The two pillars of 20th-century physics, one is general relativity, and the other is the quantum mechanics I'm going to talk about here, and there is a world of difference between them.
Both theories tell us that the subtle structure of nature is more subtle than we see. General relativity is a small gem: it was conceived by Einstein on his own, a concise and unified view of gravity, space, and time. Quantum mechanics, or "quantum theory," on the other hand, has been incredibly successful in experiments and its applications have changed our daily lives (the computer I use to write articles, for example, is relevant to it). But more than a hundred years after its birth, this theory is still shrouded in a mysterious and bizarre atmosphere.
Born in 1900, quantum mechanics has led almost an entire century of intense thinking. The German physicist Max Planck calculated the electromagnetic field in equilibrium within a "hot box" . To this end, he used a clever method: it is assumed that the energy of the electromagnetic field is distributed on the "quanta", that is, the energy is packed or piece by piece. The results calculated by this method are completely consistent with the measured data (so they should be correct), but they are contrary to the perception of the time, because people at that time thought that energy was continuously changing, and it was nonsense to say that it was composed of a pile of "broken bricks".
For Planck, treating energy as a collection of energy masses was just a special strategy used computationally, and even he himself didn't understand why it worked. Five years later, however, it was Einstein who finally realized that these "energy packs" were real.
Einstein pointed out that light is made up of packets of light particles, which today we call "photons". In the introduction to that article, he wrote:
"In my opinion, if we assume that the distribution of light's energy in space is discontinuous, we can better understand the phenomena related to blackbody radiation, fluorescence, cathode rays produced by ultraviolet rays, and other phenomena related to the emission and conversion of light." According to this hypothesis, the energy of a beam of light emitted by a point light source is not continuously distributed in an increasingly wide space, but consists of a limited number of "energy quanta", which are distributed in a point-like manner in space, as the smallest unit of energy emission and absorption, and the energy quanta are no longer indivisible."
These few words are simple and clear, and they are the true declaration of the birth of quantum theory. Notice that this passage begins with the extraordinary words "in my opinion", which is reminiscent of Darwin's great ideas on the evolution of species beginning with the words "I think" in his notes, while Faraday referred to his "indecision" when he first introduced the revolutionary concept of electromagnetic fields in his writings. Great geniuses know how to think twice.
At first, Einstein's achievement was ridiculed by his peers, who thought the young talent was talking about it. Later, Einstein won the Nobel Prize for this research. If Planck was the father of quantum theory, Einstein was the nurturer who made it thrive.
Like all children in the world, quantum theory grew up to be on its own path, and later Einstein no longer recognized the child. In the 1910s and 1920s, danish Dane Niels Bohr led the development of this theory, he learned that the energy of electrons in atomic nuclei, like light energy, can only be specific values, and more importantly, electrons can only "jump" from one atomic orbit to another under specific energy, and release or absorb a photon at the same time, which is known as "quantum jump". Bohr's institute in Copenhagen is home to some of the most talented young scientists of the 20th century who work together to try to establish order for the perplexing phenomena of the atomic world in a bid to create a self-consistent theory.
In 1925, the equations of quantum theory finally emerged, replacing the entire newtonian mechanics. It's hard to imagine anything greater than that. Suddenly, all phenomena have found their home, and everything can be calculated. Here's just one example: You remember the periodic table, right? It was Mendeleev's one. It lists all the elements that may appear in the universe, from hydrogen to uranium, and many school classrooms have this table hanging. So why are these elements listed on the table? Why is the structure of the periodic table like this? Why do these elements and cycles have such characteristics? The answer is that each element is a solution to the most important equation of quantum mechanics. The entire discipline of chemistry is based on this one equation.
The first to outline the equations for this new theory was a very young German genius, Werner Heisenberg, whose ideas were simply dizzying.
Heisenberg imagined that electrons didn't always exist, only when someone saw them, or rather, only when interacting with other things. When they collide with something else, they appear somewhere with a computable probability. The "quantum jump" from one orbit to another is the only way they appear: an electron is a series of jumps under an interaction. If it is not disturbed, the electron has no fixed shelter, and it does not even exist in a so-called "place".
It seems that God did not draw a heavy stroke when designing reality, but only vaguely outlined it with dots.
In quantum mechanics, nothing has a definite position unless it hits something else. To describe the leap of an electron from one interaction to another, an abstract formula is used that exists only in abstract mathematical space and not in real space.
To make matters worse, the leaps of these things from one place to another are mostly random and unpredictable. We cannot predict where an electron will appear again, but only calculate the "probability" that it will appear here or there. This probabilistic problem strikes at the heart of physics, but all the problems of physics are controlled by the universal and immutable iron laws.
Isn't that ridiculous? Einstein thought the same thing. On the one hand, he nominated Heisenberg for the Nobel Prize, admitting that he had explored some of the most essential things in the world. But on the other hand, he complains at every opportunity that it is too unreasonable.
The spirited young scientists in Copenhagen were frustrated: Why was Einstein against them? Their spiritual father, who had the courage to think about questions that no one else dared to think about, now retreated, afraid of this new step toward the unknown world, a path he had personally opened up? Why Einstein? It was he who taught us that time is not universal, that space can be bent, but now he says that the world cannot be so absurd and bizarre.
Bohr patiently explained these new ideas to Einstein, and the cute Instein did not agree. To prove that these new ideas are contradictory, Einstein devised thought experiments: "Imagine a box filled with light, and we allow a photon to escape in an instant..." The "photon box" thought experiment began, one of his famous examples. But in the end Bohr was always successful in refuting Einstein's views. Through lectures, letters and papers, the dialogue between the two scientists continued for many years... In the process of communication, both great people had to make concessions and change their views. Einstein had to admit that there was no contradiction in these new ideas; and Bohr had to admit that things were not as simple and clear as he had originally thought. But Einstein was reluctant to make concessions where the most critical matters were, insisting that there was an objective existence independent of interaction. Bohr insisted that this new and profound way of being, as determined by the new theory, was valid. Finally, Einstein acknowledged that quantum theory was a huge advance in the process of human understanding of the world, but he was convinced that things could not be so absurd and bizarre, and that there must be a more reasonable explanation "behind" all this.
A century later, we are still at the same point. The equations of quantum mechanics and the results derived from them are applied every day in physics, engineering, chemistry, biology, and beyond. Quantum mechanics is of vital importance to the overall development of contemporary technology. Without quantum mechanics, transistors would not have emerged. However, these equations remain mysterious because they do not describe what happens within one physical system, but only how one physical system affects another. What does this mean? Does this mean that the real existence of a system cannot be described? Does that mean we're still missing a piece of the puzzle? Or, in my opinion, does that mean that we have to accept that "the so-called truth is nothing but the result of interaction"?
There is no doubt that our knowledge is growing. We can do things that we didn't even dare to think about before. The growth of knowledge also opens up new questions, new mysteries. People who apply quantum mechanical equations in the lab are usually less concerned with the equations themselves, but in recent years, more and more physicists and philosophers have continued to explore this issue in papers and conferences. Quantum theory has been around for more than a hundred years, but what exactly is it? Is it an extraordinary and profound inquiry into the nature of the world? Was it a beautiful mistake of luck? Is it part of the mystery of incompleteness? Or is it a clue to such a difficult problem as the structure of the world, but we can't digest it yet?
When Einstein died, Bohr, his strongest rival, expressed his admiration for him, which was deeply moving. A few years later, when Bohr also died, someone took a picture of the blackboard in his study. On the blackboard is a picture of the "box full of light" from Einstein's thought experiment. Until the last moment of his life, he was still challenging himself and still wanted to know more. Until the last moment, he did not stop doubting.