Albert Einstein is often held up as the epitome of the scientist. He’s the poster child for genius. Yet he was not perfect. He was human and subject to many of the same foibles as the rest of us. His personal life was complicated, featuring divorce and extramarital affairs.
Though most of us would sell our in-laws to achieve a tenth of what he did, his science wasn’t perfect either: while he was a founder of what came to be called Quantum Mechanics, he disagreed with other scientists about what it all meant, and he once thought he had proved that gravitational waves could not exist (an anonymous reviewer of his paper found his mistake and set him straight). Yet Einstein did create one thing that, as far as we can tell, is as correct as anything can be in science. That is his theory of gravity, called General Relativity.
He presented the theory to the world over four consecutive Wednesdays in November 1915 in lectures at the Prussian Academy of Sciences. Einstein was by then well respected in European physics circles, and one can imagine more than one person in the audience that November thinking that he’d gone bonkers. Einstein’s theory purported to replace the hugely successful 1687 gravity theory of Isaac Newton, which posited gravity as an attractive force between masses, with one where gravity was a result of the curving and warping of space and time by massive objects. And the evidence for this new theory? It managed to account for a tiny discrepancy of 120 kilometers per year in the spot where Mercury makes its closest approach to the Sun. The concepts behind this new theory were so radical and unfamiliar that it was said that only three people in the world understood it.
Yet a few people, like David Hilbert in Germany, Willem de Sitter in the Netherlands, and Arthur Eddington in England grasped the startling implications of this theory. Within four years, Eddington would propel Einstein to science superstardom with the announcement that his team of astronomers had detected the bending of starlight by the Sun’s gravity and had found that it agreed with Einstein’s prediction, not Newton’s. Newspapers around the world proclaimed, “Einstein theory triumphs.”
And that was pretty much it for General Relativity for the next 40 years. Because it was perceived as predicting only tiny corrections to Newtonian gravity, and as being virtually incomprehensible, the subject receded into the background of physics and astronomy. Einstein’s theory was quickly superseded by other areas, such as nuclear, atomic and solid-state physics, which were viewed as of both fundamental and practical importance.
Yet in the 1960s, a remarkable renaissance began for Einstein’s theory, fueled by discoveries such as quasars, spinning neutron stars (pulsars), the background radiation left over from the big bang, and the first black holes. Precise new techniques, exploiting lasers, atomic clocks, ultralow temperatures, and spacecraft, made it possible to put General Relativity to the test of experiment as never before. During the subsequent decades, researchers performed literally hundreds of new experiments and observations to check Einstein’s theory. Some of these were improved versions of Einstein’s original tests involving Mercury and the motion of light. Others were entirely new tests, probing aspects of gravity that Einstein himself had never conceived of. Many were centered in the solar system using planets and spacecraft, or in sophisticated laboratories on Earth. Others exploited systems called “binary pulsars,” consisting of two neutron stars revolving around each other. More recently we have witnessed numerous gravitational wave observations by the LIGO-Virgo instruments, the study of stars orbiting the supermassive black hole at the center of the Milky Way, and the stunning image of the black hole “shadow” in the galaxy M87.
In this vast and diverse array of measurements, scientists have not found a single deviation from the predictions of general relativity. When you consider that the theory we are using today is the same as the one revealed in November 1915, this string of successes is rather astounding. After more than 100 years, it seems Einstein is still right.
Will this perfect record hold up? We do know, for example, that the expansion of the universe is speeding up, not slowing down, as standard general relativity predicts. Will this require a radical new theory of gravity, or can we make do with a minimal tweak of general relativity? As we make better observations of black holes, neutron stars and gravitational waves, will the theory still pass the test? Time will tell.
Featured Image Credit: by Roman Mager via Unsplash