As we have seen in the previous chapters, many aspects of quantum mechanics run counter to our physical intuition, which is formed from our experience living in the classical world. The probabilistic nature of quantum mechanics does not agree with the certainty of the classical world-we have no doubt that the sun will rise tomorrow. Moreover, the disturbance of a  quantum mechanical system through the action of measurement makes us part of the system, rather than an independent observer.

These issues and others make us wonder what is really going on in the quantum world. As quantum mechanics was being developed in the early twentieth century, many of the world's greatest physicists debated the "true meaning" of quantum mechanics. They often developed gedanken experiments or thought experiments to illustrate their ideas. Some of these gedanken experiments have now actually been performed and some are still being pursued.

In this chapter, we present a  few of the gedanken and real experiments that demonstrate the spookiness of quantum mechanics. We present enough details to give a flavor of the spookiness and provide references for further readings on these topics at the end of the chapter.