The more precise the measurement of position, the more imprecise the measurement of momentum, and vice versa. - Werner Heisenberg, on the Uncertainty Principle.

The physicist may be satisfied when he has the mathematical scheme and knows how to use for the interpretation of the experiments. But he has to speak about his results also to non-physicists who will not be satisfied unless some explanation is given in plain language. Even for the physicist the description in plain language will be the criterion of the degree of understanding that has been reached. - Werner Heisenberg