On Friday, representatives of more than 60 nations, gathered in Versailles, France, approved a new definition for the kilogram.
Since the 19th century, scientists have based their definition of the fundamental unit of mass on a physical object — a shining platinum iridium cylinder stored in a locked vault in the bowels of the International Bureau of Weights and Measures (BIPM) in Sevres, France. A kilogram was equal to the heft of this aging hunk of metal, and this cylinder, by definition, weighed exactly a kilogram. If the cylinder changed, even a little bit, then the entire global system of measurement had to change, too.
With Friday’s vote, scientists redefined the kilogram for the 21st century by tying it to a fundamental feature of the universe — a small, strange figure from quantum physics known as Planck’s constant, which describes the smallest possible unit of energy.
Thanks to Albert Einstein’s revelation that energy and mass are related, determining exactly how much energy is in that unit can let scientists define mass in terms of Planck’s constant — a value that should hold up across space and time — rather than relying on an inconstant metal cylinder. (Mass determines something’s weight, and for most purposes mass and weight are interchangeable.)
The redefinition is the result of a decades-long, worldwide quest to measure Planck’s constant precisely enough that the number would stand up to scientific scrutiny.
Though the newly defined kilogram won’t affect your bathroom scale, it will have practical applications in research and industries that depend on meticulous measurement.
The Planck length is the scale at which classical ideas about gravity and space-time cease to be valid, and quantum effects dominate. This is the quantum of length, the smallest measurement of length with any meaning.
And roughly equal to 1.6 x 10-35 m or about 10-20 times the size of a proton.
The Planck time is the time it would take a photon travelling at the speed of light to across a distance equal to the Planck length. This is the quantum of time, the smallest measurement of time that has any meaning, and is equal to 10-43 seconds. No smaller division of time has any meaning. With in the framework of the laws of physics as we understand them today, we can say only that the universe came into existence when it already had an age of 10-43 seconds.
Science, advancing at 1.6 x 10-35 meters at a time. A very short time, though.
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