First discovery belong to gravitational waves in 2016 provided decisive confirmation of Einstein’s theory of general relativity. But another amazing prediction remains unconfirmed: According to general relativity, every gravitational wave should leave an indelible mark on the fabric of spacetime. It would permanently stretch space, altering the gravitational wave detector’s mirrors even after the wave had passed.
Since its first discovery almost six years ago, physicists have been trying to figure out how to measure this so-called “memory effect”.
“The memory effect is absolutely a strange, strange phenomenon,” says Paul Lasky, an astrophysicist at Monash University in Australia. “It’s really profound stuff.”
Their goal is broader than just glimpses of the permanent scars of spacetime left by a passing gravitational wave. By discovering the connections between matter, energy and spacetime, physicists hope to better understand Stephen Hawking’s black hole information paradox, has been the main focus of theoretical research for five decades. “There is a close connection between the memory effect and the symmetry of spacetime,” says Kip Thorne, a physicist at the California Institute of Technology, whose work on gravitational waves made him part of the 2017 Nobel Prize in Physics. “Ultimately, it has to do with information loss in black holes, a very profound problem in the structure of space and time.”
Scar in spacetime
Why would gravitational waves permanently change the structure of spacetime? It depends on the intimate relationship of general relativity between spacetime and energy.
First, consider what happens when a gravitational wave passes through a gravitational wave detector. The Laser Interferometer Gravitational-Wave Observatory (LIGO) has two arms positioned in an L shape. If you imagine a circle encircling the arms, the center of the circle is the intersection of the arms. , a gravitational wave would distort the circle periodically, squeezing it vertically, then horizontally, alternating until the wave passed. The difference in length between the arms will oscillate — behavior that indicates the distortion of the circle and the propagation of gravitational waves.
According to the memory effect, after the wave passes, the circle will be permanently deformed by a very small amount. The reason why has to do with the characteristics of gravity as described by general relativity.
The objects that LIGO detects are very far away, their gravity is very weak. But gravitational waves have a longer reach than gravity waves. So the property responsible for the memory effect: gravitational potential.
In Newton’s simple terms, gravitational potential energy measures how much energy an object would gain if it fell from a certain height. Drop an anvil off a cliff, and the speed of the anvil at the bottom can be used to regenerate the “potential” energy that falling off a cliff can impart.
But in general relativity, where spacetime is stretched and distorted in different directions depending on the motions of the bodies, a potential energy determines more than the potential energy at a location – it determines shape of spacetime.
“Memory is nothing more than a change in gravitational potential, but it is a relativistic gravitational potential,” says Thorne. The energy of the passing gravitational wave produces a change in the gravitational potential energy; change in the potential to distort spacetime, even after the wave has passed.
How, exactly, would a passing wave distort spacetime? The possibilities are literally limitless, and confusingly, the possibilities are also equivalent. In this way, spacetime is like an infinite Boggle game. The classic Boggle game features 16 six-sided dice arranged in a four-by-four grid, with a letter on each side of each dice. Each time the player shakes the grid, the dice will ring around and arrange into a new arrangement of letters. Most of the configurations are distinguishable from one another, but all are equivalent in a larger sense. They are all in the lowest energy states the dice can have. When a gravitational wave passes, it shakes the cosmic Boggle board, changing spacetime from one difficult configuration to another. But spacetime is still in its lowest energy state.
Super symmetry
That feature – that you can change the table, but in the end everything stays essentially the same – suggests the presence of hidden symmetries in the fabric of spacetime. Within the past decade, physicists have clearly identified this connection.
The story begins in the 1960s, when four physicists wanted to better understand general relativity. They wondered what would happen in a hypothetical region far from all mass and energy in the universe, where the pull of gravity could be ignored, but gravitational radiation could not. They start by looking at the symmetry this region follows.
