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.