The theory of General Relativity , by the German physicist Albert Einstein , is today one of the most precise and beautiful theories known . We know that it is not perfect, that it is not complete, but it has still turned out to be one of the most successful physical theories of the 20th century. But we don’t always see it that way. If it has become so valued it is precisely because it has survived all the harsh tests that we have put before it . Today we are talking about one of these tests, one of the earliest, and how it is a sign that the paths of science are, like others, inscrutable.
On May 29, 1919 , 103 years ago, a total solar eclipse would take place. This event would be the ideal occasion to verify one of the (many) predictions of General Relativity. This theory says, at the most basic level, that it is the content of the universe that tells spacetime how it should curve and that it is the curvature of spacetime that tells this content how it should move . This theory replaced the Law of Universal Gravitation, by the Englishman Isaac Newton, a theory that also told us about how some bodies influence the movement of others, through gravity.
However, for Newton the only quantity capable of generating and experiencing gravity was mass . It was the especially massive objects, like planets or stars, that had enough gravity to dictate the dynamics of other nearby masses. However, Einstein goes one step further . Already a decade before he proposed his general theory of relativity he had formulated Special Relativity . This serves as a more general version limit. It serves as the limit in which the gravitational field is negligible (or said in more appropriate terms within these theories, it applies when the curvature of spacetime is zero).
This means that all the premises of Special Relativity can be transferred to General Relativity. One of these premises, one of the best known specifically, is that mass and energy are related magnitudes . This is made explicit in the famous formula E=mc 2 . That is, General Relativity will tell us that not only mass is affected by the curvature of spacetime (by gravity), but also energy . Therefore, two objects with the same mass, but one of them having more internal energy (because it is at a higher temperature, because of its chemical composition, or for whatever reason), will not be influenced in the same way. More importantly, it will mean that objects that have no mass , such as photons or other fundamental particles, will also be affected by gravity .
Well, that is what they set out to discover on May 29, more than a century ago, taking advantage of that solar eclipse. The experiment organized by the British Eddington and Dyson consisted of taking advantage of the totality phase of the eclipse , in which the Moon completely hides the Sun behind it, to observe distant stars that would appear close to the Sun at that time . The idea was that if gravity was capable of bending light and changing its path, the light from these stars would be bent by passing close to the Sun on its way to our telescopes. This would result in a translation of these stars, which would appear in the sky slightly displaced from their expected position.
To do this, two expeditions of scientists were established to take measurements from different points along the route of the totality of the eclipse. One of these teams was located in the Brazilian town of Sobral , located in the northeast of the country, near the coast. The other team settled on Principe Island, the smaller island of São Tomé and Príncipe , west of the Central African coast.
After months of planning, weeks of preparation and a few frantic minutes of measurement, several photographic plates were obtained, showing the solar eclipse and some nearby stars. The results were immediate and showed that the light coming from the observed stars deviated exactly what Einstein had predicted. This result was so spectacular that the most important newspapers around the world echoed it, making Albert Einstein and his theory of General Relativity world famous .
This experiment and the prediction that motivated it are one of many proofs that scientific theories do not arise solely as an explanation for new phenomena without previous explanation . Sometimes it happens, as on this and other occasions, that a new theory, developed to explain certain observations or certain phenomena, is capable of predicting a whole series of new behaviors. It happened on this occasion, in which General Relativity predicted that light should be affected by the gravity of the Sun, it also happened with the prediction of this same theory of the existence of gravitational waves and that they could be measured one day . Also the Higgs boson was predicted in the 1970s but was not observed until forty years later, in 2012.
References:
Earman, C. Glymour, 1980, Relativity and eclipses: the British eclipse expeditions of 1919 and their predecessors, Historical Studies in the Physical Sciences, 11 (1): 49–85. doi:10.2307/27757471
