After a discussion of Einstein’s theory of relativity, which says that the speed of light is always constant and therefore, there is no absolute time (and, indeed, time and space are interrelated), Hawking discusses in more detail modern models of the Universe. Hubble’s discovery, along with fact that the Universe appears pretty much identical in all directions (verified by the constant microwave radiation from the early Universe discovered later) determined that the Universe was expanding. There were some attempts to avoid a beginning of time, but ultimately, general relativity implies that there must be a beginning of time.
Next is a discussion of quantum mechanics and the uncertainty principle. Quantum mechanics views particles as probability waves, so one can never measure both a particle’s velocity and location exactly. After this there is a discussion of elementary particles, namely that all particles appear to be made of quarks, which come in six flavors, each with three colors. Each particle is composed of three quarks and comes is four varieties of spin: 1/2, 0, 1, and 2. Spin 1/2 particles make up matter, spin 0 particles; the rest are massless particles that are manifestations of the four fundamental forces. Spin 1 particles are photons (electromagnetism), bosons (weak nuclear force) and gluons (strong nuclear force). Spin 2 particles are gravitons (gravity). Theories have been developed that unify the spin 1 forces but cannot be verified because the forces unify at incredibly high energies. It turns out, though, that particles can decay into other particles (generally particles and antiparticles), and while this is mostly symmetric, there is a slight bias in favor of particles.
With this information, black holes can now be discussed. Black holes are created by stars larger than a certain size whose mass is so large that gravity is stronger than all of the other forces and it collapses into a singularity when it dies. As one approaches the singularity, there is an event horizon, beyond which not even light can pass. At the singularity, the laws of physics break down, due to the infinities involved there. When this happens, time ceases to have a meaning and we cannot perform useful analysis. This would cause similar problems with the beginning of the Universe, if it began with a singularity.
Quantum mechanics becomes important here. Because the energy density of space cannot be exactly zero due to the uncertainty principles, virtual particles and antiparticles are constantly being created. Usually these must annihilate, giving a net 0 energy. However, if the antiparticle falls into a black hole, the particle is free to continue its existance (or vice-versa), effectively sucking energy from the black hole. The smaller the black hole, the easier it is for the particle to escape, so small black holes radiate more energy than large ones, possibly ending life in a big explosion. Non-uniform densities of matter in the early Universe may have formed black holes smaller than those from stars and some of them may actually be decaying in our time. No such decay has been observed, which gives an upper limit on how non-uniform the early Universe was—apparently it was very smooth.
The usual model of the Universe states that temperature is inversely proportional to size. So at the beginning, when the singularity had no area, it was infinitely hot. One second afterwards, it had cooled enough to allow photons, electrons, and neutrinos. One hundred seconds after the beginning protons and neutrons began condensing into nuclei of hydrogen, helium, and other light elements. After a few hours this would have stopped. Nothing would have happend for the next million years as the Universe expanded enough to cool down to the point where atoms could form (several thousand degrees). At this point stars could begin to form.
There are some difficult questions that this model raises, however. First, why was the early Universe so hot (i.e. why is temperature inversely proportional to size)? Second, why is the Universe so uniform (at large scales)? Since light has not been able propogate between all regions of the Universe, no communication could have happened for the density to become uniform, so it must have started that way. This is surprising, since a non-uniform density seems so much more probable. Third, why did the universe expand at almost exactly the critical rate for it to always expand? This required great precision at the beginning, again unlikely. How did the local irregularities (i.e. the galaxies) form from the uniformity? Unfortunately, since the laws of physics break down at singularities, general relativity cannot answer these questions.
Some of these questions can be answered by the anthropic principle, which come in two flavors. The weak anthropic principle says that a large system like the Universe is likely to have conditions for sentient life in few areas, so we should not be surprised that we live in on of those areas. The strong anthropic principle says that we see the Universe as it is, because if it were otherwise, we would not be here to see it. It would be preferable, though, to know that many different intial conditions would give rise to results similar to what we see. This would happen if there is a spin 0 field. High densities would have a repulsive force and would expand faster than low densities, which would smooth them out.
We still cannot look beyond the singularity, but Hawking proposes that we may be able to get around the problem with quantum theory. Since we do not have a viable theory unifying quantum theory and gravity, this is a bit speculative. But we do know that such a theory requires summing of the probabilities and that space-time must be curved. In order for us to be able to do the mathematics, space-time must be imaginary, in which case it might be curved similar to the Earth. The Earth is “flat” and has finite area, but has no boundaries. If this model is correct, the Universe would have no boundary condition, i.e. no beginning. “The universe would be completely self-contained and not affected by anything outside itself. It would neither be created nor destroyed. It would just BE.”
Hawking finishes with a description of why the arrow of time must always point in the future for sentient beings, discusses string theory as a possible unifying theory, and notes that complicated life can only exist in three dimensions: in two dimensions the digestive tract and circulatory system separates the organism, and in dimensions larger than three gravity falls off too rapidly with distance to permit stable orbits. He finishes with “if we do discover a complete [unifying] theory, ... it would be the ultimate triumph of human reason—for then we would know the mind of God.”
A Brief History of Time is a very readable explanation of complicated physics. Hawking explains the relevant laws of physics as part of a coherent plan leading to his main discussion, always giving a quick background of how these theories came to be. The explanations are simple, accurate, well-illustrated, and peppered with short insights into the character of the discoverer. In fact, the explanations are so good that readers with a background in Physics will likely find that their understanding of the concepts increases, particularly in why it is the way it is.
Christian readers will likely find themselves both intriuged and disturbed. While he admits that our knowledge is not sufficient to label his ideas of the Universe as God as anything other than proposals, they are mathematically compelling, even to those who believe that God is more personal than a mathematical Universe can be. An intellectually honest person must consider arguments that his views are correct and Hawking’s suggestions have an internal consistency that must at least be considered, even if they do not address other observations of the world, such as why people universally believe in the supernatural.
This book will challenge readers, both scientifically and in their faith. It is quite accessible, even to the non-technical reader. It is very well organized, concise, dotted with humor, and truly makes a rather dull topic into an intruiging discussion. Definitely a must-read for anyone with an interest in Physics or beginnings.
The flow of the discussion is very well done; even the necessary background digressions feel necessary and are interesting. The writing, while not a paragon of the use of the English language, is very readable and certainly is not overly academic. The content is excellent, concise, accurate, interesting, and relevant (in more ways than just the flow of argument). My only complaint is that phrases are occasionally repeated, as if the editing were stopped prematurely. A small complaint, however, and really does not detract from the book. One could argue that he is a bit too consumed with eliminating the unseemly need for God in scientific theory, but he does not let it unduly interfere with the book, and he does have valid points. Well-worth the time spent reading, as the points he raises are sure to be pondered by a thoughtful reader for some time.