Among them,1On Electrodynamics of Moving Objects published on June 30th, 905 was later called Special Relativity.
Today is the fourth anniversary of the publication of the special theory of relativity1/kloc-0. This theory was 100 years ago, so we have no reason not to understand it. Today, I will give you a good talk: What does the special theory of relativity say?
A feud that lasted 200 years.
In fact, the birth of special relativity mainly stems from a feud that spans 200 years of scientific history.
Let's switch the camera to17th century. The first supporting role is Galileo, who is known as the father of modern physics.
Galileo once put forward a "galilean transformation?" ：
The laws of physics established in a reference frame can be applied to any reference frame through proper coordinate transformation.
Is there an unconscious feeling? In fact, this is all a bluff. For example, you can understand that if you are on a train and there happens to be a train next to you, it is difficult for people sitting on the train to tell whether their own train is moving or the train next to you is moving.
In fact, it can be understood that motion is relative. If a simple model is used, it is:
A and b are close to each other. If we choose A as the frame of reference, we can conclude that A is static and B is moving. If you choose B as the reference system, B is stationary and A is moving.
Yes, this is the "frame of reference" or "reference object" that will be talked about in physics classes in junior and senior high schools.
If you go forward in the car, the children standing on the ground will think,
Your speed = the speed of the train+your speed in the car, your speed =10+5 =15m/s. Have you found that in this theory, the speeds can be superimposed?
Later, Newton brought galilean transformation into his own mechanical system. When we apply Newton's law, we must first specify a reference system.
However, we should be clear that Newton actually made an assumption that space and time are absolutely independent.
To put it bluntly, all objects on the earth feel the same about time. Space is the same, and the distance of space is the same for everyone. If you have to sum it up simply:
Space and time have nothing to do with the motion state of objects!
Space and time have nothing to do with the motion state of objects!
Space and time have nothing to do with the motion state of objects!
(Say the important things three times)
Newton's theory was later widely used, and even predicted the existence of Neptune, which became a firm cornerstone theory of physics.
Later, scientists began to study electricity and magnetism. Especially in Maxwell's time, Maxwell put forward Maxwell's equation.
He unified "electricity" and "magnetism" and put forward the concept of electromagnetic wave, predicting that light is electromagnetic wave.
Physicist Hertz verified Maxwell's point of view through experiments. But that's the problem. Maxwell's equation does not need a reference frame. Say it bluntly:
The speed of electromagnetic waves, or the speed of light, does not need to be relative to a certain reference frame. In any inertial reference system, the speed of light is 3× 108 m/s.
This is in contradiction with Newtonian mechanics. However, Newtonian mechanics is so correct that observation and theory match perfectly. Maxwell's equation is also rock-solid, which can explain electromagnetic phenomena well. So what went wrong?
Compromise of scientists
You know Galileo, Newton and Maxwell are the top five absolute gods in the history of physics. When the gods fight, the average physicist can only eat melons. It's just that physics always has to move forward, and the great god can't afford to offend, so a bowl of water is even.
Therefore, scientists believe that the propagation of water waves needs a medium, namely water. Does light propagation also need medium?
So scientists at that time believed that the speed of light propagation should be relative to its medium, not absolute. Therefore, scientists believe that space is full of a substance called "ether". Ether is to light (electromagnetic waves) what water is to water waves. It looks perfect, doesn't it? But science can't just rely on imagination, it needs to find evidence to prove that "ether" really exists.
And the result? Sorry, scientists have tried everything, and finally come to a conclusion: ether doesn't exist!
It's over now. After a long time, the contradiction between Newton and Maxwell has not been solved. As a result, scientists began to open their minds and hold back big moves. The most famous of them are Lorenz and Poincare. If you have to find something in common between two people, it must be: the person closest to the special theory of relativity.
Lorenz is a master who can make ends meet. He held a "galilean transformation" in his left hand and a "constant speed of light in inertial reference system" in his right hand, and then combined them to create something that even he could not understand. This thing is called Lorentz transformation.
Poincare put forward some viewpoints from the philosophical level, especially the relativity of simultaneity. It is the same event, and what different people (frame of reference) see may not happen at the same time, depending on their motion state. However, poincare also thought about it, but no one can really come up with a satisfactory result.
Yang Zhenning once wrote in his article Opportunity and Vision:
Lorenz has mathematics, but no physics; Poincare has philosophy, but not physics. It was 26-year-old Einstein who dared to question the original concept of time and insisted that simultaneity is relative, thus opening the door to a new physical world.
Third-class technician of patent office
Yes, the hero of our story was born after the failure of a bunch of great academic gods. However, before telling his legend, let's get to know him.
1on June 30th, 905, Einstein published a paper on electrodynamics of moving objects. He should also want to make ends meet, just like Lorenz and Poincare. Therefore, starting from two basic assumptions:
1.？ Principle of Relativity (galilean transformation)
2.？ Principle of light speed invariance (light speed invariance in inertial reference system)
One of these two hypotheses was put forward by Galileo and the other was based on Maxwell's theory. Then the Lorentz transformation is deduced (after all, the methods used are the same), and just now Lorenz said that he couldn't understand this thing. But Einstein, unlike Lorenz and Poincare, was able to strike a balance and create a whole new world.
So what is this brand-new world?
Einstein's rebellion
It should be said that Einstein's rebellion, before him, no one dared to question space and time. Everyone thinks that space and time are absolute. What do you mean that space and time are absolute? That is to say,
Your second is also someone else's second, and everyone's second is the same.
But Einstein thought it was wrong. Let's imagine a picture. You are standing on the ground and your friends are on the spaceship. At this time, your friend took out a lamp clock, which does not exist in real life, but the principle is the same as clock timing. After all, Einstein liked this "thought experiment" very much. The timing method of this light clock is:
The time for the lamp to rise and fall once is set to one second.
In fact, the truth is the same as that of a clock running once. If we assume that the speed of light is the same in any reference frame (the principle of constant speed of light), then the light people see on the spaceship is up and down, while the light they see on the ground is actually inclined.
Einstein thought that time = distance/light speed is constant in any inertial reference system, and so is light speed. Therefore, it takes 1 second for people on the spacecraft to see the light back and forth, while the path for people on the ground is longer, so we assume it is 2 seconds.
If someone on the spaceship is doing broadcast gymnastics with the rhythm of "light clock", then the action that can be completed on the spaceship 1 second will be watched by people on the ground for 2 seconds. To put it bluntly, what they saw was the slow motion of broadcast gymnastics.
Conversely, if people on the ground also hold the "light clock", in fact, because the movement is relative, the situation will be just the opposite. People on the ground see it as 1 second, and people on the spaceship see it as 2 seconds. In other words, if people on the ground are also doing broadcast gymnastics with the rhythm of "light clock", people on the spaceship will also see the slow motion of broadcast gymnastics.
This effect is called time dilation. It does exist, and scientists have proved this through μ(miω) quantum experiments. If we replace the spaceship with a high-speed rail, the clock in the high-speed rail will actually be one billionth of a second slower. It is precisely because of this slight difference that we can't feel it. When the speed is particularly fast, especially the closer to the speed of light, the more obvious the effect of time expansion is.
This experiment tells us a truth:
Time is related to the motion state of an object!
Time is related to the motion state of an object!
Time is related to the motion state of an object!
(Say the important things three times)
Einstein said that not only time is related to the motion state of objects, but also space. Let's take the spaceship as an example. It's also distance. Due to the time dilation effect, it may take us 2 seconds to walk on the ground, but the people in the spaceship will walk in 1 second. And no matter the people in the spaceship or the people on the ground, the speed of the spaceship relative to this distance is the same. This shows that the distance seen by people on the spacecraft is actually shorter than that seen by people on the ground. This is length contraction.
So we will find that the closer the speed is to the speed of light, the more serious the length contraction is. This shows that:
Space is related to the motion state of objects!
Space is related to the motion state of objects!
Space is related to the motion state of objects!
(Say the important things three times)
Einstein went further and put forward a very subversive concept: the theory of relativity of simultaneity. Specifically:
Two things happen at the same time in one coordinate system, and probably won't happen at the same time in another coordinate system.
So what's going on?
We are also here to learn from Einstein and do a thought experiment. First of all, we can imagine two trains of the same size, with opposite directions and the same speed relative to the ground.
It's just that the two trains are not on the same track, but on a double-deck parallel track, with one train above and one train below. We stipulate that "event A" is when the front of the upper train meets the rear of the lower train; "Event B" is that the front of the lower train meets the rear of the upper train.
So, the question is, which happens first, event A or event B?
Of course, if you look at it from the ground, these two events really happened at the same time.
However, if you are on the train above, the train below is moving relative to you. We also talked about the scale effect above. So, what you see is that the train below is shorter than yours. Therefore, the scene you see will be like this:
In other words, what you see in the train above is that "Event A" happened before and "Event B" happened after.
If you are on the train below, the train above is moving relative to you. Or because of the scale effect. So, you will find that the train above is shorter than yours. So, what you should see is this:
In other words, what you see in the train below is that "Event B" happened before and "Event A" happened after.
However, it should be noted that this effect will be more obvious only when the speed is very high and the closer it is to the speed of light. At low speed, we can't see the difference with the naked eye.
Therefore, "simultaneity" is also a relative concept, based on the frame of reference. Different frames of reference have different situations.
Based on this cognition, Einstein's former math teacher Minkowski put forward the concept of "light cone".
We can establish a coordinate system based on any event, with the abscissa representing space and the ordinate representing time, and draw the temporal and spatial position of the event in the coordinate system.
It should be noted that this light cone is specially designed for sports events, and the future light cone refers to:
The impact of the present on future events.
For example, event A in the figure below is likely to have an impact on event B. ..
The past light cone refers to past events that have an impact on the present.
This means that only events that happen in the "past light cone" will affect the present. Past events outside the "past light cone" cannot affect the present because of the limitation of light speed.
For example: I write this article as event A, and you regard this article as event B.
So, there is a famous saying:
In the cone of light is fate.
All the status quo is caused by the light cone incident in the past. The past has happened and we can't change it. If we go further, we will find that we can never live in the present, because what we call "the present" is all caused by the past.
For example, when you look in the mirror, you actually see yourself in the past, not yourself now. This is because your face reflects light to the mirror, and the mirror reflects light to your eyes. It takes time for light to travel this distance, so what you see is actually your past self.
In other words, all the events you see actually happened in the past, and it takes time for the past events to affect the present; What is happening now affects not the present, but the future, which is a revelation from the time cone. It can be said that Einstein only unified time and space through special relativity, but this is not over.
Quality is energy.
1905 In September, Einstein published a paper "Is the inertia of an object related to the energy it contains?" In this article, Einstein unified mass and energy and put forward the famous mass-energy equation:
So how do we understand the mass-energy equation?
Before Einstein, lavoisier put forward the law of conservation of mass, while in Newtonian mechanics, energy is conserved. However, Einstein thought:
Energy and mass are not independent, they are actually the same thing.
The famous scientist Dr. Dali once gave such an example:
If you have deposit accounts in China and the United States, the deposit value of the two accounts will not change. But because they belong to two countries, if you want to transfer money from one account to another, you need to convert it through exchange rate. Here, we can regard RMB as energy and USD as quality. If the sum is constant, energy and mass can be transformed. Then e = MC 2 represents the exchange rate of energy and mass, where the speed of light c is the exchange rate system.
This formula explains why the atomic bomb is so powerful, because the mass before and after the nuclear explosion is deficient, and these masses are converted into energy.
There are actually a lot about special relativity, so let's talk about it this time. If you want to understand the special theory of relativity deeply, you actually need to do mathematical calculations, because relativity is counter-common sense. Why is it against common sense?
We live in a macro-low-speed world. On this scale, the relativistic effect is too small. People can't feel it, even instruments are hard to detect. In the case of macroscopic low speed, relativity is equivalent to Newtonian mechanics. The relativistic effect is more obvious only when the speed is closer to the speed of light.
It is for this reason that we feel that the theory of relativity is very anti-common sense. This tells us a truth:
Don't be deceived by the present life, and look at the outside world more. Only by jumping out of your own life can you understand the world more deeply.