Without Einstein, we would have dealt with gravity for decades

In 1905, Albert Einstein turned the world of theoretical physics upside down, publishing work on a discipline, which would later be called the special theory of relativity. She showed that space and time cannot be considered as absolute entities: time can accelerate or slow down, standard lengths can be reduced, masses can increase.

And, the most famous result is the equivalence of the mass of energy, and their proportion is expressed through the equation E = mc².
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No one doubts the genius of Einstein, who formulated GTR, 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 is bent around the galaxy, located closer to us, working as a gravitational lens.

Only in 1915 did Einstein demonstrate his genius by publishing his general theory of relativity. She argued that the curvature of space-time is proportional, and also occurs due to "energy-momentum density," that is, 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 with the light of stars bending around the Sun.

Over the past hundred years, GR has been tested with tremendous accuracy and has withstood every test. GTR has become such a giant leap forward that one can say - if Einstein had not formulated it, it could have remained undiscovered for a long time.

The path to the general theory of relativity

In 1907, “the happiest thought of a lifetime” came to Einstein when he was sitting on a chair at the patent office in Bern:

If a person falls freely, he does not feel his weight.

She led him to the formulation of the " equivalence principle ", which states that it is impossible to distinguish between an accelerating reference system and a gravitational field. For example, if you are standing on Earth, it will feel just like if you were standing in a spacecraft moving with an acceleration of 9.81 m / s² - with the acceleration of free fall 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 one can think about space-time and the Universe in geometrical terms. The most surprising conclusion of GR, the dynamic nature of time and space, apparently led Einstein to the need to rethink the "geometric" space-time.

Einstein conducted a series of accurate thought experiments comparing observations made by observers in inertial and rotating reference frames.

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 learn in schools. We need to introduce “curved space” into the argument in order to take into account the anomalies predicted by relativity. Curvature becomes the second most important assumption supporting its GRT.

To describe the curved space, Einstein turned to the earlier work of Bernard Riemann , a nineteenth-century mathematician. With the help of his friend Marcel Grossman , also a mathematician, Einstein spent several tedious years studying the mathematics of curved spaces - what mathematicians call "differential geometry." Einstein noted that "compared with the understanding of gravity, the special theory of relativity seemed like a child's toy."

Now Einstein had a mathematical apparatus to bring the theory to completion. The equivalence principle claimed that the accelerating reference frame is equivalent to a gravitational field. As a result of studying geometry, he believed that the gravitational field was a simple manifestation of a curved space-time. Therefore, he could show that the accelerated reference systems were non-Euclidean spaces.

Development

The third most important step was the elimination of difficulties in the application of GR to Newtonian gravity. In the special theory of relativity, the constancy of the speed of light in all reference systems and the statement that the speed of light is the maximum attainable speed contradicted the Newtonian theory of gravity, which postulated the instantaneousness of the action of gravity.

Simply put, Newtonian gravity said that if you remove the sun from the center of the solar system, the gravitational effect of this event will instantly be felt on Earth. But the 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 follows from the Newtonian F = G * M * m / r². Therefore, the mass clearly determined the force of the gravitational field. The 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 used by Einstein to formulate his theory were:
1. In rotating (non-inertial) reference systems, space is curved (non-Euclidean).
2. The principle of equivalence says that accelerating reference systems are equivalent to gravitational fields.
3. The SRT implies the equivalence of mass and energy, and from Newtonian physics it follows that the 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 an “insight”, when he was able to put this puzzle together and connect mass / energy with the curvature of space.

From 1913 to 1915, Einstein published several papers, simultaneously working on the completion of GR. Some papers encountered errors, which is why Einstein spent time on unnecessary distractions in theoretical reasoning.

But the final result, that the energy-momentum density bends space, time, like a bowling ball - a stretched rubber sheet, and that the movement of mass in a gravitational field depends on the curvature of space-time - this is without doubt the greatest guesswork made by human intelligence.

Handicap

How long would we understand gravity if we weren't with Einstein's genius? It is possible that we would have had 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 on which gravitational lensing was observed.



This amazing discovery can only be explained by the curvature of space-time. For it would certainly have given the Nobel Prize, if not for the genius of Einstein. Or maybe it still worth to issue.