The speed of light may define a meter and atomic clocks may define a second, but a century-old cube of metal still defines the kilogram–at least, until scientists give this antique lump a 21st-century makeover.
As far as the kilogram is concerned, the year is still 1879, but scientists are meeting in London this week to discuss a change for this humble unit. As the Guardian reports:
“The kilogram is still defined as the mass of a piece of platinum which, when I was director of the International Bureau of Weights and Measures, I had in a safe in my lab,” said Terry Quinn, an organiser of today’s meeting. “It’s a cylinder of platinum-iridium about 39mm high, 39mm in diameter, cast by Johnson Matthey in Hatton Garden in 1879, delivered to the International Committee on Weights and Measures in Sevres shortly afterwards, polished and adjusted to be made equal in mass to the mass of the old French kilogram of the archives which dates from the time of the French Revolution.”
The problem with this dusty old cube is that it isn’t immune to aging: since 1879, it has shed 50 micrograms. In other words, if the President of the United States at the time–Rutherford B. Hayes–were to time travel to the present, he’d officially have more mass because it takes more of today’s kilograms to equal yesteryear’s kilograms–all because our standard is a vulnerable physical object. Adding to the confusion, other units–the newton, for example–rely on the kilogram and multiply the amorphous state of seemingly standard measurements.
Scientists, though, think they have an answer. As MSNBC reports:
“International consensus has been achieved, that in the near future the kilogram shall be redefined, based on a fixed value of the Planck constant,” Michael Stock, a physicist at the International Bureau of Weights and Measurements (BIPM), said in a statement. Stock said researchers have been conducting experiments that establish a link between mass and the Planck constant by comparing measurements of electrical and mechanical power.
The Planck constant reflects the size of quanta, the smallest particles of physical matter, in quantum physics. To connect the quantum world with our everyday world, scientists have been conducting experiments with a device called the watt balance, which can be used to relate the mass of an object to the electrical energy required to move it.
In the coming year, the General Conference on Weights and Measures will meet to vote on changing the kilogram’s definition–until then, we’re still stuck with a shard of metal stored in France.
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Image: flickr / s2art