In 1927, Heisenberg proposed a very interesting principle, which is direct consequence of the dual nature of matter, known as uncertainity principle. According Heisenberg’s uncertainity principle, ”position and momentum of electron cannot be determined simultaneously with absolute accuracy”.
∆x∆p ≥ ℏ/2
Similarly, we have
∆E.∆t ≈ h
∆J.∆θ ≈ h
where ∆E and ∆t are the uncertainities in determining the energy and time while ∆J and ∆θ are the uncertainities in determining the angular momentum and angle.