: If you want to deeply understand general relativity , first answer these 6 questions about equivalence principle .
Let’s talk about the relationship between gravity and quantum mechanics
- Spirit Escaper
If you believe that gravity is a geometric effect caused by the curvature of space and time, not a real force, then graviton is a redundant concept! However, many scientists from various countries believe in the existence of gravitons and spend a lot of money to design graviton detectors. Why do you think this is?
And the bigger problem is that gravity is not a force, so there is no unification of the four basic forces! Because gravity is an illusion. But today's scientists are working hard to unify the other three forces. They have already unified them. Why don't they declare that they are done? If gravity is not a force, string theory is also redundant, because string theory is also an effort to unify the four forces.
Whether they are scientists or civilians, whether they believe this or that, they are confused! In other words, the people who overwhelmingly promote gravity as a geometric effect caused by the curvature of space and time actually don’t know what they are talking about. Einstein himself was also confused. If the direction of
is wrong, everything will be unclear. First, let’s really understand whether gravity is a force? Is the interpretation of general relativity accurate? Not solving the problem fundamentally will not only waste material and resources, but also cause many people who love exploration to spend their lives without results. Our generation should focus on solving this problem rather than leaving it to the next generation.
Is gravity a force? Think carefully now and then give yourself an answer. There are three possible answers to the question: gravity is a force, gravity is not a force, and gravity is a force but not a force. Someone will definitely criticize at this time: "How can gravity be a force and not a force?" Yes, I also find it incredible. This is asking for trouble. But when I think about Schrödinger's cat can be both dead and alive, light can be a wave or a particle, and it can also be in the state of wave-particle duality coexistence. [Scientists from the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland successfully took the first-ever photo of light showing wave-particle duality at the same time. This breakthrough result was published in the journal Nature Communications on March 2. 】
Which of the above three possibilities is correct? We have to go back to the history of gravity exploration to determine the answer step by step. And everyone already knows this history very well. Regarding the mainstream theories of gravity, there is Newton's universal gravitation, and then there is Einstein's general theory of relativity. The disagreement arises between the two of them, so who is right? We never seem to say that Newton was wrong and Einstein was right. Nor did we say that Newton was right and Einstein was wrong. What we are saying is that at low speeds, Newton's theory of universal gravitation approximates the results of general relativity; at high speeds, general relativity is used. Our textbooks also teach Newton's theory of universal gravitation, which means that at this stage we accept Newton's theory of universal gravitation and we do not think it is a wrong theory.
But the two theories have completely different views of space and time. Obviously, Einstein's view of space and time has withstood all current experimental tests. And the reason why we haven't given up on Newton's theory of gravity is because it's easy to use. The gravitational force between two objects can be easily calculated using Newton's gravity theory. If you try to use Einstein's gravitational field equation to calculate it, you will be completely confused. If we think that Einstein's gravity theory is the most essential gravity theory, then Newton's gravity theory is naturally wrong. That means gravity is not a force, or gravity is a force but not a force. Let’s take a walk in this direction first.
You must clearly understand that whether it is Newton's theory of gravity or Einstein's theory of gravity, they are inseparable from two concepts: inertia and acceleration.
A deep understanding of these two concepts is the key to understanding what gravity is. Let’s first look at the definition of inertia.Newton's definition of inertia: The nature of an object remaining stationary or moving in a straight line at a uniform speed is called inertia. Textbooks now believe that inertia is an inherent property of an object, expressed as a degree of resistance of an object to changes in its motion state, and mass is a measure of the inertia of an object.
In my book "Change", I extended the definition of inertia. Inertia is defined as: In the gravitational field , objects have the property of maintaining their current state of motion, which is called inertia.
Let’s take a look at the definition of acceleration: Acceleration is the ratio of the change in velocity to the time taken to occur, Δv/Δt, and is a physical quantity that describes how quickly the speed of an object changes.
Here you must understand that force is not what keeps objects moving, inertia is. Force is what changes the state of motion of an object, that is, force is what causes acceleration.
Spirit Escaper "Change"
First of all, you will find that inertia is a very magical existence. Inertia is what maintains the motion of an object, but inertia is what resists changes in the state of motion of an object. This is why an object's motion state needs to change and work must be done by overcoming inertia.
Why do a feather and an iron ball fall to the ground at the same time in a vacuum? It's because the acceleration due to gravity is the same. The fundamental reason is that the greater the mass of an object, the stronger its ability to resist changes in its motion state. But if the feather and the iron ball are placed in different dimensions of the earth, even in a vacuum environment, they will not fall at the same time. Because the acceleration due to gravity changes with latitude, there are changes.
Newton's theory of gravity does not explain how the gravity between objects is generated. Newton believed that gravity existed without explanation and could not be explained.
Similarly, Newton's theory of inertia does not explain how inertia is generated. Newton believed that inertia inherently exists and is a property of objects. No explanation is needed and no explanation is possible.
What about Einstein’s theory of gravity? Einstein believed that gravity is a geometric effect of space and time, not a real force. We say that massive objects will deflect light and prevent it from traveling in a straight line. This is Einstein's prediction and has been confirmed. If you agree that the curvature of space-time produces gravity, then the accurate understanding should be this. Massive objects cause space-time to curve, and light propagates along the curved space-time, that is, it is deflected. In other words, it is not gravity that deflects light, but the curvature of space and time that causes light to only take a curved path [geodesic path]. In the same way, the same is true for the movement of other stars or objects, as if they all have their own curved "orbits".
At the same time, you also need to understand that to deeply understand the general theory of relativity, you must deeply understand the special theory of relativity. Because the general theory of relativity is based on the special theory of relativity. Then we need to list the conditions for the establishment of special and general relativity and analyze them one by one.
The Four Parts of Popular Science for Spirit Escapers
1. The special theory of relativity is based on the principle of special relativity and the principle of the invariance of the speed of light . So what is the principle of special relativity, and what is the speed of light is constant ?
Principle of special relativity: All physical laws (laws of mechanics, laws of electromagnetism, laws, and other dynamic laws of interaction) are equivalent (equal) in all inertial reference systems. No inertial system has Superior status, there is no absolutely static reference system (ether), thus denying " ether says " and absolute space.
It is worth mentioning that this principle is actually a generalization of Galileo's principle of relativity, which means that Galileo was the first person to think about changes in the motion of objects in inertia. It can be said that if you do not understand inertia, inertial frames, and non-inertial frames , you cannot have a deep understanding of the theory of relativity.
Principle of constant speed of light: The speed of light in a vacuum is the same for any observer.
2. The general theory of relativity is based on the general principle of relativity and the equivalence principle.So what is the general principle of relativity? What is the equivalence principle?
General principle of relativity: All physical laws take the same form in any reference system.
Equivalence principle: The dynamic effects of the inertial force field and the gravitational field are locally indistinguishable.
The equivalence principle is divided into a weak equivalence principle and a strong equivalence principle. The weak equivalence principle holds that the dynamic effects of the inertial force field and the gravitational field are locally indistinguishable. The strong equivalence principle believes that "kinetic effects" are promoted to "any physical effects". It should be emphasized that the equivalence principle only holds true for local inertial systems, and does not necessarily hold for non-local inertial systems.
The equivalence principle is a physical property of Ehrlich's gravity theory, that is, an appropriate reference system can be selected at any point in space and time, so that the equation of motion of all matter no longer contains a gravitational term, that is, gravity can be eliminated locally. If this reference system that eliminates gravity is considered to be an inertial system, then the equivalence principle tells us that at any point in space and time, there must be a local inertial system. Galileo was the first to notice that the sliding motion of different objects along an inclined plane is the same, that is, the gravitational acceleration has nothing to do with the composition of the object.
The equivalence principle is the first basic principle of general relativity and the core of the entire general theory of relativity. The basic meaning of is that the gravity field is equivalent to the reference frame moving with appropriate acceleration.
Why should we focus on the equivalence principle here? In fact, if you carefully consider the four principles of special and general relativity, the most confusing one is the equivalence principle. Among the other three principles of , the constant speed of light has now withstood experimental verification. The principle of special relativity and the principle of general relativity can also be proven through mathematical derivation. Only this equivalence principle does not actually have strict experiments. This idea was inspired by Einstein's elevator experiment.
The special theory of relativity is based on uniform linear motion in four-dimensional straight space. It is applicable to all inertial systems, but it cannot explain the motion phenomena in non-inertial systems. In order to overcome the shortcomings of the special theory of relativity and extend the special theory of relativity to non-inertial systems with acceleration, Einstein used the elevator thought experiment to open up ideas for people.
Imagine that you are in a sealed elevator and find yourself floating in it. So how do you distinguish whether you are weightless in space or whether you are following the elevator in free fall in the earth's gravitational field? On the contrary, if you are sucked firmly to the ground of the elevator, how can you tell whether the elevator is accelerating in space or whether the elevator is stationary on the surface of the earth? This is where the thought experiment of Einstein’s Equivalence Principle comes from.
The elevator thought experiment tells us that the effects of gravitational field and acceleration are equal. General relativity has been successfully extended to non-inertial frames.
Let me ask you a few questions. If you answer these questions, you will be able to clearly understand whether you really understand the general theory of relativity.
First question: The equivalence principle is divided into a weak equivalence principle and a strong equivalence principle. So, is the general theory of relativity based on the weak equivalence principle or the strong equivalence principle? The equivalent statement of the weak equivalence principle is "gravitational mass and inertial mass are equal". And "gravitational mass and inertial mass are equal" has undergone a large number of rigorous experimental tests. In the 1960s, Dicke improved Eotvos' experiment and further improved the accuracy to 10-11. After that, Braginsky and Basseler respectively improved the experimental accuracy to 10-12 accuracy. However, general relativity should be based on the strong equivalence principle, because it is a basic theory, must be strict, and its scope of application must be universal. The strong equivalence principle is now a stronger hypothesis and has not undergone as rigorous experimental testing as the weak equivalence principle. However, we should point out that any success of general relativity is effective support for the strong equivalence principle, including the weak equivalence principle. Improvement of experimental accuracy.
Therefore, according to the strong equivalence principle [all physical effects of gravitational field and inertial field are locally indistinguishable. 】The test of all physical effects is a huge challenge. For example, we don't actually know whether a certain quantum mechanical effect is different in the gravitational field and the inertial field.
The second question: Why does the definition of the equivalence principle say "locally indistinguishable"? Obviously, the word "local" is a limitation, a limitation of a specific environment and state. This will lead to the third question: "Since it is limited, can it be called broad?"
The fourth question: Why, for so many years, when explaining the equivalence principle, we always use the elevator As for experiments, is there no better experiment than the elevator experiment? Or after so many years, can we not think of a better explanation than Einstein? I tried to think about it, but when I think about concepts like gravitational field and acceleration, it seems that I can only follow the idea of "elevator experiment", indicating that this is also a "limitation". Just like Newton, he actually knew the fact that gravitational mass and inertial mass were equal, but he couldn't explain it, so he thought it was accidental.
Einstein didn't think it was a coincidence, so he kept thinking and came up with the idea of the elevator experiment. But if we can't think of ideas and ideas other than the "elevator experiment", is there also an element of "accidental"?
The fifth question: Why is the equivalence principle the key to extending the special theory of relativity to the general theory of relativity? It's because there is the concept of acceleration, which is the transition from an inertial frame to a non-inertial frame. But it should be noted that the free fall of and the acceleration of in the elevator experiment are all constant accelerations, not the acceleration is fast and slow. If this is the case, then the gravitational field and the inertial field can be distinguished. Always remember that a non-inertial frame can be defined as a reference frame that has relative acceleration to an inertial reference frame.
So the elevator experiment is actually a very special situation, and the restrictions are actually quite large. At this time, I will answer the third question, "Since it is limited, can it be called broad?" The answer is yes. Although the elevator experiment is an ideal and special situation, it can indeed reflect a The fact that gravitational and inertial fields are locally equivalent. So based on the idea of differential , there is such a point, and there are countless such points. For example, if the acceleration is 9 meters per second, 10 meters per second, or even 100 meters per second, there will be exactly one gravitational constant on the planet. g values are equal to these numbers, then the elevator experiment is universal. It achieves the purpose of seeing the whole leopard with one spot, that is, changing from "differential" to "integral" and realizing the theoretical "generalization."
Sensitive people have been able to discover the direct consequences of the local equivalence of the gravitational field and the inertial field. , which is to turn gravity into "inertia". When gravity is "inertial", you will find that gravity seems to be a kind of "gravitational potential energy", and its root is space-time. So Einstein concluded that gravity is the geometric effect of the curvature of space-time. How can I give you a more popular description? Just like we often say inertial force , everyone is familiar with it. Inertial forces are common in daily life and have a great impact on us. A force that has a real impact on us is not a real force, that is, we think that the inertial force is not a force. Now that gravity has a real effect on us, we also say that it is not a force. It should be easier for you to understand this explanation, because it has no force actor.
If we really talk about it this way, then gravity is not a force.
But have you ever thought that if gravity can be explained in this way, can I also explain inertia in this way? Explain the inertial force? You haven't thought about it? It doesn't matter, there is a person named Mach , he thought about it. In fact, Einstein also drew on his ideas to construct his general theory of relativity.
At this point, you understand. Gravity is not a force, but a geometric effect caused by the curvature of space-time. If this is true, then gravitons do not exist, and neither does the gravitational field. Gravitational waves can still exist because gravitational waves are ripples in space-time, which is evidence of fluctuations in the curvature of space-time. And now it has been proven that the transmission speed of gravitational waves is the speed of light.
Now you finally understand why Einstein said imagination is more important than knowledge. But what I want to say is that although imagination is more important than knowledge, imagination that cannot be imagined makes people feel vaguely uneasy.Through the above discussion, so many "restrictions" and "idealizations" are used to support this modern pillar theory, which always makes people feel uneasy and fear that one of its "legs" will be unstable.
Let me ask you the last question, which is the sixth question in this chapter: "Is general relativity a theory of gravity?" Yes, Einstein wanted to incorporate gravity into the special theory of relativity, but he couldn't, so he thought hard. Developed the general theory of relativity. But in our opinion, this general theory of relativity that incorporates "gravity" is not so much a gravity theory as a space-time theory. It must be a theory of space and time first, and then a theory of gravity.
My vague uneasiness comes from the same suspicion as Einstein, that is, we all believe that the equality of gravitational mass and inertial mass is not an accident, which also explains the cause of gravity, that is, gravity is caused by the curvature of space and time. But why do we still believe that inertia does not need to be explained and cannot be explained? Who gives the inertia of an object? how come? If I don't explain this, I will often feel vaguely uneasy.
So how do I explain the cause of inertia? As I analyzed in the previous chapters of "Change", we cannot make the same mistake as Mach. I agree with Einstein's theory of the speed of light. Then it is a metaphor that inertial propagation has the speed of light. It is natural that abandoning inertia is a choice dictated by the time and space of the entire universe. So what exactly makes an object have inertia? I naturally thought that gravitational mass and inertial mass are equal, and also thought of the equivalence principle. Then the answer seems clear, it is gravity. Only this thing can meet the requirements. That is, gravity makes objects have inertia! This is a thought written N times in "Change".
But as soon as I thought about it and wrote it, the problem came again. Gravity is not a force, but a geometric effect of the curvature of space-time. Then gravity makes objects have inertia, which means that the curvature of space and time makes objects have inertia. But once it is connected with time and space, the problem of distance will inevitably arise.
So I have to question what is the cause of gravity? In other words, gravity is a geometric effect caused by the curvature of space and time, so there is a question mark. So gravity is a force, so it must be discussed. When gravity is a force, the particles that propagate gravity are gravitons, and the concept of gravitational field must be present. In this way, the cause of gravity becomes a force in which objects interact by transmitting gravitons in the gravitational field. The gravitational field is of course global, so it points to space-time. John Wheeler's classic interpretation of general relativity: "Matter tells space-time how to curve, and space-time tells matter how to move." That is, the gravity of massive objects causes space-time to curve, and the curvature of space-time affects the movement of objects.
So I wrote in the book that Einstein established an amazing theory, but made mistakes when explaining it. Some people say you question the theory of relativity? I admire and pay tribute to Einstein with a skeptical attitude. General relativity is first of all a theory of the universe, and the field equations are also field equations of the universe.
At this time, you will find out why Newton's theory of gravity is correct and get the answer. It's just that Newton considered the static super-light universe, while Einstein considered the four-dimensional space-time theory combined with time.
I believe you will understand why general relativity is difficult through the above discussion. Just one equivalence principle is worth asking a lot of questions. Then there are many questions that can be asked about the constant speed of light, time, the principle of general relativity, etc. You must have a harmonious understanding of the relationship between them to help you better understand the general theory of relativity.
After reading this, you should know clearly that Newton failed to explain why gravitational mass and inertial mass are equal, and Einstein also failed to explain it. Although he believed that this was not an accident, he established the equivalence principle based on it. But does the equivalence principle explain why gravitational mass and inertial mass are equal? No.
So you will understand that the establishment of the principle of equivalence comes from a "strictly equal fact", but this fact has not been strictly reasoned and demonstrated. Therefore, the true general theory of relativity is called general relativity, not Einstein's theory of gravity, which is very correct.General relativity is not a theory of gravity in the first place, it is a theory that includes the theory of gravity! That is, the expanded theory from the inertial frame to the non-inertial frame, that is, the global field theory. So it's called Einstein's field equation. When we call it Einstein's gravitational field equation, it's actually a "restricted" interpretation of it.
So what is the reason why gravitational mass and inertial mass are strictly equal? Friends who have read my "Change" already know that the reason why gravitational mass and inertial mass are strictly equal is that inertia is produced by the action of gravity, that is, gravity is the source of inertia. The essence of gravity is the gravitational field, that is, the effect of the space-time field on matter. In this way, the existence of graviton is reasonable. Gravity must be force! So naturally, scientists will think of efforts to unify gravity with the other three fundamental forces. It is so big that it has no outside, and its small has no inside. How can gravity run counter to quantum mechanics? No, gravity, as a basic effect, must be harmonious with quantum mechanics.
is an independent scholar and popular science writer who works on popular science. This chapter will be included in the spiritual escaper's quantum mechanics book "Seeing the Small Things".
About the author: Spirit Escaper, formerly known as Wang Yin, male. A native of Shaanxi, now living in Xi'an. His main representative works are the popular science quartet "Change", "Seeing the Small Things", "Reconstructing the World" and "Exploring Life". Other literary works include "Looking at the World", "Nonlinear Waves", "My World", "The Pen Has a Thousand Key Points", and "Touching the World".
