Quantum physics Schrödinger's cat. Quantum Cheshire cat. The solution to the Schrödinger's Cat paradox - the Copenhagen interpretation

Don't look for "eastern mysticism", spoon bending or extrasensory perception here. Look for the true story quantum mechanics, the truth of which is more amazing than any fiction. This is science: it does not need outfits from another philosophy, because it itself is full of beauties, mysteries and surprises. This book attempts to answer the specific question: “What is reality?” And the answer (or answers) may surprise you. You may not believe it. But you will understand how modern science looks at the world.

The cat that appears in the title is mythical creature, but Schrödinger really existed. Erwin Schrödinger was an Austrian scientist who, in the mid-1920s, played a major role in creating the equations of a particular branch of science now called quantum mechanics. However, to say that quantum mechanics is only a branch of science is hardly true, because it underlies all modern science. Its equations describe the behavior of very small objects - the size of atoms and smaller - and represent the only thing description of the world of smallest particles. Without these equations, physicists would not be able to develop workers' designs nuclear power plants(or bombs), create lasers, or explain how the temperature of the Sun does not decrease. Without quantum mechanics, chemistry would still be in Dark Ages and molecular biology would not have appeared at all: there would be neither knowledge about DNA nor genetic engineering- Nothing.

Quantum theory is the greatest achievement of science, much more significant and much more applicable in a direct, practical sense than the theory of relativity. And yet she makes some strange predictions. The world of quantum mechanics is indeed so unusual that even Albert Einstein found it incomprehensible and refused to accept all the consequences of the theory derived by Schrödinger and his colleagues. Like many other scientists, Einstein decided it was more convenient to believe that the equations of quantum mechanics were just a kind of mathematical trick that, by chance, provided a reasonable explanation for the behavior of atomic and subatomic particles, but they contain a deeper truth that relates better to our everyday sense of reality. After all, quantum mechanics states that there is no reality and we cannot say anything about the behavior of things when we do not observe them. Schrödinger's mythical cat was intended to clarify the differences between the quantum and ordinary worlds.

In the world of quantum mechanics, the laws of physics familiar to us from the ordinary world cease to work. Instead, events are governed by probabilities. A radioactive atom, for example, may decay and, say, release an electron, or it may not. You can conduct an experiment by imagining that there is exactly a fifty percent probability that one of the atoms of a bunch of radioactive substance will decay at a certain moment and the detector will register this decay if it occurs. Schrödinger, equally upset by the findings quantum theory, like Einstein, tried to demonstrate their absurdity by imagining that such an experiment takes place in a closed room or box where there is a live cat and a bottle of poison, and if decay occurs, the bottle with the poison breaks and the cat dies. In the ordinary world, the probability of a cat's death is fifty percent and, without looking into the box, we can safely say only one thing: the cat inside is either alive or dead. But this is where the strangeness of the quantum world reveals itself. According to the theory none Of the two possibilities that exist for the radioactive substance, and therefore the cat, it does not seem realistic unless there is observation of what is happening. Atomic decay did not happen and did not happen, the cat did not die and did not die, until we look into the box to find out what happened. Theorists who accept a pure version of quantum mechanics argue that the cat exists in some indeterminate state, being neither alive nor dead, until an observer looks into the box and sees how the situation has turned out. Nothing is real unless observation is made.

This idea was hated by Einstein, as well as many others. “God doesn’t play dice,” he said, referring to the theory that the world is determined by the totality of the results of an essentially random “selection” of possibilities at the quantum level. As for the unreality of the state of Schrödinger's cat, Einstein did not take it into account, suggesting that there must be some deep “mechanism” that determines the truly fundamental reality of things. For many years he tried to develop experiments that would help show this deep reality at work, but he died before it became possible to conduct similar experiment. Perhaps it was for the best that he did not live to see the result of the chain of reasoning he had set in motion.

In the summer of 1982, a group of scientists from the University of Paris-Sud, led by Alain Aspé, completed a series of experiments designed to reveal the underlying reality that defines the unreal quantum world. This deep reality - the fundamental mechanism - was given the name “hidden parameters”. The essence of the experiment was to observe the behavior of two photons, or particles of light, flying in opposite directions from a source. The experiment is described in full in Chapter Ten, but overall it can be considered a reality check. Two photons from the same source can be detected by two detectors that measure a property called polarization. According to quantum theory, this property does not exist until it is measured. According to the idea of ​​"hidden parameters", every photon has a "real" polarization from the moment of its creation. Because two photons are emitted simultaneously, their polarization values ​​depend on each other, but the nature of the dependence that is actually measured differs according to the two views of reality.

The results of this important experiment are clear. The dependence predicted by the theory of hidden parameters was not discovered, but the dependence predicted by quantum mechanics was. Moreover, as quantum theory predicted, measurements made on one photon had an immediate effect on the nature of the other photon. Some interaction inextricably linked the photons, although they scattered into different sides at the speed of light, and the theory of relativity states that no signal can be transmitted faster than light. Experiments have proven that there is no deep reality in the world. “Reality” in the ordinary sense is not suitable for thinking about the behavior of the fundamental particles that make up the Universe, and these particles at the same time seem to be inextricably linked together into some indivisible whole, where each knows what happens to the others.

The search for Schrödinger's cat is the search for quantum reality. From this short review it may seem that this search was not crowned with success, since in the quantum world of reality in in the usual sense the word doesn't exist. But the story doesn't end there, and the search for Schrödinger's cat may lead us to a new understanding of reality that transcends—and at the same time includes—the conventional interpretation of quantum mechanics. However, the search will take a long time, and you need to start with a scientist who, perhaps, would be more frightened than Einstein if he had a chance to find out the answers we have now given to the questions that tormented him. Studying the nature of light three centuries ago, Isaac Newton probably had no idea that he had already set foot on the path leading to Schrödinger’s cat.

Anyone who is not shocked by quantum theory has not understood it.

Niels Bohr 1885-1962

Isaac Newton invented physics, and the rest of science rests on it. While Newton certainly built on the work of others, it was his publication of the three laws of motion and the theory of gravity over three centuries ago that set science on the path that eventually led to space exploration, lasers, atomic energy, genetic engineering, the understanding of chemistry and everything else. . For two centuries, Newtonian physics (what is now called "classical physics") ruled the world of science. Revolutionary new ideas advanced twentieth-century physics well beyond Newton, but without those two centuries of scientific growth, these ideas might never have appeared. This book is not the history of science: it talks about the new physics - quantum, and not about those classical ideas. However, even in Newton's work of three hundred years ago, there are already signs that change is inevitable: they are contained not in his works on the movement of planets and their orbits, but in his studies of the nature of light.

"Anyone who isn't shocked by quantum theory, does not understand it,” said Niels Bohr, the founder of quantum theory.
The basis of classical physics is the unambiguous programming of the world, otherwise Laplacean determinism, with the advent of quantum mechanics it was replaced by the invasion of a world of uncertainties and probabilistic events. And here thought experiments came in handy for theoretical physicists. These were the touchstones on which the latest ideas were tested.

"Schrodinger's Cat" is a thought experiment, proposed by Erwin Schrödinger, with whom he wanted to show the incompleteness of quantum mechanics in the transition from subatomic systems to macroscopic systems.

IN closed box cat placed The box contains a mechanism containing a radioactive nucleus and a container with poisonous gas. The probability that the nucleus will decay in 1 hour is 1/2. If the nucleus disintegrates, it activates the mechanism, it opens a container of gas, and the cat dies. According to quantum mechanics, if no observation is made of the nucleus, then its state is described by a superposition (mixing) of two states - a decayed nucleus and an undecayed nucleus, therefore, a cat sitting in a box is both alive and dead at the same time. If the box is opened, then the experimenter can see only one specific state - “the nucleus has decayed, the cat is dead” or “the nucleus has not decayed, the cat is alive.”

When does the system cease to exist? How does one mix two states and choose one specific one?

Purpose of the experiment- show that quantum mechanics is incomplete without some rules indicating under what conditions the wave function collapses (an instantaneous change in the quantum state of an object that occurs when measured), and the cat either becomes dead or remains alive, but ceases to be a mixture of both.

Since it is clear that a cat must be either alive or dead (there is no state intermediate between life and death), this means that this is also true for the atomic nucleus. It will necessarily be either decayed or undecayed.

Schrödinger's paper “The Current Situation in Quantum Mechanics,” presenting a thought experiment with a cat, appeared in the German journal Natural Sciences in 1935 to discuss the EPR paradox.

Articles by Einstein-Podolsky-Rosen and Schrödinger indicated the strange nature of " quantum entanglement"(the term was introduced by Schrödinger), characteristic of quantum states, which are a superposition of the states of two systems (for example, two subatomic particles).

Interpretations of quantum mechanics

During the existence of quantum mechanics, scientists have put forward different interpretations of it, but the most supported of all today are the “Copenhagen” and “many-worlds” ones.

"Copenhagen Interpretation"- this interpretation of quantum mechanics was formulated by Niels Bohr and Werner Heisenberg during their joint work in Copenhagen (1927). Scientists have tried to answer questions arising from the wave-particle duality inherent in quantum mechanics, in particular the question of measurement.

In the Copenhagen interpretation, the system ceases to be a mixture of states and chooses one of them at the moment when the observation occurs. The experiment with the cat shows that in this interpretation the nature of this very observation - measurement - is not sufficiently defined. Some believe that experience suggests that as long as the box is closed, the system is in both states simultaneously, in a superposition of the states “decayed nucleus, dead cat” and “undecayed nucleus, living cat,” and when the box is opened, then only then does the wave function collapse to one of the options. Others guess that the "observation" occurs when a particle from the nucleus hits the detector; however (and this key moment thought experiment) in the Copenhagen interpretation there is no clear rule that says when this happens, and therefore this interpretation is incomplete until such a rule is introduced into it, or is told how it can be introduced. The exact rule is that randomness appears at the point where the classical approximation is first used.

Thus, we can rely on the following approach: in macroscopic systems we do not observe quantum phenomena (except for the phenomenon of superfluidity and superconductivity); therefore, if we superimpose macroscopic wave function to a quantum state, we must conclude from experience that the superposition is destroyed. And although it is not entirely clear what it means for something to be “macroscopic” in general, what is certain about a cat is that it is a macroscopic object. Thus, the Copenhagen interpretation does not consider that the cat is in a state of confusion between living and dead before the box is opened.

In the "many worlds interpretation" quantum mechanics, which does not consider the measurement process to be something special, both states of the cat exist, but decohere, i.e. a process occurs in which a quantum mechanical system interacts with environment and acquires information available in the environment, or otherwise becomes “entangled” with the environment. And when the observer opens the box, he becomes entangled with the cat and from this two states of the observer are formed, corresponding to a living and a dead cat, and these states do not interact with each other. The same mechanism of quantum decoherence is important for “joint” histories. In this interpretation, only a “dead cat” or a “live cat” can be in a “shared story.”

In other words, when the box is opened, the universe splits into two different universes, one in which the observer is looking at a box with a dead cat, and in the other, the observer is looking at a living cat.

The paradox of "Wigner's friend"

Wigner's Friend's Paradox is a complicated experiment of the Schrödinger's cat paradox. Nobel Prize Laureate, American physicist Eugene Wigner introduced the category of "friends". After completing the experiment, the experimenter opens the box and sees a live cat. The state of the cat at the moment of opening the box goes into the state “the nucleus has not decayed, the cat is alive.” Thus, in the laboratory the cat was recognized as alive. There is a "friend" outside the laboratory. The friend does not yet know whether the cat is alive or dead. The friend recognizes the cat as alive only when the experimenter tells him the outcome of the experiment. But all the other “friends” have not yet recognized the cat as alive, and they will only recognize it when they are told the result of the experiment. Thus, the cat can be recognized as fully alive only when all people in the Universe know the result of the experiment. Up to this point in scale Big Universe the cat remains half alive and half dead at the same time.

The above is applied in practice: in quantum computing and in quantum cryptography. A light signal in a superposition of two states is sent through a fiber-optic cable. If attackers connect to the cable somewhere in the middle and make a signal tap there in order to eavesdrop on the transmitted information, then this will collapse the wave function (from the point of view of the Copenhagen interpretation, an observation will be made) and the light will go into one of the states. By conducting statistical tests of light at the receiving end of the cable, it will be possible to detect whether the light is in a superposition of states or has already been observed and transmitted to another point. This makes it possible to create means of communication that exclude undetectable signal interception and eavesdropping.

The experiment (which can in principle be carried out, although working quantum cryptography systems capable of transmitting large amounts of information have not yet been created) also shows that “observation” in the Copenhagen interpretation has no relation to the consciousness of the observer, since in in this case A completely inanimate branch of the wire leads to a change in statistics at the end of the cable.

And in quantum computing, the Schrödinger cat state is a special entangled state of qubits in which they are all in the same superposition of all zeros or ones.

("Qubit" is the smallest element for storing information in a quantum computer. It admits two eigenstates, but it can also be in their superposition. Whenever the state of a qubit is measured, it randomly transitions to one of its own states.)

In reality! Little brother of "Schrodinger's cat"

It's been 75 years since Schrödinger's cat appeared, but still some of the consequences of quantum physics seem at odds with our everyday ideas about matter and its properties. According to the laws of quantum mechanics, it should be possible to create a “cat” state in which it is both alive and dead, i.e. will be in a state of quantum superposition of two states. However, in practice, the creation of a quantum superposition of such large quantity atoms have not yet been achieved. The difficulty is that the more atoms there are in a superposition, the less stable this state is, since external influences tend to destroy it.

To physicists from the University of Vienna (publication in the journal Nature Communications", 2011) for the first time in the world it was possible to demonstrate the quantum behavior of an organic molecule consisting of 430 atoms and in a state of quantum superposition. The molecule obtained by the experimenters looks more like an octopus. The size of the molecules is about 60 angstroms, and the de Broglie wavelength for the molecule was only 1 picometer. This “molecular octopus” was able to demonstrate the properties inherent in Schrödinger’s cat.

Quantum suicide

Quantum suicide is a thought experiment in quantum mechanics that was proposed independently by G. Moravec and B. Marshall, and was expanded in 1998 by cosmologist Max Tegmark. This thought experiment, a modification of the Schrödinger's cat thought experiment, clearly shows the difference between two interpretations of quantum mechanics: the Copenhagen interpretation and the Everett many-worlds interpretation.

The experiment is actually an experiment with Schrödinger's cat from the cat's point of view.

In the proposed experiment, a gun is pointed at the participant, which fires or does not fire depending on the decay of some radioactive atom. There is a 50% chance that the gun will go off and the participant will die. If the Copenhagen interpretation is correct, then the gun will eventually go off and the participant will die.
If Everett’s many-worlds interpretation is correct, then as a result of each experiment conducted, the universe splits into two universes, in one of which the participant remains alive, and in the other dies. In worlds where a participant dies, he ceases to exist. In contrast, from the perspective of the non-dead participant, the experiment will continue without causing the participant to disappear. This happens because in any branch the participant is able to observe the result of the experiment only in the world in which he survives. And if the many-worlds interpretation is correct, then the participant may notice that he will never die during the experiment.

The participant will never be able to talk about these results, since from the point of view of an outside observer, the probability of the outcome of the experiment will be the same in both the many-worlds and the Copenhagen interpretations.

Quantum immortality

Quantum immortality is a thought experiment that stems from the quantum suicide thought experiment and states that, according to the many-worlds interpretation of quantum mechanics, beings that have the capacity for self-awareness are immortal.

Let's imagine that a participant in an experiment detonates a nuclear bomb near him. In almost all parallel Universes, a nuclear explosion will destroy the participant. But despite this, there must be a small number of alternative Universes in which the participant somehow survives (that is, Universes in which a potential rescue scenario is possible). The idea of ​​quantum immortality is that the participant remains alive, and thereby is able to perceive the surrounding reality, in at least one of the Universes in the set, even if the number of such universes is extremely small compared to the number of all possible Universes. Thus, over time, the participant will discover that he can live forever. Some parallels to this conclusion can be found in the concept of the anthropic principle.

Another example stems from the idea of ​​quantum suicide. In this thought experiment, the participant points a gun at himself, which may or may not fire depending on the outcome of the decay of some radioactive atom. There is a 50% chance that the gun will go off and the participant will die. If the Copenhagen interpretation is correct, then the gun will eventually go off and the participant will die.

If Everett’s many-worlds interpretation is correct, then as a result of each experiment conducted, the universe splits into two universes, in one of which the participant remains alive, and in the other dies. In worlds where a participant dies, he ceases to exist. On the contrary, from the point of view of the non-dead participant, the experiment will continue without causing the participant to disappear, since after each split of universes he will be able to recognize himself only in those universes where he survived. Thus, if Everett's many-worlds interpretation is correct, then the participant may notice that he will never die in the experiment, thereby "proving" his immortality, at least from his point of view.

Proponents of quantum immortality point out that this theory does not contradict any known laws physicists (this position is far from unanimously accepted in scientific world). In their reasoning, they rely on the following two controversial assumptions:
- Everett's many-worlds interpretation is correct, not the Copenhagen interpretation, since the latter denies the existence parallel universes;
- all possible scenarios in which a participant may die during the experiment contain at least a small subset of scenarios in which the participant remains alive.

A possible argument against the theory of quantum immortality is that the second assumption does not necessarily follow from Everett's many-worlds interpretation, and it may conflict with the laws of physics, which are believed to apply to all possible realities. The many-worlds interpretation of quantum physics does not necessarily imply that “anything is possible.” It only indicates that at a certain point in time the universe can be divided into a number of others, each of which will correspond to one of the many possible outcomes. For example, the second law of thermodynamics is believed to apply to all probable universes. This means that, theoretically, the existence of this law prevents the formation of parallel universes where it would be violated. The consequence of this may be the achievement, from the point of view of the experimenter, of a state of reality where his further survival becomes impossible, since this would require a violation of the law of physics, which, according to the previously stated assumption, is valid for all possible realities.

For example, in an explosion nuclear bomb described above, it is quite difficult to describe a plausible scenario that does not violate basic biological principles in which the participant will survive. Living cells simply cannot exist at the temperatures reached in the center nuclear explosion. In order for the theory of quantum immortality to remain valid, it is necessary that either a misfire occurs (and thereby avoid a nuclear explosion), or some event occurs that is based on as yet undiscovered or unproven laws of physics. Another argument against the theory under discussion can be the presence of natural biological death in all creatures, which cannot be avoided in any of the parallel Universes (at least in at this stage development of science)

On the other hand, the second law of thermodynamics is a statistical law, and nothing is contradicted by the occurrence of fluctuations (for example, the appearance of a region with conditions suitable for the life of an observer in a universe that has generally reached a state of thermal death; or, in principle, the possible movement of all particles resulting from nuclear explosion, in such a way that each of them will fly past the observer), although such a fluctuation will occur only in an extremely small part of all possible outcomes. The argument regarding the inevitability of biological death can also be refuted on the basis of probabilistic considerations. For every living organism in this moment time there is a non-zero probability that he will remain alive during the next second. Thus, the probability that he will remain alive for the next billion years is also non-zero (since it is the product large number non-zero factors), although very small.

What is problematic about the idea of ​​quantum immortality is that according to it, a self-aware being will be “forced” to experience extremely unlikely events that will arise in situations in which the participant would seem to die. Even though in many parallel universes the participant dies, the few universes that the participant is able to subjectively perceive will develop in an extremely unlikely scenario. This, in turn, may in some way cause a violation of the principle of causality, the nature of which in quantum physics is not yet clear enough.

Although the idea of ​​quantum immortality stems largely from the “quantum suicide” experiment, Tegmark argues that under any normal conditions, every thinking being before death goes through a stage (from a few seconds to several years) of decreasing level of self-awareness, which has nothing to do with quantum mechanics. and the participant has no possibility of continued existence by moving from one world to another, which gives him the opportunity to survive.

Here, a self-conscious intelligent observer only in a relatively small number of possible states in which he retains self-consciousness continues to remain in, so to speak, “ healthy body" The possibility that the observer, while retaining consciousness, will remain crippled is much greater than if he remains unharmed. Any system (including a living organism) has much more possibilities function incorrectly than to remain in perfect shape. Boltzmann's ergodic hypothesis requires that the immortal observer will sooner or later go through all states compatible with the preservation of consciousness, including those in which he will feel unbearable suffering - and there will be significantly more such states than states of optimal functioning of the organism. Thus, as philosopher David Lewis suggests, we should hope that the many-worlds interpretation is wrong.

There was a kind of “secondary” quality. He himself rarely engaged in certain scientific problem. His favorite genre of work was response to someone else's scientific research, development of this work or its criticism. Despite the fact that Schrödinger himself was an individualist by nature, he always needed someone else’s thought, support for further work. Despite this peculiar approach, Schrödinger managed to make many discoveries.

Biographical information

Schrödinger's theory is now known not only to students of physics and mathematics departments. It will be of interest to anyone who is interested in popular science. This theory was created by the famous physicist E. Schrödinger, who went down in history as one of the creators of quantum mechanics. The scientist was born on August 12, 1887 in the family of the owner of an oilcloth factory. The future scientist, famous throughout the world for his riddle, was fond of botany and drawing as a child. His first mentor was his father. In 1906, Schrödinger began his studies at the University of Vienna, during which he began to admire physics. When the First came World War, the scientist went to serve as an artilleryman. In his free time, he studied the theories of Albert Einstein.

By the beginning of 1927, a dramatic situation had developed in science. E. Schrödinger believed that the basis of the theory of quantum processes should be the idea of ​​wave continuity. Heisenberg, on the contrary, believed that the foundation for this field of knowledge should be the concept of discreteness of waves, as well as the idea of ​​quantum leaps. Niels Bohr did not accept either position.

Advances in science

For his creation of the concept of wave mechanics in 1933, Schrödinger received Nobel Prize. However, brought up in the traditions of classical physics, the scientist could not think in other categories and did not consider quantum mechanics a full-fledged branch of knowledge. He could not be satisfied with the dual behavior of particles, and he tried to reduce it exclusively to wave behavior. In his discussion with N. Bohr, Schrödinger put it this way: “If we plan to preserve these quantum leaps in science, then I generally regret that I connected my life with atomic physics.”

Further work of the researcher

Moreover, Schrödinger was not only one of the creators of modern quantum mechanics. It was he who was the scientist who introduced the term “objectivity of description” into scientific use. This is an opportunity scientific theories describe reality without the participation of an observer. His further research was devoted to the theory of relativity, thermodynamic processes, and nonlinear Born electrodynamics. Scientists have also made several attempts to create unified theory fields. In addition, E. Schrödinger spoke six languages.

The most famous riddle

Schrödinger's theory, in which that same cat appears, grew out of the scientist's criticism of quantum theory. One of its main postulates states that while the system is not being observed, it is in a state of superposition. Namely, in two or more states that exclude each other’s existence. The state of superposition in science has the following definition: this is the ability of a quantum, which can also be an electron, photon, or, for example, the nucleus of an atom, to simultaneously be in two states or even at two points in space at a moment when no one is observing it.

Objects in different worlds

It is very difficult for an ordinary person to understand such a definition. After all, every object material world can be either at one point in space or at another. This phenomenon can be illustrated as follows. The observer takes two boxes and puts a tennis ball in one of them. It will be clear that it is in one box and not in the other. But if you put an electron in one of the containers, then the following statement will be true: this particle is simultaneously in two boxes, no matter how paradoxical it may seem. In the same way, an electron in an atom is not located at a strictly defined point at one time or another. It rotates around the core, located at all points of the orbit simultaneously. In science, this phenomenon is called an “electron cloud.”

What did the scientist want to prove?

Thus, the behavior of small and large objects is implemented in a completely different rules. In the quantum world there are some laws, and in the macroworld - completely different ones. However, there is no concept that would explain the transition from the world of material objects familiar to people to the microworld. Schrödinger's theory was created in order to demonstrate the inadequacy of research in the field of physics. The scientist wanted to show that there is a science whose goal is to describe small objects, and there is a field of knowledge that studies ordinary items. Largely thanks to the work of the scientist, physics was divided into two areas: quantum and classical.

Schrödinger's theory: description

The scientist described his famous thought experiment in 1935. In carrying it out, Schrödinger relied on the principle of superposition. Schrödinger emphasized that as long as we do not observe the photon, it can be either a particle or a wave; both red and green; both round and square. This principle of uncertainty, which directly follows from the concept of quantum dualism, was used by Schrödinger in his famous riddle about the cat. The meaning of the experiment in brief is as follows:

• A cat is placed in a closed box, as well as a container containing hydrocyanic acid and a radioactive substance.
• The nucleus can disintegrate within an hour. The probability of this is 50%.
• If atomic nucleus decays, it will be recorded by a Geiger counter. The mechanism will work, and the box of poison will be broken. The cat will die.
• If decay does not occur, then Schrödinger's cat will be alive.

According to this theory, until the cat is observed, it is simultaneously in two states (dead and alive), just like the nucleus of an atom (decayed or not decayed). Of course, this is only possible according to the laws of the quantum world. In the macrocosm, a cat cannot be both alive and dead at the same time.

The Observer's Paradox

To understand the essence of Schrödinger's theory, it is also necessary to understand the observer's paradox. Its meaning is that objects of the microworld can be in two states simultaneously only when they are not observed. For example, the so-called “Experiment with 2 slits and an observer” is known in science. The scientists directed a beam of electrons onto an opaque plate in which two vertical slits were made. On the screen behind the plate, the electrons painted a wave pattern. In other words, they left black and white stripes. When the researchers wanted to observe how electrons flew through the slits, the particles displayed only two vertical stripes on the screen. They behaved like particles, not like waves.

Copenhagen explanation

The modern explanation of Schrödinger's theory is called the Copenhagen one. Based on the observer's paradox, it sounds like this: as long as no one observes the nucleus of an atom in the system, it is simultaneously in two states - decayed and undecayed. However, the statement that a cat is alive and dead at the same time is extremely erroneous. After all, in the macrocosm the same phenomena are never observed as in the microcosm.

Therefore, we are not talking about the “cat-nucleus” system, but about the fact that the Geiger counter and the atomic nucleus are interconnected. The kernel can choose one state or another at the moment when measurements are made. However given choice does not take place at the moment when the experimenter opens the box with Schrödinger's cat. In fact, the opening of the box takes place in the macrocosm. In other words, in a system that is very far from atomic world. Therefore, the nucleus selects its state precisely at the moment when it hits the Geiger counter detector. Thus, Erwin Schrödinger did not describe the system fully enough in his thought experiment.

General conclusions

Thus, it is not entirely correct to connect the macrosystem with the microscopic world. In the macrocosm quantum laws lose their power. The nucleus of an atom can be in two states simultaneously only in the microcosm. The same cannot be said about the cat, since it is an object of the macrocosm. Therefore, only at first glance does it seem that the cat passes from a superposition to one of the states at the moment the box is opened. In reality, its fate is determined at the moment when the atomic nucleus interacts with the detector. The conclusion can be drawn as follows: the state of the system in Erwin Schrödinger’s riddle has nothing to do with the person. It depends not on the experimenter, but on the detector - the object that “observes” the nucleus.

Continuation of the concept

Schrödinger theory in simple words is described as follows: while the observer is not looking at the system, it can be in two states simultaneously. However, another scientist, Eugene Wigner, went further and decided to bring Schrödinger’s concept to the point of complete absurdity. “Excuse me!” said Wigner, “What if his colleague is standing next to the experimenter watching the cat?” The partner does not know what exactly the experimenter himself saw at the moment when he opened the box with the cat. Schrödinger's cat emerges from superposition. However, not for a fellow observer. Only at the moment when the fate of the cat becomes known to the latter can the animal be finally called alive or dead. In addition, billions of people live on planet Earth. And the final verdict can be made only when the result of the experiment becomes the property of all living beings. Of course, you can tell all people the fate of the cat and Schrödinger’s theory briefly, but this is a very long and labor-intensive process.

The principles of quantum dualism in physics have never been refuted thought experiment Schrödinger. In a sense, every being can be said to be neither alive nor dead (in superposition) as long as there is at least one person not observing it.

John Gribbin

In search of Schrödinger's cat. Quantum physics and reality

I don't like all this, and I regret that I was involved in this at all.

Erwin Schrödinger 1887-1961

Nothing is real.

John Lennon 1940-1980

IN SEARCH OF SCHRÖDINGER'S CAT

Quantum Physics and Reality

Translation from English by Z. A. Mamedyarova, E. A. Fomenko

© 1984 by John and Mary Gribbin

Acknowledgments

My acquaintance with quantum theory took place more than twenty years ago, while still at school, when I discovered that the theory of the shell structure of the atom magically explained all periodic table elements and almost all chemistry, which I struggled with in many boring classes. I immediately began to dig further, resorting to library books said to be "too complex" for my limited scientific training, and immediately noticed the beautiful simplicity of the explanation of the atomic spectrum from the perspective of quantum theory and discovered for the first time that the best in science is simultaneously beautiful and simple, and this is a fact that too many teachers - accidentally or on purpose - hide from their students. I felt just like the hero of the novel “The Search” by C. P. Snow (although I read it much later), who discovered the same thing:

I noticed how mixed up random facts suddenly fell into place... “But this is the truth,” I said to myself. - This is wonderful. And this is the truth." (Edition A, 1963, p. 27.)

It was partly because of this insight that I decided to study physics at university. In due course, my ambitions were realized, and I became a student at the University of Sussex in Brighton. But there the simplicity and beauty of deep ideas was eclipsed by the variety of details and mathematical methods solving specific problems using the equations of quantum mechanics. Applying these ideas to the world modern physics gave, perhaps, about the same idea of ​​deep beauty and truth that piloting gives Boeing 747 about hang gliding. Although the power of the original insight remained the most significant influence on my career, for a long time I did not pay attention to the quantum world and discovered other delights of science.

The embers of that early interest were reignited by a combination of factors. In the late 1970s and early 1980s, books and articles began to appear that tried, with varying degrees of success, to explain the strange quantum world to non-scientific audiences. Some of the so-called “popular texts” were so monstrously far from the truth that I could not even imagine that there would be a reader who would understand the truth and beauty of science by studying them, and therefore wanted to tell it like it is. At the same time, information emerged about a long series of scientific experiments that proved the reality of some of the strangest aspects of quantum theory, and this information forced me to go back to the libraries and refresh my understanding of these amazing things. Finally, one Christmas, the BBC invited me to appear on a radio program as a sort of scientific opponent to Malcolm Muggeridge, who had just announced his conversion to Catholicism and was the chief guest for the festive season. After this great person made his point, emphasizing the mystery of Christianity, he turned to me and said: “But here is someone who knows all the answers - or claims to know them all.” Time was limited, and I tried to give a decent response, pointing out that science does not claim to have all the answers, and it is religion, not science, that relies entirely on boundless faith and the belief that the truth is known. “I don’t believe in anything,” I said and began to explain my position, but at that moment the program came to an end. Throughout the Christmas holidays, friends and acquaintances reminded me of these words, and I spent hours repeating that my lack of boundless faith in anything does not prevent me from living normal life, using the perfectly reasonable working hypothesis that the sun is unlikely to disappear overnight.

All this helped me sort out my own thoughts about the nature of science during long discussions about the basic reality - or unreality - of the quantum world, and it was enough to convince me that I could write the book you now hold in your hands. While working on it, I tested many of the more subtle arguments during my regular appearances on the British Forces Broadcasting Corporation's science radio program, hosted by Tommy Vance. Tom's inquisitive questions quickly revealed imperfections in my presentation, and with their help I was able to organize my ideas in the best possible way. The main source of reference material I used in writing the book was the University of Sussex library, which contains perhaps one of the best collections of books on quantum theory in the world, and more rare materials were selected for me by Mandy Caplin from the magazine New Scientist, who persistently sent me teletype messages while Christina Sutton corrected my misconceptions about particle physics and field theory. My wife not only provided me with invaluable assistance in reviewing the literature and organizing the material, but also softened many sharp corners. I am also grateful to Professor Rudolf Pearls for explaining to me in detail some of the intricacies of the clock-in-a-box experiment and the Einstein-Podolsky-Rosen paradox.

All that is good about this book is due to: "difficult" chemistry texts, the names of which I no longer remember, which I discovered in the Kent County Library at the age of sixteen; woe to the “popularizers” of quantum ideas who convinced me that I could describe them better; Malcolm Muggeridge and the BBC; University of Sussex Library; Tommy Vance and BFBS; Mandy Caplin and Christina Sutton and especially Min. Any complaints regarding those shortcomings that still remain in this book should, of course, be addressed to me.

John Gribbin

July 1983

Introduction

If you add up all the books and articles on the theory of relativity written for ordinary people, then the stack will probably reach the moon. “Everyone knows” that Einstein’s theory of relativity is the greatest scientific achievement of the 20th century, and everyone is wrong. However, if you add up all the books and articles on quantum theory written for ordinary people, they will easily fit on my desk. This does not mean that quantum theory has not been heard outside the walls of academies. Quantum mechanics even became popular in certain sectors: with its help they tried to explain telepathy and bending spoons, and they drew inspiration from it for many science fiction stories. In popular mythology, quantum mechanics is associated - if at all - with the occult and extrasensory perception, that is, a strange, esoteric branch of science that no one understands and for which no one can find practical application.

This book is written to counter this perception of what is essentially the most fundamental and important field scientific knowledge. This book owes its origin to several circumstances that arose in the summer of 1982. First, I just finished reading a book on the theory of relativity called The Curvatures of Space and decided it was time to take on the task of demystifying the other great branch of twentieth-century science. Secondly, at that time I was increasingly irritated by the incorrect ideas that existed under the name of quantum theory among people far from science. Fridtjof Capra's excellent book The Tao of Physics gave birth to many imitators who understood neither physics nor the Tao but felt that money could be made by linking Western science with Eastern philosophy. And finally, in August 1982, news came from Paris that a group of scientists had successfully carried out a crucial experiment that confirmed - for those who still doubted - the accuracy of the quantum mechanical concept of the universe.

Don't look for "eastern mysticism", spoon bending or extrasensory perception here. Seek the true story of quantum mechanics, the truth of which is more amazing than any fiction. This is science: it does not need outfits from another philosophy, because it itself is full of beauties, mysteries and surprises. This book attempts to answer the specific question: “What is reality?” And the answer (or answers) may surprise you. You may not believe it. But you will understand how modern science looks at the world.

Nothing is real

The cat in the title is a mythical creature, but Schrödinger really existed. Erwin Schrödinger was an Austrian scientist who, in the mid-1920s, played a major role in creating the equations of a branch of science now called quantum mechanics. However, to say that quantum mechanics is just a branch of science is hardly true, because it underlies all modern science. Its equations describe the behavior of very small objects - the size of atoms and smaller - and represent the only thing description of the world of smallest particles. Without these equations, physicists could not design working nuclear power plants (or bombs), create lasers, or explain how the temperature of the Sun does not decrease. Without quantum mechanics, chemistry would still be in the Dark Ages and molecular biology would not have appeared at all: there would be no knowledge of DNA, no genetic engineering, nothing.

As a hypothetical example of how a macroscopic object (a cat) quite familiar to us in everyday life could exhibit quantum properties.

The very essence of these properties is the so-called quantum entanglement or entanglement. The name of this phenomenon, in general, reflects its essence. Indeed, in the example considered, the states of the radioactive nucleus and the cat turn out to be entangled (in other words, rigidly connected to each other). An important aspect It is quantum entanglement that is the presence of uncertainty in these states. That is, we don’t know whether the cat is alive or not, and we also don’t know whether the nucleus has decayed or not. However, we know for certain that if the nucleus disintegrates, the cat will die; if it does not disintegrate, the cat will live.

There is great interest in this phenomenon among modern scientists, and it is associated with the idea of ​​​​creating quantum computer, as well as the organization of secure communication channels. This is what forces attempts to be made over and over again in laboratories to create, if not cats, then at least Schrödinger’s kittens, i.e. objects are more tangible and large (mesoscopic), and therefore amenable to simpler control than individual microparticles, but exhibit the same properties of quantum entanglement as Schrödinger's cat.

But nature has created plenty of examples of quantum entanglement that are less exotic than laboratory Schrödinger kittens. Perhaps the most accessible manifestation of entanglement takes place in the same atom that we all love. Let's take the simplest of atoms - the first element of the periodic table - hydrogen. Like all other atoms, it consists of a nucleus and electrons, but the beauty of the hydrogen atom is that it has only one electron, and the nucleus is, again, a single and almost completely elementary particle - a proton, which differs from an electron in the main way positive sign electric charge and a very large mass (almost 2000 times the mass of an electron).

In one of mine, I talked about the fact that some microparticles, in particular the electron, have such a characteristic as spin, or, to use a simple analogy, they rotate around their axis in one of two directions (clockwise or counterclockwise), which , in turn, is determined by one of two values ​​of the so-called spin projection. So a proton, like an electron, has a spin and can “rotate” to the right or left. Moreover, it turns out that the “most comfortable” state with the lowest energy for the electron and proton forming the hydrogen atom is the one in which they rotate in opposite directions, as if compensating each other’s spins, so that its overall projection is zero (this fact , by the way, is used for various astrophysical observations).

It is this feature of hydrogen that hides the treasured entanglement and the tiny, atom-sized Schrödinger’s kitten. Indeed, until we have carried out appropriate experiments and measured the projections of the spin of particles, we do not know whether the proton rotates to the right or to the left. We can say the same about the electron. However, what we do know for sure is that if an electron rotates counterclockwise, then a proton rotates counterclockwise, and vice versa.

In their famous article of 1935, A. Einstein, B. Podolsky and N. Rosen pointed out the flaws in quantum theory, which operates with such entangled states (they are called EPR pairs after the first letters of the names of the authors of the article), in particular, leading to an apparent contradiction with the theory of relativity and the paradoxical violation of cause-and-effect relationships. But more on this already in.

And this is how some artists imagine quantum entanglement...