Good morning everyone! I'll speak about time measurement.
To talk about measuring time, I will ask an obvious question, which is:
what time is it?
It may seem like a trivial question,
but if we performed an experiment today, and had everyone looking at their watch,
everyone would have a different time from his or her neighbor.
Particularly, you would have a different time from the one
on this rather strange clock.
This is a clock that gives you the atomic time.
I wasn't able to bring an atomic clock along with me today.
The atomic time is built at the Paris Observatory.
It is broadcast by radio waves, and here, we receive that information.
If you compare the time - that's the date, November 27 -
to the time here, you see there's a difference.
My presentation is not only going to explain to you how to reset the clock.
I am going to explain to you how to measure time with high precision.
You will see that the magnitudes in the precision are astonishing.
To provide you with some original and fascinating applications,
I'll start with a very simple thing. To measure time, we use a ruler.
I use the analogy between a time ruler and a spatial ruler.
To measure a distance, you take a ruler that's been calibrated.
You'll count the number of graduations, for example, in centimeters,
so if you count five graduations,
assuming that one graduation equals one centimeter,
you will deduce a length of five centimeters.
With time, it's going to be the same. We are going to use a temporal ruler.
A temporal ruler can take the form of an oscillator.
An oscillator is a physical device
that gives you a periodic signal with timing
- whose parameter is reproduced in a periodic way with time.
I brought one with me, Professor Calculus's pendulum.
This is an oscillator.
As you see, we can count time by counting the number of round trips.
If we say a round trip takes one second, we can count one, two, the time passing.
Using this time ruler,
which elementary calibration - called period -
we are able to measure time.
We can imagine that if we want a better precision,
we will need more graduations.
It is equivalent to what we have with a ruler.
If your ruler, instead of being calibrated in centimetres,
is calibrated in millimeters,
and that you measure 51 - tiny millimeter calibrations -
you can assume that your length is 5.1 centimeters.
It is exactly the same for measuring time.
If you take a faster oscillator than the one I showed you,
therefore, having a shorter period, meaning a frequency,
a greater number of pulses per second
you will achieve a much better resolution in time measure.
This is what has led research
since the invention of time measure with oscillators.
Typically, given the magnitudes, you take a mechanical clock,
it doesn't need to be a Swiss clock,
just a mechanical one, similar to my little oscillator pendulum,
it beats at a pulse per second,
so you don't have a huge precision, when you want to measure a second.
If you take an oscillator that you carry with you,
like your watch or your mobile phone.
It has a quartz oscillator using piezoelectricity,
it has the shape of a vibrating diapason at a millimeter scale.
It is going to beat at 32,768 pulses per second.
You are going to cut one second, into 32,768 small elementary periods.
Why such a weird number?
Because it is easy to divide by two, 15 times,
to get to one pulse per second, and get the tick-tock of your watch.
And if we go to the ultimate, to the fastest oscillators known today,
called lasers - oscillators, in the optics field -
you see that a laser is an oscillator that gives you an electromagnetic wave
beating extremely fast until it cuts
your second to 500,000 billions little pulses.
You can see the elementary calibration is extremely small
We will count 500,000 billion and say that one second has passed,
we will count again 500,000 billion etc.
You see, in measuring time,
having a frequency as high as possible is what gives you greater precision.
We can say we have almost solved the problem.
Not at all! In fact, what confidence can we have in this measure?
I go back to the example with two rulers.
You buy two rulers, in two different places, different countries, and measure.
Make the experiment and you will see.
It might not be as blatant, but you will see it works very well.
For the same length, you won't have the same calibrations.
So which should we trust?
Which rule should we trust, which one has the right measure?
With the oscillators, whilst measuring time,
we face the same problem.
Your oscillator, two different oscillators,
won't give you exactly
the same number of period for measuring a given duration;
each little calibration is different.
Or, if you take an oscillator,
according to the place, or the moment you use it,
it won't give you the same measurement.
For example, the pendulum I showed you,
whether you use it at the equator or at the poles,
because the oscillation's period depends on the gravitational force,
after one year, you will have around two days' difference
between the measures... that's huge.
Maybe not in everyday life, though two days is significant.
But for the applications I am going to show you,
it is something very annoying.
How do we solve this problem?
This is where we build atomic clocks.
The atom is the solution to this problem,
since the atom is going to be our reference.
What happens in an atomic clock? It is relatively simple.
You still have an oscillator, but we will compare its frequency
to one that is infinitely stable, universal, and extremely well-known.
It is the frequency of resonance to hop from one atomic level to the next.
Why is this atomic frequency very well known?
Well, because quantum mechanics tells us that the states of energy
that is, energy levels between which atoms transit,
these states of energy have
extremely stable and well determined values.
Thus, the frequency of resonance to go from a level to another,
will too be extremely well fixed.
Here you have a photo of the atomic clock
which is at the Paris Observatory.
Today, using atoms which are a bit specific,
since they are cold atoms.
We cool them by laser, to extremely low temperatures,
and we trap them with the laser light,
using optical oscillators beating extremely fast.
We manage to have a precision in measuring time
which is very impressive,
since a clock amongst the best in the world today,
will only go off one second after 3 billion years.
In other words, we are capable of giving
the value of a small graduation, or of the frequency of the clock,
with 17 digits after the decimal point.
As you see, it is an application,
that is very highly impressive, a very high level of stability
and which further more has many many applications.
The first application is the speaking clock.
It is an application which generally speaks to the public.
Where does the speaking clock come from?
It was created at the Paris Observatory in 1933,
At that time, it was the role of astronomers to give the time.
It was not atomic physics yet.
The line of the Paris Observatory always took care of it,
Because everybody called Ernest Esclangon,
who was the Director of the Observatory, to get the time.
Ernest Esclangon had the idea of developing this speaking clock.
There has been several generations of speaking clock.
Today, the speaking clock presented here gives you the time
with 50 milliseconds of uncertainty.
As metrology is an experimental science, we will call the speaking clock.
I have the authorization to keep it connected, it is my privilege!
It is always a risk, in experiments: it might not work.
Clock: It is 5 pm, 7 minutes 10 seconds.
ND: We are going to wait just a little,
But you can see the red lights there.
Speaking clock: It is 17 hours, 7 minutes, and 20 seconds.
ND: There, it works! Thank you!
(Applause)
This application might be harmless, but it is important especially
at the time of changes between summer time and winter time.
If we want to give the time in a more precise way,
we can also use Internet, telecommunication satellites, or GPS, etc.
This is the first application.
The second application, which is very fashionable,
and is also very important
is the use the atomic clocks to test Einstein's law of relativity
that tells you, for the last 100 years, that time is not absolute.
That is, if you take identical clocks,
and you put them in different frames of reference,
which move relative to one another,
or which have different environmental parameters,
you will find and measure differences
between the times and frequencies of the clocks.
This non-absolute character of time is already tested on the ground.
We are going to test it extremely precisely in space,
by installing in a few years, an ultra-precise clock
aboard the International Space Station.
And by comparing the time and frequency of this clock in space
with the time and frequency of clocks situated all around the Earth,
it will become possible to validate Einstein's theory.
Knowing that all modern theories
predict a violation of Einstein's theory.
So there is a real scientific benefit in doing that.
We are going to test various aspects of general relativity.
For instance, we will test a rather interesting property, which says:
fundamental constants are constant.
This is not trivial!
In physics, a whole set of constants is supposed to be constant.
In fact, all modern theories predict these constants vary in time and in space.
We will be able to test this precisely.
We will also test an original effect of general relativity,
time passes at a different rhythm according to the altitude.
For instance, you, who are sitting in the first row,
you do not age at the same speed as those sitting in the last row,
since you are sitting at different altitudes.
But to reassure you, on the length of my presentation
the difference of aging is around one picosecond.
10 to the power of -12 seconds, a billionth of a billionth of a second.
So, there is no need to run up and down, remain in your seats!
We will also test the speed of light is constant.
This is an extremely strong postulate of special relativity:
the speed of light is independent from the frame of reference
against which the measure is taken.
This is an extremely important property,
which is used to measure distances from time measurements.
If you want to measure a distance you use a signal which will propagate,
and by knowing the time of propagation, knowing the speed of propagation,
which is the case with the speed of light, you can infer the distance.
One could say there is no need
for an ultra-stable clock to do that. But yes, there is a need for it.
Light goes fast, at 300,000 km per second.
If you make a nanosecond error, a billionth of a second,
you are wrong by 30 centimeters.
Typically, this kind of application, measuring distances from measures of time,
is used to measure Earth-Moon distance.
By sending impulses to the Moon,
which are reflected by retro-reflectors installed by the Apollo missions
to measure the Earth-Moon distance better than to the centimeter.
When we have a distance to measure, we know where to position.
How do we do this?
With the GPS, for instance,
if you have a cluster of satellites with synchronized atomic clocks,
by measuring the travel time of each wave,
from each satellite to your receptor,
you measure your distance from each satellite,
and by triangulation, you measure your position.
You need four satellites, because in the time-space
there are four coordinates: x, y, z and t,
since time is also needed to position one self in time-space.
You can see that the GPS's applications
are not only to position oneself in one's car
an area where we need a resolution of a few meters.
There are also applications in geophysics.
We will be able to analyze the movement of tectonic plates
with resolutions down to a few centimeters per year,
which is an excellent resolution.
It is interesting,
since from time measurements, we know the functioning of the Earth,
we infer fluctuations of the rotation of the Earth.
It is interesting because historically, it was the exact opposite.
It was the Earth's rotation which gave the hour.
At present it is the opposite.
The measurement of time gives us the fluctuations of the Earth's rotation.
Another thing you might have heard, on the radio or on television,
are the famous intercalary seconds.
As the Earth does not go perfectly round, as we all know,
and the atomic time is infinitely stable,
meaning that both time scales linked to Earth's rotation and to atomic clocks
are going to diverge from one another.
To prevent them diverging too much,
we voluntarily add, at the international level,
a extra second, called the intercalary second.
This means that generally every two years,
either on June 30, or December 31, - December 31 is less bothersome -
one minute is actually made of 61 seconds.
This leap must be made everywhere, all over the Earth.
So as a conclusion,
I would like to show you
time measurement has left the field of astronomy
to land in the domains of atomic physics, and of quantum mechanics.
Since the invention of clocks, around the mid-20th century,
we have gained a factor of 10 every 10 years.
It is really impressive progress.
Each time we improved the precision, we said to ourselves:
there is no need for all these figures after the decimal point.
It is not true.
Each time, an application appeared, 10 years later, 20 years later,
which used that precision.
I think that I'll conclude and say
that those who measure time are ahead of their time
is totally appropriate in this case.
Thank you very much.
(Applause)