Gravitational waves must permanently distort spacetime

first detection From gravitational waves In 2016, he provided a decisive confirmation of Einstein’s general theory of relativity. But another astonishing prediction remains uncertain: according to general relativity, each gravitational wave should leave an indelible imprint on the structure of spacetime. It must permanently strain space, clearing the gravitational wave detector mirrors even after the wave has passed.

Since that first discovery nearly six years ago, physicists have tried to figure out how to measure the so-called “memory effect.”

“The memory effect is certainly a strange and strange phenomenon,” he said. Paul Lasky, an astrophysicist at Monash University in Australia. “It’s really deep stuff.”

Their goals are broader than just glancing at the permanent spacetime scars left by a fleeting gravitational wave. By exploring the connections between matter, energy, and spacetime, physicists hope to gain a better understanding of Stephen Hawking. The black hole information paradox, which has been a major focus of theoretical research for five decades. “There is a close relationship between memory effect and space-time symmetry,” he said. Cape Thorne, a physicist at the California Institute of Technology, whose work on gravitational waves earned him part of 2017 Nobel Prize in Physics. “It’s ultimately related to the loss of information in black holes, and it’s a very deep issue in the structure of space and time.”

scar in spacetime

Why does a gravitational wave permanently change the structure of spacetime? It is about the intimate connection of general relativity between spacetime and energy.

First consider what happens when a gravitational wave passes through a gravitational wave detector. The Laser Gravitational-Wave Observatory (LIGO) has two arms positioned in an L-shape. If you imagine a circle surrounding the arms, and the center of the circle at the intersection of the arms, the gravitational wave would periodically distort the circle, compressing it vertically, then horizontally, alternately until the wave passed. The difference in length between the arms will oscillate – behavior that reveals the deformation of the circuit, and the passage of the gravitational wave.

According to the memory effect, after the wave has passed, the circuit should remain permanently distorted by a small amount. The reason for this has to do with the properties of gravity as described by general relativity.

The objects that LIGO detects are very far away, and their gravitational pull is significantly weak. But a gravitational wave has a longer range than the gravitational force. Also, the property responsible for the memory effect: the gravitational potential.

In Newton’s simple terms, gravitational potential measures how much energy an object would gain if it were to fall from a certain height. Drop an anvil from a cliff, and the velocity of the anvil at the bottom can be used to rebuild the “latent” energy that a fall from a cliff can transmit.

But in general relativity, where spacetime is stretched and compressed in different directions depending on the motions of objects, the potential dictates more than just the potential energy of the location—it dictates the shape of spacetime.

“Memory is nothing but the change in the gravitational potential, but it is the relative gravitational potential,” Thorne said. The energy of the transient gravitational wave creates a change in the gravitational potential; This change in voltage distorts spacetime, even after the wave has passed.

How, exactly, will a passing wave distort spacetime? The possibilities are literally endless, and the puzzling thing is that these possibilities are also equivalent to each other. In this way, space-time is like an endless game of perplexity. The classic Boggle game has 16 six-sided dice arranged in a 4-by-4 grid, with a letter on each side of each dice. Each time the player shakes the grid, the dice are rolled and settled into a new arrangement of letters. Most of the formations can be distinguished from each other, but they are all equivalent in a larger sense. They are all at rest in the lowest energy state a dice can be in. When a gravitational wave passes by, it shakes the plate of cosmic aberration, changing space-time from one wonky configuration to another. But spacetime remains at its lowest in terms of energy.

super symmetries

This property – that you can change the palette, but in the end things remain essentially the same – indicates that there are subtle symmetries in the structure of space-time. Over the past decade, physicists have explicitly come to this connection.

The story began in the 1960s, when four physicists wanted to better understand general relativity. They wondered what would happen in a hypothetical region quite far from mass and energy in the universe, where gravitational pull could be neglected, but gravitational radiation could not. They began by looking at the symmetries that were obeyed in this region.

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