Physics 11 U1L1 What is Physics 2026

And apparently there's no countdown. Apparently we're live. So let's go like this. Record lesson. Here we go. I started teaching math in 1997 here at Pit Meadow Secondary School. I'm a math teacher by training. And uh in 2003, the physics teacher here retired and nobody in the science department wanted to take physics on. In fact, we're going to have to lay off a good young science teacher to hire a physics teacher. I had a phenomenal physics teacher in high school. My physics teacher, Mr. Cowell, won a prime minister's teachers award for being one of the best teachers in Canada, one of my nerd heroes. And so that summer, I sheepishly raised my hand and I said to the principal, I'll try teaching physics. And when I went home and I told my parents that, my mom said, you know, Kelvin, I know what math is. It's adding and subtracting and exponents and square roots. What the heck is physics? And I'm willing to bet some of you, if I asked you to define what physics was, wouldn't be able to give a good explanation. I've learned a lot of people, oh, Dick is pretty fun. They're signing up because they've heard my name. I'm going to tell you the definition that my prof, my physics prof told me. It's not the official definition, but I like it because it kind of encompasses everything. My prof used to say that physics was the science of measurement. He said, "If you're measuring something, you're doing physics." It's a really arrogant explanation or definition, Brady, because it means that that includes chemistry. We would say that's physics, too. Haha. It includes biology, that's physics, too. It's kind of an arrogant thing. We physicists think that everything is physics. We kind of look down at the other sciences. But it leads to a question. What do we measure? So we have something in science that we you think of it as the quote metric system but in reality Liam it's the SI system it's French it stands for system international it's a system of units and prefixes that works really well there are seven fundamental SI units the first three the ones that we're going to look at quite a bit here well the first thing we're going to measure is length Does anybody know what the SI system internal fundamental unit for length is? Make a guess and you're probably right. Guess no, we're not allowed to shrug. Yeah, of course it's meters. In Canada, we spell meters re e, not e r. But we're never going to write that word out again. We're going to a brief. In fact, we're going to be lazy but organized. I will never ever have you write down unnecessary stuff. Lily, Lily, Lily, I will obsess over what's the minimum amount of writing for the maximum bang for our buck. You're going to hear me use the phrase lazy but organized over and over. Last year at grad, two grade 12 girls gave me that poster behind me of duisms. They call stuff I say over and over. And I'm sure if you look somewhere on there, you'll see the phrase lazy but organized. We're going to do that. And so we're going to a brief. We're going to use a lowercase m as our brief for meters. But we're also going to run into a problem. We're going to run into a problem. Hater. The problem is this. We have 26 lowercase letters and 26 uppercase letters. And we got more than 52 things out there. So we're going to have to reuse certain letters. We're already going to start to run into that today. And so we're going to have to also look at the context or where the letters show up to know what they're in the brief for. We also measure time. Serena, welcome to my class. Make a guess. What do you think the fundamental SI unit for time is? >> Yes. What would be a good abbreviation? [snorts] >> No. that uh for as an abbrevation, we're going to use a lowercase m for mass. So, this is already where we're going to start to reuse letters. If you see this, that's an abbrev. If you see this, that's an abbrev. And we're just going to have to get used to, we don't have enough letters. We're going to have to reuse them. You're going to hear me say that all throughout the year. Uh the worst one is the capital letter W, which is west, work, and watts, depending on where it's shows up in an equation or in a situation. But what are the units for mass? >> GS >> which is an abbrev. >> Okay. So >> okay it was originally grams lowercase g but then they realized the gram was too small and all of the answers that they were getting were just way too big. And so they eventually decided on the kilogram kg lowerase K lowerase G. Everything you learn this year is going to be made up of those jewels or kilogram meters/s squared meters. Newtons are kilogram meters/s squared. Everything that you learn this year is going to be some some combination of kilograms, seconds and meters. And in fact, there are only seven fundamental units in all of science. Everything else is made up of combinations of those sevens. So we call this the SI units or the system internal. The base quantities we already said uh meters, seconds, and kilograms. If you want to, you can draw a little arrow right there. That's where physics 11 is going to stop. Physics 12, the second half of physics 12 is electricity and magnetism. And so we bring in the unit for electric current. Does anybody know what the correct unit for electric current is? Make a guess if you're not sure. I told you yesterday I want to create a classroom where you're comfortable getting something wrong and learn. IT'S HOW WE LEARN. >> WATTS. >> Watts is power. watt is a jewel per second which is a kilogram meter/s squared meter/s. So no, but good thought. That's a common guess. >> Yep. >> Jewels is a measure of energy. Jewels is uh force time distance. So it's kilogram meters/s squar time meters. You don't need to memorize all of those. By the way, I'll show you how you can figure them all out from the equations that I've given you on your green sheet. There's a lot of information packed in there. Starts with letter A. Not that many electric nerds here. So we measure electric current in amps. And that abbrev is a capital letter A. Lowercase A is angstroms which is something different. Named after a physicist whose last name was Ampier. One of the biggies in the discoveries of electricity. The next one is my favorite because I have a massive ego. The temperature of an object. What is the SI fundamental unit for temperature? It's measured in degrees but a specific scale. >> Kelvin. >> What? >> Kelvin. >> What? >> Kelvin. >> What? My first name is Kelvin. My first name happens to be one of the fundamental SI units. So yes, it's degrees Kelvin or degrees lowercase K. I could not I could not Mila have planned that any better. Thank you, Mom and Dad. I'm old school. You call me Mr. Dick. However, if you're ever solving an equation, getting an answer in degrees Kelvin, you are absolutely allowed to say, "Mr. Dick, the answer is 7° Kelvin Kelvin." I can't resist that stupid joke where you tack on my name to the I will live with that. Absolutely can't resist that one. Amount of a substance. This one you'll see in chemistry. Your hint is Mr. Perua has one at the front of his classroom hanging above I think above his little projector light. Yes. Mole M. We won't be touching on that here but you will in chem. The last one is kind of cool. Um, meter. Unless you've seen a meter stick, you don't know that a meter is about this long. You have to have seen a meter stick before you know how long a meter is. Unless you've looked at a watch hand, you don't know that a second is about that long. You have to have seen that. But this next one, as soon as I tell you the name, you'll know how bright it is. Does anybody know the SI fundamental unit for light intensity or brightness? It's candles. You know how bright one candle is? Exactly how bright you're thinking right now. It's actually candelas. But one candela is one candle and that a brief is lowerase C lowerase A. Yeah. How bright is one candle? Yeah, that bright. We don't look at that in physics 11 or 12 either. But these seven fundamental units are defined. A thousand years from now, if by then humans have moved to other planets or maybe even out of our solar system, if the Earth exploded, we could recreate those seven units to a ridiculous level of accuracy so that all of our engineering textbooks would still be correct, all of our science textbooks would still be correct. And that's why we we've we've done this with one exception. Six of those units were defined. One was only defined in 2019. And I'll tell you the story about that later in class under nerd trivia. It's not something you need to know. Turn the page. Here's science humor. Says, "Here's an easy way to compare temperature units on the Fahrenheit scale, which the US uses. If it's 100° Fahrenheit, that's really hot outside. If it's 0 degrees Fahrenheit, that's really cold outside. On the Celsius scale, if it's 100° Celsius outside, you're dead. If it's 0 degrees Celsius outside, that's fairly cold outside. On the Kelvin scale, if it's 100° Kelvin outside, you're dead. If it's 0 degrees Kelvin outside, you're dead. Just saying. Turn the page. Uh in addition to the base units, the SI system uses a prefix system to allow you to multiply by big or small powers of 10. Prefixes can be placed in front of a base unit. They represent a multiplying factor. For example, kilogram, kilo is an abbrev the 3. And so it means times by 1,000 or times by 10 the 3. Wyatt, what is less writing? Writing out all of the zeros or just using an exponent? >> Exponent. >> Which one do you think I'm always going to go with? Lazy but organized. I'm going to use the exponents. These are the SI prefixes and you have to memorize them. No, you don't. I told you yesterday. I think memorizing useless stuff is a waste of time. Madison's figured it. Madison. Yes. >> Yeah. >> Maddie, do you prefer Maddie? Okay, I'll change that. Uh, look at the back page of your green sheet, please. Brady, you too. They're all on there. You might memorize a few out of laziness. In fact, in physics 12, there's one in one particular unit. I'll tell you memorize this one because you get a sick of looking it up over the next few weeks, but you don't need to. I do know Mr. Perua might make you memorize a few of them, but not all of them. We're going to write these out once so you know they exist, but Ben, you do not need to commit these to memory. In the middle we have the base unit and that could be seconds or grams or jewels or volts or watts or or whatever your base unit is. There's a whole bunch of them out there that are made up of those seven fund. Oh, by the way, typically what we do although everything comes from those seven fundamental units, rather than write them out, we give it a name. As an example, forces measured in Newtons, which is actually a kilogram meter/s squared. But a 100 years ago, people got sick of writing a kilogram meter/s squared. And so they renamed that after a famous scientist, the Newton. And so you're going to notice like watts is jewels per second, which is kilogram meters/s per uh squared meters/s. Nobody writes that out anymore. We abbreve the base unit. You're multiplying by one. But because Brady I want to keep a power of 10 pattern, I'm going to say you're multiplying by 10 to the 0. The initial SI system started in the in France um right around the time of the French Revolution, which I'm going to say was 1760 1770 I think. I'm going from memory. And they didn't quite know what they were doing. It was happening organically. There's now an international committee and so some of the stuff isn't going to follow a pattern and then it's going to follow a pattern. I'll show you what I mean. Phoebe, can you look at your green sheet? What's the abbreviate? And be make sure when I ask you tell me uppercase or lower case because it's going to make a difference. And Phoebe is that times 10^ the 1. Yes. which the same as dividing by 10, but I want to keep powers of 10, so I'll use negative negative exponents. No one ever uses that. I never say that's one decimeter. I say it's 0.1 meters, but they're there. Centi we use a little bit because you're probably familiar with centiers, lowercase c and mason, that's 10 to the -2. Yes. >> Yeah. >> That negative in front of the negative one hardly showed up. Let me do that a little better. Millie. I I Sorry, I can't resist. Mie. Millie. Uh, what's the abbrevi Sorry, I couldn't resist. >> Wait a minute. Isn't that meter? This is our first situation. We're already running out of letters. So if you see the lowercase m in front of a letter that's millie. If you see the lowercase m after a letter that's meters. It's what we got. Lowercase m and milly mlly is 103. Yes. And thank you for indulging my stupid joke there. Micro. Lily. What letter does the word micro start with? What letter does the word micro start with? Just look at the word micro. What's the first letter, kiddo? M. Why can't we use an M? Already taken from Millie. So already now we're going to start pulling in some Greek letters. Okay. The Greek letter is called a Greek letter mu. Looks like this. If you look up, I have one cut out. A student did this for me a few years ago. It looks like a lowercase u with a really droopy tail. So it looks like that and it's 10^ the -6. There's all sorts of terrible jokes. What does a Greek cow say? Mew. What does a Greek cat say? Mew. You're going to hear my favorite joke later on this year. Hey, what's me with you? I don't know. What's me with you? Nothing. Okay, tough audience. Tip your waitress. Try the deal. I'm old enough that I remember when computers were called micro computers. And what they were saying was the computer chips had just reached micro is 1 1 millionth because you're dividing by 10^ the 6, tsing by 106. They had reached 1 millionth of a meter. The chips were that small. They're much smaller now. When I first started teaching, I stopped Taylor with the next one. Taylor, what's the abbrev for nano? >> Upper or lower case? >> Oh, I don't need that. And uh that's 10 to the 9. Yes. If you are ever forced to memorize I mean nanogative9. There's the triple N's there if you're looking for a stupid hook. 1 billionth. Nanotechnology was the cutting edge when I started teaching physics in 2003. I used to stop the chart here, but I've had to slowly expand it as technology has gone further. And so we now have pico pico predom. What's the abbrevi? you start to notice they go up by powers of three or in this case down by powers of three 6 9 12 and that's engineering speak. If you become an engineer you'll do everything in powers of 10 to the three and you'll probably memorize all of these. It just becomes like a second language so you just speak it fluently. Uh pico you're on the size of very large molecules. phento is uh 10 to the negative 15th. Is that correct? Later on, not today, but later on in the unit, I will show you a video of a phento camera. This is a camera that can video a phento second that's so fast that you'll actually be able to see a beam of light move across the screen slowly. That's in one phto second. It's it's very cool technology. And then ato is lowercase a uh at you're on the size of a proton and that's 10^ the 18. You'll notice as we go smaller all the letters are lowercase. Had they been thinking as we get bigger they would have made all the letters uppercase. They didn't think of that right away. It took about a hundred years and then they decided that. So there's going to be a little bit Well, Josh, what's the abbreviase? They should have made it uppercase. In hindsight, if we were doing it over again, we would have said, "Oh, if you're getting bigger, uppercase letters makes sense. That's 10 to the positive one." Yes. >> And again, no one ever uses it. I don't say oh it's a decimeter it's 10 meters hecto is also very similar hecto is a lowercase h 10^ the 2 hecto I hardly ever use I don't say a football field is one hecto yard it's 100 yards right but the next one ah the next one kira we do use what's the abbrev kilo and it's a lowercase still because it's an older one and they hadn't yet set the tradition so it's a lowercase k 10 the 3r kilo you'll use kilogram kilometers that one's going to show up when you get your licenses you'll figure out pretty quickly 2 km is 2 m then they standardized it and they said you know what if we're getting bigger it should be uppercase it makes way more sense So, oh, who haven't I picked on yet? I'm reading upside down, but I Oh, Sahil. No, Sahil. Sahil. Got it. Gez, I butchered it three times. I'll get it. Sahil. Uh, what's the abbrevi? >> Okay. And that's 10 to the 6th. Yes. In fact, now we start going up by powers of three. That was also the standard. They kind of went, why don't you just go up by powers of three? The same way our number system, every power three zeros becomes a thousand or a million or a billion or a. So they kind of standardize it that way. Mega is a million. I've memorized that because I know there's a lottery called megaillions. So I can remember that. My first hard drive was 8 megabytes. And I can remember thinking, how will I ever fill that? Is there even that much data on Earth? Now megabytes, nothing. You folks are kind of the next generation going into the one after that cuz giga all of your phones have gigs of memory. So you're probably familiar with that term. Gigs. Uh capital G. Yes. 10 to the 9th or a billion. And terra is now fairly common as well. You can't see I've got a little plug-in external hard drive here and it's for terra bytes. It's not measured in gigabytes anymore. So that's become in fact this is where I used to stop until about 7 or 8 years ago. I stopped with terra. Terra is capital T 12th. Uh what comes after a billion? It goes thousands, millions, billions. Yep. Only in it's this is weird. Only in North America. In Europe it's a thousand billion in North America. Sorry, I got to be careful. Sorry. Billion is different in North America. So we go million and then billion. In Europe they go million, thousand million million million then billion. Their billion is our quadrillion. And if you don't think that leads to confusion on the world economic markets, I thought all numbers were the same. No, they're not. It's not standardized. Uh, PETA. So, Google talks about having pedaflops of data or pabytes of data. capital P 10^ the 15th and then XA capital E 10 the 18th. Again, let me emphasize, do you need to memorize these? Everybody say no. No. Can you out of laziness? Sure. Or out of nerdness. There is a obscure pattern by the way. This is not on the test. You're going to find I love nerd trivia. In fact, what you're going to see me do is I'm going to go # nerd trivia. There is a pattern for the bigger ones. If you go up by powers of three, the first power of three kilo is 10 the 3r. The second power of three 10^ the 6th is mega pattern hasn't showed up yet. The third power of three is giga 10^ the 9th and then this is how the committee comes up with these names. The fourth power of three pen stopped working. So this is still a problem. Give me one second. I've learned if I just go like this, plug it in, then unplug it, we're working again. Okay. The fourth power of three. What came after giga? >> Okay. It's the fourth power of three. There is a Greek prefix for four. Tetra terra. the fifth power of three. What do you call a five-sided shape? You've learned this. You've got some knowledge. There's a little call in response here, folks. What do you call a five-sided shape? Starts with letter P. They said five is pentagon. They crossed out the there's where the pa comes from. The sixth power of three. What do you call a six-sided shape, boys and girls? What letter do they cross out? >> There's the exo. I haven't checked to see if they've defined the next one, but the next one would be the seventh power of three, which would be 21. Uh, it's either going to be seta or sepa. They'll cross out one of those letters. And then it would be octa. So, it's going to be a ora. I don't know which one. But if you're wondering, that's the pattern. And there's an international committee that decides these things. You can survive your life just fine without knowing that. But I like to think that I just enriched your life a little bit maybe. Okay. Nothing. Okay. Back to here. Any base unit can be used with a prefix. So for example, 1 kilogram kilo is 10 the 3r or a,000. I can just multiply the one by a,000. That's 1,000 grams. 100 cm centi is 10^ the -2. If I go 100 * 10 -2 I get 1 meter 50 mega me capital m lowercase m me m me m me m me m me m me m me m me m me m me m me m me m me me me me me me me me me me me me me me me me me me me me me me me me me me mega is 10 the 6 or a million it's 50 times a million or 50 million meters 200 milliliters this and this is where I was saying that the context matters a little m in front of a letter means millie a lowercase m after a letter means meters we're going to have to get used to it so millie is 103 if you 200 * 103 on your calculator or in your head you get 02 L.1 terrac again terra is 10 12th if you go 0.1 * 10 12th you get that 100 million 100 centto it's that put your pencils down look up of the seven fundamental units. Six were very defined. The weird one is the one that Brady already pointed out at the beginning was kind of weird because he said grams because it didn't have a letter in front of it, but then you kind of caught yourself and you went, "No, I think we use the kilogram. What? Why is that the one that has the metric prefix in front? What are the others do?" It's because the mass unit is weird. Put your pencils down. Really? I got to download this. I told it to download this already. New laptop. This is going to drive me crazy. >> Sorry YouTube for the dead space. >> Can I hold it? I think you promise to be really really careful. >> I promise I will be so incredibly careful. I will be incredibly careful with >> I promise. >> All right. >> So, it's slippery. Be careful. [laughter] >> All right. Are you ready? I'm about to touch a 1 kilogram sphere of silicon 28 atoms. They're about 2.15 * 10 25 of them. It feels absolutely incredible. Wow, that is amazing. Besides its creators, I am one of only a handful of people ever to hold this sphere. The raw material used to make it was worth €1 million, but now that it has been so precisely sculpted, how much is that worth? >> It's priceless. This you're looking at now is the roundest [music] object in the world. >> If there isn't intelligent life out there, that is the roundest object in the universe by far. How can you say for sure that it's the roundest object? I mean, the Earth is pretty round, isn't it? >> If this was the Earth, >> if this were the Earth, then the highest mountain in the lowest valley would be >> about 14 [clears throat] m apart. So if you blew it up to the size of the earth, the difference between the highest and lowest only about 14 m. They shaved this down to the nearest atom. They were that smooth, that round. [clears throat] [laughter] >> That is shocking. That is shockingly round. But why would you invest €1 million and thousands of manh hours perfecting a pure polished silicon sphere? Well, the answer is grave. or rather grav as it would have been pronounced in the original French. You see the grav was the original name for the base unit of mass in the metric system which became the system international junite [music] or SI units. In 1793 a commission which included notable scientist and aristocrat Antoine Lavoisier defined the base unit of mass as the weight of a cubic decimeter of water at the melting temperature of ice. essentially just a liter of ice water. The name grav came from the Latin gravitas meaning weight, [music] but it wasn't to last. It sounded too similar to the aristocratic title graph, which is the equivalent of an earl or a count. And with the French Revolution in full swing with a rallying cry of equality for all, you couldn't exactly have one unit noboler than the others. At this, Lavoisier lost his head literally. I got to pause. If you ask Mr. Perua who his favorite scientist is, he'll often say Antoine Lavoisier, who was a very good chemist who helped invent the metric system, but who went out in style. The story goes that he knew he was going to get guillotined. And so he said to his friends, "Look, as soon as they cut my head off, run and grab my head and ask me yes or no questions. I'll blink twice for yes and once for no. Let's see how long I remain conscious." and they did it and he didn't respond. The reason that we know that if you're decapitated, death is instantaneous. That's the experiment right there. Talk about going out in style. You're going to kill me. I'm going to have some fun with my execution. I I kind of got to hashtag respect that, I think. >> Not because he helped deise one of the greatest systems of measurement of all time, but because he was collecting taxes as a nobleman. So things really were grave. The new Republican government believed a gra would be too big for the things they wanted to measure anyway. And so they settled on a gram, which was just a thousandth of a gra as Brady suggested. But then they realized, as Brady did too, but soon they realized that a gram was too small and so they returned to the grav. But since [music] they couldn't call it that, they invented the kilogram, 1,000 g. And that is why out of the seven base SI units, the kilogram is the only one to have a prefix in its name. In 1799, the kilogram definition was refined to be the mass of a liter of water at 4° C, the temperature at which it is densest. But water itself is obviously not the most sensible thing to use as a mass standard. So a pure platinum cylinder was created to have the same mass as the water definition and it was declared kilogram of the archives. Now it's important to note at this point the kilogram is no longer tied to the mass of a volume of water. The kilogram of the archives is by definition the kilogram. 90 years later in 1889 the kilogram was upgraded to a platinum aridium alloy cylinder. Now, it was much harder than the original, but was otherwise basically identical. And to this day, it remains the definition of the kilogram. It is officially called the international prototype kilogram, though it's affectionately known as Lrand C or Big K. Oh, and it's about this big. It is the only thing in the entire universe with a mass of exactly 1 kilogram because it is the kilogram. It is also the only SI unit that is still defined by a physical object. It sits under three bell jars next to six sister kilograms in a climate controlled vault locked by three infinitely controlled keys in the basement of the International Bureau of Weights and Measures on the outskirts of Paris. Now, if you were able to break into the vault and tamper with big K, you would actually be changing the definition of the kilogram, a definition on which many of our measurements rely. That is the best James Bond movie plot ever. You even got the name of the villain, the Grand C. He breaks into the international vault of measures. He substitutes the one and only kilogram with his own. World markets plunge into e economic disarray. Nobody knows how much anything weighs. Oh, wouldn't that be a great James Bond plot? I'm telling you. Or maybe not. And so you would throw the world into chaos. Well, no, not actually. But how would anyone ever know if the mass of Big K changed? Well, when it was first created, 40 identical replicas were also made. Well, they weren't quite identical. They had a mass which was slightly different to Big K, but those offsets were recorded. Now, these replicas were sent out to countries around the world to serve as their national standards. Canada has one. In 1948, the kilograms were reunited for a weigh-in. And this is when the problem started because even though all the cylinders were made of the same alloy and stored under virtually the same conditions, their masses had diverged over time. The mass of Big K wasn't even the same as the six sister cylinders stored with it. And to make matters worse, when they were brought together again 40 years later, their masses had further diverged up to about 50 microgram. That's about the weight of a fingerprint. But fingerprints were not the culprits since the kilograms were carefully washed before their weigh-ins. So some physical process must have actually changed the mass of the cylinders. But how that exactly works remains a matter of speculation. One thing is for certain, the mass of a platinum aridium cylinder is not stable over time. And this is a big problem. You can't have a unit which changes its value. And the fallout isn't limited to measurements of mass since of the seven base SI units, four of them depend on the mass of the kilogram. Not to mention all the derived units like Newtons, jewels, volts, and watts. At this point, those of you in countries that have not adopted the metric system, yes, I'm speaking to you, Liberia, Burma, and the US. >> Burma went metric about 7 years ago. So, it's just Liberia and the US that haven't adopted the metric system. All of you can collectively roll your eyes if you want to because you may be feeling rather smug that your base unit of mass the dua pound is no longer defined by a physical object. No, instead it is defined as precisely 0.45359237 kg sucked in. So clearly something needs to be done to eliminate the kilograms dependence on a physical object. And this is where the silicon sphere comes in. But how exactly does that help? Here you have a physical object and it's beautiful, but you know it's still a physical object. You're trying to get get away from that. >> We're trying to get away from the physical object, but what we're doing with this particular object is counting how many atoms are in there. [laughter] >> You can't actually count how many are in there, can you? >> You can't count how many are in there, but you can calculate how many are in there because this material is silicon. There's no voids or dislocations. >> So, this is like a perfect crystal of silicon. >> That's right. >> Not only is it pure silicon, it contains only one isotope of silicon, silicon 28. And that explains why the original material was so expensive. [music] >> And why a sphere? >> Well, a sphere is a pretty simple object. If you know the diameter of the sphere, you can characterize the entire dimension of the object. Well, that explains why the sphere has to be the roundest object ever created. But how do you actually make something that round? >> We actually start with an oversized sphere. So, it was about 2 mm larger in diameter. And then we just grind it progressively finer and finer using um abrasive. It's actually massaging atoms. You're down at that level of of trying to control the shape of an object down at the atomic level. But making the sphere is only half the battle. Then you need to accurately measure its diameter. >> The diameter is actually measured by a laser. So you're actually measuring um having the sphere in the center of the cavity and a laser is hitting both sides and you're actually measuring the gap. [music] >> By knowing the diameter, you can determine its volume. And since the atom spacing of silicon is known to high precision, you can then calculate how many atoms make up the sphere. This allows you to redefine Avagadro's constant. At the moment, Avagadro's constant is defined based on a kilogram. It is equal to the number of atoms in 12 g of carbon 12. But using this approach, the number of silicon atoms in the sphere would be used to fix Avagadro's constant, which would then define the kilogram. So even if the silicon spheres were lost or damaged, it would have no effect on the definition of the kilogram because it would be defined not by a physical object but by a concept. If you would like to see the official definition of the kilogram, say a kilogram is the mass of 2.15 * 10 25 silicon 28. >> Yes. >> There's a there's a likelihood a high likelihood that it's going to happen. But there is another approach to redefining the kilogram which involves fixing planks constant and it's done using something called a watt balance. These two approaches are complimentary. Each one provides a check on the other and if they show good agreement and are able to bring their uncertainties down to about 20 micrograms, they may redefine the kilogram as early as 2014. And then the kilogram finally will be an unchanging unit no longer defined by a physical object in the basement vault of some place in Paris. It took till 2019, but in 2019 the kilogram became the last of those seven fundamental units to be based on an object. Instead, it's now based on a procedure. And if Earth blew up, we could recreate it to a ridiculous level of accuracy. There are people that think about this. There are committees that think about this and although most people aren't aware of that it actually is really important because everything else all of our engineering and measurement kind of depends on getting those things right. Just saying. Okay, I am going to go rightclick stop recording.