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IN THIS CHAPTER

Squaring off: Gravity and quantum physics just don’t get along

Seeing four types of particle interactions

Hoping to tie all of physics into one equation with quantum gravity

Physicists like to group concepts together into neat little boxes with labels, but sometimes the theories they try to put together just don’t want to get along. Right now, nature’s fundamental physical laws can fit into one of two boxes: general relativity or quantum physics. But concepts from one box just don’t work together well with concepts from the other box.

Any theory that can get these two physics concepts to work together would be called a theory of quantum gravity. String theory is currently the most likely candidate for a successful theory of quantum gravity.

In this chapter, we explain why scientists want (and need) a theory of quantum gravity. We begin by giving an overview of the scientific understanding of gravity, which is defined by Einstein’s theory of general relativity, and our understanding of matter and the other forces of nature, in terms of quantum mechanics. With these fundamental tools in place, we then explain the ways in which these two theories clash with each other, which provides the basis for quantum gravity. Finally, we outline various attempts to unify these theories and the forces of physics into one coherent system, and the failures they’ve run into.