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Redefining the Kilogram
The kilogram is shrinking.
The official object that defines the mass of a kilogram is a
tiny, 139-year-old cylinder of platinum and iridium that
resides in a triple-locked vault near Paris. Because it is so
important, scientists almost never take it out; instead they
use copies called working standards. But the last time they
did inspect the real kilogram, they found it is roughly five
parts in 100 million heavier than all the working standards,
which have been leaving behind a few atoms of metal every
time they are put on scales. This is one of the reasons the
kilogram may soon be redefined not by a physical object but
through calculations based on fundamental constants.
“This [shrinking] is the kind of thing that happens when you
have an object that needs to be conserved in order to have
a standard,” says Peter Mohr, a physicist at the National
Institute of Standards and Technology (NIST), who serves
on the committee that oversees the International System of
Units (SI). “Fundamental constants, on the other hand, are
not going to change over time.”
The redefinition of the kilogram will be part of a planned
larger overhaul to make SI units fully dependent on
constants of nature. Representatives from 57 countries will
vote on the proposed change this month at a conference in
Versailles, France, and the new rules are expected to pass.
What will happen to the old kilogram artifacts after the
redefinition? Rather than packing them off to museums,
scientists plan to keep studying how they fare over time.
“There is so much measurement history on these,” says
physicist Stephan Schlamminger of NIST. “It would be
irresponsible to not continue to measure them.”
Adaptado de: <https://www.scientificamerican.com/article/redefining-thekilogram/>