you've heard about the theory of everything.
which brings me to this question: is there an end to the development of physical theories to explain the universe?
in other words, is physics ((permanently(( incomplete?
I'd say yes.
we have different theories to explain different aspects of physical phenomena: newtonian mechanics explains macro phenomena in general; einstein (general) relativity represents a definite refinement to newton's classical mechanics. quantum mechanics better explains subatomic particles. then, various string theories reconcile einstein's theories with quantum mechanics, etc.
let's suppose in some future we had T, the set of all theories (T1, T2, ...Ti, ...) to explain physical all phenomena.
T would become the mother of all theories, that is to say, there would be nothing new to explain. nothing deeper or different.
and yet, how would we know that? it is a platitude that theories explain phenomena, but T makes an extraordinary claim not made by previous theories: it contains and explains all theories.
and yet, if T explains all preceding theories (and consequently all phenomena) it would not be explaining theories or phenomena --as much as explaining itself!
T cannot present itself as the bound of all theories unless we dispose of Tk, making that provision. Moreover Tk would need to, sort of pull the rug from underneath it by claiming a condition of possibility that now it denies any further possible theory.*
someone may counter that Tk is just a meta theory on T & not on phenomena, but that ignores the Duhem/Quine thesis, i.e., T can not be tested in isolation (nor would Tk).
so, physics must remain ((permanently(( incomplete.
* I am referring to Popper's falsifiability principle.