Stephen Hawking originally published A Brief History of Time in 1988. Supposedly his editor told him that “each equation [he included] in the book would cut the sales in half.” True to form, the book is filled with plain English explanations of theoretical concepts, but very few equations.
He talks a lot about the history of science, including anecdotes like the following:
Newton was very worried by this lack of absolute position, or absolute space, as it was called, because it did not accord with his idea of an absolute God. In fact, he refused to accept lack of absolute spacem even though it was implied by his laws. He was severely criticized for this irrational belief by many people, most notably by Bishop Berkeley, a philosopher who believed that all material objects and space and time are an illusion. When the famous Dr. Johnson was told of Berkeley's opinion, he cried, "I refute it thus!" and stubbed his toe on a large stone.
He also helpfully reminds us how many things in modern society are basically completely arbitrary:
In effect, the meter is defined to be the distance traveled by light in 0.000000003335640952 second, as measured by a cesium clock. (The reason for that particular number is that it corresponds to the historical definition of the meter-in terms of two marks on a particular platinum bar kept in Paris.)
Eventually, he starts to dig into the science behind famous experiments. For example, he describes the double-slit experiment which was a big breakthrough in understanding whether electrons acted as particles or waves:
One might therefore think that opening another slit would just increase the number of electrons hitting each point of the screen, but, because of interference, it actually decreases it in some places. If electrons are sent through the slits one at a time, one would expect each to pass through one slit or the other, and so behave just as if the slit it passed through were the only one therefore giving a uniform distribution on the screen. In reality, however, even when the electrons are sent one at a time, the fringes still appear. Each electron, therefore, must be passing through both slits at the same time!
As an astrophysicist, he provides wonderful explanations about the large-scale structure of the universe. I particularly enjoyed the explanation of how stars can create other stars:
More massive stars would need to be hotter to balance their stronger gravitational attraction, making the nuclear fusion reactions proceed so much more rapidly that they would use their hydrogen in as little as a hundred million years. They would then contract slightly, and as they heated up further, would start to convert helium into heavier elements like carbon or oxygen. This, however, would not release much more energy, so a crisis would occur, as was described in the chapter on black holes. What happens next is not completely clear, but it seems likely that the central regions of the star would collapse to a very dense state, such as a neutron star or black hole. The outer regions of the star may sometimes get blown off in a tremendous explosion called a supernova, which would outshine all the other stars in its galaxy. Some of the heavier elements produced near the end of the star's life would be flung back into the gas in the galaxy, and would provide some of the raw material for the next generation of stars. Our own sun contains about 2 percent of these heavier elements, because it is a second- or third-generation star, formed some five thousand million years ago out of a cloud of rotating gas containing the debris of earlier supernovas. Most of the gas in that cloud went to form the sun or got blown away, but a small amount of the heavier elements collected together to form the bodies that now orbit the sun as planets like the earth.
He also provides a clear argument for why the universe has a beginning:
The difficulty is that in an infinite static universe nearly every line of sight would end on the surface of a star. Thus one would expect that the whole sky would be as bright as the sun, even at night. Olbers's counterargument was that the light from distant stars would be dimmed by absorption by intervening matter. However, if that happened the intervening matter would eventually heat up until it glowed as brightly as the stars. The only way of avoiding the conclusion that the whole of the night sky should be as bright as the surface of the sun would be to assume that the stars had not been shining forever but had turned on at some finite time in the past. In that case the absorbing matter might not have heated up yet or the light from distant stars might not yet have reached us. And that brings us to the question of what could have caused the stars to have turned on in the first place.
This inevitably leads to the question of “why are we here?”. For this, he leans on the anthropic principle:
There are two versions of the anthropic principle, the weak and the strong. The weak anthropic principle states that in a universe that is large or infinite in space and/or time, the conditions necessary for the development of intelligent life will be met only in certain regions that are limited in space and time. The intelligent beings in these regions should therefore not be surprised if they observe that their locality in the universe satisfies the conditions that are necessary for their existence. It is a bit like a rich person living in a wealthy neighborhood not seeing any poverty.
One example of the use of the weak anthropic principle is to "explain" why the big bang occurred about ten thousand million years ago-it takes about that long for intelligent beings to evolve. As explained above, an early generation of stars first had to form. These stars converted some of the original hydrogen and helium into elements like carbon and oxygen, out of which we are made. The stars then exploded as supernovas, and their debris went to form other stars and planets, among them those of our Solar System, which is about five thousand million years old. The first one or two thousand million years of the earth's existence were too hot for the development of anything complicated. The remaining three thousand million years or so have been taken up by the slow process of biological evolution, which has led from the simplest organisms to beings who are capable of measuring time back to the big bang.
From here, he works his way into modern science, including the four fundamental interactions (gravity, electromagnetism, strong and weak nuclear forces) that make up the Standard Model of particle physics. We’re taught the basics in high school, and college physics does a better job of explaining how to calculate some of the more complicated scenarios, but if you start asking why the answers tend to be hand-wavey. Hawking dives into this question with gusto, providing helpful analogies to macro-level effects like water freezing to ice:
As we discussed earlier, one would expect that at such high temperatures the strong and weak nuclear forces and the electromagnetic force would all be unified into a single force. As the universe expanded, it would cool, and particle energies would down. Eventually there would be what is called a phase transition and the symmetry between the forces would be broken: the strong force would become different from the weak and electromagnetic forces. One common example of a phase transition is the freezing of water you cool it down. Liquid water is symmetrical, the same at every point and in every direction. However, when ice crystals form, they will have definite positions and will be lined up in some direction. This breaks water's symmetry.
The Standard Model works well for large-scale structures like astronomy but fails to accurately explain all of the phenomena we’ve observed in subatomic particles. For this, we need quantum mechanics. Quantum mechanics (QM) is a theory of physics that describes how subatomic particles act in a probabilistic universe. This is very unintuitive, even to professional physicists. In response to QM, Einstein is known to have said “God does not play dice with the universe.” Einstein rejected the idea of quantum mechanics, even though it was his theory of relativity that paved the way for its discovery. To this day, we are still grappling with its implications. Hawking describes one possible way to reconcile the two major theories of physics:
We don't yet have a complete and consistent theory that combines quantum mechanics and gravity. However, we are fairly certain of some features that such a unified theory should have. One is that it should incorporate Feynman's proposal to formulate quantum theory in terms of a sum over histories. In this approach, a particle does not have just a single history, as it would in a classical theory. Instead, it is supposed to follow every possible path in space-time, and with each of these histories there are associated a couple of numbers, one representing the size of a wave and the other representing its position in the cycle (its phase). The probability that the particle, say, passes through some particular point is found by adding up the waves associated with every possible history that passes through that point.
He also delves into his academic specialty and the book’s namesake: time. Specifically, why time runs forward and not backward. Like most other concepts, he explains this one using a metaphor:
It is rather difficult to talk about human memory because we don't know how the brain works in detail. We do, however, know all about how computer memories work. I shall therefore discuss the psychological arrow of time for computers. I think it is reasonable to assume that the arrow for computers is the same as that for humans. If it were not, one could make a killing on the stock exchange by having a computer that would remember tomorrow's prices! A computer memory is basically a device containing elements that can exist in either of two states. A simple example is an abacus. In its simplest form, this consists of a number of wires; on each wire there are a number of beads that can be put in one of two positions. Before an item is recorded in a computer's memory, the memory is in a disordered state, with equal probabilities for the two possible states. (The abacus beads are scattered randomly on the wires of the abacus.) After the memory interacts with the system to be remembered, it will definitely be in one state or the other, according to the state of the system. (Each abacus bead will be at either the left or the right of the abacus wire.) So the memory has passed from a disordered state to an ordered one. However, in order to make sure that the memory is in the right state, it is necessary to use a certain amount of energy (to move the bead or to power the computer, for example). This energy is dissipated as heat, and increases the amount of disorder in the universe. One can show that this increase in disorder is always greater than the increase in the order of the memory itself. Thus the heat expelled by the computer's cooling fan means that when a computer records an item in memory, the total amount of disorder in the universe still goes up. The direction of time in which a computer remembers the past is the same as that in which disorder increases.
I enjoyed Dr. Hawking’s book very much. You don’t need me to tell you it’s good — it broke every record on the planet when it first came out. But if you’re like me and still haven’t gotten around to reading it 33 years later, consider this a friendly reminder. And even if you don’t consider yourself a “science” person you should check it out. Even among science people, Hawking is in a league of his own.
People all over the world tell me how much they have enjoyed my book though they didn't understand it all.
—Dr. Stephen Hawking