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Last week, it was announced that we've detected gravitational waves on Earth. Now, Hank explains what that means for the future and why it's such a huge deal.
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Hosted by: Hank Green

NSF/LIGO press conference in Washington, DC

Last week, a teeny tiny, unimaginably small little blip made headlines everywhere.  That blip was humanity's first direct evidence of gravitational waves, and the whole world freaked out about it.  The day of the announcement, we were just as excited as everybody else, probably a little bit more excited than average, and we uploaded a video covering the basics of the discovery, but there is so much more to talk about.  Like how can we be sure that the measurements are legit?  Why exactly is this discovery so important?  And what's next?


Around 1.3 billion years ago, two black holes collided, but we just found about it this September and in a big way.  On September 14th, the whole planet Earth was stretched and squeezed by just the tiniest bit, way too small for you or me to feel, but it was enough that some of the most advanced technology in the world could measure it.  Like Caitlin said in last week's video, gravitation waves are distortions in spacetime, the combination of space and time that makes up the universe.  According to Einstein's theory of general relativity, mass curves spacetime, and when mass moves, it can compress and stretch spacetime in the form of ripples that we call gravitational waves.  Using Einstein's equations, physicists can look at certain scenarios and predict the patterns of waves that they would create, and in this case, the pattern matched what we would expect to see if two black holes collided.  That signal was really hard to find, though, because the effects of gravitational waves are super tiny.  

This whole discovery is based on some mirrors that moved by a few thousandths of the diameter of a proton.  But even though those twitches were small, two state of the art detectors were able to pick them up.  One is in Washington State, the other in Louisiana, and each of them is basically a giant L with four kilometer arms.  The idea is that if a gravitational wave comes along, it will expand space along one arm and squeeze it along the other.  Laser light travels along each arm of the detector, bouncing off mirrors at either end.  Eventually, the two beams of light recombine and the waves of both beams should be traveling together synchronously.  But if one arm's distance is suddenly longer than the other because a gravitational wave stretched it, then one beam of light will have spent more time traveling, and the waves will be out of sync.  

Using this awesome system, the detectors picked up on something that looked like a gravitational wave in the data, but that didn't necessarily mean that's what it was.  There are all kinds of other factors that can affect the data, like if a distant earthquake made the mirrors jiggle a little, plus, there are four people whose job it is to make fake signals that look like gravitational waves, just to make sure that researchers are analyzing the data properly.  

That's why a team of about a thousand scientists then spent months trying to figure out what really caused the signal on September 14th.  But they didn't find any earthquakes, there was a lightning strike in Africa, but it wasn't strong enough to affect the detectors, and nobody had stuck a fake signal into the data either.  Eventually the researchers realized this had to be the real thing, that the odds of recording this signal and having it not be a gravitational wave were less than 1 in 3.5million.  

So that's it!  And now that we know how to detect these waves, there are all sorts of new things we might discover.  Right now, we mainly observe the universe using the electromagnetic spectrum, searching for patterns in light waves that range from the lower energy radio waves to high energy X-Rays, and those observations have taught us a lot, but there are some things, like black holes, that don't produce electromagnetic radiation, but they do give off gravitational waves.  So what we need to do is measure the gravitational waves coming in, then use Einstein's equations to work backward and figure out what caused any particular pattern.  Eventually, we might be able to observe all kinds of things, like more black holes merging or neutron stars crashing into each other and other things that we definitely could not otherwise detect.  

According to the researchers in charge of the project, LIGO, the pair of detectors that made the discovery, has already detected more signals that look like they could be coming from gravitational waves, and bigger, better gravitational wave detectors are in the works.  The plan for the Einstein telescope, for example, is to have three arms, each 10km long, arranged in a triangle.  It's still in the early stages of planning, they don't know where exactly it's going to be built yet, but it could be online by the late 2020s.  

Then there's ELISA, an ESA mission planned for 2034, which would put three satellites in orbit around the Sun to look for gravitational waves in space.  Its arms would be a million kilometers long.  So while this is the first event we've observed using gravitational waves, it definitely will not be the last.  

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