When subatomic particles are considered as probabilities, they can do strange things, such as quantum tunneling. Suppose an electron requires some extra energy to get to the other side of some energy barrier. In ordinary mechanics, the electron has to have the extra energy or it is not going anywhere. However, with quantum mechanics, there is some probability that the electron could get through to the other side of the barrier without the extra energy. Sometimes this does happen. However, it’s more likely to happen if the amount of extra energy needed is not very much. Another strange part of quantum mechanics was the uncertainty principle discovered by Werner Heisenberg. Suppose a physicist tries to measure the location of an electron. As the physicist measures the electron with greater and greater precision, the momentum of the electron is known with less and less precision. The converse is also true. Measurement of the momentum with greater precision leads to poorer knowledge of the position. In fact, the product of the uncertainties can never be less than a quantity called Planck’s constant divided by 2times pi. This was a somewhat disquieting result to some. There was a limit to what could be measured, and there was no way around the limit. Some physicists at the end of the 19th century said that their filed would only con-sits of measuring what was already known to greater and greater precision. That was a pipe dream. Beyond a certain precision, one could go no further without throwing away other knowledge. There would always be a tradeoff.

There were quite a few physicists who were not happy with matter being constructed out of probabilities, with the universe as one giant casino. Einstein was chief among these and loudly asserted that “God does not play dice.”

(Niels Bohr supposedly replied “Don’t tell God what to do!”) Einstein and other physicists sought “hidden” variables that underlay quantum mechanics and behaved in a more sensible way. However, no trace of the hidden variables has found, and the theories that postulate them are somewhat like the attempts of astronomers in the late Middle Ages to save the Earth-centered solar system by adding extremely complicated motions to it that would agree with the observations

Both relativity and quantum mechanics arose in one of the great flowe rings of science. In the early 20th century, scientists all over the world changed how humanity thought about how the universe began, how motion could be described, what matter was, and what the lime-it’s of physical knowledge were. The biographies of many of those who broke this new ground are in this volume. Much of today’s physics, astronomy, and chemistry is following in the paths that these pioneers trail blazed.