Programming the Universe: A Quantum Computer Scientist Takes on the Cosmos

by Seth Lloyd

Apparently it is not possible to have a definite value of spin about two different axes at the same time. This intrinsically chancy nature of quantum mechanics was immortalized by Werner Heisenberg, one of the founders of quantum mechanics, as the “uncertainty principle.” The uncertainty principle states that if the value of some physical quantity is certain, then the value of a complementary quantity is uncertain. Spin about the vertical axis and spin about the sideways axis are just such complementary quantities: if you know one, you can’t know the other. Another pair of complementary quantities are position and speed: if you know the position of a particle exactly, then you know nothing of how fast it’s going. (Traffic cop pulls over Heisenberg’s car: “Professor Heisenberg, do you have any idea how fast you were going?” Heisenberg: “No, but I know exactly where I am.”)