Roughly speaking, a scientific theory is a self-contained explanation of some natural facts and the simpler and more predictive it is, the more powerful. This means that, if the theory is correct, it can forecast the occurrence of some distinguishing events, whose empirical confirmation would strike a blow for the theory itself. The smoking gun of Einstein’s General Theory of Relativity has been a phenomenon observed during the solar eclipse of May 1919.
It’s a very interesting story also from a historical and sociological point of view, other than being one of the greatest turning points in modern science. Einstein finished working at his General Theory of Relativity right in the beginnings of the World War I, and presented his results at the Prussian Academy of Sciences in 1915. His theory broke away from the Newtonian concept of absolute space and time in which natural phenomena just “happen” in favour of a more comprehensive scenario in which the space and time are tied to each other and the resulting space-time is shaped by the matter (and therefore the energy) it contains.
Although the math behind the General Relativity is awesomely daunting, the underlying concept is simple and elegant: the spacetime of the universe with no matter around (as in an empty universe) is just flat, and the light rays propagate in straight lines. Instead, in presence of a massive body (for example, a star), the spacetime right around it will be distorted. In a two-dimensional analogy, the spacetime can be represented by a billiard table: in the empty universe case, a ball that was thrown will roll smoothly over it, following a straight line. In the same analogy, the massive object, the star, might be depicted as a dip in the middle of the table. The closer you get to it, the more curved the surface will be: the ball will now deviate from a straight line trajectory, and the more, the closer to the dip.
Leaving metaphors aside, if a light ray happens to pass close to a massive object such as a star, it will be forced to bend in order to follow the curved spacetime around it, as it cannot travel anywhere else: it has to comply with the warps of the spacetime and cannot just “detach” from it, as there’s nothing else, “outside” it.
The consequence is that a star, or a group of stars normally seen at a certain position when the Sun is in another part of the sky, would appear to be slightly shifted when the Sun passes in front of them.
Ironically enough, this idea of the light bending had been already formulated by Sir Isaac Newton himself in his Opticks book in 1704. He also computed the predicted value for the light rays bending of background stars when grazing the surface of the Sun: the background stars would appear to be shifted by an angle of 0.87 seconds of arc (when the Sun itself occupies an angle in the sky of about half of a degree, that is 1800 arcseconds). At that time, a measurement of such a tiny value was certainly out of the technological possibilities, but at the beginning of the 20th century, it was within reach.
The value predicted by General Relativity instead is twice as large as the Newtonian one (1.74 arcseconds) and therefore even easier to be detected, in principle.
As many German scientists during the World War I, Einstein and his theory, before the solar eclipse of 1919, were known only in the German speaking wing of the science community. As Britain and Germany were at war, there was no direct contact of any type between the two countries.
A British astronomer, Sir Arthur Stanley Eddington, holder of the most prestigious chair in Britain, and director of the Cambridge Observatory, learned about Einstein’s Theory of General Relativity through a Dutch scientist, Willem De Sitter, who was in the neutral Holland at the time.
Eddington was fascinated by Einstein’s theory and became its first promoter among the scientists of the Royal Astronomical Society, highlighting the importance of testing it against Newton’s theory using light bending measurements. These measurements could only be performed during a total solar eclipse, as otherwise the stars close to the Sun wouldn’t have been visible at all.
Right after the end of the war, two expeditions were organised to observe the eclipse of the 29 May 1919. One was directed to Sobral, north of Brasil, and a second, led by Eddington, was heading to the Principe Island, part of the present Sao Tome and Principe, off the coast of Equatorial Guinea, in West Africa.
Their mission was very well defined, in theory: taking picture of the stars around the Sun during the total eclipse, and then compare them with pictures of the same stars, in the same patch of the sky, taken when the Sun was not there. The amount of displacement of the position of the stars, if measured with enough precision, would have pointed to one of the concurrent theories of gravity, with a substantial degree of credibility.
In November 1919, the long-waited results were presented at a special joint meeting of the Royal Astronomical Society and the Royal Society of London. Both measurements in Sobral and Principe were greatly compatible with the Einstein’s value and extensively away from the Newtonian one.
Even though part of the scientific community was doubtful about the reliability of the measurements, which indeed were taken in non-optimal conditions and among several difficulties, the results triumphantly conquered the headlines of the major newspapers at the time. It was a tremendous press success and Einstein himself was deemed as an undisputed genius. The entire story was also greeted as a seed for a faster reconciliation between Britain and Germany.
So far, the Einstein’s General Relativity is the most complete description of gravitation and spacetime that we have, but it might eventually be found to break down in extreme physical cases, which we will not be able to explain in its framework, and we would need to replace it with another, more general theory of gravity. Stay tuned!
Stefania Pandolfi (@StefaniaP20) earned a PhD in Cosmology, after which she has been a postdoctoral researcher at the University of Copenhagen for three years. She is a member of the ESA’s Planck and Euclid collaborations. Notwithstanding her happy and fruitful astrophysics career, the burning flame for science communication has grown too large to be held back. Therefore, she got a Masters in Journalism and Institutional Science Communication and now she is trying to turn my innate passion into a new exciting career.