Special Relativity 's view of time and space in space, it subverts the view of time and space proposed by Galileo and Newton . That is to say, it subverts the view of time and space that is more in line with people's daily life experience in the past 300 years.
Any view of time and space brings not only our understanding of time and space, but also brings a more basic part of physics - change in dynamics!
1905, Einstein also published another paper on the special theory of relativity, the title of the paper is "Is the inertia of an object related to the energy it contains?" In this paper, Einstein proposed the relationship between mass and energy for the first time. He derived his famous formula from a few thin pages of paper, concluding that an object's energy is equal to its mass multiplied by the square of the speed of light. Since the speed of light is a huge number, its mass-energy equivalent relationship means that an object has huge potential.
Einstein's mass-energy relationship has dual meanings: the first meaning is that when an object is in a stationary state, it also contains huge energy, because the mass of a stationary object is not equal to zero; the second meaning is that when an object moves, its effective mass is greater because its energy is greater.
In this book, we are not suitable to give Einstein's deduction of the machinist relationship in 1905. However, 41 years after Einstein discovered the special theory of relativity, in 1946, he published a paper entitled "A Simple Derivation of the Relationship of Mass-Energy Equivalence."
Einstein's new derivation is indeed simple enough, so simple that we can retell it verbally here, so simple that there is no simpler derivation than it today.
So, let’s see how Einstein did it? This deduction is a thought experiment, that is, you don’t need to go to the laboratory to do experiments, just imagine it. We know that any object absorbs light, puts a stationary object on a horizontal table, and then lets the two sides of the object receive two beams of light from the opposite direction respectively. Assuming that the energy contained in these two beams of light is equal, according to Maxwell's theory, these two beams also contain momentum, but their momentum is equal in magnitude and opposite in direction.
Therefore, according to the law of conservation of energy, when a stationary object absorbs two beams of light, its energy increases significantly, but remains still. This is because the momentum of the two beams of light adds up to zero, and here we assume that momentum is also conserved.
But this fact is not enough to allow us to deduce the relationship of mass-energy equivalent. Then, a more important step comes. Imagine that we are in another reference frame, and the direction of this reference frame is perpendicular to the desktop. What do we see in this reference system? First, the object placed on the table has a speed perpendicular to the table; second, the direction of movement of the two beams of light absorbed by the object is no longer parallel to the table, and they also have a speed perpendicular to the table. In other words, in this new reference system, the momentum of the two beams of light cannot cancel out each other. Therefore, the momentum of the object after absorbing two beams of light must increase - of course, we still assume here that momentum is conserved.
However, in the moving reference frame, the speed of that object does not change before and after absorbing light, because it is always placed on the table. The energy and momentum of this object have increased, but the speed has not changed. What does this mean? It can only be said that its effective quality has increased. Einstein derived that the increased energy is proportional to the increased mass, and this proportional coefficient is the square of the speed of light. Why does the speed of light appear? Because we used two beams of light during the derivation process.
Needless to say, we all know the story that Einstein's mass-energy relationship completely changed the world because the subsequent development of physics confirmed the mass-energy relationship, especially the emergence of nuclear power plants and the nuclear bomb .
When talking about quantum mechanics , we know that Planck is the first person to propose light quantum . He said that the energy of a light quantum is proportional to the frequency of light. Later, Einstein generalized Planck's formula to the momentum of photon .Interestingly, in 1905, when Einstein contributed his paper on relativity to the German physics journal "Physics Yearbook", Planck himself recommended this paper. In 1907, Planck also gave a new form of mass-energy relationship.
Einstein's first paper on special relativity is very distinctive, not only logically clean, but also different from other papers. Einstein did not cite any literature in this paper, but just thanked for the discussion with his friend Besso.
Next, let’s talk about the mass-energy relationship of moving objects. In the last class, it was mentioned that in the special theory of relativity, the moving ruler will be shortened and the moving clock will be slowed down. When the speed of the moving ruler approaches the speed of light, the ruler becomes infinitely short. Similarly, when the speed of the moving clock approaches the speed of light, the clock becomes infinitely slow. So, what is the relationship between the energy and speed of moving objects? The mass-energy relationship tells us that when the speed of a moving object approaches the speed of light, the energy will become infinitely large. This is completely different from what Newtonian mechanics tells us. Of course, if the speed of a moving object is much smaller than the speed of light, the kinetic energy of the moving object is very close to that of the kinetic energy in Newtonian mechanics, in addition to the static energy.
Therefore, when we accelerate the object, the closer the object is to the speed of light, the greater the energy it needs, and when the object is accelerated to the speed of light, infinite energy is required. This is also the reason why an object's speed cannot exceed the speed of light in relativity.
Next, let’s talk about the verification and use of special relativity. Let’s talk about the slowing effect first. In the universe, in addition to the electrons that make up molecules and atom and atom nucleus , there are many particles, these are elementary particles, because they are very small like electrons and nuclei. The earliest elementary particles that exist outside the earth were discovered called Miaozi, which is the pronunciation of a Greek letter. This particle is very much like an electron, but it is about 200 times heavier than an electron. Another difference between it and electrons is that it has a very short lifespan, only one-half-millionth of a second. Next, we can verify the theory of relativity. Because one-five hundred thousandth of a second is the lifespan of the muffler when he is still, let it move at a speed close to the speed of light and its lifespan will become longer. The moving clock will slow down, which looks like slow motion, so the lifespan of a particle is also shown in "slow motion".
If we want to turn the lifespan of the muffen into 1 second so that we can see it, how fast does it need to be? The mummy needs to be only 3/5 mm slower than the speed of light per second. Think about it, the speed of light is 300,000 kilometers per second, and the difference is too small. A quick-running muff can survive for 1 second, so if the muff runs about 300,000 kilometers, the instrument can easily see it.
Relativity also means that every elementary particle has its antiparticle . Dirac is the first person to predict the existence of antiparticles using relativity. In trying to combine quantum mechanics and relativity, Dirac discovered that there must be a particle in the world with a charge opposite to electrons and a mass the same as electrons. Because the properties of this particle are opposite to electrons, it is called an antiparticle of electrons. This particle is called positron because it has a positive charge.
Of course, when Dirac predicted this particle, there was an interesting process. At the beginning, he said that electrons must have an antiparticle, the charge is opposite to it and the mass is the same. If electrons encounter this antiparticle, disaster will occur, and they will disappear after they gather together and become photons. This process is a bit like suicide, so physicists call this process annihilation. However, after a while, no one discovered positrons, so Dirac was a little anxious and wanted to modify his theory, saying that the antiparticle of electrons should be protons, that is, hydrogen atom nuclei. The proton is just positively charged, but its mass is almost 2,000 times larger than that of electrons. Four years later, in 1932, experimental physicist Anderson discovered positrons in cosmic rays.
However, Dirac's prophecy is too abstract.American physicist Feynman gives a simple explanation. Feynman said that particles can walk in the direction of time, because relativity allows particles to walk in this way. Feynman believes that electrons that go against time are actually positrons. Since any particle can go against time, any particle has antiparticles. You see, we don’t actually need to understand Dirac’s abstract methods, but we can also understand the existence of antimatter .
With particles and their antiparticles, we can directly see the role of the relationship between mass and energy. It is devastating that particles and antiparticles collided together. They will annihilate each other into light, that is, mass becomes energy, and the mass of particles and antiparticles becomes energy of light. In fact, all nuclear power plants are also verifying Einstein's mass-energy relationship. When a relatively large nuclear crack becomes some small nuclei, the total mass of the small nuclei is smaller than that of the large nuclei, and the excess mass of the large nuclei becomes energy released. Daya Bay Nuclear Power Plant uses nuclear fission to create a continuous stream of energy.
Special theory of relativity is not only important in particle physics, but also in astrophysics . The explanation of a large number of high-energy astrophysical phenomena is inseparable from the special theory of relativity. Einstein's special theory of relativity not only changed our view of time and space, but also completely changed the world of mechanics. An object contains huge potential energy, its energy equals its mass multiplied by the square of the speed of light. This simple relationship is the basis for research on nuclear power plants, particle physics and astrophysics.
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