Date: Mon, 20 May 2019 02:14:01 -0400
Subject: [WSFA] Re: Tonight's big change
From: "Elspeth Kovar" <ekovar at panix.com>
To: "WSFA members" <WSFAlist at KeithLynch.net>
Reply-To: WSFA members <WSFAlist at KeithLynch.net>

Keith F. Lynch wrote:

I've just printed this out as a document so I can look things up and follow along!

What's your source on this? Sounds fascinating.

Elspeth

> 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.
>