Brief Answers to the Big Questions

by Stephen Hawking

A nuclear war is still the most immediate danger, but there are others, such as the release of a genetically engineered virus. Or the greenhouse effect becoming unstable.

But we are now entering a new phase of what might be called self-designed evolution, in which we will be able to change and improve our DNA.

Nevertheless, I am sure that during this century people will discover how to modify both intelligence and instincts like aggression.

If the human race manages to redesign itself, to reduce or eliminate the risk of self-destruction, it will probably spread out and colonise other planets and stars.

a round trip from us to the nearest star would take at least eight years, and to the centre of the galaxy about 50,000 years. In science fiction, they overcome this difficulty by space warps, or travel through extra dimensions. But I don’t think these will ever be possible, no matter how intelligent life becomes.

These machines would be a new form of life, based on mechanical and electronic components rather than macromolecules. They could eventually replace DNA-based life, just as DNA may have replaced an earlier form of life.

We are used to thinking of intelligent life as an inevitable consequence of evolution, but what if it isn’t? The Anthropic Principle should warn us to be wary of such arguments. It is more likely that evolution is a random process, with intelligence as only one of a large number of possible outcomes.

It is not even clear that intelligence has any long-term survival value.

If this figure is correct, it would mean that intelligent life on Earth has developed only because of the lucky chance that there have been no major collisions in the last sixty-six million years.

Meeting a more advanced civilisation, at our present stage, might be a bit like the original inhabitants of America meeting Columbus—and I don’t think they thought they were better off for it.

A scientific law is not a scientific law if it only holds when some supernatural being decides to let things run and not intervene.

In practice, however, our ability to predict the future is severely limited by the complexity of the equations, and the fact that they often have a property called chaos.

The energy in the packets or quanta is higher for ultra-violet and X-rays than for infra-red or visible light. It means that unless a body is very hot, like the Sun, it will not have enough energy to give off even a single quantum of ultra-violet or X-rays. That is why we don’t get sunburn from a cup of coffee.

This is summed up in the Uncertainty Principle that Heisenberg formulated; the uncertainty in the position of a particle times the uncertainty in its speed is always greater than a quantity called Planck’s constant, divided by twice the mass of the particle.

Although quantum mechanics leads to uncertainty when we try to predict both the position and the speed, it still allows us to predict, with certainty, one combination of position and speed.

It is said that fact is sometimes stranger than fiction, and nowhere is that more true than in the case of black holes. Black holes are stranger than anything dreamed up by science-fiction writers, but they are firmly matters of science fact.

The escape velocity is just over 11 kilometres per second for the Earth, and about 617 kilometres per second for the Sun. Both of these are much higher than the speed of real cannon balls. But they are low compared to the speed of light, which is 300,000 kilometres per second. Thus light can get away from the Earth or Sun without much difficulty. However, Michell argued that there could be stars that were much more massive than the Sun which had escape velocities greater than the speed of light. We would not be able to see them, because any light they sent out would be dragged back by gravity. Thus they would be what Michell called dark stars, what we now call black holes.

All our theories of space are formulated on the assumption that space–time is smooth and nearly flat, so they break down at the singularity, where the curvature of space–time is infinite. In fact, it marks the end of space and time itself.

The Einstein equations can’t be defined at a singularity. This means that at this point of infinite density one can’t predict the future.

When John Wheeler introduced the term “black hole” in 1967, it replaced the earlier name of “frozen star.” Wheeler’s coinage emphasised that the remnants of collapsed stars are of interest in their own right, independently of how they were formed.

A black hole has a boundary called the event horizon. It is where gravity is just strong enough to drag light back and prevent it from escaping.

If you fall towards a black hole feet first, gravity will pull harder on your feet than your head, because they are nearer the black hole. The result is that you will be stretched out lengthwise, and squashed in sideways. If the black hole has a mass of a few times our Sun, you would be torn apart and made into spaghetti before you reached the horizon.

The famous second law of thermodynamics says that entropy always increases with time.

If one measures exactly where something is, then its speed is undetermined. If one measures the speed of something, then its position is undetermined.

This means that a black hole must contain a lot of information that is hidden from the outside world. But there is a limit to the amount of information one can pack into a region of space. Information requires energy, and energy has mass by Einstein’s famous equation, E = mc2. So, if there’s too much information in a region of space, it will collapse into a black hole, and the size of the black hole will reflect the amount of information. It is like piling more and more books into a library. Eventually, the shelves will give way and the library will collapse into a black hole.

Now, in the presence of a black hole, one member of a pair of virtual particles may fall into the hole, leaving the other member without a partner with which to engage in mutual annihilation. The forsaken particle or antiparticle may fall into the black hole after its partner, but it may also escape to infinity, where it appears to be radiation emitted by the black hole.

People have searched for mini black holes of this mass, but have so far not found any. This is a pity because, if they had, I would have got a Nobel Prize.

Even worse, if determinism breaks down, we can’t be sure of our past history either. The history books and our memories could just be illusions. It is the past that tells us who we are. Without it, we lose our identity.

The same is true of space–time. If one puts objects into a space–time, the translational and rotational symmetries get broken. And introducing objects into a space–time is what produces gravity.

space–time far away from any matter has an infinite collection of symmetries known as supertranslations.

A conserved quantity is a quantity that does not change as a system evolves. These are generalisations of more familiar conserved quantities. For example, if space–time does not change in time, then energy is conserved. If space–time looks the same at different points in space, then momentum is conserved.

We know that the three discernible properties of black holes are their mass, their charge and their angular momentum.

If it were a stellar mass black hole, you would be made into spaghetti before reaching the horizon. On the other hand, if it were a supermassive black hole, you would cross the horizon with ease, but be crushed out of existence at the singularity.

But today’s science fiction is often tomorrow’s science fact.

Einstein had realised in 1905 that space and time are intimately connected with each other, which is when his theory of special relativity was born, relating space and time to each other. One can describe the location of an event by four numbers. Three numbers describe the position of the event. They could be miles north and east of Oxford Circus and the height above sea level. On a larger scale they could be galactic latitude and longitude and distance from the centre of the galaxy.

The fourth number is the time of the event. Thus one can think of space and time together as a four-dimensional entity called space–time.

However, in a remarkable paper written in 1905 when he was a clerk in the Swiss patent office, Einstein showed that the time and position at which one thought an event occurred depended on how one was moving. This meant that time and space were inextricably bound up with each other.

space–time was warped or distorted by the matter and energy in it, and this theory is known as general relativity. We can actually observe this warping of space–time produced by the mass of the Sun in the slight bending of light or radio waves passing close to the Sun.

So can space and time be warped enough to meet the demands from science fiction for things like hyperspace drives, wormholes or time travel?

The idea is that according to quantum theory the universe doesn’t just have a unique single history. Instead the universe has every single possible history, each with its own probability.

We have presented our planet with the disastrous gift of climate change.

The universe is a violent place. Stars engulf planets, supernovae fire lethal rays across space, black holes bump into each other and asteroids hurtle around at hundreds of miles a second.

It is this driven curiosity that sent explorers to prove the Earth is not flat and it is the same instinct that sends us to the stars at the speed of thought, urging us to go there in reality.

For example, the film 2001: A Space Odyssey showed us with a base on the Moon and launching a manned, or should I say personned, flight to Jupiter.

you stacked the new books being published next to each other, at the present rate of production you would have to move at ninety miles an hour just to keep up with the end of the line.

Even what we think of as empty space is full of particles moving in closed loops in space and time. That is, they move forwards in time on one side of the loop and backwards in time on the other side.

At the moment computers have an advantage of speed, but they show no sign of intelligence. This is not surprising because our present computers are less complex than the brain of an earthworm,

Not to leave planet Earth would be like castaways on a desert island not trying to escape. We need to explore the solar system to find out where humans could live.

We could have a base on the Moon within thirty years, reach Mars in fifty years and explore the moons of the outer planets in 200 years.

Going into space won’t be cheap, but it would take only a small proportion of world resources. NASA’s budget has remained roughly constant in real terms since the time of the Apollo landings, but it has decreased from 0.3 per cent of US GDP in 1970 to about 0.1 per cent in 2017. Even if we were to increase the international budget twenty times, to make a serious effort to go into space, it would only be a small fraction of world GDP.