## Tuesday, September 14, 2010

### Should We Redefine The Kilogram?

What is a kilogram?  For those who don't know, it is the mass of a certain cylinder of platinum sitting in France. (Seriously.)

However, surely the mass of that cylinder is changing.  What about the atoms that fall off every time it is picked up?  Do we really want the definition of something as important as the SI unit for mass to be something that changes ever so slightly with time?

Therefore, I found this article to be interesting from the Physics Arxiv Blog:
Why not make the kilogram equal to the mass of a certain number of carbon-12 atoms, specifically 2250× 28148963^3 of them?
Then a kilogram would be a cube of carbon 8.11cm on each side (8.11cm is roughly the length of 368,855,762 carbon atoms laid side by side).
With that definition, almost anybody could make a kilogram in their own kitchen given some carbon and a knife.
I think this would be a good idea.  Then the definition of the kilogram would remain fixed forever and would be something anyone could compare to with relative ease. (That is, easier that comparing with a cylinder in France hardly anyone is ever allowed to touch.)

Thoughts?

1. Why not just use that cylinder of platinum and estimate how many atoms of platinum it is?

2. Quantum_Flux,

I guess we could do that too.

3. What's a kilogram? Yes, I am an American.

4. Stan,

What confuses me is when we have visitors to the group who walk in and say: "Wow, what nice weather here, I think it is supposed to be 24 degrees today." It always takes me a second to remind myself where they are getting a number like that since it is *never* in the 20s here. :) (Seriously, I don't think once in the entire 3 years I've been here, even as the low.)

5. Joe,

I've gotten pretty good at thinking in SI (or CGS, crazy astronomers) units for everyday things such as speeds, masses, distances, etc., but the one I can never seem to wrap my brain around is temperature for some reason. I even spent 2 years in Brazil converting temperatures back to Fahrenheit and I still can't "think" in Celsius (or Kelvin).

6. "Holden wrecks and boiling diesels steam at 45 degrees"

An Australian song that always reminds me that we are different and behind the times here in the States.

7. Here's an off-the-wall question - why do we like carbon so much? I understand that it's a great element, but we already define the "mole" using carbon. Carbon seems to be causing global warming. Maybe we should spread the love around and use a less-popular element to define the kilogram. My vote is 1 kg = 3.44181374 × 10^24 yttrium atoms.

8. Nick,

You make a good point. My guess is the practicalness of using carbon. I mean, carbon-12 is plentiful, stable, not-dangerous and a solid at room temperature.

9. I think a lot of people are looking at different ways to redefine the kilogram. Re-defining units is something that happens from time to time in SI. (See here, here and here.)

In reality, the definition probably has next to no practical value. Even though if that platinum cylinder in France suddenly split in half, technically the numerical value of everyone's mass would suddenly double, what would most likely actually happen is that some technician / caretaker would see the problem, grumble under his breath in French, take the thing out of its casing, and have it re-cast such that it would have the mass of exactly 1 kg as measured by some electronic analytical balance in that lab. (Granted, I might be oversimplifying a bit, but minus the scientific media blitz that would surely ensue if word got out, I imagine that the situation would actually be dealt with similarly to this.) This is why it is kept underground in a temperature and humidity-controlled vault. (Actual picture here.)

The issue of how to deal with uncertain definitions is something that we are having to deal with for other units as well. We are getting to the point in atomic physics, where we can tell the difference in frequency between a Cesium transition in an atom in Boulder, Colorado, and one in Champaign, Illinois. (They will be slightly different because the local acceleration due to gravity is slightly different, and this has relativistic consequences.) Will we need to define the second not only by what transition in what atom, but by where or under what conditions that atom is in?

Should we re-define the kilogram? Probably. Will it make a huge difference? I doubt it. What should we use to re-define the kilogram? Well, a definition like that should be (1) stable, and (2) practically measurable with high precision. A lot of people like counting atoms. Another way we could do it is in terms of fundamental constants. The meter is currently defined by defining a value of c. We could just as easily define the kilogram by defining a numerical value for G or h or h-bar (sorry, I don't know how to typeset that one here).

Anyways, good stuff to think about. As always, great post.

10. Look up NIST. They do this day in and out.

I like Madhatter's way of thinking, though...

11. @Bill: There are copies of it.