The Copenhagen Institute is 100 years old The quantum world is still foggy

The "Copenhagen spirit" created by Bohr cannot be replicated. This is an academic atmosphere that is improved in the discussion, perfected in the debate, equal and unrestrained discussion and close cooperation, that is, to promote the development of quantum mechanics in the debate, which is very in line with Bohr's personality and proposition.

In 1900, in order to solve the problem of blackbody radiation, the German physicist Planck proposed the "energy son" model. At that time, Einstein was 21 years old and had just graduated from college; Bohr was just 15 years old, probably still in middle school.

In 1905, Einstein proposed the "light quantum" hypothesis, which successfully explained the photoelectric effect by imagining light that "should be an electromagnetic wave" as a particle of light. Immediately after, Bohr proposed the quantized atomic structure in 1913, successfully explaining the energy level structure and spectra of hydrogen atoms, and won the Nobel Prize that year.

In 1921, Bohr refused the offer of his mentor Rutherford to work and decided to create the Institute of Theoretical Physics of the University of Copenhagen to continue his in-depth study of quantum mechanics. As soon as the institute was established, Bohr's personality charm soon attracted a large number of outstanding young physicists like a magnetic field, and quantum mechanics giants such as Heisenberg, Pauli, Born, and Dirac all came from this institute, forming the world-famous "Copenhagen School". In addition to Bohr, the institute produced nine Nobel Laureates in Physics, an unprecedented event. To this day, 100 years later, Copenhagen remains a "pilgrimage site" for physicists.

"Did God roll the dice?"

The Copenhagen School began to think philosophically about quantum problems at the beginning of its founding. What does it mean that light and electrons are sometimes like waves, and sometimes like particles? What is its origin? Why can we only see that it is a wave or a "side" of a particle every time? Eventually, the "Copenhagen Interpretation" of quantum mechanics surfaced:

The quantum state of a quantum system can be fully expressed as a wave function. The wave function represents all the information an observer knows about a quantum system, and the description of a quantum system is probabilistic.

In a quantum system, the position and momentum of a particle cannot be determined at the same time.

Matter has wave-particle duality, and an experiment can show the particle behavior or wave behavior of matter. But you can't exhibit both behaviors at the same time.

- Measuring instruments are classical instruments that can only measure classical properties, such as position, momentum, etc.

The quantum physical behavior of large-scale macroscopic systems should be approximate to classical behavior.

The Copenhagen School's "probabilistic interpretation" of quantum mechanics has a core idea of "measurement collapse". A quantum-scale object whose current state is actually a "probabilistic superposition" of several possible states; we cannot predict the true state of the object until it is measured, which is a true randomness; once the measurement is initiated, the object "collapses" into a possible state, as if it had always been that state.

Although the "probabilistic interpretation" is similar to the "dice experiment", quantum mechanics is more pure. The randomness of the dice is actually pseudo-random, and the "thousand masters" can completely control the results of the rolling of the dice. But the Copenhagen School believes that going back to the microscopic quantum world, no matter how absurd the facts, may appear in the quantum world.

Thus began the famous controversy in the history of science. The classics, led by Einstein, launched a fierce attack on the Copenhagen School, and each tit-for-tat confrontation sparkled with wisdom and even art.

Albert Einstein: God doesn't roll dice!

Bohr: Please don't tell God what to do!

This classic dialogue is the beginning of this controversy and the core of the debate that follows.

Schrödinger, who was in the same line as Einstein, gave an example of what he himself considered "absurd" in order to refute the theory of "probabilistic interpretation": a cat was placed in a box with a poison release system driven by radioactive elements, and before opening the box to observe, the cat was in a superposition of "alive" and "poisoned" at the same time, which was the famous "Schrödinger's cat".

It is worth mentioning that Schrödinger certainly did not expect that the experiment he proposed to refute the Copenhagen doctrine, which was absurd and incompatible with reality, was mistaken by many people as a classic thesis of "probabilistic interpretation".

It is generally believed that what one person has not achieved in science will certainly have another person succeed in the future. But the "Copenhagen spirit" created by Bohr cannot be replicated. This is an academic atmosphere that is improved in the discussion, perfected in the debate, equal and unrestrained discussion and close cooperation, that is, to promote the development of quantum mechanics in the debate, which is very in line with Bohr's personality and proposition.

Today, many years after the controversy of the century, the copenhagen school of physics, the "probabilistic interpretation" theory, which is most widely recognized, still faces many controversies.

Although Bohr and Einstein argued all their lives, it did not affect their friendship in the slightest. Whoever leaves whom, this quantum galaxy will become bleak.

"No one really understands quantum mechanics"

In his 1958 book Physics and Philosophy: Revolution in Modern Science, Werner Heisenberg recounts how, after those long-day conversations with Nils Bohr, he kept saying to himself, "Is it really possible for nature to be so absurd?" Because the quantum world seems so irrational and so counterintuitive, Richard Feynman famously said about it: "No one really understands quantum mechanics." ”

Although quantum mechanics has proven its predictive power time and time again, this does not diminish the fact that in addition to the Copenhagen interpretation, there are many interpretations of quantum theory in the scientific community.

The multi-world interpretation holds that the wave function is physically real, and the Schrödinger equation is precisely a description of reality. When you measure stacked particles that are in different positions at the same time, the particles being measured actually appear in different versions of reality in all those locations. In other words, this means that the two "realities" you are in forked into two different branches. It sounds like if you've done something bad in this reality

Decide, don't worry, maybe in another reality, you can still get a perfect result. Multi-world interpretation is also one that is often borrowed by popular culture. The multi-world interpretation raises a problem – it makes probability meaningless.

In order to solve the problem of probability in the multi-world interpretation, some scientists have developed a cosmological interpretation. This interpretation holds that if there are infinite universes, then the interpretation of so many worlds must hold, because there are infinite "you" experimenting, and reality will split according to the proportion of probability. In this way, classical probabilities still make sense.

There are also scientists who have proposed the conjecture of "hidden variables": perhaps the properties of particles have some "secret" variables, which are actually a definite state, but we do not know it, and we will not find it until we measure it.

Scientists have proposed the "non-localized hidden variable theory" on the basis of the hidden variable theory, that is, the De Broglie-Bohm interpretation. This interpretation holds that particles are real, and that they move under the guidance of a guided wave that we cannot see.

In addition, there are quantum Bayesianism, quantum Darwinism, transaction interpretation, relational interpretation, etc., and many physicists have put forward different ideas and perspectives. But some scientists therefore believe that the emergence of so many different interpretations means that there are still some very basic and fundamental parts of quantum mechanics waiting to be discovered.

"Multiple Worlds" vs. "Multiple Histories"

The core idea of these various interpretations of quantum mechanics is essentially derived from a multi-world interpretation that is logically concise, physically profound, but the imagery is very counterintuitive. Multi-world theory expresses "one wave function, many worlds", and the various interpretations developed by it are mostly based on self-consistent history, talking about "one world, multiple histories", but their respective focuses and emphases are different.

Due to improper interpretation and false rumors, many world interpretations have been stigmatized for a long time. In particular, many people feel that the Copenhagen interpretation is correct, and the multi-world interpretation is considered metaphysical, even pseudoscience.

It is precisely because the Copenhagen interpretation of quantum mechanics emphasizes the need to draw on the classical world that it is logically inconsistent. Philosophically, the Copenhagen version of quantum mechanics is a dualism, and an ideal perfect theory should be monism: everything stems from quanta, and classics are just "derivative" phenomena under the macroscopic limits of quantum systems.

In any case, by introducing unobservable properties into physics, physics becomes metaphysics. It is imperative to find an explanation for the wave function that can be measured and verified, and that observations do not cause the wave function to collapse.

Quantum mechanics has brought about change in the world in many fields. From the first transistors to today's tech society, to quantum computers that may become a reality in the near future. It doesn't matter that we can't fully understand it, because physicists themselves can't fully understand it. All we can do is follow Feynman's advice, "relax and enjoy," and then look forward to the birth of a "new physics" that will open a new chapter in the quest for the world's universe. (Reporter Wu Changfeng)

Source: Science and Technology Daily