 The problem is that our fundamental theories of matter and energy, the theories describing the interactions of various particles via the electromagnetic force and the strong and weak nuclear forces, are all quantum theories. In  quantum theories,  these physical quantities do not in general have definite values. For example, in quantum mechanics, the position of an electron may be specified with arbitrarily high accuracy only at the cost of a loss of specificity in the description of its momentum, hence its velocity. At the same time, in the quantum theory of the electromagnetic field known as quantum electrodynamics (QED), the electric and magnetic fields associated with the electron suffer an associated uncertainty. In general, physical quantities are described by a quantum state which gives a probability distribution over many different values, and increased specificity (narrowing of the distribution) of one property (e.g., position, electric field) gives rise to decreased specificity of its canonically conjugate property (e.g., momentum, magnetic field). This is an expression of Heisenberg’s  Uncertainty Principle.  In the context of quantum gravity the fluctuating geometry is known as “spacetime foam”. Likewise, if one focusses in on the spatial geometry, it will not have a definite trajectory.