 Good afternoon. I would like to welcome all of you to our viewing of the 2021-2022 Hitchcock Lecture entitled Time, Einstein, and the coolest stuff in the universe. This is being presented by Dr. William Phillips. We are pleased to present this lecture to the public and hope very much that you'll enjoy it. Now, my name is Vincent Resh. I'm a professor in the Graduate School in the Department of Environmental Science, Policy, and Management here at the University of California, Berkeley, and I'm also chair of the Hitchcock Lecture Committee. The Hitchcock Lectures were endowed by Dr. Charles M. Hitchcock in 1885 to institute a professorship at UC Berkeley. The Hitchcock Foundation Lectures began in 1909 and were later expanded and retitled the Charles M. and Martha Hitchcock Lectures in 1932. Thanks to a generous gift from Dr. Hitchcock's daughter, Lily Hitchcock-Coyt, and of course, Coyt is a very famous name in this area. Biochemist lioness Pauling, astrophysicist Stephen Hawking, and oceanographer Sylvia Earle are among the many distinguished scholars who have served as Hitchcock professors in the more than a century since the lectures began. I'm very pleased to now be able to present to you with Dr. William Phillips, our Hitchcock lecturer for this semester. Dr. Phillips received a BS in physics from junior to college in 1970, and his PhD from Massachusetts Institute of Technology in 1976. After two years as a CHI-Invitesman postdoc at MIT, he joined the National Bureau of Standards to work on precision electrical measurements and fundamental constants. There, he initiated a new research program to cool atomic gases with laser light. He founded the National Institute of Science and Technology, which of course is the successor of the National Bureau of Standards, laser cooling and trapping group. And later was a founding member of the Joint Quantum Institute, which is a cooperative research organization of the National Institute of Standards and Technology and the University of Maryland. Now this institute is devoted to the study of quantum coherent phenomenon. His research group has been responsible for developing some of the main techniques now used for laser cooling and cold atom experiments and laboratories around the world. Dr. Phillips is a fellow of the American Physical Society, the American Association for the Advancement of Science, and the American Academy of Arts and Science. He's also a fellow, an honorary member of the Optical Society, a member of the National Academy of Sciences, and the Pontifical Academy of Sciences, and he is a corresponding member of the Mexican Academy of Sciences. In 1997, Dr. Phillips shared the Nobel Prize in physics for development of methods to cool and trap atoms with laser light. And so without further delay, I present to you Dr. William Phillips and his presentation time Einstein and the coolest stuff in the universe. Thank you very much for joining us Dr. Phillips. Thank you very much for that. That very kind introduction. Let's share my screen and let's hope that this all works. Okay, I think you're seeing a test pattern now. And there's the title slide. Okay, time Einstein and the coolest stuff in the universe. Well, as you've heard, I'm from a place called the Joint Quantum Institute, which is joint between the University of Maryland and the National Institute of Standards and Technology. And at NIST, I'm a member of the Laser Cooling and Trapping Group, and Gretchen Campbell is now the leader of that group. And the other members, Paulette, Trey Porto, Ian Spielman, Ida Titzinger, Charles Clark, and Nicole Younger-Halpern are the permanent members with whom it's my great pleasure and honor to work on a daily basis. The work that I'm going to be telling you about has been funded by a number of people over the years, mainly NIST, but the Office of Naval Research has provided crucial funding throughout the development of this work. And at the Joint Quantum Institute, we've had a physics frontier center from the National Science Foundation. So we're very grateful to them. Now, one of the things I want to emphasize right at the start is that this lecture is only part of the story of Laser Cooling and Ultra-Cold Atoms. As you've heard, I shared the Nobel Prize in 1997, and the people I shared it with were Steve Chu and Claude Comptonugy, and they have much more of that story to tell. But the fact is that there are research groups all over the world who are pursuing what I really consider to be an adventure in Ultra-Cold Atoms. And some of the greatest work is going on right here at Berkeley. Ahud Altman and Norman Yauer are doing theory relating to Ultra-Cold Atoms. Hartman Hefner, and I'm so pleased to say that Hartman is an alumnus of our Laser Cooling and Trapping Group at NIST. Dan Stamper-Kern, Hoger Mueller, and Hoger is formerly worked with Steve Chu. You see, we really have a kind of a community here in this Cold Atom business. Well, time Einstein and the coldest stuff in the universe. So what does time have to do with Einstein? Well, time put Einstein on the cover of its magazine as the other person of the century. And I really think that was a great choice because Einstein has done so many things that have really changed the way we think about the world and have changed our lives. All of these things, Brownian motion, the understanding of stimulated emission gave us lasers. Where would we be without lasers, without general relativity? GPS wouldn't work. The photoelectric effect was the beginning of quantum mechanics, which dominates our lives. But probably the thing that Einstein is best known for is his theory of special relativity. With that theory, he changed the way we think about space and time itself. Before Einstein, people thought that space and time were like a fixed stage on which the events of the universe played themselves out. And what Einstein taught us was that space and time are actually part of the action that the stage is not fixed. It depends on who's looking at it and what's going on, what the nature of that stage is. And part of that understanding is what is called the relativity of time. The fact that time changes depending on who's looking at it. And Einstein came to his understanding about the relativity of time in part by asking himself a question, a question that I think people have asked themselves since the beginning of at least human time. And that is, what is time? What is this strange thing that is always the present but will soon become the past? Why is it that tomorrow is always a day away? And the answer that Einstein gave to that question might seem superficial to you. What Einstein said was, time is what a clock measures. And while that may seem superficial, by taking that idea seriously, Einstein came to his profound understanding about the nature of time. But if time is what a clock measures, then you may ask, what is a clock? Well, for me, a clock is something that ticks, something that gives you a series of events, a periodic train of events. The first clock is the rotating Earth. Now, ancient people didn't know that the Earth rotated, but they knew that the sun rose and set every day. And that set of periodic events allowed them to tick off days. When they became more sophisticated, they did things like make sundials and they could measure hours. Much later, the story is that Galileo was sitting in the cathedral in Pisa, watching the chandelier in the cathedral swing back and forth. Apparently the worship service wasn't very interesting and he had better things to do. So looking at the cathedral and timing it with his own pulse, he discovered that the period of the pendulum was independent of how far the pendulum was swinging. What I don't understand is, with such an exciting discovery, how did his pulse rate stay constant? But nevertheless, he figured this out. And it wasn't long after that, Christian Huygens decided that this could make a clock. And tall clocks like this, this beautiful grandfather's clock, used this feature of the pendulum as being a very good ticker. There may be some of you who are still wearing wristwatches and inside that wristwatch is a tiny quartz crystal shaped like a tuning fork and the vibration of that quartz crystal is the ticker for this quartz watch. Well, some of these clocks are marvels of engineering and of art. This tall clock is one example. Another clock, one of my favorites, is this clock imaginatively called H4 because it was John Harrison's fourth attempt to make a clock that could win the famous 20,000 pound longitude prize in the 18th century. The British government offered this prize, 20,000 pounds, which was a fortune at that time. And the reason they offered it was because they were losing ships at sea because the ships did not know where they were. It was easy to determine their latitude, but determining their longitude was really difficult. And in the year 1707, famous shipwreck occurred off the Silly Islands in England, and three ships went down. Over 1600 sailors were lost, including the Admiral who was in command of this fleet. And that really got the British Admiralty worried. They decided that they really needed to solve this problem of longitude. And they convinced the British government to offer a prize of 20,000 pounds. And the prize was for somebody who could figure out what the longitude was to within half a degree, which is about 30 nautical miles. And that required knowing what time it was to two minutes. Now, why does it need time? Well, think about this. You all are in California. And what is it? It's, I don't know, 430 or something like that in California now. And it's 730 here where I am. If we could communicate to each other what time it was, and we were keeping time by the sun, we would know exactly what the difference in our longitude was. But in the 18th century, they couldn't communicate like that. The only way you could know the difference in the time between two different places was to carry a clock with you that preserved the time in, say, Greenwich. So a navigator could synchronize their clock to what the time was in Greenwich and then determine what the local time was anyplace else and looking at the difference between the Greenwich time and the local time determine their longitude. That was the idea. The trouble was getting a clock that would work on an 18th century sailing ship was not an easy task. 20,000 pounds and people as famous as Newton thought that you'd never make a mechanical clock that would do the job. It would have to be something you do by observing the heavens. But in fact, this guy, John Harrison, thought he could make a clock and he spent his entire career trying to make a clock that would do the job. And finally, oh, you've got to read this book. This is a book about this wonderful adventure, this scientific and technology adventure of trying to make a clock that can do this. I highly recommend this book Longitude. But Harrison in trials in 1762 and 1764 proved that this device, this H4 chronometer could do what was needed to navigate that well. It was good to 40 to avoid a 47 days. This clock was good to 39 seconds. So it easily won the Longitude prize 20,000 pounds. Well, the British government delayed giving his money. Does that sound familiar? He finally had to appeal to the king in order to get all of his money, but he got all of his money finally. And Captain Hook, one of the greatest explorers in history, got one of the first copies of Harrison's chronometer and he loved it, used to navigate his way around the South Pacific. This was amazing, an amazing clock. Now think about this. Today, a decent quartz watch is better than Harrison's clock. So what we've seen is that throughout history, clocks are wonderful and they've been getting better. But all of these clocks are imperfect in one way or another. The length of a pendulum may stretch or shrink and that will change its ticking rate. Every quartz crystal in a quartz watch is a little different. And it may change depending upon whether you wear it on your wrist or whether you put it on the bedside table. Even the rotation of the earth is not constant. It's slowed by the tides. It's affected by storms and changes in ocean current. The fact that the earth is not constant in its rotation was brought home to me in a rather dramatic way one day when I was visiting the US Naval Observatory. The Navy has been interested in clocks since the time of Harrison. The Navy has funded me to make better clocks. And so I went to visit them to see what the latest was in clocks. And as I was walking with my hosts down the hallway to his laboratory, we passed by a door. And on the door was written, Director of Earth Rotation. I thought, wow, that's a pretty responsible job. But the point is that somebody keeps track of these things because people still navigate using the stars. And if you do that, you need to know these details about the rotation of the earth and people still keep track of that because the earth is not constant. Well, we have all these wonderful clocks. There is one kind of clock that is better than all of them. And that's the atomic clock. Atoms are the best tickers. There are specific frequencies that correspond to the difference in energy levels in atoms that constitute the ticking of these of these atoms that are in atomic clocks. And the beauty of this is that every atom of the same kind is identical to every other atom of the same kind in the entire universe. Every cesium-133 atom is identical as far as we know to every other cesium-133 atom in the universe. And cesium atoms are the ones we use to define what we mean by a second. Every quartz crystal is a little bit different. Every mechanical clock, every copy of Harrison's clock was a little different. But these atoms are all the same. And they're very little affected by the environment. Here's the oversimplified version of how an atomic clock works. Imagine an atom here, a cesium atom. And it's got a nucleus and a chord. It's got a valence of electron. And when you send in microwaves at a frequency of about 9 gigahertz, if it's just the right frequency, it'll make the electron flip its spin. That'll change the energy level of the atom. But if the frequency is not just right, it won't cause the electron to flip. So the basic idea of an atomic clock is you're shining the microwaves, you tune the frequency until it's making the electron flip its spin. And when it's just the right frequency, then this is that frequency by definition. Because we use that definition of what this frequency is to define what we mean by a second. So this is 9 billion, a certain number of million cycles per second. That's what we use to define what we mean by a second. Now, you may ask, how good are these clocks? We know how good Harrison's clock was. For less than $100, you can buy a quartz clock that's good to a part in a million, about 30 seconds in a year. That's amazing. But for $100,000, you can buy an atomic clock that's good to a part in a million million. That's 30 seconds in a million years. And you may say $100,000 for a clock. That's a lot of money. But think about it this way. You spent a thousand times more money and you get a million times better performance. I think that's a bargain. But you still may ask yourself, who needs a clock that's good? That is that good? Because after all, we don't need to know what time it is to that level of precision in order to go about our daily lives. Or so you may think. I now want to convince you that in fact you are very happy that somebody is keeping track of time to that level of precision to do things that are important to you in your daily lives. I found this advertisement in a magazine, an advertisement for some high-end car. And it says if you get into trouble with this car, don't worry, because help is only 10,000 miles away. And the 10,000 miles they're referring to is the orbiting altitude of the satellites of the satellite navigation system that we call GPS or the global positioning system. How does it work? It works by having atomic clocks on board inside these satellites, a constellation of 24 satellites minimum orbiting the Earth with atomic clocks on board. And here's how it works. So this is a cartoon. Here's a couple of GPS satellites. They've got atomic clocks on board. Here you are with your GPS receiver on the ground. Now, you don't have a clock, but for the sake of argument, let's imagine that you do. And all of these clocks are synchronized. And that's indicated by the fact that you see all these clocks are showing the same time. Now, the clocks are broadcasting information about what time it is. And they're also broadcasting information about their location, where they are in space. So as they broadcast that information, the information takes a certain amount of time to get to the receiver. And so when you receive information from a satellite, from this satellite, telling you what time it is, it tells you the time it was when the signal left the satellite. And now you compare that to the time it is on your clock when it gets received. And because you know the speed of light, you know how far away you are from the satellite. And because you know where the satellite is, you know you're somewhere along this surface. Okay, that's if you have one satellite, you've got another satellite. And when you get the signal from that satellite, it tells you how far away you are from that satellite. So that means you know that you're at the intersection of this curve and this curve, so you know you're here. Now, if we lived in a two-dimensional universe, that would be all you'd need to know, but we live in a three-dimensional universe, so you need another satellite. And if you had a clock, that would be all you would need, but you don't, so we need a fourth satellite. So when you turn on your GPS receiver and it says looking for satellites, it's trying to find four. When it finds four satellites, it's ready to go, and it can tell you where you are, anywhere on the face of the earth, within a few meters. And people are using these everywhere. I use them just to find my way to the local car dealer, taxis, delivery trucks, commercial aircraft and ships, military vehicles. Even people are going hiking out in the wilderness, take these things with them. Golfers use it to figure out how far they are from the green. And earth scientists can study continental drift using the global positioning system. It's that good, and it's all because of atomic clocks. So you care about time at that level. So now you may ask, what do these clocks look like? Well, I found another ad in the magazine once it was an ad for an airline. And it said, recently, scientists in Braunschweig, Germany set the atomic clock back one full second. And we've adjusted our schedules accordingly. Yeah, right. That airline, by the way, isn't in business anymore. But what they say about setting the atomic clock back a second is true. Why they chose Braunschweig, Germany, when this was an American company. And the same is done at the National Institute of Standards and Technology, where we keep track of time for the United States. I have no idea why they use Germany, but that's not the point. The point is that the instrument in front of which these two dorks are standing looks nothing like an atomic clock. And if you were to go to our laboratories in Boulder, Colorado, where we have the atomic clocks and look at the best atomic clock from a few years back, it would look like this. This long tube that you see here is where the cesium atoms form an atomic beam. And this outside cylinder protects them from any magnetic fields that might be around. And if you look a little bit deeper into this to see how it works, we've got a cesium oven, it's just a can of metal, and there's cesium in there it's heated up the, the atoms evaporate from the metal, and, and come out a little hole in the side, make a beam of atoms, moving along here at more than 100 meters per second. Now for the reasons that we talked about each one of these atoms is like a little tiny clock. It's a perfect clock because it's the thing that defines what we mean by, by time itself, and the atoms pass through a place where there's microwaves we call the microwave cavity. And what that microwave cavity does essentially is synchronize the ticking of the cesium atoms with the ticking of the microwaves. And then the clock, the atoms go to the other end of the apparatus and see the microwaves again. What happens here is that the ticking of the microwaves is compared now to the ticking of the, of the cesium atoms, and if they're a little bit off, then you make an adjustment to change the ticking of the quartz clock because it's the atoms that are perfect. It's sort of like what would happen if you wanted to see whether your own watch was keeping the right time. You go online to www.nist.gov and you get the time.gov website, and you'd synchronize your watch to that website. But that doesn't tell you how fast or slow your watch is running. You've got to come back later to compare. Well, obviously it's useless to come back a minute later. I usually do this about a month later and then see whether there's a difference. And then I know how fast or slow my watch is. It's the same way with the atoms. You don't want to check too soon after you synchronize. And that's why you have something like a meter between the place where you synchronize and the place you compare. But the atoms are going at 100 meters per second. So that means it's only a few thousands of a second between the time you synchronize and the time you can compare. But these clocks are amazing. These clocks are good to a part in 10 to the 14. But the reason why they're limited to a part in 10 to the 14 is just this fact that the atoms are moving so fast and that it limits the amount of time you have to make the measurement, which means you can't do quite as well. But there's more things. There's the Doppler effect. When these atoms are moving, because the Doppler effect, it seems like their ticking is different. And there's relativistic time dilation. Einstein, in his special theory of relativity, taught us that time is relative. And a moving clock seems to run slow and these atoms are moving clocks. So let's say a little bit about the Doppler effect. If you've got a clock, like atoms, and if you were moving toward it, or if it was moving toward you, there would be a shift in the frequency and the fractional shift in the frequency is equal to the ratio of the velocity of the clock to the velocity of light. Usually that's pretty small. And it doesn't matter whether the receiver moves or whether the clock moves. By the way, this is how the cops tell how fast your car is going. If you shine microwaves on your car, the microwaves reflect off your car and that means your car is like a moving clock. The cops receive the microwaves. Look at the difference in the frequency between what they sent out and what comes back and they see how fast you're going. Now, as I said, the fractional Doppler shift is the ratio of the atoms velocity to the speed of light. Now, a typical atomic velocity, say the molecules in the air in the room are moved about 300 meters per second. The speed of light is 300 million meters per second. So the fractional shift is a part in a million. That may not sound like very much, but wait a minute, we want to do a part in 10 of the 14. This is a disaster. Fortunately, there are all kinds of tricks to get rid of it. You make sure that you got the microwaves coming in both directions at the same time, so it cancels out. And this gets rid of most of it, but not all of it. And the fact that you can't get rid of it all is one of the reasons why these clocks are only good to a part in 10 of the 14. And there's Einstein. Einstein's theory of spatial relativity says that the clocks run slow by the ratio of the square of the velocity of the atoms to the velocity of light. That's around 10 to the minus 12. Again, that may not seem like very much, but we want to do 10 to the minus 14 or better. And there aren't any tricks to get rid of this. The only way you can get rid of this is measure all the velocities and correct for it. And that's really hard to do. So what we want to do, and here's where it really starts to get interesting. We want to slow the atoms down so that we don't have these problems. And slowing the atoms down means cooling them. Remember the title of the talk? The coldest stuff in the universe or the coolest stuff in the universe. And the reason is because the difference between hot and cold is the difference between fast and slow. If you've got a hot gas, it means the atoms are moving around really fast. And if you've got a cold gas, then it means the atoms are going much more slowly. In fact, temperature is simply a measure of the thermal kinetic energy of the atoms in a gas. That's all temperature is. And so if we want to make the atoms go more slowly, we've got to cool them down. And so in order to give you an idea about how cold we want to get some things, I'm going to show you this movie of a talk that I gave in person in a town in Germany called Lindau. And really cool town, by the way, some cool stuff happens there. And so I'm going to show you that movie and I'm going to narrate it myself. So what I'm doing is I've brought along some really cold stuff in this thermos bottle, this thing we call a doer. This is liquid nitrogen. And compared to the floor, this liquid nitrogen is so, whoops, I wanted to stop it, but I made it start over again. I'm sorry. The point is compared to the liquid nitrogen. The floor is burning hot. The liquid nitrogen is that cold. So imagine what would happen if you were to pour cold water onto a red hot stove, because the floor is like a red hot stove. It boils immediately. That's what's happening here. The liquid nitrogen is boiling. So the idea is you got something this cold, this incredibly cold, then why not use it to cool down some gas. So here I've got a traditional container for hot gas, a balloon. It's green because green is one of the colors of the town of Lindau, one of their official colors. And so I'm going to fill this balloon with hot gas. And, you know, I'm from Washington DC or just outside of Washington DC. So we're really familiar with the concept of hot air here. So I fill the balloon with hot air. And now what I'm going to do is I'm going to stuff it into a bucket that is full of liquid nitrogen. That's the key thing. Remember, it's full of liquid nitrogen. And as I press it down, maybe you can see the liquid nitrogen is spilling out because the bucket is full. And so I got to push it into the bucket. And I put a lid on the bucket. And now the other color of Lindau is white. So we got some white balloons. And it turned out that the balloons that we were able to find in this town, the only white balloons that we could find were so hard to blow up that I had to get this young physics student to help me blow the balloons up. So, so Roman being young and strong was able to blow up these balloons, which I was not capable of doing. And so I'm going to stuff this white balloon into the bucket again. Okay, and you can see liquid nitrogen is pouring out. Okay, but you know, let's keep going. If we want to do something we might as well do it with a lot of gas. So let's fill up another balloon with with hot air. And, meanwhile, Roman is blowing up this, it's out of the balloon, and I'm going to stuff this green balloon in. It looks like I maybe blew it up a little bit too much. So I got to squeeze it a little bit to blow it up to get it to fit in. But eventually I get it squeezed in and put the lid on. Now I take the balloon from Roman. These balloons are supposed to be heart shaped, but I don't know who designed these things. They don't blow up to look like a heart at all. They look like something that's got little ears on it. But anyway, I'm stuffing that balloon into the bucket as well. And why not? Let's blow up another green balloon and get some more hot air into this bucket. But this liquid nitrogen is really, really cold. In fact, if you haven't been in a low temperature physics laboratory, this is probably the coldest stuff you've ever seen. So it seems obvious this would be a great thing to use to cool down a hot gas. Now I made this balloon a little bit too big again, so I'm squeezing it up and getting it to go in. Now I decided, because I saw there were some kids in the audience, and I hope there's some kids watching today, that I really ought to say something about safety. So let me, I'll just let you listen to what I said. Actually, I should probably take a moment to say something about safety, because you're seeing us do all these crazy things up here on stage and you're wondering, wow, that really looks cool. I'd like to do that. No, no, no, no, no. I've been working with liquid nitrogen for 50 years. I get safety training on a regular basis. So if you want to do this, you get the safety training, you learn how to use liquid nitrogen and you can do cool things like this too. In fact, if you're young, and you think this is cool, and what could be more cool than the things that we're seeing here, then you should grow up to be a scientist and learn how to do all this stuff. And what scientists do is play with cool toys all day. Well, true enough. Now let's, let's go back to stuffing the balloons in. So we'll take this last balloon, those funny little ears on it. Again, I just squeeze it and stuff it in. But, but we got, we got three green balloons and three white balloons in there. But now what I want to do is to pause for just a moment and talk about how cold this stuff is cold enough that it boils when you pour it out on the, on the ground because the ground is so much hotter than it. And in order to talk about how cold it is, we have to talk about the temperature scale that physicists like to use to describe temperatures. Now you know that in, in daily life, we generally describe things using the Fahrenheit scale or the Celsius scale. You know, on a cold day, maybe a cold day in Berkeley, it might get down to zero Celsius on a really cold day in Washington, it might get down to zero Fahrenheit. Or even below. But physicists do not like to have temperatures below zero, it just doesn't seem to make sense. So we have a temperature scale we call the Kelvin scale, where the degrees are the same as Celsius degrees, but the zero of the Celsius scale, I mean of the Kelvin scale is the lowest temperature you could possibly have. So there's no such things as negative temperature. Well, it's a little bit of a lie. But, but let's just for the moment say that zero is the lowest temperature you have that's true. Now you might ask why is there lowest temperature. Well remember what temperature is about it's about the energy of the atoms the kinetic energy the atoms. The faster the atoms are going the hotter the temperature the slower the atoms are going the lower the temperature. Well what's the slowest you can go the slowest you can go is stopped. And so there's a lowest possible temperature, when the atoms stop. Now it turns out because of things having to do with quantum mechanics and Heisenberg's uncertainty principle, the atoms never really stop. But let's just say that between us friends that absolute zero is when the motion stops. So on this scale room temperature is about 300 degrees. Let's talk about some cold stuff. Ice melts at about 273 degrees. That's zero Celsius. A really cold snowy day in Washington might be typically around 260 Kelvin ice cream in your freezer, probably 255 Kelvin. Dry ice pretty cold stuff, 195 Kelvin. Antarctica, the coldest temperature ever measured for the air temperature in the earth anywhere on the surface of the earth was 185 degrees. Think of that 10 degrees colder than dry ice 185 degrees. Coldest temperature anywhere on earth. Liquid nitrogen is 77 degrees above absolute zero. This stuff is really, really cold. And it's obvious that with something that cold, it would be a good thing to use to cool down some gas, which is what we were doing by putting all those balloons in. So now let's take the balloons out. I'm guessing that some of the more observant people among you have realized that the volume of the balloons that went into this bucket was considerably greater than the volume of the bucket. And the reason for that is that these balloons have turned into pancakes. These things are like frisbees. I think that was, I think that was the the the sixth balloon that I put in here. They were green and white right you saw them come out. How many, what about red balloons. How about blue ones. How about red again. There's red again. There's orange. There's another orange one. There's a yellow one. There's another yellow one. I loaded that thing up with balloons before we started and I could have probably put 100 balloons in there because every one of those balloons became as flat as a pancake but as you can see those balloons did not become flat because the air went away because once it warmed up those balloons So what's happened is that the gas that was in those balloons condensed and this will happen with any gas. If you put the gas in some kind of a container, put that container in contact with something cold. In this case liquid nitrogen. If it's cold enough, then the gas is going to condense into a liquid or a solid or it's going to stick to the inside of the container and you will not have a gas anymore. And that won't do you any good for making a clock because in order to get this perfect ticking frequency, the atoms have to be floating freely in space, not stuck on to other atoms, not stuck on to some kind of container. So how are we going to cool something down without touching it. This is not going to work. And the fact is there are other problems with this. 77 degrees the coldest stuff you've ever seen if you haven't been in a low temperature physics lab, because the velocity of the atoms goes like the square root of the temperature. That means that the velocity of those nitrogen molecules when they boil off is half as fast as the as the nitrogen in the air in this room half as fast. Well, the factor of two isn't bad, but I have not spent my entire professional career trying to make half fast atoms. What I wanted to do was to make things that were really, really slow. So how are we going to do that and in particular, how are we going to cool something without touching. And the answer has been staring at us from the heavens for centuries, because since the time of Kepler, people have known that the tales of comets always point away from the sun. When a comet comes in from somewhere out in the Oort cloud, and gets close enough to the sun, the dust and gas that makes up the comet heats up, then the pressure from the sunlight pushes the dust and gas out forming a tail that always points away from the sun. And as the comet comes around, the sun goes back out the tail streams in front of it, because it always points away from the sun. We're going to do the same thing with our atoms. We're going to use the pressure of laser light to push on the atoms, so as to make them slow down. And if they all slow down, then that means the gas is colder. So in order to understand how this works, I need to tell you about two important things, one of which we've already discussed. The first one is resonance. If I've got a gas, typically the gas is transparent, like the air in this room. Light passing through the gas is not absorbed. And if the light is not absorbed, then it can't push on the atoms. In order to push on the atoms, I've got to have the atoms absorb the light. If I've got a gas of atoms, the atoms will only absorb the light if the color of the light, which is to say the frequency of light. Light is an electromagnetic wave just like radio waves and microwaves. So it has a frequency, very high frequency, about 10 to the 15 hertz. Unless that frequency is just right, the atoms will not absorb the light. In fact, it better be within about 10 megahertz of the correct frequency, which means that it's got to be right to a part in 100 million. If you were looking at two colors that were different by a part in 100 million, you would never be able to tell the difference between those two colors. But the atoms can easily tell the difference between those two colors because they are so exquisitely sensitive to the color of the light. They only absorb just the right color. The light has to be in resonance with the atoms as to match the frequency of the atoms or it won't be absorbed. The second thing is the Doppler shift. We've already talked about that. If I've got a moving receiver, say the atoms, moving toward some source of light, it looks to that receiver as if the frequency of the light is higher. This effect is very small. It would be a really bad idea if you went through a red light and were stopped by the police to explain to the officer that because you were moving toward the red light that the Doppler shift made it look green. In order for the shift to be that big, you would have to be moving close to the velocity of light. And I think the speeding violation would be worse than the red light violation. So it's very tiny. You could never see this change, but the atoms can because the atoms are so exquisitely sensitive. So now you know what we need to know in order to understand laser cooling. So here's a cartoon of laser cooling. We're imagining a gas of atoms in one dimension. Some of the atoms are going to the right. Some of the atoms are going to the left. And we're bringing in laser light and the laser has been tuned. So it's a little bit below the resonant frequency of the atoms. So it's not at just the right frequency of the atoms like to absorb it. But this atom moving toward the laser beam sees the laser beam frequency shifted up because the Doppler shift. And so it does absorb strongly from this laser beam, which slows it down. And this atom moving the other way, if it absorbed, it would speed up. But you see, because it's moving away, the Doppler shift shifts this laser frequency down as far as the atom is concerned. And so it won't absorb very much. Now, if we bring in laser light from the other side, then this atom sees this laser beam is having the right frequency. It absorbs it and it slows down. And this atom sees this laser beam is having the right frequency. It absorbs it and it slows down. And this works just fine in three dimensions as well. If you bring laser beams in from the top and the bottom and backwards and forwards, then no matter which way the atom goes, it slows down. The atom feels as if it is in a viscous fluid. Imagine that you were in a swimming pool full of molasses. If you tried to move in any direction, you would immediately feel a force that was opposing your motion. The atoms feel the same way. And Steve Chu, at his colleagues at Bell Labs in 1985 when they first did this, understood this feature and named this optical molasses. Now, if that was all there was to it, the atoms would simply come to rest. But things are never that simple. And because of something Einstein taught us in 1905, that we should think of light as being a stream of particles. This beginning of wave particle duality. The laser beams not only cool the atoms, but they heat them as well. When the atoms absorb light, they absorb one photon at a time. When they emit light, they emit one photon at a time. And every time they absorb and emit, they get a little kick. The kick is random because the number of photons absorbed in any given time is random. The direction in which the photons are emitted is random, and that random process heats the atoms. That balances against the cooling process that we already described to reach a final temperature. And this is what the temperature should be. The energy associated with the movement of the atoms is at least as big as the range of energies of the photons that will be absorbed by the atom. What that means is something amazingly wonderful, that the atoms can get down to a temperature as cold as 240 degrees for sodium atoms, which was the first atoms that we laser cooled. 240 degrees. The velocity of sodium atoms at 240 degrees is 30 centimeters per second. The normal velocity of sodium atoms is hundreds of meters per second. And now if we could laser cool them, they'd only be going at 30 centimeters per second. 240 micro Kelvin, 240 millions of degree above absolute zero. This is amazingly cold. So everybody got interested. And then Steve Chu was doing this at Bell Labs. We were doing it in Gaithersburg. Here is a picture of laser beams coming in from all directions onto a glowing ball of sodium atoms. This is about a centimeter across. It's got about 100 million atoms. And the question is, can we measure what the temperature is? Well, this is the method that Steve Chu devised. He said, let's start with the atoms in this glowing ball of sodium atoms with the laser beams on. The atoms are jiggling around with some velocity. We turn the laser beams off. No more molasses. The atoms are moving with whatever velocities they have. They sort of explode in all directions. We turn the laser beams on again after a few milliseconds. And the number of atoms that are left compared to the number of atoms that started tells us what the temperature is. Because the fewer the atoms are left, the higher the temperature has to be. Okay, so they did that. And they got the temperature measured to be 240 micro Kelvin. Now there was a big uncertainty because it's a hard measurement to make. But it was exactly what the theory said was the lowest temperature you could get. Well, we repeated those measurements a little bit later, doing exactly the same thing that they did. And we got exactly the same answer, 240 micro Kelvin. And other people in other places did the same experiments, and they got results consistent with the theory. Until we started to make some more measurements. We started to do things like measure how sticky the molasses was, and it wasn't working out. It was not behaving anything like what the theory predicted it should. And so Paulette said, hey, let's measure the temperature again. We didn't do a very good job of measuring the temperature before. At Bell Labs, they didn't do a very good job of measuring the temperature. Let's measure the temperature. So here's the technique that we came up with. We start with the atoms in the optical molasses. We turn the laser beams off. The atoms are going to drop because there's always gravity. At the same time that the atoms are dropping, they're expanding because of their thermal velocities. They get down to a place where we have another laser beam. When they get to that other laser beam, they light up, just like the other picture you saw, except now this one's separated by a little bit. And if you look at the number of atoms that hit here, which you learn by looking at the brightness that comes from here, and you look at it as a function of time, it tells you what the temperature is. So we did that. This is what it should look like if the temperature was 240 degrees. Now I want to remind you 240 degrees is the lowest temperature you can get. And this gray curve is what it would have been if the temperature was 40 micro degrees, 240 millionths of a degree above absolute zero, instead of 240 micro degrees, 40 micro degrees. Given what our uncertainties were. In other words, we were measuring something that was six times colder than it was supposed to be. We felt like the devils in this cartoon who have seen something that is way colder than the environment in which they are. We had literally seen a snowball in hell. And we figured out three more ways of measuring the temperature just to make sure we weren't making a mistake. People in other laboratories thought we were crazy, so they repeated the measurements and found out that yes, it was true the temperature was way colder than what the theory allowed. And so it seemed clear that what we needed was a new theory, and there was a lot of discussion about what the theory should be, and I wish I had time to tell you about it. But if you're interested, ask me, and I'll tell you about the new theory that came out. But the key thing is that guided by this new theory that was the result of Jean-Dalibard and Claude Coenten, who are now a community in Paris. And Steve Chu was at Stanford at that time. Guided by that theory, we eventually got caesim Adams the Adams we make clocks with down to 700 nano Kelvin. 700 nano Kelvin, think about this this is 200 times colder than what was expected for caesim Adams's CZ madness were supposed to go down to about 130 micro Kelvin. it down to 700 nano kelvins, so about 200 times colder than what was supposed to be possible. 100 million times colder than liquid nitrogen, which is the coldest stuff you've ever seen. 4 million times colder than outer space, the coldest natural temperature in the universe. So when I say the coolest stuff in the universe, yes, literally the coolest stuff in the universe. The coolest stuff in the natural universe is about three degrees above absolute zero, the cosmic microwave background radiation. The thermal velocity of these atoms is one centimeter per second. This is what we were dreaming of. Cesium atoms, the clock atoms, one centimeter per second instead of over a hundred meters per second. What kind of a clock can we make with this atom? Well, here's a reminder of what the old clocks look like. We had a microwave cavity here, a meter away. We have another one. We shoot cesium atoms through here at over 100 meters per second. And because they're going so fast, the time between synchronizing and comparing is only milliseconds. And so it's really hard to measure. So now we've got atoms instead of 100 meters per second, they're less than one centimeter per second. What kind of a clock can you make? And the answer is no clock at all because the atoms drop like stones. If you've got something that is only moving at one centimeter per second, it's just it's going to go almost straight down. And that's what the atoms would do. But fortunately, many years ago, a guy named Gerald Zacharias at MIT had the key idea. He said, what if we had slow atoms? And what if we launch those atoms up through the microwave cavity? And they go up a meter or a couple meters and they come back down after a second or a couple seconds. And we'll have this really long time and we'll make a really great clock. But it never worked for Zacharias because he didn't have slow atoms. But now we do. And in experiments with Steve Chu, did at Stanford that we did with colleagues in Paris, we made the first fountain clocks. And now those clocks are keeping time for the entire world. Here are Steve Jeffords and Don Mikoff at the Atomic Clock Labs at NIST in Boulder, Colorado. They cool season atoms down to below a microkelvin here. They toss them up in the vacuum. They come back down after a second. And these clocks are good to about a part in 10 to the 16. That corresponds to one second in 300 million years. But what if you want to make these atoms hang around for even longer than the one second you get by tossing them up a meter and having them come down? What if we want to put them in a box in a container? What kind of a container would work? Well, you can't use a hot container because it would heat the atoms up. You can't use a cold container because the atoms would stick to it. You can't use any container at all that's made out of material. We use a container that's made out of magnetic fields. And this little demonstration illustrates that. This thing is a spinning top. And this is a spinner spins it up. This is a big magnet. And the magnet pushes up on the on the top to keep it held in space without anything touching it. So let's run the movie. So we're spinning it. And now I'm going to lift it to the point where the vertical force of the magnetic field cancels out gravity. And then this thing floats. And these middle school students were impressed. And I passed my hand under it to show that there's nothing holding it up. And this is actually how we trap our atoms because our atoms are like little tiny spinning magnets just like this thing. But then I say, well, you know, any magician can levitate a woman and then pass a ring around her that carefully avoids the wires that are holding her up. But you never see a magician do this. You see, there's nothing holding this thing up. This is the real deal. This is being held up by a magnetic field. And that's how we hold up our atoms. Now that was the toy version. Here is the actual atoms. This is a ball of cesium atoms about two millimeters across, cooled down to about a micro kelvin. And it's bouncing around in this magnetic trap because we release a little bit off center so that it bounces around. And you notice that as time goes on, it fades out. And that's because the vacuum isn't perfect. So we get some more atoms in there. They fade away after about a second because of the imperfect vacuum. And so that was the first experiment that we did with this particular magnetic trap. We learned how to make the vacuum better. We learned other ways of trapping. Steve Chu made the first optical trap. We made the first optical molasses. Dave Weinlein in our labs in Boulder, Colorado traps single ions. Hartmut Hefner here at Berkeley is also trapping charged atoms, ions. Junyi and our Boulder laboratories traps strontium atoms and looks at a transition in the optical region of the frequency spectrum close to 10 to the 15 hertz and makes a clock that is good to two parts in 10 to the 18. And Dave Weinlein looking at a single aluminum ion does better than a part in 10 to the 18, nine parts in 10 to the 19. This is better than one second in the age of the universe. This kind of quality at a lab like the National Institute of Standards Technology, an agency of the United States government, this is what we call close enough for government work. And it doesn't stop there. My colleagues out in Boulder at Nisten Boulder have done even better with these strontium atoms eight times 10 to the minus 21. This means they can tell the difference in the gravitational redshift that Einstein predicted in the 1920s for a height difference of less than one millimeter. Well, trapping atoms has opened up all these exciting avenues, including things besides great clocks and Einstein again, it offers another connection to Einstein with another way of cooling things. It's a cooling system that you already understand very well. If your coffee is too hot, you blow on it. What's happening? What's happening is the water molecules that are most energetic leave the surface of the coffee. And that means that what's left behind has a lower average energy, which means it's colder. That's how you cool coffee. We cool atoms in the same way. We hold the atoms in a trap. We allow the most energetic of the atoms to escape and the atoms get colder. They get so cold that they can do something that Einstein himself thought was crazy. Back in 1924, he predicted that a certain kind of gas, a gas of atoms like sodium atoms or rubidium atoms, very common. If you got them cold enough and dense enough, something amazing would happen. A large fraction of the atoms would simply stop moving. Now it turns out they don't stop moving because of Heisenberg's uncertainty principle, but Einstein didn't know about Heisenberg's uncertainty principle back then. And when he didn't know about it, he didn't much like it. But the point is that today we would say that the atoms go into the lowest possible energy state that quantum mechanics allows. And then over 70 years after Einstein's prediction, teams at NIST in Boulder, Colorado and at MIT in Cambridge, Massachusetts did it. They got a super cold gas cold enough using evaporative cooling after first laser cooling it. So these guys, Eric Cornell and Carl Weiman in Boulder and Wolfgang Ketterli at MIT, cooled gas in Boulder. It was rubidium atoms. This is the velocity distribution that they measured when the temperature was 400 nano Kelvin. So below a millionth of a degree, too hot still. It produces this nice broad velocity distribution. But when they got down to 200 nano Kelvin, this huge peak comes up. That's the atoms that are moving as little as quantum mechanics allows. And as they cool it even further, this residual broad part almost disappears. So that here is a gas at about 50 nano kelvins. And for many purposes, it looks like zero Kelvin. So we're coming to the end now. We've been on a kind of an odyssey to get to lower and lower temperatures. And that odyssey is illustrated here in this logarithmic thermometer. And by logarithmic, I mean each tick mark on the thermometer represents, in this case, a factor of 10 change in the temperature. We start off here at the surface of the sun. It's not the hottest thing there is, but it's pretty hot, 6,000 degrees, room temperature 300 degrees. I want you to notice that on this scale, room temperature is just a little bit colder than the surface of the sun and liquid nitrogen. The coldest thing you've ever seen is just a little bit colder than room temperature. In fact, outer space, the coldest natural temperature is just a little bit colder than liquid nitrogen, a little bit colder than the surface of the sun compared to this temperature scale. The first measurements with optical molasses at 240 micro Kelvin were colder than outer space by more than outer space is is cold compared to the temperature of the sun. And that was just the beginning. I haven't explained what Sisyphus cooling means. It's how we get to these incredibly low temperatures. Keep pushing the temperature down By 2003, Bose-Einstein condensates were being cooled down to 500 pico Kelvin. Recently, getting a couple of seconds of near zero gravity in a drop tower, these guys over in Germany have gotten down to 38 pico Kelvin. But we're hoping that with experiments in space, we're going to get down to one pico Kelvin. Where we are right now is a trillion times colder than room temperature. And Hager-Mueller is looking forward to doing these experiments in space with these really incredibly cold atoms to do things that we just can't imagine doing on Earth. Well, what has all this given us? The best clocks in national laboratories all over the world, they're using laser cooled atoms to keep time. Those atoms are being used to improve our ability to do navigation, and just beginning to improve our ability to measure what's going on with the gravity field on the earth. We're using these atoms to test whether the fundamental constants are really constant. And people like Hager-Mueller are using Adam and her from me to measure things like like the acceleration of gravity, or Newton's gravitational constant and other fundamental constants, which relate to the next talk, tomorrow's talk, which is partly about the new definition of the kilogram. Doing atom interferometry is one of the ways in which we can now define the kilogram. How does that work? Come tomorrow and find out. Quantum computers. Oh, I'd love to spend an hour telling you about quantum computers. These things are so amazing. Quantum computers are more different from your high-end laptop than that laptop is from the ancient abacus. That's how amazing these quantum computers are. And a lot of those quantum computers are being made with laser cooled atoms. And things that we haven't even thought of yet, maybe some of the young people who are listening today will be the ones who figure this out, the most important and most incredible things. Here is a picture of some of the young people who have joined us a few years ago to pursue this adventure of cold atoms. There are people from all over the world, from all over the United States. I just want to point out Trey Porto and Ian Spielman, Gretchen Campbell and Paul Lett, who are permanent members of the laser cooling group. And so we come to the end, but you know, it's not really the end because there's always something new to learn. Thank you very much. Thank you very much, Bill, for a wonderful lecture. I hope we've all enjoyed it. And one of the weird things about these times is that Bill and I, we actually cannot see anyone in the audience. What we can see is there is a spreadsheet with questions and I very much encourage you to put your questions there. Bill, while you're all thinking about the questions to ask, let me ask a first one, though, because I was just amazed to hear firsthand this incredible story of the problem started hundreds of years ago, perhaps with the longitude problem gearing up in your own work towards the discovery of laser cooling, right? What is it like to see that you're edging closer and closer to where you want to be? Is there a moment of, yes, I figured it out, or is there a slow reduction of doubt? How is it like? Yeah. So, you know, I think a lot of people have this idea that science advances through eureka moments. And it's true to some extent. But my experience has been that it's a lot of really hard work where finally you have figured out what's going on and you can then move on to the next stage. So, for example, take the low temperatures, which was really a revolutionary discovery, the fact that the temperatures were not what the theory had predicted. When we first saw that, our first reaction was, oh, we've done something wrong. We'd better figure out what we're doing wrong. And so we devised all these experiments to try to figure out what we were doing wrong. And when we couldn't see what we were doing wrong, then it started to dawn on us that maybe we were right and maybe the temperature was really low. And then we thought, we'd better try really harder to see whether we're doing something wrong because no one's going to believe us if we tell them that the temperature is this low. Because I talked to a lot of people before we did these experiments and everybody assured me that there was no way that you could possibly get a temperature lower than what we now call the Doppler cooling limit. So we had to be really, really careful. And so there was months of work when we had to make sure that there was just no way it could be otherwise. And then finally, we just slowly, little by little, got more and more confidence, enough confidence that we were ready to publish. Of course, we didn't first publish. What we first did was we called up our friends in Paris and said, you know what's happening? This is crazy. You got to think about it to figure out what is going on. And they did. But you know, sometimes things happen that just make you feel so good. I remember the day that we first magnetically trapped atoms. So people have been talking about magnetic trapping for years. Wolfgang Paul, not Pauli, but Paul in Bonn had been talking about trapping atoms and trapping neutrons for years and years and years. But it never worked. And we finally did it. But the evening that it happened, it was really late at night and nothing was working. We had everything set up. The atoms were slowed down. We brought them down to essentially zero velocity with a little bit of a spread. We turned on the magnetic trap and there would be nothing there. And we couldn't understand what was going on. So we said, okay, let's go out, get some to eat. It's about midnight. We went out, got a hamburger, came back and it immediately worked. And we just took a whole bunch of data and we then realized what was going on was we were working so hard that the magnetic trap was heating up and messing up our vacuum. It's what we call outgassing. You know all about it. And the outgassing was making it so we couldn't see anything. When we went away and let it get cold for a half an hour, then everything worked perfectly. And as soon as we realized that, then we knew just what to do. And we took a whole bunch of data for the rest of the night. It was fantastic. Fantastic. Let me maybe jump right to the questions from the audience. And the way I'm going to ask them is the first question is a little bit off topic, but it is the one that got asked first. And after that we'll get closer to the center of topics of your talk. But the first question is, and I personally don't know how to answer it. I don't know if you know, but it's about the possibility that white holes actually exist. I guess that was triggered by the Einstein in your title. Are they still considered to be likely or is there now evidence piling up against their existence? Okay. So now, first of all, my understanding of what a white hole is, and I'm asking you to help me here is that a white hole has to do with strong localization, that is Anderson localization of light in a disordered optical medium. Is that the way you understand a white hole to be? That's a new one to me. Okay. So tell me what you think a white hole is in the context of this question. Excellent. Excellent. The next question is from Kyle in Australia, actually, and it is plain and simple. What is the strongest atomic clock ever made? Okay. Well, so by strongest atomic clock, I'm going to interpret that to mean the best atomic clock, as opposed to the most robust atomic clock. And we could we could talk about that as well. I mean, one of the things that's happened at NIST is they learned how to make clocks that are really good, that are on a on a chip. So the actual atomic clock is the size of a grain of rice. And there's a little bit more stuff that that has to be for the electronics and the lasers and stuff, but the thing is really small. And these things are now being made by a commercial company. And they're being used in the field. I mean, these things are really robust. Soldiers can carry these things around. So so so as far as as the the the the toughest atomic clock, that may be the answer. They're not super good. But they're a part in 10 of the 12, which is the kind of thing that that we pay $100,000 for in a laboratory, the kind of thing that goes into the the satellites. And it doesn't cost anywhere close to that for this this really compact version. But anyway, let's go to the best ones. So the best ones are, well, you know, they're they're sort of competing because they're almost the same. The the single ions and an ion is just a charged atom or an array in an optical lattice of of individual atoms that are sometimes held in individual lattice sites and sometimes allowed to communicate a little bit with each other because they're held in in planes. And these these atomic clocks are are measured to be accurate. Now, what do I mean by accurate? What I mean is that they're giving the true atomic frequency to about a part in 10 of the 18. Now, that's amazing. I mean, I wasn't sure that I was going to live to see clocks that were good to a part in 10 of the 18. A part in 10 of the 18 is, you know, is like it's about a second in the age of the universe. But that's not the end of the story. The clocks are getting better every time you open fissure of letters, it seems somebody's made some advancement in clocks. So I mentioned clocks that were good to parts in 10 of the 21. Now, these are clocks where they can tell the difference between two sections of the cloud of atoms with a precision of, let's say, a part in 10 of the 20th. That doesn't mean they're accurate to a part in 10 of the 20th. What it means is that that they're stable and that you can you can measure the frequency that precisely. But it doesn't mean it's necessarily exactly right. You got to do a bunch of other things to make sure the frequency is exactly right. But what it tells you is that you're on the road, that you probably can make a clock that's good to a part in 10 of the 20th. So, you know, the answer to what's the best clock, it keeps changing because people keep working and they keep getting better and better. There's a woman at the National Standards Laboratory in Germany, the PTB, and she's putting a string of ions together. So she's got more ions and she's got the advantage of having a single ion, but because they're in a string, they don't talk to each other too much, but you've got more ions. So that means you get more signal, which you can make a better measurement. And so that's looking really good. And the ions are really fantastic because you get just one ion. And it's so isolated that everything just works out so well. And this just keeps getting better. It's just so exciting. It's amazing to see what the field of cooling atoms has made possible. So among the questions that I see on my screen, I'm going to pick a few from the, there's a few that I think are asked by grad students. So dear grad students, I apologize, but I will ask the questions asked by non grad students first because so the next one is how cold actually does the universe get at the end of inflation? That's again, one of those above my favorite, but maybe you know something about that. I don't. I don't know what the temperature is at the end of inflation, but I'm pretty sure it's still really, really hot. And I say that because the thing that I do remember is that about 400,000 years after the Big Bang and inflation happens really early. Okay. So the 400,000 years would be the same, whether I said it was after the Big Bang or after the end of inflation, because the inflation happens quite early. That 400,000 years is when the universe has cooled down enough that the plasma that you had, which was mainly protons and electrons, is cold enough that you can form atoms. And that's when the universe becomes transparent. So when we look back, which we do by having telescopes that can can look further and further away, it's like looking back in history. And you know, this James Webb telescope that just went up is going to allow us to look further and further back when we look back. The furthest we can look back is to about 400,000 years after the Big Bang, because after that the universe was opaque because it was this this plasma of charged protons and charged electrons and light doesn't get through that kind of a medium. So only when things got cold enough that the thermal energy was lower than the energy required to pop an electron off of a hydrogen atom, when the thermal energy got lower than that, then you could form hydrogen atoms and then the universe is transparent, as most gases are. But a plasma is a gas that is not transparent. So I don't know what the answer is, but it was really hot. Yeah, that's also what I remember. Let's move to more. Here's actually one by Joseph Robinson from South Australia. There seem to be some people from down under here. Is it possible to get a miniature atomic clock? Yeah, the answer is yes. These things that I was talking about that are only as big as a grain of rice are being made commercially. Now the whole package is bigger than a grain of rice, but yes, miniature atomic clocks absolutely. And I don't know whether I should say this, but Holger and I are on a panel trying to think about what we should be doing in space for the next 10 years. And one of the things that we're very concerned about is putting atomic clocks into space. And if we're going to put atomic clocks into space, we can't put a whole laboratory. So when you see these atomic clocks that are doing a part in 10 of the 18, it's like a whole laboratory. So we got to take those things, which we know how to do. I mean, we know how to make a laboratory full of equipment. We got to make that into a compact package. It might not have to go down to a grain of rice, but it probably has to go down to a package that looks something like this. And so not only do we have miniature atomic clocks, we're working all the time to make the better atomic clocks smaller. So this is what always happens, smaller, better. This is the way technology has progressed for a long time. Look at your computers. Think about what computers were like when I was a boy. They filled a whole room and they weren't as powerful as your watch, let alone your cell phone. And now we have these things that it's just, it's that big. And it's an amazingly powerful computer. Well, that's the way it's going to go with atomic clocks. I have no doubt at all. Well, here's one that might actually be asked by a grad student, but it's a nice one. What are some things that can become possible if we call to one Pico Kelvin and that's from prime Berkeley? Yeah. Well, actually, I should make you answer that question, because I think you're one of the people who's going to make the best use of a cloud of atoms at one Pico Kelvin, the beauty of one Pico Kelvin is that the atoms will not spread out very much as you just have them sit there. Now, the trouble is if you were doing this on earth, the atoms sure enough wouldn't spread out very much, but they would fall like rocks. But if you're in space, then they don't. So why don't you say a few words about what you would like to do with the one Pico Kelvin cloud of atoms? I cannot do better than you will, but I will say exactly. It's about spreading out. So on the ground, experiments with atoms are limited in two ways. The first thing is they fall to the bottom. That problem is gone if you go to space for obvious reasons. But the other problem is even if your sample doesn't fall, it still spreads out thermally. So we're talking microkelvins. Microkelvins means the atoms move so slow that you can work with them conveniently maybe for a tenth of a second, maybe for a second. Then we've been talking about nano-kelvins. That means 30 times slower. So now you can work with them for a couple of seconds. But at a Pico Kelvin, theoretically, if all the other technology improved as much as we need to, we could work with them for minutes. And now, I think it was that light. So if you ever go to Berkeley, make sure they don't install the motion sensor lights. I just wave my hands. We have them at Maryland as well. One of the mantras of this day and age of quantum information is keep the quantum state alive. An atom as a Pico Kelvin in a space-borne laboratory would keep the quantum state alive much longer than we can do now on Earth. That would be one answer. But I think that they really want to hear from you and not from me. They can do that any day. So let's switch to the next one. Could one use a strong enough magnetic field to cool atoms? Okay. So we often use magnetic fields in the process of cooling the atoms, but it's not the magnetic fields per se that do the cooling. And sometimes when we do use magnetic fields directly to cool atoms, it's not by making them strong, it's by making them weak that we get things cold. So let me explain what I mean. One of the ways in which we start to cool atoms is we start with an atomic beam of atoms, and they're going really fast, maybe 500 meters per second, sometimes 1,000 meters per second, and we have to slow them down. And we slow them down, atoms are moving this way, laser beam going this way, the atoms absorb the laser beam, and slow down. Now the trouble with that is that after they absorb a few hundred photons, they haven't slowed down very much. They have to absorb tens of thousands of photons to slow down enough to be useful to us. But after absorbing just a hundred photons, they've slowed down so much that the Doppler shift takes them out of resonance with the laser beam. We can use a magnetic field to compensate that effect, and this thing is called a Zaman slower or a Zaman cooler. And that allows us to get the atoms down to the place that we can start to use some other tricks, like bringing the laser beams in from all directions, and we do that. Then one of the things we do, and we don't always do the same thing, we can then dump those atoms into a magnetic trap after we've cooled them off. Now they don't heat or cool, because the magnetic trap has this wonderful feature that it is what we call benign. It doesn't heat the atoms up, it doesn't cool to me either, but it means we can keep them for a long time and they will stay cold, as long as we're reasonably careful. But here's the beauty. If I make that trap weaker, then the atom cloud will expand, and in the process of expanding, it will cool down. You might have heard the term adiabatic expansion. Let's say you've got some kind of a spray can, and you spray the stuff that's inside the can, it's cold when it comes out. That's because it's compressed inside the can. When you let it out, it expands, and that process of expansion cools it down. It's because, in a sense, the energy of the atoms is used to push against the thing that's expanding, and it does some work on it, and that reduces its energy. But you know, study thermodynamics and you'll understand it all. And that adiabatic expansion allows us to cool the atoms even more, and you know what? It's going to work even better in space. Well, there's a trick that we use that's almost like adiabatic expansion. It's not going to make any sense. It's called delta cooling, just to not tell a lie. It's almost like adiabatic expansion, except it works faster. And it gives the atoms really, really cold, but by making things weaker, not by making them stronger. Great. Let's move to the next assignment. I see that there are four more questions right now. Maybe we'll get through them. Maybe we won't, right? And maybe there will be more questions by the time we're done. They can come tomorrow. Exactly. Exactly. So Karl App, also from Australia, asks, is it completely impossible to make an atom to stay completely still? Okay. And the answer to that is yes, it's impossible to make an atom stay completely still. And the simple answer is it's because of Heisenberg's uncertainty principle. Now, what does Heisenberg's uncertainty principle tell us? It says that the uncertainty that we have in where something is times the uncertainty we have in its momentum. What is its momentum? It's mass times its velocity. So basically, it's the uncertainty in the velocity. But to get the right units, we have to multiply by the mass. The product of those two uncertainties, the uncertainty in position where it is times the uncertainty in momentum, how fast it's going. That product can never be smaller than Planck's constant. It's actually Planck's constant divided by 4 pi. But that doesn't matter. Okay. So in other words, there's the smallest number that that product can be. Now, let's say that the atom was at rest, exactly at rest. Then that would mean that we would have zero uncertainty in where it is. Because I've got to have the product of the uncertainty in the velocity times the product times the uncertainty in the position. The product of those two uncertainties has always got to be bigger than this quantity Planck's constant. So if one of the uncertainties is zero, the other one better be really big. It has to be infinite. And it can't be because our atom is somewhere. Okay. We know that it's inside our apparatus. We know that, you know, usually we know that it's inside a trap. And so there's no way that the velocity of the atom can be zero. It always has to be some nonzero number. Now, here's the beauty of the experiments we do. We routinely get that uncertainty in the atom's velocity to be just what Heisenberg said the minimum uncertainty could be. So the fact that we actually get them to be the minimum possible, that's astounding. I mean, when I started doing this, that was a dream. Nobody imagined you could do that. And now, you know, because of people like Eric Cornell, we do it routinely. Isn't that amazing? Those were the things that Heisenberg ensured a hundred years ago, thought of as the crazy predictions of quantum mechanics, right? A routine now. Let's move on to the spinning magnet video that you showed. I love that, right? And Kahnep also from Australia again asked, would it be possible to do this in a normal environment? And I read this as, how can I do this? Okay. And the answer is yes, you can do it in a normal environment. This is something you can do. Okay. So I'm going to go to that. Now, can you see the video? Right now, we can see you. Oh, okay. Oh, darn it. You see, I'm just wondering whether I can, okay, I can't, I've got to see, I want to show the video. So I went, so I now see the video on my screen. Unfortunately, I'm not sharing it anymore. I don't know how to change that. I'm trying to do it right now. And I'm hoping, trouble is I'm doing it blind. Okay, look, I'm just going to keep, I'll talk while I'm doing this. The thing that I showed is actually a commercial toy. It's called a Levitron. Now, I'm not sure you can still buy it, but you can get it on eBay. And so all the things you need to do this, you can, you can find on commercially. And now I understand that there are new versions of it being made. And okay, I'm soon going to, going to get to show your thing. Oh, dear. Okay, almost there. See, I thought that was okay. So, so isn't technology wonderful? Okay, anyway, you can buy this thing. And you can also make one yourself. But, but the thing is cheap enough. I think that the new versions maybe cost $25 US. So look on your, your favorite online store and put in something like magnetic levitation. And there's a lot of different magnetic levitators. You don't want the one that levitates by, by feedback. You see, you can make something float, but it won't be stable. But if you sense where it is, you can change the magnetic field and keep it levitated. So people actually have levitated platforms that you can put, put really heavy things on. You don't want that kind. You want this one that's a spinning top. So you just have to look around. You can find this thing. The original version was called a Levitron. I don't know what the new ones are called, but you can certainly find it online. And I find that if someone has never done it before, opening the box, it'll probably take you about an hour to get enough skill to make this work. I do it all the time. So I get, I usually get it to work, you know, in about a minute. So, so don't give up. It's not easy, but, but you can make it work. So the answer is absolutely yes. There are two questions about fundamental limitations. And I'm going to ask them all at once to make it harder for you. The first is about the limitations on the accuracy of clocks. One estimate is that it takes 10 to the negative 24 seconds for a photon to cross a proton. That's a fundamental limit on clocks. And the other is, do you think that Femto or even Atokelvin will become possible? Yeah. So I don't see any reason why the time that it takes a photon to cross a proton would have anything to do with the fundamental accuracy of a clock. In fact, what you want to get high clock accuracy is wrong times, not short times. So typically when you, when you make a clock, the quality of the clock is something that is seen after you average for a very long time. And what you hope is that during that very long time that you average, something else doesn't go wrong, which it always does. And so it's those things that are the things that limit how good the clocks are, is the things that are constantly going wrong that you have to control in your laboratory. So let's say that the temperature of your laboratory changes by a tiny bit. It turns out that shifts the clock. So you've got to start controlling the temperature of your laboratory to milli degrees. Now we never thought when we started this thing that that was going to be an issue. In fact, when I first started working on this kind of stuff, it wasn't even something we thought about. We didn't even think about the temperature of the laboratory as being something that affects the clock. But now it's a really important feature. So it's taking care of all of those little things that affect the accuracy of the clock. Now, is there something I'm forgetting, Holger? Would you like to chime in on this thing about whether there's some other fundamental limits to the accuracy of a clock? There is one which isn't as fundamental certainly for us that are living on the ground. But that's actually the subject of the next question. So, well, maybe let's ask that now. Do our atomic clocks affect it due to the change in the Earth's velocity? Okay. And the answer is yes. And it's even worse than that. So there's so many funny little things that are happening. We live on a rotating almost sphere, okay? And we have laboratories that are at different heights above the center of that sphere. And it's rotating. So that means the velocity of any given laboratory is going to be different in principle from the velocity of some other laboratory that's at a different latitude or at a different altitude. And part of that is because they're moving. So that's, you know, there's a relativistic time dilation. And there's Doppler shifts. So you have to take all those things into account if you want to compare two clocks that are at different places on the Earth. Now, it turns out that we know how to do that reasonably well. But there's another one. And that's the gravitational field. So I think this is where you really get into trouble that comes from living on a rotating sphere, not just that it's rotating, but gravity. One of the things that I didn't talk about, maybe I alluded to it a little bit, was that in 1923-24, Einstein told us that clocks will run slower when they are lower in the gravitational potential. So a clock at sea level runs slower than a clock up in the mountains, say in Boulder, Colorado. When I first started at NIST in 1978, the best clocks in the world were good to a part in 10 of the 13. And there were those clocks, well, there was only one of them, it was in Boulder, Colorado. If you had had a similar clock at sea level, then the ability to compare the two things would be at about one and a half parts in 10 of the 13 for the two clocks. That was about the difference in the clock rate between sea level and Boulder. It was about one and a half parts in 10 of the 13. So nobody worried about it because you couldn't possibly see it. Today, what you can see is less than a millimeter. And so that means that, I mean, of course the reason is because clocks got so much better by many orders of magnitude, but what it means is we now have to worry about gravity in a way that we did not before. So somebody like Junyi in Boulder who wants to compare his clock to somebody at a much lower altitude, let's say in Braunschweig at the PTB or in Paris at, I forget what the laboratory's called in France, but anyway, wherever those laboratories are, they have to account for the fact that the gravitational potential is different. And because the Boulder laboratories are right next to these mountains and they don't know what exactly the composition of the mountains is, it's very difficult to make that correction. If you were probably in the middle of Iowa, maybe it wouldn't be so hard, but in Boulder right next to the mountains, it's not so easy. Now that doesn't mean they're not going to be able to do it because we've got satellites that are measuring the gravitational field and we can now use clocks to map out the gravitational potential. So all these things are being worked on, but now we've got global warming and the sea level is rising. And what does it even mean to be at sea level? I don't even know the answer to that question, but we're going to have to figure it out because we have to define what we mean by time. And today, we define time to be at sea level, but that definition was made when we weren't worrying about global warming. Maybe we're going to have to put our clocks in space. Maybe the reference point is going to have to be at one of these, you've probably heard of these Lagrange points because this is where though the James Webb telescope is going, it's a place where Earth's gravity doesn't exactly cancel out the sun's gravity, but it cancels it out enough so that the Webb telescope rotates around the sun at the same rate that the Earth does. So it cancels out sort of the extra change in gravity that the James Webb telescope has because it's further away. But anyway, the point is that these are very special points that are not going to be so sensitive to redistributions of masses on the Earth. And so people are dreaming about that. I don't think anybody knows how we're going to do this, but when you start to talk about things at the part in 10 to the 18 level, you've got to be careful about all these things. And it turns out that your velocities are important, but these other things that have to do with us living on this this gravitating sphere are even worse. Let me jump in with two gravity questions that just popped up on my screen. And I know we're running out of time, but they are so good, I can't not ask them. The first is, can the gravitational mass of a researcher walking towards the atomic clock affect the timing? Okay, so that's a good question. And offhand, I do not know what the answer is. I could probably figure it out, but it would take me, you know, a couple of minutes on the back of an envelope to figure that out. But one of the things I am confident of is that that kind of gravitational field is something that can be sensed by atomic sensors. So the reason I say this is that atom interferometers, which are the specialty of Holger Mueller, that atom interferometers can detect the change in the acceleration of atoms due to putting a, you know, sort of human size mass around the path of the atoms. And groups, the one I'm thinking of is Tino's group in Italy, have done these experiments where they've measured the Newtonian gravitational constant, not quite as well as you can measure it by the more classic means, but getting close. And I know that Mark Kassovich thinks that he can do a whole lot better. And I think you probably can do a lot better, too. And especially in space. So the answer is, yes, these things, you can certainly measure the gravitational effect. I'm not sure if it shows up directly on atomic clock. I'd be surprised if it didn't at the level that the clocks are now at the part in 10 of the 20th level. My guess is you can see that, but I'd have to do a calculation to be sure. And let's make this the final one, and that could be straight out of the NASA panel that you mentioned. If current clocks are so sensitive to gravity, could they be used to detect gravitational waves? Yes. And the answer is yes. And this is one of the things that I find so amazing. A couple of decades ago, people asked the same question, and the answer was no. And now the answer is yes. That was asked by Bill and Tallahassee. So you might have been observing the NASA panel, Bill. The one in Tallahassee, I mean now. Great. But that one comment from the audience that I see here is great lecture. Thanks so much, and I cannot say it in any better words. Well, it's been a pleasure, and the questions have been fantastic. Well, Bill, I think all of us enjoyed the lecture tremendously. And certainly one of the things that did to me was wish that we could have had you out here in person and, you know, met you in the hallway and asked a hundred other questions about this. But it was a great lecture. Thank you so much. And to remind everybody, there's another talk tomorrow at the same time, four o'clock Pacific time. And that lecture is going to be on a new measure, the revolutionary quandary form of the modern metric system. So Bill, on behalf of the Hitchcock Committee, on all of the staff that have worked on this, who I want to thank personally, especially Jane Fink. Thanks so much. And we look forward to your lecture tomorrow. And please, please join us in Berkeley when all this is over and we're back to whatever new moral is. Well, I'm looking forward to being in Berkeley in person. Maybe we can figure out a way of having a lecture where a whole bunch of school children come in person and be one of the whole bit with the liquid nitrogen and. Well, thank you so much for from all of us. And we look forward to hearing you tomorrow. It's been a pleasure. See you tomorrow. See you tomorrow. Bye bye.