With the development of artificial intelligence, virtual reality, etc., the amount of data to be processed is increasing, but the limits of integrated circuits are approaching. So instead of gates made of transistors, quantum computers that use quantum as their operational rules are emerging as an alternative. What exactly is quantum, and what can we do with it as an alternative? A reporter who has no connection with science takes a close look at everything from quantum to quantum computers, which are a recent issue, with a learning heart.


Copenhagen interpretation
1. The state of a particle is determined by its wave function. The square of the wave function represents the probability density for the measured value.
2. All physical quantities are meaningful only when they can be observed. The physical quantities possessed by physical objects are not objective values unrelated to observation, but values that are affected by the observational action.
3. It is impossible to measure physical quantities that are related to each other simultaneously and accurately according to the uncertainty principle proposed by Heisenberg.
4. Particles such as electrons have complementary particle and wave properties.
5. Quantum jumps are possible. The states allowed in quantum physics can only have discrete, specific physical quantities. Therefore, in order to change from one state to another, one must disappear from one state and appear in the other state at the same time.


Scientists' quintessence of energy, quantum mechanics
As we have seen above, quantum mechanics was triggered when Max Planck proposed the quantization hypothesis while solving the black body radiation problem. In the early 20th century, quantum mechanics, which is different from classical mechanics, was established in physics, which was considered to have nothing more to discover. Quantum mechanics was not created by a single genius scientist. Of course, the theory of relativity was created by a single genius scientist. The foundation of quantum mechanics was established through the overlapping of research results of numerous scientists. Even the research results of scientists who are negative about quantum mechanics.

The quantum mechanics system established in the 1920s was successful in the phenomena, predictions, and applications of microscopic systems. However, the results of the quantum mechanics formulas were very different from the intuition that humans have had since birth. Even outstanding physicists like Albert Einstein had difficulty accepting them. Therefore, various methods of interpreting quantum mechanics emerged to explain the results of the quantum mechanics formulas.

The Copenhagen interpretation refers to the interpretation centered around Niels Bohr, Werner Heisenberg, and Max Born. The Copenhagen interpretation is currently the mainstream interpretation of quantum mechanics. However, it is not absolute. This is because there exists the many-worlds interpretation, which Hugh Everett argued in 1957. The many-worlds interpretation will be discussed in the next article.
Bohr became famous for his research on the quantum nature of atoms. In 1918, the Danish government approved the establishment of Bohr's Institute for Theoretical Physics. Bohr established the institute in Copenhagen. Many physicists from all over the world flocked to the Copenhagen Institute and stayed there for many years to conduct research.

Copenhagen immediately became a center for quantum mechanics research, where a group of scientists led by Heisenberg later created the Copenhagen interpretation.


The big picture, the principle of complementarity and the principle of uncertainty
The Copenhagen interpretation is based on Bohr's complementarity principle and Heisenberg's uncertainty principle. First, let's look at what the two principles are.

The complementarity principle states that particles that make up atoms have two completely different properties, such as wave and particle, but both properties are necessary to completely describe the phenomena related to the particles that make up atoms. Light shows the property of a 'wave' in experiments such as interference or diffraction, and the property of a 'particle' in experiments with the photoelectric effect. However, both properties do not appear at the same time in one experiment. It was confirmed that particles such as electrons and protons have the same properties. Bohr summarized this duality of light and particles as the complementarity principle.

The uncertainty principle is the principle that there is always a certain degree of uncertainty between the observer and the observed. It is not a theory of a specific person, but rather a basic premise of quantum theory, and was organized by Heisenberg based on the research results of Bohr, Cramer, Slater, and others.

Bohr and Heisenberg worked together on quantum mechanics in Copenhagen from around 1927. By studying the frequencies at which light is emitted under various conditions, they generalized the conditions for the quantization of photon energy that had been assumed in the work of Planck, Einstein, and Bohr himself.
The two men broke away from the classical mechanics view of physical objects as either particles or waves and came up with the idea that particles could be both waves and particles. Bohr's new theories were based on many experiments at the time and the fact that matter had dual properties of waves and particles. And then Heisenberg announced the uncertainty principle, which states that both the position and momentum of a particle cannot be measured accurately.

The Copenhagen interpretation of quantum mechanics, which was opened by Bohr and Heisenberg, concluded that human 'observation activities' of events 'change the reality' of events. The core of the Copenhagen interpretation is that it is unnecessary to say that the value of a physical quantity exists before the act of measurement. On the contrary, in classical mechanics, the physical quantity expressed in a formula exists independently of human measurement activities. In other words, according to the Copenhagen interpretation, in quantum mechanics, both the observer and the object should be considered, not just one.

Quantum mechanics has met with resistance from many physicists. At the 5th and 6th Solvay conferences held in Brussels in 1927 and 1930, Bohr preached to the physicists of the time his interpretation of quantum mechanics based on the principle of complementarity that he had proposed. At this time, a debate broke out between Bohr and Einstein, and this debate solidified the foundation of quantum mechanics.


Building a new foundation with philosophy
Bohr and Heisenberg were devoted to laying the philosophical foundations for broadening our intuition about quantum mechanics.

According to Heisenberg's uncertainty principle, we must use a microscope with a short wavelength to know the exact position of an electron. However, due to the influence of the Compton effect, we cannot help but obtain an inaccurate value for the momentum of the electron. In other words, the position and momentum are in an uncertain relationship within a very small range.

Meanwhile, Bohr, who had been preoccupied with the philosophical foundations of quantum mechanics since losing to Einstein in the debate over the existence of light quanta, also reached a similar view. We can only describe the microscopic system of atomic phenomena based on the terms of the macroscopic system and the concepts obtained from it. Therefore, our terms for describing the microscopic system are limited. A term that is consistent with contradiction is restricted by the complementary relationship between definability and observability.
For example, when microscopic phenomena, such as the interaction between light and matter, are defined by observational propositions of macroscopic systems, such as particles or waves, certain restrictions are imposed. With this complementarity principle, Bohr was able to escape the dilemma of the duality of waves and particles.


Einstein was not satisfied at all
Einstein could not accept the indeterministic nature of quantum mechanics represented by the two principles of Bohr and Heisenberg. Einstein was not the only one to criticize the indeterministic nature of quantum mechanics. Max von Laue, Erwin Schrödinger, and Planck were also critical of the Copenhagen interpretation. Einstein, in particular, was the most famous and widely known scientist among them, and his position was particularly prominent. And Einstein continued to argue with Bohr endlessly, not believing in quantum mechanics until his death.

In 1935, Einstein, along with Boris Podolsky and Nathan Rosen, made a sharp criticism of quantum mechanics.
The three, who are abbreviated to EPR by taking only the first letters of their last names, assumed that a theory is complete if it satisfies the following conditions: "Every element of physical reality must have a counterpart in a physical theory. If, without disturbing a system in any way, the value of a physical quantity can be predicted exactly, that is, with a probability equal to 1, then there exists a physical reality corresponding to this physical quantity." After examining quantum mechanical descriptions based on this criterion of completeness, EPR came to the conclusion that the quantum mechanical description of physical reality given by the wave function is not complete.

This is the EPR paradox, which states that when there are two quantum entangled particles, there must be some element of reality that quantum mechanics cannot explain, assuming the criterion of reality proposed by EPR and the special theory of relativity.

In the 1950s, David Bohm proposed the deterministic hidden variable theory, which attempted to revive Einstein's causal position. In 1964, John Bell proposed the so-called 'Bell's inequality', which can be used to verify problems in quantum mechanics through experiments.

Let's assume the locality assumption that EPR made, that is, if a physical quantity is measured on one side, the other side always gives the opposite value for the same physical quantity. When considering these three physical quantities, it was shown that the common sense inequality that simply correlates physical quantities does not hold in quantum mechanical calculations. The generalization of this is Bell's inequality.

Since the 1970s, several experiments have been conducted to verify this Bell's inequality. The results of the experiments? Bohr's victory. All quantum mechanics experiments failed to satisfy Bell's inequality.


The cat may be alive or dead.
Schrödinger also strongly criticized the Copenhagen interpretation in 1935, which states that the observer's act of measurement affects the object. The thought experiment he proposed at that time was later called 'Schrödinger's cat'. Schrödinger really hated the Copenhagen interpretation based on the uncertainty principle. So, in order to reveal the absurdity of the Copenhagen interpretation, he expanded the events of the microscopic system to the events of the macroscopic system.
“A cat is locked in a box. The box is connected to a machine containing a radioactive nucleus and a canister containing poison gas. At the beginning of the experiment, the probability of the nucleus decaying within one hour is adjusted to 50%. If the nucleus decays, poison gas will be released and the cat will die.”

In this situation, Schrödinger criticized the expression of the wave function as a combination of the cat's alive and dead states, and said that a "cat that is both dead and alive" does not actually exist.

“The cat must be either alive or dead. Quantum mechanics is incomplete and unrealistic, since there can be no cat that is both alive and dead.”

The Copenhagen interpretation states that a cat is a superposition of being alive and dead, and that its being alive or dead is determined by the observation.

Ironically, Schrödinger's cat has become the classic quantum mechanics experiment that everyone, except cat owners, likes to talk about because the metaphor is so catchy.

We have seen how quantum mechanics was established through the Copenhagen interpretation and its criticisms. In the next article, we will look at the many-worlds interpretation.