# The atomic beauty of the new kilogram

##### by Faye Flam

Bloomberg

Last week the kilogram got a new definition — one that rests on a quantity of light. The old kilogram was defined by a platinum cylinder kept in a vault in Paris, and at first glance, that system might seem more intuitive, but it was crude and prone to error. The new definition harnesses the elegance of the universe.

Not everyone is explaining the new kilogram as a quantity of light, but MIT physicist Wolfgang Ketterle makes a convincing case that this is the best and simplest way to understand it. Ketterle, who shared the 2001 Nobel Prize for creating a long-theorized form of matter called Bose-Einstein condensate, said he was worried that some other explanations offered up for the new kilogram were leading the public to believe that physicists were making life more complicated than it has to be.

Some physicists are explaining that the new kilogram is defined through a quantity called Planck’s constant (more on that later). But to Ketterle, it’s more intuitive to define mass in terms of a quantity of stuff, and in this case, the stuff is light.

The new kilogram, he said, is equal to the mass of a number of photons — 1.4755214 times 10 to the 40th power — of a particular frequency. Yes, that’s a lot of photons, and you have to explain how the laws of physics allow light to have mass — but he says this is doable. In fact he has done it, at a talk at MIT that I couldn’t attend, and he graciously went through it again in an interview.

First, however, it helps to explain why scientists would want to change the definition of the kilogram. They’re changing it so they can keep this measure the same — so that it will not be subject to change.

The reasoning is parallel to a move in 1983 to change the meter from a definition based on a platinum bar to one based on the speed of light. People used to measure the speed of light in meters per second using set values for a second and a meter. Seconds could be defined perfectly by the vibrations of an atomic clock. Meters, however, had been defined by an artifact made by humans.

But thanks to Einstein, we know the speed of light is a fundamental constant of nature. So scientists switched things around, defining the speed of light as a fixed constant — exactly 299,792,458 meters per second — and using it to define the meter as the distance light travels in 1/299,792,458th of a second. It sounds circular, but it does away with the need for a platinum bar.

That left just one scientific unit for basic physical traits that was based on a created object: the kilogram. Until now, all scales were calibrated to copies of a platinum alloy cylinder, created in 1879, known as the International Prototype Kilogram, or IPK. Because all physical objects, no matter how coddled, will lose or gain a few atoms to or from their surroundings, discrepancies keep cropping up between the IPK and its various copies. For a time, people weren’t sure whether the copies were gaining weight or the original was losing weight.

In a story last year in the New Yorker magazine, the writer wonders, briefly, why scientists didn’t just weigh the IPK to keep it accurate. As one of the scientists explained to him, all scales are ultimately calibrated to the IPK — so if the cylinder itself doesn’t show as weighing 1 kg, that means the scale is wrong. There is no platonic ideal kilogram — or there wasn’t until now.

The world of sub-atomic particles is platonic. All electrons, protons, neutrons and photons have perfectly uniform masses and other properties. They don’t age or get dented or scratched. And yes, particles of light — photons — can have a mass under certain circumstances, as Ketterle explains.

Light is massless when it travels the speed of light, he said, but you can confine photons in a sort of mirrored box, so that they bounce back and forth but have an average speed of zero relative to the box. In that case, the photons indeed have predictable mass. The box with the photons will have more mass than the box alone.

Still, connecting our useful measuring tools to subatomic quantities is a lot easier said than done. More than a century ago, the founder of quantum mechanics, Max Planck, realized it could be done, at least in theory. Planck had discovered one of the fundamental connections that would make this possible, now expressed through a constant named for him: The energy of a photon is equal to its frequency times the Planck constant.

Just as scientists had to set the speed of light at a fixed number in order to define the meter without the platinum bar, so on Monday they set Planck’s constant at a fixed value to get rid of the mutable kilogram. There were two different kinds of devices that had been used to precisely measure Planck’s constant, and both can be used to measure the kilogram now that Planck’s constant is fixed.

One device, called the Kibble balance, can measure either the kilogram or Planck’s constant by balancing weight against electromagnetic forces. The other apparatus uses a sphere of precisely measured size — so precise that scientists can approximate the number of atoms it contains.

The now-universal and permanent definition of the kilogram will allow much more precise measurements in the future, though scientists aren’t sure yet what those will entail. The kilogram shift seemed inevitable because it became possible, and it harnesses something of the perfection and predictability of the atomic-scale world to make our human-scale world more elegant.

Science writer Faye Flam is a Bloomberg Opinion columnist.