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Black holes are some of the most extreme mysterious objects in the universe. And when two black holes collide, shockwaves get sent out through the very fabric of space. This weekend, scientists are celebrating the 10th anniversary of the first time they ever detected these waves. And as NPR's Nell Greenfieldboyce reports, they've gotten so good at measuring them, they've just been able to test a key idea about black holes first proposed by physicist Stephen Hawking.
NELL GREENFIELDBOYCE, BYLINE: These waves are called gravitational waves. They're like the ripples in a pond when you throw in a pebble. Max Isi is an astrophysicist at Columbia University. He says the waves move through everything - the Earth, even our own bodies - stretching and squeezing distances.
MAX ISI: At one moment it makes me taller and thinner. The next moment it makes me shorter and fatter.
GREENFIELDBOYCE: This is imperceptible to us, of course. Isi says that Albert Einstein, who proposed the existence of these waves, thought they'd never be detected.
ISI: Just because it sounded ludicrous. So I'm sure that if we told him that we are detecting gravitational waves from colliding black holes every two or three days or so, it would have been mind-blowing to him.
GREENFIELDBOYCE: Yet that is just what researchers have been doing in recent years, with two enormous detectors known as LIGO. One is in Washington state. The other is in Louisiana. These facilities send lasers down 2 1/2-mile tubes to detect the tiny squeeze and stretch that occurs when a gravitational wave rolls through. Their first detection was back on September 14, 2015. The waves came from two black holes that circled each other and then merged.
Gabriela Gonzalez is a gravitational wave researcher with Louisiana State University. She says they initially expected LIGO would sense a bunch of extreme cosmic events other than black hole collisions.
GABRIELA GONZALEZ: But since then, it's almost the only thing we have seen.
GREENFIELDBOYCE: Hundreds of pairs of colliding black holes have been registered by the LIGO detectors. Katerina Chatziioannou is a physicist at Caltech. She says earlier this year, they logged the strongest signal to date - two black holes, each about 30 times the mass of the sun, merging together about 1.3 billion light-years from Earth.
KATERINA CHATZIIOANNOU: It looks very similar to the black holes that created the first signal 10 years ago.
GREENFIELDBOYCE: What's different now is that over the years, LIGO's equipment has been upgraded and improved.
CHATZIIOANNOU: Because the detectors are so much better today, we can record the signal so much more clearly.
GREENFIELDBOYCE: That let them test some major theories about black holes, like a famous prediction that Stephen Hawking made in 1971 about the area of a black hole.
CHATZIIOANNOU: Which says that the event horizon of a black hole - the region beyond which nothing can escape from the black hole - only grows with time.
GREENFIELDBOYCE: This is exactly what they saw when they analyzed this particular burst of gravitational waves. Max Isi says, to him, it's really striking.
ISI: All of these ideas that people had thought up in the '70s, thinking it was just idle speculation, now they are manifested in actual way that we see these things happening.
GREENFIELDBOYCE: Gravitational wave researchers have plans for the next 10 years. They want bigger and more powerful detectors, assuming they can get the money to build them. Funding for the existing detectors is currently under threat, with the Trump administration proposing steep cuts for 2026.
Nell Greenfieldboyce, NPR News.
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