• Author: Stephen Hawking
  • Full Title: A Brief History of Time
  • Tags: #Inbox #books

Highlights

  • CHAPTER 1 OUR PICTURE OF THE UNIVERSE (Location 76)
  • forever, whether or not the universe had existed forever. As we shall see, the concept of time has no meaning before the beginning of the universe. (Location 185)
  • This was first pointed out by St. Augustine. When asked: “What did God do before he created the universe?” Augustine didn’t reply: “He was (Location 186)
  • An expanding universe does not preclude a creator, but it does place limits on when he might have carried out his job! (Location 204)
  • A theory is a good theory if it satisfies two requirements. It must accurately describe a large class of observations on the basis of a model that contains only a few arbitrary elements, and it must make definite predictions about the results of future observations. (Location 208)
  • Any physical theory is always provisional, in the sense that it is only a hypothesis: you can never prove it. No matter how many times the results of experiments agree with some theory, you can never be sure that the next time the result will not contradict the theory. On the other hand, you can disprove a theory by finding even a single observation (Location 214)
  • a good theory is characterized by the fact that it makes a number of predictions that could in principle be disproved or falsified by observation. Each (Location 217)
  • The eventual goal of science is to provide a single theory that describes the whole universe. (Location 227)
  • Today scientists describe the universe in terms of two basic partial theories—the (Location 241)
  • CHAPTER 2 SPACE AND TIME (Location 272)
  • Both Aristotle and Newton believed in absolute time. That is, they believed that one could unambiguously measure the interval of time between two events, and that this time would be the same whoever measured it, provided they used a good clock. Time was completely separate from and independent of space. This is what most people would take to be the commonsense view. (Location 325)
  • However, we have had to change our ideas about space and time. Although our apparently commonsense notions work well when dealing with things like apples, or planets that travel comparatively slowly, they don’t work at all for things moving at or near the speed of light. (Location 327)
  • The fundamental postulate of the theory of relativity, as it was called, was that the laws of science should be the same for all freely moving observers, no matter what their speed. (Location 359)
  • For this reason, any normal object is forever confined by relativity to move at speeds slower than the speed of light. Only light, or other waves that have no intrinsic mass, can move at the speed of light. (Location 370)
  • In other words, the theory of relativity put an end to the idea of absolute time! It appeared that each observer must have his own measure of time, as recorded by a clock carried with him, and that identical clocks carried by different observers would not necessarily agree. (Location 378)
  • We must accept that time is not completely separate from and independent of space, but is combined with it to form an object called space-time. (Location 395)
  • To someone high up, it would appear that everything down below was taking longer to happen. This prediction was tested in 1962, using a pair of very accurate clocks mounted at the top and bottom of a water tower. The clock at the bottom, which was nearer the earth, was found to run slower, in exact agreement with general relativity. (Location 505)
  • Roger Penrose and I showed that Einstein’s general theory of relativity implied that the universe must have a beginning and, possibly, an end. (Location 526)
  • CHAPTER 3 THE EXPANDING UNIVERSE (Location 528)
  • the stars in its spiral arms orbit around its center about once every several hundred million years. (Location 554)
  • light emitted by an object that is glowing red hot. (In fact, the light emitted by any opaque object that is glowing red hot has a characteristic spectrum that depends only on its temperature—a thermal spectrum. This means that we can tell a star’s temperature from the spectrum of its light.) (Location 563)
  • even the size of a galaxy’s red shift is not random, but is directly proportional to the galaxy’s distance from us. Or, in other words, the farther a galaxy is, the faster it is moving away! (Location 588)
  • The discovery that the universe is expanding was one of the great intellectual revolutions of the twentieth century. (Location 590)
  • Many people do not like the idea that time has a beginning, probably because it smacks of divine intervention. (The Catholic Church, on the other hand, seized on the big bang model and in 1951 officially pronounced (Location 706)
  • to be in accordance with the Bible.) (Location 707)
  • It is perhaps ironic that, having changed my mind, I am now trying to convince other physicists that there was in fact no singularity at the beginning of the universe—as we shall see later, it can disappear once quantum effects are taken into account. (Location 766)
  • CHAPTER 4 THE UNCERTAINTY PRINCIPLE (Location 779)
  • In other words, the more accurately you try to measure the position of the particle, the less accurately you can measure its speed, and vice versa. Heisenberg showed that the uncertainty in the position (Location 806)
  • particles no longer had separate, well-defined positions and velocities that could not be observed. Instead, they had a quantum state, which was a combination of position and velocity. (Location 817)
  • The only areas of physical science into which quantum mechanics has not yet been properly incorporated are gravity and the large-scale structure of the universe. (Location 829)
  • CHAPTER 5 ELEMENTARY PARTICLES AND THE FORCES OF NATURE (Location 900)
  • CHAPTER 6 BLACK HOLES (Location 1151)
  • We now know that really both theories are correct. By the wave/particle duality of quantum mechanics, light can be regarded as both a wave and a particle. (Location 1154)
  • 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. (Location 1329)
  • The number of black holes may well be greater even than the number of visible stars, which totals about a hundred thousand million in our galaxy alone. The extra gravitational attraction of such a large number of black holes could explain why our galaxy rotates at the rate it does: the mass of the visible stars is insufficient to account for this. We also have some evidence that there is a much larger black hole, with a mass of about a hundred thousand times that of the sun, at the center of our galaxy. (Location 1373)
  • black holes are not really black after all: they glow like a hot body, and the smaller they are, the more they glow. So, paradoxically, smaller black holes might actually turn out to be easier to detect than large ones! (Location 1404)
  • CHAPTER 7 BLACK HOLES AIN’T SO BLACK (Location 1406)
  • A precise statement of this idea is known as the second law of thermodynamics. It states that the entropy of an isolated system always increases, and that when two systems are joined together, the entropy of the combined system is greater than the sum of the entropies of the individual systems. (Location 1438)
  • One such black hole could run ten large power stations, if only we could harness its power. This would be rather difficult, however: the black hole would have the mass of a mountain compressed into less than a million millionth of an inch, the size of the nucleus of an atom! (Location 1530)
  • CHAPTER 8 THE ORIGIN AND FATE OF THE UNIVERSE (Location 1597)
  • What really happens during the very early or late stages of the universe, when gravitational fields are so strong that quantum effects cannot be ignored? Does the universe in fact have a beginning or an end? And if so, what are they like? (Location 1604)
  • in 1981 my interest in questions about the origin and fate of the universe was reawakened when I attended a conference on cosmology organized by the Jesuits in the Vatican. (Location 1606)
  • At the end of the conference the participants were granted an audience with the Pope. He told us that it was all right to study the evolution of the universe after the big bang, but we should not inquire (Location 1609)
  • into the big bang itself because that was the moment of Creation and therefore the work of God. I was glad then that he did not know the subject of the talk I had just given at the conference—the possibility that space-time was finite but had no boundary, which means that it had no beginning, no moment of Creation. (Location 1610)
  • We are therefore fairly confident that we have the right picture, at least back to about one second after the big bang. (Location 1653)
  • 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. (Location 1675)
  • The first primitive forms of life consumed various materials, including hydrogen sulfide, and released oxygen. This gradually changed the atmosphere to the composition that it has today, and allowed the development of higher forms of life such as fish, reptiles, mammals, and ultimately the human race. (Location 1687)
  • This picture of a universe that started off very hot and cooled as it expanded is in agreement with all the observational evidence that we have today. Nevertheless, it leaves a number of important questions unanswered: Why was the early universe so hot? Why is the universe so uniform on a large scale? Why does it look the same at all points of space and in all directions? (Location 1689)
  • 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. (Location 1736)
  • 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. (Location 1737)
  • Most sets of values would give rise to universes that, although they might be very beautiful, would contain no one able to wonder at that beauty. (Location 1759)
  • One can take this either as evidence of a divine purpose in Creation and the choice of the laws of science or as support for the strong anthropic principle. (Location 1760)
  • It would be very difficult to explain why the universe should have begun in just this way, except as the act of a God who intended to create beings like us. (Location 1784)
  • In order to predict how the universe should have started off, one needs laws that hold at the beginning of time. (Location 1884)
  • In the classical theory of gravity, which is based on real space-time, there are only two possible ways the universe can behave: either it has existed for an infinite time, or else it had a beginning at a singularity at some finite time in the past. (Location 1927)
  • In the quantum theory of gravity, on the other hand, a third possibility arises. Because one is using Euclidean space-times, in which the time direction is on the same footing as directions in space, it is possible for space-time to be finite in extent and yet to have no singularities that formed a boundary or edge. Space-time would be like the surface of the earth, only with two more dimensions. The surface of the earth is finite in extent but it doesn’t have a boundary or edge: if you sail off into the sunset, you don’t fall off the edge or run into a singularity. (I know, because I have been round the world!) (Location 1928)
  • On the other hand, the quantum theory of gravity has opened up a new possibility, in which there would be no boundary to space-time and so there would be no need to specify the behavior at the boundary. (Location 1935)
  • universe had a beginning, we could suppose it had a creator. But if the universe is really completely self-contained, having no boundary or edge, it would have neither beginning nor end: it would simply be. What place, then, for a creator? (Location 2011)
  • CHAPTER 9 THE ARROW OF TIME (Location 2013)
  • The increase of disorder or entropy with time is one example of what is called an arrow of time, something that distinguishes the past from the future, giving a direction to time. (Location 2041)
  • There are at least three different arrows of time. First, there is the thermodynamic arrow of time, the direction of time in which disorder or entropy increases. Then, there is the psychological arrow of time. This is the direction in which we feel time passes, the direction in which we remember the past but not (Location 2042)
  • the future. Finally, there is the cosmological arrow of time. This is the direction of time in which the universe is expanding rather than contracting. (Location 2044)
  • why does disorder increase in the same direction of time as that in which the universe expands? (Location 2051)
  • So if disorder were to decrease in the contracting phase of the universe, one might also expect it to decrease inside a black hole. (Location 2115)
  • CHAPTER 10 WORMHOLES AND TIME TRAVEL (Location 2169)
  • However, other more reasonable space-times that are allowed by general relativity and which permit travel into the past have since been found. One is in the interior of a rotating black hole. Another is a space-time that contains two cosmic strings moving past each other at high speed. (Location 2189)
  • We can accelerate particles to 99.99 percent of the speed of light, but however much power we feed in, we can’t get them beyond the speed-of-light barrier. (Location 2230)
  • The idea of wormholes between different regions of space-time was not an invention of science fiction writers but came from a very respectable source. (Location 2241)
  • therefore that as we advance in science and technology, we would eventually manage to build a time machine. But if so, why hasn’t anyone come back from the future and told us how to do it? (Location 2270)
  • Of course, some people would claim that sightings of UFOs are evidence that we are being visited either by aliens or by people from the future. (If the aliens were to get here in reasonable time, they would need faster-than-light travel, so the two possibilities may be equivalent.) (Location 2273)
  • On the other hand, the future is unknown and open, so it might well have the curvature required. This would mean that any time travel would be confined to the future. There (Location 2280)
  • Thus the possibility of time travel remains open. But I’m not going to bet on it. My opponent might have the unfair advantage of knowing the future. (Location 2335)
  • CHAPTER 11 THE UNIFICATION OF PHYSICS (Location 2337)
  • I think that there is a good chance that the study of the early universe and the requirements of mathematical consistency will lead us to a complete unified theory within the lifetime of some of us who are around today, always presuming we don’t blow ourselves up first. (Location 2515)
  • CHAPTER 12 CONCLUSION (Location 2542)
  • In effect, we have redefined the task of science to be the discovery of laws that will enable us to predict events up to the limits set by the uncertainty principle. The question remains, however: how or why were the laws and the initial state of the universe chosen? (Location 2572)
  • Up to now, most scientists have been too occupied with the development of new theories that describe what the universe is to ask the question why. On the other hand, the people whose business it is to ask why, the philosophers, have not been able to keep up with the advance of scientific theories. (Location 2596)
  • However, if we do discover a complete theory, it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason—for then we would know the mind of God. (Location 2603)
  • For example, in 1998, our picture of the universe’s future was radically revised. Two competing teams using the Hubble Space Telescope independently reached the conclusion that the expansion of our universe is accelerating. (Location 2677)
  • Space, it seems, will expand forever. (Location 2680)