# Why can't we define fundamental unit of mass? [duplicate]

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?

## 2 Answers

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.

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.

• is it nice for a definition to depend on other definitions? – Abhishek May 25 '18 at 9:29
• How would you do otherwise? – Stéphane Rollandin May 25 '18 at 9:30
• using exact value of avogadro's number? – Abhishek May 25 '18 at 9:31
• So you would be using the definition of the mole. – Stéphane Rollandin May 25 '18 at 9:35
• 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 9:38