From: "Keith F. Lynch" <kfl at KeithLynch.net>
To: WSFA members <WSFAlist at KeithLynch.net>
Subject: [WSFA] Tonight's big change
Date: Sun, 19 May 2019 17:02:40 -0400 (EDT)
Reply-To: WSFA members <WSFAlist at KeithLynch.net>

Tonight's the night our system of units changes.  The definitions of
nearly all SI (metric) units changes tonight at 8 pm EDT, just under
three hours from now.  Only the units that depend only on time and
distance will remain unchanged, e.g. time, frequency, distance, area,
volume, velocity, acceleration, jerk, snap, crackle, and pop.  All
other units will change.

The kilogram will stop being the mass of a particular cylinder in
Paris, and will start being the mass-equivalent of 299792458^2
joules of energy.  This has the interesting side effect of changing
Einstein's famous equation E=mc^2 from a (well tested) conjecture
to being true by definition.

(Don't think you're immune if you don't use the metric system.  The
avoirdupois pound has long been defined in terms of the kilogram,
(it's exactly 0.45359237 kilograms).  Similarly with other avoirdupois
and troy units of mass.  Similarly with the like-named units of
weight, which are defined in terms of the units of mass by assuming
gravity is always 9.80665 meters per second per second.)

The coulomb of charge will stop being one ampere-second, where an
ampere is that constant electric current which, if maintained in
two straight parallel conductors of infinite length, of negligible
circular cross-section, and placed one meter apart in vacuum, would
produce between these conductors a force equal to 200 nanonewtons per
meter of length.  Instead it will become 1/(1.602176634 * 10^-19)
times the charge of an electron, and the ampere will be defined in
terms of the coulomb, as a current of one coulomb per second.  So
the charge of an electron will cease being a measured quantity and
start being a defined quantity, while the force between two current-
carrying wires will cease being a defined quantity and state being
a measured quantity.  This will have effects on nearly all other
electrical units, including the volt, farad, henry, and ohm, and
all magnetic units including the gauss and tesla.

Planck's constant will change from a measured quantity to a defined
quantity.  It will become exactly 6.626070151 * 10-34 joule-seconds or
joules per hertz.  (This makes the reduced Planck constant, h-bar, a
transcendental number, the above number divided by 2*pi.)

Boltzmann's constant becomes exactly 1.380649 * 10^-23 joules per
degree of freedom per Kelvin.  This means that the Kelvin temperature
scale is redefined.  And the Celsius temperature scale has long been
defined in terms of Kelvin.  And the Fahrenheit temperature scale has
long been defined in terms of Celsius.  The triple point of water will
change from being a defined temperature to a measured temperature.

Avogadro's constant becomes exactly 6.02214076 * 10^23.  The mass of
a carbon-12 atom will cease being a defined quantity and become a
measured quantity.

The definition of the candela also changes, hence so do the
definitions of the lumen and the lux.

The permeability of free space, mu-naught, will change from being
defined as exactly 4*pi * 10^-7 henries per meter to being a
measurable constant.  The dimensionless fine structure constant,
alpha, can't change since it's dimensionless.  It has to be measured.
It depends on the speed of light (c), Planck's constant (h), the
charge of the electron (e), and the permeability of free space (mu).
Before 1983 when the speed of light became a defined constant,
people could debate whether a hypothetical change in alpha could be
attributed to a change in c, h, e, or any mixture of them.  After
1983, when c became a defined constant, it could only be attributed
to a change in h or in e.  After tonight, when h and e become defined
constants, it will have to be blamed on changes in mu.  This shows
that much of physics is arbitrary.

Tonight's change will make no practical difference to anyone (except
those who work with extremely high precision measurements).  But it
has profound philosophical consequences.  It implicitly *defines*
special relativity and much of quantum mechanics as being true.  And
while its changes in the size of various units are very small, in a
powerful sense it changes what those are units *of*.  After tonight,
when you talk about voltage, force, temperature, or nearly any other
physical measure, you'll be talking about something subtly different
than what those things were before.