Now is the centenary of the eclipse that confirmed the theory of general relativity. "Every day, when we search the mobile phone map for a trajectory from our position, we are benefiting from Einstein's being right in making such a powerful and precise theory about what space and time are like," explains astrophysicist and disseminator Javier Armentia.
Image of the 1919 eclipse taken by Dyson, Eddington and Davidson. Wikimedia Commons
On May 29, 1919 there was a total eclipse of observable Sun in a strip of the Earth ranging from South America to Central Africa. At that time the passion for eclipse observation was, let us say, low on the list of people's concerns.
To lapse into an easy pun I can't help making, the terrifying events of the Great War had logically eclipsed almost any kind of intellectual activity. Looking at it from the perspective of these one hundred years, however, the year in which the end of the war was experienced was especially profitable for astronomy. That same year the International Astronomical Union was created and, once again, an observation of a celestial phenomenon made it possible to confirm a new physical theory.
In 1915 Albert Einstein had published a revolutionary theory that has become part of possibly the greatest intellectual adventure of the twentieth century: space and time were so by virtue of the action of gravity. Or, thinking about it in a new way, the gravity that had served to lay the foundations of modern science from Newton onwards, that universal force that shaped the cosmos, became the reason why we have a specific geometry and history.
The mathematical development of this theoretical framework became the major headache of many physicists, and it continues to be so even (or especially) for Physics degree students at any university anywhere in the world. But as early as the first years of the Einstein Era, there was already a perception of the measurable consequences of the concepts of relativity.
Thus, relativity explained that the anomalous precessional movement of Mercury was due to the dragging of space-time caused by the proximity of the Sun, about which there had previously been attempts to explain it with the presence of a non-existent unknown interior planet, Vulcan. Another of the surprising predictions of these curvatures of the texture of the universe around a mass was that it was possible to calculate how much the same light would deviate when it passed close to it.
Swarzschild even calculated that, with a sufficiently dense mass, the light could become trapped in a gravitational well. And in March 1919 the English astrophysicist Arthur Eddington proposed that this curvature of light could be observed in a solar eclipse. It was enough to obtain images of the stars near the solar disk, taking advantage of the fact that the Moon would spend a few minutes ahead hiding the light from the photosphere.
Se suele decir que Eddington fue la primera persona que entendió la relevancia de la teoría de la relatividad general. Se suele contar, además, que siendo de confesión cuáquera estaba en la disposición perfecta para poder soslayar la barrera entre naciones que había impuesto el escenario de la guerra. Dyson, el responsable del observatorio real británico, entendió que era posible montar dos expediciones a la franja de totalidad del eclipse a ambos lados del Atlántico, una en Sobral, Brasil y otra en la isla del Príncipe. Las imágenes y las mediciones de ese día de hace justo un siglo permitieron comprobar los cálculos relativistas.
It is often said that Eddington was the first person to understand the relevance of the theory of general relativity. It is often said, moreover, that, as a Quaker, he was in the right position to circumvent the barrier between nations that wartime circumstances had imposed. Dyson, the head of the British Royal Observatory, understood that it was possible to mount two expeditions to the entire strip of the eclipse on both sides of the Atlantic, one in Sobral, Brazil, and the other on Príncipe Island. The images and measurements of that day just a century ago made it possible to check the relativistic calculations.
This was something that, moreover, reached the media. Einstein was not well-known at the time, while Eddington was much more so in the United Kingdom, but the recognition of the theory provided by this astronomical observation was what allowed journalists to write, for the very first time: "Einstein was right". A headline that has been repeated throughout this century each time relativistic physics fulfils one of its predictions.
This year when, for one thing, the international team of the Event Horizon Telescope presented its image of the black hole in the nucleus of the galaxy M87. Or four years ago, when gravitational waves were detected by the international LIGO team. And even if we do not mention it, every day, when we search the mobile phone map for a trajectory from our position, we are benefiting from Einstein's being right in making such a clear theory.
It is ironic that Albert Einstein found the mental experiments and mathematical strength of theoretical physics more interesting than the uncertain and modest obsession of experimental physics to advance through real experiments. Because if his theory really gained ground, that was because of his ability to respond accurately to the challenges of a universe that, without his theory, would not have a correct explanation.
Today, a century ago, that eclipse showed that physics is one of the occupations most likely to change human history in the long term. Therefore, it is worth celebrating.
Javier Armentia Fructuoso (Vitoria-Gasteiz, 1962) is an astrophysicist and disseminator of Spanish science.
Since 1993, he has been the director of the Pamplona Planetarium.