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  • [MUSIC PLAYING]

  • And all of you here are in for a treat today.

  • Sam Gralla, who I'm very impressed with,

  • is going to tell us about one of the most amazing things

  • that science has done in a long time.

  • And that's the measurement of gravity waves.

  • And he will describe it to us, and at the end of the evening

  • we'll have a quiz.

  • [LAUGHTER]

  • But Sam is a theorist working on gravitational physics

  • and relativistic astrophysics.

  • He's interested in the strongest gravitational

  • and electromagnetic fields in the universe,

  • which occur near black holes and neutron stars.

  • And you heard a little bit about this last week.

  • Sam applies techniques from diverse communities,

  • relativity, astrophysics, particle physics

  • to study the physical processes occurring

  • in these extreme environments.

  • His PhD is from Chicago, and his postdoctoral work

  • is from Maryland and Harvard.

  • And we're really lucky to have Sam as one of our faculty

  • members.

  • Sam, where are you?

  • [APPLAUSE]

  • Well, what a pleasure to be here in this beautiful hall,

  • in this great university in this wonderful city.

  • I'm a newcomer to Tucson.

  • I've been here less than two years.

  • So I thought I'd start by telling you

  • the story of how I came to the University of Arizona.

  • When you're a young theoretical physicist, you apply widely

  • and you hope you get some interviews--

  • which, fortunately, I did.

  • Now, a job interview for a professor job

  • is a little different from a normal job interview.

  • It's really more like 20 or 30 interviews right

  • after each other.

  • You meet with everybody-- professor

  • after professor, half an hour meetings.

  • You barely have time to pee.

  • If you're lucky, in the midst of all this

  • you get to meet with the dean.

  • Now, I had a thing I did with deans on job interviews.

  • The first thing I would do when I got in the dean's office

  • is I'd take out a book and I'd start reading it.

  • That would get the conversation started, usually

  • with something like, what are you doing?

  • And then I'd say, you tell me one thing--

  • if you're in a vehicle moving at the speed of light

  • and you turn on your headlights, do they work?

  • [LAUGHTER]

  • And Dean [INAUDIBLE] said the U of A

  • was the only one who did not immediately

  • throw me out of the room.

  • OK, I made that one up.

  • That's a recycled Steven Wright joke.

  • But the comedian Steven Wright has his interviewee in his joke

  • ask a really deep question.

  • You go at the speed of light, you

  • turn on your headlights, what happens?

  • This is a good question for two reasons.

  • First, it pushes us outside of our comfort zone, right?

  • You think you kind of know how light

  • works from living with light all around you,

  • but as soon as you ask a question involving where

  • you're moving like the speed of light,

  • you get a little confused.

  • Maybe you don't understand light so well,

  • because you're used to not moving that fast.

  • The second reason Steven Wright's question is a good one

  • is because it's reasonably precise.

  • It's not some vague, can you make light stop?

  • It's some pretty precise sequence of actions.

  • You get in your car, you get up to light speed,

  • and you pull the lever that would normally

  • turn the lights on.

  • What happens?

  • What happens?

  • These kinds of questions that are reasonably precise

  • yet push us outside our comfort zone

  • are called thought experiments.

  • And Einstein really was the pioneer

  • of the thought experiment.

  • He called them gedanken experiments in German.

  • This is not exactly the kind of thought experiment

  • Einstein did, but it's close.

  • And the answer to this particular conundrum

  • was given to us by Einstein.

  • The answer is you can't go that fast.

  • [LAUGHTER]

  • Seems a little unsatisfying, but wait.

  • Einstein didn't mean your car won't go that fast.

  • That's not what he meant.

  • He meant you just can't go that fast,

  • nothing can go that fast-- not your car, not a truck,

  • not a motorcycle, not a plane, not a spaceship,

  • not an ambulance, not a backhoe--

  • we have lots of those in my neighborhood right now--

  • not a garbage truck, not a fire truck--

  • I have a two-year-old, he really likes trucks.

  • He knows about suction excavators and so forth.

  • None of your favorite desert animals

  • can go that fast-- coyotes, road runners.

  • You know those wolf spiders that seem like they're

  • going the speed of light?

  • Those ones can't go that fast.

  • You can't hit a baseball that fast, a hockey puck.

  • No elementary particle can be accelerated that fast--

  • absolutely nothing can go the speed of light.

  • That's what Einstein taught us.

  • There's a universal speed limit.

  • [LAUGHTER]

  • Now, this speed limit is not Einstein's limit

  • where he'll come give you a ticket if you violate it.

  • This is a law of nature.

  • You simply cannot go this fast, no matter how hard you try.

  • That seems well and good.

  • We have the authority of Einstein to back it up.

  • But if you were a physicist around Einstein's time,

  • you might say, hey, wait a minute, mister.

  • That doesn't make any sense, because of the following.

  • I don't know about you guys, but I

  • have memories from when I was a kid riding

  • in the backseat of my car, parents

  • driving me on the freeway.

  • You're watching the scenery go by, and then

  • all of a sudden, whoosh, a car whizzes by you.

  • Of course, they're just driving 60 miles an hour or so too,

  • but they're going in the opposite direction.

  • So to you, they're going 120 miles per hour.

  • Then there's a formula, v equals v1 plus v2,

  • that if you wanted to, will tell you how to compute this.

  • And it's really kind of indisputable, this formula.

  • What is speed?

  • It's how far you go in how much time.

  • So if the car up here on the left is going 60 miles an hour,

  • and in an hour then he'll be 60 miles to the right,

  • and the other car will then in that same hour

  • be 60 miles to the left, so in one hour

  • the relative distance is 120 miles.

  • So they're going 120 miles an hour--

  • seems obvious.

  • The person who is skeptical of Einstein

  • now says, well, what if I'm going

  • at 75% the speed of light?

  • Now this obvious formula tells me

  • that I measured the other car, [WHOOSH] the one going

  • by me, moving at 150% the speed of light,

  • which is supposed to be impossible.

  • So here's another thought experiment

  • that seems to reveal something self-contradictory

  • about this universal speed limit.

  • We now have a situation that theoretical physics

  • calls a paradox.

  • We have two laws that we really believe,

  • but they're incompatible.

  • You can't have the universal speed limit

  • and the formula v equals v1 plus v2.

  • If you were a lesser physicist you might think,

  • OK, well we know v is v1 plus v2 That's

  • utterly trivial and obvious, so Einstein must be wrong.

  • But if you're Einstein, you think,

  • we don't really have any good experimental evidence

  • that v equals v1 plus v2 when you're

  • moving near the speed of light.

  • We've only done those experiments

  • at 60 miles an hour, which might as well be standing still

  • compared to light.

  • So which of these do we keep and which do we discard?

  • We keep the speed limit.

  • It turns out, that formula is wrong.

  • Now, as Professor Dienes emphasized in the first lecture

  • in this series, if you were here,

  • physics doesn't discard laws .

  • We don't establish laws and then find out they're false.

  • Instead, we realize that certain laws may not

  • apply as widely as we thought they did.

  • In this case, v equals v1 plus v2 is perfectly

  • true at 60 miles an hour.

  • But it is not true near the speed of light.

  • And Einstein derived the correct law.

  • It's now up there on your right.

  • You don't have to worry about the details in the law.

  • The point is, it reduces to the old law when you're going slow,

  • and it's compatible with the other law of nature.

  • So we've resolved the paradox.

  • So now, you're not going to measure 150% lightspeed.

  • If you did the measurement, you would in fact

  • get 96% lightspeed.

  • If you actually got your Doppler radar,

  • this is what you would measure, the other car moving relative

  • to you.

  • So everything is happy with the idea

  • that nothing can go faster than the speed of light.

  • So what have we learned?

  • Is it about cars?

  • Well, it would also work for trucks--

  • [LAUGHTER]

  • --motorcycles, planes, spaceships, ambulances,

  • backhoes, dump trucks, fire trucks, wheel trenchers,

  • suction excavators.

  • All of your favorite desert animals would observe this law.

  • All of your favorite balls, if you played sports,

  • would observe this law.

  • Elementary particles.

  • It's true for everything.

  • When you think about it, it's not really

  • about the stuff of the universe.

  • This law doesn't care if you're a Gila monster or a backhoe.

  • This law is about something more fundamental.

  • It's really about space and time itself.

  • Now, why do I say that?

  • Well, think about how I argued a minute ago

  • that this original v equals v1 plus v2 law was so obvious.

  • I said, well, the car up there is

  • going to be 60 miles to your left after an hour,

  • and the other car's going to be 60 miles to your right.

  • So they're 120 miles apart in one hour--

  • 120 miles an hour.

  • That argument gives a wrong conclusion.

  • The only slop in that argument are

  • the concepts of space and time, the idea

  • that if one goes 60 this way in an hour and the other

  • goes 60 that way in an hour, they're now exactly

  • 120 apart in exactly one hour.

  • That seems obvious to us, but that's not actually

  • the way space and time work.

  • Einstein taught us that you can't just

  • add space and time together in the simple way that you think.

  • The theme of our lecture series is rethinking reality.

  • And this was a big one.

  • This was 1905, special relativity.

  • Einstein really rethought reality in a big way.

  • Let me ask you another question.

  • If I have a feather and a hammer and I let them go,

  • which hits the ground first?

  • Well, in this room, the feather would float down

  • and the hammer would go wham and hit the floor,

  • maybe damage the stage.

  • But if we did this experiment on the moon,

  • where there's no air--

  • there's just gravity-- in fact, the feather and the hammer

  • would fall at the same rate.

  • Now, this is something that has actually

  • been known about gravity since the time of Galileo.

  • But the Apollo astronauts actually did this on the moon

  • to dramatize it.

  • There is actually a video of dropping a hammer and a feather

  • on the moon.

  • So this dramatizes something that's unfamiliar about gravity

  • but absolutely true-- that gravity, like the other laws we

  • have been talking about, acts on everything in the same way.

  • Can you guess what I'm going to put up on the screen now?

  • [LAUGHTER]

  • That's right.

  • I could drop a truck, a motorcycle, a plane,

  • an ambulance, a backhoe, a Gila monster, a bark scorpion,

  • and a wolf spider.

  • They would hit the floor at the same time if there were no air.

  • Gravity acts in the same way on everything.

  • Now, here's Einstein's perhaps greatest insight

  • in a life full of great insights.

  • Gravity, since it acts the same way on everything,

  • is also somehow not about the stuff it's acting on.

  • It's more fundamental.

  • It too is just about space and time.

  • Einstein had this idea, and five minutes later it

  • was still an idea, and 10 minutes later it

  • was still an idea.

  • And it took him five or 10 years,

  • really, to flesh this idea out and turn it into equations.

  • And the very first thing he had to do to flesh this idea out

  • was to start listening to his mathematician friend

  • Hermann Minkowski, who had been harping on him saying,

  • no, no, no, time and space aren't separate in your theory.

  • In your theory, Einstein, you should think of time and space

  • as spacetime.

  • They need to go together.

  • So Einstein got on board with this idea that there should be

  • spacetime-- three dimensions of space and one dimension

  • of time--

  • and that somehow, gravity should just be spacetime.

  • And through much hard work, he hit on the idea

  • that what gravity is, is the curvature of spacetime.

  • Now, that's a kind of weird set of words.

  • Why would he say that?

  • Well, we all kind of know what curvature

  • is for objects in space.

  • The surface of the earth is curved,

  • or I could take a piece of paper and curve it in front of you.

  • We'd all agree on what curvature meant.

  • And there are mathematical equations

  • that have been known for a long time that

  • describe the curvature.

  • What Einstein realized was that if he

  • uses the same mathematical equations that

  • describe curvature of two-dimensional sheets of paper

  • to the four dimensions of spacetime,

  • he gets a theory of gravity.

  • And the equation is up there on the top right.

  • So this little cartoon up here you've seen in other lectures

  • if you've been coming regularly.

  • This flexible fabric you see looks two-dimensional

  • on the board, on the screen, but it's just an analogy.

  • It's supposed to represent that the four dimensions, all three

  • of space and time, are curved.

  • And what makes the curvature is matter.

  • So the big red blob in the middle might be the Sun.

  • The Sun makes spacetime curve, and then the Earth

  • orbiting the Sun, say that white dot,

  • just follows the straightest possible trajectory

  • in this curved spacetime.

  • And that makes it go around the Sun, giving rise to gravity.

  • Well, this was the hard part.

  • Once Einstein had these equations

  • he had done the hard part and then the real fun begins.

  • And at some level, the fun hasn't stopped since he died.

  • He and many others after him, including me,

  • are working out the amazing consequences of that equation

  • on the top right.

  • And the consequence I want to tell you

  • about today is called gravitational waves.

  • If you really buy into the analogy,

  • it's actually almost trivial that there

  • will be gravitational waves.

  • Because if I wiggled that big red mass in the middle there,

  • clearly the fabric is going to wiggle.

  • Waves are going to go out, waves in gravity.

  • And indeed, Einstein got that equation in 1915.

  • And in 1916, one year later, he had gravitational waves.

  • Another good analogy for gravitational waves

  • is waves on a pond.

  • I don't know about all of you, but I don't float very well.

  • So if I go out in a pond and try to float,

  • I end up kind of thrashing all around

  • and I look a little bit like the gentleman in that picture.

  • There's waves going out all around me.

  • Now, if it was a really big pond-- maybe a huge lake--

  • and we're on shore and we can't see the man thrashing around,

  • we might still be able to learn that there

  • was some thrashing going on, because back at shore

  • we would see some waves.

  • So you have to imagine a really still day

  • where the pond doesn't have any waves on it at all.

  • You're sitting on the shore, then all of a sudden you

  • see some ripples come in.

  • You might be able to learn from those ripples--

  • you could put a bob in the water to measure the ripples--

  • and you might be able to learn that there was thrashing

  • around going on out there.

  • And if you're really good, you might

  • be able to tell certain properties, like how

  • big the man was, how hard he was thrashing,

  • what frequency, and so forth.

  • That's really the basic idea of gravitational waves,

  • except now, instead of a pond, we have spacetime.

  • So you have some thrashing around in spacetime.

  • Here we have a pair of distant black holes--

  • more on that later.

  • But they're thrashing around in spacetime.

  • And so gravitational waves go out, and back on Earth

  • you can try to measure these waves.

  • Well, how do we do that?

  • This is physics.

  • We clearly need lasers.

  • That's how you do it.

  • You build two machines-- one in Washington state,

  • one in Louisiana-- with big lasers.

  • I'm going to explain that in a minute.

  • But here's a movie, just to get you oriented to the process.

  • Two black holes orbit and merge.

  • A burst of radiation goes off across the universe.

  • Maybe it encounters some other civilizations on its journey,

  • millions or billions of years.

  • Finally, the gravitational radiation

  • reaches us here on Earth.

  • So we'll zoom in to Earth, and you

  • will see an absurdly exaggerated movie

  • of what the waves do to Earth.

  • They stretch and shrink it.

  • They distort it tremendously in this absurdly exaggerated

  • video.

  • Here's the burst.

  • [LAUGHTER]

  • And then the waves are gone off on their journey

  • across the rest of the universe.

  • Now, those waves were made by black holes.

  • But we don't need black holes to make waves.

  • I'm going to make some waves right now.

  • I am shaking my fist, and I'm launching gravitational waves.

  • I'm launching gravitational waves to the back of the hall.

  • I am launching them to the people in the front.

  • Can you feel the power, the awesome power,

  • of the gravitational waves washing over you?

  • I'm shaking harder.

  • I've doubled the strength of the waves.

  • I am now changing the frequency.

  • The waves are now in resonance with the human gut.

  • Soon you will feel very nauseous,

  • and it will get very messy.

  • Is anyone feeling nauseous?

  • [LAUGHTER]

  • Yeah.

  • So it is true that moving matter makes waves.

  • Any motion of matter disturbs spacetime, sends waves out.

  • And it is true that the gravitational waves

  • stretch and compress and stretch and compress.

  • This is among the many things that Einstein calculated.

  • This is what the waves actually do to you.

  • First you feel a little tall, then you feel a little wide,

  • then you feel a little tall and wide.

  • And you were all in fact getting stretched and compressed

  • by the awesome power of my gravitational waves.

  • But you didn't feel it.

  • So why is that?

  • Well, for the answer, we're going

  • to turn to Einstein himself, 1916, who gave us this formula.

  • You don't have to worry about the details,

  • but let me explain what the symbols mean.

  • h on the left is the strain.

  • The strain is the same thing an engineer would

  • talk about the strain on a bar.

  • It's a measure of how much stretching or compressing

  • force.

  • Here we're quoting the amount of fractional stretching

  • or compressing if the bar didn't have any internal resistance

  • to stretching, if it were really just a free-floating blob

  • of particles and the gravitational wave washed by.

  • If this h on the left was 1/10, then that free-floating blob

  • of particles would stretch and compress by 1/10 of its length.

  • So that's what age the strain is on the left.

  • And on the right, with this i double dot and this D,

  • you have some measure of how hard I'm shaking my fist

  • and how far away you are from my shaking fist.

  • But the elephant in the room here

  • is the number on this slide, 10 to the minus 44.

  • 10 to the minus 44 is a number so small

  • I don't think it comes up even in physics.

  • I have never heard of a number this small.

  • This number is as if you took an atomic nucleus

  • and you lined up atomic nuclei all the way

  • across the visible universe.

  • That would take about 10 to the 44 atomic nuclei.

  • That number, that fractional stretching and compressing,

  • is as if the whole universe changed by an atomic nucleus.

  • That is not a lot of stretching and compressing.

  • And so when Einstein saw this, he surely

  • thought, well, that was a nice theoretical point I made,

  • but we are never going to see these.

  • This is far too small.

  • This is just theoretical physics, never

  • experimental physics.

  • Everything changed when neutron stars and black holes

  • were discovered in the '60s and '70s.

  • If you came last week to hear Professor Ozel,

  • you learned a lot about neutron stars and black holes.

  • They're unimaginably dense objects.

  • A neutron star has the mass of the Sun

  • in the size of Manhattan.

  • That makes the density of a neutron star

  • the same as an atomic nucleus.

  • It's like an atomic nucleus the size of Manhattan.

  • It's a totally bizarre object.

  • But if you took two such atomic nuclei the size of Manhattan

  • and you had them orbit each other at 100 or 1,000 times

  • a second-- which they can do--

  • that's a pretty hard fist shake, right?

  • That might make some gravitational waves.

  • That can turn that 10 to the minus 44 into a whopping 10

  • to the minus 21.

  • [LAUGHTER]

  • 10 to the minus 21 is also an extremely small number.

  • And if you're me, you give up.

  • But if you are one of the ingenious experimenters

  • and theorists who thought up the experiments I'm

  • going to tell you about next, you say, wait a minute.

  • 10 to the minus 21--

  • if I build a kilometer-scale device,

  • that's distance changes of less than a proton radius.

  • But I think I might be able to do that.

  • And Ray Weiss at MIT drew this sketch

  • on the bottom left in an internal MIT journal,

  • and together with Ron Drever, Kip Thorne,

  • and many, many other important players,

  • they put together the Laser Interferometer Gravitational

  • Wave Observatory, LIGO.

  • Now, it's a little glib to do this in one slide

  • and say 40 years, a billion dollars, and 1,000 scientists.

  • So I want to take some time to really emphasize

  • how amazing this project is.

  • As I'm going to describe in the rest of my lecture,

  • this experiment has made a real breakthrough the likes of which

  • comes along in my opinion, once every 100 years at most.

  • This is a real breakthrough.

  • And this experiment really was 40 years in the making,

  • and the National Science Foundation

  • and research universities in the US

  • provided all the seed funding for the initial prototypes.

  • The NSF picked it up and funded it starting in 1994.

  • And over 20 years, they kept it going over changes of Congress,

  • changes of administration, changes

  • of NSF program director.

  • Everybody had the vision to see this risky but high-reward

  • project through.

  • And we should be proud, as American taxpayers,

  • that through our elected and appointed representatives

  • we supported this project.

  • So what is LIGO?

  • There's two LIGOs.

  • There's one in Washington state.

  • There's one in Louisiana.

  • And you can kind of make out that there's a lot of tubes.

  • In those tubes are lasers.

  • It's the Laser Interferometer Gravitational Wave Observatory.

  • Let me tell you why.

  • This device is called a Michelson interferometer.

  • This is something that here at the U of A

  • we teach our undergraduate physics

  • majors in the second year of their study.

  • So it's not something I can fully

  • explain in the next three minutes,

  • but I can give you the gist.

  • The basic idea is you shine laser light out,

  • you split it up, and then it bounces off mirrors and gets

  • recombined.

  • And then as a wave wiggles those mirrors, the amount of light

  • read out changes.

  • The reason you can do this is because light is a wave.

  • Light has peaks and troughs.

  • And when two beams of light meet,

  • if those peaks and troughs line up, then you get more light.

  • It's brighter.

  • But if those peaks and troughs anti-align,

  • as shown here on the right of your screen,

  • you get no light at all.

  • So whether the peaks and troughs are aligned or not

  • is a very sensitive function of the distance

  • the light has traveled in this setup.

  • And so as the wave comes through and wiggles those mirrors,

  • the peaks and troughs change their alignment and the light

  • read out on the right of your screen changes in time.

  • That is the light that LIGO actually measures in order

  • to measure the changes in distance of the mirrors

  • as that gravitational wave washes through.

  • Here's LIGO in a kind of CAD drawing.

  • It's the same concept, but with lots of bells and whistles.

  • These tubes are in vacuum--

  • it's a very large vacuum--

  • and there's all kinds of tricks they

  • play to increase the power in the interferometer

  • and to isolate it from sources of noise.

  • That's really the hardest part of this whole business.

  • You're trying to measure distance changes

  • of less than a proton diameter.

  • How do you do that when a truck could drive by

  • and wiggle your mirrors by way more than a proton diameter,

  • or an earthquake could happen?

  • What do you do?

  • You need to be isolated from environmental noise somehow.

  • And the way they do it is as follows.

  • They have this amazing gizmo, this four-stage pendulum,

  • which you see the design for on the left

  • and some photos on the right.

  • And this provides two kinds of isolation.

  • There's active and passive.

  • Passive isolation is like the shock absorbers in your car.

  • When you drive over a bump, the bump hits your car pretty hard.

  • But you don't feel much of that bump,

  • because there's springs in your car that damp that out

  • and give you a smooth ride.

  • They use that kind of passive noise isolation.

  • And they also use active isolation,

  • which is like your noise-cancelling headphones.

  • When you put them on and turn on the power,

  • they actively cancel out incoming sound waves.

  • Similarly, they actively drive this system

  • to cancel out any incoming vibrations in the ground

  • that it's attached to.

  • Now, this talk has a lot of eye candy,

  • a lot of really nice pictures, movies.

  • But I have to tell you, this is the slide

  • that still gives me goosebumps.

  • I remember vividly, almost a year ago

  • exactly, when this slide was released in a press conference

  • at the National Science Foundation, which I streamed

  • here at the U of A for students and faculty and anyone who

  • was interested.

  • And when I saw that signal, I thought,

  • oh my gosh, that's a gravitational wave.

  • Now, for you, it may not jump out

  • to be a gravitational waves so obviously.

  • But let me explain to you why.

  • But before I do that, the most important thing about this plot

  • are those numbers on the top left, GW150914.

  • That is 2015, September 14--

  • my birthday.

  • The gravitational waves arrived on my birthday.

  • [LAUGHTER]

  • Thank you, universe, for that birthday present.

  • OK, that's only the most important part

  • of that slide to me.

  • To the rest of you, let me take you through it.

  • First, look at the vertical axis.

  • It says "strain."

  • That's that fractional stretching and compressing,

  • and it says 10 to the minus 21.

  • This is from the scientific paper.

  • They did it.

  • They measured those wiggles that you

  • see in red are changes in distance of a part in 10

  • to the minus 21, less than a proton diameter.

  • The x-axis, the horizontal axis, is time.

  • Notice, all this happened over just a fraction of a second.

  • And now let's look at the red signal.

  • The red signal is what the device in Washington state

  • measured.

  • That's the readout, which represents the changes

  • in distance of the two mirrors.

  • Is that a gravitational wave?

  • Is it a truck driving by?

  • Is it an earthquake?

  • How do we know?

  • Well, on the second slide, you see that same data on the right

  • over-plotted with the data from the other observatory

  • in Louisiana, all the way across the US.

  • And they line up perfectly.

  • There's no way that this same truck drove in front of Hanford

  • and drove in front of Louisiana at exactly whatever

  • it is, three milliseconds apart, the time

  • for the gravitational wave to come from one to the other

  • in exactly the same way.

  • They did a bunch of fancy statistical analysis

  • to put some numbers to say that, yes, that's not what happened,

  • and yes, that's really a gravitational wave.

  • But this is why I knew right away

  • when they put it up on screen that this

  • was a gravitational wave.

  • This signal was so loud that you didn't

  • have to do any of that fancy statistical stuff.

  • It just says, wow, gravitational waves from outer space.

  • When we physicists try to understand these signals,

  • we often convert them to sound.

  • It's a quick way to get all the information

  • about amplitude and frequency.

  • So now I'm going to play the sound for you,

  • and you're going to hear a bit of a thump.

  • That's what it sounds like if you just

  • take the frequency of the wave and convert it

  • to a sound wave at the same frequency.

  • You're going to hear that thump twice,

  • and then you're going to hear the same sound

  • but upshifted into a frequency range

  • where your ear is more sensitive to the features.

  • [SOUND PLAYS]

  • Hear it again.

  • [SOUND PLAYS]

  • So that little whoop--

  • we have a name for that.

  • We call it a chirp.

  • [LAUGHTER]

  • That chirp's going to come back in about 10 minutes,

  • so let's remember that name.

  • Let me play it for you again in case you missed it.

  • Again, the first thing you hear is a kind of thump.

  • That's the actual signal.

  • And you're going to hear that twice.

  • Then you're going to hear an upshifted version,

  • then you're going to hear the whole thing again.

  • [SOUND PLAYS]

  • Those are the waves from a binary black hole,

  • two black holes that merged 1.3 billion years ago,

  • 1.3 billion light years away.

  • The waves came all the way across the universe,

  • they came to the LIGO experiment,

  • and they made that sound.

  • How do we know these are actually merging black holes,

  • like I said?

  • Well, again, they did a lot of fancy statistical analysis.

  • But for this signal, it was so loud they really

  • didn't have to.

  • Again, this is a snapshot from the scientific paper.

  • On the top line, you see the same two plots

  • that I showed you before--

  • the data from one interferometer on the left,

  • and the two stacked on top of each other on the right.

  • On the second line, you see a theoretical prediction,

  • a calculation from Einstein's equations

  • of what the waves look like if two black holes merge.

  • And then what do you do?

  • You subtract the two.

  • You subtract the prediction from the model, and what's left

  • should just be noise.

  • And that's on the bottom.

  • So you see right away that, what is this?

  • It's the normal noise in the interferometer plus a signal,

  • which is merging black holes.

  • Now, the way these theoretical calculations are done

  • is by a supercomputer.

  • We take the equations that Einstein wrote down in 1915,

  • and we solve them on the state-of-the-art super

  • computers.

  • Here's a simulation of the orbiting

  • and merging black holes of the same size and distance

  • from each other that was measured

  • by the LIGO experiment.

  • Now, you see the black blobs and the tracers.

  • That's just to orient your eye as to what's going on.

  • All the interesting information is in the arrows and the colors

  • and the shape of the sheet below them.

  • That's representing the curvature of spacetime.

  • You'll see now the simulation will slow way down near merger

  • so you can see just how distorted spacetime is getting.

  • It's like a storm in spacetime as the black holes finally

  • merge, and there's really some cusp-y sharp features.

  • There's a burst of radiation, and then the waves

  • are off on their journey across the universe to LIGO.

  • Here's another fun video.

  • This is the question, suppose we were actually

  • near these black holes and we were watching them

  • in a spaceship.

  • What would you see?

  • Well, mostly you'd see the stars behind you.

  • And you don't see any stars from where the black hole is,

  • because all the light has been swallowed up by the black hole.

  • But you also see a very distorted pattern of stars,

  • because the black holes bend light.

  • So the stars don't look like their actual positions.

  • They look very distorted.

  • And I think we'll just watch the end of this beautiful movie

  • together.

  • Let me tell you what they actually

  • measured for this system.

  • It's not about just detecting gravitational waves.

  • It's also about doing astrophysics, finding out

  • what's out there.

  • So how heavy were these black holes?

  • How big were they?

  • The larger black hole was 36 solar masses,

  • meaning it had the mass of 36 suns.

  • So I've drawn 36 dots to represent that.

  • This was reported in the scientific paper.

  • The other black hole was slightly smaller.

  • It had the mass of 29 suns.

  • And then they also measured the mass

  • of the final merged black hole, which was 62 suns.

  • So everything hangs together.

  • If you take 36 and you add 29, you get 62.

  • It's simple arithmetic.

  • [LAUGHTER]

  • Wow, you guys are good at arithmetic.

  • Yeah, 36 plus 29 is not 62.

  • It's 65.

  • What happened to those three suns of mass?

  • Where did they go?

  • Believe it or not, all of those three suns of mass,

  • the weight of three suns, was radiated away

  • in pure energy carried by those gravitational waves.

  • This is e equals mc squared energy,

  • Einstein's other equation.

  • Right?

  • Except if you think about the normal way we use e equals

  • mc squared energy-- you know, something really impressive,

  • like atomic weaponry-- atomic weaponry releases

  • some tiny fraction of the e equals mc

  • squared energy inside your warhead, which

  • is not the mass of the sun.

  • Here you're releasing all of the e equals mc

  • squared energy of three suns.

  • That is a ton of energy.

  • And you're releasing that in just a fraction of a second.

  • And in fact, during that fraction

  • of a second when these black holes merge,

  • they're releasing more energy per unit time.

  • They're brighter than the whole rest of the universe combined.

  • As theorists, we knew this.

  • Einstein's equations are well-established.

  • We know how to solve them on computers.

  • We can calculate that when you merge a 36 and a 29

  • solar mass black hole, you radiate away

  • three solar masses of energy.

  • And we talked about this, but now we've actually

  • observed it happen in nature.

  • It's fantastic.

  • Another really fun part of LIGO's story

  • is this amazing signal I've been telling you

  • about which arrived on my birthday, September 14,

  • we almost missed.

  • The LIGO experiment decided to start their observing run

  • on September 12.

  • And the founder of LIGO, Ray Weiss, was dragging his feet.

  • He said, oh, it's not quite calibrated.

  • We don't want to start.

  • Let's get it perfect.

  • But the younger crowd convinced him to go for it.

  • And we're glad he did, because two days after they turned it

  • on--

  • this thing's been in the works for 20, 40 years--

  • they measured this spectacular signal.

  • They saw something else in October,

  • which was probably a signal-- they're not sure yet--

  • and then another confirmed one in December.

  • Then they took the experiment down,

  • upgraded some of the optics, improved the seismic isolation,

  • and they're observing again now.

  • Already, just from one measurement,

  • we learned a little astrophysics.

  • First of all, we learned that black holes can find each other

  • and merge.

  • That's not obvious.

  • Predictions for how many black holes there would be

  • and how often they would merge were all over the map.

  • Now we have tight constraints on that.

  • And we really discover a new type

  • of black hole, because previously, from x-ray studies

  • there in the purple, we knew there were black holes

  • of around 10, 15 solar masses.

  • And from other studies, like what Professor Ozel told you

  • about last week, we knew there were giant black holes

  • of a million solar masses.

  • And now we know that there are also 40

  • and 60 solar mass black holes.

  • So that's the weight of these black holes.

  • What about the size?

  • Well, to set the scale, I'll give you a map

  • of the Eastern United States.

  • I think that's the part I'd like to blot out with black holes

  • right about now.

  • [LAUGHTER]

  • There's the signal I've been telling you about.

  • That's how big those black holes are.

  • There's the candidate signal, and there's

  • the smaller black holes that were also measured.

  • So these are state-sized objects with the mass of 30, 40, 50,

  • 60 suns.

  • LIGO is just the beginning.

  • We have two detectors, LIGO Hanford and LIGO Livingston.

  • Those are the ones that made the detection,

  • but there are many more detectors around the world,

  • either operational, coming online, being built or planned.

  • And in five to 10 years, we should have all six

  • of these detectors working.

  • In Germany, the GEO detector; Virgo in Italy;

  • LIGO India in India, and KAGRA in Japan.

  • It's great to have more detectors,

  • because gravitational waves are a little bit like sound.

  • You don't really know where a sound is coming from.

  • If you have two ears, you can sort of

  • tell where it's coming from, but not very precisely.

  • If you have three ears or four ears,

  • you could start to tell more precisely.

  • And if you have six ears, you can really

  • start to get a good handle on where

  • that sound is coming from.

  • So if we have six detectors, we can really

  • tell where in the sky the gravitational wave is coming

  • from, and we can tell our astronomer friends, hey,

  • go point your telescopes over there.

  • You might see something interesting.

  • When we have these six detectors--

  • and even before-- we have a lot of fun ahead of us.

  • We're super excited about the first signal

  • we saw, merging black holes, but we're going to see a lot more.

  • I'm sure LIGO has already seen more black holes, although they

  • don't tell people who aren't officially

  • in the collaboration.

  • But I've heard rumors.

  • There are supernova explosions.

  • When stars end their lives and explode,

  • they make these giant explosions.

  • That probably makes some gravitational waves.

  • We'll hope to observe those.

  • My personal favorite is merging neutron stars.

  • Again, a neutron star is matter at

  • the density of an atomic nucleus.

  • We don't know very much about matter

  • at the density of an atomic nucleus.

  • So we're going to learn a lot of nuclear physics

  • if we can start seeing a lot of merging neutron stars,

  • because those waves carry information

  • about the process of merging.

  • We'll see rotating neutron stars.

  • And perhaps most excitingly is the question, question,

  • question mark on the right, because we've really

  • opened a completely new window on the universe

  • here with gravitational waves.

  • And every other time in the history of astronomy that we've

  • gotten a new way of looking at the universe,

  • like when radio astronomy started or x-ray astronomy

  • started, every time there were expectations,

  • and there were huge unexpected discoveries.

  • So perhaps most exciting are the things

  • we don't know about that we will discover with this new way

  • of seeing the universe.

  • I'm going to change gears a little bit

  • and tell you about my own research.

  • I work a lot on gravitational waves,

  • and one question we asked recently

  • was, what if a black hole spins really fast?

  • You can spin a black hole, and you can spin it really fast.

  • You can't spin it at the speed of light.

  • Nothing can go faster than light.

  • But you can get it really, really, really close,

  • at least in principle.

  • So here's the sound of a wave form

  • that is from a normal black hole.

  • This is theoretically generated.

  • We just made this on our computers

  • as part of the research.

  • But this is just a comparison signal

  • for what a normal black hole merger sounds like.

  • [SOUND PLAYS]

  • Right at the end there, there's that [SOUND EFFECT].

  • That's the chirp.

  • Chirp is the hallmark, the calling card,

  • of merging black holes.

  • All the theoretical predictions have always predicted chirps.

  • And that's what LIGO measured.

  • But when we got one of these black holes

  • spinning really fast, when we finally cracked the mathematics

  • to let us explore this regime, something very interesting

  • happened.

  • It starts the same, but you're not

  • going to hear a chirp from this rapidly spinning black hole.

  • [SOUND PLAYS]

  • It just sits on a single note, and it fades away.

  • We call that a song.

  • [LAUGHTER]

  • It's very different.

  • It's not [SOUND EFFECT].

  • It's [SOUND EFFECT].

  • [APPLAUSE]

  • So why is this exciting?

  • Well, in astrophysics, we don't have

  • great ways of measuring how fast these black holes spin.

  • But now we can tell that if you hear

  • a sound that doesn't chirp--

  • it just sings-- you're looking at a black hole

  • spinning really, really, really fast.

  • Now, we gave this black hole a nickname.

  • And our scientific paper has a funny title

  • up there on the right, "Inspiral into Gargantua."

  • If you've seen the movie Interstellar,

  • there's a black hole called Gargantua.

  • And it turns out, for the plot of Interstellar

  • to make any sense at all, that black hole Gargantua has

  • to spin very rapidly.

  • It has to spin with 99.999999999999%

  • the speed of light, 14 nines.

  • So, is Gargantua out there?

  • How would we know?

  • Well, if the next thing LIGO hears, or some other experiment

  • in the future, if the next thing they hear is this song,

  • then we've discovered Gargantua.

  • When I started thinking about this problem,

  • I wasn't thinking about astrophysics at all.

  • Rapidly spinning black holes turned out

  • to be a really good theoretical testing

  • ground for the following sort of pure theoretical physics

  • question that I'd like to describe for you.

  • It's called the black hole information paradox.

  • At the start of the talk, I gave you

  • a paradox involving the speed of light being the upper limit

  • that you can physically go, and this formula, v is v1 plus v2.

  • That paradox was resolved 100 years ago.

  • This paradox is still with us today,

  • and it's really driving theoretical physics,

  • I'd say more than any other paradox.

  • In its simplest form the question is,

  • can you tell what was thrown into a black hole?

  • So these are thought experiments, not

  • real experiments.

  • But we used to do them with cats.

  • We would throw the cat into the black hole.

  • And then we started feeling bad about that,

  • so now we use graduate students.

  • [LAUGHTER]

  • That seems to go over a little better.

  • But in these thought experiments--

  • not real experiments--

  • we either throw a cat in or a graduate student in,

  • and then we ask you to come back later

  • and look at the black hole.

  • And can you tell--

  • in principle, not in practice--

  • if it was a cat or a graduate student that I threw in?

  • Einstein's theories say no.

  • The black hole-- anything that goes into it

  • is, for all intents and purposes,

  • gone from the universe.

  • You will never be able to tell, in principle, whether I threw

  • in a cat or a graduate student.

  • But there's another theory of physics

  • that you'll hear about a lot more in two

  • weeks with Professor Cheu's lecture, which

  • is quantum mechanics.

  • And all the formulations of quantum

  • mechanics that we know about have a fundamental law

  • that information is preserved.

  • In quantum mechanics, you have to be

  • able to tell, in principle if not in practice,

  • whether it was a cat or a graduate student.

  • So there's a fundamental tension here.

  • And it's really Stephen Hawking who first pointed this

  • out after discovering some fantastic

  • properties of black holes that made it very apparent.

  • His paradox is really still with us today.

  • Now, I did not solve this paradox, alas,

  • when I was thinking about rapidly spinning black holes.

  • But it led to a new prediction in astrophysics,

  • which just shows the interconnectedness of science.

  • In the last few minutes, I want to take a step back and think

  • about where in the history of science and astronomy

  • this gravitational wave discovery lies.

  • You may know that light is just one

  • wavelength of electromagnetic radiation,

  • and radio and microwave and infrared and ultraviolet

  • and x-ray and gamma ray are other wavelengths

  • of electromagnetic radiation.

  • There's a whole electromagnetic spectrum.

  • Astronomy really began when Galileo first

  • turned his telescope to the heavens,

  • and among other things, discovered

  • the moons of Jupiter.

  • You can see his sketches there.

  • There's Jupiter with four little moons that he observed.

  • Since that time, we now have astronomy in all

  • of these different wavelengths.

  • In radio astronomy, we've discovered incredible things.

  • That was the discovery of neutron stars, in fact.

  • Up there in the top left, that speck there is a whole galaxy.

  • That galaxy is shooting out particles

  • moving near the speed of light in a straight line for hundreds

  • of times the galaxy's size until they make this beautiful plume

  • that we see in the radio.

  • Some of my other research is aimed

  • at trying to understand what the heck is going on there.

  • In the microwave, we can see light left over

  • from after the Big Bang.

  • 13 billion years ago when the universe was just

  • a hot plasma--

  • no stars, no planets, just hot plasma, no elements even.

  • In the infrared, we can see galaxies,

  • learn about their structure, trace them through cosmic time.

  • In the visible, well, we've come a long way since Galileo.

  • Now if we look at Jupiter with our best instrument, the Hubble

  • Space Telescope, we get that beautiful image.

  • That's not a painting.

  • That's a picture of Jupiter.

  • In the ultraviolet we can see the storms

  • on the surface of the Sun and learn about plasma physics

  • at high temperature.

  • These coronal mass ejections that wipe out communications

  • here on Earth-- we can study those, among other things.

  • And in x-ray and gamma ray, we see the high-energy events

  • in the universe.

  • The gamma rays sky looks like that big image

  • on the upper right there, except every once

  • in a while when there's a bright flash called a gamma ray burst.

  • Those gamma ray bursts were actually

  • first discovered by military satellites

  • looking for Soviet nuclear tests.

  • But they pretty quickly figured out

  • that the gamma rays they were measuring

  • were actually coming from outer space.

  • The origin of those gamma rays is still pretty much a mystery,

  • and it's a mystery LIGO might help unravel.

  • But that's the electromagnetic wave spectrum.

  • Gravitational waves also have a spectrum.

  • And where are we in observing this spectrum?

  • Well, we're just like when Galileo first pointed

  • his telescope to the sky.

  • We've made one observation.

  • It's tremendously exciting, these merging black holes,

  • but it's one observation at one tiny frequency

  • band in this whole spectrum.

  • As LIGO becomes more sensitive, we'll

  • start to hear higher frequency things--

  • maybe supernovae and merging neutron stars.

  • There are space-based detectors in the works that

  • will measure the monsters of the universe-- supermassive

  • black holes, black holes with millions or billions of times

  • the mass of the Sun.

  • There are other detectors that get us

  • to even lower frequencies, where we might hear cosmic strings--

  • or if we're very lucky, gravitational waves

  • from the Big Bang itself.

  • Let me summarize.

  • I started the evening telling you

  • about how a century ago Einstein discovered

  • a deep new reality of interconnected space, time,

  • and gravity.

  • He discovered new things about physics.

  • Those consequences of Einstein's equations were revolutionary,

  • and they are still with us today.

  • And 100 years later, one of those consequences

  • has now been turned into a tool.

  • We are now using Einstein's gravitational waves

  • to hear the universe in a completely new way.

  • And in closing, I want to leave you with two questions.

  • What will we learn in the next 100, 200

  • years with this new astronomy?

  • And what will be the next revolution in space, time,

  • and gravity?

  • Thank you very much.

  • [APPLAUSE]

[MUSIC PLAYING]

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