### Overview of the chapter

The rules of quantum theory may appear strange, but at first glance they do not appear *problematic*. We will now see however that combining these rules leads to a conclusion that sounds very bizzare and alarming. In fact this conclusion is so bizzare that at first many scientists, including Einstein, refused to accept that quantum theory could be correct.

Suppose we set up an experimental apparatus, perform some measurement with this apparatus, and get a result of this measurement. Then we repeat the process exactly the same way; i.e., set up the same experiment, and perform the same measurement procedure. Should we get the same result as we obtained the first time?

Our experience with classical physics suggests that the answer should be *yes*; if we repeat an experiment, the laws of classical physics say that we will get the same result.

But such is not the case in quantum theory. Suppose an experiment has two possible outcomes. If we repeat the experiment several times, we will find that we get one outcome or the other at *random*. We can determine the *probabilities* for the two different outcomes, but we cannot predict which outcome we will get in any one instance of the experiment.

In this chapter we will see how this randomness is forced upon us by the basic principles of quantum theory. In the next chapter we will see that while this situation is certainly novel, it does not imply a complete breakdown of the basic idea that physical laws should predict a definite evolution in time.