Stephen Hawking – Quotes

‘The Simpsons’ appearances were great fun. But I don’t take them too seriously. I think ‘The Simpsons’ have treated my disability responsibly.” — Stephen Hawking

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?” — Stephen Hawking

A zero-gravity flight is a first step toward space travel.” — Stephen Hawking

According to ‘M’ theory, ours is not the only universe. Instead, ‘M’ theory predicts that a great many universes were created out of nothing.” — Stephen Hawking

All my adult life people have been helping me.” — Stephen Hawking

Although September 11 was horrible, it didn’t threaten the survival of the human race, like nuclear weapons do.” — Stephen Hawking

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.” — Stephen Hawking

Among physicists, I’m respected I hope.” — Stephen Hawking

As Irving Good realised in 1965, machines with superhuman intelligence could repeatedly improve their design even further, triggering what Vernor Vinge called a ‘singularity.’” — Stephen Hawking

As a child, I wanted to know how things worked and to control them. With a friend, I built a number of complicated models that I could control.” — Stephen Hawking

As scientists, we step on the shoulders of science, building on the work that has come before us – aiming to inspire a new generation of young scientists to continue once we are gone.” — Stephen Hawking

Because there is a law such as gravity, the universe can and will create itself from nothing.” — Stephen Hawking

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.” — Stephen Hawking

Before I lost my voice, it was slurred, so only those close to me could understand, but with the computer voice, I found I could give popular lectures. I enjoy communicating science. It is important that the public understands basic science, if they are not to leave vital decisions to others.” — Stephen Hawking

Before we understand science, it is natural to believe that God created the universe.” — Stephen Hawking

Cambridge is one of the best universities in the world, especially in my field.” — Stephen Hawking

Computers double their performance every month.” — Stephen Hawking

Cosmology is a rapidly advancing field.” — Stephen Hawking

Earth might one day soon resemble the planet Venus.” — Stephen Hawking

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

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?” — Stephen Hawking

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.” — Stephen Hawking

Exploration by real people inspires us.” — Stephen Hawking

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.” — Stephen Hawking

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.” — Stephen Hawking

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

God not only plays dice, He also sometimes throws the dice where they cannot be seen.” — Stephen Hawking

God not only plays dice, but also sometimes throws them where they cannot be seen.” — Stephen Hawking

However difficult life may seem, there is always something you can do and succeed at.” — Stephen Hawking

I am in touch with a company that hopes to replicate my voice. However, they are not replicating my original voice – if they did that, I would sound like a man in his 20s, which would be very strange! They are actually trying to replicate the synthesizer that sits on my wheelchair.” — Stephen Hawking

I am just a child who has never grown up. I still keep asking these ‘how’ and ‘why’ questions. Occasionally, I find an answer.” — Stephen Hawking

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.” — Stephen Hawking

I believe everyone should have a broad picture of how the universe operates and our place in it. It is a basic human desire. And it also puts our worries in perspective.” — Stephen Hawking

I believe in universal health care. And I am not afraid to say so.” — Stephen Hawking

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.” — Stephen Hawking

I believe there are no questions that science can’t answer about a physical universe.” — Stephen Hawking

I believe things cannot make themselves impossible.” — Stephen Hawking

I can’t disguise myself with a wig and dark glasses – the wheelchair gives me away.” — Stephen Hawking

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.” — Stephen Hawking

I don’t care much for equations myself. This is partly because it is difficult for me to write them down, but mainly because I don’t have an intuitive feeling for equations.” — Stephen Hawking

I don’t have much positive to say about motor neuron disease, but it taught me not to pity myself because others were worse off, and to get on with what I still could do. I’m happier now than before I developed the condition.” — Stephen Hawking

I don’t have much positive to say about motor neurone disease. But it taught me not to pity myself because others were worse off, and to get on with what I could still do.” — Stephen Hawking

I don’t think the human race will survive the next thousand years, unless we spread into space.” — Stephen Hawking

I don’t want to write an autobiography because I would become public property with no privacy left.” — Stephen Hawking

I enjoy all forms of music – pop, classical and opera.” — Stephen Hawking

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.” — Stephen Hawking

I first had the idea of writing a popular book about the universe in 1982. My intention was partly to earn money to pay my daughter’s school fees.” — Stephen Hawking

I grew up thinking that a research scientist was a natural thing to be.” — Stephen Hawking

I had a bet with Gordon Kane of Michigan University that the Higgs particle wouldn’t be found.” — Stephen Hawking

I had not expected ‘A Brief History of Time’ to be a best seller.” — Stephen Hawking

I had not expected ‘A Brief History of Time’ to be a best seller. It was my first popular book and aroused a great deal of interest. Initially, many people found it difficult to understand. I therefore decided to try to write a new version that would be easier to follow.” — Stephen Hawking

I have a full and satisfying life. My work and my family are very important to me.” — Stephen Hawking

I have found far greater enthusiasm for science in America than here in Britain. There is more enthusiasm for everything in America.” — Stephen Hawking

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.” — Stephen Hawking

I have so much that I want to do. I hate wasting time.” — Stephen Hawking

I have visited Japan several times and have always been shown wonderful hospitality.” — Stephen Hawking

I have wanted to fly into space for many years, but never imagined it would really be feasible.” — Stephen Hawking

I have wondered about time all my life.” — Stephen Hawking

I hope I have helped to raise the profile of science and to show that physics is not a mystery but can be understood by ordinary people.” — Stephen Hawking

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.” — Stephen Hawking

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.” — Stephen Hawking

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

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.” — Stephen Hawking

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

I think the human race doesn’t have a future if it doesn’t go into space.” — Stephen Hawking

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.” — Stephen Hawking

I think we have a good chance of surviving long enough to colonize the solar system.” — Stephen Hawking

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

I want my books sold on airport bookstalls.” — Stephen Hawking

I want to know why the universe exists, why there is something greater than nothing.” — Stephen Hawking

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.” — Stephen Hawking

I was never top of the class at school, but my classmates must have seen potential in me, because my nickname was ‘Einstein.’” — Stephen Hawking

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

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

I wouldn’t be here today if it were not for the NHS. I have received a large amount of high-quality treatment without which I would not have survived.” — Stephen Hawking

I’m an atheist.” — Stephen Hawking

I’m never any good in the morning. It is only after four in the afternoon that I get going.” — Stephen Hawking

I’m not afraid of death, but I’m in no hurry to die. I have so much I want to do first.” — Stephen Hawking

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.” — Stephen Hawking

If I had to choose a superhero to be, I would pick Superman. He’s everything that I’m not.” — Stephen Hawking

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.” — Stephen Hawking

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.” — Stephen Hawking

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.” — Stephen Hawking

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.” — Stephen Hawking

If you believe in science, like I do, you believe that there are certain laws that are always obeyed.” — Stephen Hawking

If you understand the universe, you control it, in a way.” — Stephen Hawking

Imaginary time is a new dimension, at right angles to ordinary, real time.” — Stephen Hawking

In Britain, like most of the developed world, stem-cell research is regarded as a great opportunity. America will be left behind if it doesn’t change policy.” — Stephen Hawking

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.” — Stephen Hawking

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

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.” — Stephen Hawking

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.” — Stephen Hawking

Intelligence is the ability to adapt to change.” — Stephen Hawking

It is extremely important to me to write for children.” — Stephen Hawking

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.” — Stephen Hawking

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.” — Stephen Hawking

It is not clear that intelligence has any long-term survival value.” — Stephen Hawking

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

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.” — Stephen Hawking

It’s a pity that nobody has found an exploding black hole. If they had, I would have won a Nobel prize.” — Stephen Hawking

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.” — Stephen Hawking

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

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.” — Stephen Hawking

Life would be tragic if it weren’t funny.” — Stephen Hawking

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.” — Stephen Hawking

M-theory is the unified theory Einstein was hoping to find.” — Stephen Hawking

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

Many people find the universe confusing – it’s not.” — Stephen Hawking

Maybe I don’t have the most common kind of motor neuron disease, which usually kills in two or three years.” — Stephen Hawking

Most people don’t have time to master the very mathematical details of theoretical physics.” — Stephen Hawking

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.” — Stephen Hawking

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.” — Stephen Hawking

My discovery that black holes emit radiation raised serious problems of consistency with the rest of physics. I have now resolved these problems, but the answer turned out to be not what I expected.” — Stephen Hawking

My father was a research scientist in tropical medicine, so I always assumed I would be a scientist, too. I felt that medicine was too vague and inexact, so I chose physics.” — Stephen Hawking

My first popular book, ‘A Brief History of Time,’ aroused a great deal of interest, but many found it difficult to understand.” — Stephen Hawking

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

My ideal role would be a baddie in a James Bond film. I think the wheelchair and the computer voice would fit the part.” — Stephen Hawking

My three children have brought me great joy.” — Stephen Hawking

My wife and I love each other very much.” — Stephen Hawking

My work and my family are very important to me.” — Stephen Hawking

No one can resist the idea of a crippled genius.” — Stephen Hawking

No one undertakes research in physics with the intention of winning a prize. It is the joy of discovering something no one knew before.” — Stephen Hawking

Not only does God play dice, but… he sometimes throws them where they cannot be seen.” — Stephen Hawking

Nothing cannot exist forever.” — Stephen Hawking

Observations indicate that the universe is expanding at an ever increasing rate. It will expand forever, getting emptier and darker.” — Stephen Hawking

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.” — Stephen Hawking

One can’t predict the weather more than a few days in advance.” — Stephen Hawking

One cannot really argue with a mathematical theorem.” — Stephen Hawking

Only black holes of very low mass would emit a significant amount of radiation.” — Stephen Hawking

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.” — Stephen Hawking

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.” — Stephen Hawking

People who boast about their I.Q. are losers.” — Stephen Hawking

People won’t have time for you if you are always angry or complaining.” — Stephen Hawking

Perhaps one day I will go into space.” — Stephen Hawking

Philosophers have not kept up with modern developments in science. Particularly physics.” — Stephen Hawking

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

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.” — Stephen Hawking

Science is increasingly answering questions that used to be the province of religion.” — Stephen Hawking

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

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.” — Stephen Hawking

Scientists have become the bearers of the torch of discovery in our quest for knowledge.” — Stephen Hawking

So long as the 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?” — Stephen Hawking

Some forms of motor neuron disease are genetically linked, but I have no indication that my kind is. No other member of my family has had it. But I would be in favour of abortion if there was a high risk.” — Stephen Hawking

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.” — Stephen Hawking

Some scientists think it may be possible to capture a wormhole and enlarge it many trillions of times to make it big enough for a human or even a spaceship to enter.” — Stephen Hawking

Someone told me that each equation I included in the book would halve the sales.” — Stephen Hawking

Sometimes I wonder if I’m as famous for my wheelchair and disabilities as I am for my discoveries.” — Stephen Hawking

Stem cell research is the key to developing cures for degenerative conditions like Parkinson’s and motor neuron disease from which I and many others suffer. The fact that the cells may come from embryos is not an objection, because the embryos are going to die anyway.” — Stephen Hawking

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.” — Stephen Hawking

The Paralympic Games is about transforming our perception of the world.” — Stephen Hawking

The Planck satellite may detect the imprint of the gravitational waves predicted by inflation. This would be quantum gravity written across the sky.” — Stephen Hawking

The cyclic universe theory predicts no gravitational waves from the early universe.” — Stephen Hawking

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

The fastest manned vehicle in history was Apollo 10. It reached 25,000 mph.” — Stephen Hawking

The human race may be the only intelligent beings in the galaxy.” — Stephen Hawking

The media need superheroes in science just as in every sphere of life, but there is really a continuous range of abilities with no clear dividing line.” — Stephen Hawking

The missing link in cosmology is the nature of dark matter and dark energy.” — Stephen Hawking

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

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.” — Stephen Hawking

The universe is governed by science. But science tells us that we can’t solve the equations, directly in the abstract.” — Stephen Hawking

The universe is not indifferent to our existence – it depends on it.” — Stephen Hawking

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?” — Stephen Hawking

The voice I use is a very old hardware speech synthesizer made in 1986. I keep it because I have not heard a voice I like better and because I have identified with it.” — Stephen Hawking

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.” — Stephen Hawking

Theology is unnecessary.” — Stephen Hawking

Theoretical physics is one of the few fields in which being disabled is no handicap – it is all in the mind.” — Stephen Hawking

There are grounds for cautious optimism that we may now be near the end ofthe search for the ultimate laws of nature.” — Stephen Hawking

There are no black holes in the sense of regimes from which light can’t escape to infinity.” — Stephen Hawking

There are plenty of dead scientists I admire, but I can’t think of any living ones. This is probably because it is only in retrospect that one can see who made the important contributions.” — Stephen Hawking

There are too many accidents that can befall life on a single planet.” — Stephen Hawking

There could be shadow galaxies, shadow stars, and even shadow people.” — Stephen Hawking

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.” — Stephen Hawking

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

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.” — Stephen Hawking

There is no unique picture of reality.” — Stephen Hawking

There is nothing bigger or older than the universe.” — Stephen Hawking

There’s no way to remove the observer – us – from our perceptions of the world.” — Stephen Hawking

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.” — Stephen Hawking

Time can behave like another direction in space under extreme conditions.” — Stephen Hawking

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.” — Stephen Hawking

Time travel was once considered scientific heresy, and I used to avoid talking about it for fear of being labelled a ‘crank.’” — Stephen Hawking

To confine our attention to terrestrial matters would be to limit the human spirit.” — Stephen Hawking

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.” — Stephen Hawking

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

Using e-mail, I can communicate with scientists all over the world.” — Stephen Hawking

Wagner manages to convey emotion with music better than anyone, before or since.” — Stephen Hawking

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.” — Stephen Hawking

We are all now connected by the Internet, like neurons in a giant brain.” — Stephen Hawking

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.” — Stephen Hawking

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.” — Stephen Hawking

We are the product of quantum fluctuations in the very early universe.” — Stephen Hawking

We live in a bewildering world.” — Stephen Hawking

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.” — Stephen Hawking

We must develop as quickly as possible technologies that make possible a direct connection between brain and computer, so that artificial brains contribute to human intelligence rather than opposing it.” — Stephen Hawking

We only have to look at ourselves to see how intelligent life might develop into something we wouldn’t want to meet.” — Stephen Hawking

We should seek the greatest value of our action.” — Stephen Hawking

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.” — Stephen Hawking

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

What I’d really like to control is not machines, but people.” — Stephen Hawking

What was God doing before the divine creation?” — Stephen Hawking

When one’s expectations are reduced to zero, one really appreciates everything one does have.” — Stephen Hawking

When we understand string theory, we will know how the universe began. It won’t have much effect on how we live, but it is important to understand where we come from and what we can expect to find as we explore.” — Stephen Hawking

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.” — Stephen Hawking

Why are we here? Where do we come from? Traditionally, these are questions for philosophy, but philosophy is dead.” — Stephen Hawking

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.” — Stephen Hawking

Women. They are a complete mystery.” — Stephen Hawking

Work gives you meaning and purpose and life is empty without it.” — Stephen Hawking

You can’t regulate every lab in the world.” — Stephen Hawking