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?
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5$\begingroup$ You can always play this game. Technically, the sensor isn't even measuring deformation either; it contains a circuit that measures a resistance. But you can't "really" measure a resistance, you measure the current at a given voltage. But you can't "really" measure current, you measure the magnetic deflection of a current-carrying wire. And so on and so on. $\endgroup$– knzhouCommented May 6, 2017 at 19:08
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1$\begingroup$ It's an endless word game. If you keep playing it, you have to either conclude that nothing is real or that the game is meaningless. $\endgroup$– knzhouCommented May 6, 2017 at 19:09
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$\begingroup$ Forces are simply our way of explaining and calculating interactions. They aren't even necessarily "real" things. $\endgroup$– BlaviusCommented May 6, 2017 at 19:46
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$\begingroup$ Thank you, makes sense. I am not a physicist so I was wondering whether there is more to it than simply being a concept that "works" well with the theories we use to explain and predict phenomena around us. $\endgroup$– DragosCommented May 7, 2017 at 23:22
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$\begingroup$ Sure it can, millions of scales measures body weight (which is force), not body mass. Mass is extrapolated afterwards. $\endgroup$– Agnius VasiliauskasCommented Jun 2 at 21:19
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2 Answers
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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.
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$\begingroup$ Kira, thank you - this is a lot of information to process, I will do some reading on the topics you mentioned. $\endgroup$– DragosCommented May 7, 2017 at 23:14
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Your question, whether forces can be measured directly, is perfectly justified but requires some elaboration (In short, the answer is no).
First, the basic equation of mechanics (Newtons Second Law)
$$m \ddot{x} = F$$
relates the three notions mass, position (acceleration) and force. If this is to have any meaning, we should be able to say what $m$, $\ddot{x}$ and $F$ are, and we should also be able to specify a procedure that allows us to verify (falsify) whether this equation holds.
The acceleration $\ddot{x}$ can be defined purely mathematically in terms of the position $x = x(t)$ as a function of time, so it suffices to define what $x$ is, but position can be defined using extensions and subdivisions of a ruler.
Inertial mass $m$ is a bit harder to define, but a theoretical (but not very practical) definition would be $$m = \frac{|\ddot{x}_t|}{|\ddot{x}|}$$ where $\ddot{x}_t$ denotes the acceleration of a fixed test mass and $\ddot{x}$ the acceleration of the object whose mass is to be measured, if both masses are put close to each other but are isolated from their environment.
Now finally and most importantly, the force $F$ simply cannot be given an a priori definition at all. Of course you can measure the acceleration it produces on an object of mass $m$ and then compute $F$ as $F = m\ddot{x}$, but then Newtons Second Law would be a tautology. So in this sense, forces cannot be measured "directly".
Now I've said earlier that if $m\ddot{x} = F$ is to have a meaning we should be able to say what both sides are, and I just said that we don't know what $F$ is, so it seems that I'm saying that $m\ddot{x} = F$ is meaningless. However I'm not saying this, or rather, I'm saying that there is no general definition of $F$ and that $m\ddot{x} = F$ therefore has no meaning as a general equation. However of course in the various situations in which we are able to find definitions of $F$ (gravity or Lorentz force), $m\ddot{x} = F$ is an equation and has a (falsifiable) meaning.
