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What do you consider the fundamental quantities in physics to be? By fundamentals, I mean quantities that cannot be described by a combination of other quantities. Fundamentals are things that just are.

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    $\begingroup$ This question (v1) seems like a list question. $\endgroup$ – Qmechanic Oct 2 '16 at 16:47
  • $\begingroup$ Mass and charge aren't fundamental, check out pair-production and annihilation. $\endgroup$ – John Duffield Jan 16 '17 at 20:03
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With fundamental quantities, I could imagine that you mean properties that differentiate various particles.

In particle physics, there are multiple charges:

  • electric charge
  • color charge
  • weak isospin
  • mass (“Higgs charge” so to speak)

Then also discrete symmetries like parity and charge conjugation that give you more quantum numbers:

  • parity
  • charge conjugation parity
  • spin
  • $g$-parity (although that is a combination of the other ones)

Then one could look at like the core concepts of QFT:

  • spinor fields
  • gauge fields
  • spin-0 fields

All this needs the spacetime with its curvature and the various symmetry group manifolds.

One could also take things like the action to be fundamental. From the action or the Lagrange density one can derive the equations of motion. Using the action one can compute (using Feynman's path integral') all the possible interactions. Using lattice field theory one can simulate it on the computer. It is not completely clear how the microscopic theory of quantum chromodynamics (QCD) generates the mesoscopic degrees of freedom that we see: the proton, neutron and other hadrons. It is believed that the microscopic theory can explain it. But is the theory fundamental if one cannot (yet) compute how the emerging structures are going to be?

I think it depends on your perspective. You can take the stance that the standard model is an effective theory which one gets by integrating out all the string theory physics. Then string theory would be fundamental.

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  • $\begingroup$ Martin, the magnetic dipole moment of all subatomic particles is a (intrinsic) fundamental property too. This property seems to be forgotten often. (Why?) $\endgroup$ – HolgerFiedler Oct 2 '16 at 16:59
  • $\begingroup$ Isn't the magnetic dipole moment just proportional to spin and electric charge? I would call the latter two more fundamental than the magnetic moment. Of course one could define any one of the three as non-fundamental. Is that what you meant? $\endgroup$ – Martin Ueding Oct 2 '16 at 17:11
  • $\begingroup$ A neutron has an intrinsic spin and a related magnetic dipole moment but not charge (or zero charge). I argue that in bonded states the electrons electric charge will weaken and the magnetic dipole moment is the more influencing property. $\endgroup$ – HolgerFiedler Oct 2 '16 at 17:29
  • $\begingroup$ In my view, one should not consider any theory in physics or its elements to be fundamentals, because physical theories always change as more is learned about the nature of physical reality. For example, "fields" are not fundamentals, since they are merely a theoretical convenience employed to explain influences that occur over a distance. $\endgroup$ – John Petrovic Oct 2 '16 at 18:09

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