Undergrad Are physical constants 'constant' axiomatically, or is there a theory?

[cmb]

I can see why it would be pretty illogical to speculate that physical constants change over time, but is there more to it than just being 'illogical' to assume otherwise? Is it axiomatic in physics to presume certain physical constants are constant, because otherwise stuff like atoms and things start falling apart (and they don't!)? I get that, if that's the response.

But I was just wondering if there was a specific theory which actually puts forward the proposal that certain physical constants cannot change?

 

[olgerm]

they do not change that is defintion(basically axiom). If these seem to change then it is another model in which these are not constants.
numeric valeu of constants with unit depends of unitsystem. most units-ystems are arbitary and historically created (like SI).

 

[DaveE]

Since you posted in the classical physics section, then I would say constants are constant in classical physics.
However, there are people studying cosmology now that are dealing with exactly this question. Different measurement techniques that imply different values at different times in the evolution of the universe.
Models are always susceptible to change as our knowledge improves.

 

[olgerm]

However, there are people studying cosmology now that are dealing with exactly this question. Different measurement techniques that imply different values at different times in the evolution of the universe.
Models are always susceptible to change as our knowledge improves.

it is question of notation, but I woluld say that in these models the these values are not constants. Even if name of the value includes "constant".

 

[Nugatory]

Is it axiomatic in physics to presume certain physical constants are constant,

It's easy to overuse the word "axiom" in discussing physics because physics is an empirical discipline, so does not follow the mathematician's practice of limiting conclusions to the logical consequences of the axioms. We see this in the occasional debates about what exactly Einstein's "postulates" in his 1905 paper "mean". A mathematician will find the question absurd - a postulate "means" exactly what it implies, no more and no less - and attribute the confusion to the error of calling good but informally stated heuristics "postulates".

But with that said... If there's much experimental evidence suggesting that something is constant and no convincing counterexamples, then it's sensible to make the assumption that it is a constant and proceed accordingly. So yes, in that sense we do presume that the physical constants are constant. If one of them didn't behave as if it were a constant, we wouldn't say it was constant.

 

[DaveE]

it is question of notation, but I woluld say that in these models the these values are not constants. Even if name of the value includes "constant".

Yes, kind of by definition. Constants are considered constant until you have evidence that they aren't actually constant. Then you rename them as you change your models.

 

[Ibix]

Then you rename them as you change your models.

Or not - see the Hubble constant, which is definitely a (very slowly) time dependent quantity.

@cmb - to paraphrase @Nugatory, we measure a quantity and find that it doesn't change, so we label it a constant. Somebody inevitably wonders if it's just changing too slowly to detect and works out the consequences for our theories if it weren't constant. Sometimes that might explain some puzzling observation. For example, there was a thread a few weeks back about a paper claiming that if you let the $8\pi G/c^2$ in Einstein's field equations vary then you don't need dark energy. No idea if that's going to go anywhere, but it's an example.

So in short, we call constants constant because they don't seem to change. But we aren't philosophically wedded to the notion.