Are there really still only four forces in the universes? Or are all of them except for gravity reducible now to one force? What is a force anyway? What is their medium of transmission across space? Is charge a force? What about inertia? What about the Casimir Effect? Are quantum fluctuations a force? Tks for your time..
There appear to be four fundamental interactions, sometimes referred to a bit loosely as four forces: strong nuclear, weak nuclear, electromagnetic and gravitational. Though there has, I think, been some success at combining the electromagnetic and weak nuclear interaction into a single "electroweak" one. Each of them seems to have a mediating entity, though that for gravitation remains tentative: https://en.wikipedia.org/wiki/Force_carrier Charge is not a force but the attraction and repulsion between charges is part of the electromagnetic interaction. Inertia is a manifestation of one effect of mass. The Casimir effect is accounted for by electromagnetic interactions (London or van der Waals forces q.v.). Quantum fluctuations are not a force.
Sure, the forces between magnetic objects are another part of the electromagnetic interaction, just as electrostatic forces are. Magnetic forces are the result of the effect of relativity on electric charges. This illustrates why it is better to speak of 4 interactions rather than 4 forces: there are all sort of forces (in Newtonian mechanics). But there are only 4 fundamental interactions, so far as we know.
So far, we have managed to explain all observed interactions with reference to just four basic interactions (although, now I come to think about it, I don't know exactly where the Higgs interaction fits into that). There is currently no compelling scientific reason to introduce a "fifth force" into our models. That's not to say we have a complete understanding of our universe, of course. Currently, it appears we have quite a big problem with dark matter and dark energy, but we currently don't have comprehensive models of either of those things. The electromagnetic and weak interactions are unified in electroweak theory. In a sense, the Standard Model of particle physics explains the electroweak and strong interactions but not, as far as I'm aware, in a completely unified way. A unified theory of the strong and electroweak interactions would be what is often called a "Grand Unified Theory" or GUT. Our best current theory of gravity is not a quantum theory, and that presents a significant barrier to the creation of a so-called "Theory of Everything" (TOE). There are, of course, attempts to incorporate gravity into the quantum picture, such as string theories. But those are still speculative. Each of the four interactions, in quantum theory, is "mediated" by the exchange of "force carrier" particles that are transmitted between the particles that are affected by each interaction. None of the fundamental forces require a particular "medium" for transmission; all can be transmitted in a vaccum. No. Charge is a property of fundamental particles that are affected by the electromagnetic interaction. Fundamentally, electromagnetic forces only act on things that have electric charge. In a similar vein, "colour charge" is a property possessed by quarks, and only particles that have that property are affected by the strong interaction. Mass is the kind of "charge" that the gravitational interaction acts on. Inertia is an effect of mass. Mass, in the modern picture, is a property that some particles gain through their interaction with the Higgs field. I'd have to look that one up. No, but I guess forces can result from some kinds of quantum fluctuations.
Good overview of the Casimir Effect... https://physicsworld.com/a/the-casimir-effect-a-force-from-nothing/
This is interesting to me as a chemist. I have read that the Casimir effect is not at all surprising to chemists, because it can be accounted for in terms of London dispersion forces. These have been known for about a century to be responsible for the intermolecular attraction between molecules with no permanent dipole. My understanding is that the derivation of London forces does not require any consideration of the vacuum. They are ascribed to random fluctuations in the electron cloud of a molecule, thereby creating an instantaneous dipole that can induce a dipole in a neighbouring molecule, creating attraction. But I have also read that the two explanations of the attraction are equivalent, in quantum-mechanical terms. This leaves me rather unclear to me whether dear old Casimir discovered something profound or not, or whether the credit belongs to Fritz London, back in 1930! Anyone who can shed light on this would be most welcome to explain the connection.
