# If Einstein hadn't wasted 30 years on a unified field theory

What would 21st-century physics look like if Einstein hadn't wasted 30 years on a unified field theory

Einstein was a great scientist after Archimedes and Newton. His theory of relativity can be said to be the greatest achievement in the 20th century. The theory of relativity and quantum mechanics brought revolutionary changes to physics, and together they laid the foundation of modern physics. Relativity has greatly changed the "common sense" concept of the universe and nature, and put forward new concepts such as "simultaneous relativity", "four-dimensional space-time" and "curved space-time".

However, Einstein's greatest achievements were almost all completed before 1920, and nothing remarkable was achieved after that. Many people say that Einstein was obsessed with theology, but he was not. Einstein continued to follow maxwell's path to complete the unification of the universe.

We said that if we were to choose the two greatest physicists in the world, Newton Einstein would be the one who built the mechanics of the macro world, and the one who took us into the micro world and explored the universe. And the link between the two is undoubtedly Maxwell, the terminator of the old era and the pioneer of the new era.

The electromagnetic field theory established by him unified electricity, magnetism and optics, which was the most glorious achievement of the development of physics in the 19th century and the first great unification in the history of physics. It can be said that without Maxwell, there would be no modern civilization.

A Cambridge physicist is said to have once complimented Einstein by saying, "You are standing on Newton's shoulders." Einstein replied, "No, I am standing on Maxwell's shoulders!"

Einstein's whole life of scientific research is in the path of Maxwell did not go, special relativity is obviously to solve the contradiction between Maxwell's electromagnetic theory and classical mechanics was established, and general relativity is the continuation of the previous thought. The unified field theory, which he spent half of his life researching, is also aimed at unifying the gravitational field and electromagnetic field, realizing the second great unification in the history of physics.

The concept of field theory also originated from Maxwell's electromagnetic field theory. The core concept of Maxwell's electromagnetic field theory is "field". In a certain space area, objects with certain properties can exert a force on similar objects that are not in contact with them, which is called "field". Then one charged body exerts a force (an attractive or repulsive force, depending on the polarity) on another charged body. A magnet has a magnetic field around it.

The magnetic field

Because influenced by the macroscopic field exists in the form of (not involving molecules, atoms, electron, etc., internal structure, or mechanism, maxwell era of physics is not in-depth micro field) of maxwell electromagnetic field theory knowledge of field or fuzzy, stay in the "field theory" stage, electric and magnetic field lines just described the macroscopic properties of electric field and magnetic field, The nature and origin of the field are not elucidated.

However, Maxwell's theory of electromagnetic fields unified the action of electricity and magnetism, and it was the first unified theory of several kinds of interaction in history. This allowed many physicists of the time to break free from the Newtonian "at a distance" and to accept the idea that electromagnetic and gravitational action were both "at a distance".

Later physicists put it in the microscopic realm, where when one charge moves, the other does not immediately sense it. The first charge senses a reaction and gains momentum, but the second charge does not until the effect of the first charge's movement reaches the second charge at the speed of light, giving it momentum. So before the second charge moves, the momentum is hidden in the field. So that explains the existence of electromagnetic fields. Later the concept of the field became the paradigm for the whole of modern physics.

In his special theory of relativity, Einstein asserted that there was no medium for electromagnetic waves in nature, and that the electromagnetic field itself was the medium. Einstein proposed a specific state of a field in a region of space and time, with neither electromagnetic radiation nor matter particles. This is called a vacuum. Einstein said that a field is the space in which matter exists. It is manifested by the interaction of various factors in the material space-time environment.

On this basis, Einstein introduced the field idea into the theory of gravity to create general relativity. General relativity led to an upsurge in research that had already unified field theory.

Because in general relativity the gravitational field was described as the curvature of space-time, plus the only basic interactions known at the time were gravitational and electromagnetic. So many scientists wanted to be the next man after Maxwell to complete the unification.

Such as weyl was inspired from the symmetry of Einstein's general theory of relativity, aware of the maxwell equations of electromagnetic field should also be like Einstein field equations, corresponding to a kind of basic symmetry, outside, has established the one called "scale invariance" symmetrical transformation, aiming at unifying gravity and electromagnetic force, from the point of view, based on the specification. Although it did not achieve the grand goal of unifying gravity and electromagnetic interaction, it gave birth to normative field theory.

In the 30 s, because of delays in implementing the unification of gravity and electromagnetic effect, coupled with the empirical evidence by quantum mechanics continuously, many physicists have turned to the study of quantum mechanics, so for the unified field theory research is gradually weak, already unified field theory to study the effect of the physical is not big, but plays an important role in the development of differential geometry.

But Einstein did not give up, since the 1920s, he has been committed to the research of unified field theory, after winning the Nobel Prize, Einstein once said that "the existence of two kinds of fields (guiding force field and electromagnetic field) independent of each other cannot satisfy the mind seeking unity." "We look for a unified field theory in mathematics, The gravitational field and the electromagnetic length are just different components of the same field." !

As we know, Einstein's general theory of relativity made outstanding achievements in describing the gravitational field by Riemannian geometry. He used Riemannian geometry of curved space-time to describe the gravitational field, gave the physical laws in the gravitational field, and then proposed the equations of the gravitational field.

Einstein thought that if the gravitational field could be described in terms of Riemannian geometry, then there must be a new geometry to unify the forces of gravity and electromagnetism.

In 1929 he published his new paper on Unified Field Theory, which attempted to obtain a unified theory of gravity and electromagnetism under the condition that spacetime manifold had absolute parallelism in addition to the Riemannian metric. Einstein himself was very proud of the paper, and on January 5, 1929, he wrote to his friend M. Basso thought that the unification of gravity and electromagnetism had been accomplished.

However, things did not go as Einstein predicted, and there was no real progress in unifying gravity and electromagnetism. Einstein later moved on to the work of Kalucha Klein, an early exponent of unified field theory, T. Carucha and O. Klein treated the electromagnetic potential as a component of the five-dimensional space-time metric tensor. Instead of establishing a new geometric theory to unify gravity and electromagnetism, as Einstein thought, he generalized Riemannian geometry from four to five dimensions. Revolutionized the concept of higher dimensional space.

Einstein spent 10 years from 1931 to 1941 trying unsuccessfully to start with the Carucha klein theory. By this time, Einstein had been working on a unified field theory for nearly 20 years, and in 1945 he had written "A Generalization of Relativistic Gravity," an attempt he considered logical but with physical difficulties.

In addition to continuing his polemic with Bohr of the Copenhagen School, Einstein at this time occasionally explored gravitational waves and the equations of relativity, for example in his 1952 book Relativity and The Problems of Space. Most of my time was spent in unified field theory.

It is a pity that Einstein's interpretation of the Copenhagen school of quantum mechanics was at odds with much of the Copenhagen school of quantum mechanics that had already been subjected to experimental controversy. And unified field theory was no longer part of the mainstream of physics by this time, and Einstein was very isolated in physics for the next 20 years of his life.

On April 18, 1955, Einstein died, the unified field theory that he had spent half his life working on failed. What a boost it would have been if Einstein had spent those 30 years working on something else.

On why Einstein failed in unified field theory, because the only basic interactions known at the time were gravitational and electromagnetic. Weak interaction was not put forward until the 1930s by Fermi theory, which regarded weak interaction as a point interaction between four particles (such as neutrons, protons, electrons and neutrinos in neutron decay), and the concept of weak interaction was still very vague at that time.

It was not until 1954 that Yang Zhenning and Mills found a unique way, which was no longer limited to the unification of gravity and electromagnetic force, but wanted to start with weak interaction and electromagnetic force. Based on the theory of outer, they extended the concept that electromagnetic action is determined by the local norm invariance to the local symmetric groups that cannot be commutated. It is revealed that gauge invariance may be the common nature of electromagnetic and other interactions, thus opening up a new way to unify various interactions using gauge principles. Yang Zhenning perfected the canonical field theory, and the physics field officially changed from the study of Einstein's unified field to the canonical field theory, which became a new trend of physics. The canonical field was later considered by many people to be the most likely to achieve the unification of the four forces.

Young Mills equation

Later, after Li Zhengdao, Yang Zhenning, Wu Jianxiong and others proved the parity and CP nonconservation of weak interaction, physicists considered establishing a weak interaction theory based on quantum field theory. In 1968, Weinberg and Salam established a perfect theory of electroweak unification based on the unified model of Electroweak. Their theory has been confirmed in many experiments. The electroweak unified theory is a scientific theory that has been tested quite rigorously by experiments.

Among the four basic interactions, the strong interaction was proposed in 1973, when the electromagnetic field theory of electromagnetic force had already developed into quantum electrodynamics, while the strong interaction had quantum chromodynamics and the weak interaction had Fermi point interaction theory. On the basis of the unified theory of electroweak, Glashow combined the three into a unified theory describing the interaction of weak, electric and strong. The standard field theory has also been extended to provide a unified mathematical formalization -- the Standard Model -- for electromagnetic interactions, weak and strong interactions, and the fundamental particles that make up all matter. It is a gauge field theory with the gauge group SU(3) × SU(2) × U(1).

Elementary particles under the Standard model

So why didn't Einstein complete the unified field theory? It was way ahead of its time. At that time, the strong force of the four fundamental interactions had not been proposed, the concept of weak force was not clear, and Einstein chose the most difficult of them, gravity, for reasons of relativity. Remember, the Standard Model, at this point in its development, still doesn't describe gravity. Compared with the electroweak unified theory, the unified standard model is far from mature, and it is only placed in a theoretical framework.

In addition, Einstein is from geometry Angle and collaborators from hair, more considering the material characteristics of space and time, ignore the quantum nature of matter, and ignore many experimental data, combined with his quantum theory of Copenhagen school does not accept, no absorbing achievements of the theory of quantum field theory, always trying to electromagnetic field geometry, Establishing the electromagnetic and gravitational fields as a geometric field was Einstein's undoing.

In any case, Einstein's spirit of persistent exploration is worth learning from, but if he had spent his 30 years in other areas of research, or had accepted the Copenhagen interpretation of quantum mechanics and restudied it, with Einstein's wisdom, would physics be different today?

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