The Kilogram is not a Kilogram!

Though it may sound paradoxical, physicists have known for decades that a kilogram just isn’t what it used to be. That’s because it’s lighter—or at least lighter than its copies—by fifty micrograms. After all, worldwide agreement on experimental results is only possible because there are standardized (SI) units like the meter, the second, and the kilogram. But when the standard kilogram, a cylinder of metal alloy (platinum and iridium), is compared to manufactured copies (with the same composition and size), the scale tips, very slightly, toward the copy. Thus, the original has lost mass (perhaps to polishing) or the copies have gained mass (perhaps by absorbing air), but of course, there’s no way to tell which; they are the standards by which scientists would make such a judgment.

Philosophers should take note. Does the standard cylinder weigh one kilogram because scientists were careful when they made it or because it was defined that way? According to National Public Radio, the former. “In Paris, scientists polished and weighed [the cylinder] 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.” But according to Richard Steiner, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, MD, the latter. “If somebody sneezed on that kilogram standard, all the weights in the world would be instantly wrong.” So which is it? Neither. The original cylinder could not have been determined to be exactly one kilogram before there was a standard, and a single sneeze wouldn’t invalidate experiments around the world.

Luckily, scientists are working on a solution. By using a Watt balance (a device which measures mass by how much electric and magnetic field it takes to counteract gravity) Steiner and others hope to settle the matter once and for all by relating the infamous cylinder to Planck’s constant. This would make the official kilogram another of nature’s fundamental constants like the speed of light. It has taken decades—they must account for earthquakes, the gravity of the moon, etc.—but the scientists should soon have a number that is eight digits long. Then the number will be, by definition, the kilogram and no one can sneeze on it!

2 thoughts on “The Kilogram is not a Kilogram!”

  1. I don’t know if I buy it. Matter is in constant flux unlike a “law” and should Planck shed some light on this whose to say mother nature won’t disrupt some surface molecules and change the cylinder’s mass?

  2. That’s the great thing about this new approach—once the number is determined, via experiment, the cylinder can be thrown away (though we’ll likely want to keep it for historical reasons!). Since the number will be, by definition, the kilogram, we can just use it like we do the speed of light. Matter can keep changing, but the unit of one kilogram will be forever fixed.

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