There’s so much science in the news right now, I don’t know what to write about: A massive crater discovered under Greenland’s ice sheet that caused climate chaos for early humans? A new planet around Barnard’s star, a mere six light years away? A method for cooking turkey using nothing more than a car battery and a vat of vegetable oil?
Ok, I made that last one up.
But with Thanksgiving this week and the holidays right around the corner, I just can’t help it:
We are getting a new kilogram for Christmas!
I know this might not seem all that important, but to scientists around the world, this is A Very Big Deal. Physicists depend on accurate measurements to determine how the laws of nature actually work. And for a while now we’ve had very accurate accounting of space and time.
A second is 9,192,631,770 periods of the radiation of a cesium atom.
A meter is how far light travels in 0.0000000033356 seconds.
Both of these are defined so fundamentally that even an alien living on a planet orbiting Barnard’s Star would understand them. All you’d need to do is send them a sample of cesium and a primer on the mathematics of counting by 10s.
But a kilogram, our most basic unit of mass? We put a piece of metal in a bell jar 129 years ago and called it Le Grand K. And it’s been sitting in France ever since.
When a physicist wants to know how massive something is, she creates another slug of metal. She then travels to France and uses something as simple as a balance to make sure it has the same weight as Le Grand K. She then takes that mass back to her lab to measure everything else against it.
It’s not a very sophisticated system. But for over a century, scientists have soldiered on, dutifully measuring things with our reference kilograms, and occasionally checking their mass against Le Grand K and making the appropriate adjustments. What a great excuse to travel to France!
It has started to become a problem though because every time scientists trot out Le Grand K for another check, it loses a few atoms. So much so, it’s down about 50 micrograms already, about the same mass as a hair from your eyebrow. That’s not such a big deal if you are calculating the cooking time for your turkey but significant if you are measuring the gravitational ripples of colliding black holes pulsing through the universe.
But this month, the General Conference on Weights and Measures met in France to approve a new definition of a kilogram that uses something called a Kibble balance. It measures the force gravity exerts on an object — what we call weight — against the force created by an electromagnetic field. This device accurately determines a fundamental constant of the universe called the Planck constant, and along with our other definitions of meters and seconds, can be used to define a kilogram.
I imagine you think I’m already deep into the holiday hard cider to be so excited about this, but then again, my physicist spouse and I spent our first Thanksgiving calibrating the heat in our oven — to several decimal places.
This is such a big deal for physicists because now, in principle, any properly equipped laboratory can construct a Kibble balance and create a kilogram standard with the appropriate mass, and we don’t need to worry about Le Grand K shedding weight over the years, which means measurements from different laboratories will be that much more precise and easier to compare with one another.
From a physicist’s point of view, we are getting to where we need every conceivable decimal place to verify experimentally our ideas about how the universe works. This brings us closer to a standard reference that will help us push those experiments to new limits.
Of course, the precise value of a kilogram was still defined by a lump of metal in France, and it will be difficult to communicate that to our friends orbiting Barnard’s star. But with a better understanding of the physical universe, who knows? Maybe we’ll use our warp drive to zip over and invite them to a Thanksgiving meal, complete with a properly weighed turkey.
And wouldn’t that be some news?