# Why is the kilogram the last SI unit which is defined in terms of a physical prototype? [duplicate]

All elementary SI units, except the kilogram, have been redefined depending of a physical constant.

However kilogram still depends on the International Prototype Kilogram. Why is this? Is there no possibility of defining it depending on Planck's constant, for example, or some other constant?

• Possible duplicates: physics.stackexchange.com/q/32120/2451 and links therein. – Qmechanic Aug 9 '16 at 22:53
• The most promising constant to redefine the kilogram is the planck constant. In order to do so, the planck constant has to be related to the kilogram prototype and that turned out to be rather difficult. Recently, there has been some progress though, see for instance the following Nature News item: nature.com/news/kilogram-conflict-resolved-at-last-1.18550 – Paul Aug 9 '16 at 22:58
• Wen we have a quantum theory of gravity, I would expect the kilogram to be redefined as your question suggests. – R. Rankin Aug 9 '16 at 23:45
• @fauve, btw, see my edited answer in the duplicate. – Emilio Pisanty Aug 10 '16 at 13:50

This is essentially a duplicate of Why do we still not have an exact definition for a kilogram?, but the answers there are somewhat outdated. There is, in fact, a programme to overhaul the SI and make it wholly independent of physical artifacts; this programme is currently ongoing, having started in ~2013 and with a proposed date of 2018 for the formal redefinitions.

In particular, the kilogram will be redefined by fixing a value for Planck's constant (and subsequently also dependent on the definitions for the metre and the second, which in turn are given by fixed values of the speed of light and some (new) atomic transition).

For more details on this process, and what the new definitions will look like in practice, see What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?, as well as the Wikipedia page on the proposed redefinitions. For more details on why the kilogram was the last unit to get this sort of definition, see the proposed duplicate, Why do we still not have an exact definition for a kilogram?.

The short story for that, though, is well explained in Paul's link to this Nature News piece:

The kilogram is the only SI unit still based on a physical object. Although experiments that could define it in terms of fundamental constants were described in the 1970s, only in the past year have teams using two completely different methods achieved results that are both precise enough, and in sufficient agreement, to topple the physical definition.

Simply put, it turns out that old-school balances are surprisingly accurate, and that building experiments that are precise enough to beat them (and doing that in a scalable, metrologically sustainable way, with multiple independent routes that verify each other) takes several decades to do well.

• Would you mind adding this excellent addition to the question which this is a duplicate of? I just voted to close, but I think your remarks should be preserved. – CuriousOne Aug 10 '16 at 0:41
• @CuriousOne I suppose I should, but to be honest it'll just stay buried under Steve's answer there. – Emilio Pisanty Aug 10 '16 at 12:24
• @CuriousOne OK, fine, there you go. Now go show that answer some love, and bug the OP there to accept it, or something. – Emilio Pisanty Aug 10 '16 at 13:36