This reasoning process is a bit brain-burning. But this is a theory decades or hundreds of years ago. So it should be no problem for modern people to understand. If you still don't understand, you can only say that I didn't make myself clear enough.
One question that must be understood is: Is light a wave or a particle?
Young's double-slit experiment proves that light is a wave. First, Maxwell used equations to solve electromagnetic waves; Thirdly, it is found that the speed of electromagnetic wave is the same as the speed of light; So it is reasonable to speculate that light is electromagnetic wave. So, light is a wave.
No problem in theory. But this theory needs to be tested. Physics is not mathematics, and there is always something unreasonable. Cows run as fast as rabbits. Can you say he is a rabbit? Therefore, tests are needed to prove that cattle people are rabbits. At least you should prove that he has four legs and a short tail.
1803, British Thomas? Yang made a famous double-slit experiment. The experimental tools at that time were very rough. The light source is a lighted candle, with a shutter behind it and a screen behind it. There are two gaps in the shielding plate, and the light of the candle can only hit the back screen through these two gaps.
Then, a magical scene happened. On the screen, alternating light and dark, circular and orderly stripes are formed. What does this mean? This shows that light passes through two gaps in the form of waves, and then two sets of light waves interfere on the screen. When the peaks meet, they just overlap, and when the peaks meet, they just cancel.
Therefore, light is a wave, like water waves.
Planck to Einstein proved that light is a blackbody in particle blackbody radiation, that is, it does not reflect light, but only emits its own light. Like the sun, the hot old iron, and the human body in the dark, all can be approximated as blackbody. The light emitted by a blackbody is due to its heat, which is a kind of thermal radiation.
However, problems have arisen. At a given temperature, the frequency of light emitted by a black body forms a curve that no one can explain.
You regard light as an electromagnetic wave, and then use statistical mechanics to calculate it, but you can't solve the equation of blackbody luminescence anyway.
So, Planck shot, he only looked at the curve, only used mathematics, and only worked out an equation. However, to satisfy this equation, there is a premise that the light emitted by the black body must be one. Simply put, the light radiated by a black body is continuous, not like a wave, but like a bullet.
So the explanation of blackbody radiation is that the light radiated by blackbody is a copy.
At the same time, physicists found a strange phenomenon: when a beam of light hits the metal plate, the metal plate will emit electrons. The explanation is simple. Light is an electromagnetic wave. Electromagnetic waves have energy. The energy of electromagnetic waves pushes electrons, which are knocked away. But the problem is how electrons run, which has nothing to do with the intensity of light, but only with the color of light. In terms of physical quantity, it is related to the frequency of light. High-frequency green light and blue light, no matter how weak the intensity, will knock out electrons; No matter how strong the low-frequency red light is, electrons will not be knocked down.
Light is electromagnetic wave, and the energy of electromagnetic wave is only related to the intensity of light, not to the frequency. So the higher the intensity, the higher the energy, and then the more electrons are knocked out. But whether electrons can be knocked out has nothing to do with the intensity of light waves, but only with the frequency of light. So electromagnetic waves can't explain this problem.
Next thing you know, Einstein did it.
Einstein thought that not only the light emitted by blackbody radiation is a copy, but as long as it is light, it is a copy. Light is not a continuous wave, but is made up of photons. The energy of each photon is its frequency multiplied by Planck constant. Therefore, the higher the frequency, the higher the energy. As for the intensity of light waves, it is not important. What matters is the frequency of light. If the frequency is enough and the photon energy is enough, the electrons can be knocked out naturally. It can be inferred that light is a particle, just like a bullet.
What is light: wave-particle duality. Next, we have to ask: Is light fluctuating or particle? You can think that the light formed by photons is fluctuating, just like water polymerized by water molecules; And a single photon is a particle, just like a bullet.
However, Young's double-slit experiment was later accurate to the extent of a single electron, but it still showed the nature of waves.
There are all kinds of experiments and theoretical reasoning in the middle, which have been brushed aside. A physicist named de Broglie put forward an insight that electrons, like photons, have volatility, and all substances have volatility. The reason why you can't see the fluctuation of matter is because the fluctuation is too small to be observed. For example, a ball with a mass of 3 kg moves at a speed of 10 meter per second. According to the calculation, its wavelength is 10 minus 35 meters.
So light has wave-particle duality.
196 1 year, physicists did the young's double-slit experiment with a single electron. Experimental results: Even if you only emit one electron at a time, the more electrons you accumulate, the interference fringes will still appear on the screen.
If electrons are particles, then the back of the screen should be like target paper, and there is no possibility of interference fringes. So, the only explanation is that a single electron passes through two gaps at the same time: it is here and there.
You're burning your brain. However, the above only needs to remember one conclusion, that is, the world is made up of atoms, which have wave-particle duality.
Then, more brain burning problems and experiments appeared.
When is a wave and when is a particle? You put a detector behind the screen of Young's double-slit experiment and stare at the single slit to see how photons move. What will happen at this time? Photons shoot out from a gap like bullets, and there are no interference fringes on the screen, which are particles rather than fluctuations.
If there is no screen in the experiment, two detectors are used to stare at two gaps respectively, and then photons either pass through this gap or that gap, and the behavior of photons is particles.
If a screen is placed in the experiment, and the screen blocks the viewing angles of two detectors, photons will pass through two gaps at the same time, and then interference fringes will be formed on the screen.
What's the matter with you?
When I observe this way, your photons fluctuate and form interference fringes on the screen; When I look at it like that, your photons are particles, flying like bullets.
In other words, whether a photon behaves as a wave or a particle depends entirely on my observation. Wave or particle, photon said: I can do it, see if you like it or not. You put a screen in the back, and then I'll give you wave interference fringes; You put a detector on me, and I'll give you particle motion.
Conjecture and experiment of delayed selection So, in the late 1970s, physicist Wheeler put forward the delayed experiment. To put it simply: not putting the screen at first is staring at the gap, which is equivalent to staring at how photons move; When your photon passes through the double slit completely, suddenly put it on the screen to see if you can still wave out interference fringes.
There are many experiments, all kinds of complicated designs and advanced instruments are used, because physicists have to toss photons to death.
Too complicated operation design will not be explained, and the general steps can be summarized as follows:
There is no screen behind the double seam, and the detectors are used to stare at the two seams respectively: at this time, photons will definitely be like bullets, either from this seam or from that seam;
In this process, photons appear as particles;
After the photon passes through the double slit like a bullet, it can't turn around or start again, and then suddenly open the screen to see what the photon will leave on the screen.
How can photons feel so embarrassed? Should I behave as a wave or a particle?
Photons can't turn around, start over, or be hooligans, so they can only shoot like bullets, and the screen will be like bullets. However, no, interference fringes appear on the screen.
In other words, photons pass through the double slits at the same time, causing fluctuations. Our observation changed the historical trajectory of photons.
Use a popular example to illustrate this problem:
When the teacher calls the roll, I will definitely go to class; When the teacher doesn't call the roll, I will definitely play; Whether to attend class or play ball depends on the information of whether the teacher calls the roll or not.
At first, the teacher said that I wouldn't call the roll, so I put on my sportswear and went to play on the court. Suddenly, the teacher called the roll again, but I was already sitting in the classroom in a formal suit.
What "history" it will be before putting on formal clothes, going to class, putting on sportswear and playing ball depends entirely on whether the current teacher will call the roll now.
The choice now can change the past.
If the time scale of this example is not large enough, then use Wheeler's idea:
A galaxy one billion light years away from the Earth, its starlight is split into two beams by Einstein's gravitational lens, and it reaches the Earth one billion years later. Gravitational lens is equivalent to the double slit in Young's double slit experiment. It's just that the gap is a little big.
If we observe a beam of light alone, it appears alone and in the form of particles.
If a screen is set on the earth, then the two beams of light will be combined and appear in a wave form.
Does this mean that our behavior can determine the state of photons when they set off a billion years ago, whether it is fluctuating or particle?
The choice of the moment can determine history.
However, there is no need to worry too much.
First of all, this only stays at the microscopic particle level; Secondly, the choice can only determine whether the photon is a wave or a particle; Third, choose not to change the direction of light. The enlightenment we can get is that we may need to think about the past and the future and wonder if this is an illusion.
If the universe is not perfect, it has bugs. Do you believe it? The double-slit interference experiment seems to have discovered the "loophole" of this universe step by step.
What is the double-slit interference experiment? When we throw a stone into the water, the water surface will ripple. If we drop two stones at the same time, there will be cross interference fringes between the two water waves. This is the characteristic that waves can interfere with each other.
In the double-slit interference experiment, an opaque baffle with two slits is placed in front of the light source, and an observable background is placed behind the baffle. When we turn on the light source, we will see bright and dark stripes on the background. This is a simple double-slit interference experiment. This experiment proves that light is a wave! Because the light passes through two slits and only Porter interferes, the opposite wave is cancelled and enhanced, resulting in alternating light and dark stripes on the background. (Active noise-reducing headphones in daily life use this principle and use the opposite sound waves to cancel the noise. )
Let's upgrade the experiment, the light source becomes very small, and the background is replaced by a high-sensitivity and high-resolution negative. After turning on the light source, at first we saw countless randomly distributed dots, then more and more, and finally formed stripes of light and dark! After upgrading the experiment, it is proved that light is a particle and has the characteristics of wave, that is, the wave-particle duality of light!
Double-slit interference delay experiment Although the double-slit interference experiment has been praised, scientists have upgraded this experiment. Turn the light source into an electron at a time! If electrons want to pass through this baffle, they can only pass through two gaps at random.
We know that if we want to intervene, we must have someone. How can we be disturbed without an object? This time, however, the experimental results are wrong. Even if a single electron randomly passes through two gaps, interference fringes are finally formed.
This result shocked the scientific community! Why can a single electron interfere with itself? Does he have another doppelganger? Stranger still, when we observe which gap the electrons pass through, the interference fringes disappear. When the observation was cancelled, the interference fringes appeared magically again! It seems that there are a pair of eyes peeping at us, which can only show us the path of electrons passing through the gap (particle characteristics) or the interference fringes of electrons (wave characteristics)!
Delay-selective quantum erasure of double-slit interference. Seeing this, you may think that there will be many unknown loopholes in the above experiment. When we observe electrons, we have disturbed the normal movement of electrons and caused changes in their properties, but we have no way to find out this factor. Next, scientists use more complicated and sophisticated methods to do the double-seam experiment. Separate a photon into a pair of entangled photons A and B (entangled photons can influence each other regardless of the distance).
AB does the double-slit interference experiment (an environment that does not affect each other), while B is farther away from the sensing screen than A, so A will reach the sensing screen before B. When we put a camera in experiment B to observe the path of B through the double slit, the interference image of experiment A disappears. Obviously, when two entangled photons interact, the wave property A that B can't get can't be obtained. Next, we erase the path information obtained by B through technical means, and then interference fringes appear in both A and B ... There are two very strange phenomena here. The measured photon path information is just a "leak". Without the supervisor's consciousness to check, the interference fringe will disappear! Erase this path information and interference fringes will appear again!
Even weirder, when we set the path information from B in the experiment, A has reached the sensing screen and formed an image! At this time, the path information of B is erased, and the image that has been "photographed" on the screen of A sensor will become a ghostly interference fringe!
The incomprehensible "cosmic program" Many people initially thought that observing photon paths meant that human consciousness was involved in experiments. However, we know from the last experiment that in the delayed selection experiment, the measured path information, whether you look at it or not, has been determined by the cosmic program that you leaked the secret! Photon fluctuation property is hidden! We can't get the interference image. If we erase this leaked secret, the cosmic program will immediately repair the fluctuation of photons and let us get the interference image. Unexpectedly, we humans use quantum entanglement to drill a hole in the laboratory, so that we can get the path information after the image is formed. Then we choose to leak or erase, and the cosmic program is still executed according to the original instructions. Let the image that has been formed change back (photons that once did not interfere, now interfere again. Is this a detour? Does this mean that we found a BUG in the cosmic program and changed the past with our current decision! Or is there another reason? Is the universe we live in, a world that is so real that we can't see the edge, a set "program"? Or the universe, a seemingly perfect world, actually has some loopholes. What will the future world look like if human beings take advantage of these loopholes in the future?
Many people will feel that "the mystery is both mysterious" after hearing the double-slit interference experiment, so there is "quantum mechanics". In fact, quantum mechanics is a stepping stone for human beings to understand the underlying logic of the universe, and the double-slit interference experiment is the embodiment of the core of quantum mechanics. Let me talk about how "weird" the double-slit interference experiment is. What core does it reveal in the universe?
Schrodinger's cat, God rolls the dice, where does the parallel universe come from? Because quantum is too abstract, we transfer the quantum phenomenon to Schrodinger's cat, and then go back to the double-slit interference experiment, which is easy to understand. This is a good example of Schrodinger's understanding of quantum mechanics.
In other words, there is a cat in a closed box, and then a quantum device is connected to the poison bottle. The life and death of a cat depends on quantum properties. If quantum decays, the cat dies, and vice versa. In other words, the life and death of a cat indirectly illustrates the nature of quantum. The question of the experiment is whether the cat is dead or alive in the end.
Big names from all walks of life have expressed their views, and there are three mainstream views:
Bohr, Copenhagen School: This is a quantum cat. The probability that it is in the box is 100% alive and 100% dead. The two States exist at the same time and are superimposed together. The moment the box is opened, the life and death of the cat will be displayed, and the result of life and death is random.
Einstein and Schrodinger: Cats are 50% dead and 50% alive. Before we open the box, it is dead or alive. When we open the box, we see the result, not the induced result.
Einstein: Bohr, you mean that when you opened the box, God found that someone was coming to see the results, so he quickly shook the number to decide the cat's life and death?
Bohr: Never mind what God can do!
Hugh Everett: Quiet, quiet, I haven't said it yet! First of all, I agree with Bohr's superposition state, but 100%+ 100%=200% should be conserved before and after unpacking, so I think that if the cat dies when unpacking, then the living cat should exist in another world-parallel universe.
Einstein, Schrodinger, Bohr: You are so amazing that we don't know how to prove what you said is wrong!
The story came first. It doesn't matter if you can read it. Let's talk about the result first: Bohr is right! Parallel universes cannot be proved, but at best they are a hypothesis. There are several points in this story:
1. Is the cat dead or alive-superposition state
2. Opening the box means observing, and observing will make the superposition state collapse into a single state at random. (god shakes the number! )
3. The first two points, before and after opening, also imply wave-particle duality. (later)
What is light? -the "weird" of double-slit interference Next, let's look at double-slit interference. This matter begins with Newton, and stems from a seemingly simple question that no one can answer-what is light?
Figure: Newton prism experiment
Newton, as a contemporary scholar, made many contributions to optics. For example, he did a prism experiment. In this experiment, sunlight is a mixture of all kinds of light. He thinks that light can be reflected and refracted, and its trajectory will change, just like table tennis will bounce back when it is thrown on the wall, so its smallest unit should be particles.
In the19th century, Thomas Young refuted Newton. He only did one thing, let a beam of light pass through two small slits, and there was a sensing screen behind it. According to Newton, the result of this experiment should be two stripes, as shown below:
In fact, the following results have appeared:
So Lao Yang said that light is like the waves below, but it is actually waves:
The light wave passing through the gap becomes two waves, which interfere with each other and appear the same phenomenon as water, so interference fringes are displayed on the screen.
This is the double-slit interference experiment, but what is strange is the double-slit interference experiment of quantum mechanics.
The good times didn't last long. With the help of blackbody radiation experiment, Planck found that light energy is discontinuous, and Einstein found photoelectric effect, that is, when light interacts with atoms, it exchanges energy in the form of particles. So everyone re-examined the double-slit experiment and upgraded it.
Since light is grain by grain, what happens when we let photons pass through a double slit? The actual experiment uses electrons, and the reason is the same. )
The bosses quickly emitted shuttle electrons like machine guns according to two slits, and a large number of particles appeared randomly on the display screen, but from a distance, these particles also formed interference fringes. Since it is a particle, why does it interfere?
So some people think that many electrons are crowded together, so there is interference, which is a bit like an ocean ball in a children's paradise. When you jump in, the ocean ball will spread like a wave. Although it is a particle, it will also cause interference. But is this really all?
Figure: Experimental results
The scientist did another experiment and changed it to "pistol" launch. An electron was hit with a bang, the electron reached the sensing screen, and then shot again, which prevented the two electrons from interfering with each other in motion. However, scientists are puzzled. Whether you type fast or slow, the result is the same. The screen will still fluctuate, and there will be new interference stripes instead of two stripes! In other words, a single electron interferes, so who does it interfere with? Just two seams, it can only choose one to pass, and there is no electron coming out of the other seam. Where can it interfere?
In order to solve the problem, the bosses installed a photodetector in the experiment to "see" it and see how the electrons interfere! It was found that electrons honestly formed two stripes on the sensing screen. Everyone: Oh, my God, tell me what happened!
The above story has given the answer: the wave-particle duality is first explained by the unreliable parallel universe theory: when you are not looking, electrons pass through seam A, then through seam B, and then interfere. You can understand that there is a doppelganger in quantum. If you look at it, the universe will split. If electrons enter through slot A, then electrons in the parallel universe will enter through slot B. It was our exploration that caused the universe to split, making it impossible for electrons in the two universes to interfere.
Bohr's explanation: In the first half, like the parallel universe, the electrons are in a superposition state, which is a wave dynamic, but when you watch it, it collapses into a particle state at random.
Einstein: I can't explain it! There must be something we haven't figured out yet. God won't shake the number anyway.
Figure: In our impression, the electrons in the atom are like this.
Figure: Actually, this is the case, so it is also called electronic cloud, which has probability and fluctuation.
Judging from the current scientific research results, Bohr is right. Quantum has wave-particle duality, which is the core of quantum mechanics. Electrons have the properties of both waves and particles.
When it does not collapse into particles, although it is also emitted as a single particle, the nature of the wave is also at work. When you emit a single electron, it is similar to emitting water waves. When you emit a beam of electrons, you are actually emitting a beam of waves, which will be displayed on the sensing screen according to the interference results. When you detect an electron, it will collapse into the nature of a single particle, so a beam of electrons is knocked out without interference, only two stripes appear.
If you don't understand the nature of quantum, you will think that I don't look at interference fringes, but I don't interfere. It seems a little scary. Naturally understand that quantum mechanics is the lowest logic for human beings to discover the universe. It can explain the origin of the universe, from the composition of the universe to the formation of particles that constitute the smallest structure of the universe.