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Physicist fight: how heavy is a kilogram?

Laura June

Physicists are currently hotly debating a topic some of us never think about -- or if we did -- surely we'd think 'there's an answer for that, even if I don't know what it is.' The question? The question at hand is 'how heavy is a kilogram?' The currently accepted answer is the mass of a cylinder of platinum and iridium called the International Prototype Kilogram. The problem with that definition, of course, is that not just anybody can measure it -- since most of us don't have an International Prototype Kilogram laying around, especially since every time the thing is picked up a few atoms rub off of it making it a little bit lighter. Because of this, the actual International Prototype Kilogram (yes, there does only seem to be one in existence) is stored in a vault in Sevres, France, limiting the lay person's ability to determine the actual weight of a kilogram. There are other options on the table, of course, including one involving a two-stories high piece of equipment that costs around $1.5 million -- which isn't much better, when you think of it. Enter Ronald Fox and friends over at the National Institute of Standards and Technology. They're suggesting a rather shocking solution: make the kilogram equal to the mass of a certain number of carbon-12 atoms (2250× 28148963^3 of them), also known as a cube of carbon measuring 8.11 centimeters on each side. This would mean, of course, that pretty much anybody could determine the weight of a kilogram at home. But we wouldn't want that, now, would we?

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