Okay. Thank you for coming, everybody. I'm Richard on the East Coast, director of this week's module.
And as part of the week we've organised a seminar and delighted to introduce Dr. Martin Saini,
Deputy Chief Executive of the National Physics Laboratory.
And I think his talk will make clear why I was so keen that somebody from a physics laboratory should come and talk on a medical science program.
Okay. So yes, thanks for the introduction and what I'm going to try and do in the next three quarters of an hour or so is three things.
One, I want to give you some idea about why having trusted measurements is quite so important.
The second thing is I want to tell you about how it is, what it is, what,
what exist globally to ensure that we can all have confidence in the measurements that we make or the measurements get made on our behalf.
And the third thing I want to do is to explain to you why 2019 is possibly
the most exciting year in the science of measurement for at least 140 years,
and potentially is the culmination of a project that started in 1791 during the French Revolution.
So measurement is something that we probably all take for granted.
It's something that has been part of human society for at least 5000 years.
Almost certainly what happened was that soon as humans started to interact outside their family groups or their very local groups,
they realised they needed to have some means of exchange for two reasons.
Firstly, for trade and this is some of the earliest archaeological evidence for measurement.
These are church weights that come from the Indus Valley somewhere between two and a half thousand and 3000 BCE.
And they are an illustration of people's obsession with ensuring their very accurate obsession with ensuring that trade was fair,
that you got what you paid for.
The second thing that people probably needed to have measurements for very early on was construction building projects.
Again, as soon as those projects went outside of the local family group.
This is probably the most obvious example of that. The pyramids are a remarkable feat of of construction.
And the way that it worked was the work was distributed by very through a very large number of groups of masons.
Each of those masons had a wooden standard, which was a copy of one of these.
This is a copy. This is a reproduction of the Egyptian cubit.
It was the distance between here and the fingertips of the pharaoh.
I will pass this around to see if I don't. I don't match up to a ferret.
But if you want to pass this around, you can see if you match up to a fairer.
And in order to ensure in order to ensure that these all to ensure that this consistent measurement was used and was being maintained,
every full moon, the heads of the different mason yards had to return and compare their standard to the Master Cubit.
Failure to do so was punishable by death. They took consistent measurement pretty seriously.
Congratulations in in the UK, one of the first places that you have real documented evidence of how important measurement is,
is this document Magna Carta 1215.
In the English speaking world, it's looked back to as one of the documents that the roots of the freedoms that we have.
So in here comes the first habeas corpus.
In here comes a real requirement of people to be trialled by their peers, a whole host of freedoms.
But about two thirds of the way down somewhere down here are these words.
I have to realise that just as important as other things,
having consistent measurement and having confidence in the measurements that being made was important for the functioning of a society.
And I also like and I also think it's interesting that the priorities are wide I'll
and corn and some things don't change today measurement is part of everyday life.
There's lots of pictures up here where we come across measurement and probably take it for granted.
Sometimes we wish it wasn't quite as accurate as it could be on the scales.
Does anybody know who the lady in the top is? Top in the middle there?
I didn't know that lady. Ellie Simmons.
Absolutely right. That's Ellie Simmons. And in 2016 at she in the Paralympics she got a world record for the 200 metre Im six swimming and that
was 2 minutes 59.8 1 seconds now for her and for her competitors and the predecessors and the successes.
In Olympics, it is important that it really was 2.50 9.81 and 2 minutes, 59.8 1 seconds, so that a world record at 2 minutes,
59.80 will be absolutely confidently achieved if that happens in the next time we have these Olympics.
So those are examples where it's pretty obvious that we are involved in a measurement and we're taking a
measurement and it's and we can see how important it is to our lives and the lives of individuals and and so on.
But most of the time, the measurements that are going on around us in our developed society are probably completely hidden.
There's an enormous iceberg of measurements on which we all depend.
Every single day, 3600 billion cubic metres of gas is traded annually was 2017.
That was the figure. 3600 billion cubic metres of gas was traded across the world in 2017.
It's traded not on volume, it's traded on calorific value,
which is a combination of accurate measurement of volume and accurate measurement of the constituents of the gas.
And over the last 25 or 30 years, there have been an enormous amount of very detailed,
very complex, very technical, very scientific research done to help have the standards.
So you can do calorific value with all the different constituents of gas from across the
world at the accuracy you need for trade so that it's completely transparent to us.
Similarly, building construction. This is a Rolls-Royce, Trent 1000 engine.
It has 18,000 components. The 18,000 components come from across the world when they arrive either at Derby in the UK or Singapore,
whether they have assembly plants, these and all those different parts have to fit.
They have to be the right size. They have to be the right shape. They have to have the right materials properties.
They have the right electrical properties. And that doesn't happen by accident.
It is amazing and something that we take for granted that global manufacturing can do this kind of thing across the world.
The other thing about these engines is that Rolls-Royce no longer sell engines.
Rolls-Royce sell power by the hour. So you buy power from the engine.
So when the engine is on, the amount of power you generate is what Rolls-Royce build the aircraft companies for literally thousands of measurements,
a second being taken from this engine. And they are all either downstream immediately or more likely download it when it
lands back to the Rolls-Royce company so that they can do their both billing.
But all the diagnostics on how the engine is performing. We take weather forecasting for granted.
Interestingly, we make a lot of fun of it. The UK Met Office is either number one or number two.
Whenever this any assessment is done in the most accurate forecasting organisations in the world.
The Rolls-Royce, the the Met Office has its headquarters in Exeter and it has two of the largest supercomputers in the company,
has two just in case one breaks down and they both run in parallel four times a day.
A global atmospheric model in order to predict the weather.
That global atmospheric model sucks in 125 million measurements every day from across the globe.
Those measurements have to be consistent.
So one degree in Sydney, one millibars in Toronto has to be the same as one degree in Slough, one millibars in Paris.
Without that consistency, a quite a high accuracy with confidence.
Then the global atmospheric model would just turn out gibberish.
We all take this for granted. We all have satnav in our pocket.
If I bet that most of you us are pretty certain just about everybody has sat nav in their pocket.
Again, something we take for granted that wherever you are in the world, you can find out your position to within a few metres to tens of metres.
That is possible because orbiting the earth are at least 24 satellites.
I think there's 26 at the moment. There has to be at least 24 satellites in the global positioning system constellation.
Those satellites each have on them an atomic clock.
That clock is accurate and synchronised with the others.
Two within 100 nanoseconds, 100,000,000,000th of a second of the global, the universal, coordinated time for the globe.
That is an astonishing feat of measurement on which we all depend when we open up our phones and we take it for granted.
This is the of the table in a radiotherapy clinic in the UK, 330,000 people a year are diagnosed with cancer.
Approximately half of them will have radiotherapy. And if you from a medical background, you'll know better than I.
Radiotherapy in concept is very simple.
You deliver enough dose to the tumour to kill it without delivering too much
dose to the surrounding tissue and compromising the health of the patient.
It is the most demanding application for measurement of ionising radiation.
You need to be able to measure and deliver in the patient on the table in the clinic every time, ideally to within about 3% of your target dose.
That is an astonishingly challenging measurement to have to do.
But I can tell you that if you or anybody you know is ever unfortunate enough to lie on a table like this in the UK,
the dose in the UK is delivered more accurately and more consistently than anywhere else in the world.
And that's the result of about 30 people who work at my institution and their predecessors who have been working with medical physicists,
equipment manufacturers, professional bodies to put in place the infrastructure to ensure that people's lives get saved every single day.
So just a few examples of the fact that beneath the surface of our everyday life,
our economy, our quality of life and in situations like this are very lives,
depend on confidence in measurement that either we make or more often than not is made on our behalf.
So how is that confidence assured?
Well, in 1791, the French Revolution was just underway starting in 1789.
In 1789, there were it was the start of about 100 years, almost 100 years of turmoil in France.
At least two monarchies were deposed, one dictator in the middle, an incredible amount of turmoil.
And this picture, which the famous picture painted by Dell'acqua in the middle of that to celebrate
the the bringing down of Charles the 10th called liberty leading the people.
And in France this is seen as one of the pictures that kind of really summarises and is an icon of that turbulent time of the history.
But at the beginning of that, in 1791, these people, a group of people,
including these two famous mathematicians whose names you've probably come across if you've got a mathematical background,
wrote a paper to the French Academy of Science, and I would leave you for 15 or 20 seconds to read what they said.
Their vision was that you could set up.
They talked only about length here, but they were trying to set up a system of measurement that was for all time and for all people.
I suspect that the somewhat turbulent times that went on between then and the 1870
was one of the reasons that starting to really do something about this was delayed.
But in 1875, 17 nations signed the metre, the Treaty of the Metre Convention.
This is the UK copy. Interestingly, the UK didn't sign in 1875 because the UK was very, very suspicious of European projects.
We signed in 1879 and this set up a framework, the international measurement system, which has continued to evolve since then when it was set up.
They, the French, offered some headquarters for this international measurement system, and they offered it very, very generously in Paris.
And the international community who had signed this said they would contribute to making this this this headquarters fit for purpose.
So this was the building that the French offered to the international community.
It was in a little bit of an I suspect to the international community, was a bit shocked about quite how much they had to spend.
This is what it looks like today. It's on the outskirts of Paris.
It has several servers, probably more famous for its porcelain than it is for measurement,
but it overlooks a server, a factory, server, porcelain factory.
And it's the Bureau Internationale de Marzio,
and it's the headquarters of probably the single largest science and technology cooperative activity on the planet.
So this is how we ensure that there is some confidence in measurements that we make or get made on our behalf.
If you want to make a measurement, if you're a business or government and society and you want to make a measurement in which you can have confidence,
there are three or four things that you need. You need to calibrate an instrument.
Unless it's trivial, you need an agreed procedure. How are you going to make the measurement?
And again, unless it's trivial, you need a trained practitioner about making that measurement.
And if it has any regulatory component to it, you need a fourth thing.
You probably need some kind of verification or accreditation. There are two fundamental concepts that enable that to happen.
The first is called traceability, and that is an unbroken chain of calibration from the measurement that's being
made back to special institutes in the in the members of the data convention,
national metrology institutes and images.
And in the UK, NPL where I work is the National Metrology Institute for the UK and we are that top of that chain for most measurements,
certainly all physical measurements. There is an organisation called LGC who are the top of the chain for chemistry measurements designated for them.
And there's an organisation in Scotland who at the top of the chain for flow measurements,
but to a large part that one organisation, the enemy responsible for this.
Now these chains can be quite long.
If you take a ruler and you measure the width of an alcove in your house so that you can order a couple of the internet and be sure it fits.
There's probably half a dozen steps between your ruler and the National Metrology Institute,
but if you lie on that table in a radiotherapy clinic, there's nobody between you and NPL.
We collaborate directly.
We interact directly, partly because there's not much difference between the uncertainty that we can get and the uncertainty they need.
And partly because it's absolutely life critical and the thing that sits at the top of the chain on national or primary standards,
they are realisations, they are artefacts or ways of realising the measurement at a national level or at an
international level that are demonstrated to be at the highest and highest accuracy.
Now, how do you the second concept is called equivalence, and that's coordinated by the headquarters of the international measurement system.
And it's the domain solely of these national metrology institutes.
It's a complex set of calibrations and checking of quality systems,
of comparisons to demonstrate that the top of the chain in each of these countries is comparable equivalent within the uncertainties of the.
Quote, some countries, quote, higher uncertainties than others. But the uncertainties have to be able to be demonstrated.
The measurements have to be equivalent within the uncertainties. I said these are complex, these comparisons because of the checking,
because of the importance, because of the complexity, because they are done at the most.
The highest level of measurement can take years, sometimes a decade, to complete and demonstrate that that is the fact.
And so there's constantly a rolling cycle of these measurements across the whole of the system.
And above that, there is the general conference of the meta convention,
which is the the meet the conference that happens every four years and is when all of the member
states to get together to make significant decisions about the global measurement system.
Between the conference,
they delegate the running of the global measurement system to the International Committee of Weights and Measures Caps Committee,
International Diplomas Year in French.
And that's 18 people who are responsible for operating and overseeing the global measurement system between conferences.
And I'm privileged to be one of the people sitting on that committee.
Now, if there were no new activities that humans undertook,
if there was never any new science and technology, if there were never any new global challenges,
this system could be set up and it would run like clockwork, and national laboratories would just be engaged in this.
And indeed, there's a large percentage of the national laboratories,
national metrology institutes in the world who just do this for their nation, particularly in the emerging nations.
Nothing else. But there are new challenges.
There are new activities. There are new problems.
There's new science and technology. So the most advanced national metrology institutes of which I'm pleased one.
Work with academia and business to futureproof the system. To put in place new science,
new technology to verify data and to have world leading facilities so that as science and technology and global challenges appear,
the measurement system is there, ready to assist.
And at the heart of this whole system is the set of base units, the set of units that enable you to have consistent measurement around the world.
There are seven of those. The number increased slowly over a period of time.
But since 1971, there have been seven kg, the unit of mass mater, the unit length.
Second, the unit of time and per unit of current. Okay, the unit count kelvin the unit of temperature mole, the unit of quantity of matter,
and Candela about which I will say very little in this because it's a very, very curious unit that is solely related to human perception of light.
However, these are the units and all measurements within the system can be expressed in combinations of those.
So we will probably be familiar with velocity metres per second.
Energy, which has a special note in the joule is actually kilogram metre, second metre squared second to the most to resistance.
The ohm is kilogram metre squared, section two minus three up to minus two.
And for those who are chemists, Catholic activity, concentration, whatever that is, kettles per cubic metre is this mile per second per cubic metre.
So those enable you that, that framework enables you to have a way of measuring everything.
And the PSI is incredibly successful. There are 60 member states of the convention.
There are 42 associate members of the metre convention.
And then through regional organisations, the the different regional organisations you can see on this picture,
there are other nations who are aspiring to be part of the metre convention who are engaged at a regional level as well.
So 98 point something percent of GDP of the global GDP is encompassed for the PSI system.
So it is an incredibly successful system.
It's very nearly utterly lipophilic for all people, but it's a bit messy.
We don't really need all those units if you really if you just wanted to measure anything,
if you are a mathematician or a physicist and you were saying, we really don't need all of these, what's the minimum number of units we need?
We there are three and we really don't need the Candela is how bright the lights appear to humans.
That's what it's measuring.
It's a unit based on human perception, and it's basically defined at one wavelength of like near the sensitivity of the eye.
And it's just a conversion factor from a measured measurement of light intensity per the radiant
to how it appears to the human eye in terms of how candles appeared in Victorian times.
It's a very strange unit. It's really important for the lighting in the health and safety industry, but it is a strange unit.
The mole basically links chemistry to the PSI.
It's the number of it's an avogadro number of elementary entities and it's the number of atoms in 12 grams of carbon 12.
It basically links what's going on atomic scale to what's going on in a macroscopic scale.
It's incredibly useful and it's absolutely embedded in the whole of chemistry and increasingly in biology, but in principle, you don't need it.
And finally, Kelvin, temperature is a fascinating measurement.
Temperature was measured before anybody really knew what it was, as temperature is actually the average energy of molecules,
the average molecular energy of the molecules interacted in an object.
And so you you can measure average energy, but because temperature is something again that you want to have macroscopic,
this is a way of linking a macroscopic measurement down to a microscopic activity.
The energy of molecules and the scale is set at the moment by one point, the triple point of water,
which is just defined as 273.16 Kelvin and the whole of the scale bootstraps off of that.
So it's a bit messy. However, it's completely impractical to abolish those unnecessary units.
They are all embedded. The whole of global manufacture, global trade, science, technology,
everything has these things so embedded in them that it's impractical to get rid of them.
More serious problem is that some different.
Missions explicitly limit how you can realise a primary standard.
Those things that sit at the top of the chain and are the absolutely most accurate way that you could
move with measurements below which all the other measurements are flow from them and are less certain.
Some definitions limit how you can realise a primary standard the most accurate measurements.
And one definition is an artefact. And I'll tell you why that's a real problem at the moment.
But these things, one or both of those create an absolute limit to reducing the uncertainty of measurement.
So that can limit the applications of science and technology and trade.
We can find that these limitations on measurement are actually a barrier to science moving forward or technology moving forward,
or people being able to address global challenges.
So a little bit about that more in a moment. So the kilogram is one artefact left,
and it illustrates both of these things about both about the problem with an artefact and the problem of you can only define it in one particular way.
This is the definition of the kilogram and this is it.
This is how big it can't get a sense of scale. So I brought one that's not a real kilogram.
So because they're made of platinum, iridium, so it's a stainless steel plank.
But if that's to give you an idea of the size.
And certainly nobody would let me loose with anything that's even close to one of the copies of the copies of the kilogram.
But this is the master KG. It's it's held at the headquarters of the International Measurement System in Paris.
It's in a safe in a vault in the basement of a building.
And at least once a year, there's a very strange ritual where the president of the committee, the international committee UPM,
the rector of the laboratory by PM and somebody from the French Academy of Science bring their three independent keys and they open the this,
they open the basement, they open the vault and they open the safe, and they confirm that the kilogram is being kept.
And last year I was at this it's a it's a very interesting table.
So I saw the kilogram sitting in. There also are these six, which were six copies made very early.
And they're the ones that get used. The kilogram gets used maybe once every ten years because it is the definition of the kilogram.
So if it gets scratched, we all weigh more.
That's. And the other problem with artefacts is they're not for all time.
So this is an interesting graph.
This is 1889 when the kilogram was created with a kilogram and there were some copies made that in 1946, a couple of other copies were made.
So there are now these official copies one, two, three, four, five, six, and they are regularly into compared.
And so because the kilogram is the definition of the kilogram, it's always right.
So this is how far over a period of time in 2014 was the last time it was done.
You can see. So they've drifted apart by around 60, just over 60 micrograms now because the kilogram has to be right.
It's got a be. But so all these things really for all time there's a that's a problem about artefact
even one that's kept and controlled and looked after in the way that the kilogram is.
The second problem is those traceability chains that I talked about,
the fact that you have to have an unbroken chain of calibration from a primary standard in order for something to have confidence in the measurement.
If the only definition of the kilogram is a particular artefact, that chain, every single chain has to start ultimately with that.
And every time you make a comparison, you add uncertainty to your measurement.
So even here where you are comparing the copies to the kilogram,
you've got an uncertainty of five parts in ten to the eight, which isn't a problem for mass measurement.
But if you want to measure something really small down here, but a milligram because the traceability chain to get there is so long,
you are already at five in ten to the four at one milligram.
That is now. Hasn't been in the past.
That's now starting to get significant for some applications of measurement.
Drug companies are making measurements down at micrograms and want to ensure they're traceable and consistent globally.
So these are stuff that's just starting to get a problem further. KG So what's the solution?
What's the solution to this problem of having units defined in these strange ways, including one that sits there as an artefact?
Well, the solution is to lock all the units to unchanging properties of the universe.
What Lagrange and Laplace thought was a good idea in 1791 to lock them to the universe,
not to an artefact, not to a definition, to lock them to the universe.
But because, as I said earlier, the whole of our lives are dependent on confidence and consistency of measurement.
You have to do it without any perceptible step change in any application.
That is a massive challenge.
You need to select a suitable set of universal concerts that cover all the seven units and measure them with a small enough uncertainty.
So you use the existing measurement system to measure fundamental constants.
Then you fixed the value of the constants and let them define the units.
This project, which was conceived either in 1791 or in 1875 when the convention was signed or really in earnest in 1960, has been absolutely mammoth.
It makes finding the Higgs boson like falling off a log.
It has an absolutely enormous challenge, and it was achieved in 2018 for implementation this year.
So how do you do it? Let me explain how you do this.
And the first step on the journey is 1967, when the second was defined by reference to a particular transition,
a microwave transition in the Season 133 item in the 1950s.
Example, Lewis. Lewis created the atomic clock. We've all got used to the atomic clock.
Instead of using a pendulum or a fruit or a vibrating material of some kind that uses as its pendulum,
the vibrations or the oscillations of a particular in this microwave transition.
So you get a microwave and you use that and you count the number of oscillations that microwaves.
So the second was defined in 1967 as 9,192,631,770 periods of the microwave emission from the hyper fine transition of the ground status.
Caesium 133 You probably won't do that, but I just robot did.
And so you are locking the second to a property of caesium atoms that is the same everywhere for all time as far as we know.
This was really interesting because there was some argument from the astronomers who wanted it all still locked to the earth in some way,
but they gave in and this second was defined in 1967, and that unleashed scientific research and applications in the most amazing way since then.
Onto five Nobel prizes have been physics have been won by people researching ways in which you can improve this kind of measurement and apply it.
Not only GPS is the outcome of this, but all of telecoms, all of the Internet,
all absolute relies on the fact that we can make measurements really, really quite accurately.
This picture here is Ampere's primary clock.
So this is our clock. This is where we capture and we hold stationary caesium atoms.
We interrogate them and we lock microwave radiation onto those.
We lock the frequency of a microwave onto the caesium atom. And then we use that to, to define our second.
And we can this clock is accurate to one second in 168 million years.
So that's a reasonably accurate clock.
So that's the second the next part of the story came in 1983 when once you defined the second, if you can measure the speed of light,
which is believed to be a fundamental constant in the universe, you can measure the speed of light accurately, then you can define the metre.
From reference to how far light travels in a second. So then the metre becomes part dependent on the caesium frequency and the speed of light, see.
And so this is a speed light was measured. This is this rather fuzzy because this is a 1970s picture again from NPL.
There were experiments like this done in earnest in the US and in various other places all across the world.
They all had to agree. The guys who did this sort of story, the mist finished theirs just about a month before these guys finished theirs,
and they were so worried about consistency and no chance of conclusion next sent them their results.
No internet or anything nice, sent them their results in a brown paper envelope and they sealed it and put it in the director's
office and would not look at it until they had their number and the number numbers agreed fortunately.
So 299792458 metres per second is the speed of light.
And so the second was defined like this. The Ampere is a really interesting one.
The Ampere that is something that's literally only just been well only redefined in 2019.
The Ampere is one over either charge on the electron.
So again, you need time which you can get from the caesium and you need the charge on the electron.
And the ampere is defined as the number of electrons per second one over e the charge in the electron electrons per second.
You can't actually measure, you can't actually do this yet.
This is really interesting. You can't actually measure what may this year for the first time in a couple of labs,
including ours, be able to realise this unit by counting the number of electrons per second.
But fortunately there are a couple of Nobel Prizes 1973 and 1985 that enabled you to have
quantum realisations of the Volt and the Volt and the Resistor Neo so you could create
situations in which you absolutely knew what the voltage across a junction was from the
Josephson Junction or the resistance of a piece of material from the Von in effect,
the Quantum Hall effect. So you could have and they were dependent on A and C upon the speed of light and upon the charge on the electron.
So actually you were able to create the directly you're not able to measure the ampere by counting electrons,
but you can definitely relate it back to the charge on the electron.
The biggest challenge by far is this one replacing IP.
Okay, the kilogram. That was the single biggest challenge in the whole of this exercise of there's two ways that you can do it.
Option one is you create an artefact of known mass because if you remember the muscle, 12 grams of the number of atoms of carbon, 12 and 12 grams.
So you could see that you could link by counting atoms, you could link back to the kilogram through there.
And option one was a recipe for creating artefacts and in mass.
Fortunately, the semiconductor industry can create really ultrapure silicon 28 not quite pure enough for this experiment,
so they had to do even more work on that. And then you build a sphere, you build a sphere, and then you measure it size.
This, these few objects were the most accurately measured objects on the planet.
Just so you get some sense of scale. That's the kind of scale of them I've held.
I've held a one of the first ones, but I've not held one of the edge.
They don't let anybody hold one of these because you can't get anything on the outside at all.
So you have to know exactly its dimensions. You have to understand surface properties and oxygen and oxidation of surface.
It's an incredibly complicated thing, but this was the Avogadro project and that was one way of measuring the the kilogram,
the other way of measuring the kilo with measuring mass in terms of something else, something other than masses.
And this guy here, Brian Keppel, who sadly died in 2016, if he hadn't died, he would have been a candidate for the Nobel Prize.
You can't win the Nobel Prize posthumously.
He came up with the mechanism that is that was the most practical way of realising the kilogram by measuring the gravitational force.
So if you can about imbalance,
that's got a gravitational force measuring it in terms of electrical forces and the electrical forces could be related to fundamental constants C,
A and H. So I said C and D speed like the electron charge.
Already defined for the metre and the impasse he bootstrap from that a new just need h now bought Planck's constant.
Now I'm just going to try and I'm just going to click this if it works.
Yes, it will. I can't explain very in the time available exactly how the cable balance works,
but I can give you a clear idea of how you might think mass and this thing Planck's constant could be related together.
This is the most famous physics equation in history. See my only equations in the whole of the talk.
So equals M.C. squared. Where is the energy of an object at rest?
M is the mass of the object and C is the speed of light.
I imagine everybody in the rooms see this equation before.
A slightly less famous physics equation equals HSF.
So the energy of a light quantum, the photon is Planck's constant times,
the frequency of light and this constant Planck's constant like speed of light appears to be a fundamental property of the universe.
If you combine those two equations, you end up with an equation that looks like this m equals h.f. obasi squared.
So a change in the mass of a particle when it emits a photon a frequency f is that so you can see how frequency which is time, speed of light and h.
So if you've already locked frequency to the second and speed of light to the metre,
then h is the only the constant that gives the whole system locks it into mass.
An interesting thing this this equation at the top here equals empty squared was of course, Einstein's famous equation.
This equation, although it's got Planck's name associated with it,
the first time it was ever used by anybody was by Einstein for the photoelectric effect.
And that's actually what he got his Nobel Prize for. He didn't get his Nobel Prize for this.
He got his Nobel Prize for understanding the photoelectric effect and coming up with an equation like that.
So so in order to do that, you have to have this balance,
this device that can compare weighing with incredibly accurate measurements of electrical forces.
It is a fiendishly complex experiment. There are people who have spent 40 years their entire career doing this experiment.
There was a guy, an apple, who worked with Brian Kebble.
This guy has been working 30 years. Brian Cable was working ten or 15 years before him.
It is fiendishly complicated because you have to do it all to get an uncertainty of one part in ten to the eight.
And it's a mechanical and electrical experiment at one part in 100 million.
It is fiendishly complicated. And in 2017, when I sat down with the CPM, this was the data that we were shown.
So I'll try and explain this graph. This graph are all the experiments that have been done since 1990.
There were more before that, but they have bigger error bars. These are all experiments done since 1990 and these are all measurements of H.
So taking that CPU balanced measurements or those silicon sphere measurements and because
of the way work worked all locking it all back on to we're measuring h Planck's constant.
These are the measurements from all of that time this scale here.
There was a, there was an assessment of what Planck's constant was in 2014 by the Global Physical Constants Group, the co data.
And so this scale along here is parts in ten to the eight away from the previous definition of eight.
So you could see they all cluster.
And there was I can tell you there was 3 hours of discussion at the consulting committee on units and then at least half a day of discussion at
CPM about whether we really believed that this was consistent enough to lock finally locked the last of the units to a fundamental constant.
And this was the number we came out with.
That's Planck's constant, because this was this was go no go for the whole conference to say yes or no to doing this.
And it was an interesting discussion, but we did say yes.
So we have seven units and seven constants. The second related to that fundamental, that constant of nature, we could have chosen another atom,
but caesium the metre was related to that caesium property and speed of light.
The ampere again for the caesium property and the electron to the electron, the kilogram to the speed of light,
the caesium transition and Planck's constant, the Boltzmann constant is a scaling constant.
The Boltzmann constant is the constant, that is the scale between temperature and average energy.
In order to get that constant, there have to be some of the most accurate measurement,
one of the most accurate measurements of temperature ever done at about 20 laps around the world.
The Candela is a technical constant. As I said, that hasn't changed.
That's just this relationship between light energy and how we perceive it.
And the Avogadro constant, which came out of the silicon sphere and other things was fixed.
So this is seven constants, seven units, this was the end of the project.
And so from next year it was agreed last year from 20th of May 2019, the anniversary of the signing of the original beach convention 20.
As of May, the site will be the system of units in which the following constants have exact values.
So these are now fixed, these have zero uncertainty, these are exact numbers.
And so you infer units of measurement from those rather than measure these with units of measurement.
So we have a new size. So there was a historic and a unanimous vote on the 16th of Ember 2018.
This was it in Paris. I've never known so much press.
I guess everybody probably heard about the KG finally being pensioned off.
If you work in metrology used to people not knowing what measurement is or how important it is, and it's just kind of goes on under the surface.
But this was mopped, literally mopped by the press, this conference.
It was bizarre, but this is a once in a generation moment to futureproof the system and to enable and catalyse new science,
technology and applications. Because when you can measure things more accurately,
you get that synergistic relationship between more accurate measurement and people drawing on that to improve technology,
which in turn can help you to make more accurate measurements,
you get a synergy between the two that has developed and created some of the most astonishing technology we have today.
So everybody was very excited about this. And this was a video that was created about how excited I was about 130 years groundbreaking
science and the agreement from the world's scientific community that tells it's impossible,
accurate, precise measurements. Anytime, anywhere, fast, we can identify that the atmosphere you're going to get to see a body, one missing.
The line rolled down on my knees.
Yeah, but it was on me. You know, I'm really trying to come up with my thought, you know, how hard is that?
And now? So it was clear that itself.
But I thought, I'm going to have to fly through this country.
Oh, yeah. Well, I made a point to show off my work.
How? My I'm. Oh, very special.
Congratulations. So there we go.
For all time and for all people. Maybe the first one I talked to you about was the second locked to the hyper fun transition to the season 133 up.
Because it's a microwave emission, it puts an absolutely fundamental limit on how accurately time can be measured.
It's still a measurement that is defined where the realisation is implicit in the way it's defined.
So across the world at the moment there are a large number of people trying to create clocks that instead of relying on a transition,
a microwave transition, that particular micro transition in caesium can be locked to optical transitions.
This is the MPL, one of the Ampere clocks, and this is the heart of it.
And in here we can hold a single atom, a single atom of strontium 90, we can call it to absolute zero with lasers,
and then we can probe it and lock a clock to its frequency of that emission that will
enable measurements with an accuracy of one second in the lifetime of the universe,
and probably in 2026 the site will be tweaked again.
But that shouldn't take away from the excitement of luck year. So I'll finish.