1
$\begingroup$

What are all of the fundamental properties, that is all measurable quantities which are not derived from anything else? Many quantities are derived e.g. area is length squared, velocity is length per unit time, force is mass times acceleration (which is velocity per unit time), etc. Even temperature may be seen as derived (a measure of kinetic energy per unit mass, which amounts to velocity squared)

I have seen a list of only four properties which can be called fundamental:

  • Length
  • Time
  • Mass
  • Charge

Is this the complete list, or are there any others?

$\endgroup$
  • 1
    $\begingroup$ There are about 30 of these, like lepton charge, barion charge, 3 types of color charge (red, green, and blue), etc. BTW, length is not one of them, as it is defined by time via the postulated speed of light. $\endgroup$ – safesphere Mar 6 '18 at 3:55
  • 1
    $\begingroup$ I would like to point out that temperature is not defined as kinetic energy per unit mass, nor does it have these units. Temperature is defined as the change in the internal energy of a system with respect to a corresponding change in entropy. $\endgroup$ – Bob Knighton Mar 6 '18 at 3:57
  • $\begingroup$ This isn't really a well-posed question. Speed can be derived from length and time if you consider those to be fundamental (as SI does), or length can be derived from speed and time instead (as is perhaps more natural). $\endgroup$ – Chris Mar 6 '18 at 3:58
  • $\begingroup$ I suspect 'Mass' is not consider a property because I read it from somewhere that it is actually some sort of interactions of some fields, but never mind that I must go by the textbook or else detention. $\endgroup$ – user6760 Mar 6 '18 at 9:24
  • $\begingroup$ Possible duplicate of To what extent are quantities fundamental? $\endgroup$ – sammy gerbil Mar 8 '18 at 3:17
0
$\begingroup$

There's nothing truly fundamental about that list- you can take different properties to be fundamental and derive other units from those.

The seven base units, as defined in SI, are:

  • meter
  • kilogram
  • second
  • ampere
  • kelvin
  • mole
  • candela

and all other units are defined from these. There's no reason you couldn't have a different system, however. For instance, in one system of natural units, the fundamental quantities are:

  • angular momentum ($\hbar$)
  • speed ($c$)
  • gravitational constant ($G$)
  • Boltzmann constant ($k$)
  • permittivity ($\epsilon_0$)

There are also various other charges (quantum numbers) in particle physics that you could see as fundamental, but which are traditionally expressed as dimensionless numbers rather than attaching a unit to them.

$\endgroup$
0
$\begingroup$

I think one should be careful with the "fundamental-ness" of quantities as regards measurement vs properties. What we call a base quantity does not necessarily equate nor connote the quantity with a fundamental property. You yourself note that temperature is a complicated concept when put under scrutiny.

A physical quantity is something physical that can be measured in some way and that we then quantify. The choice of such units are arbitrary and tend to have a lot to do with history (although that's about to change). The list of four "properties" you have given are better phrased as base units and if you're talking base units, you have seven as given by the SI: metre, kilogramme, second, Ampere, Kelvin, mole and candela.

As units, they are a bit more "solid" to breaking down (and this allows us to perform useful things like dimensional analysis) but are by no means impervious; your system of base units can be anything you want, it merely has to be coherent and consistent. Of course, you'd have to convince the world to adopt your system as opposed to the one that's already in place to get anyone to work efficiently with you, but that's beside the point.

Charge is a fundamental property but not, I believe, in the way you've phrased it. The electromagnetic force is a fundamental property because it is one of the four fundamental forces: it appears irreducible to a more basic form or interaction. But it's not because of the base unit of charge. It arises from discrete symmetries, is additive, countable and is relativistically invariant. In fact, it's the other way around, the unit of charge is (will) be because of the elementary electric charge.

Thinking from the roots of units, what is charge ? "Amount of zap-zappiness", sure, but its unit is the Coulomb. What is a Coulomb ? Why, it's the amount of current transported by an Ampere in one second ! What's an Ampere ? An Ampere is flow of constant current such that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 newtons per metre of length.

Is then your charge a derivative of the metre and kilogramme ? Is then your candela merely a weighted summation of contributions of wavelengths of light ? Is then your kilogramme defining your newton, or should it be the other way around ? In special relativity, there's even a notion of expressing time in dimensions of distance, $ct$, the speed of light multiplied by time. Should we take that, then ?

Base units are base units, determined largely by consensus, convenience and usefulness. "Fundamentality" often comes from deeper arguments: symmetries, theories, equivalences, laws, etc. The "fundamentality" of a thing might inform the unit choice of something, but not vice versa.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.