It was gravity, but not as we know it. 100 years ago this December, physicist Albert Einstein published his theory of general relativity, which redefined what gravity is, forcing the world to abandon the long-standing concept developed by Isaac Newton.
Newton thought gravity was simply a force that exerts a pull uniformly in all directions. General relativity turns this idea on its head, interweaving gravity with space and time. Einstein’s equations led scores of theoretical physicists to completely change their ideas on the inner workings of the universe, and shed new light on our understanding of planetary motion and even such enigmatic substances as dark matter.
Einstein’s theory has practical applications, too – for example in making time corrections for signals from the Global Positioning System (GPS) of satellites, which enable accurate navigation for military, civil and commercial users around the world.
“If this system had been in place in 1915, Albert Einstein would have won the Nobel Prize for his theory of gravity instantly, because it would have explained many timing discrepancies that are otherwise unaccounted for by Newtonian gravity,” says Avi Loeb, theoretical physicist and chair of the astronomy department at Harvard University. “Instead, the explanation of the precession of the perihelion of Mercury [an observation that conflicted withNewton’s equations] was not powerful enough to convince the physics community about the validity of the theory and its far-reaching implications.”
As timing precision improves, satellites could be used in the future to detect so-called gravitational waves, ripples in spacetime fabric thought to be produced by the merger of pairs of supermassive black holes, formed when galaxies collide.
Much as our theory of electromagnetism predicts that electromagnetic waves – or light – should exist, Einstein’s theory of general relativity predicts that gravitational waves should exist too. “The difference, of course, is that whereas light is very easy to detect, we still haven’t managed to directly observe gravitational waves,” says Dan Hooper, a physicist at the USA’s Fermilab particle physics laboratory.
To carry this analogy further, an electromagnetic wave consists of electric and magnetic fields that oscillate as it moves through space. In contrast, a gravitational wave warps spacetime as it travels, compressing and stretching the distances between points.
“Pulsars are already being used as clocks in detecting subtle motion of the Earth as it is shaken slightly by passing gravitational waves,” says Loeb. “The existence of these ripples in spacetime as well as the existence of black holes are two other remarkable consequences of general relativity.”
For cosmologists, the most significant outcome of Einstein’s theory of gravity is that it provides a framework for our understanding of the evolution of the universe.
As for potential applications of gravitational waves, if they are discovered, Hooper says that at the moment it’s hard to think of any. “That being said, when the electron was discovered in 1897, no one could have imagined that it would lead to transistors and the birth of the information age,” he adds.
Meanwhile, the hunt to detect gravitational waves is heating up. In northern Italy, a huge experiment called Advanced Virgo will be using a 3km laser-powered interferometer to directly detect the miniscule ripples in spacetime that, according to the theory, gravitational waves would cause as they pass through the Earth.
A parallel experiment in the United States called Advanced Ligo, the upgraded Laser Interferometer Gravitational-Wave Observatory, is already up and running. Scientists there predict that it will take them about two years to prove the final piece in Einstein’s puzzle explaining gravitation.