Confirmation of Einstein's general theory of relativity was provided by the first detection of the waves. According to general relativity, every wave should leave a mark on the structure of spacetime. The space should be strained permanently, even after the wave has passed.
Physicists have been trying to figure out how to measure the memory effect since that first detection.
The memory effect is a strange phenomenon, according to Paul Lasky, an astronomer. It is really deep stuff.
Their goals are more than just looking at the permanent scars left by a passing wave. Physicists hope to come to a better understanding of the black hole information paradoxes by exploring the links between matter, energy, and spacetime. The memory effect and the symmetry of spacetime are related, according to a physicist who earned a piece of the 2017 Nobel Prize in physics. It is connected to the loss of information in black holes, a very deep issue in the structure of space and time.
There is a scar in space.
Why would a wave change spacetime? It comes down to the relationship between spacetime and energy.
What happens when a wave passes by a detector? The L shape of the two arms of the Laser Interferometer Gravitational-Wave Observatory. If you imagine a circle circumscribing the arms, with the center of the circle at the arms intersection, a wave will periodically distort the circle, squeezing it vertically, then horizontally, alternating until the wave has passed. The difference in length between the two arms will change as the circle is distorted.
The memory effect says that after the wave passes, the circle should be permanently altered by a small amount. The particularities of gravity are described by general relativity.
The objects that LIGO finds are so far away that their pull is weak. The force of gravity has a shorter reach than the force of a wave. The property that is responsible for the memory effect is also responsible.
A gravitational potential is the amount of energy an object would gain if it fell. If you drop an anvil off a cliff, the speed of the anvil at the bottom can be used to reconstruct the potential energy that falls off the cliff.
In general relativity, where spacetime is stretched and compressed in different directions, a potential is more important than the energy at a location.
The change in the gravitational potential is what the memory is about. The change in potential caused by the energy of a passing wave distorts spacetime even after the wave has passed.
How will a wave affect spacetime? The possibilities are equivalent to one another, and it's puzzling. Spacetime is like a game of Boggle. The Boggle game has 16 six-sided dice arranged in a four-by-four grid with a letter on each side of each die. Each time a player shakes the grid, the dice clatter around and settle into a new arrangement of letters. All configurations are equivalent in a larger sense. The dice could possibly be in the lowest-energy state, so they are at rest. When a wave passes through, it shakes the Boggle board, changing spacetime from one configuration to another. Spacetime is in its lowest-energy state.
It's a characteristic that you can change the board, but things stay the same. Physicists have made this connection in the past decade.
The story begins in the 1960s when physicists wanted to understand general relativity. They wondered what would happen in a region infinitely far away from mass and energy in the universe, where gravity's pull can be neglected, but the radiation cannot. They looked at the symmetries this region obeyed.