A highlight of 20th century’s science – The confirmation of Einstein’sGeneral Theory of Relativity 100 years ago

Equipped with data and analyses, a team of astronomers led by Sir Arthur Stanley Eddington stoodin front of an overcrowded room at the Royal Society in London on the evening of November 6th, 1919 and outlined its measurement results. About half a year earlier their members had observed and photographed a total solar eclipse on the remote West African island of Príncipe (a second group had travelled to Brazil in order to take pictures of the same event in Sobral). What millions of people are doing today with their mobile phones on such an occurrence was at that timestill very difficult. The photographic technology was still ratherprimitive, and scientific resources were generally scarce after World War I. But Eddington’s researchers had taken these efforts upon themselves, as they believed that their observations could prove or refute the most revolutionary scientific idea in modern science: Albert Einstein’s General Theory of Relativity.

At a meeting of the Prussian Academy of Sciences in November 1915, Einstein had presented his “general theory of gravity” in which he had connected the effect of gravitational forces with the structure of space and time. He had formulated that gravitycorresponds tonothing else but to a change in the geometric structure of the unified four-dimensional space-time continuum. Einstein had thus united what remained sharply separated in Isaac Newton’s classical theory: space and force, geometry and gravitation. Masses are no longer solely connected with each other by forces in some absolute space and absolute time as in Newton’s theory, but they change the very structure of space-time by bending or „curving“ it, which in turn influences other masses „gravitatively“. The classical flat, so-called „Euclidean geometry“ of space loses its validity in Einstein’s theory and is replaced by a locally curved geometry the respective curvature of which depends on the prevalent mass distribution. Space is therefore no longer the container of the physical world, time no longer the internal parameter of motion, but both are integrated objects of physics with their own dynamics.

Einstein’s abstract theory can be illustrated with the help of an analogy: A lead ball on a rubber mat causes a deformation at the point where it lies. This curvature in turn influences the movement of other balls on the mat. A second lead ball will (assuming to friction) circle around the center of the curvature caused by the first ball. The balls on the rubber mat thus do not attract each other because of some forces acting between them, but because they themselves change the shape of the space they are in. Similarly, the deformation of the geometric structure of three-dimensional space by a massive body leads to a perceived force upon other massive bodies. But while it is clear that the two-dimensional mat is deformed into the third spatial dimension,where is spaceitself to be deformed into? The answer is: into a fourth dimension. Instead of describing only the geometry of a three-dimensional space, in which the bodies move within a one-dimensional time independent of that space, we must in Einstein’s geometric description of gravity regard space and time as inseparably connected with each other within a four-dimensional world. And in the presence of massive bodiesthis space-time continuum is (locally)bended. This means, for example,that a ray of light bends when it leaches through a space curved by a mass.

To support his theory, Einstein had used existing astronomical observations that contradicted Newton’s classical theory of gravitation, including the already back then long known anomalies in Mercury’s orbit around the sun. But these were post hoc rationalizations. What was needed to convince people that his theory was correct was a verified prediction of a previously unknown phenomenon. A bending of light through the gravitational force of the sun was such.

The solar eclipse in May 1919 offered Eddington the perfect opportunity for measuring such a phenomenon. Because during a total solar eclipse the moon disk runs right in front of the sunfading out the sun’s bright rays and allows astronomers to study the relatively weak light of the background stars. By comparing existing photographs of a particular group of stars (in this case the stars of the Hyades cluster in the constellation of Taurus) with images of them taken during a solar eclipse, it should be possible to determine whether the position of the starshas shifted due to space being bent by the sun.

An Englishman’s defense of the German Einstein’s theory was politically a very delicate matter. First, direct communication between the two men had been impossible for years, as Germany had been at war with Great Britain at the time the General Theory of Relativity was published. Eddington had received a smuggled copy and had secretly planned with his colleagues in the middle of World War I to confirm the controversial ideas of the German physicist, in a time in which the journal Nature had even gone as far asproclaiming the inferiority of all German science.

But the curvature predicted by Einstein was very difficult to measure, as Newton had also predicted a bending of light by the effect of gravity. Calculations based on Newton’s theory by the German astronomer Johann Georg von Soldner of 1801 had shown that light rays from a distant star passing by the edge of the sun should be deflected by an angle of 0.9 arc seconds (one arc second corresponds to 1/3,600 of a degree). Was the value obtained from the measurement going to be sufficiently precise to confirm the value of 1.8 arc seconds predicted by Einstein and distinguish it from Newton’s value?

Everybody was anxiously awaiting Eddington’s results on that November evening in 1919. And indeed, the value measured corresponded exactly the light deflection Einstein had calculated and thus clearly contradicted Newton’s theory. The audience was stunned. Suddenly, two hundred years of Newtonian physics had been overthrown by the collaboration of the tireless perseverance of the theorist Einstein, who was unwilling to accept some not widely perceived inconsistencies and minimal inconsistencies of the existing physical theory, and the amazing precision of the astronomer Eddington’s measurements, who spared no effort and expense to put Einstein’s theory to the test. What a triumph for the science of the early 20thcentury at the time of so much unrest!

Like the scientists, journalists from all over the world were impressed. „Revolution in science: New theory of the universe. Newtonian ideas overthrown,“ headlined the London Times the next day, while the New York Times reveled: „Lightsall askew in the heavens: Einstein’s theory triumphs.“ It was these results byEddington and bis colleagues that turned Albert Einstein into a global celebrity like no other physicist had been since Isaac Newton, whose theory he had overturned.

Even in our days, Einstein’s theory still experiences spectacular confirmations, such as the measurement of gravitational waves – deformations of space-time propagating in waves, also caused by massive bodies – in 2016 and the very recently published first photograph of a black hole. But other than at the time of Eddington scientists are no longer surprised and awed bus this today.

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