 We now move to the second talk of the morning, which will be delivered by Professor Amol Dighay from the Tata Institute of Fundamental Research. Professor Amol Dighay is a professor of physics at TIFR. He is also currently the dean of graduate studies at TIFR. His research area is high-energy physics, which aims to understand the nature of fundamental interactions by studying properties of elementary particles. His recent research has focused on neutrinos, their nature, and the role they play in astrophysics and cosmology. So he also looks for signals for new physics at experiments like the Large Hadron Collider at CERN, Europe, you may have heard about it, and in the particles that come from the sky. Professor Dighay completed his BTEC in Engineering Physics from IIT, Bombay. He went on to do an MS and a PhD from University of Chicago, where he explored signals of charge parity violation in particle physics interactions. He was a post-doctoral fellow at ICTP in Trieste, and CERN in Geneva, and Max Planck Institute for Physics in Munich, Germany. And he joined TIFR as a faculty member in 2003. And since then, he has been at TIFR. Amol is one of our first Olympiad medalists, actually. He was the first Indian bronze medalist in the International Mathematical Olympiad, which was held in Germany in 1989. That was the first year that India participated in any international Olympiad. That was a mathematical Olympiad. He has received the Institute Silver Medal from IIT, Bombay, and the World Lab CERN, John Bell Scholarship. He was the leader of the Max Planck India Partner Group in Neutrino Physics and Astrophysics for five years. He has been elected fellow of the Indian Academy of Sciences in the Indian National Science Academy. He has won the Swarna Jayanti Fellowship from the Department of Science and Technology and is the recipient of the Shantish Swaroop Bhatnagar Award. I must mention that Prasad Dige is also actively involved with our Olympiad program. He has been a resource person and a mentor in as many as three of our Olympiad's physics, astronomy and astrophysics, and junior science. Prasad Dige. Thanks, Advish. And thanks for having me here. As he told you, I am almost a local person. I have been involved in many of the advocacy activities, and therefore, I closely know what this institution does. Today, I'm now going to talk about the kind of things that do in my research. What about something interesting that has been happening as we speak, and which is that the kilogram is changing. The unit of mass that we all have grown up with for so long is changing. As we speak, we are in a transition period, and something is going to happen in the next year. So that's something that I would like to tell you about. As Professor Ganesh told you, at the beginning of his talk, the things that a scientist does is ask questions. So my talk will be focused on these questions about Mr. Kilogram. So I will first try to ask why. How? Why now? What now? And what's the big deal? Why is it that we worry so much about changing kilogram that no people are going to be first about it and have events about it and try to do these things? So what will happen during the course of this talk is presumably what I want to convey to you is some idea about how we think about units and measurements, and what all goes behind actually constructed these measurements, and how so-called advanced things in physics, chemistry, and mathematics actually are behind simple things that we speak about like time, distance, and mass. So it's going to be something very simple. So I said goodbye, and you usually say goodbye to somebody who retires. The person who retires is actually called La Granca. It's the French name for what we call as the International Prototype Kilogram. So this person is actually referred to in masculine as a person, this weight. This is what this weight looks like. It's also called as the IPK, or International Prototype Kilogram. It is this small cylinder that you see. It's a cylinder which has a diameter of about 4 centimeters, more like 39 millimeters, because you have to be very, very precise now. Height also of our order same, so it basically looks something like this. It's made up of iridium and platinum. If you see here, it's kept inside evacuated bell jar. This bell jar you will see inside another bell jar evacuated. This bell jar is inside a third bell jar, and somebody thought that he was enough to protect it from any of that possible disturbances. So we are here, and this is kept in a vault. The vault is opened very, very rarely, and it is opened to prepare replicas of this kilogram. So periodically there are some other kilograms that you want to spread all around the world, which will carry the signature or the brand of exactly 1 kilogram. So every masses that we see in our grocery shops finally come up to the very great chain of these kilograms which are weighed against each other at various places. This, however, is the so-called standard kilogram, which is the international prototype. This has been there since 1889, so it has history of more than 125 years. So it looks a good idea, but there always are problems with having a physical object as standard. There are some very clear disadvantages. One is you're putting all the eggs in one basket. So if tomorrow something happens to this and no its weight changes a bit, then everything that you're doing is going to be off by certain quantities. There are things that we have to do now for which we need very good precision, and therefore precision of the order of sometimes one is a million, and whether a billion is required in while doing certain things. And therefore changing the definition of as basic thing as kilogram actually might be very precarious. And we are just to be careful that this object is known to as good a precision as possible. Second thing, which is not really scientific in a sense, but philosophical, is that it's very inconvenient and it's very undemocratic. Inconvenient simply in the sense that if tomorrow I do want to check whether something I have is let's say 500 grams or not, I have to go back the whole chain. And finally, it just lead back to this vault in Paris. And somehow this thing kept in the vault in Paris has to be brought out only when those people will allow me to. Therefore, as a scientist, it's not that at any point of time I want, I have access to what is the standard of one kilogram. So in that sense, it's very undemocratic. Those who hold that piece have the standard available. Of course, it's inconvenient. What is more important scientifically is that in the last 125 or so years, this IPK could have changed by as much as about 50 micrograms. Now, how can it change? Of course, you take utmost care to make sure that nothing happens to it. But see, all said and done, when you want to make more replicas, you take it out. It's possible that a few dust particles will come settled on it. It's possible that there will be some evaporations from the surface. It's possible that whatever you touch it with may be some oil, but something will stick to it. So it's quite possible that the weights would change. We know that the weights have changed. The reason is because since 1889, we have made many, many, many replicas of this. And these replicas do not match to each other within about 50 micrograms. Now, of course, most of the replicas also have been taken care of as replicas to a great extent. In spite of this, if you find a variation of about 50 micrograms within these replicas, it means that quite likely that our standard kilogram might also change by about 50 micrograms. The worst part of it is we will never know because by definition, that is the kilogram. So if that has changed over the last 100 years, we will never know. And that is a big disadvantage of having a physical object at the standard because we know physical object change. And that is the principle with which the meteorologist, which means those who decide on the measurements actually act. The question is what is the alternative? Everything, as was said in the last talk, as scientists in the natural world, which means all that we have is things in nature. And we observe that things in nature change. Therefore, a priori, there is a similar way out of this to define something like mass. You must have something that you use as a standard. So the question is, is there a way out of it? And that's what we'll try to look at in this range. Let me tell you who are the people in charge of this. There is a CGPM, which is the General Conference of Weights and Wages. Then there is a CIPM, which is the International Committee for Weights and Wages. There's a BIPM, which is the International Bureau for Weights and Wages. And there's NIST in the US, which is the Institute for Sanderson Technology. All of these three things are in Paris. French were the people who started all of this. And therefore, somehow historically, French have been the leaders in looking at standards. These names that you see also are French names. So this is Bureau in the Nassiaudal, des Pois et Mezure, and so on. But we'll refer to them only as these four. All the information I have in this talk is taken from the websites of the organizations, plus, of course, some papers, plus Wikipedia comments. So it's good to always say in your paper, for example, what you have been referring to. And that's also true for talks, when you're referring to something better. So the next question is, how do we achieve, how do we go to a stage where we can define the kilogram without referring to any standard measure? To understand that, we will step back and we will look at another example. And that example is going to be the definition of a standard meter. So let's just go through some history and see how things developed. So of course, more than 4, 5 centuries ago, people in various lands used various units. This was a digit. This was span, foot. I was talking to Anves today about why is foot equal to, this is an inch. Why is the foot equal to 12 inches? We never know. Maybe there was some person. Those foot and inch had this ratio of 12. This was the qubit. This was hand. This was the phantom. This was a yard, and so on and so forth. So units were defined not very accurately, but sort of conventionally. The first time that people thought of actually making nice units was after French Revolution. So somehow, at the time of French Revolution, there was also some change in mindset going. There were discussions happening in the French scientific community about actually having units which are standard. Before French Revolution, the second was actually defined, the meter was defined as half period of a, as a length of a second pendulum. A second pendulum is the one where you take one second to go from one end to the other. So in the modern pantheon, it means that the time period should be equal to two seconds. That was, therefore, half period of a second, the second half period of a, that's, it has half period of a second. So that length was one meter. As we know, if you take a one meter pendulum, it's time period is equal to two seconds. The problem with this was obvious even at that point of time, because if you measure it in Paris, and if you measure it in some other place, since of course we know that G changes from place to place, the time period was different. So it was even observable because it depends on gravity. So very clearly that could not have been a good definition of a second. The second definition that people thought of making depended on the earth. So the second definition was actually based on the melody of the earth. So what decided was we will take the circle that passes, great circle that passes through Paris, of course because Paris was the center of the world at that point of time. We know that the circumference of earth is about 40,000 kilometers. That is not a coincidence. It is around 40,000 because the meter as was defined first was defined as one over 10 million of this. This part, which is one quarter of the circle is called as the half meridian. Meridian basically means pole to pole. This was half meridian. You measure this, you measure, take one over 10 millionth part of it, that would be a meter, okay? Of course this means that you have to actually measure the length of the meridian. There were people who went and did that by actually using rulers. There were expeditions that went around, trying to go through each and every part of this meridian. So wherever it was sea, they went over sea with land, they went over land. These expeditions actually tried to measure what this distance was. And from that figured out approximately what a meter was. So a meter actually has been defined with respect to earth, which is something I learned very recently, which I didn't know of this. It's kind of fascinating, okay? And from this, they decided what a length of a meter should be. In 1739, which is where this exercise was kind of over, they made this meter scale, which was called as the Matilda archives, which is meter in the archives. And it was claimed to have accuracy of about 0.1 millimeters, which is about 1 in 10,000, okay? So which is reasonable. So which means that the circumference of the earth, this meridian, half meridian, was measured to have accuracy of about 1 in 10,000, which is fair compared to what we had at that point. Then of course people kept on making better and better measurements, or in fact making rods, which were equal to that original rod, which will not change its dimensions. So in 1875, there was a so-called international treaty of the meter. This is called as one of the scientific treaties. It's also called as a meter convention. I think there were 17 countries which were parted to this. We decided that now we will hold this particular object and call it as a meter. It's called as a standard meter. This was the prototype meter, which was made in 1889 and last until 1960. You see it as an expiry date, and expiry date I'll come to later as to why it had an expiry date. This meter, which looked like this, was also made up of indelipatism for the simple reason that these things are very stable. But of course, you know, an object like a meter depending on temperature will change the length, right? It has some expansion coefficient. So to define it very, very properly, it has to be at zero degrees temperature, so zero degrees Celsius. Depending on the pressure you have, there might be some expansion. You press it from top. This much thing might get spent. It has to be exactly at one standard atmospheric pressure. It also depends on how you measure it. If you put it on two supports, it is going to bend slightly. But you have to put it on two supports. One support is not possible. Depending on where you put the support, the length would change slightly. So it is very important to decide how you measure it. It's not just the object, but how you measure the length of the object when you are trying to make other meters, okay? So it was supported by two cylinders separated at 571 centimeters. So they were, so you place one cylinder here. It's a very small one. One here, you balance it on these two, put them exactly in the center, and you measure the length, okay? So you have to specify all of these three things. And when you do that, then you can measure the length of this meter to the accuracy of two micrometers, okay? So in about 1880, we had gone to this level. However, again, we know that this already has many difficulties. You have to satisfy these three conditions. Again, it depends if you take zero or 0.1, it's going to make some difference, and we do need to be as accurately as possible. Things were made but simple when we had quantum mechanics and chemistry, okay? Atomic physics, okay? So it was found with the advent of atomic physics that when there are atomic transitions, they correspond to very particular wavelengths, okay? Which are very, very narrow. So if we choose a very narrow wavelength, we will know what that wavelength is. That will be maybe some nanometers, and then we say that if that is a nanometer, you multiply by a sub-number, we're going to get a meter. That's the principle, okay? So this was based on the krypton lines. So these are the Barber series transitions of krypton lines. Basically, you are coming from N equal to five-shell to N equal to two-shell, this particular line, which has a wavelength of 434 nanometers. So in 1960, when the earlier meter was discontinued, the new definition was the following. It said, okay, fine. You measure the wavelength of light, which was this. Then you multiply that by this number, 1,650,000, blah, blah, blah. Exactly these many wavelengths are going to form a meter, okay? Now, of course, this has one great advantage that you don't have to go anywhere and measure the length of the meter. You can do the experiment in your own lab. All you need is a source like this, a krypton source. And in fact, many of these replicas were made and sent to many laboratories where they could make a meter scale on their own. Accuracy of this from two micrometers actually went down to 0.01 micrometers. So an increase in the factor by factor of about 200 was still not enough because as was found later on with the technology developed and we found that with lasers, we can make much sharper lines. It was found that this krypton line that was earlier thought to be a very standard line is not standard. It has some asymmetry and therefore there were problems in defining things well if you wanted to go to accuracy more than 0.01 micrometers. So in fact, in 1983, even this definition was discarded. So what did we do? Where did we go from here? So idea came from a very strange source. It came from what Einstein propounded in 1905, which was the special theory of relativity. One of the principles of special theory which all of us know is that the distance traveled by light, the speed of light is vacuum is a constant. And therefore, distance traveled by light in one second is going to be a constant. What does that mean? That means that if you know what is a second and if you know what is the speed of light, you can define the meter by just saying that it is the distance traveled by light in so much time. So in fact, the definition adopted in 1983 was that a meter is a distance traveled by light in the vacuum in this much fraction of a second. It is basically made that this is the speed of light. In other words, meter now is that length, is that unit in which speed of light is 299792458 meters per second. From 1983 onwards, the speed of light has not changed. Well, it did not change since the beginning of the universe, but the number corresponding to speed of light has not changed since the 1983. And by definition, it will never change now. At least to the accuracy of one in a billion, these numbers are not going to change because their speed of light is now not a quantity to be measured. It's a quantity used to define a meter when you are given a second. So it looks like a colluded way of thinking, but actually it happens to be a dice that consistent way is going to last forever. So one of the motos of this meteorologist is units for all and units forever, which means units should be such that they're accessible to everybody. And units should be something that should last forever. The definition will not have to change. Speed of light, as far as we know, is a universal constant. It's a constant of nature that does not change. So remember, we were looking for something which will be a standard, which will not change because of handling of human beings or because of rotation of earth or because of no falling on the floor. And we know that speed of light does not change. It's an abstract quantity. It's a constant of nature, but we know it exists and we know that it does not change. So that principle is now going to be used in talking about all the rest of the units. So we are in some sense back to defining distance in terms of time, right? When you thought of second pendulum, you first define the second and then you define this length. So it might look that we are back to this, but it's slightly, of course, better than that because now there is no dependence on what the source is. Doesn't depend on Krypton or Xenon or CJM or anything. Special relativity has been checked and is always checked by a multitude of experiments. So therefore, we are very, very confident that the value of C does not change and what is more important is that a second can be determined with an accuracy of more than 1 in a trillion. So as we'll see later, we'll keep on defining all our units in terms of a second, okay? Because of course, we need to somehow go to nature and ask it some question though, what is the unit? We ask the question to nature only for second. So that's our compromise, okay? So once you have defined this second very well, everything else will come out naturally, okay? So that's what we are trying to see. So let's first understand how a second is measured and then we'll come back to Newton again. So the general idea is the following. A day has 24 hours, hour has 60 minutes, minutes has 60 seconds. Basically means that 86,400 seconds is a day. So you take whatever is average day or a mean day, whatever the meaning is and you take this fraction of this, okay? Of course, extremely unsatisfying definition because day change, okay? Definition of mean day is variable and therefore this cannot be very, very accurate. So here also we go back to a quantity which you can measure very accurately almost to one part in a trillion and which is again going back to atomic transitions. So what is done is you look at CGM source, okay? CGM 55, this is a nucleus of CGM 55. These are orbits, n equal to 1, 2, 3, 4, 5. You look at the so-called 6S electron of CGM. It has some spin. You look at the proton of CGM. It has spin half, okay? The transition which takes you from this where both are speed up to what is speed down is called hyperfine transition. This transition happens to have very, very accurate measurement. Its frequency is known to, as you see here, 10 decimal places and it is defined to be nine trillion, 192 million, blah, blah, blah. So many hertz. This is now taken as the sort of benchmark for measuring time and the second is defined as this number, nine, 192, blah, blah, blah. These many periods of the radiation correspond into this particular CGM matter, okay? This is an important measurement because this is a measurement that is going to decide all your units, okay? So at the end of the talk, we will see once you can do this measurement, it means that you basically know all the units in a science system, okay? But this is one that you really, really have to go back and measure. So what is the principle now? Principle that we learnt from the definition of meter. Firstly, you choose definition of time measured on very, very accurate experiment, which is what we have done, okay? Next step is you choose a fundamental constant of nature that does not change, which connects time and distance. So in this case, we have time, so t, distance is meters, so speed of light connects distance and time. We take that as a fundamental constant of nature and that gave us the definition of meter, okay? Now next step, you have to measure this fundamental constant as accurately as possible, okay? Because we know that once we fixed it, it's not going to change, the value will not change. We do not want to change the current values of meter and second as far as possible. So first thing you do is measure the value of this constant as accurate as possible and freeze the value, okay? Just like I said, the speed of light now, well, let me say it's not going to change, okay? So you freeze the value of this constant and now through this fundamental constant, you define one meter. So this definition of meter now is going to be everlasting definition. You will not have to change because your standard has not changed, okay? What could happen is you could maybe measure the time more accurately, but that's it. That's the only one measurement that you can do more accurately. Once you have that, everything else will follow. So let's come back to kilogram, okay? So we are going to use that to define the kilogram now. So now you want to connect mass with distance and time because those two you have defined well. So you can think of fundamental constants of nature which connect mass with time and distance. Turns out that such a constant exists and it's called as Planck's constant, okay? So anybody who has done quantum mechanics would know Planck's constant, okay? This connects energy and time. So it tells you that if you have got a photon with frequency F, this energy is E and E is equal to Planck's constant time the frequency. E is equal to h nu, okay? So this constant connects energy and frequency. Special energy also tells you that E is equal to mc squared, okay? So therefore, if you connect these two, you will see that a mass, therefore, can be defined in terms of Planck's constant frequency. Frequency means time, right? One over time. And c, another constant of nature, okay? So what you have done therefore is your related mass to these measurements which are basically constants of nature and just one time measurement, okay? So how will I define the kilogram based on Planck's constant, okay? We use the same technique as before. We want Planck's constant to be equal to this, okay? Six point, this is the accurately measured value. And now that we have this value, we are going to freeze this value, okay? In July 2017, the value was frozen, okay? So since July 2017, your Planck's constant will not change. In fact, a kilogram will be defined in terms of Planck's constant, okay? So definition of kilogram can be defined in two ways. One way is this. It is that mass whose rest mass energy would be equivalent to a light with the frequency of one over so many, so many hertz. That's what we are looking at. A second definition which is exactly the same as this is this. See h is written like this. H is some number kilogram meter square per second. If I change my unit of kilogram, h will change, okay? If only one particular definition of kilogram will h have this value, okay? So I say that kilogram is that unit of mass in which Planck's constant has this value. We already defined meters and seconds earlier, so that all the distance, okay? So that's the principle on which Planck's constant is will has been defined, sorry, in the principle on which kilogram has been defined from Planck's constant, okay? It's actually a big deal in the sense, first thing that you need for this is to be able to fix on what this value of Planck's constant is without changing what we currently think as kilogram. We don't want to suddenly start calling something as a kilogram. We want to be as close to current kilogram as possible. The question now is why now? I mean, why is it that this was not done long ago? No, why is it suddenly people woke up in 2017 and said, okay, okay, let's not fix Planck's constant. No, this is this value. You were earlier, right? That's one question we can ask. Why didn't people think of this? 100 years ago, people were good thinkers, they wake up with ideas. But of course the fact is that 100 years ago, we did not have the means or the technology to be able to do this, okay? So for example, special relativity just getting established. No, it's 2018, no quantum mechanics. So of course, people didn't know of Planck's constant, okay? Planck's constant was though, okay? So that's not the correct statement, there was no quantum mechanics as such. General relativity was just proposed, okay? So we are in just in front stages of what our model science consists of, okay? Could you have done it 50 years ago? Or 25 years ago? Or 10 years ago, okay? Let's look at this question. Why is it that suddenly now we can do this and we are not able to do this earlier, okay? So let's see what technology is needed to measure the value of H, okay? So measuring H uses a very good technique which is called as watt balance. What a watt balance does? In fact, I'll show you a movie about this because the movie can explain that thing much better than me, it has some nice car tools also. Instead of trying to balance by using masses on two sides, you use electromagnetic forces on one side, okay? And that will allow you to measure the value of Planck's constant. Let me show you this movie. I think the sound is connected. So let's just... An ordinary beam balance works by adjusting the amount of mass on one side of the beam so that its weight exactly balances the weight of a test mass on the other side. Gravitational force against gravitational force. A watt balance for all its sophistication does basically the same thing. In this case, however, the force that balances the weight of the mass is not gravitational but electromagnetic. This force is produced by a coil of wire that is suspended in a strong magnetic field created by stationary permanent magnets. The watt balance lets researchers determine the mass of an object indirectly by determining two quantities, the strength of the magnetic field and the current running through the coil of wire. It does this in two separate measuring modes. The first, called velocity mode, uses an electric motor to move the coil through the magnetic field at a constant velocity. This movement induces a voltage in the coil that is exactly proportional to the field strength. Measuring the voltage indicates the field strength. In the second mode, called weighing mode, an electrical current is run through the coil to turn it into an electromagnet. As the coil's field interacts with that of the permanent magnets, an upward force is exerted, proportional to the current, which can be measured. NIST's new watt balance uses a wheel instead of a balance beam. Attached to the wheel on one side is the coil and a platform for the test mass. On the other side is the motor that moves the coil in velocity mode. Both the coil and the motor are surrounded by metal enclosures. In velocity mode, laser sensors track the coil's motion using a technique called interferometry. This detects differences in position as small as a fraction of the wavelength of the laser light. As the motor moves the coil, the interferometry system ensures that the coil's motion remains perfectly constant. That constant velocity makes it possible to calculate the strength of the magnetic field. In weighing mode, a test mass is placed onto the platform on the same side of the balance as the coil. An electrical current runs through the coil, producing an upward force that is proportional to the current. By carefully adjusting the current, the upward force is tuned until it exactly offsets the weight of the test mass and the system reaches equilibrium. I had to watch the video about 17 times to be able to really figure out what's happening. So I don't expect you to have figured out in the first time, but of course, there are people sitting here who are smarter than I am. So I would be surprised if you have. But I wanted to show it to give you a feeling of what is it that was done in the last few years, from 2010 onwards around that time, to be able to balance the mechanical force by the time in the force of gravity with the electrical force, which was used to push this mass up because of electrical forces. And that gives you the value of H. Now people sitting here, like Ganesh, he will immediately tell me that I am cheating because I talked about mechanical forces, I talked about electrical forces, and as many of you know, they don't involve Planck's constant. So I showed you this slide, this movie, which actually did not use, did not mention Planck's constant. It said mechanical forces, electrical forces. So never get cheated by people who tell you that you do this and you measure Planck's constant. I didn't cheat, but I just hid something from you, which I will tell you very soon. So the question was this, where does H enter? As physicists know, H basically enters when you are addressing some quantum phenomena. If the phenomena is completely classical, then H doesn't come into picture. There was somewhere was H hidden in all of these things. And in fact, that lies in how you measure these quantities. So remember that in all of this, what was very important is measurement of magnetic field, measurement of voltages and currents. Now the most accurate measurements of magnetic field voltage and currents actually needs quantum mechanics. So how? The current is measured using what is called as Josephson junction. So this is for people to go back and Google and see what is Josephson junction. This was discovered, if it was discovered by Josephson in 1962, mobile was given in 1973. So this is something that happened about 50, 60, 70 years ago. That gives you Josephson constant, which is called KJ, which is two times E divided by H. E is the electric charge and H is Planck's constant. So exact measurement of current actually needs the factor of H, which is Planck's constant. There was a measurement of voltage here. The voltage is measured using what is called as fractional quantum Hall effect. Hall effect, this fractional quantum Hall effect is a quantum phenomenon, not just simple Hall effect. Discovered by Van Kielsen in 1980, the Nobel Prize was given to, well, not to him, but to Laughlin, Storbett, Sui in 1998 for explaining theoretically quantum Hall effect in 1998. This actually involves the Van Kielsen constant, which is again H by E squared. So Planck's constant appears here. So the measurement of these voltages and currents involved Planck's constant, and therefore this measurement needed fundamental discovery in quantum mechanics. So clearly, even if somebody had thought about balancing mechanical forces with electrical forces, before we knew quantum mechanics, especially these two phenomena, we could not have measured things to the accuracy of one in a billion that we needed. So therefore the advent of quantum mechanics are very, very essential. This experiment, for example, as I say, it was, this was discovered only about 30 years ago, could not have been done more than 30 years ago. So of course things are discovered, things are accepted, people become comfortable with it, scientists accept it worldwide, and only when there is no doubt can it be used for standard or measurement. Therefore that thing takes a few decades. Therefore now the time was ripe to be able to use this knowledge and this technology that we had. So this is how the Planck's constant measurement has become better since 1979. So that is 79, for example. So this line is the current value of Planck's constant. These are error bars. So that is 79, error bars are so big. Then the experiment done by many, many different ways. Till finally now you will see here that error bar is of this order, which is about one part in the billion. So the fact that we were now able to determine H to accuracy of one in a billion was an important factor in doing this change now. That was the answer to what now? Such an important thing should also be confirmed by something else. And that was done by another beautiful experiment, not by using Planck's constant but by using Avogadro's number. And this is a beautiful thing called as Avogadro project and let me tell you what this was done. This is by the way supposed to be the most spherical object ever created. So this is a sphere of pure silicon, pure silicon-28, the particular isotope. The diameter is about 98 millimeters but not here. It is exactly spherical because accuracy of one nanometer. So from all sides it is 94 millimeters to accuracy of one nanometer. This accuracy has been confirmed by interference and using extended diffraction. So again, just to make this, we need an extended diffraction. To be able to polish this, we actually needed a particular chemical whose name I have forgotten. I remember this somewhat of time. It's a very long name, which has a four-liter short form but I am not good with names. But however, so this was polished by expert lens makers to accuracy of one nanometer. And so what do you do now? So you make this exact spherical ball. You measure its mass, which you can measure. Then you calculate the number of atoms inside this. Now this has a perfect crystalline structure so that you exactly know what is how where the atoms are placed. This has no defects. I mean, it took many years to make. And therefore, you can calculate the number of atoms inside this sphere. So first of all mass, you know the number of atoms. You can use that to find out Avogadro's constant in A. So in fact, this was another measurement that I will come to, which will be revised very, very soon. Now you have measured mass in whatever units you have. You have measured Avogadro's constant. They should match. Therefore, this relation from this, you can find out what is one kilogram. You say that one kilogram is equivalent to one by twelfth mass of N A number of carbon twelfth atoms. So this project was called as Avogadro project because its principal aim was to find out the value of Avogadro's constant. Once you do Avogadro's constant, then you could multiply that by the mass of C 12 atoms and divide by 12. And that answer is going to be one kilogram. These two measurements of kilogram that we decided, one from Black's constant, one from Avogadro's constant, should match to within one part in the billion. That's the requirement. The fact that they matched is sort of evidence of the success of people who are doing these measurements and measure of consistency. So this BIPM, which is the convention, which the bureau, which decides on what unit should be, actually self-imposed some conditions. Well, there is a committee which self-imposed conditions on BIPM that there should be at least three independent experiments. They should include both what balance and Avogadro project because they do things independently, one through H and one through N A. They should yield it to accuracy of at least 50 parts of the billion. So I keep on saying one part of the billion, though just as a demo thing, but it's actually 50 parts of the billion. So you get three measurements, these two plus one more. And in fact, they were the second one balance at a different place. At least one of these research should have uncertainty of not larger than 20 parts of the billion. So at least one of them should be more accurate. And all experiments should be consistent at 95 percent developed confidence. So it was decided that if once these three conditions are satisfied, we will take the next step towards defining kilogram in terms of plant constant or Avogadro's constant. So what happened? July 2017, all the conditions were fulfilled. There were three experiments that gave consistent values. A meeting happened not more than a month ago. So that's fresh of the press. November 2018. So a meeting of BIPM, which are the contributing states of this so-called Treaty of the Meter or Meter Convention. India is also part of it. It was a reasonable decision by 60 member states to revise SI units. Units will be revised starting from May of next year. That's why I said that we are still in transition state. So what now? So we have solved this problem in principle or in practice of redefining kilogram. So while we are doing this, since changing units is not something you do all the time, people had a relook at all the other units and in fact thought that some of the good ideas that we were talking about for many, many years can actually be implemented even now. This is how we defined SI units when we were in school or in fact till last year. Second was given by CGM line. Meter was given through speed of light, as I explained sometime back. Kilogram was given through the International Prototype Kilogram that is for the Paris vote. Bowl was defined as through mass of carbon, which you have to measure. Ampere was defined through this force on wires. It requires some mu dot by four pi. That's permittivity of free space. There was Kelvin, which was defined to triple point of water and Candela was defined through something else. So this was the sort of network by which you defined SI units. Meter needed information for seconds and speed of light. Kilogram needed information for this. Bowl needed information for kilogram and mass of carbon and so on. But remember if you have to define bowl as one place of the mass, you also need to know what is the kilogram. So definition of bowl needs kilogram and mass of carbon. So this is the network which tells you how the old SI units were defined. And now we will see how things will change. So this map is now slowly going to change. Let's change it one by one. Ampere. Definition of ampere if I remember from your school textbooks was the following. It's that constant current which if maintained in two straight parallel conductors, so you have two straight parallel conductors, infinite length, negligible cross section, one meter apart in vacuum, have a force of two type demo by the same Newton's per meter. I'm sure you hated this when you read this in your textbooks. I hated it. Metrologists have been hating it for almost a century now. And therefore everyone jumped on the opportunity because we have a very simple definition available now. Because now we know what an electron is. We know charge on an electron. So simple definition that will appear now is this. Is that amount of current such that electron charge is these many ampere seconds. This is something that everyone thought we should do long ago. However, note that to do this, we needed measurement of electron charge to the accuracy of one in a million. This has happened long ago. However, about 50 years ago. However, now people took up the courage to do this. And therefore, from July onwards, the students in standard 11 will not have to remember this. You will actually be fine with remembering this number. And that's a very complicated definition. One mole. As I mentioned sometime back, mole was defined as you take carbon atoms, of 12 grams, major number of atoms in that. So then the mass and so on, use kilogram and blah, blah, blah. So that's the amount of substance of a system that contains as many, let's say, atoms as their atoms in 0.01 to kilogram of carbon 12 which needs carbon. But you know there's something much simpler which is just this. It is the amount of substance that has number of units equal to this. So this, in fact, is a number, 10.23, beyond 857. All the remaining, I think, 16 digits are zeros. Exactly zeros. So number NA has now been fixed forever to this quantity. And that's it. You will not now refer to carbon or to silicon or to hydrogen-defined mole. Mole is just this number. Because after all, it was just a number. Somehow, at some point from this convention. So mole is just going to be equal to this number. It's not 6.023 anymore for those. It's 0.22140. So those who are interested in the fifth significant figure should note that 6.023 has changed. Anyway, so this number will now become. So we did hampere, we did mole. Now we're going to Kelvin. Why leave things? One Kelvin was the following. You look at the triple point of water. Triple point of water is the point at which when you have ice, water, and steam exists. You take the temperature at that point of time. You take 1 over 273.16 of that. That is defined as one Kelvin. It is because of this that our zero Kelvin is minus 273.16 degrees Celsius. So this was taken as one Kelvin. Again, this is not a good definition because it is a very hard job to actually get triple point of water. It also depends on many other conditions. Atmosphere, the fact that there should not be any currents, water should be stationary, and so on and so forth. However, now that we have this great tool in our hand, though, I just go back and point out that tool that we have used without any noticing. Here, we use this tool that charge of electron is a fundamental constant. Does not change. So charge of electron does not change. Was used to define ampere. Here, we use Avogadro's constant. Number of particles that does not change. We used it to, well, keep Avogadro's constant as a fixed number. For Kelvin, we use another constant, which we know is fundamental and we know accurately, and that is Boltzmann's constant. Value of Boltzmann's constant, if you see I always write eight or nine decimal places, is now known to about one in a billion. Now is the time to freeze Boltzmann's constant to this value. Kb is blah, blah, blah, Joules per Kelvin. You know what Joules are because you know what is kilogram, meter, and second. You say that one Kelvin is that temperature, that temperature difference such that in these units, Boltzmann constant is this. Now I define Kelvin through Boltzmann constant. So here, we are using the principle that energy is the, temperature is the measurement of energy. So let's say in the monotomic gases, E is equal to three-halves kT. So we know that Boltzmann's constant actually is fixed constant of energy. There's something called as luminous intensity, which we don't come across often, but optics people do it. So we define as you take a source of particular frequency. You look at its emission in one particular angle and you, when the intensity is one by eight, a 683 watt, first area you call this as one candle. It's complicated, but it's not so big a deal. So these have not been changed. They're simply reordered in our new language. Our new language is you define a constant in some units and you say that I define this unit such that this constant has a frozen value. So now one candle is that unit where the fixed numerical value of luminous efficacy is 683 and that means 683.000 to at least 10 decibel basis. So new SI system therefore will be the following. Second, you measure my cesium line. Ampere, you take constant of nature, which is electron charge. Meters, you take speed of light. Kelvin, you take Boltzmann's constant. Candela, you take luminous efficacy. Kilograms, you take Lang's constant. For Boltzmann, you take Avogadro's number. So all these units have not been defined through one measurement and six constants of nature. That's going to be the new world that we're going to live in starting from May of 2019. So what's the big deal? What does this teach you? Did we do something great or did we just do it to change something for fun? So of course I don't get heavier or taller or brighter. My luminous efficacy does not change, but that's the whole idea. Idea of changing definition of kilogram is not to change kilogram. One should not have to change any units that are currently intact. The meter scale that you have in your lab will stay the meter scale. What we ensure that the meter scale stays the meter scale for millions of years, not that you change it. So units should not be changed values of units will not change to one part of the billion. That's why we go to all this trouble to fix the value of life constant H. We go down to measure all these physical constants to at least nine decimal places so that we know they will not undergo a change. So the motto is we change it for posterity. So which means that even if no physical standards of today survive, both will survive, but of course basis go out of space. People after centuries will still know what exactly we made by a second, what we made by meter or what we made by a kilogram or what we made by ampere. Only thing that they will need perhaps is a CGM atom. So that's the only measurement based on that everything is. So we only need to measure this frequency of CGM oscillations. This can be done by anyone, anymore. If you are CGM here you can do the HBCSC and that means that you will have defined all the fundamental units in HBCC lab and you could do that job in, once you measure the CGM line in five minutes. You don't need to go to Paris, open the vault and see what the kilogram is. You know everything, you know kilogram, ampere, you know mole, you know candela, blah, blah, blah. So once you do this, the forever fixed fundamental constants allow us to reconstruct all the units. So it gives you a different way of thinking about units. It means all units are basically the same. Second is not very different from kilogram. It's not very different from meter. Then the question we face is, why do we have all these stage values? So these are our stage constants. C is some, I don't know, some nine digit number. H is somewhere, E has some nine digit representation. A, they, blah, blah, blah. So why are these constants of nature so strange? Why are they not simple? And answer actually lies in our ignorance. We did not know. When we defined second and meter, we did not know that they will be related by speed of light, which will be constant of nature. When we defined Kelvin and Joule, we did not know that they will be a Boltzmann constant, which will relate Kelvin and Joule. We defined all these things independently. We did not know that they are connected. It is now, after maybe about 100 years, depending on which units they are, that we know that all the units are connected by laws of nature and therefore, by fundamental constants. So what this change is doing? It is giving prominence to laws of nature as opposed to facts in nature or objects in nature. We don't give prominence to a meter scale or to iridium platinum ball of one kilogram. We give prominence to laws of nature and that's a major way in thinking about this. That's what I wanted to make up. You could think make this game up. Now that I have laws of nature, why do I need these units? I can use some better units. I can define natural units. I call in that units. So maybe I define on that second. I say okay, I define on that meter such that speed of light is one that meter upon that second. Why not? I define that kilogram so that Planck's constant is one, that joule, that second. I define that ampere, so electron charge is one, that ampere, that second. Similarly, that mole, that Kelvin and that Kandela. Why not? In principle, if the world is agreeable to change, I can define that unit like this. Of course, it's difficult to do practically but I want to point out in principle that nothing against doing this. This could happen, which will make algebra quite simple but of course, I'm not saying that we should do this. But as physicist, you do it very often. You might have seen at some point of time you use C equal to one, H equal to two pi, Boltzmann constant equal to one, KT equal to one. You would see if you haven't seen now you will see people doing it very often. People like me who are lazy to keep tag of constants always do that. These are so-called natural units. So there's nothing great in the knowing that the speed of light was blah, blah, blah. That value of two, nine, nine, nine, nine came simply by coincidence. So happened, that somebody defined second in a particular way, somebody else defined middle in a particular way. It so happened that the speed of light was two, nine, nine, blah, blah, blah. I could have chosen units if I was brilliant enough. So that all costs of the method actually are one. We do this. Can you think of some place where you actually use natural units while talking about... Anybody from Astronomy Olympiad here? Okay. How far is the nearest star to the sun? About four light years. So what we did? We used years, which was the unit of time. We multiplied that by distance covered by light in one year and we called it light year. So this conversion of time to distance unit is something that we are used to doing this. We say the four light years, that's really right. That's the answer that people would have thought of. One would not think of trying to look at the distance as something 10 to the power 27 meters, right? We would think of light years. So it's not unnatural to think of natural units. People do think of natural units. And in fact, the fact that the units are related is actually therefore important. So the thing to emphasize is that the simple thing that we use in everyday life actually needed knowledge of physics, chemistry, and many of the different aspects. Defining seconds, we needed atomic physics. For meter, we needed a special theory. Kilogram, we needed quantum mechanics. Ampere's, electromagnetism. Moles, we needed chemistry or atomic molecular physics for Kelvin. We needed thermodynamics, terrestrial physics for candlelight engineering optics, okay? Which is to say that even for making any simple measurement in everyday life, not other physicists, but simply as somebody who goes and buys groceries, I'm actually using the so-called esoteric physics that we think in everyday life, but it's actually is behind, even though the grocer has a mass of kilogram, all of these things actually go behind that, okay? And that's the message that I think we should all tell our friends who are not scientists that these pieces of science do go behind even defining what you do in the everyday life, okay? So this old SI that you have now has sort of become this new SI, this new connection, okay? So to end with the final provocative question, okay? All of these things, dependent on having a fundamental consciousness that do not change, okay? So the question would be what if the constants change, okay? The reason that we have Barossa on this is we have been testing them continuously, okay? That the job of meteorologists to do continuous laboratory testing, we always get indirect evidences by observing the universe. For example, the test for like constant can be done by looking at sodium lines coming from stars, which are at very, very large red shifts, for example. More than that, these fundamental constants are not constant just because they're major. They're enshrined in theories like relativity or quantum mechanics or thermodynamics. So they are integral part of these theories. The constants are not constants, the theories will collapse and experiments, therefore, will go wrong. So any test of these theories, and everyone around the world, many physicists are doing this all the time, is a test of constancy of the fundamental constants, okay? And therefore, we are fairly confident that these constants will not change, okay? Of course, it does observe to change. We will have exciting times ahead. And as a theorist, I hope for these times to come. I want meteorologists to be free for now, okay? Yeah, so let's wait till May 2019. 20th May 2019 is the world meteorology day, because on this day, the tree of the meter was signed in 1875, if I'm correct. And on that day, we will enter a new era where our kilogram will be different, where our ampere will be different, where our mole will be different. Yeah, but our world will stay the same. Thank you. I just want to know about that silicon sphere. Where is it located? And can you tell something about how it was polished, because polishing would destroy the crystallinity of the surface layers? Yeah, so there was, as I said, this name of the chemical which I forget, which I found on Google. Yeah, so this is some chemical which can scrape things off and evaporate, okay? So I'm not an expert in this, but maybe we can figure it out and ask Google then. I think it's an NIST in the Boulder, but I'm not sure, okay, so I don't know, I don't know. It's CZM. I think it's just because this line, perhaps, would be very, very narrow, and the narrower the line you have, the better accuracy you have. The suspect is one of the narrowest lines. Okay. Any other questions? No questions from the students? Okay, if not, let's thank, oh, there's one question. I heard that there is a standard meter scale using Fabry-Barré Tetillon's interferometer, which is also kept in the museum, Paris, which is used for the standardization of a meter scale. So any updates on that? No, so see, for example, a standard of kilogram like this, or a standard meter like this, will always exist, okay? So it will always exist except that now that will not be the standard. That is something that will be checked, so maybe frequently, whether it corresponds to whatever we have now. Okay, so I suspect that the kilogram retirement does not mean that the kilogram is cease to exist. It will still be kept there. I think it will still be kept in the beijer. It will perhaps still be in the museum. It will not be used to define the kilogram. That's it. Similarly, I think the meter scale still exists in the same BIPM. The meter still exists, but that's not a standard one. That is checked against the speed of light, this is the level of light in certain times. You've shown a picture of the measurement of H, Planck's constant, and the error bars. I mean, not the latest one, but the ones previous to that actually, the error bars are not coinciding with the current value. So does it mean those measurements were wrong? Well, so that's a question for the experts, but of course, everything is within about two sigma. See, remember that the condition there, which is expected that there will be some fluctuations. In fact, if you see the condition, that they should be consistent within 95 percent, about 95 percent of confidence, which is about two sigma. So the expectation was, let me just see if I show this here. So this is the perhaps something that you're referring to. Then the slightly off by the only this much amount. But remember that, so this experiment was perhaps repeated again. In fact, the two, this is your, this is MetaS, NPN, and I think it has done what? It has to what? It has to what was repeated again to get a better measurement. Textbook will change, probably year 2020. Science textbook will have change, major change, and a lot of work for publishing house, for publishing the new books and all that. So that's what I am looking at. I don't know how long it will take for this thing to come to change the textbook. This could be random, that's perhaps. Definitions can be. Yeah, some definitions will be same, okay. You said that the fundamental constants might change. But for example, the Avogadro number, Avogadro's constant may not change, right? So how do we know when something changes? I mean, do you want to say something about that? So I'll give you an example, okay? So I gave you an example of, so you look at particular atomic transitions which are coming from stars, which were at a large red shift, which means that light started from them about a few billion years ago. Now suppose, okay, so let me tell you, take this for granted that the frequency of the atomic transition lines that you see here depends on the value of h cross. So it depends on h cross and c and e, some combination of e square by h c, which is alpha. So now, the atomic transition lines that you see now from that star started from there 10 million years ago, let's say. Now 10 million years ago, if light constant was different, then what I observe of those lines now is going to be different, okay? So that's the way of checking whether the constants of nature change in time. In fact, measurement like this has been done a few 10 years ago, there were papers which said that these measurements showed that the value of this e square by h c could be different by one part into minus nine, minus five over 10.9 years, but of course it's more data, it turned out it is for the fluctuation. But in principle, there exist ways of checking if, see simply because we can do experiments here with things that are happening here. We can do experiments, when you do astrophysical experiments, we are basically doing experiments that happened a million years ago. So we have access to experiments done now and experiments, what are events that happened million years ago. We compare the two and see if they correspond to the same physical laws. So there are ways of checking this. Sir, what exactly is a charge and how do we define a charge? A charge, difficult question. An electric charge is a property of an electron because of which it experiences electromagnetic force. So a charge of an electron, okay, let me put. A charge of an electron is the strength of interaction of electron with light. So that will be a good definition. Sir, do we know the origin of charge? Like where it has come from? So that will go back to Ganesh, for example. His talk, but at some point of time. So in science, you define certain things and then you proceed from here, okay. You can understand what a charge is in many, many different directions, okay. What was this as electron and photon coupling? One is saying that there's something called as, finish it like to describe nature in terms of what is called as gate symmetries. And this charge comes as the strength of the gate symmetry of an electron's wave function. You can think of this as the strength by which electrons or particles will repel or attack each other. Origin of charge is a very difficult question because the question I think is, it takes some time to define the question. What do you mean the origin of charge? What kind of answer are you looking for? We know that particles gain mass by interacting with the Higgs field. So where does the charge come from exactly? That is interacting with the electromagnetic field. Exactly the same. The theories we have, there is no difference between particles getting mass from Higgs field to particles getting charged from. So particles mass is proportional to its interaction with the Higgs field. Similarly, particles charge is proportional to its interaction with electromagnetic field. A very practical question. See, we have standards, primary standards kept somewhere in Paris as of now. And finally, I get a meter scale from a stationary store. What could be the error? I mean, will it impact the measurements? Okay, Fargate Grocer's shop, but the laboratory experiments which you do, what kind of error can it introduce? So this particular change? Not the change. Even in the old system, you are making primary standard, secondary standard. Finally, you get a meter scale from a shop. And use it in the lab also. So what is the kind of error I can expect? Well, so usually the error in the instrument is at least expected to be less than its least count. Yeah, what I call a meter on a meter scale, which I buy from my shop, and the real meter, how much difference can be there? I don't know. I would expect it to be smaller than a millimeter simply because the least count scales are of millimeter. And of course, the lab equipment which claim to be able to measure it to greater accuracy, I would expect that they have reproduced the meter to that much greater accuracy. There are no numbers like, I mean, it's a one percent, I mean, one percent offer. I mean, any such... Yeah, so this error will decide what is the standard of the equipment. High standard equipment will have low error. Okay, and that will also, I guess the cost of equipment also will be proportional to. It's difficult to say just by your cover. Okay, I think now we'll take the tea break for about 20 minutes. Prasad Digha will be available outside. Let's first thank Prasad Digha for his talk again. Thank you.