Tomorrow The Definition of The
Kilogram Will Change Forever. Here's What That Really Means
MICHELLE STARR
19 MAY 2019
Finally, 130 years after it
was established, the kilogram as we know it is about to be retired. But it's
not the end: tomorrow, 20 May 2019, a new definition will be put in place - one
that's far more accurate than anything we've had until now.
After the shift was
unanimously voted
in at the General Conference on Weights and Measures in Versailles at
the end of last year, the change is now finally about to become official. Le
kilogramme est mort, vive le kilogramme.
Most people don't think about
metrology - the science of measurement - as we go about our day. But it's
vastly important. It's not just the system by which we measure the world; it's
also the system by which scientists conduct their observations.
It needs to be precise, and it
needs to be constant, preferably based on the laws of our Universe as we know
it.
But of the seven base units of
the International
System of Units (SI), four are not currently based on the constants of
physics: the ampere (current), kelvin (temperature), mole (amount of substance)
and kilogram (mass).
"The idea,"
explained Emeritus Director of the International Bureau of Weights and Measures
(BIPM) Terry Quinn to ScienceAlert, "is that by having all the units based
on the constants of physics, they are by definition stable and unaltering in
the future, and universally accessible everywhere."
For example, a metre is
determined by the distance light travels in a vacuum in 1/299792458 of a
second. A second is determined by the time it takes for a caesium atom to
oscillate 9,192,631,770 times.
A kilogram is defined by… a
kilogram.
No, literally. It's a kilogram
weight called the International
Prototype of the Kilogram (IPK), made in 1889 from 90 percent platinum
and 10 percent iridium, and kept in a special vault in the BIPM headquarters.
In fact, the kilogram is the
only base unit in the SI still defined by a physical object.
There are copies of the IPK in
various locations around the world, which are used as national standards and
occasionally sent back to France to be compared against the prototype.
And that's where things get
interesting - the mass of these copies has been observed to be drifting away from
that of the IPK locked away in the vault. It's unclear whether the
copies were losing mass or the IPK was
gaining mass, but neither scenario is ideal for scientific precision, even
if we're dealing with mere micrograms.
For the last
few years, metrologists have been talking about the need for a new
standard. Now, they're finally ready to redefine the kilogram based on the
Planck constant, the ratio of energy to frequency of a photon, measured to its
most precise value yet only
last year.
"It is only now that we
can define the kilogram in terms of a constant of physics - the Planck
constant, the speed of light and the resonant frequency of the caesium
atom," Quinn explained.
"Why all three? This is
because the units of the Planck constant are kgm2s-1, so we need first to have
defined the metre (in terms of the speed of light) and the second (in terms of
the caesium atom in the atomic clock)."
So under the new definition,
the magnitude
of a kilogram would be "set by fixing the numerical value of the
Planck constant to be equal to exactly 6.626 069… × 10–34 when it is
expressed in the SI unit s–1 m2 kg, which is equal to J s."
That won't make any
perceivable difference to most people's lives at all - a kilogram of apples
before the change is still going to be a kilogram of apples after the change -
but it will make a difference to metrologists in particular, and scientists in general.
Because, as noted, base unit
standards can rely on other base units. The candela, the ampere, and the mole
will be redefined to greater accuracy based on the kilogram. And, as for
scientists...
"[The new definition]
will considerably improve the understanding and elegance of teaching about
units," Quinn said. "It will open up the way to unlimited
improvements in accuracy of measurements, it will improve greatly the accuracy
and extend the possibilities of making accurate measurements at very small and
very large quantities."
It will be the end of an era,
truly - and also the beginning of a new one.
As for the IPK itself, the
small piece of metal that has been so important for so many years will continue
to be kept in the same conditions it always has, under two bell jars in a
climate-controlled vault.
That's partly to honour its
legacy; but scientists will always be scientists. It will also be studied
"in future years and decades we can observe how much its mass
changes," Quinn said, this time against the new, immutable definition of
the kilogram. So finally we'll be able to tell for sure if it has actually been
losing mass all this time.
Quinn also noted that, while
it may look complex, the new system can actually be easily understood by
anyone. He himself built a simple
balance out of Lego in his basement that can measure directly against
the Planck constant, within 5 percent.
"School children,"
he said, "will be able to have immense fun with this."
The new kilogram definition
will come into effect on World
Metrology Day: 20 May 2019.
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