Claming that the "e-mark" stands just for estimated is a little bit confusing as it states per EU Directive 76/211/EEC that the filling of the product container is controlled so that it should fall with in given limits. Also the the directive states that the quantity cannot be altered without opening or destroying the packing material.
My dad once complained to me that since speed of light is so close to 300,000,000 m/s, we should have just redefined the meter to make it exactly so. Physicist dad complaints.
I'd love to have a dad like that. My dad just loves cars, meaning I know an awful lot about cars. I'd rather know an awful lot more about physics and mathematics.
I was thinking exactly in the same line, if you decide to peg the unit to a constant, then let's make the constant round and change the unit accordingly.
Kevin Higby You could, but you could hardly call it a metre anymore ( you couldn't really do it for the 3e8 thing either honestly). Call it a Higby and see if it catches on.
IamGrimalkin Yeah, changing the unit so dramatically would take a long time to adjust to. I don't think anybody would want to switch to that. Just look at how long the US has been just accept the SI system.
The ℮ mark explanation is incorrect in that it does not mean an estimated weight. As this excellent video is about precise definitions, I felt the need to give a precise definition of the ℮ marking. Packages that meet the requirements of the Packaged Goods Regulations and are between 5g to 10kg and 5ml to 10l can apply the ‘℮’ mark. Packaged goods labeled with the ‘℮’ mark are declaring their compliance with the requirements of the ‘average system’ under Directive 76/211/EEC and are not subject to further weights and measures regulation. It is an optional choice for the packer whether to display the ‘℮’ mark. Packages which do not display the ‘℮’ mark and are to be traded outside the UK must meet the regulations set by the destination country. The ‘℮’ mark is a metrological passport to trade allowing free access within the EEA and its respective markets.
I wouldn't call it the demise of the kilo, but an improvement. You not only discard legacy means of definition, but you can clearly explain what a kilo is with no risk of distortion from outside influences.
You are correct, but not the demise of the Kilo as a standard of measurement as the title stipulates. And what does it matter if the object's kicked to the "kerb" when a better means of definition is in place. Also it will sit in a museum anyway so it's not wasted:P
I like how this is basically pulling out constant values in the universe and using them as the numbers we need to define our practical measurements. In a true mathematical sense we will always share the same baseline for our measurements as it is open to anyone to measure that baseline with indefinitely precision.
He seems to say around 6:29 that the speed of light in m/s is still measured, contradicting the definition of the meter he presented at 3:00. The speed of light has been fixed at 299,792,458 m/s for over thirty years.
what about sending a a wave signal to liens and telling them that a meter is the wave lenght of that signal? if the are smart they can calculate the doppler effect due to the fact that our planets are runnuing away from ach other, and so the can figure out the real wavelenght of the wave and so they can know what a meter is
The values of h, v and c affrirm that m should be about 6,78 x 10^-41 and this value elevated to -1 should affirm that 1,47552 x 10^40 photons' mass is equal to 1 kg, but at 7:45 there are 45 digits instead of 40! Is that an error?
TheTornado121According to wikipedia, "The gram was originally defined in 1795 as the mass of one @wiki/Cubic_centimeter of water at 4 @wiki/Celsius, making the kilogram equal to the mass of one @wiki/Litre of water." Therefore, the kilogram was chosen to make things fit -- one liter is 1m^3 of water; kilogram is the mass of a liter of water.Why use the kilo prefix (i.e. why not make the mass of a liter of water be a gram)? Presumably this was done to make working with weights easier as a kilogram is heavy, so you would be mostly working in milligrams. Also, gram derives from the Greek "gramma" meaning "a small weight" which would not make sense if the cubic liter was used as a gram.
6:10 A second can be defined as 9,192,631,770 wavelengths of a Caesium atom. 6:36 But the speed of light, C, requires a unit of length. Leaving aside the earth based definition of a meter, where does that come from?
Not true. It will now be defined using the Watt-scale. A brilliant solution actually since it will no longer be based on a physical object. As for distance/meter it is now based on the speed of light
That stylised e is actually about the methods used for quality control confirming the mass of the product. All products are obviously approximate to some degree but not all carry that lable, its generally better for large scale production and things that are difficult to dose out (chips/crisps, powders) as it is slightly more permissive of occasional underweight items than some other systems
this just blown my mind , which came first , the unit or the process and result and since they have been used to measure these constants , then we probably have some units at least not accurately presented , and now they are interactive . my brain is burning.
I'm sure someone has already thought of this, but couldn't we measure the volume of the current kilo and then measure how much light bends around it due to the kilo's mass?
Volume of the kilo? Different materials weighing 1 kg have different densities, and thus different volumes. There is no such thing as volume of a mass unit.
The flaw in the original definition of the metre is that the distance from the north pole to the equator isn't a constant as the poles drift and tectonic plate shift also changes where Paris is. They should be specifying the date and time of the original measurement I would imagine. Also I read somewhere that the speed of light is changing, so there's spanner in the works of the new definitions.
I remember seeing a video on a UA-cam science channel. It says that they were about to change the kg to a pure Si sphere of a precise radius because Si is stable and volume is an idea that others can recreate
The 'E' Stands for the German Word "Einwage" which translates approximately to "inmeasured". It means they put 1kg of sugar in when the packed the box. It indicates that the weight might have changed winced packed, primarily due to moisture loss/gain.
I'm asking myself why I didn't know this earlier. I've been reading about the Watt balance and counting silicon atoms in a near perfect crystalline silicon sphere as ways of defining the kilogram, but I wasn't aware that the equation m = hν0 / c^2 (shown at 7:32 in the video) could give us a direct answer.
I wonder if there is any fundamental numbers we can assign to those time and those 3 constants so that the numbers are easy to work with and not ongoing decimals.
They do, when doing the scientifically accurate experiments that define these terms. You can measure mass without using weight as a proxy. That's how it is done in the international space station, where they use a spring to apply a known force, and measure your body's acceleration.
This is a little confusing. How is this not already the case? The brilliant thing about SI units is they can be used to define each other. If one litre of water (at 4 °C) occupies one cubic decimetre and has one kilogram of mass, then isn't the kilogram already defined by constants and not the Parisian weight? If this is not the case, then how can we safely convert between units?
Metrology. Of course, even more ideal would be to replace this ν₀ with G, the universal gravitational constant, which, like h and c, is a fundamental property of the universe, rather than of a particular sort of physical structure (a neutral atom of ¹³³Cs). G, h (or ℏ = h/2π, which is the quantum of angular momentum), and c are the basis of the Planck units, so this would amount to defining the kg, second, and meter in terms of Planck units. Problem is, G is notoriously hard to measure with enough precision to make this practical; so that adopting it would subject the SI units so defined, to a life of too much jitter, as new measurements refine G.
But the whole point is, how will you *know* how much mass IS a kg, if you adopt a number times the Planck mass for it, when G (and therefore, the Planck mass itself) is too uncertain for some of the more precise applications?
So... the kilogram could be defined in terms of the mass/energy of a photon emitted by cesium in the same conditions that are used to define the second?
The assertion that you can't "communicate" what a kilogram is since it's not defined in terms of fundamental constants isn't quite accurate. You can express a kilogram in fundamental constants, even if it isn't officially defined that way.
Weight was depending on the gravity. With this definition one kilo on earth and one kilo on the moon will be the same because the mass doesn't change. Weight is relative... nice thought...
The fundamental "constants" will provide a more accurate and stable basis for our measurement units than physical objects ever could. Metrologists aren't just doing this for fun; it's genuinely better.
Did only I noticed, that, Planck's constant contains kilogram itself, soo... Actually kilogram itself defines how acurately we measured it's value? That is the same for Boltzmann constant. Joules have a kilogram, so I think that kilogram was predefinied by discovering those constants currently I know that there a margin of mistake, but what it includes? Measurement, or problem with kilogram too?
If we are going to redefine how we define a metre then why don't we say it is the distance light travels in 300,000,000th of a second and change the definition of a second slightly so that calculations for a meter are now a lot easier?
Why was the meter initially specified as a fraction of the line passing through Paris from the North Pole to the Equator? Would it have mattered from which point the line passed as long as the end points had the same latitude?
My guess is it wouldn't have made a difference. But I guess that just like the USA can plant a flag on the moon then the French could state that the meter crosses through _their_ country XD
The first definition of the metre as a fraction of the meridian from the North Pole to the Equator never included the phrase 'the line through Paris". Yes, the measurements were done on that line between Dunkirk and Barcelona, but the real definition of the metre was: 1 metre is the 1/40 000 000 part of the meridian. Period! I have seen the French law of the time around 1790 - 1795 that defined the metric standards. All meridians were deemed equal.
If length is defined as the distance travelled by light in a fixed amount of time, could we not do away with having distance as an SI unit since it's just a derivation of time?
Could we use this opportunity to define a metre as the distance light travels in 1/300 million seconds? I calculate a meter would only become less than 0.1% longer, and the value is nice and round. I don't know if that amount of change is enough to cause problems or not?
From what I remember from my chemstry lesson are "mole". Wouldn't that help? But then mass differs on different gravitational measurements (don't know the word) in different parts of the word or different places.
When he gives that example of explaining through radio to another being what a meter is, makes sense to be the distance the light travel in x time. But why would we assume that those being would now what a second or an hour is?
Juan Pablo Morbelli As explained in the video, a second is defined as the rate of decay of Cesium. So you could tell them to find that specific atom and count 9~ billion photon emissions and the time that passed would be one second.
I have a problem understanding why using the simplified mass-energy relation (E=mc²). Which means that a photon has a mass while we know it does not. Shouldn't we use the more general équation ( E²=m²c^4 + p²c² ) ?
There's a mis-statement at about 06:30 in the video. The speed of light is *already* defined to be exactly 299,792,458 m/s, and the frequency of the caesium atom oscillation is *already* defined to be exactly 9,192,631,770 Hz. The latter officially defines the second, and the two together officially define the meter. All that remains is to nail down the numerical value of Planck's Constant and this will officially define the kilogram.
Didn't they also originally define the meter as the pendulum length which gives a half-period of one second? (basically, they defined the meter in such a way that pi squared=g)
In the past, yes, but right now the second is defined as the duration of a certain amount of periods of radiation of a Caesium atom. This way the second becomes a unit which is independent of all other units, and is used now to define the meter.
Arturo Gutierrez No, no, I know. I was just commenting on how he said the meter was originally defined as a fraction of the length of a specific meridian, but failed to mention that other definition that existed for a while.
The definition of a second itself might not quite be so constant. Time slows in the presence of a massive object (such as the Earth). As Earth gains mass (albeit, relatively slowly), time itself will very gradually slow down on the surface. Distance, however, changes in the presence of velocity. So, measure the distance light travels in 1/299,792,458th of a second might come up with a slightly different value than if one did so, say, on the surface of the moon, or Europa.
Its not the "demise of the kilogram", its the demise of the definition of the kilogram", the kg is too common and too widely used (both publicly and scientifically) for it to be replaced.
The problem with conceptualizing physical units is that you require quite precise and contrive way to measure those concepts in term of constants as time.
Aren't we about to change the kilogram to an orb of silicon 28. As I understand it, it will be based on the mass of the orb i.e. Particles of silicon 28 in the orb and not the weight?
One problem question for the speed of light definition to give a measurement of a second. We know that space is expanding according Hubble's observation. So, the exact same 2 points in space will take a beam of light longer and longer time to travel. But the 2 exact points should have the exact distance. Does this mean that our definition of a second will become longer and longer because we need to give the same number of seconds for light to travel the same distance between the 2 points? If a second becomes longer and longer, does that mean our life expectancy will become shorter and shorter in reference to the number of seconds we live on average? The problem with all these definition is that nothing is constant. We only fool ourselves by saying speed of light is constant to make the math look pretty and "elegant" when the exact 2 points in space will take light longer and longer to travel. Take for example, near the big rip moment at the end of our universe's life, it can take light 1 second to travel from earth to moon one day, and it will take light 2 seconds to travel from earth to moon the very next day. And yet the orbit of the moon has not changed because of mass conservation and energy conservation. Then what is the point of saying that speed of light is constant when the same distance measured by light it can change 200% per day? That is equivalent of a hyper-inflation scenario when a country sovereign economy is breaking down and the same bottle of water cost twice as much as the previous day, and yet you keep saying that a dollar is a constant because a dollar is defined as a dollar. It is meaningless to say that a dollar is constantly a dollar. You are not fooling any one else but yourself.
You really don't need that remaining arbitrary time measurement. Throw it out, too. You've got distance and speed as universal constants. Use the thing that travels at that speed, measured for that distance, as the basic unit of time. (Yes, you need to scale it up to get any sort of actual result, but then you just scale it back down and set that as one time unit.) Then you get the energy unit and the mass unit (expecting both to eventually be replaced by the smallest possible amount).
Brady, was it one of your videos that discusses the leap second? More generally just time but it included a comparison of UTC and astronomical time. I'm dying trying to find the video.
Surprised no one brought up another problem; Uncertainty Principle. By measuring something, we've changed it. Questions now are, by how much, and does it... unchange... back to its pre-measured form over time, or when it's no longer being measured?
How can the speed of light be used to base the calculation required to judge how long a meter is if the speed of light is based on the meter in the first place (i.e. approx 3 x 10^8 m/s)? I get that it is a physical constant in the universe so in essence, what is used to calculate the speed of light for it to be then used as a calculation for the meter (which the speed of light is then based on)? Are physics calculations therefore only proportionally correct?
So, we define the fundamental units of measurement using the fundamental constants of nature, but those constants are expressed in values depending on the fundamental units! E.g. we use the speed of light, which needs the meter in order to be expressed, as a tool that defines the meter itself... Isn't it a bit annoyingly recursive?
@about 5:59 "anywhere in the Universe" - very pedantic point, but the Earth's gravitational field slows time down by (from memory) something like 2E-10 which (using the 9,192,631,770 figure in the slide at 5:59) would make the second about 2 ticks shorter in flat spacetime. So I guess that part of the definition actually *does* include the fact that the second is that many ticks at sea level on Earth. If you were talking to someone on another planet you would have to convert to the number of ticks in flat spacetime and let them convert back for their own gravitational field. If the measurement was a few orders of magnitude more precise then you would either have to re-define it in flat spacetime anyway, or you would have to include a point (probably Paris!) on the Earth where the second was to be measured because the gravitational field changes (with latitude and with local geology).
The speed of light governs all physical processes, and so something that takes 1 second will always take one second relative to a local observer regardless of gravity. The alien might think we are a little slow, but something we measure as one second here will also be one second when they do the same thing there. The variability is in how time works, not the definition of 1 second. Defining 1 second in Paris will not help.
One thing that i've been thinking about recently is how do we define the length of a week? I mean i know that a week is 7 days but why? one year is based on the revolutions of the earth around the sun and one day on the revolutions of the earth around itself, and in between there are months and weeks, months i believe are based around the revolutions of the moon around the earth, but why do we split them up into 4 weeks?
I Understand the the speed of light is the same in all referencials just like the plank constant, but the frequency. Can we define the based units to be independent from your referencial? It would make lot of things easier...
If atomic clocks change there time due to speed, then wouldnt the cesium used to define the time constant also need to be defined at a certain relative speed?
So what if we want to measure a kilogram, how will that work? I mean its nice knowing that 147190471904713904713904 photons is one kilogram, but that doesn't really help with measuring does it?
We could literally pick any random numbers of our choosing and redefine the SI units from those. It just happens that scientists are wanting to use numbers that they've been using for years to redefine the units.
Oniontears123TNG Because avogadro's number is really nice and round. It might work if you set avogadro's number as exactly 10^24, but then it really wouldn't be a kilogram anymore.
Finally! I've been thinking about a system of measurement based on the planck length for years. Hopefully they'll redefine the mole too, so chemists won't have to use Avogadro's Number.
I would appreciate someone explaining how a speed or velocity can be squared. Using miles and seconds is arbitary - where does the number that can be squared come from?
Defining meter from the speed of light would have to take take gravity in to account as it changes the space, inwith the light is traveling. There is no straight line in space, as gravity alters the space itself.
I thought the speed of light (c) was already a defined value, since the length of a meter is defined as the distance light travels in 1/299,792,458 of a second.
YtterbiJum for the definitions of speed of light and the meter, the new definitions really don't change anything, except that a better measurement of how far light travels in 1/299,792,458 of a second would change the definition of the meter rather than the speed of light.
Wouldn't the constants be circular referencing themselves? Speed of light is 299 792 458 m/s but the meter is defined by the seed of light? That doesn't make sense...
since we're redefining it in terms of the speed of light, why not just define it as 1/300,000,000 of the distance light travels in a second and make it nice and rounded?
It may not be "broken" for you, but it was "broken" for scientific applications that require such precision. They try to keep the previous definition as best as they can, when re-defining the units. So what you previously thought a kilogram is, is still *about* a kilogram, to the degree that it makes a difference in your everyday life.
"The kilogram or kilogramme (SI unit symbol: kg; SI dimension symbol: M), is the base unit of mass in the International System of Units (SI) (the Metric system) and is defined as being equal to the mass of the International Prototype of the Kilogram (IPK)." - Wikipedia
The "e" symbol does't just mean that the weight is estimated. It's a quality assurance symbol that states that the producer has had it's manufacturing process verified by an accredited entity to have a very small std. deviation and a mean weight of at least the nominal weight of a package.
So right now the speed of light is an integer (299792458) measured in meters per second, and since the meter is defined based on the speed of light, there should be an infinite number of 0s after the decimal place, even if those 0s eventually cease to make physical sense after a certain point.
I´ve got two (maybe dumb) questions: Why wasn´t this transition made earlier? The constants are known long enough, and really every aspect of life on earth will benefit from the adaptation of the new definitions. Why do the relevant masses and lengths have to stay the same? Why do we have to go for 9192631770 periods and not for 10.000.000.000. Sure, real-world distances would change accordingly, but the long term benefit of being able to do fractions so much easier would far outweigh the confusion that would maybe last for one generation or so. Or am I alone on that?
You're not alone. A significant proportion of the comments to this video talk about that. All and all, I think your question can be answered with another question. You ask: "Why wasn´t this transition made earlier?" to which I respond: "Why are there still _countries_ which haven't switched to the metric system already?" ;)
Jeremy J. - The transition waited for the technology needed to implement it to be developed. You are essentially alone in wanting to make substantive changes to the SI units. A handful of people commenting in a UA-cam forum don’t amount to anything. Standards bodies and scientists and engineers around the world agree that there must be minimal deviation from established values of the SI units.
The meter is defined as the distance light travels for a certain fraction of _seconds_. Seconds it seems are the *really* fundamental unit here. Their definition is independent from both meters and light, and thus they are used to measure the other two.
Yes, then you invert the definition and no longer need the original 'meter' object from that point forward. Same thing with second. You measure some really fast universally accessible thing that happens within a second, then you invert the definition and can now define a second in terms of the universally accessible thing.
They had to maintain consistency with existing definition of 1/86400th of a mean solar day, when defining that number. That's the previous definition of a second, before the atomic clock was invented.
If we redefine the length of a meter, doesn't this change the speed slightly? for example, if c = 299,792,458 m / s and you define a meter based on this, the meter is lengthened or shortened and the speed at which light passes the set points for the "new" meter would change every time you redefined the distance.
No. We are defining c to be a constant number of meters per second. The meter would be redefined as a result of measurements which show we were slightly wrong about the speed of light, but instead of changing the speed of light, we simply say we were slightly wrong about the meter.
yeah, like I stated after the fact, they use the current model for the speed of light per meter per second to create a new standard measurement. Math is cool.
wait, you define the speed of light by using metre's/sec but then define a meter with the speed of light over a distance in a second. um. how does that work?
You are forgetting about isotopes [Wikipedia lists 9 for silicon, 3 of them stable] AND impurities which would make any OBJECT created as a "standard" flawed. It is much better to have non-object-definitions (the decay of Caesium is specific for each isotope, so you can't have an error there and the speed of light is not "object based"). Also: it is a HUGE effort to create a totally pure silicon sphere AND keep it clean from impurities (surface reactions, fingerprints, dust, ...) and so it is much easier to define units by something you can measure instead of an object.
@@Muck006 Does the atomic clock Cesium atom's frequencies depend on the specific isotope of Cesium? Or is this exclusively an electromagnetism property of Cesium that doesn't "care" about the neutron count?
Problem is, these constants are over a defined unit of time: a second. So you have to communicate that first. And what if a second changes, since those are based of of on the orbit and rotation of the Earth, which is also always changing?
I wonder if anyone can explain how c squared is calculated - how a velocity can be squared, if it is a distance per a unit of time. Also, how a weight can be defined when it changes as the force of gravity upon it changes. The mass may be unchanging at a given speed through space, but how is a weight constant?
Weight is gravity pressure, while mass is t amount of stuff in t object. Something is still a kilogram, even in space, but zero pounds weight. Weight is measured via a spring balance, like at a grocery store's checkout counter, but mass via a balance beam scale; example, tomatoes on one side Vs 1kg on t other.
I agree with Brady. It really would make it hard to do work on those new constants. What happens if we have a new measurement but their isn't consensus in the community and the implications are inconvenient. Right now publishing a new estimate is as easy as publishing it and some disagreement is allowed.
I wonder if it will effect my unit converter on my calculator. Right now it says on my 10-digit calculator, that 1lb = 0.45359237kg. Once it's defined, I guess this particular decimal answer will change then?
Scootaloo The pound-mass is always going to be the same number of kilograms. And all SI-derived units, and their US customary counterparts will remain defined the same, relative to it. What will happen, is that the existing accuracy of the definition of the kg will remain, while how it relates to physical reality will be redefined.
don't understand! if we define the speed of light to some value instead of measuring it and sometime down the road we realize our speed of light measurement was slightly off, then that defined speed of light value is no longer the actual speed of light and we're back to square one dealing with arbitrary values that aren't fundamental?
Work with plank length and give it a unit name Plank and then do a naming convention kind of like kilo mega deca for length. Then redefine second as the the time taken for light to travel a certain Planks. Weight units as the difference in attractions between 2 masses of silicon or cesium etc. independent of earths gravity.
With the exception of Planck mass, where you could express common masses in units of Mega Planck mass units (about 46 grams), we don't even have a prefix large or small enough to express ordinary quantities in Planck units. The named SI prefixes only extend to 10^(-24) and 10^(+24), and most people don't even know the names of anything beyond trillion or trillionth. You'd have to use scientific
Since the mole is defined as 12 grams of C-12, surely we could continue such a definition such that 1.2 kg of C-12 is 100 mol and then 100/1.2kg moles of C-12?
The other leading movement for changing the kilogram is trying to do something similar to what you're saying: make an incredibly round silicon sphere that is exactly 1 kilogram, then count its diameter using a laser with incredible precision, use that to figure out how many atoms are in the sphere, and then define the kilogram to be the mass of that many silicon atoms. The mole would just be some simple fraction of that number, so it would be essentially defining the kilogram in terms of the mole. The only real problem relative to this method is that the relationship between the kilogram and the fundamental constants would not remain stable by definition. I almost wonder if this is a bit of a physics/chemistry spat where chemists want the mole to remain locked while physicists would prefer the fundamental constants to.
Quantities are arbitrary as they only reflect what we traditionally thought given units were, if it turns out that light travels a tiny bit faster then physicists thought it won't have to affect the definition of meter as the distance that light travels in an arbitrary fraction of a second, it's no longer relevant to ask what the speed of light is - it is what it is, it's fundamental and definitions of meter and other units are relative to it,
forgive me if im being dumb but couldn't we use e=mc^2 for the base of our mass e.g 1kg = the X energy. c is constant and the energy equivalence to 1kg would also be kept constant, could we just find an energy value close to what we use as a kg?
I thought that 1 cubic centimeter of water was equal to 1 gram of mass. Am I missing something? If you have the meter defined by the speed of light why can't the gram be defined by the mass of 1 cubic centimeter of water?
It has been pretty annoying to me that fundamental constants have these many significant figures. And since this was a results of having our units arbitrary (poorly) defined in the first place, then as we are going to do it the other way around by defining units with fundamental constants, can't we just give these constants "better looking" rounded numbers? E.g. set c=3E8 m/s
Theoretical physics does it in a way not so helpful with everyday life. Hopefully there is a way to make these numbers looks nice, and at the same time stay close enough that the difference doesn't affect the practical everyday use.
You can define a second as the time it takes for a specific number of cycles of a light from a specific source. You can also define the meter as the length of another specific number of cycles of light from the same source.
Ive seen few of the measurements of speed of light done in the history of science. All of them seem to be declining in time(the older are higher, the latter are lower). The situation changed in the 1950's, when scientists started to use atomic clocks to measure the speed of light. From that point the speed of light is constant(not a single change). The options are - first: speed of light is a constant and the older measurements were inaccurate, second: the speed of light is not constant and latest measurements are constant because the atomic clock is something directly related to the speed of light. The conclusion: either we will be getting shorter and heavier(im talking about relating the length of meter and the mass of kg to the speed of light and Planck's "constant" - also related to the speed of light) or we have a really nice unification of two most known and used values on earth. Also, very nice video Im going to watch more :)
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Claming that the "e-mark" stands just for estimated is a little bit confusing as it states per EU Directive 76/211/EEC that the filling of the product container is controlled so that it should fall with in given limits. Also the the directive states that the quantity cannot be altered without opening or destroying the packing material.
My dad once complained to me that since speed of light is so close to 300,000,000 m/s, we should have just redefined the meter to make it exactly so.
Physicist dad complaints.
I agree with your father entirely. Better yet, lets set it to exactly 1,000,000,000, so the meter is approximately one foot.
I'd love to have a dad like that. My dad just loves cars, meaning I know an awful lot about cars. I'd rather know an awful lot more about physics and mathematics.
I was thinking exactly in the same line, if you decide to peg the unit to a constant, then let's make the constant round and change the unit accordingly.
Kevin Higby You could, but you could hardly call it a metre anymore ( you couldn't really do it for the 3e8 thing either honestly). Call it a Higby and see if it catches on.
IamGrimalkin Yeah, changing the unit so dramatically would take a long time to adjust to. I don't think anybody would want to switch to that. Just look at how long the US has been just accept the SI system.
The ℮ mark explanation is incorrect in that it does not mean an estimated weight. As this excellent video is about precise definitions, I felt the need to give a precise definition of the ℮ marking.
Packages that meet the requirements of the Packaged Goods Regulations and are between 5g to 10kg and 5ml to 10l can apply the ‘℮’ mark.
Packaged goods labeled with the ‘℮’ mark are declaring their compliance with the requirements of the ‘average system’ under Directive 76/211/EEC and are not subject to further weights and measures regulation. It is an optional choice for the packer whether to display the ‘℮’ mark.
Packages which do not display the ‘℮’ mark and are to be traded outside the UK must meet the regulations set by the destination country.
The ‘℮’ mark is a metrological passport to trade allowing free access within the EEA and its respective markets.
I wouldn't call it the demise of the kilo, but an improvement. You not only discard legacy means of definition, but you can clearly explain what a kilo is with no risk of distortion from outside influences.
Well, it IS the demise of the use of an object to define the standard kg.
That object would be "kicked to the curb."
You are correct, but not the demise of the Kilo as a standard of measurement as the title stipulates. And what does it matter if the object's kicked to the "kerb" when a better means of definition is in place. Also it will sit in a museum anyway so it's not wasted:P
I like how this is basically pulling out constant values in the universe and using them as the numbers we need to define our practical measurements. In a true mathematical sense we will always share the same baseline for our measurements as it is open to anyone to measure that baseline with indefinitely precision.
He seems to say around 6:29 that the speed of light in m/s is still measured, contradicting the definition of the meter he presented at 3:00. The speed of light has been fixed at 299,792,458 m/s for over thirty years.
This was actually a quite eye opening video, I had never thought about this sort of thing before!
what about sending a a wave signal to liens and telling them that a meter is the wave lenght of that signal?
if the are smart they can calculate the doppler effect due to the fact that our planets are runnuing away from ach other, and so the can figure out the real wavelenght of the wave and so they can know what a meter is
The values of h, v and c affrirm that m should be about 6,78 x 10^-41 and this value elevated to -1 should affirm that 1,47552 x 10^40 photons' mass is equal to 1 kg, but at 7:45 there are 45 digits instead of 40! Is that an error?
I don't quite understand why the length is defined with the meter, volume with the liter but weight with the kilogram and not the gram...
TheTornado121According to wikipedia, "The gram was originally defined in 1795 as the mass of one @wiki/Cubic_centimeter of water at 4 @wiki/Celsius, making the kilogram equal to the mass of one @wiki/Litre of water." Therefore, the kilogram was chosen to make things fit -- one liter is 1m^3 of water; kilogram is the mass of a liter of water.Why use the kilo prefix (i.e. why not make the mass of a liter of water be a gram)? Presumably this was done to make working with weights easier as a kilogram is heavy, so you would be mostly working in milligrams. Also, gram derives from the Greek "gramma" meaning "a small weight" which would not make sense if the cubic liter was used as a gram.
TheTornado121 - A kilogram object is more convenient to work with.
6:10 A second can be defined as 9,192,631,770 wavelengths of a Caesium atom.
6:36 But the speed of light, C, requires a unit of length. Leaving aside the earth based definition of a meter, where does that come from?
Nehmo Sergheyev - If you have a defined second, and you define the speed of light to an exact constant, then you have defined the length of a meter.
Not true. It will now be defined using the Watt-scale. A brilliant solution actually since it will no longer be based on a physical object. As for distance/meter it is now based on the speed of light
How am I expected to weigh my cocaine now? pounds? like a pilgrim?
Just measure it by the spoonful. If you're in the USA you don't use weghts or volumes for cooking anyways, but standardized cups.
People will tell you to eyeball it, but I tried that, and it makes my eyeball numb.
I'd assume you'd measure it by Volume and not Mass
You need the planck constant and a laser.
That stylised e is actually about the methods used for quality control confirming the mass of the product. All products are obviously approximate to some degree but not all carry that lable, its generally better for large scale production and things that are difficult to dose out (chips/crisps, powders) as it is slightly more permissive of occasional underweight items than some other systems
this just blown my mind , which came first , the unit or the process and result and since they have been used to measure these constants , then we probably have some units at least not accurately presented , and now they are interactive . my brain is burning.
I'm sure someone has already thought of this, but couldn't we measure the volume of the current kilo and then measure how much light bends around it due to the kilo's mass?
Volume of the kilo? Different materials weighing 1 kg have different densities, and thus different volumes. There is no such thing as volume of a mass unit.
Don't worry everyone, I have a dumbbell weight that says 1kg so we can just use that.
The flaw in the original definition of the metre is that the distance from the north pole to the equator isn't a constant as the poles drift and tectonic plate shift also changes where Paris is. They should be specifying the date and time of the original measurement I would imagine. Also I read somewhere that the speed of light is changing, so there's spanner in the works of the new definitions.
I remember seeing a video on a UA-cam science channel. It says that they were about to change the kg to a pure Si sphere of a precise radius because Si is stable and volume is an idea that others can recreate
The 'E' Stands for the German Word "Einwage" which translates approximately to "inmeasured". It means they put 1kg of sugar in when the packed the box. It indicates that the weight might have changed winced packed, primarily due to moisture loss/gain.
I'm asking myself why I didn't know this earlier. I've been reading about the Watt balance and counting silicon atoms in a near perfect crystalline silicon sphere as ways of defining the kilogram, but I wasn't aware that the equation m = hν0 / c^2 (shown at 7:32 in the video) could give us a direct answer.
I wonder if there is any fundamental numbers we can assign to those time and those 3 constants so that the numbers are easy to work with and not ongoing decimals.
But we dont use mass as an everyday measurement. We use "weight".
So you would have to take into account of acceleration or gravity surely??
They do, when doing the scientifically accurate experiments that define these terms.
You can measure mass without using weight as a proxy. That's how it is done in the international space station, where they use a spring to apply a known force, and measure your body's acceleration.
Right, and the true metric unit of weight is the newton, not the kilogram.
I noticed "Atlas of Creation" by Adnan Oktar aka Harun Yahya on the shelf. That's there for laughs right?
Nice catch! Let's hope so. I _ I
This is a little confusing. How is this not already the case? The brilliant thing about SI units is they can be used to define each other. If one litre of water (at 4 °C) occupies one cubic decimetre and has one kilogram of mass, then isn't the kilogram already defined by constants and not the Parisian weight? If this is not the case, then how can we safely convert between units?
Metrology.
Of course, even more ideal would be to replace this ν₀ with G, the universal gravitational constant, which, like h and c, is a fundamental property of the universe, rather than of a particular sort of physical structure (a neutral atom of ¹³³Cs).
G, h (or ℏ = h/2π, which is the quantum of angular momentum), and c are the basis of the Planck units, so this would amount to defining the kg, second, and meter in terms of Planck units.
Problem is, G is notoriously hard to measure with enough precision to make this practical; so that adopting it would subject the SI units so defined, to a life of too much jitter, as new measurements refine G.
But if you use G the mass of a kilogram is still a kilogram even if it is weightless.
But the whole point is, how will you *know* how much mass IS a kg, if you adopt a number times the Planck mass for it, when G (and therefore, the Planck mass itself) is too uncertain for some of the more precise applications?
I see big G I was thinking G little g
So... the kilogram could be defined in terms of the mass/energy of a photon emitted by cesium in the same conditions that are used to define the second?
The assertion that you can't "communicate" what a kilogram is since it's not defined in terms of fundamental constants isn't quite accurate. You can express a kilogram in fundamental constants, even if it isn't officially defined that way.
Weight was depending on the gravity. With this definition one kilo on earth and one kilo on the moon will be the same because the mass doesn't change.
Weight is relative... nice thought...
The fundamental "constants" will provide a more accurate and stable basis for our measurement units than physical objects ever could. Metrologists aren't just doing this for fun; it's genuinely better.
Did only I noticed, that, Planck's constant contains kilogram itself, soo... Actually kilogram itself defines how acurately we measured it's value? That is the same for Boltzmann constant. Joules have a kilogram, so I think that kilogram was predefinied by discovering those constants currently I know that there a margin of mistake, but what it includes? Measurement, or problem with kilogram too?
A kilogram is a metric unit of mass equal to 2.2 pounds on Earth, translated into newtons, the TRUE metric unit of weight, a kilo is about 9 newtons.
If we are going to redefine how we define a metre then why don't we say it is the distance light travels in 300,000,000th of a second and change the definition of a second slightly so that calculations for a meter are now a lot easier?
Why was the meter initially specified as a fraction of the line passing through Paris from the North Pole to the Equator? Would it have mattered from which point the line passed as long as the end points had the same latitude?
My guess is it wouldn't have made a difference. But I guess that just like the USA can plant a flag on the moon then the French could state that the meter crosses through _their_ country XD
Earth isnt "perfectly round".
The first definition of the metre as a fraction of the meridian from the North Pole to the Equator never included the phrase 'the line through Paris". Yes, the measurements were done on that line between Dunkirk and Barcelona, but the real definition of the metre was: 1 metre is the 1/40 000 000 part of the meridian. Period! I have seen the French law of the time around 1790 - 1795 that defined the metric standards. All meridians were deemed equal.
If length is defined as the distance travelled by light in a fixed amount of time, could we not do away with having distance as an SI unit since it's just a derivation of time?
+Jonny Bell It's really cumbersome to write (299 792 458 m/s)*(1s/299 792 458) everytime you want to write m.
Yeah. Like the distance travelled by light after a certain whole number of clicks of an atomic clock.
Could we use this opportunity to define a metre as the distance light travels in 1/300 million seconds? I calculate a meter would only become less than 0.1% longer, and the value is nice and round. I don't know if that amount of change is enough to cause problems or not?
But speed of light is already defined to be exactly 299 792 458 m/s from the definition mentioned in the video.
From what I remember from my chemstry lesson are "mole". Wouldn't that help? But then mass differs on different gravitational measurements (don't know the word) in different parts of the word or different places.
When he gives that example of explaining through radio to another being what a meter is, makes sense to be the distance the light travel in x time. But why would we assume that those being would now what a second or an hour is?
Juan Pablo Morbelli Because of planck.
Juan Pablo Morbelli As explained in the video, a second is defined as the rate of decay of Cesium. So you could tell them to find that specific atom and count 9~ billion photon emissions and the time that passed would be one second.
I have a problem understanding why using the simplified mass-energy relation (E=mc²).
Which means that a photon has a mass while we know it does not.
Shouldn't we use the more general équation ( E²=m²c^4 + p²c² ) ?
It's going to suck when I look down at the scale and see that I weigh 11036900578625743570599644546878423670905544 hv/c^2 =( Jk
There's a mis-statement at about 06:30 in the video. The speed of light is *already* defined to be exactly 299,792,458 m/s, and the frequency of the caesium atom oscillation is *already* defined to be exactly 9,192,631,770 Hz. The latter officially defines the second, and the two together officially define the meter.
All that remains is to nail down the numerical value of Planck's Constant and this will officially define the kilogram.
can anyone tell me what is/are the advantage(s) of using the Caesium to calculate its mass? why not use another element?
The second and the speed of light are already defined exactly, so defining the kilogramme will have to be done by defining Planck's constant.
Didn't they also originally define the meter as the pendulum length which gives a half-period of one second? (basically, they defined the meter in such a way that pi squared=g)
In the past, yes, but right now the second is defined as the duration of a certain amount of periods of radiation of a Caesium atom. This way the second becomes a unit which is independent of all other units, and is used now to define the meter.
Arturo Gutierrez No, no, I know. I was just commenting on how he said the meter was originally defined as a fraction of the length of a specific meridian, but failed to mention that other definition that existed for a while.
***** Oooh. I didn't catch that subtlety in your comment. My mistake :)
Arturo Gutierrez np :)
It was originally defined as 1/10'000'000 of the distance from the north pole to the equator, passing through France.
This got me in a good mood, I was smiling at the end.
The definition of a second itself might not quite be so constant. Time slows in the presence of a massive object (such as the Earth). As Earth gains mass (albeit, relatively slowly), time itself will very gradually slow down on the surface. Distance, however, changes in the presence of velocity. So, measure the distance light travels in 1/299,792,458th of a second might come up with a slightly different value than if one did so, say, on the surface of the moon, or Europa.
Its not the "demise of the kilogram", its the demise of the definition of the kilogram", the kg is too common and too widely used (both publicly and scientifically) for it to be replaced.
It's the demise of the importance of Le Grand K, in its role in defining the kilogram.
The problem with conceptualizing physical units is that you require quite precise and contrive way to measure those concepts in term of constants as time.
Aren't we about to change the kilogram to an orb of silicon 28. As I understand it, it will be based on the mass of the orb i.e. Particles of silicon 28 in the orb and not the weight?
One problem question for the speed of light definition to give a measurement of a second. We know that space is expanding according Hubble's observation. So, the exact same 2 points in space will take a beam of light longer and longer time to travel. But the 2 exact points should have the exact distance. Does this mean that our definition of a second will become longer and longer because we need to give the same number of seconds for light to travel the same distance between the 2 points? If a second becomes longer and longer, does that mean our life expectancy will become shorter and shorter in reference to the number of seconds we live on average?
The problem with all these definition is that nothing is constant. We only fool ourselves by saying speed of light is constant to make the math look pretty and "elegant" when the exact 2 points in space will take light longer and longer to travel. Take for example, near the big rip moment at the end of our universe's life, it can take light 1 second to travel from earth to moon one day, and it will take light 2 seconds to travel from earth to moon the very next day. And yet the orbit of the moon has not changed because of mass conservation and energy conservation. Then what is the point of saying that speed of light is constant when the same distance measured by light it can change 200% per day?
That is equivalent of a hyper-inflation scenario when a country sovereign economy is breaking down and the same bottle of water cost twice as much as the previous day, and yet you keep saying that a dollar is a constant because a dollar is defined as a dollar. It is meaningless to say that a dollar is constantly a dollar. You are not fooling any one else but yourself.
You really don't need that remaining arbitrary time measurement. Throw it out, too. You've got distance and speed as universal constants. Use the thing that travels at that speed, measured for that distance, as the basic unit of time. (Yes, you need to scale it up to get any sort of actual result, but then you just scale it back down and set that as one time unit.) Then you get the energy unit and the mass unit (expecting both to eventually be replaced by the smallest possible amount).
Brady, was it one of your videos that discusses the leap second? More generally just time but it included a comparison of UTC and astronomical time. I'm dying trying to find the video.
Surprised no one brought up another problem; Uncertainty Principle. By measuring something, we've changed it. Questions now are, by how much, and does it... unchange... back to its pre-measured form over time, or when it's no longer being measured?
How can the speed of light be used to base the calculation required to judge how long a meter is if the speed of light is based on the meter in the first place (i.e. approx 3 x 10^8 m/s)? I get that it is a physical constant in the universe so in essence, what is used to calculate the speed of light for it to be then used as a calculation for the meter (which the speed of light is then based on)? Are physics calculations therefore only proportionally correct?
So, we define the fundamental units of measurement using the fundamental constants of nature, but those constants are expressed in values depending on the fundamental units! E.g. we use the speed of light, which needs the meter in order to be expressed, as a tool that defines the meter itself... Isn't it a bit annoyingly recursive?
the speed of light is mesured in km/s, If you redefine the meter won't the speed of light change? It may sounds stupid, but just curious
@about 5:59 "anywhere in the Universe" - very pedantic point, but the Earth's gravitational field slows time down by (from memory) something like 2E-10 which (using the 9,192,631,770 figure in the slide at 5:59) would make the second about 2 ticks shorter in flat spacetime. So I guess that part of the definition actually *does* include the fact that the second is that many ticks at sea level on Earth.
If you were talking to someone on another planet you would have to convert to the number of ticks in flat spacetime and let them convert back for their own gravitational field.
If the measurement was a few orders of magnitude more precise then you would either have to re-define it in flat spacetime anyway, or you would have to include a point (probably Paris!) on the Earth where the second was to be measured because the gravitational field changes (with latitude and with local geology).
The speed of light governs all physical processes, and so something that takes 1 second will always take one second relative to a local observer regardless of gravity. The alien might think we are a little slow, but something we measure as one second here will also be one second when they do the same thing there. The variability is in how time works, not the definition of 1 second. Defining 1 second in Paris will not help.
Jonathan Cohen opps! you are right.
Thanks!
One thing that i've been thinking about recently is how do we define the length of a week? I mean i know that a week is 7 days but why? one year is based on the revolutions of the earth around the sun and one day on the revolutions of the earth around itself, and in between there are months and weeks, months i believe are based around the revolutions of the moon around the earth, but why do we split them up into 4 weeks?
I Understand the the speed of light is the same in all referencials just like the plank constant, but the frequency. Can we define the based units to be independent from your referencial? It would make lot of things easier...
If atomic clocks change there time due to speed, then wouldnt the cesium used to define the time constant also need to be defined at a certain relative speed?
It would need to be defined in the inertial reference frame.
Okay, so the kilogram is defined as 1.4 x 10 to the whatever photons emitted by Cs137. How the hell are you supposed to measure that?
So what if we want to measure a kilogram, how will that work? I mean its nice knowing that 147190471904713904713904 photons is one kilogram, but that doesn't really help with measuring does it?
We could literally pick any random numbers of our choosing and redefine the SI units from those. It just happens that scientists are wanting to use numbers that they've been using for years to redefine the units.
Oniontears123TNG Because avogadro's number is really nice and round.
It might work if you set avogadro's number as exactly 10^24, but then it really wouldn't be a kilogram anymore.
Finally! I've been thinking about a system of measurement based on the planck length for years. Hopefully they'll redefine the mole too, so chemists won't have to use Avogadro's Number.
Ive always wondered where/how we obtained/discovered a perfect straight edge.
I would appreciate someone explaining how a speed or velocity can be squared. Using miles and seconds is arbitary - where does the number that can be squared come from?
Defining meter from the speed of light would have to take take gravity in to account as it changes the space, inwith the light is traveling.
There is no straight line in space, as gravity alters the space itself.
I have actually thought of this idea before. However, I would say the time should be calculated as the time light uses to travel one planck length.
I thought the speed of light (c) was already a defined value, since the length of a meter is defined as the distance light travels in 1/299,792,458 of a second.
YtterbiJum for the definitions of speed of light and the meter, the new definitions really don't change anything, except that a better measurement of how far light travels in 1/299,792,458 of a second would change the definition of the meter rather than the speed of light.
To my knowledge it is indeed defined.
YtterbiJum The video is discussing changing the definition of the kilo, not changing the definition of the metre.
Did you not watch the video? He established that.
Wouldn't the constants be circular referencing themselves? Speed of light is 299 792 458 m/s but the meter is defined by the seed of light? That doesn't make sense...
since we're redefining it in terms of the speed of light, why not just define it as 1/300,000,000 of the distance light travels in a second and make it nice and rounded?
khajiit92 - Because the deviation from prior usage would be too great.
Why do these units have to be defined to such an accuracy?
If it isn't broken, why try and fix it?
It may not be "broken" for you, but it was "broken" for scientific applications that require such precision. They try to keep the previous definition as best as they can, when re-defining the units. So what you previously thought a kilogram is, is still *about* a kilogram, to the degree that it makes a difference in your everyday life.
"The kilogram or kilogramme (SI unit symbol: kg; SI dimension symbol: M), is the base unit of mass in the International System of Units (SI) (the Metric system) and is defined as being equal to the mass of the International Prototype of the Kilogram (IPK)."
- Wikipedia
and 4 years later, its finally changing
The "e" symbol does't just mean that the weight is estimated. It's a quality assurance symbol that states that the producer has had it's manufacturing process verified by an accredited entity to have a very small std. deviation and a mean weight of at least the nominal weight of a package.
So right now the speed of light is an integer (299792458) measured in meters per second, and since the meter is defined based on the speed of light, there should be an infinite number of 0s after the decimal place, even if those 0s eventually cease to make physical sense after a certain point.
jk991234 - It suffices to say that the number is exact, by definition.
I´ve got two (maybe dumb) questions:
Why wasn´t this transition made earlier? The constants are known long enough, and really every aspect of life on earth will benefit from the adaptation of the new definitions.
Why do the relevant masses and lengths have to stay the same? Why do we have to go for 9192631770 periods and not for 10.000.000.000. Sure, real-world distances would change accordingly, but the long term benefit of being able to do fractions so much easier would far outweigh the confusion that would maybe last for one generation or so. Or am I alone on that?
You're not alone. A significant proportion of the comments to this video talk about that.
All and all, I think your question can be answered with another question. You ask: "Why wasn´t this transition made earlier?" to which I respond: "Why are there still _countries_ which haven't switched to the metric system already?"
;)
Jeremy J. - The transition waited for the technology needed to implement it to be developed. You are essentially alone in wanting to make substantive changes to the SI units. A handful of people commenting in a UA-cam forum don’t amount to anything. Standards bodies and scientists and engineers around the world agree that there must be minimal deviation from established values of the SI units.
So the speed of light, which is defined in meters, is used to define the meter?
The meter is defined as the distance light travels for a certain fraction of _seconds_. Seconds it seems are the *really* fundamental unit here. Their definition is independent from both meters and light, and thus they are used to measure the other two.
Yes, then you invert the definition and no longer need the original 'meter' object from that point forward. Same thing with second. You measure some really fast universally accessible thing that happens within a second, then you invert the definition and can now define a second in terms of the universally accessible thing.
Why is the length of a second defined as 9,192,631,770 periods? How was that particular number decided on?
They had to maintain consistency with existing definition of 1/86400th of a mean solar day, when defining that number. That's the previous definition of a second, before the atomic clock was invented.
How constant are those constants? We're basing them on our perception over time, and our perception is limited to our recorded history.
If we redefine the length of a meter, doesn't this change the speed slightly? for example, if c = 299,792,458 m / s and you define a meter based on this, the meter is lengthened or shortened and the speed at which light passes the set points for the "new" meter would change every time you redefined the distance.
I guess it would work out if you defined it once and set the original value as your reference. I can kinda see how it could be used.
No. We are defining c to be a constant number of meters per second. The meter would be redefined as a result of measurements which show we were slightly wrong about the speed of light, but instead of changing the speed of light, we simply say we were slightly wrong about the meter.
yeah, like I stated after the fact, they use the current model for the speed of light per meter per second to create a new standard measurement. Math is cool.
you're not using brown paper anymore?
karnematch that's Numberphile! ua-cam.com/users/numberphile
Sixty Symbols that's Numberwang!
+PaxetLux 6, 8, 8 That's NUMBERING
karnematch Brown paper was originally made from the wrappings of Egyptian mummies
wait, you define the speed of light by using metre's/sec but then define a meter with the speed of light over a distance in a second. um. how does that work?
I don't see why a silicon atom count cant be the standard for a Kg. That's a constant, isnt it?
You are forgetting about isotopes [Wikipedia lists 9 for silicon, 3 of them stable] AND impurities which would make any OBJECT created as a "standard" flawed. It is much better to have non-object-definitions (the decay of Caesium is specific for each isotope, so you can't have an error there and the speed of light is not "object based").
Also: it is a HUGE effort to create a totally pure silicon sphere AND keep it clean from impurities (surface reactions, fingerprints, dust, ...) and so it is much easier to define units by something you can measure instead of an object.
@@Muck006 Does the atomic clock Cesium atom's frequencies depend on the specific isotope of Cesium? Or is this exclusively an electromagnetism property of Cesium that doesn't "care" about the neutron count?
Problem is, these constants are over a defined unit of time: a second. So you have to communicate that first. And what if a second changes, since those are based of of on the orbit and rotation of the Earth, which is also always changing?
Bruce Crossan - A second is defined by an atomic clock.
I wonder if anyone can explain how c squared is calculated - how a velocity can be squared, if it is a distance per a unit of time. Also, how a weight can be defined when it changes as the force of gravity upon it changes. The mass may be unchanging at a given speed through space, but how is a weight constant?
Weight is gravity pressure, while mass is t amount of stuff in t object. Something is still a kilogram, even in space, but zero pounds weight.
Weight is measured via a spring balance, like at a grocery store's checkout counter, but mass via a balance beam scale; example, tomatoes on one side Vs 1kg on t other.
Too, acceleration is squared, as you are -increasing- velocity; speeding up.
1m/s ---> 2m/s = ?^2, but 1m/s ---> 1m/s = 1m/s, no need for ?^2.
I agree with Brady. It really would make it hard to do work on those new constants. What happens if we have a new measurement but their isn't consensus in the community and the implications are inconvenient. Right now publishing a new estimate is as easy as publishing it and some disagreement is allowed.
I wonder if it will effect my unit converter on my calculator. Right now it says on my 10-digit calculator, that 1lb = 0.45359237kg. Once it's defined, I guess this particular decimal answer will change then?
The pound is defined in terms of the kilogram. So no.
Scootaloo The pound-mass is always going to be the same number of kilograms. And all SI-derived units, and their US customary counterparts will remain defined the same, relative to it.
What will happen, is that the existing accuracy of the definition of the kg will remain, while how it relates to physical reality will be redefined.
don't understand! if we define the speed of light to some value instead of measuring it and sometime down the road we realize our speed of light measurement was slightly off, then that defined speed of light value is no longer the actual speed of light and we're back to square one dealing with arbitrary values that aren't fundamental?
Derek Johnson - A definition can’t be off.
The wink to Veritasium made me smile. :)
I know who it is but I don't get the relevance of the reference
about half-way through with a glowing sphere of Si silicon
Work with plank length and give it a unit name Plank and then do a naming convention kind of like kilo mega deca for length. Then redefine second as the the time taken for light to travel a certain Planks. Weight units as the difference in attractions between 2 masses of silicon or cesium etc. independent of earths gravity.
With the exception of Planck mass, where you could express common masses in units of Mega Planck mass units (about 46 grams), we don't even have a prefix large or small enough to express ordinary quantities in Planck units. The named SI prefixes only extend to 10^(-24) and 10^(+24), and most people don't even know the names of anything beyond trillion or trillionth. You'd have to use scientific
Since the mole is defined as 12 grams of C-12, surely we could continue such a definition such that 1.2 kg of C-12 is 100 mol and then 100/1.2kg moles of C-12?
The definition of a mole must be based on the kg, not the other way around. We can't count a mole atoms, so it can't be a primary unit.
The other leading movement for changing the kilogram is trying to do something similar to what you're saying: make an incredibly round silicon sphere that is exactly 1 kilogram, then count its diameter using a laser with incredible precision, use that to figure out how many atoms are in the sphere, and then define the kilogram to be the mass of that many silicon atoms.
The mole would just be some simple fraction of that number, so it would be essentially defining the kilogram in terms of the mole. The only real problem relative to this method is that the relationship between the kilogram and the fundamental constants would not remain stable by definition.
I almost wonder if this is a bit of a physics/chemistry spat where chemists want the mole to remain locked while physicists would prefer the fundamental constants to.
Quantities are arbitrary as they only reflect what we traditionally thought given units were, if it turns out that light travels a tiny bit faster then physicists thought it won't have to affect the definition of meter as the distance that light travels in an arbitrary fraction of a second, it's no longer relevant to ask what the speed of light is - it is what it is, it's fundamental and definitions of meter and other units are relative to it,
forgive me if im being dumb but couldn't we use e=mc^2 for the base of our mass e.g 1kg = the X energy. c is constant and the energy equivalence to 1kg would also be kept constant, could we just find an energy value close to what we use as a kg?
i should stop commenting before the video finishes, they literally just started talking about e=mc^2 when it started playing again
No need for the Pt/Ir kg and m standards anymore?
I will give you the address where to send them.
I thought that 1 cubic centimeter of water was equal to 1 gram of mass. Am I missing something? If you have the meter defined by the speed of light why can't the gram be defined by the mass of 1 cubic centimeter of water?
Avant-Tom - With that method, you cannot produce a physical kilogram with sufficient precision.
It has been pretty annoying to me that fundamental constants have these many significant figures. And since this was a results of having our units arbitrary (poorly) defined in the first place, then as we are going to do it the other way around by defining units with fundamental constants, can't we just give these constants "better looking" rounded numbers? E.g. set c=3E8 m/s
Theoretical physics does it in a way not so helpful with everyday life. Hopefully there is a way to make these numbers looks nice, and at the same time stay close enough that the difference doesn't affect the practical everyday use.
How can you define a meter using a fraction of light speed since light speed depends on meter ?
You can define a second as the time it takes for a specific number of cycles of a light from a specific source. You can also define the meter as the length of another specific number of cycles of light from the same source.
Ive seen few of the measurements of speed of light done in the history of science. All of them seem to be declining in time(the older are higher, the latter are lower). The situation changed in the 1950's, when scientists started to use atomic clocks to measure the speed of light. From that point the speed of light is constant(not a single change). The options are - first: speed of light is a constant and the older measurements were inaccurate, second: the speed of light is not constant and latest measurements are constant because the atomic clock is something directly related to the speed of light. The conclusion: either we will be getting shorter and heavier(im talking about relating the length of meter and the mass of kg to the speed of light and Planck's "constant" - also related to the speed of light) or we have a really nice unification of two most known and used values on earth. Also, very nice video Im going to watch more :)