One of the main reasons why physicists are obsessed by this topic is that position and momentum are mapped to the phase space coordinates of Hamiltonian mechanics, which appear in the matrix mechanics formulation of quantum mechanics, which offers insight into the theory, particularly when generalizing to relativistic quantum mechanics.

One way to think is: what is the definition of space space? It is a way to write the wave function $ψ_{x}(x)$ such that:And then, what is the definition of momentum space? It is of course a way to write the wave function $ψ_{p}(p)$ such that:

- the position operator is the multiplication by $x$
- the momentum operator is the derivative by $x$

- the momentum operator is the multiplication by $p$

https://physics.stackexchange.com/questions/39442/intuitive-explanation-of-why-momentum-is-the-fourier-transform-variable-of-posit/39508#39508 gives the best idea intuitive idea: the Fourier transform writes a function as a (continuous) sum of plane waves, and each plane wave has a fixed momentum.