Can Force be measured directly? Most of the Force sensors I am aware off really measure deformation, not Force. Is Force merely a concept but not something we can really measure directly? Since Gravity is a consequence of spacetime and not a Force - could the same be true of all the other "Forces" we came to accept as "real"? Could electromagnetic forces be simply another tweak in spacetime? Or maybe spacetimecharge - or something more complex than spacetime?
 A: The 'deformation' you are talking about that the force sensors measure is actually due to the electromagnetic interactions between the action-reaction pairs which indeed is a force and at the microscopic level it involves exchange of photons in between the participating particles.
According to the standard model of particle physics all forces occur due to exchange of the force carrier particles called 'bosons'.Four fundamental forces of nature have been found yet and each was expected to occur via exchange of bosons.The electromagnetic force occurs due to exchange of photons.The weak force acts via gluons.The strong interactions are mediated by W and Z bosons and the gravitational force is expected to be caused by the elusive graviton.Macroscopically all the forces propagate in space-time via the "field" which is defined mathematically as mostly a vector field.It is the quanta(the smallest unit that carries the force) of these fields that are called bosons which leads to the occurrence of these forces hence the concept of forces is not merely hypothetical,it has a physical significance.The "force sensors" in general are the particle accelerators(such as the L.H.C.) which measure the forces implicitly from the energy changes in their interactions.The catch here is that all the force carriers for all the forces have been found in the L.H.C. except for the gravitational force hence the elusive graviton.This is because the gravitational force in those scales is the weakest of all forces which is causing a difficulty in detecting it. You'll be amazed to know that infact all the forces except gravity have been unified into a single field force via the "Unified Field Theory" and you can intuitively see why gravity hasn't been Incorporated in it yet.The physical significance of unification is that not long after the big bang only one force was dominant in nature and as the universe cooled down,symmetry broke and gave rise to the other forces that we see today.Unification is one of the most interesting and painstaking task that has been going on for decades now but hasn't been completely done yet and the biggest challenge is incorporating gravity in the theory.Alternatives such as loop quantum gravity,string theory and others have been suggested but known has been proved yet.String theory seems to be the most probable candidate for a "theory of everything" but for it to be true the universe has to have more dimensions than what meets our eyes.It's a really exciting time to be in the world right now more so for the physicists as anything can happen in the next few decades.
