Quantum mechanics introduced the strange notion of grounding physics in probability . For many physicists, this represented a step backwards since it implied that we could no longer predict for sure how nature would behave. Physical systems were subject to random fluctuations, meaning that chance, rather than certainty, was the rule of the day. It was for this reason that Einstein raised his oft-repeated objection to quantum mechanics: “God does not play dice with the universe.” Quantum mechanics is also heavily counterintuitive, even to modern physicists and some of its leading practitioners. As Richard Feynman famously declared, “Anyone who says that they understand quantum mechanics is lying!” Nevertheless, the physics community has long embraced quantum mechanics for the simple reason that it agrees with experiments amazingly well.
The clash between quantum mechanics and established principles of physics led to some interesting conundrums. In the 1920s, physicists noticed that electrons seemed to possess an additional “degree of freedom”–an independent, defining feature that they called spin. Though it was similar to the conventional meaning of the term, it also had striking differences.
