Talk:Physical constant

Time to move the tables into List of physical constants?

As the tables of listed constants grows, this article has taken on the nature of a list-class article. It seems appropriate to move these tables to List of physical constants (currently a redirect), leaving behind a short section with a hatnote linking to it, and a short table of frequently used physical constants. Opinions? —Quondum 23:01, 21 June 2019 (UTC)

 Done

SI time vs Uzan?

The SI system uses Δν_Cs for its time definition. The second on "Number of fundamental constants" is based on Uzan's long paper, but it does not call out a time or frequency definition. I think we should have some comment about how these two compare. Johnjbarton (talk) 17:26, 10 January 2024 (UTC)

I'm afraid I'm having difficulty following what you are trying to say. Δν_Cs is an emergent quantity, that is directly determined by all the fundamental constants (in a hypothetical correct model), but is used experimentally as a time reference. Notice that ε₀ does not occur in the list either, but is also considered fundamental (as would be any function of fundamental constants). That there are a few constants in common should not be taken as that there is anything special about their status as fundamental constants. All that we could say is that the SI constants are in principle fully determined by whatever list of constants one chooses as the "base" fundamental constants, but I'm not sure we should even do that. —Quondum 17:44, 10 January 2024 (UTC)

The article current mentions kilogram based two values, h and c, listed among the ones on Uzan's list and one not listed. I think a legitimate question by readers would be: "Why isn't it listed?" I'm saying that if "ΔνCs is an emergent quantity, that is directly determined by all the fundamental constants", it is not evident to readers. Johnjbarton (talk) 18:15, 10 January 2024 (UTC)

Okay, so the narrative you might be looking for is that since 2019, the SI is in principle wholly determined by fundamental constants. Three of the SI dimensions (temperature, amount of substance, and luminous intensity) are technically determined by the other four, and can be ignored. Two are directly from the list (c and h), one is directly determined (e, directly determined by the coupling constants of the gauge groups or may even be one of them, or so I guess), and the fourth is an emergent property of a complex system, namely a caesium atom, whose properties are in principle determined by the fundamental constants but which is beyond us to derive. We realize those units by using physical properties that are directly related to these constants, aside from the experimentally determined property of the caesium atom. —Quondum 20:22, 10 January 2024 (UTC)

LOL, probably not something we want in the article ;-) Johnjbarton (talk) 01:50, 11 January 2024 (UTC)

Yes, it is quite a mouthful just to put in anticipation of the reaction to the SI happening to have moved in this direction – and kinda off-topic. Maybe we should actually trim down what is said on the SI to, in effect, that the SI has moved towards using fundamental constants in preference to artefacts for defining units? —Quondum 01:57, 11 January 2024 (UTC)

Dated image for SI constants.

The current page has a nice image of the SI constants on an index card, but it's out of date. Maybe use

SI base units (outer ring) and constants (inner ring)

instead?

@Countercheck Johnjbarton (talk) 01:56, 17 September 2024 (UTC)

Done. Thanks. Countercheck (talk) 15:15, 17 September 2024 (UTC)

Done. Thanks. Countercheck (talk) 15:16, 17 September 2024 (UTC)

The opening definition.

The current lead sentence:

• A physical constant, sometimes called a fundamental physical constant or universal constant, is a physical quantity that cannot be explained by a theory and therefore must be measured experimentally.

is obviously false or the speed of light (set by SI) or the Planck constant (set indirectly) would not be physical constants. A subclass of physical constants, dimensionless physical constant are the ones which must be measured experimentally. Dimensioned physical constants are set arbitrarily or depend upon other physical constants which are set. Thus SI.

Other parts of the article which supposedly source this line are incorrect. For example the source

• Uzan, J. P. (2011). Varying constants, gravitation and cosmology. Living reviews in relativity, 14(1), 2.

is not about "physical constants", but rather about the subclass dimensionless physical constant. Johnjbarton (talk) 04:24, 7 August 2026 (UTC)

@User:Johnjbarton The fundamental constant is a specific quantity and shall be considered not only as a numeric value but together with the unit, regardless of the applied unit system. Vacuum permittivity has in SI a specific dimenion, in CGS it is a dimensionless quantity.

The theory (either its fundamentals - postulated laws, or its results) can for example explain, that there must be a limiting velocity - speed of light, can provide the relations to other fundamental constants (vacuum permittivity and permebility), which can only limit the number of fundamental constant, but there is a set of independent constants, the value of which (in any chosen system of units) cannot be explained by the theory. Be careful, some constants can be used in individual system of units for the definition of the base units (SI, Planck units etc.) and its value is fixed, but there are always other constants which must be kept unfixed to have a coherent system of units without internal contradictions. You cannot fix together vacuum permittivity, vacuum permeability, speed of light and the elementary charge. Petr Karel (talk) 08:42, 7 August 2026 (UTC)

Thanks, but I am unable to connect your reply to the issue I raised. Every source I have read considers the speed of light a physical constant; I've never read that it is not. The speed of light is not measured experimentally, contradicting the opening sentence.

If a constant cannot be explained by theory, it does not follow that it must be measured. It can be assigned an arbitrary value, exactly as is done in all systems of units. Our opening sentence is incorrect.

The underlying problem here is deeply ingrained but ultimately inappropriate terminology. Both base units and dimensionless quantities appearing in theories are called "physical constants" but they have very different characteristics.

You make a separate point which is also not reflected our article: a physical constant is not meaningful alone, but is part of a set. Only the set has meaning. Many sources hint at this but few take it up head on. Johnjbarton (talk) 15:26, 7 August 2026 (UTC)