6th HLF – Laureate Lectures: Time, Einstein and the Coolest Stuff in the Universe


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6th HLF – Laureate Lectures: Time, Einstein and the Coolest Stuff in the Universe
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Lindau Lecture: William D. Phillips
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Phillips, William D.
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William D. Phillips: "Time, Einstein and the coolest stuff in the universe" At the beginning of the 20th century Einstein changed the way we think about Time. Now, early in the 21st century, the measurement of Time is being revolutionized by the ability to cool a gas of atoms to temperatures millions of times lower than any naturally occurring temperature in the universe. Atomic clocks, the best timekeepers ever made, are one of the scientific and technological wonders of modern life. Such super-accurate clocks are essential to industry, commerce, and science; they are the heart of the Global Positioning System (GPS), which guides cars, airplanes, and hikers to their destinations. Today, the best primary atomic clocks use ultra-cold atoms, achieve accuracies of about one second in 300 million years, and are getting better all the time, while a new generation of atomic clocks is leading us to re-define what we mean by time. Super-cold atoms, with temperatures that can be below a billionth of a degree above absolute zero, use, and allow tests of, some of Einstein's strangest predictions. This will be a lively, multimedia presentation, including exciting experimental demonstrations and down-to-earth explanations about some of today's hottest (and coolest) science.

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it's my pleasure to welcome you to our lino lecture those of you who have attended previous HLS know that as part of the collaboration with the Linda lue Nobel laureate meeting we have one presentation each year at HLF bought given by Nobel laureate normally visits the Linda meetings and in exchange there is one presentation by a Heidelberg laureate that's the Linda meeting in June and this year this was given by les valiant and in return we wind our lecture this year will be given by Merlin Phillips from the University of Maryland he was awarded the Nobel Prize in Physics in 1997 together with Steven Chu a claude cohen-tannoudji for his work on cooling and trapping atoms with laser light and in the spirit of this work people talk about some as you
can see some really cool stuff today and this will not be without side effects with optical and acoustical and for that reason we have to urge you not to come down the aisles during the presentation if for any reason you have to leave the room during the presentation please use the upstairs doors but this is a hazard zone so we're looking forward though thank you for coming [Music] so I'm very happy to be here in in Heidelberg to give the lindahl lecture I was very pleased that the last time that I was in lindau that Vint Cerf gave the Heidelberg lecture and I hope that I can do as good a job giving the lindau lecture as vent did giving the the Heidelberg lecture so this is I think the comic relief for the the Heidelberg laureate forum there's going to be almost no mathematics no computer science but there may be a little bit of fun so time Einstein and the coolest stuff in the universe so I should say to begin with I'm competing against the Sun I realize here that in fact it's true that I'm from the University of Maryland I'm also from the National Institute of Standards and Technology which is the the American equivalent of the of the PTB here in Germany it's our national metrology laboratory and we're part of the joint quantum Institute which is a joint operation between NIST and the University of Maryland and I'm part of the laser cooling and trapping group and the permanent members Gretchen Campbell Paulette and Trey Porto are people with whom I have the pleasure of working every day now of course it's important
when giving these talks to acknowledge the people who give you the money and the Navy has given us money for a long time and the National Science Foundation supports a physics Frontier Center but I also want to say because I think it's
relevant to a number of people here that we have another joint institute which I'm not part of but I work closely with them this is the joint Institute for quantum information and computer science and this is combining ideas from from quantum computing and other kinds of quantum information areas with computer science to really do something interesting and useful and so some of you people may be
interested in in working with this this great organization so let me repeat the
warning that I've posted before there may be some interesting thing happen as some interesting things happening now because I'm not sure how long this talk is going to take so I may run over into the question time I want to invite everyone but especially the the young rabbits to to ask me questions not just after the talk but just any time you see me if you ask me a question I will give you a prize but more than that if you want to talk about stuff then just say why don't we have lunch together why don't we have dinner together why don't we sit together on the boat ride okay so that's what I'm really looking forward to you I've been having a wonderful time some of you we've had meals together and it's been wonderful and so I want to extend that as much as we can so so please ask me questions and you'll get a prize as long as the question is not what's the prize okay so now time
Einstein and the coolest stuff in the universe okay so you might ask well what does time have to do with Einstein well time put on stein in the cupboard
and I think it was a really good choice because Einstein did all sorts of things that really changed our our lives in in important ways and each one of these things that I've noted here would be worth a one-hour lecture in itself but probably the thing that Einstein is most famous for is his theory of special relativity that theory really changed our notions of space and time before Einstein people thought that space and time were like an unchanging stage on which the events of the universe played themselves out like the stage on which I'm standing here but what Einstein taught us was that the stage was part of the action that time and space depended upon what was happening and how people were looking at it and Einstein came to this understanding about the relativity of time by asking himself a question that I suppose people have asked
themselves since the beginning of human time what is time what is this strange thing that is always in the present but will soon become the past how does it turn out that the future becomes the present why is tomorrow always one day away tomorrow tomorrow you know and to answer this question Einstein gave an answer which you may find to be a little superficial he said the time is what a clock measures but by taking seriously that idea Einstein came to his understanding about the relativity of time but if time is what a clock measures then we can ask ourselves what is a clock well for me a clock is
something that ticks something that gives you a periodic set of events the earliest clock is the rotating earth now ancient people did not know that the earth was rotating but they the sunrise and set every day and in that way they were able to tick off days later Galileo discovered that you know the legend is the Galileo was in the Cathedral in Pisa and he was watching the chandelier of the church swing back and forth and apparently he wasn't paying so much attention to the worship service and and he was timing the period of the chandelier with his own pulse and found that whether the chandelier was swinging a lot or whether it was only swinging a little that the time to go back and forth was essentially the same this discovery of the independence of the period of a pendulum on its amplitude led other people to make clocks like this beautiful tall clock or grandfather's clock that you see here using a pendulum as the ticker of the clock now some of you like me maybe some of the older ones because I realize that young people don't wear watches anymore but but but some of you may be wearing a clock on your wrist and inside this quartz watch is a a tuning fork shaped crystal and vibrations of that crystal are the ticking for this this quartz watch so all of these all these clocks have tickers some of these clocks are marvelous works of engineering and apart some of them are just silly like this
cuckoo clock this isn't even a clock it's a refrigerator magnet and yes it's it's right twice a day but that's not particularly useful but all of these clocks are imperfect in one way or
another so so the the swinging of a pendulum is almost independent of how far it swings but it is not at all independent of how long the pendulum is and so the pendulum may stretch or shrink and that's going to change the the period of the pendulum the ticking rate every quartz watch is a little bit different manufactured a little bit differently and it may keep a different time depending upon whether you keep it on your wrist or whether you put it on the bedside stand at night even the rotation of the earth is not constant it is slowed by the tides it's changed by things like earthquakes or changing our ocean currents or storms the fact that the Earth's rotation is so variable was made clear to me in a way that was rather dramatic for me one day I was visiting the US Naval Observatory the Navy of all countries has been interested in timekeeping for navigation for centuries and they've always been interested in clocks and still are today and I was visiting a colleague who was going to show me the latest in clocks at the Naval Observatory and as we were walking to his laboratory we passed by a door and on the door was written director of Earth rotation it sounds like a rather responsible job you wonder what happens if he goes on vacation well anyway the point is that somebody is keeping track of these variations in the rotation of the earth because people like astronomers want to know where to point their telescopes some people are still navigating by the Stars and so this is important to do so all of these clocks are are imperfect in one way or another sorry of what oh yeah
so the question is what forces of nature caused the vibration of a quartz crystal and the answer is the same as the answer for almost everything that we observe in our daily lives its electromagnetism so electromagnetism is is the force that controls almost everything that we experience that isn't gravitational of course gravity keeps us from floating away but almost everything that has to do with the way we live and and the way we operate is electromagnetic of course there are two other forces of nature the weak force and the strong force and they control things that happen in the nucleus but we usually don't see those things even though they do affect us we don't see their effects so the vibration of a quartz crystal is electromagnetic forces the way it works is that you have a feedback mechanism in there and you get the quartz crystal oscillating and there's a feedback that keeps it that keeps it oscillating at the natural frequency of the quartz okay so anyway but it's not perfect because every quartz crystal is a little bit different and temperature and humidity may affect it okay sorry where's the gift oh well later because I view so to come down and ask me later about the gift but the thing is I've got a lot of material to cover and and and I want to make sure that we get to it all because it's all wonderful so okay so among all of these possibilities of different kinds of tickers for clocks the best tickers are autumns so why are Adams tickers well as you know atoms have certain energy levels and in order to go from one energy level to another one of the ways you can do that is to shine in excuse me shining light or microwaves or radio frequency to induce a transition between the levels and the frequency of that light has to match the difference in the energy levels and the great thing about atoms is that every atom of the same kind is absolutely identical to every other atom of the same kind we define the second to be a certain number of vibrations of cesium 133 as far as we know every cesium 133 atom in the entire universe is identical to every other cesium 133 atom so you don't have to worry about the origin of your cesium if you make a cesium clock in your laboratory and you do a good job you are going to get exactly the same frequency ticking as everyone else in the world or in the universe who makes such a cesium clock and that's one of the reasons why these atomic clocks are so good the other reason is that they are very little affected by the environment and we protect them for most of the things in the environment so they really make great clocks now you may ask how good
are these clocks well if you want to buy a retro watch like mine this has a calculator on it it's hard to imagine something more retro than my watch you can get this for less than a hundred euros and it is so good that it will only gain or lose about 30 seconds in a year that's about a part in a million for less than a hundred euros but if you're willing to spend a hundred thousand euros you can buy a commercial atomic clock that will keep time to a part in 10 to 12 now you may say a hundred thousand euros for a clock that's a lot of money but think about it this way you spend a thousand times more money and you get a million times better performance I think that's a bargain but you may still ask yourself why does anybody want a clock that is that good because after all we don't
really 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 in fact I now want to convince you that you are very happy that somebody is keeping track of what time it is to that level of precision for things that matter very much to you in your daily life I found this advertisement in a magazine
once an advertisement for some expensive car and it says if you get into trouble in our car don't worry because help is only ten thousand miles away so I apologize for the ten thousand miles you know the United States is actually one of the original signatories of the 1875 colección de mettre the metre treaty that we agreed at that time that we would keep all of our standards in terms of metric units and we have done that there is there is no legal mile in the United States it's defined in terms of the meter okay and yet we persist in using these foolish units so I apologize for that but the point is that the the ten thousand miles the 1,600 kilometers 16 thousand kilometers that they're talking about is the orbiting altitude of the satellites of the global positioning system so there's a a cartoon of one of those satellites how does this global positioning system work
well here's a here's a cartoon version of the global positioning system there are at least 24 satellites each of which have several atomic clocks on them that are of this quality that you can buy for a hundred thousand euros now the ones that go into the satellites cost a lot more because you know they have to be space qualified but it's the same idea these are things that are good to a part in 10 to the 12 and they're all synchronized so all the satellites are synchronized and now just for the sake of argument let's imagine that you on the earth have a GPS receiver and it has a clock in it it does not maybe in the future it will but just for the moment let's imagine that it has a clock and all the clocks are synchronized so you see all the clocks are reading one now let's animate this and see what happens these clocks these satellites are broadcasting what time it is and that information about what time it is is traveling to the receivers at the speed of light they're also broadcasting where they are so let's see what happens so the broadcast what time it is and because the speed of light is finite it takes a certain amount of time for the information about what time it is to get to you so your clock says that it's 4:00 and all the other clocks say that it's 4:00 but the signal that you got from that clock says it's 1:00 that means you know how far away that clock is from you and you know where it is so that means you know you're somewhere along here so that's great but there's more there's more satellites of course so when you get the signal from the other satellite then you know how far away you are from the first satellite so you're on this curve and you know how far you where you are from this satellite so you're on this curve so you know you're here now if we lived in a 2-dimensional world and you had a clock then that would be all we would need but we live in three dimensional world so you need one more satellite to complete the triangulation and you don't have a clock so you need one more satellites you need four satellites so if you turn on your GPS receiver and it says looking for satellites that's what it's doing it's trying to find four if it finds more so much the better a little bit of redundancy never hurt so this allows you to determine where you are anywhere on the face of the earth to within a few meters this is incredible people use this for all sorts of things the the car that brought me here this morning had a GPS and guided it right well I'm pretty sure the driver knew how to find his way to university possibly but still I think that we're getting to the point that people won't know how to find their way home from the grocery store if they don't have their GPS on and it's all because of atomic clocks and people are using this for all sorts of things commercial aircrafts military vehicles people take them when they're hiking in the mountains so they don't get lost all kinds of things the global positioning system is so good that Earth scientists can use it to study continental drifts if you average for long enough time you can see changes of centimeters in a year that's how good the global positioning system is now you might ask what do
these atomic clocks look like well I found another advertisement in a
magazine it said this was an advertisement for an airline it said that the atomic clock in Braunschweig Germany that's where the PTB is the the German metrology Institute that recently are they set the atomic clock back wonderful second our flight schedules have been adjusted accordingly yeah right this airline is no longer in business but but they're right about this adjustment of the second this is what's called a leap second so remember I told you that the rotation of the earth is changing whenever the rotation of the earth gets out of sync with the atomic clocks which are keeping better time whenever they get out of sync by a large fraction of a second then they reset the the standard time by one second to keep atomic time and Sun time the same because if you didn't do this then it would no longer be noon when the Sun was highest in Greenwich and people would start to get confused about when to have tea in England so there's a big controversy as to whether we should continue to have leap seconds but it makes great cocktail conversation because most people don't know about it but but that's really not the point by the way the last time we had a leap second was New Year's Eve of 2016 that day December 31st you got one extra second in the day I hope you all used it wisely but I'm not showing you this in order to tell you about leap seconds the reason
I'm showing this is to assure you that the instrument in front of which these two dorks are standing looks nothing like an atomic clock I mean isn't this marvelous is this big clock face and then there's this 1940s electronics here is just amazing if you were to go to our atomic clock laboratories in Boulder Colorado a few years ago you would have seen an
instrument that looks like this now inside this long tube is an atomic beam of cesium and because remember cesium is the is the element whose ticking is what we used to define a second and here is the oversimplified version of how it
works so you have a cesium atom and it has two different states well it has lots of different states but I only care about two these two states correspond to the nuclear spin and the electron spin pointing in two different relative directions so if you flip the spin of the electron relative to the nucleus that corresponds to the two different states that we have here and the frequency difference between those two is this that's the definition of what the frequency difference between those two states is that's how we define how long a second is now the way the atomic clock works in this oversimplified version is you put the atoms all in this state you shine in microwaves if the microwaves have exactly the right frequency then the state of the atoms changes and if it doesn't then the state of the atoms stays the same so you just tune the microwaves until it's changing the state of the atoms and now your microwaves are ticking at just the right frequency now that's a grossly oversimplified version here's a version that's simply simplified which means wrong of course you understand but the point is that you have this atomic beam so these atoms are moving along at about 200 meters per second and over the the the region in which you you have the microwaves you irradiate these atoms with microwaves and if the atoms are changing state then the microwaves are the right frequency and if they're not then you have a feedback loop and that makes the corrects the frequency of the microwaves until it's making the atoms change State now here's the point even these atomic clocks are imperfect and the main reason that they are imperfect is because the atoms are moving so fast someone said making an atomic clock is sort of like trying to tell time with a clock that is going past you at the speed of sound and crashing into the wall it doesn't sound so easy but but scientists and engineers have been working on this problem for many decades and have gotten these things so they work better than a part in 10 to the 14 so that's more than a hundred times better than what you can buy if you go into a laboratory you can find atomic clocks that are good to apartment and of the 14 or better why are they limited to a part in 10 to the 14 well because the atoms are going so fast the you might think of it this way the atoms only spend about five milliseconds here in the in the apparatus the apparatus was about a meter long and it's going about 200 meters per second so they spend about five milliseconds the frequency that the atoms see is the Fourier transform of that pulse of of microwaves and it turns out to be about a hundred Hertz wide this is out of a frequency of about ten gigahertz so that means it's a part in in let's see 10 gigahertz is is 10 to the 10 so this is 10 this is a part in 10 to the 8 and we want to do a part in 10 to the 14 that means we have to find where the center of that frequency distribution is to a part in a million of its width that's not easy but these people are wonderful and they can do it to a part in a million but not much better and the Doppler shift well you all know about the Doppler shift right I think one of the easiest examples of the Doppler shift is imagine you are on the shore of the the sea or a lake and waves are coming in and hitting the beach you could time what the frequency of those waves hitting the beach is if you get into a boat and go into the the surf they will hit your boat at a higher frequency if you turn around and come back they'll hit at a lower frequency so when you're moving toward a source of waves the frequency goes up and when you're moving away the frequency goes down this is how we know that the universe is expanding because when we look at distance stars we see that the frequency of the light from those stars is lower than the frequency from the same kinds of sources on the earth and we know that those stars are moving away from us well it works the same way with the atoms if the atoms are moving relative to the microwaves it makes it look like the microwaves have a different frequency and that limits the the the precision it's a horrible effect but there are all kinds of tricks to get rid of it but not perfectly and so at the level of apartment of the 14 we have a problem and another thing is Einstein Einstein taught us that time is relative moving clocks run slow our atoms are like moving clocks this is an effect that's on the order of a part in 10 to 12 but we want to do a part in 10 to the 14 and there are no tricks to get rid of it you have to measure the velocity of the atoms and correct for it and that's not easy to do and that limits the performance to a part in 10 to the 14 and we are the National Institute of Standards and Technology we are not satisfied with a part in 10 to the 14 we want to do better and if we're going to do better the only way that we can do better is to well that just says the same thing that I've already said the only thing that we can do to make it better is to make the atoms move more slowly and making the atoms move more
slowly means cooling them down because the difference between hot and cold is the difference between fast and slow if we have a hot gas like the air in this
room and in fact it's feeling kind of hot to me so I think now let's see unfortunately somehow I'm caught up on my the cord from my microphone let's hope that I can extricate myself from that sure yes and no so the main accuracy comes from the time between the preparation and the time when you make the measurement it's during the time that the microwaves are on we do the detection afterwards so if you if you're doing the detection while the microwaves were on then that would have an effect now there's a more subtle effect that has to do with the fact that your microwave standard itself is moving around and the fact that you aren't always looking at the atoms causes a an extra uncertainty it doesn't actually cause a an error but it causes an additional uncertainty because of the fact that your microwave source is drifting around and there are a number of tricks to get rid of that problem as well but but yes there are a lot of subtle things like that that affect things that's exactly right okay so as I said the difference between hot and cold is the difference between fast and slow or more precisely temperature is a measure of the kinetic energy of the atoms in the gas of which you're trying to measure the temperature it's true of solids as well if we look at the vibration of of atoms in a solid it's proportional to the the temperature that is the energy of that so what we need to do is to cool our gas if we were
able to cool the air in this room it would mean that the the molecules and atoms in the air in this room were
moving more slowly so in order to give you some idea about how cold we want to make these things I've brought along courtesy of friends at the University some really really cold stuff this what I have in a number of these containers here is liquid nitrogen so this container is full of liquid nitrogen now liquid nitrogen remember the major constituent of the air is nitrogen so this is effectively like liquid air when I pour it out here it boils immediately and it's really exciting for the people in the front row and only a little bit more dangerous oh and there's wonderful things going on I wish I had time to explain the Leidenfrost effect to you but anyway this is so cold that compared to it this surface the floor everything in this room is burning hot and if you've got something this cold if you haven't been in a low temperature physics laboratory the chances are this is the coldest stuff you've ever seen so if you've got something this cold it seems quite reasonable to use it to cool down against so now what I've got here is a bucket of of liquid nitrogen let me just refill it a little bit because well yeah because it's it's boil off a little bit so let me let me refill that a little bit and now what I've got here is a traditional container for hot gas and what I'm going to do is to take
this balloon full of hot air and remember I'm from near Washington DC so hot air is a is a staple of our existence okay so I'm gonna put it into the liquid nitrogen to cool it down so as to make the atoms and molecules move more slowly and that way if they're moving more slowly then we should be able to make a better job of measuring them and be able to make better at Sonic locks okay so let's let that cool down and let's see what else we can do now okay so here is another Dewar flask it's basically a thermos bottle it's been sitting out at room temperature all day that means that compared to the liquid nitrogen this thing is burning hot so what would happen if you took a bucket a metal bucket and it heated it up in a fireplace until it was glowing red and then poured cold water into it well what would happen is it would boil over and that's what's happening here okay don't try this at home so let's see what else can I do to show you what's going on okay yes so here I have a nice stretchy rubber band okay now what I'm gonna do is take the rubber band and dip it into the liquid nitrogen I've got liquid nitrogen in here so I'm gonna dip it in here now I don't think you can see what's going on but what's happening is that it's making the liquid nitrogen boil because their aura band is so hot but it doesn't take very long before the the rubber band is cooled down to the temperature of the liquid nitrogen so the boiling stops and when I take it out this thing is frozen so hard that it snaps as if it were a dry twig now all I have to do is to warm it up in my hands and it's a nice stretchy rubber band again this stuff is really really cold and you know if you've got something that's that's that cold then it makes perfect sense to use it to cool down your gas so let's cool down some more gas to make the atoms and molecules move move more slowly so that we can we can study them better because that's the whole point if they move more slowly we can make a better job of measuring them so let's just stuff this in here and and and get the the atoms and molecules to move more slowly okay great now I look in here and I can see that the a lot of liquid nitrogen has boiled off so let's just top it up a little bit now I've got these nice fresh flowers so here's a flower that has been sitting out at room temperature all day it's nice fresh flower so imagine but but compared to the liquid nitrogen this this flower is red-hot in fact as you can see it's even red and so imagine what would happen if you took a fireplace poker and a metal rod and heated it up in the fire until it was glowing red and then took it out and plunged it into a bucket of cold water what would happen it would make the water boil and that's what's going on here so let's let that boil away while cooling down and let's cool down some our gas what because you know if we're gonna do experiments with cold gas we want to make sure that we've got lots of lots of sample so let's let's let's get this get this so that it's nice and cold and we got lots of lots of gas okay yeah okay sometimes I worry that I blow them up too big so they won't fit but this one's going in just nicely okay fine now let's see what else can I do I'm sure that when you were young and you were learning from your parents how to do things in the kitchen that you were warned that you should never ever take a closed container of liquid and put it into the oven well here we have a container here we have some liquid so now we're going to pour the liquid into the container very carefully that's a good amount and now what I'm gonna do is to put the lid on really tight and now compared to the liquid nitrogen this room and everything in it is like an oven okay now let's come back to the flower so I look in here and I see that the boiling has subsided that means that the flower is now cooled down to the temperature of the liquid nitrogen when I take it out the flower is frozen so hard then I can break it like it was
made out of glass this stuff is really really cold and I see that I've got a lot of debt reduce here but that's okay okay okay now so this stuff is incredibly cold and it's pretty clear that if you got something that cold then it makes perfect sense to to use it to cool down your gas to make the atoms and molecules more move more slowly which is what we've been doing with these with these balloons trying to get the atoms molecules to move more slowly so that we can do a better job of of measuring them okay so now I just want to show you one more thing here is a nice nice bouncy rubber ball okay nice nice bouncy rubber ball let's put this rubber ball into the liquid nitrogen to see how how things go where where's the is the thing that I wanted to use maybe yeah okay let's let's just yeah I'm forgetting what I'm doing here so so we'll just okay fine let's let that let that cool down now let's talk a little bit about how cold this stuff is I've said that unless you've been in a low temperature physics laboratory it's probably the coldest stuff you've ever seen to understand how cold it is we need to think about the temperature scale that physicists like to use so you know environmentally we talked about how cold it is in degrees Celsius so I don't know how cold it gets here in Heidelberg in the winter but it probably gets below zero Celsius on a cold day right physicists don't like this below zero stuff so we want to keep all the temperatures positive so we have a temperature scale called the Kelvin scale the absolute temperature scale where the lowest possible temperature is called zero we call that absolute zero now why is there a lowest possible temperature scale well because what is temperature it's about motion right and what's the slowest you can go well the slowest you can go is stopped and so there's the lowest possible temperature and roughly speaking absolute zero is when the motion stops now it's not really true as it turns out because of quantum mechanics and Heisenberg's uncertainty principle even at Absolute Zero the motion doesn't really stop but let's just say among us friends that absolute zero is when the motion stops and so measuring up from absolute zero room temperature where we are right now is about 300 Kelvin 300 degrees above absolute zero ice melts at about 273 the coldest temperature that was ever measured anywhere on the face of the earth is about a hundred and eighty-five degrees above absolute zero ten degrees colder than dry ice 185 degrees above absolute zero my friends this stuff that is so cold that when I pour it out here it just boils and excites the people in the front row this stuff is 77 degrees above absolute zero 77 degrees above absolute zero the the coldest stuff you've ever seen let me see what's happened to our our racquetball is this anyway let's just see how it bounces now it breaks like it was made out of porcelain this stuff is really really cold [Applause] your mother was right you should never put a closed container of liquid in the oven now also I should say that that you know you see me doing all this stuff with liquid nitrogen and and you think all that would be cool to do that yes it is but it's also something that those of us who are doing this have been safety trained in the use of liquid nitrogen so it's not something you should just do casually if you want to play with liquid nitrogen I really encourage that but you do need to to have the right safety training and do the right things now okay so the point was that this stuff is really really cold and we would want to use it to cool down a gas but I think that a number of you have noticed that the volume of the balloons that I put into this liquid nitrogen in order to cool down the gas the volume of the balloons rather exceeded the volume of the bucket by a good bit and the reason is that these balloons have essentially turned into frisbees these things are as flat as pancakes so does anybody remember how many balloons I put in for right and they were red and and purple right what about what about yellow how about white what about another yellow what about another white one how about an orange one how about another orange one so you see I put a whole bunch of balloons in before we started and they're all flat okay let's go back to the where on the there okay so here is what goes on if
you take any if you take any container of gas and put it in contact with something cold this is standard refrigeration then if it's cold enough then the gas will condense into a liquid or a solid or it will stick to the walls of the container and you will not have a gas anymore now the ticking frequency that we want to get from the cesium atom is for cesium atoms that are isolated floating freely in a vacuum if they're stuck on to other cesium atoms or stuck on to a container they are not going to give us that perfect ticking frequency so this is not going to work this is not the way that we can cool down atoms in order to to get them to move more slowly we have to find a way of cooling the atoms without touching them because it's the touching them with something else that's cold that makes them condense and the answer for how we're going to do that has been
staring at us from the heavens for centuries because since the time of Kepler people have known that the tails of comets always point away from the Sun so when the Sun comes in from the Oort cloud I mean when the comet comes in from the Oort cloud and the Sun warms up the dust and gas that make up the comet it pushes on that dust to make the tail and so the tail streams behind the comet but when the comet comes around and goes back out the tail streams in front of the comet and Kepler knew this and he guessed correctly that the sunlight was pushing on the comet tail we're going to do the same thing using light from a laser to push on our atoms to make them slow down and this is what we call laser cooling now the idea of laser cooling is totally
crazy because we all know that if you shine light on something it should get hot and I'm claiming to you that we can shine light on something and it will get cold how does that work well in order to understand how it works you have to understand two things one is the idea of resonance and the idea here is if I've got a gas of atoms or a gas of molecules like the air in this room and I shine light through it the light goes right through it's transparent that's why we can see one another right that's why this laser beam makes it to the to the screen because the frequency of this light doesn't match any of the frequencies that the atoms and molecules in the air like to absorb at but if I send in just the right color so for example let's say that I had a gas of sodium atoms like you see in these yellow street lamps and you shine just the right color of yellow light into it then those sodium atoms will absorb that light and they will feel a force from because the light will transmit force onto the atom it will push on the atoms and if we can make the atoms absorb in such a way that the light is pointing against the way the atoms are moving then it'll make them slow down well how do we do that well it's the Doppler shift which you've already talked about when we move toward a source of light it looks like the frequency is higher and when we move away it looks like the frequency is lower so let's take a one-dimensional simplification of atoms in a gas some are moving to the left some are moving to the right and here we have a laser beam pointing toward the atoms that is tuned a little bit below the frequency that the atoms like to absorb but this atom that's moving toward the laser beam sees it Doppler shifted up in frequency so it's closer to its resonant frequency it absorbs the light and slows down this atom on the other hand moving this way looks and sees that the frequency is lower and it was already too low so it does not absorb the light very much if it did you see it would speed up but it doesn't absorb very much so now you bring in light from the other side and and now this atom goes this way and absorbs light from this laser beam miss slows down this atom goes this way and absorbs light from this laser beam and slows down so no matter which way the atom goes it picks out the laser beam that is opposing its motion and absorbs from it and it works just fine if you do it in in three dimensions you bring in laser beams from top and bottom and backwards and forwards and no matter which way the atom goes it sees those laser beams that are opposing its motion being the ones that are absorbed it is as if the atoms are in a viscous fluid let's hold the question until the end but remember that question okay because I'm getting a little bit worried about how this time is going but I will answer all questions it's if the atoms aren't like they're in a viscous fluid if you were in a swimming pool full of molasses and tried to move you would find that no matter which way you moved that there was a force opposing your motion the atoms feel the same way and when Steve Chu and his colleagues at Bell Labs in 1985 did this they called it optical molasses and the name sort of stuff you may recognize the name Steve Chu because he was the secretary of energy in the first obama administration the first Nobel laureate ever to be appointed to a cabinet post here is a picture of that optical molasses with
laser beams coming from all directions this is a cloud of sodium atoms about a centimeter across with about a hundred million atoms and the question is how cold are they I haven't told you about how we calculate the temperature but it's an easy calculation and you can calculate that the temperature should be as cold as 240 microkelvin 240 millionths of a degree above absolute zero three hundred thousand times colder than liquid nitrogen which boils when I pour it out on to the the ground which is the coldest stuff you've ever seen three hundred thousand times colder than that and so people got really excited about that and here's a picture from our laboratory from around that time but how do you measure the temperature of something like that and the answer is
you measure how fast the atoms are going so you start with the atoms in the optical molasses they're jiggling around they don't go very far because the laser beam stops them if they start going in any given direction but then you turn the laser beams off and now the atoms just expand freely you measure you turn the laser beams on a little bit later and the ratio of the number of atoms you start with the number of atoms you end up with tells you what the temperature is well they did that at Bell Labs and
they found that the temperature was 240 microkelvin the coldest temperature allowed by the theory we repeated those measurements and found exactly the same thing and other people made other measurements with different atoms where you could predict the different temperature and it was all consistent until we started to do some more experiments that weren't quite making sense and when we started to probe more deeply we found out much to our surprise
that the temperatures were much colder than the theory had predicted now this is clearly an a violation of Murphy's Law because we were trying to make the coldest temperatures we could and we apparently made temperatures colder than we could in fact we felt a lot like the
poor devils in this cartoon who have seen the proverbial snowball in hell something much colder than it had any right to be
and so other people confirmed the experiments and it was clear we needed a new theory and there were heated discussions about the nature of that theory and eventually a new theory
emerged claude cohen-tannoudji who shared the 1997 Nobel Prize for laser cooling and his young colleague Jean Dalia Barr figured out what was going on and once we knew what was happening we could adjust our experiments to make them even better and by 1995 we had cooled cesium atoms to 700 nano carbon 700 nano to cope that's 200 times colder than the theory had originally said was possible that is a hundred million times colder than liquid nitrogen which is the coldest stuff you've ever seen it's four million times colder than the Cosmic Microwave Background which fills all of space which you might say is the coldest natural temperature in the universe so when I say this is the coolest stuff in the universe I really mean it it's 4 million times colder than the microwave background radiation the velocity of these atoms is only one centimetre per second compared to a few hundred meters per second and so people have been able
to make atomic clocks in which they take the atoms and they throw them up in the air well not in the air it's in a vacuum and they throw them up about a metre and they come back down after about a second so instead of having 1/100 of a second 10 milliseconds they get a full second and these atomic fountain clocks are the
best clocks that that have ever been
these are the best primary standards we call them fountains because it sort of
like a water jet going up and these
things are now good to a part in 10 to the 16 1 second in 3 million years but you might ask what are we going to do if we want to keep these atoms in a container how are you going to keep the coldest stuff in the universe in a container because if you put it in a hot container it's just going to heat it up and if you put it in a cold container it's going to stick to it so the only way that you can do it is to not use a container that has any material walls at all so let's switch to the camera that's looking at this apparatus and the way we do it is yeah okay what's going on yeah okay right so what we have here is a big magnet and what we have here is a little magnet now it turns out that our atoms are just like little magnets and it's been arranged so that this big magnet will push up on a little magnet and so it should push up in such ways to make this this little magnet float right here if you ever tried this when you were a kid you had a bunch of magnets and you laid them out on a table and you tried to make other magnet another magnet float above it just that's what I'm doing here and if you ever tried that it never worked because what happens is the little magnet flips over and gets attracted to the big magnet but you learn something else when you were a child and that is that if you spin a top that it won't fall over but our atoms are actually a little tiny spinning magnets and so when we spin it we can keep it from flipping over and this thing will float that's how that's how that's how we trap our atoms and that was the toy version here is the real thing there these are a
cloud of cesium atoms that are put into a magnetic trap and they're bouncing back and forth because they were put in a little bit off center and you'll notice that as time goes on these atoms go away because the vacuum isn't perfect well holding atoms in magnetic traps
where in other kinds of traps like laser traps and electric and magnetic traps for ions we can hold atoms for many seconds and and in this way we can make
atomic clocks that are even better here's a picture of juni in his laboratory in Boulder Colorado holding atoms in a laser trap he has made a clock using an optical transition so not oscillating at ten to the tenth hurts but oscillating close to 10 to the 15 Hertz making a clock this is good to two
parts in 10 to the 18 but that's just beginning Dave Whelan also in our
laboratories in Boulder Colorado who got a Nobel Prize for something else has now
made aluminum ions in an ion trap operating as a clock at nine times 10 to the 19 this is less than one second in the age of the universe at the National Institute of Standards and Technology an agency of the US government this is what we call close enough for government work so so we come to the end we come to the
end and we've been on a kind of an odyssey to get colder and colder temperatures as illustrated by this logarithmic thermometer each tick mark is a factor of 10 in temperature and at the top we have the surface of the Sun not the hottest thing there is but pretty hot but you'll notice that room temperature is only slightly cooler than the surface of the Sun and even liquid nitrogen is only slightly cooler than that in fact on this scale outer space the Coda's natural temperature in the universe is only a little bit colder than the surface of the Sun our first experiments with laser cooling were colder compared to outer space than outer space is compared to the surface of the Sun and since then we've been getting colder and colder and I don't have time to tell you about other cooling techniques and bose-einstein condensation that have gotten us colder compared to the first laser cooling measurements then those were compared to the surface of the Sun and now we are at less than one nano Kelvin less than one billionth of a degree above absolute zero and maybe in the future these cold atoms will go into space we now have a cold atom experiment on the International Space Station and we're hoping to get temperatures that go below 1 Pico Kelvin
all kinds of wonderful things have come from this better clocks all of the clocks in the industrialized world now rely on laser cooled atoms so at Pt B and n PL in the UK at NBS at the national metrology laboratories in china and japan they're all using laser cooled atoms to to keep time for the country and for the world these things are being used for tests of some of the most fundamental theories of nature and they're being used for quantum computing so I wish I had time to talk about quantum computing and why and how cold atoms are being used but but we have to come to the and so I want to acknowledge the
wonderful group that I work with and I want to point out this picture was taken a few years ago and back here we have Fred Fred stand up he's one he was a former postdoc in our group now at the [Applause] Leo were were the ones who made all of this work and here's Trey Porto and Ian's film and Gretchen Campbell and Paulette are the the permanent members of the group with whom I'm privileged to work every day and so I want to remind you ask me I will answer we come to the
end but it's not really the end because there's always something new to learn thanks very much [Applause] [Music] my time but but but if we took some questions the only thing we believe missing is the coffee break and what's coffee compared to science so so ask me a question come down later and I'll give you your prize but but ask me a question there was one back there yeah oh well when you shine two lasers in opposite directions they cancel out in some places and they reinforce in others this is what we call a standing wave and in fact we use that feature to make a simulation of a solid with our atomic gases so by putting the lasers together we can make a kind of lattice in space and the atoms get trapped on the lattice sites just in the same way that atoms arrange themselves into a crystal in a solid and so we use that very fact that the laser cancels in some places and reinforces in others in order to make a model of a solid and to study phenomena in in solid state physics yes never because it's we're never satisfied so why not because there are all kinds of both scientific and practical reasons why you want better clocks a scientific reason is that using clocks of this quality we can ask ourselves do the constants of nature change now yesterday we heard about a calculation of the fine-structure constant well you may believe it or not but people have been measuring the fine-structure constant for a long time and one of the questions is does it change with time there is some evidence that back in like astronomical time less than a billion years after the Big Bang maybe the fine-structure constant was different but we can measure in the laboratory whether the fine-structure constant is changing by looking at two different kinds of clocks of this amazing quality so that tests one of the most fundamental things about the way we understand physics whether the constants of nature are in fact constant other things like high speed synchronous communication long baseline interferometry use a very precise atomic clock so in the atomic clock business we say it's like the field of dreams if you build it they will come when we make better clocks people find uses for them let's have young people okay so quantum computing very briefly is that instead of using bits that can be either 0 or 1 we use what we call quantum bits that can be in a quantum mechanical superposition of 0 and 1 at the same time that means in a certain sense they are both 0 and 1 and the only way we can have that feature is to use quantum mechanical objects like atoms and the only way we can preserve the information in those atoms is to have them be isolated from their environment and trapping them for example in these traps that I demonstrated here is a way of isolating them from the environment and they have to be in a pure quantum state and that means they have to be incredibly cold because if something is hot it's not in a pure quantum state and so laser cooled atoms and ions are one of the ways in which we can realize quantum information and Reinert blot over here is one of the most advanced laboratories in the world for using trapped ions as qubits to make quantum computers so ask him about what the latest things are about about realizing quantum information using using laser cooled ions but people also do it with superconducting circuits and people are trying to do it with things like ions embedded in solids so there's a lot of different platforms for making quantum information but my tastes are that the atoms and the ions are the best thing because nature gives them to us perfect and then it's up to us to mess them up whereas the other things we manufacture them so they're messed up to begin with so yes back there [Music] yeah actually it's better than that we don't only catch the things that are a specific energy we make the things be the right energy so if all we did was filter we wouldn't really call that cooling so in fact we compress the velocity distribution we don't judge wow I didn't expect that but remember I told you there would be unexpected loud noises so so so we take a wide range of velocities and make them go to the the lowest energy State now I didn't tell you all the tricks that we use to make that happen but it's not just filtering you have to make the laser beams be just a little bit lower than the resonant frequency and in fact during the cooling process we often change that frequency a little bit so as to optimize the initial capturing process and then finally the final cooling process we adjust the the laser frequency by typically a few megahertz or a few tens of megahertz out of a few times 10 to the 14 Hertz and
that might sound like extremely fine control and it is but it's easy the the techniques for adjusting laser frequencies at that level are well-established and you can actually buy the things that allow you to do that yes okay all the way in the back ah absolutely so here's a funny thing that some of you may already know one of the things that Einstein taught us with his general theory of relativity in 1915 is the clocks that are lower in a gravitational potential will run slower than clocks that are higher in a gravitational potential the effect is a part in 10 to the 16 per meter on the face of the earth so when I first went to NIST in 1978 clocks were good to a part in 10 to the 13 that meant that you could just barely see the difference between Boulder which is about 1500 meters high and sea level if you had had a cloth at sea level which we did not so in other words we couldn't tell today the distance that you can see with the best of these cloths is one centimeter that's how much things have improved in part because of laser cooling now what that means is that because the atoms are affected by the gravitational potential if the gravitational potential changes as a gravity wave comes by then you might be able to see that we're not there yet we need to improve by another few orders of magnitude but Jenny tells me no problem we're gonna have that done before he dies maybe not before I die but but but yes so people are thinking about using these clocks for gravity wave detectors but we're not there yet but but people are using it from map gravity see this is amazing thing in Boulder Colorado there's lots of mountains and we don't know what the gravitational potential the mountains is and so we can't even correct the ticking frequency from Boulder down to sea level because we don't know the difference in the gravitational potential but we will [Music] [Music] okay so the question is there seems to be a problem with dark matter that some observations about galaxies don't fit with the the dark matter hypothesis that was how we first came up with the hypothesis of dark matter that it didn't fit with the the usual gravitational calculations but then there's some some galaxies where the dark matter doesn't seem to fit and so the question is what do we know and the answer is nothing we don't know anything about dark matter but could these clocks help us and maybe I don't know I mean sending a clock to another galaxy is a really hard thing on the other hand it could be that learning something about whether dark matter accumulates around regular matter which is one of the hypotheses we might learn something about that with better cloths but the fact is that the core of what you asked which was is Einsteins theory of gravity right should we modify it that's exactly the kind of thing that we want to study with these clocks so if Einstein is right if we have two different atomic clocks side by side and we move them up and down in a gravitational potential they should change in exactly the same way if they don't there's something wrong with the very core of general relativity which is the equivalence principle and most people think that in order to make gravity the theory of gravity consistent with the theory of quantum mechanics we're going to have to give something and the thing we're probably going to have to give up is the equivalence principle which is so near and dear to all of our hearts and so this exactly but something's got to give because unless we believe that gravity is not quantum mechanical and that's heretical as well to believe that there's a force in nature that isn't properly described by quantum mechanics so something has to give this is an exciting time to be alive because things are bound to change in ways that I don't think anyone can imagine and we're hoping that these atomic clocks will give us a clue about how that is going to happen maybe we should leave it there and everyone who asked a question come down and get a prize from me and and then anybody else wants to ask questions ask me [Applause] [Music] [Applause] [Music] you


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