When mentioning the special theory of relativity, it is indispensable to talk about the relativity of simultaneity, and we must talk about the classic lightning strike train experiment: Two lightning strikes the front and rear of the train. Through analysis, we concluded that the ground system thinks that these two events happened at the same time, but the train system does not think so.
Thus, Einstein discovered the relativity of simultaneous , thus opening the door to special relativity.
Einstein mentioned this experiment in his own popular science reading on the theory of relativity, "A Brief Introduction to the Special and General Theory of Relativity":
General relativity textbooks will also mention this.
has published a similar article before, but there are many comments about this experiment in the comments of the article. Moreover, you won’t know it until you look at it, but you will be shocked when you look at it. Many people have a shocking understanding of this experiment, how Einstein discovered the relativity of simultaneousness through this experiment, and how Newtonian mechanics views this matter.
Moreover, many people feel that their understanding is particularly reasonable. I feel that this experiment is a fallacy. I feel that this is a circular argument . I feel that Einstein cannot explain anything through this...
Please, Einstein is an existence in the history of human physics. Can you give Top2 a little respect? Do you think more about special relativity, or do you think more about Einstein? If this experiment really had as many problems and flaws as you said, would Einstein still take it out to popularize science around the world?
No physicist has come forward to oppose his experiment. This experiment is still discussed in textbooks? Are you the only smart person in the world? Einstein, all the physicists in the past hundred years, and all the textbook editors can't see this problem, but you are the only one who can see this problem?
It is normal for beginners to not understand many things. If you don’t understand them, then learn them. Once you understand them, you will know them. But if you haven't figured something out and feel that their logic is wrong and you should follow yours, why don't we be so arrogant?
It is good to question something. Physics needs to be questioned, but the premise of questioning is that you already have a sufficient understanding of this thing..
If you don’t understand the theory yourself, then why are you questioning it? At this time, if you feel that there is something that you cannot understand, what you need to do is to ask others to understand it, instead of constantly suspecting that Einstein said something wrong or wrote something wrong in the book.
Okay, this is the end of the complaint conference. I will not list all the misunderstandings about this experiment one by one. Let me take you through this experiment again. Once you understand this experiment completely, other problems will be solved naturally.
Let’s first take a look at what this experiment is going to do.
is very simple. This experiment is to prove the relative nature of and at the same time.
In other words, it is necessary to prove that for the two events of lightning striking the front and rear of the car, the ground system feels that they occur at the same time, while the train system feels that they do not occur at the same time.
Then, we can draw the conclusion: is relative at the same time. One inertial system feels that they are two events happening at the same time, and the other inertial system will feel that they are not that happened at the same time.
This experiment is very simple. Let me describe it briefly again: A train is traveling at a constant speed on the ground. At a certain moment, two lightning bolts hit the roadbed where the front and rear of the train are located at the same time. Of course, this is relative to the ground at the same time (this is easy to understand).We then show that, for the train system, these two events are not simultaneous.
1 Some agreements
Before talking about this experiment in detail, we must agree on something:
, The propagation speed of light in a uniform medium (such as air) is the same . This agreement is not the principle of constant speed of light in , , but it means that as long as space is isotropic, the speed of light in this reference system will be the same. The principle of constant speed of light says that the speed of light in vacuum is the same in different inertial frames. We only talk about it in one reference frame here. How to define
, and at the same time? After agreeing that the speed of light in a uniform medium remains constant, we can use light to calibrate time and use light to define simultaneity. The method is also very simple: in an inertial frame, if the light signals emitted by two events reach the midpoint of the two events at the same time, we think that the two events occur at the same time. How to determine whether
3 and are relative at the same time? We firmly grasp: if two events are and occur at the same time in one inertial frame, we see whether they are and occur at the same time in another inertial frame. If yes, then simultaneousness is absolute, if not, then simultaneousness is relative.
I want to emphasize: What we want to compare is whether two events occur at the same time in different inertial systems. The focus is on the occurrence time of the event, the occurrence time .
I simplified the experiment of lightning striking a train into the following model:
Then, let’s analyze this experiment again.
2 Flash on the train
After lightning strikes the front and rear of the train, the midpoint C of AB on the ground will definitely receive the flash from the head and tail at the same time, so the ground system thinks that they happened at the same time. I believe everyone has no doubt about this.
What is in question is what happened on the train.
We assume that there is an observer at the midpoint C' of the train. If the train is not moving, then the observer C' will definitely receive the light from the front and rear of the train at the same time, right? There is no doubt about this.
So, now the train is moving, and the speed of light is limited. The limited meaning is that it takes a certain amount of time for light to travel from the front and rear of the train to the center, and during this period of time, the train will inevitably move forward for a certain distance.
Then, observer C' in the center of the train will definitely see the flash from the front of the train first, and then the flash from the rear, right ?
Think about it carefully, is this the case? If you think about it again, what are the conditions we use to draw this conclusion? Does the principle of constant speed of light apply? No! Is there any velocity superposition formula used? Neither!
There is only one assumption we use: The speed of light is limited !
In other words, as long as the speed of light is limited, it will take a certain amount of time to spread to the center of the train. Because the train is moving, it will definitely move a certain distance during this period of time. As long as there is this distance of movement, then it will definitely receive light from the front of the train first, and then receive light from the rear of the train.
So, Does the theory of relativity admit that the speed of light is limited? recognizes ! Newtonian mechanics Does it admit that the speed of light is finite? Also recognizes !
So, so far, is it strange that the observer C' in the middle of the train will receive the light from the front of the train first and then the light from the rear? Can it explain the relativity of simultaneity, or is it absolute? cannot !
This is a place where many people easily stumble. Do you think we can draw simultaneous and relative conclusions from the different light times at the midpoint of the train? ?
Nonono, this fact is very ordinary, Newton and Einstein both recognized , because as long as the speed of light is limited, this conclusion must be reached. The extraordinary fact is below, and in the analysis of that fact.
Why was Einstein able to derive the relativity of simultaneousness based on this fact?? Because Einstein has the speed of light remains unchanged !
The principle of constant speed of light is the basic assumption of the special theory of relativity. It says that the speed of light in vacuum does not change with the change of the inertial system. The speed of light in the ground system is c, but the speed of light measured in the train system is still c. With the principle of constant speed of light, we can proceed with the calculation.
3 Calculation of relativity
The observer C' at the midpoint of the train first receives the light from the front of the train, and then receives the light from the rear of the train. However, we judge whether the same time is relative, not to judge whether your closing time is the same, but to judge whether the time of the event is the same.
In the ground system, you think that lightning struck the front and rear of the train at the same time. So, what I need to prove in the train system is: People in the train system think that these two events happened at the same time. Note that it is the time when the event occurred, not the time when the light was turned off at the midpoint of the train..
So, do you think these two events happened at the same time?
The answer is not , why? It's very simple. For the train system, the distance between and from the front and rear of the train is the same. According to the principle of constant speed of light, the speed of light on the train is also the same.
Therefore, the distance traveled by the two beams of light is the same and the speed of light is the same, then the time taken by the two beams of light to travel is the same.
On the train, we obviously receive the light from the front of the car first, and then the light from the rear. The two beams of light move at the same time, so what does that mean? This can only be said that the two beams of light are not emitted at the same time.
This logic is very important, please understand it. We came to the conclusion that the light-emitting events of are different because the light-emitting time of is the same, but the time of light movement is the same.
and the luminous time are different, that is to say, these two events do not occur at the same time, but the ground system believes that these two events occur at the same time, which is the relativity of simultaneous .
In order to deepen everyone's understanding of this conclusion, let's take a look at what Newtonian mechanics would think about this matter.
4 Calculation of Newtonian mechanics
Newtonian mechanics is an absolute view of time and space, which means that if two events occur at the same time in the ground system, they must also occur at the same time in the train system.
I have said before that as long as we admit that the speed of light is limited, then the observer at the midpoint of the train must receive the light from the front of the train first and then the light from the rear. Some people may think, look, the people in the middle of the train have different lighting conditions. They came to the same conclusion as Einstein, so this experiment is useless.
I have repeatedly emphasized: needs to judge whether simultaneousness is relative or absolute. We need to judge whether two events occur at the same time, not whether people at the midpoint of the train light up at the same time .
For Newton, because there is no principle of invariance of the speed of light, the speed of light still satisfies the speed superposition formula derived from Galileo's transformation. Therefore, Newton would think that the speed of light coming from the front of the train is c+v, and the speed of light coming from the rear is c-v. This v represents the speed of the train.
This should not be difficult to understand. In Newton's eyes, there was no difference between light and bullets. If a bullet was fired from the rear of the train towards the middle of the train, then the people on the train would definitely feel that the speed of the bullet had slowed down. If the speed of the bullet is the same as the speed of the train, we can still see the "stationary" bullet, and the same thing is changed to light here.
Therefore, for Newton, the light emitted from the front and rear of the car ran the same distance to the midpoint of the train, which was half the length of the train. However, because the speed of light coming from the front of the car is faster, the speed of light coming from the rear is slower. Therefore, if these two beams of light are emitted at the same time, then I must receive the light from the front of the car first, and then the light from the rear .
Therefore, the observer at the midpoint of your train first sees the light from the front of the car, and then sees the light from the rear. Combined with the superposition of the light and the speed of the train, we can just conclude that light emits at the same time, which just illustrates the absolute nature of being simultaneous .
5 Summary
In other words, for the incident of lightning striking a train, both Einstein and Newton felt that the events on the ground happened at the same time, and both felt that the midpoint of the train would first receive the light from the front and then the light from the rear . However, the explanations for this phenomenon are completely different.
For Einstein , people who work on trains first receive the light from the front of the car, and then receive the light from the rear. This can be directly used to prove the relativity of simultaneity. Because the speed of light remains unchanged and the length of the train is the same, the time it takes for light to travel in the train is the same. Therefore, the time when you receive the light is different, which definitely proves that the time when the light is emitted is different.
For Newton, although I saw the light from the front of the train first and then the light from the rear. However, he cannot judge that they emit light at different times based on the different time of light collection. Why? Because Newton would think that the speed of light is different, the speed of light coming from the front of the car is c+v, and the speed of light coming from the rear of the car is c-v.
Then, the length of the car is the same, the speed of light is different, and the time of receiving light is different. Newton judged based on this: The two beams of light were emitted at the same time. Is there something wrong? Nothing wrong! ! ! This is a primary school question.
In other words, Newton believed that the two beams of light were emitted at the same time on the ground, and they were also emitted at the same time in the train system. We must not say that they emit light at different times based on the difference in the light closing time on the train. You also need to look at the speed of light .
This is the difference between Newton and Einstein.Einstein believed that the speed of light does not change, so we can directly determine the difference in luminous time by using the different light-collecting times on the train system. Lao Niu believes that the different light-collecting times are caused by the speed of light becoming larger and smaller, and their luminous time is still the same.
Newton's and Einstein's theories are both self-consistent theories and can be justified within their own systems. Everyone should be clear about this.
