It is almost an article of faith among theoretical physicists, enunciated forcefully by Einstein among others, that the fundamental laws should be or… - Anthony Zee

That the exchange of a particle can produce a force was one of the most profound conceptual advances in physics. We now associate a particle with each of the known forces: for example, the photon with the electromagnetic force and the graviton with the gravitational force; the former is experimentally well established and while the latter has not yet been detected experimentally hardly anyone doubts its existence.

One reason I went to Princeton University as an undergraduate was that I had read about a Professor John Wheeler suggesting that the atomic nucleus might take on the form of a doughnut. When I got there, I learned that Wheeler was going to give a novel type of course for freshmen. A group of us were asked a few physics questions by Wheeler, and those who answered correctly were allowed into the course. The first homework assignment consisted of standing for 15 minutes in front of the house that Albert Einstein had lived in. It turned out that we were to learn physics from the top down: For example, we were taught “F = ma” as a limiting case of special relativity. If I remember correctly, the department did not allow Wheeler to teach the course again. But I learned a lot; in particular, I learned to “never calculate without first knowing the answer.”

Normally, the entropy of a system is extensive, that is, proportional to its volume. Somehow, a black hole has an entropy proportional to its surface rather than to its volume. This fact has led to the so-called holographic principle. Many fundamental physicists believe that this mysterious property of black holes holds the key to quantum gravity.