# B Why can't we define a kg as a multiple of the electron mass

1. Apr 11, 2017

### RaamGeneral

I mean, we defined a second to be the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom [copied from wikipedia].
A meter is the distance travelled by light in 1/299792458 second.
Why can't we define a kilogram as 9.10938356E31 * Electron mass?

2. Apr 11, 2017

### cosmik debris

Maybe an Electron mass isn't constant.

3. Apr 11, 2017

### sophiecentaur

It's a matter of repeatability. To create a viable standard you would need a method of counting electrons that was highly accurate. Atoms are a bit easier to count and collect in one place. Choose an element that's well behaved - relatively inert etc.

4. Apr 11, 2017

### Morgan Patranella

First, ask why would you want to do that in the first place. The numbers would be way to big to deal with. Plus the Kg was standardized before the mass of the electron was found.

5. Apr 11, 2017

### Khashishi

6. Apr 12, 2017

### sophiecentaur

Of course it was but those kg bars in Paris are steadily deteriorating and any standard that exists as an individual sample is basically flawed. A good standard should be reproducible by a Scientist, isolated on Planet Zog, just given the right information. That principle has been observed in most cases already. Who would base the 'second' on the rotation of the Earth or the period of a pendulum of given length?

7. Apr 12, 2017

### nasu

Who said we can't? There are many things that we don't do but it does not mean we couldn't do them if it were worthwhile. :)

8. Apr 12, 2017

### ZapperZ

Staff Emeritus
When we define a unit, we try to have it defined in such a way that (i) it depends on the least number of assumptions and other quantities, (ii) it has the least number of steps to a physical, universal constant as possible, and (iii) it is based on a very accurate, well-known, and robust measurement.

Many of our SI units are being defined based on some physical constants. As stated, there are efforts to define the kilogram in terms of a physical constant, rather than a block of stuff.

The problem with defining the kg in terms of the mass of an electron is that the mass of an electron is not something that is obtained directly. One only has to look at how the mass of an electron was obtained. See, for example, the latest CODATA standards. In many cases, it is either the value of e/m, or the ratio of the values of the electron mass to the muon mass.

And believe it or not, "mass" really isn't a fundamental or clear unit or quantity. What is more fundamental is "momentum", because in all cases, this is what you actually measure. I can set a particular value of momentum, say, 20 kg m/s, and shoot into your body. You will never be able to tell if I was using a 10 kg mass and shooting at your at 2 m/s, or I'm using 5 kg mass and shooting at you at 4 m/s. It is only through identification of another variable, i.e. I need to independently verify the actual velocity to be able to deduce the mass. Similarly, in an e/m experiment, you are actually measuring the e/m ratio. To be able to know "m", you have to make use of another independent experiment to obtain "e".

So "mass" of any kind requires at least one or more additional level of measurement. It may be accurate, but it is not the most "fundamental". It is why we don't define the kg in terms of the mass of an elementary particle. And hopefully, soon enough, we won't be defining it in terms of that lump of stuff sitting in a controlled environment.

Zz.