ABSTRACT
What is usually called modern physics is based on two theories: relativity, which we have just nished discussing, and quantum mechanics (also called quantum theory). While some of the ideas of relativity, such as time dilation and length contraction, at rst seemed rather strange, we saw that they followed logically from the two postulates of relativity. We will see below that the ideas of quantum mechanics are also strange. Most physicists would agree that they are much stranger than relativity. In fact, we will see that there are no satisfactory explanations. To quote two Nobel Prize laureates, Murray Gell-Mann and Richard Feynman, “Nobody really understands quantum theory.” Despite this, it is one of the best-tested theories that we have. In some cases, the agreement between the theoretical prediction and experiment is better than 1 part in a million. There are many useful devices that are based on quantum mechanics. They include anything that uses microelectronics, such as a cell phone, DVD player, or computer, nuclear reactors, and medical imaging devices such as MRI and PET.
