These two concepts are extremely disparate in their nature, as gravitational theory deals primarily with extremely large objects and is written in terms of large curved fields and fluctuations in space-time, while quantum theory deals exclusively with the nature of the other fundamental forces and their associated point particles. On the scale of quantum mechanics, gravity breaks down and cannot function properly. Therefore, unifying them has been a monumental task since the early 1900s. Hawking deals with ideas such as string theory that attempt to resolve these differences.

One common theme is exploring the idea that there are more dimensions than the 3 (or 4) that we experience and interact with. Some prevailing theories postulate upwards of 11 or even 22 dimensions. This idea has become so popular in theoretical physics because it allows for scenarios that resolve many physical problems and solves issues about unification. In addition, it creates a wealth of exciting new possibilities in the world of experimental physics and science, so these scientists are understandably excited to study the concept.

Hawking talks at length about quantum mechanics and the probability of events occurring as predicated by the Schrodinger Wave Equationas well as the theoretical possibilities of other universes, higher dimensions as mentioned before, or the theory that we all live in a simulation. One of the more interesting ideas that he discusses, and which has been posited by several other physics is the membrane hypothesis is the theory that the 4 dimensions with which we interact is just a cross section, a membrane, of a larger group of dimensions. This resolved some of the theories that we live in a never-ending multiverse and also helps explain why we can’t see the other dimensions.