Why can't we define fundamental unit of mass?

Question

In my physics textbook of class $11^{th}$ The kilogram was defined as :-

mass of the platinum-iridium standard cylinder kept at Sevre's France

But this isn't a proper and scientific definition of an important fundamental unit.
When we have advanced so much in atomic physics that we are creating matter, Then why are we unable to define unit of mass?

Possible duplicate of Why do we still not have an exact (constants-based) definition for a kilogram? — Emilio Pisanty, May 25 '18 at 10:08
@EmilioPisanty Sorry but title was not clear so this happened — Abhishek Choudhary, May 25 '18 at 10:21
@Abhishek No need to apologize, asking duplicates is just something that happens. — Emilio Pisanty, May 25 '18 at 12:25
@Abhishek It's perfectly OK to have genuine questions, but if there's already a version that's essentially the same then it's inefficient to keep both versions open. If the answers to the existing question are insufficient then you should edit this question to explain why, and it can be reopened. If they do, then this can just stay closed and it's not a problem. — Emilio Pisanty, May 25 '18 at 12:35
It is possible but not necessarily appropriate. If you want to do that, raise a custom moderator flag asking whether it's suitable in this case. (The response will be in the flags page in your profile.) Basically, merging is appropriate when the answers to the new question are seamless answers to the old one. And really, in this case, there's no actual need for it. — Emilio Pisanty, May 25 '18 at 12:39

score 11 · Answer 1 · answered May 25 '18 at 09:25

11

The definition of the kilogram is to be changed in 2019, see Proposed redefinition of SI base units (wiki).

The current proposal is to indirectly define the kilogram via an exact specification of the Planck constant: this is possible because the dimension of that constant can be expressed as a combination of mass, time and distance. The kilogram would then be defined via the second and the meter.

answered May 25 '18 at 09:25

Stéphane Rollandin

3,742

is it nice for a definition to depend on other definitions? – Abhishek Choudhary May 25 '18 at 09:29
How would you do otherwise? – Stéphane Rollandin May 25 '18 at 09:30
1

using exact value of avogadro's number? – Abhishek Choudhary May 25 '18 at 09:31
2

So you would be using the definition of the mole. – Stéphane Rollandin May 25 '18 at 09:35
mole is simpler than others – Abhishek Choudhary May 25 '18 at 09:37
1

This is an interesting discussion, but comments are not meant for that. I suggest you ask a new question about this. – Stéphane Rollandin May 25 '18 at 09:38
@Abhishek Using an exact definition of Avogadro's number is also a proposed definition / part of the solution https://en.wikipedia.org/wiki/Avogadro_constant#International_Avogadro_Coordination – owjburnham May 25 '18 at 13:00
@StéphaneRollandin then my reputation will decrease but i love arguing – Abhishek Choudhary May 25 '18 at 13:01
@Abhishek There is a chat for that. – DK2AX May 25 '18 at 13:47

score 2 · Accepted Answer · answered May 25 '18 at 09:40

Mass is the last fundamental unit that is still defined like that.

Previously, meter was also defined as the length of a specific item, kept also at Sevres, in France (since the metric system has been popularized starting from France).

But the issue with mass, is that mass and energy are kind of interchangeable, so that defining a mass unit by using specific atoms is very tricky if you want the definition to be usable everywhere, that is fairly easily reproductible with an experiment method.

The current definition of the meter is not very easy to use, but it fairly accessible and absolutely reliable and reproductible.

The next definition of mass unit, as per the existing answer, is not using the same path, an rely on a fundamental constant and leverage on the meter, that can be defined precisely with a experiment method.

Why can't we define fundamental unit of mass?

2 Answers2