High School So what is the new definition of the kilogram?

[bbbl67]

So this article "Quantum leap for mass as science redefines the kilogramme" said that there is a new definition of the kilogram coming. But they neglected to mention what that new definition is exactly. All they said was that it's now based on Planck's Constant. So I worked my way backwards trying to figure out what that is. First I divided the Planck by the kilogram, and came up with this:

h / 1 kg = 6.62607×10^-34 m^2/s

So that unit (m^2/s) looks like I can use the standard constants the speed of light and the metre.

h / (1 kg * 1 m * c) = 6.62607×10^-34 m^2/s / (1 m * c)
= 2.2102191×10^-42

Consequently after rearranging, we get:

1 kg = h / (2.2102191×10^-42 m * c)
~ 4.5244383E+41 h / (c * 1 m)

Is that all there is to it? Just some weird huge number multiplied by the Planck divided by the speed of light and the meter? Do I need to throw some Pi's or Euler's numbers in there too?

 

[Janus]

The new definition I found is Planck's constant divided by 6.62607015e-37 m^-2s

 

[gneill]

See:

How We're Redefining the kg - YouTube

and

Redefining the Kilogram with the DIY Watt Balance

 

[Dale]

But they neglected to mention what that new definition is exactly.

The new definition will be:

“The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and ∆νCs.”

 

[Buzz Bloom]

Is the speed of light still defined in SI units as exactly 299,792,458 meters per second?

https://en.wikipedia.org/wiki/Speed_of_light​

If so, this would imply that the second would no longer be defined in terms of the cesium clock. It would then instead be defined as the time it takes for light to travel 299,792,458 meters in a vacuum.

 

[Dale]

If so, this would imply that the second would no longer be defined in terms of the cesium clock.

The speed of light is unchanged and the second is still defined in terms of the cesium hyperfine transition. I am not sure what makes you think this is implied.

The new definitions can be seen here:
https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf

You can see that the wording of the definitions of the second and meter have been changed, but not their meaning.

 

[Vanadium 50]

If so, this would imply that the second would no longer be defined in terms of the cesium clock

Changing the kilogram doesn't change the meter or the second.

 

[Buzz Bloom]

The speed of light is unchanged and the second is still defined in terms of the cesium hyperfine transition. I am not sure what makes you think this is implied.

Hi Dale:

I apologize for my senior moment brain lapse and careless reading. I somehow got it into my head that the meter was being redefined.

Regards,
Buzz

 

[Dale]

I somehow got it into my head that the meter was being redefined.

Ah, makes sense.

The excitement is all about getting rid of the international prototype kilogram.

 

[bbbl67]

So this video states that not only is kg changing, but they are also now fixing the values of Planck's constant, Avagadro's number, and even the Ampere and the Kelvin!

 

[Dale]

Yes, there will no longer be any physical prototypes and also they are harmonizing all of the definitions to be of the “defined constant” type. The units will no longer be defined either by a prototype or by a specific experiment. The experiments will serve to realize a unit with a given precision, but will not be the definition.

 

[Ray Vickson]

<Moderator's note: Merged threads.>

New standards have been adopted for the fundamental units. The nature of the changes are outlined in the article https://en.wikipedia.org/wiki/Redefinition_of_SI_base_units .

 

[Dale]

Yay! It was expected, but still it is good to have it official

 

[kith]

There was a live stream of some lectures and the final vote at the 26th General Conference on Weights and Measures today. There's a recording on youtube .

 

[Idunno]

So, if I have some matter, and I want to know (as best I can) what the mass is, I have to put it in something like a Watt balance. That is, I have to put it in something that allows me to relate it to Planck's constant, the meter and the second. There is no set way to do this, the Watt balance is just one option. Correct?

 

[Dale]

So, if I have some matter, and I want to know (as best I can) what the mass is, I have to put it in something like a Watt balance. That is, I have to put it in something that allows me to relate it to Planck's constant, the meter and the second. There is no set way to do this, the Watt balance is just one option. Correct?

That is correct. There is no special definitive measurement technique.

 

[Herman Trivilino]

So, if I have some matter, and I want to know (as best I can) what the mass is, I have to put it in something like a Watt balance.

Many will interpret this statement to mean they will have to do something differently when they weigh something.

That of course is not the case. In fact, even for the government regulators, the process they use to calibrate the standards will not change.

The only thing that will change is the standard itself, and that change is of such a small magnitude as to be totally negligible for the purposes stated above.

 

[Idunno]

Well, suppose I want to explain this to a bunch of high school students. I think that what I'd say is something like "to find the mass of an object precisely as possible, one has to place the object in a device that allows you to relate it as best you can to Planck's constant, the second, and the meter, such as a Watt balance. The Watt balance, if it uses the quantum hall effect, Josephson junctions, etc. will give you an equation where m = hp(n^2)(f^2)/(4gv) where h is Planck's constant, p and n are whatever the hell they are, f is the frequency from the JJunctions, g is local gravitational filed strength, and v is the speed that the mass went at in the Watt balance. But a different device will relate the mass to Planck's constant, the meter, and the second differently."

That's not great, but I think it gives a student a better idea of what is going on than “The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and ∆νCs.”

 

[diogenesNY]

There seems to be a rather significant underlying issue here, notable in that it has been touched on but not really explored in the coverage of this redefinition.

I understand that there has been some observed drift in the mass of the Reference Kilogram, and this addresses that rather directly, BUT---

One thing that is going on here is the shift from an empirically based definition to one that exists as a defined term. This strikes me as pretty major (I know it isn't the first quantity to be so redefined in the last century). Any thoughts on the implications or simply the evolution (pro? con? indifferent?) of moving from empirical to defined standards for metrological quantities?

diogenesNY

 

[gneill]

Defined standards based on (presumably constant) universal constants would seem to be a win. Constants don't ablate, rust, wear, dent, absorb or outgas material or do other sneaky things as physical objects tend to do over time.

 

[kith]

Any thoughts on the implications or simply the evolution (pro? con? indifferent?) of moving from empirical to defined standards for metrological quantities?

I appreciate the new SI very much. At first, it is harder to understand than a prototype-based system but basing all units on physical constants is conceptually very clean. It emphasizes that the numerical values of such (dimensionful) constants don't reflect properties of Nature but our desire for having convenient units to describe everyday situations. The new SI is also more similar to important different unit systems like Planck units and atomic units which are also based on the freedom to define physical constants.

 

[Herman Trivilino]

Well, suppose I want to explain this to a bunch of high school students.

I would tell them that Avagadro's Number is a known integer, by definition. And if you have that many atoms of Carbon-12 you have 12 grams of carbon, exactly, by definition. The precision we concerned ourselves with when we measured the mass of that 12-gram sample has been replaced with a concern over our ability to count Avagadro's Number of things precisely.

 

[CWatters]

Currently there is some traceability in the system in that you can compare your "bag of sugar" to a reference that has itself been compared with the international reference. Will some official body continue to provide the final step in the traceability tree?

 

[kith]

I would tell them that Avagadro's Number is a known integer, by definition. And if you have that many atoms of Carbon-12 you have 12 grams of carbon, exactly, by definition.

I don't think that the second sentence is true in the new SI. There isn't a definition which relates the mole and the kg anymore. The mass of one mole of Carbon-12 needs to be determined experimentally.

 

[Nugatory]

Well, suppose I want to explain this to a bunch of high school students. I think that what I'd say is something like "to find the mass of an object precisely as possible, one has to place the object in a device that allows you to relate it as best you can to Planck's constant, the second, and the meter, such as a Watt balance. The Watt balance, if it uses the quantum hall effect, Josephson junctions, etc. will give you an equation where m = hp(n^2)(f^2)/(4gv) where h is Planck's constant, p and n are whatever the hell they are, f is the frequency from the JJunctions, g is local gravitational filed strength, and v is the speed that the mass went at in the Watt balance. But a different device will relate the mass to Planck's constant, the meter, and the second differently."

For a bunch of high school students, I think the answer is even easier: "Weigh it using the most accurate scale you can find, just like before". he needle and the tick marks on its dial aren't infinitely thin so there will be some uncertainty in the value you read out.

All the new definition of the kilogram does is tell us where the tick marks would be on a mythical perfect dial with an infinitely thin needle and infinitely thin tick marks. The new definition was carefully chosen so that the tick marks on the dials of all currently working scales are correct, just too wide; this allows to build ever more accurate scales in the future.

 

[Dale]

It emphasizes that the numerical values of such (dimensionful) constants don't reflect properties of Nature but our desire for having convenient units to describe everyday situations.

Well said.

 

[Palash_85]

This is only good for theory, having a constant number but in real world it makes zero sense and gives a total of zero essence about mass. The previous definition gave us an idea what actually a kilo is and that it is measured for mass, and is measured against a standard physical weight. And if science doesn't explain physical things and is good only on paper, it defeats the very purpose of teaching science to common people. Keep it restricted to exclusive science club where scientists will drool over such impossible to comprehend definitions.

 

[fresh_42]

This is only good for theory, having a constant number but in real world it makes zero sense and gives a total of zero essence about mass.

Which by the way isn't the meaning of units. There is an essential difference between a physical quantity and a scale it is ruled with. The scale does not explain the quantity and has never been meant to do so.

The previous definition gave us an idea what actually a kilo is and that it is measured for mass, and is measured against a standard physical weight.

And now you have lost this idea? You must have an incredible understanding of unbelievable huge numbers, if you can recognize a few atoms more or less. Just saying: the prototype kilogram lost many atoms over the years as well!

And if science doesn't explain physical things and is good only on paper, it defeats the very purpose of teaching science to common people.

Again, don't confuse the object with the ruler! Rulers should not be used for teaching other than by some illustrations. And this didn't change at all. Speaking with "common people" about a meter, nobody has ever asked me about its definition. Not even if we had change from prototype to light speed. They always have been happy with a yardstick.

Keep it restricted to exclusive science club where scientists will drool over such impossible to comprehend definitions.

We will. Good luck when you buy your bread in the future in terms of "handfuls" because someone had redefined the kilogram by an unrecognizable amount.

 

[Nugatory]

This is only good for theory, having a constant number but in real world it makes zero sense and gives a total of zero essence about mass.

You are misunderstanding what the definition of the kilogram does. Nothing has changed in how we understand mass: it's still resistance to acceleration, we still measure it by comparing the unknown mass that we are weighing with a known mass (balance scale) or observing how it accelerates when subjected to a known force (spring scale), and statements that this object has this much more mass than that object still mean what they always have.

All that's changed is that we have a new and more precise rule for where we put the tick marks on the readout dial of our scales.

 

[Dale]

This is only good for theory,

On the contrary, this is eminently practical. Instead of an unreliable and privately held nearly inaccessible standard we now have a reliable standard that can be accessed by everyone anywhere. This is the most practical improvement since the abrogation of the prototype meter.