Brief History of Time begins with a striking image and a wonderful true story:
An elderly lady attended a public lecture given by an astrophysicist on how the
Earth goes around the Sun and how the Sun circles about with countless other
stars in our galaxy the Milky Way. During the question and answer session, the
woman stood up and told the distinguished scientist that his lecture was
nonsense, that the Earth is a flat disk supported on the back of an enormous
tortoise. The scientist tried to outwit the lady by asking, “Well, my dear,
what supports the tortoise?” To which she replied, “You’re a very clever young
man, but not clever enough. It’s turtles all the way down!”
Although not intended by Stephen W. Hawking, author of A Brief History of Time and distinguished cosmologist at Cambridge Universe, the image of many turtles, each supporting the next may be viewed as a metaphor. Consider the following chain of questions. (1) What causes Earth’s motion? The answer is, of course, gravity. The Sun pulls on Earth causing it to orbit in an almost perfect circle. (2) But what causes gravity? According to Albert Einstein’s general theory of relativity, it is the curvature of space. (3) But what causes space to curve? Again, according to Einstein’s theory, it is mass. (4) But why should mass cause space to deform? At this point, scientists can only say, “This is the way things are.” Apparently, the theory of gravity rests upon not an infinite tower of turtles, but only on three. Scientists do not know what holds up the third turtle, but, perhaps, someday a fourth turtle will be found.
Hawking uses the image of an infinite tower of tortoises to introduce contrasting views of the Universe. Indeed, the first chapter continues with an account of people’s varying pictures of the cosmos over the last few millennia. It is common practice for popular science books to present the historical perspective of a subject. This creates the feeling among readers with limited scientific background that they are learning something about science.
But it does not take long for Hawking to present some real physics. The first half of his book discusses some of the most important concepts: gravity, quantum mechanics, particle physics and the unification of space and time. Hawking has done well in selecting the topics for his book.
In 1905, Einstein forever changed our view of space-time. Time joined space as the fourth dimension. The consequences were mind boggling: the notion of time and length became relative. Clocks in fast-moving rockets tick more slowly, and yardsticks shrink in length. Energy and mass are equivalent, related through the famous equation E=mc2. These effects, all a consequence of special relativity, have been well confirmed by experiments. But for those who have not traveled at almost the speed of light and this includes all of us, the concepts seem foreign. Perhaps, it is at this point in the second chapter that many readers put down A Brief History of Time. Actually, the non-scientist, when reading a popular science book, should forge ahead, simply skipping what he or she does not understand; one will be surprised at how much one can still learn.
The third chapter of Hawking’s book addresses Einstein’s general theory of relativity, which is briefly described above in the second paragraph of this review. An amazing consequence of this gravity theory is the expansion of the Universe. The fabric of space is stretching, causing its contents to drift apart. We, humans, do not see the expansion because its local effect is insignificant. One needs to look at distance objects. When astronomers view faraway galaxies, they see them all moving away from Earth and away from one another.
Quantum mechanics involves what-is-known-as “wave mechanics.” A wave is associated with every entity in the Universe. Waves can interfere, meaning that their crests, when coinciding, add to form a higher crest (a phenomenon called constructive interference), or the crest of one and the trough of another, when merging, produce no wave at all (a phenomenon called destructive interference). This inference can create highly nonintuitive effects.
Because a wave extends over a finite distance, it does not have a precise location. This leads to uncertainty especially for tiny objects. If you were an electron zooming around an atom, you would not know your position exactly. You would continually ask, “Where am I now?” And the answer would be in terms of probabilities: 25% chance over there, 10% chance here, and so on. The idea that you, a microscopic particle, might be here or there is difficult to fathom.
Hawking only briefly mentions what-is-perhaps the most intuitive way of understanding quantum mechanics: the path integral. A great opportunity is missed here to explain in simple terms this very difficulty physics subject].
A great discovery of the twentieth century is that all matter is made up of only a few microscopic constituents and that only four fundamental forces control everything. The ultimate in reductionism has been achieved.
The basic building blocks are quarks and leptons. Three quarks bind tightly together to form a proton or a neutron. Protons and neutrons stick to one another to form a nucleus, the central, heavy core of an atom. Atoms join to create molecules, and molecules compose everything there is in the macroscopic world, from human flesh to jagged rocks. Of the leptons, there are two types: electrons, which are negatively charged, and neutrinos, which are -- as the name implies -- neutral or without charge. Electrons, which are relatively light, form a cloud of probability that surrounds the nucleus. Thus, an atom is a nucleus together with an electronic quantum cloud.
Neutrinos are produced in subatomic reactions, such as those that take place in the center of the Sun. Neutrinos rarely interact with anything, which is why they go unnoticed. Millions are streaming through your body at this very instance, causing you no harm since they do not collide with any of the quarks or electrons inside you. Neutrinos are like rapid-moving tiny ghosts.
The four fundamental forces are gravity, the electric-magnetic force, the strong subnuclear force and the weak subnuclear interaction. Gravity is the mutual attraction between bodies of mass.
In the nineteenth century, it was realized that the electric and magnetic forces were manifestations of a single force known as electromagnetism. Charges come in two kinds: positive and negative. The electric force is the repulsion between charges of the same kind and the attraction between charges of the opposite kind: As the saying goes, “Unlikes attract; likes repel.” All magnetism is created by currents, or the movement of charge. For example, Earth’s magnetic field is produced by currents in its liquid outer core. The most familiar manifestation of magnetism is the deflection of a compass needle.
The strong nuclear force binds quarks in a proton or neutron. It also holds these protons and neutrons together in a nucleus. The weak subnuclear interaction is responsible for certain radioactive decays of nuclei. It also participates in nuclear processes that produce the Sun’s energy. As its name implies, it is the weakest force.
The second half of Hawking’s book treats topics closely related to his research: black holes, Hawking radiation, the boundary condition for the Universe at the beginning of time, and the unification of physics.
A consequence of Einstein’s general theory of relatively is that gravity tugs on light. Because light travels so fast, it is able to speed past massive bodies such as the Earth and Sun with little change of direction. However, a very dense and heavy object can exert a significant pull on light. If enough mass is concentrated in a small region, then a black hole forms. On such a black hole, a beam of light when directed upward would rise, turn over, and fall back down like a fountain’s jet of water does on Earth. Nothing can escape the powerful force of gravity inside a black hole -- not even light.
Actually, the last statement is not strictly speaking true. Because of the uncertainty that arises from quantum mechanics, there is a minute probability that either an electron or a wave of light can find itself outside the black hole. If so, it is free to escape. This effect is known as Hawking radiation. Hence, black holes are not completely black. However, the radiation of electrons and light emerging from a black hole is extremely feeble unless the black hole is of microscopic size. This is because quantum mechanical effects are only significant for tiny objects.
If the Universe is expanding now, then in the remote past, it must have been considerably smaller. About 15 billion years ago, all matter had to have been concentrated in a tiny place. If so, it is possible that the Universe began as a black hole in which space and time were on equal footing.
The laws of physics determine how the Universe evolves. But they do not specify the initial conditions, that is, the position of each point of space at the moment of creation. Some people say that it is here that a supreme being could have played a role, that God chose the initial conditions and thereby determined the evolution of our world. However, Hawking in work with Jim Hartle believes that our Universe began with no boundary, a possibility that can arise only if space and time are on equal footing. It is easier to visualize the situation when space is a one-dimensional circle. In this case, space-time is like a hemisphere, similar to the southern half of Earth. Space consists of circles of fixed latitude, while motion in time is movement northward along a longitude. The beginning of time is the South Pole, while the present is a circle around the Earth at the northernmost latitude. ]
The no-boundary idea is highly speculative, unconfirmed by experiment and unlikely to be tested for centuries to come. But if true, it has dramatic philosophical implications. Hawking states in carefully worded language that there would be no need to invoke God. He ends Chapter 8 with “If the universe is really completely self-contained, having no boundary or edge, it would have no beginning: it would simply be. What place, then, for a creator?”
The final topic of the book is unification. Here the idea is to find a theory that incorporates all four fundamental forces in a single structure. Einstein spent the last half of his life unsuccessfully searching for such a unified theory. Today, theorists are exploring. Strings offer the hope of not only unifying the forces but also the quarks and leptons. Unfortunately, no one knows whether superstrings are the theory of everything or an appealing mathematical concept with no connection to reality.
A Brief History of Time is well written, but not better than other good popular science books. Among such books, Hawking’s is perhaps the best seller of all, having sold more than 5 million copies. A Brief History of Time has the reputation of being the most bought, least read book. Why then should it sell so well?
The reason is Stephen Hawking himself. He is a phenomenon. In his twenties, he contracted ALS, popularly known as Lou Gehrig’s disease. As his motor neutrons decayed, he lost the movement of his legs and arms and became confined to a wheel chair. Eventually, his voice faded to a barely comprehensible monotone of noise. Today, unable to speak, his condition has stabilized but he can only move his eyes and a finger.
Despite his horrible physical situation, Hawking has remained productive as a researcher. With the help of computers, he is able to communicate and give lectures. All such communications are painstakingly slow and need to be prepared in advance. Even under these circumstances, Hawking has written books, appeared in a movie about his life, performed calculations in his head, raised a family, and in his own words “had a fairly normal life and a successful career.” Hawking is the quintessential role model for a person with disabilities.
He has amazed the public by conducting top-rate research under such debilitating physical conditions. Although none of his work is worthy of a Nobel Prize, his scientific accomplishments are vast. Many would rank him among the best several hundred scientists of his generation.
A Brief History of Time is recommended to those who are interested in physics, cosmology, natural philosophy or the history of science and who have already acquired some knowledge of science either through study or by the reading of other books.
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