In 1900, Planck's paper opened Pandora's box and released the goblin 'quantum'. That year, Einstein, who had just graduated from the Zurich University of Technology in Switzerland, was 21 years old and was running around and looking for a job. The 15-year-old Bohr was just a naughty middle school student in Copenhagen. No one could have imagined that these two young men would become the two giants in the physics world more than ten years later. Moreover, in terms of the basic ideas of quantum theory, the two faced off against each other and started a battle that lasted until their death. of the eternal battle.
The quantum battle between Bohr and Einstein can be summed up in a famous question: Does God play dice? To explain this philosophical problem in quantum theory, we first introduce the famous Young's double-slit interference experiment.
Young's double slit experiment predates the history of quantum theory by 100 years. The original French physicist Thomas Young challenged Newton's theory of particles with this simple experiment, proving the wave nature of light. The original experimental setup was very simple, but the effects of this experiment have spread for hundreds of years. Thomas Young used light passing through a small hole as a point source of light, which was projected onto a screen after passing through two parallel slits in the paper. The observer can then see a series of alternating light and dark interference fringes forming on the screen. Interference is a phenomenon of wave, so the interference fringes appearing in the experiment is a strong proof of the wave nature of light.
In 2002, "Physical World" magazine named the top ten classical physics experiments, and "Young's double slit experiment for electrons" ranked first. Feynman believes that Young's double-slit electron interference experiment is the heart of quantum mechanics, "including the deepest mysteries of quantum mechanics."
The reader should remember the mysterious 'superposition state' in quantum mechanics that we mentioned in the first section of this article. The electron double slit experiment confirmed the existence of the electron superposition state. So how does this experiment relate to quantum mechanics? How to reveal the deepest mysteries in quantum mechanics? Where did the mysterious superposition state appear in the experiment? What does this experiment have to do with 'God throws dice or not'? These are all questions that need clarification, and let us take our time.
First of all, why are the interference fringes in the double-slit experiment a feature of waves? Let us briefly explain the formation of stripes.
Looking again at Figure 1(a), the light from the point source, as a wave, reaches the slit. According to Huygens' principle, every point on the wave surface is a wavelet source. Therefore, the wave after passing through the two slits can be regarded as the superposition of two series of waves emitted by the wavelet source located at the two slits. 'Superposition of waves' means 'superposition of amplitudes': if two waves reach the same position, the vibration direction is the same, and the amplitude increases after superposition; on the contrary, if the vibration directions are opposite, they cancel each other out, so that the amplitude decreases after superposition. Because the superimposed vibrations increase or cancel at different positions, light and dark interference fringes are formed on the screen. (The pattern on the right of Figure 1(a))
[Figure 1(c) shows the intensity distribution of light waves on the screen. The curve p we see is an image that vibrates up and down, which corresponds to interference fringes that vary in intensity between light and dark. 】
As mentioned above, (a) and (c) in the figure illustrate the situation of the 'double slit experiment', what is the figure (b)? That was the result of two 'single slit experiments'. If a slit is covered, two single-slit experiments can be performed respectively. We find that the results of these two single-slit experiments have no stripes. The distribution of light intensity in the single-slit experiment, that is, the square of the fluctuation amplitude, is determined by ( Curves p1 and p2 in b) represent.
We study the curves in (b), (c) again: p1, p2 are the intensity distributions of the single-slit experiment, and p is the intensity distribution of the double-slit experiment. Obviously, p is not equal to the simple superposition of p1 and p2. In fact, it is the square of the superposition of the amplitudes of the single-slit experiment. This is the characteristic of the wave and the source of the interference fringes.
What would be the result of a double slit experiment with particles? Readers will say: If you use particles, not waves, you won't get interference fringes. The answer is correct, but, don't forget, there are two kinds of so-called particles. In addition to the particles in the classical sense, there is also a particle with strange behavior in quantum mechanics. So, we followed Feynman's devised experiment and compared the different behaviors of water waves, bullets, and electrons passing through the double slits.
The case of water waves has just been described and is represented by FIG. 1 . Figure 2 below is the result of a double slit experiment with bullets (classical particles).
Suppose a machine gun is used to shoot the slit (Figure 2(a)), and the firing of bullets obeys the classical laws of probability and statistics. We assume that the bullets that are launched one by one and pass through the slit to reach the screen have a 50% chance of coming through the first slit and a 50% chance of coming through the second slit. Assuming that each bullet hitting the screen forms a bright spot, after a certain number of bullets are fired, there will be an image of a bright spot on the screen (right of Figure 2(a)). From the experimental results, we found that this image is different from the case of fluctuation. It is not an interference fringe between light and dark, but an image with a gradual decrease in brightness from the center to the two sides, as shown by the curve p in Figure 2(c).
Similar to the wave double slit experiment, we can also close one of the slits and do two bullet single slit experiments with the other slit. The two brightness distribution curves of the experimental results are shown by p1 and p2 in Figure 2(b). Express. Comparing Fig. 1(b) and Fig. 2(b), it is not difficult to see that the results of the bullet single-slit experiment are the same as the water wave single-slit experiment. However, the results of the double-slit experiment in the two cases are completely different. The result p of the bullet double slit experiment is a simple superposition of the results p1 and p2 of the two single slit experiments, which is determined by the superposition of probabilities.
To sum up the above, the results of the double-slit experiment of water waves are coherent superposition, reflecting the volatility of water; the results of the double-slit experiment of bullets are incoherent superposition, reflecting the particle nature of bullets. What if we experiment with electrons (or photons and other microscopic particles)?
We can use an electron gun to fire the electrons one by one toward the slit, similar to the bullet. As shown in Figure 3:
The results of the electronic single slit experiment are shown in (b) in Figure 3. The curves p1 and p2 are consistent with the water wave and bullet time. However, the result p of the electron double slit experiment is the same as that of the water wave, and interference fringes appear!
This result surprised classical physicists, because the electrons in the experiment, like bullets fired from a machine gun, were fired one by one from the electron gun. Because in classical physics, we think of electrons as particles. Since it is a particle, its macro orbital behavior should not be substantially different from that of a bullet. In the double slit experiment, although both slits are open, each electron, like a bullet, can only reach the screen through one of the slits. In this way, the result should be the same as that of the bullet, which should belong to incoherent superposition.
The experimental observations also show that the electrons are indeed like bullets, reaching the screen one by one, as shown in the figure below, corresponding to each electron reaching the screen, a bright spot appears on the screen. As the number of emitted electrons increases, there are more and more bright spots, more and more…. When the bright spots are too numerous to distinguish, a definite interference pattern is displayed on the receiving screen. so, what happened? Where does this interference come from? From the electron double slit experiment, we will draw a seemingly absurd conclusion: an electron passes through two slits at the same time, and then interferes with itself!
Let us use the concepts of quantum theory to understand the unusual non-classical behavior of the electron: the electron in the experiment passes through two slits at the same time, not just as we said in Section 1: 'The electron is in A superposition state, a situation in both position A and position B'? As a superposition particle in quantum theory, each electron (or photon) is really like Sun Wukong. It has a clone technique. When a Sun Dasheng reaches two slits, he becomes two great saints and passes through the two slits at the same time. Sew! Then, the two real and fake Monkey Kings fought with themselves again! The result of the battle may be a win-win situation, and a big Monkey King will be created, and the screen will be extremely bright; it may also be lost, and the real and the fake Wukong will all die. At that time, it corresponds to the dim place on the screen.
Therefore, the results of the double slit experiment show that the behavior of electrons is neither equal to classical particles nor classical waves. Like light, it is both a particle and a wave, and has the dual characteristics of both particle and wave. Iconic.
Readers may say: each electron comes through that slit, we should be able to measure it. Yes, physicists think so too. So, they put two particle detectors at the two slits to determine which side of the real Monkey King is going? But then, something strange happened again: the two particle detectors never sounded at the same time! Well, that means there is still only one Sun Wukong, and there is no clone. The experimenter felt relieved. He just wanted to think about the interference fringes. He looked back at the screen. Huh? There are no interference fringes. Physicists improved and repeated their experiments over and over again, only to wonder more and more: No matter what advanced measurement methods we use, which slit does an electron go through if we want to see it? The interference fringes disappeared immediately! That is to say, the fake Monkey King is too cunning, he seems to always know that we have set a trap to catch him, so he hides and hides. Wukong doesn't need clone technique, there is no war between true and false saints, there is no superposition and casualties on the battlefield, everything is calm, and the experiment gives the classic result: it is exactly the same as the image of the bullet experiment! Later, physicists gave this phenomenon of "observations affecting the quantum behavior of particles" an odd name: "wave function collapse." That is to say: once the quantum superposition state is measured, it will collapse to a fixed eigenstate according to a certain probability and return to the classical world. Before being measured, the particle is in an uncertain state of mixed superposition of 'both this and that'. Therefore, we cannot predict how the particle will behave in the future, only the probability that it may collapse to an eigenstate.
The above explanation basically uses the interpretation of quantum theory by the Copenhagen School represented by Bohr. In other words, Sun Wukong has the ability to go through two holes at the same time. However, you can't know what happened to his kung fu. He will never let you see the details of his avatar technique. He only lets you know a few probabilities. God sent him to the world to roll dice!
Einstein disagreed with the Copenhagenist interpretation and said angrily: "Bohr, God doesn't play dice!"
Bohr was unhappy: "Einstein, don't tell God what to do!"
Decades later Hawking, looking at the experimental records over the years, said a little dejectedly: "God not only throws the dice, he also throws the dice where we can't see it!"
Does God throw dice? Despite the affirmative answer given by Hawking's remarks above, it still seems to be an open question.