I'm not certain as to the true meaning of the Heisenberg Uncertainty Principle, and wonder if anyone can provide insight. I understand that there is a reciprocal relation between the certainties as to the position and momentum of a particle such as an electron; the more precisely position is known, the less precisely momentum can be known, and vice versa. Does this mean that a particle cannot have definite position and momentum at the same time, or that we simply cannot know one if we know the other? In other words, does the Heisenberg Uncertainty Principle pertain to reality itself or to knowledge of reality? Given that quantum physics describes reality in terms of probabilities, does this mean that randomness is an inherent property of nature? Or does it simply mean that we cannot know all the variables and unseen forces of nature? For example, take an atom of thorium-232, an isotope known to have a half life of 14 billion years. Is the precise time at which it decays to radium-228 predetermined by unknown forces, or does it depend entirely on random chance?