Just as general relativity is the scientific theory that explains how space and time work, quantum mechanics is the scientific theory that explains how matter and energy work. There are many key connections between relativity and mechanics. For example, there is the conversion relation between matter and energy, E = mc². Also, since matter causes gravity, it can be said that “spacetime tells matter how to move, and matter tells spacetime how to curve,” as American physicist John Archibald Wheeler (1911–2008) phrased it.

These two major scientific theories—general relativity and quantum mechanics—do not intersect or overlap very much in terms of what aspects of the universe they describe. In fact, describing certain physical phenomena using one theory sometimes contradicts how the phenomena are described using another theory. Unifying these two great theories is one of the topics at the frontier of scientific research today.