NPR last week had a story about the changing kilogram:
More than a century ago, a small metal cylinder was forged in London and sent to a leafy suburb of Paris. The cylinder was about the size of a salt shaker and made of an alloy of platinum and iridium, an advanced material at the time.
In Paris, scientists polished and weighed it carefully, until they determined that it was exactly one kilogram, around 2.2 pounds. Then, by international treaty, they declared it to be the international standard.
Since 1889, the year the Eiffel Tower opened, that cylinder has been the standard against which every other kilogram on the planet has been judged. But that’s creating problems. According to scientists, the cylinder’s mass appears to be changing.
This is not a new problem– it was old news when I was a student at NIST, and people at NIST and elsewhere have been working on alternatives to the physical kilogram standard for decades. The physical kilogram standard is really an artifact of a past age– all the other major standard measures have been redefined in terms of more universal interactions, as explained by Physics Buzz.
Attempts to redefine the kilogram have yet to yield anything, though. The problem, as always, is the gravity is so damnably weak.
Gravity may not seem like a weak force, but it is. The simplest illustration of gravity’s weakness is the old “rub-a-balloon-on-your-hair-and-stick-it-to-the-ceiling” trick. When you do that, the attractive force of maybe ten billion extra electrons on the balloon is enough to hold it up against the gravitational pull of the entire Earth pulling on a billion trillion atoms in the balloon. Gravity is preposterously weak compared to the electromagnetic force.
This is a nice feature for those of us who like to play basketball, but not such a good thing if you’re trying to come up with a standard for mass. Tiny electric and magnetic fields are enough to throw off any attempt to measure mass by measuring the gravitational force, simply because of the huge disparity in the strengths of the two forces.
There are a couple of approaches currently being pursued for alternatives to the existing physical kilogram. The one highlighted in the NPR piece is the Watt Balance, which relates the gravitational force on an object to the current and voltage in an electrical circuit. Voltage and current can be defined in terms of fundamental physical constants through the Josephson and Hall effects, so this lets you express a mass in terms of Planck’s constant and a bunch of other numbers. In effect, this would give you a mass standard whose precision is determined by the precision of the value of Planck’s constant.
another approach, which I kind of get a kick out of, is to basically make a better physical standard. There are a couple different approaches, the best-developed of which uses extremely precise single-crystal silicon spheres. The idea is to redefine mass in terms of Avogadro’s number, so one kilogram would be the mass of a specified number of silicon atoms, with the sphere being the physical realization of that number of atoms.
To date, neither of these approaches has managed to significantly improve on the existing mass standard, so for now, the platinum-irridium cylinder, flawed though it is, remains the standard. It remains to be seen whether any of the current efforts can replace it, or if it will be around until the kilogram is redefined in terms of the force between Planck-mass black holes produced in some sort of giant future accelerator.