 My story actually begins with a great hero of the last century, Albert Einstein. And Einstein in fact predicted he made many really outrageous predictions. And among those outrageous predictions was that gravity can travel across the universe. Ripples of space-time themselves travel across the universe and bring us information. And my story today is the quest to detect these directly. Now, you heard from Garrett that we did it, but let me just give you a little bit of a step back and tell you why we care so much. So my story begins here with this picture, and it's really the picture that we take with light. This is one of the very beautiful pictures in astrophysics. It's a supernova remnant. A supernova occurs when a star, like our own sun, runs out of nuclear fuel. It stops burning, it stops shining, and it implodes on itself. And when that process happens, these ejecta come bursting out, and in the very center of my picture, right here, you see this little blue dot. That was a new and rather exotic star being born in this process. That's called a neutron star. Now, one of the beauties of this picture is that it's in many different colors. Colors, in fact, that our eyes couldn't even see. So the yellows and greens in the picture are indeed optical wavelengths that our eyes can see. Those were taken with the Hubble Space Telescope, but then the reds are actually infrared light and the blues are X-ray light taken with the Chandra X-ray Observatory. And when you take all of those colors of light together, you construct this picture and the little blue dot, the newborn neutron star, you only see in X-rays. So you have to look with all of these different colors to see this. Now imagine for a moment that the star, the parent star that exploded was much heavier than our son, maybe 10 times heavier than our son. Then, instead of that little neutron star at the center, we would get a black hole. It would have so much gravity that it would just keep collapsing and turn itself into a black hole. How might we observe that? What are black holes? They're black. Why are they black? Because they have so much gravity that even light cannot escape. And so we must come up with some other tool for observing black holes. Why should we care about observing black holes? Because we now know that they're one of the most important building blocks of our universe. This process, this supernova process, is the process by which nature creates the heavier elements that we find here on Earth. Those elements are not formed in stars. They're formed in the explosion of stars. Understanding these processes is very important, but now we know that light cannot tell us everything about the system. And so we come to Einstein and gravitational waves. Einstein told us that gravity is not just a force like we learn in high school. Gravity is geometry. What does that mean? Einstein says if all of space-time were a trampoline, you put a heavy object like a star in the middle, the trampoline would curve. And then objects at the edges of the trampoline would fall into that curvature into our heavy star. And this was the picture. Not only does the star bend the trampoline, when the star oscillates, it causes ripples on the trampoline of space-time. So here is a star and it's oscillating up and down. And as it does this, our nice grid of space-time gets distorted by these ripples that travel outwards from this oscillating vibrating star. That was the picture that Einstein gave us. And it's been a quest of the last hundred years from the time he made this prediction for us to directly measure these ripples of space-time. That's what we've been trying to do here. Now, Einstein's theory of general relativity, which was the theory of gravity from which this was born, is actually very powerful. It not only told us that space-time should have these ripples, it actually told us what they should look like. I told you about the neutron star or black hole that's born when an ordinary star dies. Now, imagine that you had those two stars orbiting each other. Two black holes orbiting each other. That sometimes happens because they find each other and get gravitationally bound, or sometimes they're actually born as twins. So if you had that process, because the gravity around those stars is so intense, it must radiate these gravitational waves. Where does the energy come from that's carried away by the waves? It actually comes from the orbit of the star. So the stars must get closer and closer to each other. And eventually they collide. And Einstein's theory, in all its mathematical glory, taught us how that process should happen. So here is a simulation of how that works. Those two dots in the middle are two black holes. And as they're orbiting each other, all of these distortions are the space-time itself, our trampoline from nature. And what you see is right where the black holes are, space-time is very curved. We all remember these beautiful funnel diagrams for black holes. Now, as these black holes get closer to each other, they're going to collide. And at the bottom, look at the signal accumulating. As you can see, as these black holes get closer and closer to each other, general relativity tells us what the signal should look like. And eventually the black holes get very close to each other and, in fact, will collide. And the movie will actually slow down and freeze. And as that happens, notice the distortions of space-time. Space-time has become this incredibly violent, distorted system right near it. Here the black holes have just touched and the signal has accumulated to its maximum amplitude. And then, in fact, just a single new black hole forms and the radiation process turns off and these waves propagate into the rest of the universe. That's the process that Einstein told us about. I want to tell you about this signal here. So this signal here, not only did Einstein's theory tell us exactly what it should look like, this, by the way, this kind of signal is called a chirp because it gets faster in frequency and louder in amplitude. And if I were to encode this on a loudspeaker and play it back for you, it would sound like the chirp of a bird. It would sound like whoop. That very last burst at the end is when the two black holes collide. Now, one of the frightening things about Einstein's prediction was he told us how strong the signal should be. And right here, when it is at its maximum amplitude, the strongest we could possibly observe it, for black holes that are typically hundreds of millions to billions of light-years away, the signal corresponds to a quantity we call strain and that quantity is 10 to the minus 21. It's a very small number, but it's kind of meaningless. I haven't described what it is. When I describe what it is, you should think these scientists are crazy. So what does it actually correspond to? The thing that the gravitational wave does as it passes through the Earth or through any object, any region of space-time, is it causes the space-time to shrink and stretch. So now I'll put a scale on it. If the gravitational wave from these black holes were to pass through me, what they would do is they would cause me to come straight through me. I get a little bit shorter and fatter and half-cycle later, I get a little bit thinner and taller. And that's the process. This, by the way, is called the gravitational wave dance. And that's what it does. This is a very interesting attribute because this also teaches us how we might detect them. If you had a way of just measuring my dimensions, my length, say, and as the gravitational wave came by, I would get a little bit shorter and longer measurement over. I may not be a very good sensor. You might think of some other object. You just measure how its dimensions change. And that's indeed what we do. And so for that, it turns out that the changes in distance are very small. So for me, I'm an object that's about one meter big. My dimensions would change by 10 to the minus 21 meters. It's a small, small number. So that is a million times smaller than a single proton. And it's the nucleus of an atom which itself is 10,000 times smaller than a single atom. So that's a hopeless measurement, completely ridiculous. Even we are not foolish enough to try that. What we do instead is we build observatories like this. And here what we've done is we've done something that's rather dramatic instead of using a one meter scale distance like myself. We propagate laser beams for four kilometers in this L-shaped instrument. So right in that central building lives a laser. The laser light splits into two paths. One goes four kilometers in that direction and reflects off a mirror. The other beam goes down in this direction, reflects off a mirror. The light comes back together. And what we do is we measure the light travel time. If a gravitational wave comes through, that light travel time changes and we register it. Now because these detectors are kilometers long, it turns out we don't have to make this measurement at the level of 10 to the minus 21 meters, you know, a million times smaller than a proton. The kilometer scale detectors means we have to make a measurement that's a mere 10 to the minus 18 meters. That's only a thousand times smaller than a proton. And it turns out that's possible to do. For the next minute or so I'm going to walk you through how do you wrap your head around such a tiny measurement. So what are we measuring? We're measuring the distance changes between our laser and a mirror that's four kilometers away. What do we have to do to make that measurement? Two things. We have to keep the mirrors very, very still because everything on our planet wants to move them by more than that. So we have to do a lot of vibration isolation. And now once you've made such an apparatus where the mirror sits so, so still, more still than a thousandth of a proton, you still need a way of measuring that tiny motion. And that's where our laser light comes in. The laser light is our meter stick. So those are the two things you need. Very good lasers and very good still mirrors with vibration isolation. So let's just walk through that. This is the observatory. If you went into that central building there, you would see objects like these. These are vacuum chambers and they're quite big. Actually, if I were to stand beside one, the top of my head would be below this row of viewports. Why so big you say? What's in each one of these? In each one of these is a mirror. Now we're all used to seeing mirrors. They can be kind of this big in your lab. They might be a little bigger. These mirrors are quite a bit bigger. So they need big houses. And in fact, they're about this big. They're 35 centimeters in diameter. So that's this object here. And everything else you see in this structure is part of the vibration isolation. Remember, the ambient motion of these mirrors, if it just sits on the earth, is about a micron. So 10 to the minus 6, no, a few microns, 10 to the minus 6 meters. And we have to get it still to the level of 10 to the minus 18. So all these layers of vibration isolation, that would not be enough. So then we put more layers of vibration isolation. That's all inside each one of those vacuum chambers. And these are systems that are instrumented to measure the motions of these platforms and then cancel them out. Once you've done that, you need a laser that looks like this. This is actually a 200 watt laser that we propagate down the arms and probe the positions of the mirrors. And then finally, the experiment is controlled through a control room that looks like this. You can see a person sitting down here monitoring all the channels of information that we collect. Now, this was a picture of the LIGO observatory. That's the laser interferometer gravitational wave observatory in the US. It turns out that we have a worldwide network of observatories and the two in the US are called LIGO Hanford and LIGO Louisiana. Now, last year in September, these two observatories, the US observatories, were operational. There are detectors that are either under construction or planned across the planet, the next longest one. The US ones are four kilometers long. The Italian French detector, LIGO, is three kilometers long. There's a three kilometer long detector being built in Japan. But last year, these were the two that were operational. And we recorded a signal. So what did we see? Well, here are our two black holes. They collide, and they send off this wave into the universe. And now this wave propagates, and I will tell you now it propagates quite far. The black holes that we observed were 1.3 billion light years away. So that explosion happened 1.3 light years ago. Now, here comes the wave through the Earth. Now, this effect is greatly exaggerated. As you can imagine, our Earth did not wiggle like this. It wiggled in our detectors by 10 to the minus 18 meters, so an atom meter. But that was the effect, and our detectors picked this up with a process that looks like this. Here is the interferometer, the light interferes. And then as the mirrors greatly exaggerated, got longer and shorter because of the passing gravitational wave, what we observed really is the interference of light. Where the peaks of the light line up with each other or line up with troughs. And as a result, we get either more light or less light on our detector, which is right here. And that's the process by which we actually make the measurement. These mirrors that are shown here in this cartoon have a lot of vibration isolation. And then the measurement we're making is just whether we see a lot of light or a little light at the output, depending on whether the peaks and troughs line up or not. It's a classic interference measurement. And you can see how that works, and that's the process. And so here is what we saw. This was the celebrated signal. And what you see here on the horizontal axis is time. And on the vertical axis is this quantity strain, which is just the amplitude of the gravitational wave. It's the change in length per length of spacetime itself. And you see a few remarkable things. We saw the same signal in both the Livingston and Hanford interferometers, so the orange and the blue data. However, the signal came first to the Louisiana detector. Seven milliseconds later, we measured it in the Washington detector, which is exactly the light travel time between the two observatories. So the waves came through the earth. First hits the Louisiana detector. Seven milliseconds later, traveling at the speed of light, just as Einstein predicted gravitational waves to do, they come to the Washington detector. Now this signal waveform also behaves exactly the way Einstein instructed it to do. And in fact, encoded in this waveform, you see a few things. You see that this corresponds to a motion of a few times 10 to the minus 18 meters at its peak amplitude. And in fact, an enormous amount of power was radiated by this gliding black hole. And from this waveform alone, we can tell that the two black holes were very massive. They were about 30 times the mass of our sun. We can also tell how far they were. They were 1.3 billion light-shears away. So this collision happened 1.3 billion years ago. This gravitational wave traveled through the universe and registered in our detectors in September. I mean, how amazing is that that we are here in this moment in time when we got this collision? It is also because of the sheer power in this collision, it is the most powerful collision since the Big Bang. So that's the most amount of energy. For a brief moment, as these black holes collided, notice it's a brief moment. We're only measuring a few hundred milliseconds. For a brief moment, more energy was radiated than all the shining stars in the universe. Now, most importantly, the importance of this is that this is a black hole system. It gave off no light. If we had pointed out the most powerful telescopes we can imagine at this system, we would have seen nothing. And so this is a new tool, gravitational waves, a new tool for observing the universe and observing those parts of the universe that are very dark, but very warped and violent. Now, just as with light, we have a whole spectrum of gravitational waves. Like with light, it spans from the lowest frequencies of the longest periods, which is the age of the universe, down 18 to 20 orders of magnitude. And I just want to point out that this observation that we made with these kinds of interferometer are the fastest gravitational waves. They're about in the 100 hertz range. That's the frequency or the speed at which these black holes were orbiting each other. But as we open up more and more techniques for detecting gravitational waves, whether we use space detectors or other techniques like pulsar timing or cosmic microwave background, we're going to be able to construct what our universe looks like with gravity. Until now, we have used light. And finally, we now have another sense that we turned on. And I hope you'll stay tuned, because this is going to be an exciting few decades for this new technology. Thank you.