Stephen Hawking quotes:

Time travel used to be thought of as just science fiction, but Einstein's general theory of relativity allows for the possibility that we could warp space-time so much that you could go off in a rocket and return before you set out.

I would like nuclear fusion to become a practical power source. It would provide an inexhaustible supply of energy, without pollution or global warming.

It was Einstein's dream to discover the grand design of the universe, a single theory that explains everything. However, physicists in Einstein's day hadn't made enough progress in understanding the forces of nature for that to be a realistic goal.

I have noticed even people who claim everything is predestined, and that we can do nothing to change it, look before they cross the road.

God is the name people give to the reason we are here. But I think that reason is the laws of physics rather than someone with whom one can have a personal relationship. An impersonal God.

Intelligence is the ability to adapt to change.

I entered the health care debate in response to a statement in the United States press in summer 2009 which claimed the National Health Service in Great Britain would have killed me off, were I a British citizen. I felt compelled to make a statement to explain the error.

Time travel was once considered scientific heresy, and I used to avoid talking about it for fear of being labelled a 'crank.'

I regard the brain as a computer which will stop working when its components fail. There is no heaven or afterlife for broken down computers; that is a fairy story for people afraid of the dark.

Obviously, because of my disability, I need assistance. But I have always tried to overcome the limitations of my condition and lead as full a life as possible. I have traveled the world, from the Antarctic to zero gravity.

I was born on January 8, 1942, exactly three hundred years after the death of Galileo. I estimate, however, that about two hundred thousand other babies were also born that day. I don't know whether any of them was later interested in astronomy.

Keeping an active mind has been vital to my survival, as has been maintaining a sense of humor.

Scientists have become the bearers of the torch of discovery in our quest for knowledge.

There is no heaven or afterlife for broken-down computers; that is a fairy story for people afraid of the dark.

Cambridge is one of the best universities in the world, especially in my field.

Before 1915, space and time were thought of as a fixed arena in which events took place, but which was not affected by what happened in it. Space and time are now dynamic quantities... space and time not only affect but are also affected by everything that happens in the universe.

Wagner manages to convey emotion with music better than anyone, before or since.

I think those who have a terminal illness and are in great pain should have the right to choose to end their own life, and those that help them should be free from prosecution.

Life on Earth is at the ever-increasing risk of being wiped out by a disaster, such as sudden global nuclear war, a genetically engineered virus or other dangers we have not yet thought of.

In my opinion, there is no aspect of reality beyond the reach of the human mind.

It's time to commit to finding the answer, to search for life beyond Earth. Mankind has a deep need to explore, to learn, to know. We also happen to be sociable creatures. It is important for us to know if we are alone in the dark.

Throughout history, people have studied pure science from a desire to understand the universe rather than practical applications for commercial gain. But their discoveries later turned out to have great practical benefits.

I believe in universal health care. And I am not afraid to say so.

If we do discover a complete theory, it should be in time understandable in broad principle by everyone. Then we shall all, philosophers, scientists, and just ordinary people be able to take part in the discussion of why we and the universe exist.

There is no physical law precluding particles from being organised in ways that perform even more advanced computations than the arrangements of particles in human brains.

If we want to travel into the future, we just need to go fast. Really fast. And I think the only way we're ever likely to do that is by going into space.

Success in creating AI would be the biggest event in human history. Unfortunately, it might also be the last, unless we learn how to avoid the risks.

We think we have solved the mystery of creation. Maybe we should patent the universe and charge everyone royalties for their existence.

The doctor who diagnosed me with ALS, or motor neuron disease, told me that it would kill me in two or three years.

I'm never any good in the morning. It is only after four in the afternoon that I get going.

In my school, the brightest boys did math and physics, the less bright did physics and chemistry, and the least bright did biology. I wanted to do math and physics, but my father made me do chemistry because he thought there would be no jobs for mathematicians.

Although almost every theoretical physicist agrees with my prediction that a black hole should glow like a hot body, it would be very difficult to verify experimentally because the temperature of a macroscopic black hole is so low.

My goal is simple. It is a complete understanding of the universe, why it is as it is and why it exists at all.

I think the brain is essentially a computer and consciousness is like a computer program. It will cease to run when the computer is turned off. Theoretically, it could be re-created on a neural network, but that would be very difficult, as it would require all one's memories.

I think computer viruses should count as life. I think it says something about human nature that the only form of life we have created so far is purely destructive. We've created life in our own image.

A few years ago, the city council of Monza, Italy, barred pet owners from keeping goldfish in curved bowls... saying that it is cruel to keep a fish in a bowl with curved sides because, gazing out, the fish would have a distorted view of reality. But how do we know we have the true, undistorted picture of reality?

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.

Our population and our use of the finite resources of planet Earth are growing exponentially, along with our technical ability to change the environment for good or ill.

For years, my early work with Roger Penrose seemed to be a disaster for science. It showed that the universe must have begun with a singularity, if Einstein's general theory of relativity is correct. That appeared to indicate that science could not predict how the universe would begin.

With genetic engineering, we will be able to increase the complexity of our DNA, and improve the human race. But it will be a slow process, because one will have to wait about 18 years to see the effect of changes to the genetic code.

The radiation left over from the Big Bang is the same as that in your microwave oven but very much less powerful. It would heat your pizza only to minus 271.3*C - not much good for defrosting the pizza, let alone cooking it.

I used to think information was destroyed in black hole. This was my biggest blunder, or at least my biggest blunder in science.

In the past, there was active discrimination against women in science. That has now gone, and although there are residual effects, these are not enough to account for the small numbers of women, particularly in mathematics and physics.

It is generally recognised that women are better than men at languages, personal relations and multi-tasking, but less good at map-reading and spatial awareness. It is therefore not unreasonable to suppose that women might be less good at mathematics and physics.

Work gives you meaning and purpose and life is empty without it.

I think it quite likely that we are the only civilization within several hundred light years; otherwise we would have heard radio waves.

Science predicts that many different kinds of universe will be spontaneously created out of nothing. It is a matter of chance which we are in.

In less than a hundred years, we have found a new way to think of ourselves. From sitting at the center of the universe, we now find ourselves orbiting an average-sized sun, which is just one of millions of stars in our own Milky Way galaxy.

Our minds work in real time, which begins at the Big Bang and will end, if there is a Big Crunch - which seems unlikely, now, from the latest data showing accelerating expansion. Consciousness would come to an end at a singularity.

Philosophers have not kept up with modern developments in science. Particularly physics.

Science is not only a disciple of reason but, also, one of romance and passion.

Imaginary time is a new dimension, at right angles to ordinary, real time.

If the rate of expansion one second after the Big Bang had been smaller by even one part in a hundred thousand million million, it would have recollapsed before it reached its present size. On the other hand, if it had been greater by a part in a million, the universe would have expanded too rapidly for stars and planets to form.

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.

Most people don't have time to master the very mathematical details of theoretical physics.

Theoretical physics is one of the few fields in which being disabled is no handicap - it is all in the mind.

To my mathematical brain, the numbers alone make thinking about aliens perfectly rational. The real challenge is to work out what aliens might actually be like.

It is no good getting furious if you get stuck. What I do is keep thinking about the problem but work on something else. Sometimes it is years before I see the way forward. In the case of information loss and black holes, it was 29 years.

I think the discovery of supersymmetric partners for the known particles would revolutionize our understanding of the universe.

While physics and mathematics may tell us how the universe began, they are not much use in predicting human behavior because there are far too many equations to solve. I'm no better than anyone else at understanding what makes people tick, particularly women.

I had a bet with Gordon Kane of Michigan University that the Higgs particle wouldn't be found.

Science can lift people out of poverty and cure disease. That, in turn, will reduce civil unrest.

Science is beautiful when it makes simple explanations of phenomena or connections between different observations. Examples include the double helix in biology and the fundamental equations of physics.

Some people would claim that things like love, joy and beauty belong to a different category from science and can't be described in scientific terms, but I think they can now be explained by the theory of evolution.

There is a real danger that computers will develop intelligence and take over. We urgently need to develop direct connections to the brain so that computers can add to human intelligence rather than be in opposition.

I believe alien life is quite common in the universe, although intelligent life is less so. Some say it has yet to appear on planet Earth.

I have wondered about time all my life.

I have so much that I want to do. I hate wasting time.

People won't have time for you if you are always angry or complaining.

It now appears that the way the universe began can indeed be determined, using imaginary time.

Time can behave like another direction in space under extreme conditions.

I had not expected 'A Brief History of Time' to be a best seller.

Up until the 1920s, everyone thought the universe was essentially static and unchanging in time.

Even if it turns out that time travel is impossible, it is important that we understand why it is impossible.

I was not a good student. I did not spend much time at college; I was too busy enjoying myself.

If I had a time machine, I'd visit Marilyn Monroe in her prime or drop in on Galileo as he turned his telescope to the heavens.

My first popular book, 'A Brief History of Time,' aroused a great deal of interest, but many found it difficult to understand.

We lived in a tall, narrow Victorian house, which my parents had bought very cheaply during the war, when everyone thought London was going to be bombed flat. In fact, a V-2 rocket landed a few houses away from ours. I was away with my mother and sister at the time, but my father was in the house.

Science is increasingly answering questions that used to be the province of religion.

The past, like the future, is indefinite and exists only as a spectrum of possibilities.

What was God doing before the divine creation?

The missing link in cosmology is the nature of dark matter and dark energy.

The universe is governed by science. But science tells us that we can't solve the equations, directly in the abstract.

Cosmology is a rapidly advancing field.

We are all different. There is no such thing as a standard or run-of-the-mill human being, but we share the same human spirit.

My advice to other disabled people would be, concentrate on things your disability doesn't prevent you doing well, and don't regret the things it interferes with. Don't be disabled in spirit as well as physically.

Although September 11 was horrible, it didn't threaten the survival of the human race, like nuclear weapons do.

God may exist, but science can explain the universe without the need for a creator.

The cyclic universe theory predicts no gravitational waves from the early universe.

I believe the universe is governed by the laws of science. The laws may have been decreed by God, but God does not intervene to break the laws.

My work and my family are very important to me.

My wife and I love each other very much.

Many badly needed goals, like fusion and cancer cure, would be achieved much sooner if we invested more.

My three children have brought me great joy.

Computers double their performance every month.

I believe there are no questions that science can't answer about a physical universe.

There are no black holes in the sense of regimes from which light can't escape to infinity.

All my adult life people have been helping me.

I can't say that my disability has helped my work, but it has allowed me to concentrate on research without having to lecture or sit on boring committees.

We think that life develops spontaneously on Earth, so it must be possible for life to develop on suitable planets elsewhere in the universe. But we don't know the probability that a planet develops life.

Look up at the stars and not down at your feet. Try to make sense of what you see, and wonder about what makes the universe exist. Be curious.

No one can resist the idea of a crippled genius.

There's no way to remove the observer - us - from our perceptions of the world.

The Paralympic Games is about transforming our perception of the world.

I don't want to write an autobiography because I would become public property with no privacy left.

M-theory is the unified theory Einstein was hoping to find.

Many people find the universe confusing - it's not.

Exploration by real people inspires us.

We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet.

I'm not afraid of death, but I'm in no hurry to die. I have so much I want to do first.

A snooze button is a poor substitute for no alarm clock at all.

If aliens visit us, the outcome would be much as when Columbus landed in America, which didn't turn out well for the Native Americans.

Of course it is possible that UFO's really do contain aliens as many people believe, and the government is hushing it up

To my mathematical brain, the numbers alone make thinking about aliens perfectly rational,

I think [contacting an alien civilization] would be a disaster. The extraterrestrials would probably be far in advance of us. The history of advanced races meeting more primitive people on this planet is not very happy, and they were the same species. I think we should keep our heads low.

One cannot really argue with a mathematical theorem.

Everything that civilisation has to offer is a product of human intelligence; we cannot predict what we might achieve when this intelligence is magnified by the tools that AI may provide, but the eradication of war, disease, and poverty would be high on anyone's list. Success in creating AI would be the biggest event in human history. Unfortunately, it might also be the last.

The development of full artificial intelligence could spell the end of the human race.

"Sooner or later disasters such as an asteroid collision or a nuclear war could wipe us all out, But once we spread out into space and establish independent colonies, our future should be safe."

We are in danger of destroying ourselves by our greed and stupidity. We cannot remain looking inwards at ourselves on a small and increasingly polluted and overcrowded planet.