1927 In the spring, Werner Heisenberg, a 25-year-old German physicist, published a paper, which raised a profound philosophical question: How do we know the physical world around us?
The answer seems obvious-by observation and measurement. But in physical experiments, microscopic particles show the essence of fluctuation, and electromagnetic waves show the essence of particles. Quantum theory calls this strange phenomenon "wave-particle duality". Heisenberg realized that it is of great significance to answer the question of how we know the physical world.
Heisenberg pointed out that observing microscopic particles requires light or other forms of electromagnetic waves. Suppose we want to locate a moving electron. Because electrons are so small, we can only "see" them by short-wavelength electromagnetic waves. According to quantum theory, this kind of electromagnetic wave also has momentum similar to that of particles. So Heisenberg said that any observation of electrons is a "collision" between electromagnetic waves and electrons, which will inevitably change the speed of electrons. The more you want to accurately determine the position of electrons, the shorter the wavelength of electromagnetic waves used, the more intense the "collision" of electrons, and the greater the speed change of electrons. In other words, it is impossible to measure the position of an electron without affecting its speed-the more certain the measurement is, the more uncertain it is. We can never understand these two aspects accurately at the same time.
Heisenberg deduced a series of mathematical formulas, which formed the core of uncertainty principle. Heisenberg believes that if we can't get accurate knowledge about the current state of electrons, we can't predict what state it will be next-we can only know the probability of that state at most. Heisenberg's conclusion is that the uncertainty principle is an inevitable part of our observation and measurement, the limit of human understanding of the surrounding physical world, and the challenge to causality in the philosophical category. Heisenberg pointed out that if human beings expect to be as omniscient as God, the uncertainty principle will always be an insurmountable obstacle.
Heisenberg's paper caused a heated debate among physicists at that time. Niels bohr, the head of Copenhagen School, basically agrees with Heisenberg's theory, but thinks that the factors causing uncertainty are far more complicated than the "disturbance" caused by observation and measurement. Bohr believes that the basis of uncertainty is complementarity. Bohr pointed out that different mutually exclusive properties in classical theory become complementary aspects in quantum theory, and wave-particle duality is an important manifestation of complementarity. So the causal relationship in the classical sense no longer exists, which is the famous "complementary principle".
Heisenberg's opponent is Albert Einstein, the greatest physicist. As soon as Heisenberg's paper was published, Einstein began to question the uncertainty principle. Although physicists quickly solved the major scientific mystery of radioactive decay and solar nuclear fusion according to the uncertainty principle, Einstein still insisted that this theory was only a manifestation of ignorance-all these uncertainties showed that quantum theory was not perfect.
1935, Einstein proposed a hypothetical experiment that he thought could refute the uncertainty principle: imagine that a molecule consists of two atoms, A and B, and then the molecule splits and emits A and B in opposite directions. According to Heisenberg's uncertainty principle, any measurement of the exact position of A will make it difficult for us to know the exact speed of A, but Einstein thinks there is a way to do it: according to Newton's law of action and reaction, this means that A and B must move in opposite directions at the same speed. So he pointed out that we can determine the state of A by measuring the position of A and the speed of B at the same time.
In response to Einstein's challenge, Bohr put forward a rebuttal: the uncertainty principle affects both A and B, that is, when we measure the position of A, the measurement behavior will immediately affect the speed of B, so that the measurement results completely conform to the uncertainty principle. What's even more incredible is that Bohr thinks that even if two microscopic particles are far apart, this effect will happen instantly. On the surface, Bohr's argument broke Einstein's law that "the speed of motion cannot exceed the speed of light". But Bohr believes that the pair of microscopic particles have never really separated, and once they are formed at the same time, they will be "entangled" together forever. Einstein said indignantly that he could not accept Bohr's "strange" explanation at all.
1982, the French physicist Alain Aspe conducted a quantum entanglement experiment, and the results proved that Bohr was right. Today, the "quantum entanglement" effect provides a theoretical basis for a new communication method-"quantum communication" technology: so far, the transmission of secret information has to risk the password falling into the enemy. The experiment of "quantum entanglement" shows that even if the pairs of microscopic particles are far apart, one of them can immediately show whether the other is observed. If information is transmitted by entangled photons, any attempt to illegally read information will be immediately discovered.
When Heisenberg first put forward the uncertainty principle, it shocked the physics circle at that time. Over the past 85 years, it has profoundly changed human epistemology and world outlook. At first glance, the principle of uncertainty seems to be completely negative, and uncertainty is a restriction on human understanding of the world. However, only by acknowledging uncertainty can human beings make progress in the process of understanding the world.