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.
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 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Not to leave planet Earth would be like castaways on a desert island not trying to escape...Sending humans to other planets ... will shape the future of the human race in ways we don't yet understand, and may determine whether we have any future at all.
It surprises me how disinterested we are today about things like physics, space, the universe and philosophy of our existence, our purpose, our final destination. Its a crazy world out there. Be curious.
Remember to 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. And however difficult life may seem, there is always something you can do and succeed at. It matters that you don't just give up.
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 it to be in accordance with the Bible.
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.
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.
One, remember to look up at the stars and not down at your feet. Two, never give up work. Work gives you meaning and purpose and life is empty without it. Three, if you are lucky enough to find love, remember it is there and don't throw it away.
Nuclear weapons need large facilities, but genetic engineering can be done in a small lab. You can't regulate every lab in the world. The danger is that either by accident or design, we create a virus that destroys us.
In this way, Edwin Hubble worked out the distances to nine different galaxies. We now know that our galaxy is only one of some hundred thousand million that can be seen using modern telescopes, each galaxy itself containing some hundred thousand million stars.
I believe that the long-term future of the human race must be space and that it represents an important life insurance for our future survival, as it could prevent the disappearance of humanity by colonizing other planets,
I believe the simplest explanation is, there is no God. No one created the universe and no one directs our fate. This leads me to a profound realization that there probably is no heaven and no afterlife either. We have this one life to appreciate the grand design of the universe and for that, I am extremely grateful.
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?
It should soon be possible dramatically to increase the intelligence and life span of a few individuals [with the help of genetic engineering]. They and their offspring could become a master race. Evolution pays no regard to social justice. It was not fair on the Neanderthals they were replaced by modern humans.
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?
Gravity is so strong that space is bent round onto itself, making it rather like the surface of the earth. If one keeps traveling in a certain direction on the surface of the earth, one never comes up against an impassable barrier or falls over the edge, but eventually comes back to where one started.
The laws of science, as we know them at present, contain many fundamental numbers, like the size of the electric charge of the electron and the ratio of the masses of the proton and the electron .... The remarkable fact is that the values of these numbers seem to have been finely adjusted to make possible the development of life.
Now, radical forward thinking is offering hope for the future: Replacement body parts to order. A team of scientists in California believe that if you can design them on a computer, you should be able to print them out.
As we shall see, the concept of time has no meaning before the beginning of the universe. 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 preparing Hell for people who asked such questions.
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.
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.
The human race is just a chemical scum on a moderate-sized planet, orbiting around a very average star in the outer suburb of one among a hundred billion galaxies. We are so insignificant that I can't believe the whole universe exists for our benefit. That would be like saying that you would disappear if I closed my eyes.
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.
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.
The idea that we are alone in the universe seems to me completely implausible and arrogant, considering the number of planets and stars that we know exist, it's extremely unlikely that we are the only form of evolved life.
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.
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.
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.
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.