Explainer: Just what are Einstein's gravitational waves?

WASHINGTON -- Astronomers on Thursday announced that their new billion-dollar U.S. observatory has detected a gravitational wave, a phenomenon Albert Einstein predicted a century ago in his theory of general relativity.

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Here's what that breakthrough means.

WHAT IS A GRAVITATIONAL WAVE?

Gravitational waves are extremely faint ripples in the fabric of space and time that come from some of the most violent events in the universe. In this case, it is from the merger of two black holes 1.3 billion light-years away. The way to think of this is to imagine a mesh net and visualize pulling on its ends. Those kinks are sort of like what a gravitational wave does.

WHAT IS SPACE-TIME?

Space-time is the mind-bending, four-dimensional way astronomers see the universe. It melds the one-way march of time with the more familiar three dimensions of space.

General relativity says that gravity is caused by heavy objects bending space-time. And when massive but compact objects like black holes or neutron stars collide, their immense gravity causes space-time to stretch or compress.

HOW IS THIS "HEARING" THE COSMOS?

Scientists mostly use the word "hear" when describing gravitational waves, and the data does, in fact, arrive in audio form. The researchers can don headphones and listen to the detectors' output if they want. On Thursday, to prove they found a gravitational wave, the researchers played a recording of what they called a chirp.

HOW CAN THEY BE CERTAIN THIS IS REAL?

Astronomers sat on the discovery for nearly five months, since Sept. 14, checking back and forth to make sure it was right. They considered all sorts of Earth-bound interference or noise, examined the possibilities and eventually dismissed them.

The astronomers are so cautious that they routinely have other scientists deliberately inject false data to test their abilities. In those tests, the observatory team was able to show that the injected data wasn't real. In the case of the discovery announced Thursday, they are extra certain they are not seeing injected or hacked data because the system that allows false information to be inserted was down at the time.

In addition, the team of 1,004 scientists on the project looked over the data, and the results were then peer-reviewed by even more experts and published in the journal Physical Review Letters.

WHAT'S NEXT?

Expect more waves. It could be as many as a few a month or as little as a few per year. The observatory is also being further upgraded to hear even fainter, more distant waves.



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