The interpretation of gravity by the law of universal gravitation and general relativity is essentially two descriptions of the same thing. There is no right or wrong, only the scope of application-the description of Newtonian mechanics is simple in form, but it only works in weak gravitational fields , The description of general relativity is more complicated and applicable to all occasions. It can be said that general gravitational problems are solved by the law of universal gravitation, and a few intractable diseases are solved by general relativity.

Yang Zhenning, a physicist who made outstanding contributions to the Standard Model

At present, the theory of gravity can basically solve all the problems of gravity itself, but in addition to gravity itself, there are three other basic interaction forces. Only when the four forces are unified and unified under the same framework can it be regarded as physics. The ultimate goal of home. However, gravity is extremely “unruly”, and the road to unity with other fundamental forces is very difficult.

Physicists are obsessed with “unity”

Force is the interaction between objects. There are various forces in life, including tension, thrust, elasticity, pressure, gravity, etc. from a macro perspective, and Lorentz force, van der Waals force, intermolecular force, etc. from a micro perspective. But no matter what force it is, it can be classified as one of the following four basic interaction forces: gravitation, electromagnetic interaction, strong interaction, and weak interaction. We all know the universal gravitational force. What are the other three basic interaction forces?

Electromagnetic interaction force, referred to as electromagnetic force, is the force received by charged particles in an electric, magnetic or electromagnetic field; strong interaction force, also known as strong nuclear force, is the force that combines neutrons and protons in the nucleus; Weak interaction force, or weak force for short, only acts on extremely tiny particles, such as fermions such as electrons, quarks, and neutrinos, and restricts radioactivity. Strong nuclear force and weak nuclear force have a very short range of action, and their range of action is very small, and they are only forces between atoms or tiny particles. These forces are basically inaccessible to us. The basic forces that people usually see, except gravity Outside is electromagnetic force.

Just as mathematicians are obsessed with the “symmetry beauty” of various forms of mathematics, physicists are committed to putting various physical theories under the same theoretical framework, and “unity” is the ultimate beauty pursued by physicists. The discovery process of the four basic forces is the unified process of many physics fields: In the 17th century, Kepler proposed three Kepler laws describing planetary motions, which explained the laws of celestial bodies, but Kepler himself did not Know the connection between the three laws. It was not until Newton proposed the law of universal gravitation that Kepler’s three laws were essentially explained, and for the first time the laws of motion of objects in the sky and underground were unified.

Standard model

Relying on the law of universal gravitation and the three laws of Newton, the entire theoretical edifice of classical physics began to be gradually established. In 1865, the British physicist Maxwell proposed Maxwell’s equations, which completed the unification of electricity and magnetism. All the laws and phenomena of electricity and magnetism can be explained by Maxwell’s equations.

In 1954, physicists Yang Zhenning and Mills unified the electromagnetic interaction and weak interaction into a new theory-Yang-Mills theory through the analysis of electromagnetic interaction. Yang-Mills theory is an extremely important physical breakthrough in the second half of the 20th century, and it is the basis of modern gauge field theory. Gauge field theory is one step closer than Yang-Mills theory. Under the framework of quantum mechanics, it has successfully unified electromagnetic interaction, strong interaction and weak interaction into one model, which is the standard model.

The standard model is invincible in the microscopic field. The results of all experiments on electromagnetic interactions, strong interactions and weak interactions are in line with the predictions of this model. But when faced with gravity, this theory is a bit helpless-there is no way to quantize gravity at all!

Decades before the establishment of the Standard Model, after the birth of general relativity, Einstein made an attempt to unify electromagnetic interaction into general relativity. At that time, the strong nuclear power and the weak nuclear power had not been discovered, and only electromagnetic force stood in front of Einstein. Einstein was very optimistic at one time, thinking that electromagnetic force and gravity are so similar, both have long-distance effects, and both conform to the inverse square law of distance. However, until his death, Einstein did not incorporate electromagnetic force into general relativity.

If we go deeper, there are two reasons. The first is the mathematical reason: General relativity is infinitely separable and continuous. The theory of quantum mechanics is discontinuous and has a minimum unit. The second is the physical reason: General relativity is a classic definite theory, and the only state B can be deduced from state A. Quantum theory is uncertain. The transition from state A to state B is unpredictable and a random process. Starting from the general theory of relativity, it is extremely difficult to study other fundamental forces.

The three basic forces unified in the standard model all start from the perspective of quantum mechanics. So from the perspective of quantum mechanics, what is the difficulty of unifying gravity?

Can gravitons solve the problem?

The standard model theoretically completes the unification of strong interaction force, weak interaction force and electromagnetic interaction force. Quantum mechanics believes that force comes from the transfer and exchange of particles. These three basic forces have a characteristic, that is, the transfer meson of these interactions needs to be transferred. For example, the transfer meson of electromagnetic force is photon, and the strong transfer meson is gluon. , And the weak transfer meson is the w boson and z boson. Starting from the Standard Model, scientists assume that there is a meson that transmits gravity—gravitons. This graviton can ionize gravity—according to the basic principles of quantum mechanics, energy is quantized, so gravitational energy It must also be part by part. Gravitational energy is transferred to infinite distance by the graviton as a carrier.

Unfortunately, the graviton has not been discovered yet. It is still an imaginary particle. Some scientists firmly believe that gravitons exist, and some believe that gravitons cannot exist at all. Scientists of different schools hold different opinions. But once the graviton is discovered, the world of physics will undergo a big change.

General relativity believes that gravity is the curvature of space-time. When a mass object moves in space-time, the change in the curvature of space-time reflects the change in the position of these objects. Conversely, under certain circumstances, accelerating objects can also cause changes in the curvature of space-time, and this change can propagate outward in the form of waves at the speed of light. This propagation phenomenon is called gravitational waves.

Quantum mechanics believes that particles have wave-particle duality, that is, particles have both wave and particle properties. According to this condition, gravitational waves must be composed of some kind of particles. If such particles are gravitons, quantum mechanics may be able to relate to general relativity from a certain angle. But just to make a connection, the fundamental contradiction between quantum mechanics and general relativity will not change. This connection will bring even greater doubts: in the world of quantum mechanics, any physical quantity is uncertain, the gravitational field generated by the graviton should be uncertain, but in macroscopic reality, the gravitational field is certain. It is difficult to figure out how such a gravitational field is produced.

Whether gravitons exist or not is still unknown, but scientists will not give up their dream of “unification”, they will continue to try new methods.