In 1905, Albert Einstein turned the world of theoretical physics upside down by publishing a work in the discipline that would later be called the special theory of relativity. She showed that space and time cannot be regarded as absolute entities: time can accelerate or slow down, standard lengths can contract, masses can increase.
And, the most famous result, the equivalence of the mass of energy, and their proportion is expressed through the equation E = mc².
No one doubts the genius of Einstein, who formulated general relativity, but it is generally accepted that if he had not published his theory in 1905, some other physicist would soon have done it in his place.
"Einstein's Cross" - four images of one distant quasar, obtained due to the fact that the light from it bends around a galaxy located closer to us, working as a gravitational lens.
It wasn't until 1915 that Einstein demonstrated his genius by publishing his theory of general relativity. She argued that the curvature of space-time is proportional, and also occurs due to the "energy-momentum density", that is, the energy and momentum associated with any matter in a unit volume of space.
This statement was confirmed when it coincided with observations of the unusual orbit of Mercury and the starlight bending around the Sun.
Over the past hundred years, general relativity has been tested with amazing accuracy and has stood the test every time. General relativity has become such a giant leap forward that one can say that if Einstein had not formulated it, it could remain undiscovered for a long time.
The path to general relativity
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In 1907, Einstein had the "happiest thought of a lifetime" as he sat on a chair in the patent office in Bern:
If a person falls freely, he does not feel his weight.
She led him to the formulation of the "principle of equivalence", which says that it is impossible to distinguish between the accelerating frame of reference and the gravitational field. For example, if you are standing on Earth, it will feel exactly the same as if you were standing in a spaceship moving at an acceleration of 9.81 m / s² - with the acceleration of gravity on Earth.
This was the first major step towards the formulation of a new theory of gravity.
Einstein believed that "all physics is geometry." He meant that space-time and the Universe can be thought of in geometric terms. The most surprising conclusion of general relativity, the dynamic nature of time and space, apparently led Einstein to the need to rethink the "geometric" space-time.
Einstein conducted a series of neat thought experiments comparing observations made by observers in inertial and rotating frames of reference.
He established that for an observer in a rotating frame of reference, space-time cannot be Euclidean, that is, such as that flat geometry that we all study in schools. We need to introduce "curved space" into our reasoning to account for the anomalies predicted by relativity. Curvature becomes the second most important assumption supporting his general relativity.
To describe curved space, Einstein turned to an earlier work by Bernard Riemann, a 19th century mathematician. With the help of his friend Marcel Grossmann, also a mathematician, Einstein spent several tedious years studying the mathematics of curved spaces - what mathematicians call "differential geometry." Einstein noted that "compared with understanding gravity, special relativity seemed like a child's play."
Einstein now had the mathematical apparatus to carry the theory to completion. The equivalence principle stated that an accelerating frame of reference is equivalent to a gravitational field. As a result of his studies in geometry, he believed that the gravitational field was a simple manifestation of curved space-time. Therefore, he could show that the accelerating frames of reference were non-Euclidean spaces.
Development
The third most important step was the elimination of difficulties in applying general relativity to Newtonian gravity. In the special theory of relativity, the constancy of the speed of light in all reference frames and the statement that the speed of light is the maximum attainable speed contradicted Newton's theory of gravity, which postulated the instantaneousness of the action of gravity.
Simply put, Newtonian gravity said that if the sun was removed from the center of the solar system, the gravitational effect of this event would instantly be felt on Earth. But SRT says that even the effect of the disappearance of the Sun will move at the speed of light.
Einstein also knew that the gravitational attraction of two bodies is directly proportional to their masses, which followed from Newton's F = G * M * m / r². Therefore, the mass clearly determined the strength of the gravitational field. SRT says that mass is equivalent to energy, therefore the energy-momentum density should also determine the force of gravity.
As a result, the three key assumptions Einstein used to formulate his theory were:
1. In rotating (non-inertial) frames of reference, space is curved (non-Euclidean).
2. The principle of equivalence says that accelerating frames of reference are equivalent to gravitational fields.
3. The equivalence of mass and energy follows from SRT, and from Newtonian physics it follows that mass is proportional to the force of gravity.
Einstein was able to conclude that the energy-momentum density creates, and is proportional, to the curvature of space-time.
It is not known when he had his "insight" when he was able to solve this puzzle and relate mass / energy to the curvature of space.
From 1913 to 1915, Einstein published several papers, while working on the completion of general relativity. Errors were encountered in some of the works, which led Einstein to waste time on unnecessary distractions in theoretical reasoning.
But the net result, that the energy-momentum density bends space-time, like a bowling ball is a stretched sheet of rubber, and that the movement of mass in a gravitational field depends on the curvature of space-time is, without a doubt, the greatest guesses made by human intelligence.
Handicap
How long would we have understood gravity if not for the genius of Einstein? It is possible that we would have to wait for this for many decades. But in 1979, the mystery would surely come out. In that year, astronomers discovered "twin quasars," QSO 0957 + 561, the first quasar to observe gravitational lensing.
This amazing discovery can only be explained by the curvature of space-time. For him, they would probably have given the Nobel Prize, if not for the genius of Einstein. Or maybe she should still be given out.