A Brief History of Time | Study Guide

Stephen Hawking

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A Brief History of Time | Chapter 11 : The Unification of Physics | Summary



Stephen Hawking explains in this chapter that physics is drawing ever closer to a complete unified theory. This theory would fully explain the behavior of waves and particles at the very smallest levels of matter in quantum mechanics and the cosmic behavior of the largest levels of matter under the theory of relativity. Even with this promise and the knowledge of what some of the properties such a theory must have, incompatibilities between groupings of partial grand unified theories (GUTs) attributable to the problem of gravity have not yet been resolved.

Despite the progress of the last century, Hawking cautions readers to beware of overconfidence, citing how scientists in previous times believed they were on the cusp of knowing it all, only to be surprised by revolutionary discoveries. Just within the last few decades, the consequences of combining general relativity with the uncertainty principle has caused physicists (with Hawking leading the way) to believe black holes are not really as black as originally thought. Furthermore, the presence of the singularities that are self-contained but have no boundary in black holes cannot be confirmed by direct observation (Chapters 6 and 7).

The trail to determining the qualities of such a theory changed direction in 1984 with the growing acceptance of string theory, which proposes that matter is made of strings, having only length without any other dimension. These strings were believed to be either open (with each end of the string unattached) or closed (with the two ends connected). While "a particle occupies one point of space at each instant of time," a string has a history in space-time as a "world sheet," or a two-dimensional surface.

Physicists have since gone on to describe several different configurations for these open and closed strings. But string theories seem to be consistent within these configurations only if space-time has either 10 or 26 dimensions instead of the four dimensions that living beings are able to perceive and in which they exist (length, width, depth, and time compacted into space-time). Furthermore, there are several competing string theories, implying there may be more fundamental theories that unifies them.

It is through the overlapping of GUTs that Hawking goes forward with the hope that they may together form a series of maps—no one of which is conclusive in and of itself. Pieced together, these theories may offer an accurate picture of what could describe an as-yet unknown theory, the laws of which rule all the GUTs. The search is ongoing, despite the fact that direct observation and confirmation are not yet available.

Hawking again reminds us that the finding of such a unified theory that applies to both quantum mechanics and the theory of relativity is only the first step. We would still be limited by the uncertainty principle and our ability to make computations. In any case, the ultimate goal is to understand why we exist amid the events around us.


In this chapter Stephen Hawking explores string theory, an area of physics that continues to be baffling and controversial, even nearly 20 years after this expanded edition of A Brief History of Time was released. While many experts agree that the theory has merit, there are many competing string theories, and this is not a settled area of physics. Hawking helpfully walks readers through the difficult concepts of higher dimensions and what they might mean. However, even for specialists the idea of 26 dimensions is a lot to take in.

When he attended a conference in Sweden during August of 2015, Hawking proposed that the outer boundary of a black hole (its event horizon) stores information about any object that falls into a black hole in two-dimensional form. He speculates that this information could be available in another universe, although what it would look like would be completely different than how it does in our own universe. But in presenting this idea as a possibility in this 1988 version of "A Brief History of Time," Hawking seems to be foreshadowing subsequent speculations he has since expressed in later books (such as in his 2010 book, The Grand Design) that the universe is an entirely enclosed system, endlessly expanding and contracting in unlimited reiterations of itself. If indeed a black hole is a kind of miniature big crunch, is it possible that all the information of a previous universe reincarnates into a new one at its next big bang? As this model apparently precludes the hand of a divine creator, Hawking has faced considerable opposition to this idea by those holding religious beliefs.

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