in boulder, Colorado, USA made a "Schrodinger's cat" with a single beryllium ion and took a snapshot. It is found that beryllium ions are in the spin-up state in the first spatial position, but in the spin-down state in the second spatial position, and the difference between the two States is 80 nanometers! (1 nanometer is one billionth of 1 meter)-this is a huge distance on the atomic scale. Imagine that this beryllium ion is a psychic master, and it appears in new york and Himalaya at the same time. One is that he is parachuting from the top of a skyscraper. And the other is climbing to the top of the snowy mountain! -This "Dr. Jekyll and Mr. Hyde" feature of quantum is called "quantum coherence" in physics. In Young's early double-slit experiment, a single light particle passed through two slits at the same time with beautiful wave-particle duality, resulting in beautiful light-dark coherent stripes on the observation screen. Schrodinger's cat is a paradox about quantum mechanics put forward by him in 1935. Over the years, many physicists have racked their brains to try to solve this paradox. Obviously, a dead and alive cat is absurd. The physical problem that Schrodinger wants to explain is whether the macro world also follows the quantum superposition principle applicable to the micro scale. The paradox of "Schrodinger's cat" skillfully links microscopic radioactive sources with macroscopic cats, aiming at denying the existence of quantum superposition in the macroscopic world. But with the development of quantum mechanics, it was not until a series of ingenious experiments recently that this problem gradually surfaced. In July 2000, Nature reported the latest experimental results. The experiment reported in Nature this time is different from the one mentioned above. The Schrodinger cat used by J. R. Friedman and others at the State University of New York at Stony Brook is not a single particle, but a superconducting current composed of billions of pairs of electrons in a superconducting circuit near absolute zero. In the latest issue of Nature, Lai Bouverie of the National Institute of Standards and Technology and others said that they had achieved the "Schrodinger cat" state with more particles and the longest duration. In the experiment, the researchers "fixed" beryllium ions in an electromagnetic field trap every few microns, then cooled them to near absolute zero with a laser, and manipulated their movements in three steps. In order to make as many particles as possible realize the "Schrodinger cat" state for as long as possible, the researchers on the one hand improve the cooling efficiency of the laser, and on the other hand make the electromagnetic field trap absorb as much heat as possible from the ion vibration. Finally, they made six beryllium ions spin clockwise and counterclockwise at the same time in 50 microseconds, and realized the equal superposition and entanglement of two opposite quantum States, namely the Schrodinger cat state. Experiments show that this macroscopic quantum system composed of a large number of particles can also be in a superposition state-equivalent to the "dead-alive superposition state" of Schrodinger's cat. Of course, the superconducting current composed of billions of pairs of electrons can't be compared with the cat composed of hundreds of millions of atoms, but compared with a single atom, this is a big step forward. So someone exclaimed, "Schrodinger's cat has become fat!" "Researchers at the University of Innsbruck, Austria, also reported in the same issue of Nature that they realized the Schrodinger cat state in an eight-ion system, but the duration was slightly shorter. Scientists say that the "Schrodinger cat" state not only has theoretical research significance, but also has the potential for practical application. For example, the multi-particle "Schrodinger cat" state system can be used as the core component of high fault-tolerant quantum computers in the future, and can also be used to manufacture extremely sensitive sensors and precision measuring equipment such as atomic clocks and interferometers. Is the next step to experiment with real cats? No way! First of all, it can't be isolated from its surroundings-a cat in a vacuum will soon die. Secondly, unlike superconducting current near absolute zero, cat at room temperature is not a macroscopic quantum system at all. How can it be superimposed? Besides, there is no need to do such an experiment. According to the existing experimental results, physicists have been able to explain why Schrodinger's cat can't have a "dead-alive superposition state" that conforms to quantum mechanics.