# Who set the values of Physics Constants?

• B

## Summary:

Where do these Physical Constant come from? who set the values for these Constants?

## Main Question or Discussion Point

Hello,

I am writing a blog post about Physics Constant. I recently found out how you can find out the Constant of Proportionality. and had an idea that all the Physics Constants were Constants of Proportionality. but I have no idea how to confirm this because there are so many. so I don't know if I am right or wrong and it is keeping me from writing my blog post. • Boltzmann’s constant: • Mass of electron: • Speed of light: • Gravitational constant:
a • Gas constant: Thanks

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fresh_42
Mentor
Nobody set any constants. We set the units. Meter, kilogram, or seconds have been defined for convenience in our macroscopic world. Some like the second date back to the Babylonians, the meter IIRC to Napoleon. They fit in our daily business. It is an unintended consequence that the speed of light has a strange value. It's the units, not the values which determine the figures. Different units would give different values.

You should research the history of units for your blog. The constants are a consequence of that. But people prefer to have a convenient unit for $2\, kg$ of apples, rather than $G = 10^{-9}\,N\,m^2\,kg^{-2}$.

• • davenn, sysprog and anorlunda
The definition of a constant of proportionality is very simple and doesn’t really say anything about the basis of the constant. If you can write:

x = c y

then c is a constant of proportionality between x and y. Usually the units give a good indication. If the units are A/B it is probably a constant of proportionality. An exception is Avagadro’s number because “number” is unitless. With more complicated units you could still say the constant is a constant of proportionality between two quantities. For example you could say G is the proportionality between $kg^2$ and $N m^2$, i.e. the product of the masses related to the product of the force times the distance squared, but those don’t seem like useful individual things. So I would add the additional “requirement” that a constant of proportionality relates two quantities that are themselves relatively simple, identifiable, and useful. Given thatI would classify these constants in your list as constants of proportionality:

Avagadro’s number relates number of atoms to number of moles,

N = $N_A$ mol

Boltzmann’s constant relates thermal energy to temperature.

E = $k_b$ T

Speed of light relates distance travelled to time

d = c t

I don’t think being a constant of proportionality is particularly profound. For example every change of units involves a constant of proportionality. Your question of the origins of constants is a much more profound and interesting topic.

• sysprog
Nugatory
Mentor
Where do these Physical Constant come from? who set the values for these Constants?
As fresh_42 says above, we don’t set the value of the constants, we choose the units and that determines the numerical value we use for the constant. The speed of light is 299792458 meters per second because that’s how we defined the meter and the second, not because there’s anything physically significant about the number 299792458. These unit definitions are generally created and managed by various international standards organizations such as https://en.m.wikipedia.org/wiki/General_Conference_on_Weights_and_Measures.

There are constants that are independent of the units, and these are the only ones that have a direct physical significance. A good example is the “fine structure constant” - google for it, and look for threads here that discuss it in the context of the speed of light.

• sysprog and gmax137
Mister T
Gold Member
The values of many of the fundamental constants are set by BIPM. See www.bipm.org.

It is true that historically BIPM chose the units, and that those definitions were used to determine the numerical values of the constants, but as of a few months ago they changed that.

BIPM gets its authority to do this by international treaty. It all started in, if I recall correctly, in the 1870's. Do a google search for Treaty of the Meter.

I admit it: it wasn't me; I didn't make up the constants. I would be happy to take the credit, but I emphatically want to stay away from taking any of the blame. I didn't pass the second law of thermodynamics either, and I would be ok with issuing permits to violate it on an ad hoc basis, if i could get the 'powers that be' to go along with that.

• Anand Sivaram