[ISL]

It is now partially incorrect, too. Now we can no longer change Boltzmann's constant not because he's dead (indeed, until recently, it was a measurable quantity), but because k_B is now defined as a part of the redefinition of the SI unit system in 2018.

The value is 1.380649×10^{−23} J/K, exactly.

[JamesBryant]

It was correct when I wrote the RAQ in 2007.

In fact I've been making the crack about 'not being able to change it because he's dead' in my lectures for at least forty years.

[StavrosK]

It's correct now too, since we can no less change the value now than before the redefinition (we can't change it in either case, and not because Boltzmann is dead), so the joke works fine!

[thaumasiotes]

> k_B is now defined as a part of the redefinition of the SI unit system in 2018.

> The value is 1.380649×10^{−23} J/K, exactly.

Hmm. This is a trendy thing to do with SI units, indeed.

But this definition makes me wonder if there are also definitions of the Joule and the Kelvin. If there are, it seems like they could easily conflict with this definition of k_B.

And if that happened, we'd have to admit that k_B was a measurable quantity all along -- the only way to demonstrate a conflict in the definitions would be by measuring the quantity more accurately.

[fsh]

The underlying reason is that we don't actually need a unit for temperature. The temperature of a substance is simply the mean kinetic energy of its molecules which can be given in Joule. The problem is that historically thermometers were calibrated using the triple point of water and not by measuring the kinetic energy of the molecules. This is how the Kelvin scale used to be defined. The Boltzmann constant was simply a measure of the triple point of water in energy units (which could be measured). One problem was that the isotopic composition of water influences the triple point and was not well defined in the old SI system. Nowadays, we can actually calibrate thermometers by measuring the kinetic energy of molecules, so we no longer need to use the triple point of water. This is why the Kelvin is now just a rescaling of the Joule with a fixed coefficient (the defined Boltzmann constant). So the Boltzmann constant can no longer be measured. On the other hand, it is now possible to measure the triple point of water in Kelvin (this used to be 273.16 K by definition in the old SI system).

[minitoar]

Not in SI. Kelvin is defined in terms of k_B. Joule is defined in several ways, eg force & distance.