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• Many people would claim that the boundary conditions are not part of physics but belong to metaphysics or religion. They would claim that nature had complete freedom to start the universe off any way it wanted. That may be so, but it could also have made it evolve in a completely arbitrary and random manner. Yet all the evidence is that it evolves in a regular way according to certain laws. It would therefore seem reasonable to suppose that there are also laws governing the boundary conditions.
  • "The Quantum State of the Universe", Nuclear Physics (1984)

• My goal is simple. It is a complete understanding of the universe, why it is as it is and why it exists at all.
  • As quoted in Stephen Hawking's Universe (1985) by John Boslough, Ch. 7 : The Final Question, p. 77

• There ought to be something very special about the boundary conditions of the universe and what can be more special than that there is no boundary?
  • As quoted in The Anthropic Cosmological Principle (1986) by John D. Barrow and Frank J. Tipler.

• What I have done is to show that it is possible for the way the universe began to be determined by the laws of science. In that case, it would not be necessary to appeal to God to decide how the universe began. This doesn't prove that there is no God, only that God is not necessary.
  • Der Spiegel (17 October 1988)

• We are just an advanced breed of monkeys on a minor planet of a very average star. But we can understand the Universe. That makes us something very special.
  • Der Spiegel (17 October 1988)

• For millions of years, mankind lived just like the animals. Then something happened which unleashed the power of our imagination. We learned to talk and we learned to listen. Speech has allowed the communication of ideas, enabling human beings to work together to build the impossible. Mankind's greatest achievements have come about by talking, and its greatest failures by not talking. It doesn't have to be like this. Our greatest hopes could become reality in the future. With the technology at our disposal, the possibilities are unbounded. All we need to do is make sure we keep talking.
  • British Telecom advertisement (1993), part of which was used in Pink Floyd's Keep Talking (1994).

• I don't believe that the ultimate theory will come by steady work along existing lines. We need something new. We can't predict what that will be or when we will find it because if we knew that, we would have found it already! It could come in the next 20 years, but we might never find it.
  • Science Watch (September 1994)

• All of my life, I have been fascinated by the big questions that face us, and have tried to find scientific answers to them. Perhaps that is why I have sold more books on physics than Madonna has on sex.
  • The Illustrated A Brief History of Time (1996) [Disputed... see Discussion page.]

• Although September 11 was horrible, it didn't threaten the survival of the human race, like nuclear weapons do.
  • Interview "Colonies in space may be only hope, says Hawking" by Roger Highfield in Daily Telegraph (16 October 2001)

• I don't think the human race will survive the next thousand years, unless we spread into space. There are too many accidents that can befall life on a single planet. But I'm an optimist. We will reach out to the stars.
  • "Colonies in space may be only hope, says Hawking" by Roger Highfield in Daily Telegraph (16 October 2001)

• I'm sorry to disappoint science fiction fans, but if information is preserved, there is no possibility of using black holes to travel to other universes. If you jump into a black hole, your mass energy will be returned to our universe but in a mangled form which contains the information about what you were like but in a state where it can not be easily recognized. It is like burning an encyclopedia. Information is not lost, if one keeps the smoke and the ashes. But it is difficult to read. In practice, it would be too difficult to re-build a macroscopic object like an encyclopedia that fell inside a black hole from information in the radiation, but the information preserving result is important for microscopic processes involving virtual black holes.
  • "Information Loss in Black Holes" (July 2005)

• So Einstein was wrong when he said "God does not play dice". Consideration of black holes suggests, not only that God does play dice, but that He sometimes confuses us by throwing them where they can't be seen.
  • "The Nature of Space and Time" (online text)
  • Unsourced variants: Not only does God play dice with the Universe; he sometimes casts them where they can't be seen.
    Not only does God play dice, but... he sometimes throws them where they cannot be seen.

• Einstein is the only figure in the physical sciences with a stature that can be compared with Newton. Newton is reported to have said 'If I have seen further than other men, it is because I stood on the shoulders of giants.' This remark is even more true of Einstein who stood on the shoulders of Newton. Both Newton and Einstein put forward a theory of mechanics and a theory of gravity but Einstein was able to base General Relativity on the mathematical theory of curved spaces that had been constructed by Riemann while Newton had to develop his own mathematical machinery. It is therefore appropriate to acclaim Newton as the greatest figure in mathematical physics and the Principia is his greatest achievement.
  • "Newton's Principia" in 300 Years of Gravitation. (1987) by S. W. Hawking and W. Israel, p.4 Cambridge University Press, Cambridge ISBN 0521343127

• I have no idea. People who boast about their IQ are losers.
  • Response upon being questioned as to his IQ, in interview with Deborah Solomon "The Science of Second-Guessing", The New York Times (12 December 2004)

• Life would be tragic if it weren't funny.
  • As quoted in "The Science of Second-Guessing", The New York Times (12 December 2004)

• I am discounting reports of UFOs. Why would they appear only to cranks and weirdos?
  • As quoted in a TED talk, "Asking big questions about the universe"

• My expectations were reduced to zero when I was 21. Everything since then has been a bonus.
  • As quoted in "The Science of Second-Guessing", The New York Times (12 December 2004)
  • Unsourced variant: "When one's expectations are reduced to zero, one really appreciates everything one does have."

• Evolution has ensured that our brains just aren't equipped to visualise 11 dimensions directly. However, from a purely mathematical point of view it's just as easy to think in 11 dimensions, as it is to think in three or four.
  • As quoted in "Return of the time lord" in The Guardian (27 September 2005)

• I think that it's important for scientists to explain their work, particularly in cosmology. This now answers many questions once asked of religion.
  • As quoted in "Return of the time lord" in The Guardian (27 September 2005)

• It is a waste of time to be angry about my disability. One has to get on with life and I haven't done badly. People won't have time for you if you are always angry or complaining.
  • As quoted in "Return of the time lord" in The Guardian (27 September 2005)

• Equations are just the boring part of mathematics. I attempt to see things in terms of geometry.
  • As quoted in Stephen Hawking: A Biography (2005) by Kristine Larsen, p. 43

• The victim should have the right to end his life, if he wants. But I think it would be a great mistake. However bad life may seem, there is always something you can do, and succeed at. While there's life, there is hope.
  • On voluntary euthanasia as quoted in People's Daily Online (14 June 2006)

• The danger is that global warming may become self-sustaining, if it has not done so already. The melting of the Arctic and Antarctic ice caps reduces the fraction of solar energy reflected back into space, and so increases the temperature further. Climate change may kill off the Amazon and other rain forests, and so eliminate once one of the main ways in which carbon dioxide is removed from the atmosphere. The rise in sea temperature may trigger the release of large quantities of carbon dioxide, trapped as hydrides on the ocean floor. Both these phenomena would increase the greenhouse effect, and so global warming further. We have to reverse global warming urgently, if we still can.
  • ABC News interview (16 August 2006)

• The downside of my celebrity is that I cannot go anywhere in the world without being recognized. It is not enough for me to wear dark sunglasses and a wig. The wheelchair gives me away.
  • Interview on Israeli television, as quoted in "Happy 65th Birthday to Prof. Stephen Hawking!" at StarTrek.com (8 January 2007)

• As scientists, we understand the dangers of nuclear weapons and their devastating effects, and we are learning how human activities and technologies are affecting climate systems in ways that may forever change life on Earth. As citizens of the world, we have a duty to alert the public to the unnecessary risks that we live with every day, and to the perils we foresee if governments and societies do not take action now to render nuclear weapons obsolete and to prevent further climate change... There’s a realization that we are changing our climate for the worse. That would have catastrophic effects. Although the threat is not as dire as that of nuclear weapons right now, in the long term we are looking at a serious threat.
  • At a press conference for the Bulletin of the Atomic Scientists, as the Doomsday Clock is moved forward by two minutes to five minutes to midnight, as quoted in "Nukes, climate push 'Doomsday Clock' forward" MSNBC (1 January 2007)

• Space, here I come!
  • Before completing a zero-gravity flight in a specially modified plane, as quoted in "Hawking takes zero-gravity flight" BBC News (27 April 2007)

A Brief History of Time (1988)

• Someone told me that each equation I included in the book would halve the sales. I therefore resolved not to have any equations at all. In the end, however, I did put in one equation, Einstein's famous equation, . I hope that this will not scare off half of my potential readers.

• 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 that disagrees with the predictions of the theory. As philosopher of science Karl Popper has emphasized, 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 time new experiments are observed to agree with the predictions the theory survives, and our confidence in it is increased; but if ever a new observation is found to disagree, we have to abandon or modify the theory.

• It has certainly been true in the past that what we call intelligence and scientific discovery have conveyed a survival advantage. It is not so clear that this is still the case: our scientific discoveries may well destroy us all, and even if they don’t, a complete unified theory may not make much difference to our chances of survival. However, provided the universe has evolved in a regular way, we might expect that the reasoning abilities that natural selection has given us would be valid also in our search for a complete unified theory, and so would not lead us to the wrong conclusions.

• Bodies like the earth are not made to move on curved orbits by a force called gravity; instead, they follow the nearest thing to a straight path in a curved space, which is called a geodesic. A geodesic is the shortest (or longest) path between two nearby points.

• The whole history of science has been the gradual realization that events do not happen in an arbitrary manner, but that they reflect a certain underlying order, which may or may not be divinely inspired.

• One could say: "The boundary condition of the universe is that it has no boundary." 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.

• Just like a computer, we must remember things in the order in which entropy increases. This makes the second law of thermodynamics almost trivial. Disorder increases with time because we measure time in the direction in which disorder increases. You can’t have a safer bet than that!

• Maybe that is our mistake: maybe there are no particle positions and velocities, but only waves. It is just that we try to fit the waves to our preconceived ideas of positions and velocities.The resulting mismatch is the cause of the apparent unpredictability.

• Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe? The usual approach of science of constructing a mathematical model cannot answer the questions of why there should be a universe for the model to describe. Why does the universe go to all the bother of existing?

Black Holes and Baby Universes and Other Essays (1993)

• 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.
  • Last lines. Hawking later wrote: "In the proof stage I nearly cut the last sentence in the book... Had I done so, the sales might have been halved.

The Beginning of Time

Online text

• When I gave a lecture in Japan, I was asked not to mention the possible re-collapse of the universe, because it might affect the stock market. However, I can re-assure anyone who is nervous about their investments that it is a bit early to sell: even if the universe does come to an end, it won't be for at least twenty billion years. By that time, maybe the GATT trade agreement will have come into effect.

• The Steady State theory was what Karl Popper would call a good scientific theory: it made definite predictions, which could be tested by observation, and possibly falsified. Unfortunately for the theory, they were falsified.

• To show this diagram properly, I would really need a four dimensional screen. However, because of government cuts, we could manage to provide only a two dimensional screen.

• The universe would have expanded in a smooth way from a single point. As it expanded, it would have borrowed energy from the gravitational field, to create matter. As any economist could have predicted, the result of all that borrowing, was inflation. The universe expanded and borrowed at an ever-increasing rate. Fortunately, the debt of gravitational energy will not have to be repaid until the end of the universe.

Misattributed

• I have hardly ever known a mathematician who was able to reason.
  • Plato, The Republic, Book VII, 531-E.