Before 1919, cosmology was as subjective as art history. A solar eclipse, and a patent clerk's equations, changed everything.
A century ago, on May 29, 1919, the universe was momentarily perturbed, and Albert Einstein became famous.
Einstein himself apparently had no special plans for what he knew could be a momentous day. He was home in Berlin. He wrote a letter admitting a "blunder" in an ongoing debate with Theodor Kaluza, a German mathematician with a new notion of space-time that required five dimensions.
He betrayed no jitters about the fact that, on that day in May, two scientific expeditions were finally putting his theory of general relativity to the test. In Sobral, Brazil, and on Príncipe Island, off the western coast of Africa, two teams were viewing a total solar eclipse; in measuring the deflection of starlight by the sun's gravitational field, they proved Einstein right.
Einstein first received word of their preliminary results in September, and wrote his mother with the "happy news." The confirmation was officially announced in November at the Royal Astronomical Society in London, triggering headlines about the ensuing excitement. ("Men of Science More or Less Agog Over Results of Eclipse Observations," one headline in The Times noted.) Einstein bought himself a congratulatory violin.
One hundred years later, relativists the world over are rejoicing, each in their own way.
Katharine Leney, a physicist at CERN, in Switzerland, and the purveyor of @PhysicsCakes on Twitter, created a rotating solar eclipse diorama, featuring cake pops of the sun, Earth and moon.
On Wednesday, two actors, Ben Livingstone, playing Einstein, and Colin Uttley, as Arthur Eddington, the astrophysicist who led the Príncipe expedition, gave a special performance at the Royal Astronomical Society in London.
And there are week long conferences in Sobral and Príncipe featuring lectures, seminars, parades, dancing and fine dining. (On Wednesday, the scientific gatherings will merge for a special joint session via teleconference.)
In Sobral, in a lecture Wednesday afternoon, Luis Crispino, a physicist at the Federal University of Pará, in Brazil, aims to set the record straight about the importance of the Brazilian expedition.
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The Sobral measurements commonly are assumed to have played a minimal role in verifying Einstein's theory, in part because the photographic plates from the main Sobral instrument were blurry. But in fact, the backup Sobral instrument provided the crucial images, because many of Eddington's photographs from Príncipe were obscured by clouds and revealed few stars.
The Sobral photographs allowed Frank Dyson, the astronomer and co-organiser of the expeditions, to conclude that the results favoured general relativity, said Crispino: "This is common sense to everybody in Brazil. Outside Brazil it is not."
Crispino and Daniel Kennefick, of the University of Arkansas, recently laid out this argument in a paper in Nature Physics. "The role of other astronomers involved has been, so to speak, eclipsed by Eddington's (and Einstein's) fame," they wrote.
Kennefick also is the author of a new book on the subject, "No Shadow of a Doubt," and Wednesday, at a conference in Paris, he will offer a similar retelling. Anna Curir, from the Astrophysical Laboratory of Torino, Italy, on Tuesday gave a talk titled, "Arthur Eddington and the Gestalt theory."
On Príncipe Island, about 20 relativists gathered on Bom Bom beach to discuss a century's worth of exploration about Einstein's equations of general relativity.
"His equations allowed cosmology to become a science," John Barrow, the cosmologist, wrote in an email. "Before him, cosmology was like a branch of art history. You could imagine any type, shape or form of universe you liked."
But Einstein's equations, he added, "are more sophisticated than any others in science. They describe whole universes. Every solution of Einstein's equations describes an entire possible universe that is consistent with the laws of physics."
Alessandra Buonanno, a director at the Max Planck Institute for Gravitational Physics in Potsdam, was also on the beach in Príncipe, where she discussed gravitational waves, another prediction of general relativity.
"The waves are like fingerprints of the gravitational-wave sources," she said. Buonanno's research focuses on improving the accuracy of the models of the fingerprints for upcoming observations with the Laser Interferometer Gravitational-Wave Observatory and Virgo detectors, and for future gravitational-wave detectors, such as with the Laser Interferometer Space Antenna, or LISA, which will be launched in 2034.
"LISA seems very far into the future," she said. "But theoretical models and the required hardware and software need to be developed from now."
The Príncipe proceedings opened with a talk by Clifford Will, a Canadian mathematical physicist at the University of Florida, who in 1986 published the popular book "Was Einstein Right?" His new talk was titled, "Is Einstein still right?"
Ever more so, it seems. The past few decades have seen "an amazing array of experimental tests of general relativity, all of them in agreement with the predictions," Will said. But the quest continues: "There are still things we don't fully understand. And that's probably likely always to be the case."
"The more we keep testing it, the more confidence we have in the theory," he said. "And of course on the other hand, any sort of deviations from his predictions would surely tell us that there is something new to be investigated."
On Wednesday at the Institute for Advanced Study in Princeton, where Einstein spent his twilight years, the centenary of the eclipse is being celebrated with an afternoon of talks featuring a new book by Graham Farmelo, "The Universe Speaks in Numbers."
Edward Witten, Nima Arkani-Hamed, Freeman Dyson and Karen Uhlenbeck, the recent Abel Prize winner in mathematics, among others, will address duelling methods — experimental versus mathematical — of investigating our own cosmic diorama.
Those in favor of experiments have been known to view the mathematically inclined "as self-indulgent, without an obvious payoff for understanding the real world," Farmelo said. And the mathematical cosmologists traditionally have viewed their opposites as "ambulance chasers," rabidly pursuing every new experimental clue at the expense of overarching physical ideas.
"Einstein was convinced that the royal road to the laws of physics was not through looking at experimental data, but by developing the mathematical content of well-established theories," said Farmelo.
Written by: Siobhan Roberts
© 2019 THE NEW YORK TIMES