A Briefer History of Time – Stephen Hawking Black holes are one of only a fairly small number of cases in the history of science in which a theory was developed in great detail as a mathematical model before there was any evidence from observations that it was correct. Indeed, this used to be the … Read more
A Briefer History of Time – Stephen Hawking The fourth category is the strong nuclear force, which holds the quarks together in the proton and neutron, and holds the protons and neutrons together in the nucleus of an atom. It is believed that this force is carried by another spin-1 particle, called the gluon, which … Read more
A Briefer History of Time – Stephen Hawking All this seems fairly straightforward, but the remarkable fact is that there are particles that do not look the same if one turns them through just one revolution: you have to turn them through two complete revolutions! Such particles are said to have spin 1/2. All the … Read more
A Briefer History of Time – Stephen Hawking Aristotle believed that all the matter in the universe was made up of four basic elements—earth, air, fire, and water. These elements were acted on by two forces: gravity, the tendency for earth and water to sink, and levity, the tendency for air and fire to rise. This … Read more
A Briefer History of Time: Stephen Hawking The success of scientific theories, particularly Newton’s theory of gravity, led the French scientist the Marquis de Laplace at the beginning of the nineteenth century to argue that the universe was completely deterministic. Laplace suggested that there should be a set of scientific laws that would allow us … Read more
A Briefer History of Time: Stephen Hawking However, our uncertainty about the present average density of the universe is even greater. If we add up the masses of all the stars that we can see in our galaxy and other galaxies, the total is less than one hundredth of the amount required to halt the … Read more
A Briefer History of Time: Stephen Hawking The discovery that the universe is expanding was one of the great intellectual revolutions of the twentieth century. With hindsight, it is easy to wonder why no one had thought of it before. Newton, and others, should have realized that a static universe would soon start to contract … Read more
A Briefer History of Time: Stephen Hawking If one looks at the sky on a clear, moonless night, the brightest objects one sees are likely to be the planets Venus, Mars, Jupiter, and Saturn. There will also be a very large number of stars, which are just like our own sun but much farther from … Read more
Part 2: Space And Time – A Briefer History of Time An equally remarkable consequence of relativity is the way it has revolutionized our ideas of space and time. In Newton’s theory, if a pulse of light is sent from one place to another, different observers would agree on the time that the journey took … Read more
Part 3: Space And Time – A Briefer History of Time If one neglects gravitational effects, as Einstein and Poincaré did in 1905, one has what is called the special theory of relativity. For every event in space-time we may construct a light cone (the set of all possible paths of light in space-time emitted … Read more
