The problem with Quantum Physics (well, physics in general, but in classical physics it's mostly ignored) is that in order to make a measurement, you need to in

The problem with Quantum Physics (well, physics in general, but in classical physics it's mostly ignored) is that in order to make a measurement, you need to interact with the system thus changing it's state. You might for example measure perturbations in an electric field, but by applying the electric field you influenced the state of the system. This is also an important part of say the uncertainty principle. One way to think about why you can't measure position and momentum simultaneously to infinite accuracy is that perfect measurement of one would require shooting it with photons of zero momentum, while perfect measurement of the other would require shooting it with photons of infinite momentum. The real fun comes in when you leave a system alone for a while because then everything becomes probabilistic.