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 were titled the Charles M and
Martha Hitchcock Lectures in 1932.
Thanks to a generous gift from Dr. Hitchcock's daughter,
Lillie Hitchcock Coit,
and of course Coit is a very famous name in this area.
Biochemist Linus Pauling,
astrophysicists Stephen Hawking and oceanographers
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 Dr. William Phillips,
our Hitchcock lecturer for this semester.
Dr. Phillips received a BS in
Physics from Juniata college in 1970,
and his PhD from
Massachusetts Institute of Technology in 1976.
After two years as a Chaim Weizmann post-doc 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
in 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 in 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 the development of methods to
cool and trap atoms with laser light.
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 very kind introduction.
Well, as you've heard,
I'm from a place called the Joint Quantum Institute,
which is joined between the University of Maryland
and the National Institute of Standards and Technology.
At NIST I'm a member of
the Laser Cooling and Trapping Group.
Gretchen Campbell is now the leader of that group.
The other members, Paul Lett,
Trey Porto, Ian Spielman, Eite Tiesinga,
Charles Clarke and Nicole Yunger 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.
At the Joint Quantum Institute,
we've had a physics frontier center
from the National Science Foundation.
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 ultracold atoms.
As you've heard, I shared the Nobel Prize in 1997,
and the people I shared it with where
Steven Chu and Claude Cohen-Tannoudji
and they have much more of that story to tell.
The fact is that there are research groups
all over the world who are pursuing
what I really considered to be
an adventure in ultracold atoms,
and some of the greatest work
is going on right here at Berkeley.
Ehud Altman and Norman Yao are doing
theory relating to ultracold atoms.
Hartmut Haffner, and I'm so
pleased to say that Hartmut is
an alumnus of our Laser Cooling
and Trapping Group, at NIST.
Dan Stamper-Kurn, Holger Muller and
Holger is formerly worked with Steven Chu.
So you see we really have
a community here in this cold atom business.
Time, Einstein and the Coldest Stuff and the Universe.
What does time have to do with Einstein?
Time put Einstein on the cover of
its magazine as the other person of the century.
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.
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.
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.
Part of that understanding
is what is called the relativity of time,
the fact that time changes
depending on who's looking at it.
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.
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?
The answer that Einstein gave to
that question might seem superficial to you.
What Einstein said was,
time is what a clock measures.
While that may seem superficial,
by taking that idea seriously,
Einstein came to his profound understanding
about the nature of time.
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 allow 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.
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 Christiaan Huygens decided
that this could make a clock.
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.
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, which was a fortune at that time.
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.
In the year 1707,
a famous shipwreck occurred off
the Scilly Islands in England,
and three ships went down.
Over 1,600 sailors were lost,
including the admiral who was in command with his 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.
The prize was for somebody who could figure out what
the longitude was to within half a degree,
which is about 30 nautical miles.
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,
4:30 or something like that in California now,
and it's 7:30 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.
A navigator could synchronize their clock to
what the time was in Greenwich
and then determine what the
local time was any place 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.
People as famous as Newton thought that
you'd never make a mechanical clock
that would do the job.
They 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.
You've got to read this book.
This is a book about this wonderful adventure.
The 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.
A voyage 47 days,
this clock was good to 39 seconds.
It easily won the longitude prize, £20,000.
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.
Captain Hook, one of the greatest explorers in history,
got one of the first copies of
Harrison's chronometer, and he loved it.
He used it 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.
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.
So I went to visit them to see what the
latest was in clocks.
As I was walking with my host down
the hallway to his laboratory,
we passed by a door.
On the door it 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.
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.
Here's 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 atoms that are in atomic clocks.
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.
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.
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 core,
it's got a valence electron.
When you send in microwaves at
a frequency of about nine 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.
The basic idea of an atomic clock
is you shine in 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.
This is nine billion,
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.
You may say 100,000 for a clock,
that's a lot of money, but think about it this way.
You spend 1,000 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 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.
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. Here's how it works.
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.
All of these clocks are synchronized.
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.
As they broadcast that information,
the information takes a certain amount of
time to get to the receiver.
When you receive information from
this satellite telling you what time it is,
it tells you the time it was when
the signal left the satellite.
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.
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.
That means 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.
If you had a clock, that would be all you would need,
but you don't, so we need a fourth satellite.
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.
People are using these everywhere.
I use them just to find my way to
a local car dealer, taxis, delivery trucks,
commercial aircraft and ships, military vehicles,
even people who 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.
Now you may ask, what did these clocks look like?
Well, I found another ad in a 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."
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.
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.
If we 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 the cesium in there,
it's heated up the atoms,
evaporate from the metal,
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 time itself.
The atoms pass through a place where there's microwaves,
we call it a microwave cavity.
What that microwave cavity
does essentially is synchronize
the ticking of the cesium atoms
with the ticking of the microwaves.
The atoms go to the other end of
the apparatus and see the microwaves again,
and what happens here is
that the ticking of the microwaves is
compared now to the ticking of
the cesium atoms and 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 like what would happen if you wanted to
see whether your own watch was keeping the right time.
You'd 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 10^14.
But the reason why they're limited to a part in 10^14 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 of 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.
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 will be a shift in the frequency.
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.
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.
They 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 atom's velocity to the speed of light.
Now a typical velocity, say,
the molecules in the air in the room are
moving 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 10^14,
this is a disaster.
Fortunately, there are all kinds
of tricks to get rid of it.
You make sure that you've 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.
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 10^14.
Then there's Einstein.
Einstein's theory of special relativity says that
the clocks runs slow by the ratio of
the square of the velocity of the atoms to
the velocity of light that's around 10^minus 12.
Again, that may not seem like very much,
but we want to do 10^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.
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.
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.
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.
If we want to make the atoms go more slowly,
we've got to cool them down.
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.
A really cool town,
by the way, some cool stuff happens there.
I'm going to show you that movie, and I'm going
to narrate it myself.
What I'm doing is I've brought
along some really cold stuff
in this thermos bottle,
this thing we call a Dewar.
This is liquid nitrogen.
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.
Liquid nitrogen is that cold.
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.
The idea is, you got something this incredibly
cold then why not use it to cool down some gas?
Here I've got a traditional container
for a hot gas, a balloon.
It's green because green is one of
the colors of the town of Lindau,
one of their official colors.
I'm going to fill this balloon with hot gas.
I'm from Washington DC or just outside of Washington DC.
We're really familiar with the concept of hot air here.
I fill the balloon with hot air.
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 to remember.
It's full of liquid nitrogen.
As I press it down, maybe you can
see the liquid nitrogen is
spilling out because the bucket is full.
I got to push it into the bucket
and I put a lid on the bucket.
Now the other color of Lindau is white,
so we got some white balloons.
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 you blow the balloons up.
Roman, being young and strong,
was able to blow up these balloons,
which I was not capable of doing.
I'm going to stuff
this white balloon into the bucket again.
You can see liquid nitrogen is pouring out.
But let's keep going.
If we want to do something,
we might as well do it with a lot of gas.
Let's fill up another balloon with hot air.
Meanwhile, Roman is blowing up this other 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 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 didn't know who designed these things.
They don't blow up to look like a heart at all.
They look like something that
has got a little ears on it.
But anyway, I'm stuffing that balloon into
the bucket as well.
Why not? Let's blow up another green balloon and get
some more hot air into this bucket.
This liquid nitrogen is 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.
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,
then I really want to say something about safety.
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. I've
been working with liquid nitrogen for 50 years.
I get safety training on a regular basis.
If you want to do this,
you get the safety training.
You'll 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.
What scientists do is play with cool toys all day.
Well, true enough.
Now let's go back to stuffing the balloons in.
We take this last balloon.
Those funny little ears on it.
Again, I just squeeze it and stuff it in.
But 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 ground because the ground is so much hotter than it.
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 daily life,
we generally describe things using
the Fahrenheit scale or the Celsius scale.
On maybe a cold day in Berkeley,
it might get down to 0 Celsius.
On a really cold day in Washington,
it might get down to 0 Fahrenheit or even below.
But physicists do not like
to have temperatures below zero.
It just doesn't seem to make sense.
We have a temperature scale we call the Kelvin scale,
where the degrees of the same is
Celsius degrees but zero of
the Kelvin scale is
the lowest temperature you could possibly have.
There is no such thing as negative temperature.
Well, it's a little bit of a lie.
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 a lowest temperature?
Well, remember what temperature is about.
It's about the kinetic energy of 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.
On this scale, room temperature is about 300 degrees.
Let's talk about some cold stuff.
Ice melts at about 273 degrees, that's 0 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 cold.
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.
Now, let's take the balloons out.
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.
The reason for that is that
these balloons have turned into pancakes.
These things are like Frisbees.
I think that was the sixth balloon that I put in here.
They were green and white. You saw them come out.
What about red balloons?
How about blue ones?
How about red again? There's red again.
There's orange. There's another orange one.
Here's a yellow one. Here's the other yellow one.
I loaded that thing
up with balloons before we started,
and I could have probably put
a 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 they warmed up, those balloons re-inflated.
What's happened is that
the gas that was in those balloons condensed.
This will happen with any gas.
If you put the gas in some 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.
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 onto other atoms,
not stuck onto some container.
How are we going to cool something
down without touching it?
This is not going to work.
The fact is there are other problems with this.
Seventy-seven 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 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.
How are we going to do that?
In particular, how are we
going to cool something without touching?
The answer 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.
When a comet comes in from somewhere out in
the world 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.
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.
If they all slow down,
then that means the gas is colder.
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.
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.
Now, 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 the light.
Light is an electromagnetic wave,
just like radio waves and microwaves.
It has a frequency, very high
frequency, about 10-15 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
apart in a 100 million.
If you were looking at two colors
that were different by a part in a 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.
That's one thing, resonance.
The light has to be in resonance with the atoms,
it has 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.
It's very tiny. You could never see this change,
but the atoms can,
because the atoms are so exquisitely sensitive.
Now you know what we need to
know in order to understand laser cooling.
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.
We're bringing in laser light,
and the laser has been tuned.
It's a little bit below
the resonant frequency of the atoms.
It's not at just the right frequency
that the atoms like to absorb it.
But this atom, moving toward the laser beam,
sees the laser beam frequency
shifted up because of the Doppler shift,
and so it does absorb strongly
from this laser beam, which slows it down.
This atom moving the other way,
if it absorbed it, 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,
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 as having the right frequency,
it absorbs it, and it slows down.
This atom sees this laser beam
as having the right frequency,
it absorbs it, and it slows down.
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.
Steve Chu and 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 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.
Every time they absorb and emit, they get a little kick.
That 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.
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.
Now, if we could laser cool them,
they'd only be going at 30 centimeters per second,
240 microkelvin,
240 millions of a degree above absolute 0.
This is amazingly cold,
so everybody got interested in it.
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 a 100 million atoms.
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 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.
They did that and they got
the temperature measured to be 240 microkelvin.
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 microkelvin.
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.
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 we came up with.
We started 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 is separated by a little bit.
If you look at the number of atoms that hit here,
which you learn by looking
at the brightness that comes from here.
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.
This gray curve is what it would've been
if the temperature was 40 micro degrees,
240 millions of a degree above absolute
zero instead of 240 micro degrees,
40 micro degrees, given what are 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.
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.
It seemed clearly that what we needed was a new theory,
and there were 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
John Dalibard and Cohen-Tannoudji
in Paris and Steve Chu,
was at Stanford at that time,
guided by that theory,
we eventually got cesium atoms,
the atoms we make clocks with,
down to 700 nanokelvin.
Think about this. This is 200 times
colder than what was expected for cesium atoms.
Cesium atoms were supposed to go down to
about 130 micro Kelvin.
We got it down to 700 nanokelvin,
so about 200 times
colder than what was supposed to be possible.
A hundred million times colder than the liquid nitrogen,
which is the colder stuff you've ever seen.
Four million times colder than outer space,
the coldest natural temperature in the universe.
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 100 meters per second.
What clock can we make with this atom?
Well, here's a reminder
of what the old clocks looked like.
We had a microwave cavity here,
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.
Now we've got atoms instead of 100 meters per second,
they're less than one centimeter per second.
What o'clock can you make?
The answer is, no clock at all
because the atoms dropped like stones.
If you've got something that is
only moving at one centimeter per second,
it's just is going to go almost straight down.
That's what the atoms would do.
But fortunately, many years ago,
a guy named Jerrold Zacharias at MIT had the key idea.
He said, what if we had slow atoms?
What if we launch
those atoms up through the microwave cavity?
They go up a meter or a couple of meters,
and they come back down
after a second or a couple of 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.
In experiments that Steve Chu did at Stanford that we
did with colleagues in Paris,
we made the first founding clocks,
and now those clocks are
keeping time for the entire world.
Here are Steve Jefferts,
Dawn Meekhof at the atomic clock labs
at NIST in Boulder, Colorado.
They cool cesium atoms down to below a micro kelvin 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 10^16.
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 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.
This little demonstration illustrates that.
This thing is a spinning top,
and this is a spinner, spins it up.
This is a big magnet.
The magnet pushes up on the top to keep
it held in space
without anything touching it. So let's run the movie.
We're spinning it. 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.
These middle school students were impressed.
I passed my hand under it
to show that there's nothing holding it up.
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, 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 microkelvin,
and it's bouncing around in this magnetic trap.
Because we release a little bit off-center so
that it bounces around.
You notice that as time goes on,
it fades out and that's because the vacuum isn't perfect.
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 Wineland in our labs in Boulder,
Colorado traps single ions.
Heartwood Hafner here at Berkeley is also
trapping charged atoms and ions.
Jun Ye in our Boulder laboratories
traps strontium atoms and looks at
a transition in the optical region of
the frequency spectrum close to 10^15 hertz,
and makes a clock that is good to two parts in 10^18.
Dave Wineland, looking at a single aluminum ion,
does better than a part in 10^18,
nine parts in 10^19.
This is better than
one second in the age of the universe.
This quality at a lab
like the National Institute of Standards and 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 at NIST in Boulder,
have done even better with
these strontium atoms 8 times 10^ 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 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 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, 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.
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.
These guys, Eric Cornell and Carl Wieman in
Boulder and Wolfgang Ketterle at MIT,
cooled gas in Boulder,
it was rubidium atoms.
This is the velocity distribution that they
measured when the temperature was 400 nanokelvin,
so below a millionth of a degree,
too hot steel, it
produces this nice broad velocity distribution.
But when they got down to 200 nanokelvin,
this huge peak comes up.
That's the atoms that are moving as
little as quantum mechanics allows,
and as they cooled it even further,
this residual broad part almost disappear,
so that here is a gas at about 50 nanokelvins,
and for many purposes it looks like 0 kelvin.
We're coming to the end now.
We've been on 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 changing 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,
it's 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 on the surface of
the sun compared to this temperature scale.
The first measurements with
optical molasses at 240 microkelvin
were colder than outer space.
By more than outer space 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 picokelvin.
Recently, getting a couple of
seconds of near zero gravity in a drop tower,
these guys over in Germany have
gotten down to 38 picokelvin.
But we're hoping there was experiments in space,
we're getting it down to one picokelvin.
Where we are right now is a
trillion times colder than room temperature,
and Holger Müller 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 Holger Müller are
using atom interferometry to measure things like
the acceleration of gravity or
Newton's gravitational constant and other
fundamental constants, quantum computers.
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,
then 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 Spielman,
Gretchen Campbell and Paulette,
who are permanent members of the laser cooling group,
and so we come to the end.
But it's not really the end because there's
always something new to learn. Thank you very much.
I was just amazed to hear firsthand
this incredible story of
a problem starts as hundreds of years ago,
perhaps that's a longitude problem,
gearing up in your own VARK towards
the discovery of laser cooling.
But 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 slow reduction of doubt.
How is it like?
I think a lot of people
had 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.
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,
we've done something wrong.
We better figure out what we're doing wrong.
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 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.
We had to be 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,
gotten more and more confidence,
enough confidence that we were ready to publish.
Because we didn't first publish.
What we first did was we called
our friends in Paris and said,
you know it's happening?
This is crazy. You got to think about it
to figure out what is going on and they did.
The next question is from
Kyle in Australia actually
and it is plain and simple.
What is the strongest atomic clock ever made?
Well, 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 talk about that as well.
One of the things that happened at NIST is,
they learned how to make clocks that are really good,
that are 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 has to be for the electronics and lasers and stuff,
but the thing is really small.
These things are now being made by
a commercial company and they're being used in the field.
These things are really robust.
Soldiers can carry these things around.
But as far as
the toughest atomic clock that may be the answer.
They're not super good,
but they're apart in 10 to the 12 which is the thing that
we pay $100,000 for in a laboratory thing that
goes into the satellites
and it doesn't cost anywhere close to that for
this really compact version.
But anyway let's go to the best ones.
The best ones are,
well, they're competing because they are almost the same.
The single ions and
an ion is just a charged atom
or an array in an optical lattice
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 planes.
These atomic clocks 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 to the 18th.
Now, that's amazing.
I wasn't sure that I was going to live
to see clocks that were good to be part in 10 to the 18.
A part in 10 to the 18
is 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 [inaudible] letters it
seems somebody's made some advancement in clocks.
I mentioned clocks that were good
to parts in 10 to 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 to the 20th.
That doesn't mean they're
accurate to a part in 10 to the 20th.
What it means is that they're stable and that 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 to the 20th.
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.
The next one is how cold actually does
the universe get at the end of inflation?
Again one of those that might be great
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 hot.
I say that because the thing that I do remember is,
that about 400,000 years after the Big Bang,
an inflation happens really early.
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 is mainly protons and
electrons is cold enough that you
can form atoms and
that's when the universe becomes transparent.
When we look back which we do by
having telescopes that can look further and further away.
It's like looking back in history and this is
James Webb telescope that just went up it's 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 plasma of charged protons and
charged electrons and light
doesn't get through that kind of a medium.
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.
I don't know what the answer is,
but it was really hot.
That's all something like 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 on that here.
Is it possible to get a miniature atomic clock?
Yeah and 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.
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.
One of the things that we're very concerned about
is putting atomic clocks into space.
If we're going to put atomic clocks into space,
we can't put a whole laboratory.
When you see these atomic clocks that are
doing a part in 10 to the 18
this is like a whole laboratory.
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've 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.
Not only do we have miniature atomic clocks,
we're working all the time to make
the better atomic clocks smaller.
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.
Now we have these things that
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.
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 cool
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 atom sure enough wouldn't spread out
very much but they would fall like rocks.
But if you're in space then they don't.
Why don't you say a few words about what you would like
to do with a 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 as for obvious reasons.
But the other problem is even if your sample
doesn't fall it's still spreads out thermally.
We're talking microkelvins.
Microkelvins means the atoms move so slow that you can
work with them conveniently maybe for a 10th of a second,
maybe for a second.
Then we've been talking about nano Kelvin.
That means 30 times slower.
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.
An atom at a pico Kelvin in a space-borne laboratory
would keep the quantum state alive much
longer than what we can do now on Earth,
that would be one answer.
But I think though 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.
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.
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 is 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 100 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 100 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 zameon
slower or the zameon 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 cool 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 them 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.
If you, let's say you've got some
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 study thermodynamics and you'll understand it all.
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.
It's almost like adiabatic expansion.
It's not going to make any sense.
It's called Delta kick cooling.
Just to not tell a lie,
it's almost like
adiabatic expansion, except it works faster.
It gets the atoms really cold,
but by making things weaker,
not by making them stronger.
Caleb, also from Australia, asks,
is it completely impossible to make
an atom to stay completely still?
The answer to that is, yes,
it's impossible to make an atom stay completely still.
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.
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 4Pi, but that doesn't matter.
In other words, there's a smallest number
that that product can be.
Now, let's say that the atom was 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 uncertainty in the position.
The product of those two uncertainties
has always got to be
bigger than this quantity, Planck's constant.
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.
We know that it's inside our apparatus.
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 non-zero 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.
The fact that we actually get them to be
the minimum possible, that's astounding.
When I started doing this,
that was a dream,
nobody imagined you could do that.
Now, because of people like Eric Cornell,
we do it routinely.
That's amazing. Those were
the things that Heisenberg and Schrodinger,
100 years ago,
felt off as the crazy predictions
of quantum mechanics routine.
Now, 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^negative 24 seconds for a photon to cross a proton.
That's a fundamental limit on clocks.
The other is, do you think that femto or
even attokelvin, they'll become possible.
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 long times, not short times.
Typically, when you make a clock,
the quality of the clock is something that is
seen after you average for a very long time.
What you hope is that,
during that very long time that you average,
something else doesn't go wrong,
which it always does.
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.
Let's say that the temperature of
your laboratory changes by a tiny bit.
It turns out that shifts the clock.
You've got to start controlling the temperature of
your laboratory to millidegrees.
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 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.
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 that isn't as fundamental,
certainly for us, that are living on the ground.
But that's actually the subject of the next question.
Well, maybe let's ask that now.
Do our atomic flux
affected due to the change in the Earth's velocity?
The answer is, yes,
and it's even worse than that.
There's so many funny little things that are happening.
We live on a rotating almost sphere.
We have laboratories that are at
different heights above the center of that sphere.
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.
Part of that is because they're moving.
There's a relativistic time dilation,
and there's Doppler shifts.
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.
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 gravitating.
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.
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 10^13.
Well, there's only one of them,
and it was in Boulder, Colorado.
If you'd had a similar clock at sea level,
then the ability to compare the two things would be at
about 1 1/2 parts to the negative 13 for the two clocks.
That was about the difference in
the clock rate between sea level and Boulder.
It was about 1 1/2 parts in 10^13.
Nobody worried about it because you
couldn't possibly see it.
Today, what you can see is less than a millimeter.
Of course, the reason is because clocks cut 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.
Somebody like, Jun Ye 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
is called in France.
But anyway, wherever those laboratories are,
they have to account for the fact that
the gravitational potential is different.
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.
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.
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.
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,
you've probably heard of
these Lagrange points because this is
where 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.
It cancels out 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 that the part in 10^18 level,
you've got to be careful about all these things.
It turns out that velocities are important,
but these other things that have to do with us,
living on this gravitating sphere, are even worse.
Let's make this the final button,
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?
The answer is, yes.
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.
Now, the answer is, yes.
Well, that was asked by Bill in Tallahassee.
You might have been observing the NASA panel,
Bill, the one that Tallahassee.
Great. One comment from
the audience that I see here is, great lecture.
Thanks so much, and I cannot say it any better for us.
Well, it's been a pleasure and
the questions have been fantastic.