Gravitational waves should permanently distort time and space

First detection of Gravitational waves 2016 provided decisive confirmation for Einstein’s general theory of relativity. But another surprising prediction has not yet been confirmed: According to general relativity, every gravitational wave should leave an indelible mark on the structure of space-time. It should permanently tighten the space and move the mirror of the gravitational wave detector even after the gravitational wave passes.

Since the first discovery about six years ago, physicists have been trying to figure out how to measure this so-called “memory effect.”

“The memory effect is definitely a strange phenomenon,” said Paul Laski, An astrophysicist at Monash University, Australia. “This is really deep stuff.”

Their goal is not just to glance at the permanent space-time scars left by passing gravitational waves.By exploring the connections between matter, energy, and space-time, physicists hope to better understand Stephen Hawking’s Black hole information paradoxThis has been the main focus of theoretical research for fifty years. “There is a close connection between the memory effect and the symmetry of time and space,” said Kipthorn, A physicist at the California Institute of Technology, his work on gravitational waves made him 2017 Nobel Prize in Physics“It is ultimately related to the loss of information in the black hole, which is a very profound problem in the structure of space and time.”

Scars of time and space

Why do gravitational waves permanently change the structure of space-time? This boils down to the fact that general relativity closely connects space-time and energy.

First consider what happens when a gravitational wave passes through a gravitational wave detector. The Laser Interference Gravitational Wave Observatory (LIGO) has two L-shaped arms. If you imagine a circle surrounding the arm with the center of the circle at the intersection of the arms, the gravitational wave will periodically twist the circle, squeeze it vertically, and then squeeze it horizontally, alternating until the gravitational wave passes. The difference in length between the two arms oscillates-this behavior reveals the distortion of the circle and the passage of gravitational waves.

According to the memory effect, after the wave passes, the circle should remain slightly deformed forever. The reason is related to the particularity of gravity described by general relativity.

The objects detected by LIGO are so far away that their gravitational pull is so weak that it can be ignored. But the range of gravitational waves is longer than that of gravity. Similarly, the attribute responsible for the memory effect: gravitational potential.

In simple Newtonian terms, gravitational potential can measure how much energy an object gets when it falls from a certain height. Drop the anvil off the cliff, and the speed of the bottom anvil can be used to reconstruct the “potential” energy that can be transmitted by falling off the cliff.

But in general relativity, time and space are stretched and squeezed in different directions according to the movement of objects. Potential not only determines the potential energy of a certain position, but also determines the shape of time and space.

“Memory is just a change in gravitational potential,” Thorne said, “but it is a relativistic gravitational potential.” The energy of passing gravitational waves will cause changes in gravitational potential; even after the wave has passed, the potential will change. Distorting time and space.

How will the passing waves distort time and space? The possibilities are actually unlimited, and it is puzzling that these possibilities are also equivalent to each other. In this way, time and space are like an infinite Bogle game. The classic Boggle game has 16 six-sided dice arranged in a four-by-four grid, with a letter on each side of each dice. Every time the player shakes the grid, the dice will click and arrange in a new letter arrangement. Most configurations can be distinguished from each other, but are equivalent in a larger sense. They are all at rest in the lowest energy state that the dice can be in. When a gravitational wave passes through, it will shake the cosmic Boggle board, changing time and space from one unstable configuration to another. But time and space are still at the lowest energy state.

Super symmetric

This characteristic—you can change the chessboard, but in the end things remain basically the same—shows that there is a hidden symmetry in the structure of time and space. In the past ten years, physicists have clearly established this connection.

The story goes back to the 1960s, when four physicists wanted to better understand general relativity. They want to know what happens in a hypothetical region far from all the mass and energy in the universe, where gravity is negligible, but gravitational radiation cannot. They first look at the symmetry that the area follows.