What is a fundamental force?

Question

What is a fundamental force? I've been trying to find some kind of definition and the closest I've been to a definition so far is "In physics, the fundamental interactions, also known as fundamental forces, are the interactions that do not appear to be reducible to more basic interactions." from wikipedia.

I think forces in QFT can be described as particles (that are excitations of an underlying field with the appropriate charge) interacting via gauge bosons. The gauge bosons carries energy and sometimes charge and therefore the particles affected by the force might move or something.

Is there another way to think of this or does anyone have any nice analogy? There is something that just don't "click" for me.

Answer 1

At the level of particle physics interactions, the concept of force is relevant as momentum transfer, dp/dt. All Feynman diagrams that describe the mathematics of the interactions transfer a dp/dt at the vertices the momentum carried by the intermediary particles. Take the simple Compton scattering:

the force is carried by the virtual electron, from the incoming real particles to the outgoing real particles.

The term "fundamental forces" is in order to clarify that there are three coupling constants with different strengths, qualifying the vertices, electromagnetic, weak, strong, and these are identified as fundamental , because they are fundamental in building up the $SU(3)\times SU(2)\times U(1)$ model of particle physics. Each "fundamental force" with its own gauge bosons, "force", because the couplings define how strong the interaction is.

Answer 2

The "fundamental" forces are interactions that, by convention, have been labeled as "fundamental."

More seriously, you could try to make a rigorous definition along the lines of "fundamental forces are interactions mediated by bosons which are not composite particles." Except, of course, we can't really know that any of the bosons are not composite and the scale of the composite-ness is just really high. We also can't rule out that the current slate of bosons are not actually different components of a single field that is in some state of spontaneous symmetry breaking at energies we can probe. So we have to say "fundamental forces are interactions mediated by bosons which are not currently known to be composite." Except this definition would imply that there should be a Higgs force, since the Higgs is (as far as we know) a fundamental boson mediating an interaction. So we could try "fundamental forces are interactions mediated by spin-1 or spin-2 bosons which are not currently known to be composite." Except... well... at this point we are doing a lot of work to invent a definition that reproduces the convention we want. Which leads us back to my original claim: The "fundamental" forces are interactions that, by convention, have been labeled as "fundamental."

Making things even more confusing, the term "fifth force" has taken on a meaning in the literature of "interactions not mediated by standard model particles," even though this is not really correct.

Answer 3

Details aside, which are covered by links in the comments of your post, the Wikipedia definition seems like an appropriate one. To illustrate it a bit better, think about a non-fundamental force: the nuclear force caused by Pion Exchange (see the figures on the Wiki page), which keeps the nucleus together. Unlike Standard Model gauge bosons (which are fundamental, non-composite), pions are composite, non-fundamental particles. But they still mediate a force between nucleons.

PS: The term "force" is a bit misleading, I find. "Interaction" would be better. In most cases however, if two things can interact, then once can come up with a reasonable definition of a "force" (via something called the Born Approximation). In terms of a mental picture: If two particles can exchange something which changes their momenta (i.e. interact), then there's a force between them. If what they are exchanging is a fundamental particle (the Standard Model particles in our case), then the force is fundamental.

Answer 4

Fundamental forces act between elementary particles while
non-fundamental forces only act between a collective of particles. For example, consider friction, tension, drag force, air resistance. This is a major difference since fundamental forces are observed at the level of elementary particles, while the other forced at this level are meaninigless. The reason is that all non-fundamental forces are manifestations of fundamental forces. Because of this property, most of non-fundamental forces are non-conservative, while fundamrntal forces are conservative.