What is a proof for the general theory of relativity

Crucial evidence 100 years ago : As Einstein was confirmed by a solar eclipse

Everyone tests Albert Einstein's theory of relativity for accuracy almost every day. When the navigation system in our car announces: "You have reached your destination", this is only true because the atomic clocks on board the satellites of the GPS navigation system also take Einstein's formulas into account.

After many years of deliberation and calculations, Einstein presented these so-called field equations for the first time in November 1915 in a lecture at the Prussian Academy of Sciences in Berlin. They describe the workings of the cosmos quite differently than physicists have been used to for more than two centuries since the time of the English natural scientist Isaac Newton.

Einstein's new explanation for the earth's crooked orbit around the sun

In Newton's celestial mechanics, the "space" was just a container through which the celestial bodies move in the course of an evenly running "time". And if a planet like the earth does not move on a straight path, but is forced into an orbit around the sun, then, according to Newton, an attractive "gravitational force" between the sun and earth is necessary.

In contrast to this, in Einstein's theory of relativity, "space" and "time" are not absolute and mutually independent quantities. Rather, they are fused into the geometric structure of a four-dimensional "space-time".

With the help of this space-time geometry, Einstein found a completely new explanation for the movements of the celestial bodies on their crooked orbits, such as the orbit of the earth around the sun: Like all celestial bodies, the sun also bends the geometry of space-time in its environment .

And the earth follows on its orbit this curvature of space-time caused by the sun, completely free of forces on a completely natural path. But who was right now: Newton with his intuitively easy to imagine gravitational forces? Or Einstein with his difficult to imagine space-time curvature and its difficult mathematical description?

On May 29, 1919, there was a unique opportunity

In 1916 Einstein suggested several ways of testing his theory of relativity for accuracy. One of them would result in a total solar eclipse on May 29, 1919: While the moon wanders directly in front of the sun for a few minutes, covering its bright light, the warping of space-time can be seen in its surroundings.

Because in the darkened surroundings of the sun, a few stars would then be visible. In truth, of course, these stars shone far behind the sun. So your light, coming from great distances, would fly close to the sun before it arrives on earth.

And if the space-time in the area around the sun is actually bent, the light would also have to follow this curvature and be deflected from its straight light path. So it would no longer reach us from its original direction. In other words, the stars that you would see around the darkened sun would be shifted compared to their true positions.

The disadvantage: you couldn't see the darkness from Europe

Newton had also predicted such an effect. But the displacements of the star positions calculated with his gravitational forces were only half as large as the displacements that Einstein predicted on the basis of the space-time curvature. In theory, then, a solar eclipse should reveal whether Newton was right or Einstein was right.

And the solar eclipse on May 29, 1919 was particularly favorable for this. Because on this day the sun happened to be exactly in front of the star cluster of the Hyades. Therefore, during the eclipse, an above-average number of stars could be seen around the sun, and their respective shifts could be measured. The disadvantage: this solar eclipse could not be observed from Europe.

For this reason, two British science expeditions set out in the spring of 1919 - one to Sobral in Brazil, one to the island of Principe off the west coast of Africa - in order to point their telescopes at the darkened sun and to capture the stars that became visible in their surroundings on photo plates .

The evaluation of the images obtained was laborious. The positions of the visible stars were actually shifted - but only by fractions of a millimeter. But after the scientists had calculated both atmospheric disturbances and instrumental aberrations from their measurement result, they were certain: The shifted positions of the measured stars indicated a curvature of space-time according to Einstein and not the effect of gravitational forces as postulated by Newton .

A science news hit the headlines

Just a few months after the end of World War I, British astronomers of all places had shown that the new theory of the scientist from the country of the former enemy describes the world more precisely than the old theory of their compatriot Newton. And probably for the first time a message from the world of science hit the headlines around the world.

The "Berliner Illustrierte Zeitung", for example, named Einstein "a new great in world history" in its December 14, 1919 issue. And the "New York Times" headline: "Lights all askew in the heavens", loosely translated: "The stars hang crooked in the sky".

Although greatly exaggerated, such headlines made Albert Einstein world famous in one fell swoop. And rightly so: because without his theory of relativity we might not have known about the existence of black holes and would not have discovered any gravitational waves because we would not have looked for them at all.

And without Einstein's equations, we would still look into the ever-widening expanses of the universe today without knowing that they tell the story of the universe.

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