In this section, we will discuss three important experients that ultimately lead to the development of the quantum theory and a radical change in how we view the universe. In fact, it is not the experiments per se that can be attributed to this change in world view but rather tha rationalization of these experiments that were proposed by theoreticians. Thus, here we have an excellent illustration of the important role played by theory. Without the experimental data, theoreticians might not have considered overthrowing the beautiful structure embodied in classical mechanics. However, it was ultimately the new theoretical structure introduced to explain the experimental results that lead to this crucial change.