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The kilogram isn't a thing anymore. Instead, it's an abstract idea about light and energy.
As of today (May 20), physicists have replaced the old kilogram — a 130-year-old, platinum-iridium cylinder weighing 2.2 pounds (1 kilogram) sitting in a room in France —— with an abstract, unchanging measurement based on quadrillions of light particles and Planck's constant (a fundamental feature of our universe).
Sense - Achievement - Kilogram - Time - Cylinder
In one sense, this is a grand (and surprisingly difficult) achievement. The kilogram is fixed forever now. It can't change over time as the cylinder loses an atom here or an atom there. That means humans could communicate this unit of mass, in terms of raw science, to space aliens. The kilogram is now a simple truth, an idea that can be carried anywhere in the universe without bothering to bring a cylinder with you.
What's really fascinating here isn't that, practically speaking, the way most of us use the kilogram will change. It's how damn difficult it turned out to be to rigorously define a unit of mass at all.
Forces - Terms - Reality - Time - National
Other fundamental forces have long since been understood in terms of fundamental reality. A second of time? Once, according to the National Institute of Standards and Technology (NIST), it was defined in terms of the swings of a pendulum clock. But now scientists understand a second as the time it takes an atom of cesium 133 to go through 9,192,631,770 cycles of releasing microwave radiation. A meter? That's the distance light travels in 1/299,792,458th of a second.
But mass isn't like that. We usually measure kilograms in terms of weight — how much does this thing push down on a scale? But that's a measurement that depends on where you perform the actual weighing. That cylinder in France would weigh much less if you...
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