The Uncertainty Principal, proposed by Werner Heisenberg in 1927, has been one of my favorite ideas in Physics.
It explains that one can not know the precise variables of subatomic particles such as position and velocity. If you know one variable precisely you will be fuzzy on the other.
Hawkins explained it using this example, to see a particle you need to bounce something off of it, such as a photon. Of course, as you have struck it with another particle, even the tiniest particle has some mass/energy (quanta), you will have changed its direction and velocity.
Uncertainty can apply to many pairs of variables such as, position, velocity, energy, mass, quantity (even love). Such a simple concept, yet one of the corner stones of quantum Physics, it is an idea that make perfect sense to me. It has been useful as a tool or rule, to me, in explaining some of the more magical aspects of the universe, such as the notions of particles disappearing from one location and reappearing in another, the evaporation of black holes and even a way of looking at life.
This unpredictability is, in a way, almost spirtual, an anti-destiny; the idea that nothing is written, and that there are only infinite possibilities leading from moment to moment. That anything is possible and that all things happen in one universe or another.
