What is gravity? Most people probably think of it as the thing Newton discovered which makes things fall; while an accurate picture for our everyday life, this is not the full story. Gravity is actually a distortion of spacetime (the fabric of the universe) by a heavy mass like a black hole or star, which causes matter to adhere to certain trajectories, like the apple falling on Newton’s head! This theory, developed by Einstein is known as general relativity.

When massive objects like stars or black holes move, they create ripples in space-time, which are called gravitational waves. These waves are like ripples in a pond when a stone is thrown into it. Gravitational waves carry energy and information about the movement of massive objects across the universe. The first direct observation of gravitational waves was made in 2015 by LIGO. It consists of two massive detectors located in the United States that use laser beams to detect the tiny vibrations caused by passing gravitational waves. This observation provided decisive confirmation of Einstein’s theory of relativity; however, according to this theory the gravitational waves should permanently distort spacetime, leaving an observable after effect. This effect is known as the ‘memory effect’.

In theory, physicists should be able to measure this after effect and obtain information about the waves that caused them. Nobel prize winning physicist Kip Thorne states that the memory effect “is connected ultimately to the loss of information in black holes, a very deep issue in the structure of space and time”. As of yet, LIGO has been unable to detect this memory effect, which is predicted to leave an imprint on spacetime twenty thousand times smaller than the width of a proton. For reference, if a proton was the size of the Earth, then this distance is on the order of hundreds of meters!

Understanding this phenomenon could lead to exciting developments in the theory of relativity, and fix a great problem with the loss of information in black holes. In summary, gravitational waves are ripples in space-time caused by moving massive objects like black holes and neutron stars, they distort spacetime and move particles with their passing.



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