NASA astronomer Michelle Thaller talks about a heavy subject matter: neutron stars. These dead stars are so dense that just one teaspoonful of neutron star matter would equal the mass of Mount Everest.

Two neutron stars in orbit around each other will eventually collide, and when they do, they create ripples in the fabric of spacetime. Thanks to LIGO, the Laser Interferometer Gravitational-Wave Observatory, scientists can detect these gravitational ripples by detecting disturbances in laser light.

Albert Einstein correctly predicted the existence of gravitational waves in his theory of general relativity, 100 years before astrophysicists first detected them.

Michelle Thaller: A few decades ago, we actually saw explosions in the sky, somewhere out in space, that we really didn't understand at all. They gave intense bursts to something called 'gamma rays,' and gamma rays are the highest energy kind of light that is possible. 

You now, you've probably heard of, you know, ultraviolet rays from the sun - they give you sunburn. And then there are things like x-rays. Gamma rays are even more energetic and more dangerous to us than that. But gamma rays are only created in the Universe by things that are naturally in the billions of degrees. And we saw these little gamma ray pops going off in space. And at first we wondered, "Well, are they nearby? You know, could they be in our own galaxy, or are they very far away?" We really didn't know. And a few decades ago, we actually realized that these gamma ray bursts were coming from very, very distant galaxies, galaxies that in most cases were billions of light years away, and a light year is about six trillion miles, the distance that light travels in one year, so billions of light years away. 

So something was creating a lot of gamma rays, because they were bright enough to measure from that distance. And incredibly, some of these explosions were so intense - that there was one, I believe it was in 2007, that NASA observed - there was a little flash of visible light that came with the gamma rays, and it was actually visible with the naked eye for a couple of minutes. If you were actually in the Southern Hemisphere on that night, you would've seen a little star turn on and off for a couple of minutes, and then it would be gone. And that explosion happened about seven billion light years away. 

Something blew up seven billion years ago on almost the other side of the observable Universe, and it was bright enough to see with the unaided eye. We had discovered something unbelievable. What could possibly be that bright? What could possibly be that violent? That little explosion for a few minutes outshone the rest of the observable Universe - just one thing. 

So we really didn't know what could possibly create that much energy. And the theoretical physicists got to work, and they started just kind of guessing. I mean, what could explode that could make that much energy? And it turns out that if you have these things called 'neutron stars' - neutron stars are the leftover compressed cores of dead stars. They are amazing monsters. They're about 10 miles across, and they have a density that if you had about a teaspoon full of the material, that that would be about as much as the mass as Mount Everest crushed into a teaspoonful. They're amazing things. And we observe hundreds and thousands of these things in space. And so people sort of theorized that if two of these things spiraled together and collided, you would actually be able to get that much energy out. It seemed unlikely, but, you know, maybe that does happen sometime in the Universe - the two of these things collide. 

Now, Einstein came up with this wonderful idea that space and time is almost kind of like a fabric that connects everything in the Universe. And what gravity is, is gravity is kind of a pulling and a stretching on that fabric. And if you have two really massive things moving around each other very fast before they collide, say, two neutron stars, spiraling, spiraling in, they should actually make ripples in this fabric. So as they spiral closer and closer together, they actually make ripples that actually go out through space at the speed of light, and these are called gravitational waves. And they are very, very hard to find. And lucky for us, masses moving around only create tiny little distortions in space and time, the fabric of space and time itself. So what happened is we actually started building instruments that were sensitive enough, sensitive enough to detect this tiny little wobble in space and time itself. 

And to give you an idea about how hard this is to detect, we used an instrument called 'LIGO,' the Laser Interferometric Gravitational-Wave Observatory. And LIGO has two lasers, and the lasers are about two miles long, and they're actually at a right angle. So two mile-long lasers at a sort of a corner shape. And the idea was that if one of these ripples in space and time comes through, one of the sides of the laser in its corner construction would actually be warped a little more than the other. And you'd actually see that space and time itself were changing a little more in one direction as this ripple came through. The ripple is so small that over a two-mile laser, the distance space and time changes is by about a thousandth of the diameter of a proton. We have an instrument that can measure that. 

And amazingly, we started seeing these ripples coming from many different places in the sky as these neutron stars collided and spiraled together. And the thing that was so wonderful, is that one of these gamma ray bursts, one of these ultra-violent explosions that we had no idea really what they could be, went off. And at the same time, at the speed of light, with those gamma rays, came that ripple, that signal, that exactly matched two neutron stars spiraling together. 

We had guessed that the only thing that could actually make that much energy were these two dead stars colliding, and now we had evidence. And the evidence was a ripple in space and time a thousand times smaller than a proton.