It took 100years，but finally，scientists proved Albert Einstein's theory that gravitational waves（引力波）exist．
The waves were predicted as part of Einstein's general theory of relativity nearly 100years ago．It was the theory of the physics behind the workings of our world and the universe．
The idea was that the waves are like waves in space，caused by some of the violent and energetic processes in the universe．For example，two black holes crashing into each other．
Now a group of scientists，including ones from Caltech，MIT and the LIGO Scientific，Collaboration finally found the gravitational waves．
David Reitze is the executive director of the LIGO observatory at Caltech．
"We have detected gravitational waves．We did it!"
What are these gravitational waves？
Well，imagine throwing a rock into a pond．When the rock hits the flat surface of the water，it creates waves．Space-time is like the surface of the water．So that means gravitational waves are like the waves moving out from where the rock hits the water．
It might be hard to understand，but those gravitational waves expand and contract space and time as they move through space．And when they get to the Earth，the waves pass through，and they contract and expand the planet as the waves go by．
It was Einstein who said these gravitational waves should be observable．
But these are not huge waves．They are very，very small，which is why it took so long to find them．You cannot see them with your eyes．They are smaller than the size of an atom．
How did the scientists find them？
They used a giant scientific tool called LIGO，which stands for the Laser Interferometer Gravitational-Wave Observatory．The billion-dollar LIGO project is two L-shaped observatories．One is in Louisiana and the other in Washington state．
For years，scientists have been watching two black holes in another faraway galaxy．The two were spinning