Brief Answers to the Big Questions

by Stephen Hawking

Although the universe seems to be much the same at each position in space, it is definitely changing in time. This was not realised until the early years of the last century. Up to then, it was thought the universe was essentially constant in time. It might have existed for an infinite time, but that seemed to lead to absurd conclusions. If stars had been radiating for an infinite time, they would have heated up the universe until it reached their own temperature. Even at night, the whole sky would be as bright as the Sun, because every line of sight would have ended either on a star or on a cloud of dust that had been heated up until it was as hot as the stars. So the observation that we have all made, that the sky at night is dark, is very important. It implies that the universe cannot have existed for ever, in the state we see today. Something must have happened in the past to make the stars turn on a finite time ago. Then the light from very distant stars wouldn’t have had time to reach us yet. This would explain why the sky at night isn’t glowing in every direction.

Observational evidence to confirm the idea that the universe had a very dense beginning came in October 1965, a few months after my first singularity result, with the discovery of a faint background of microwaves throughout space. These microwaves are the same as those in your microwave oven, but very much less powerful. They would heat your pizza only to minus 270.4 degrees centigrade (minus 518.72 degrees Fahrenheit), not much good for defrosting the pizza, let alone cooking it. You can actually observe these microwaves yourself. Those of you who remember analogue televisions have almost certainly observed these microwaves. If you ever set your television to an empty channel, a few per cent of the snow you saw on the screen was caused by this background of microwaves. The only reasonable interpretation of the background is that it is radiation left over from an early very hot and dense state. As the universe expanded, the radiation would have cooled until it is just the faint remnant we observe today.

Why don’t we live in a history in which eight of the dimensions are curled up small, leaving only two dimensions that we notice? A two-dimensional animal would have a hard job digesting food. If it had a gut that went right through, like we have, it would divide the animal in two, and the poor creature would fall apart. So two flat directions are not enough for anything as complicated as intelligent life. There is something special about three space dimensions. In three dimensions, planets can have stable orbits around stars. This is a consequence of gravitation obeying the inverse square law, as discovered by Robert Hooke in 1665 and elaborated on by Isaac Newton. Think about the gravitational attraction of two bodies at a particular distance. If that distance is doubled, then the force between them is divided by four. If the distance is tripled then the force is divided by nine, if quadrupled, then the force is divided by sixteen and so on. This leads to stable planetary orbits. Now let’s think about four space dimensions. There gravitation would obey an inverse cube law. If the distance between two bodies is doubled, then the gravitational force would be divided by eight, tripled by twenty-seven and if quadrupled, by sixty-four. This change to an inverse cube law prevents planets from having stable orbits around their suns. They would either fall into their sun or escape to the outer darkness and cold. Similarly, the orbits of electrons in atoms would not be stable, so ...

What we normally think of as “life” is based on chains of carbon atoms, with a few other atoms such as nitrogen or phosphorus. One can speculate that one might have life with some other chemical basis, such as silicon, but carbon seems the most favourable case, because it has the richest chemistry. That carbon atoms should exist at all, with the properties that they have, requires a fine adjustment of physical constants, such as the QCD scale, the electric charge and even the dimension of space–time. If these constants had significantly different values, either the nucleus of the carbon atom would not be stable or the electrons would collapse in on the nucleus. At first sight, it seems remarkable that the universe is so finely tuned. Maybe this is evidence that the universe was specially designed to produce the human race. However, one has to be careful about such arguments, because of the Anthropic Principle, the idea that our theories about the universe must be compatible with our own existence. This is based on the self-evident truth that if the universe had not been suitable for life we wouldn’t be asking why it is so finely adjusted.

The process of biological evolution was very slow at first. It took about two and a half billion years before the earliest cells evolved into multi-cellular organisms. But it took less than another billion years for some of these to evolve into fish, and for some of the fish, in turn, to evolve into mammals. Then evolution seems to have speeded up even more. It took only about a hundred million years to develop from the early mammals to us. The reason is that the early mammals already contained their versions of the essential organs we have. All that was required to evolve from early mammals to humans was a bit of fine-tuning.

This was the development of language, and particularly written language. It meant that information could be passed on from generation to generation, other than genetically through DNA. There has been some detectable change in human DNA, brought about by biological evolution, in the 10,000 years of recorded history, but the amount of knowledge handed on from generation to generation has grown enormously. I have written books to tell you something of what I have learned about the universe in my long career as a scientist, and in doing so I am transferring knowledge from my brain to the page so you can read it. The DNA in a human egg or sperm contains about three billion base pairs of nitrogenous bases. However, much of the information coded in this sequence seems to be redundant or is inactive. So the total amount of useful information in our genes is probably something like a hundred million bits. One bit of information is the answer to a yes/no question. By contrast, a paperback novel might contain two million bits of information. Therefore, a human is equivalent to about fifty Harry Potter books, and a major national library can contain about five million books—or about ten trillion bits. The amount of information handed down in books or via the internet is 100,000 times as much as there is in DNA. Even more important is the fact that the information in books can be changed, and updated, much more rapidly. It has taken us several million years to evolve from less advanced...

According to the classical nineteenth-century ideas dating back to Laplace, a hot body, like a piece of red-hot metal, should give off radiation. It would lose energy in radio waves, the infra-red, visible light, ultra-violet, X-rays and gamma rays, all at the same rate. This would mean not only that we would all die of skin cancer, but also that everything in the universe would be at the same temperature, which clearly it isn’t. However, Planck showed one could avoid this disaster if one gave up the idea that the amount of radiation could have just any value, and said instead that radiation came only in packets or quanta of a certain size. It is a bit like saying that you can’t buy sugar loose in the supermarket, it has to be in kilogram bags. 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. Planck regarded the idea of quanta as just a mathematical trick, and not as having any physical reality, whatever that might mean. However, physicists began to find other behaviour that could be explained only in terms of quantities having discrete or quantised values rather than continuously variable ones. For example, it was found that elementary particles behaved rather like little tops, spinning about an axis. But the amo...

It was some time before people realised the implications of this quantum behaviour for determinism. It was not until 1927 that Werner Heisenberg, another German physicist, pointed out that you couldn’t measure simultaneously both the position and speed of a particle exactly. To see where a particle is, one has to shine light on it. But by Planck’s work one can’t use an arbitrarily small amount of light. One has to use at least one quantum. This will disturb the particle and change its speed in a way that can’t be predicted. To measure the position of the particle accurately, you will have to use light of short wavelength, like ultra-violet, X-rays or gamma rays. But again, by Planck’s work, quanta of these forms of light have higher energies than those of visible light. So they will disturb the speed of the particle more. It is a no-win situation: the more accurately you try to measure the position of the particle, the less accurately you can know the speed, and vice versa. 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.

One can show that to create a wormhole one needs to warp space–time in the opposite way to that in which normal matter warps it. Ordinary matter curves space–time back on itself, like the surface of the Earth. However, to create a wormhole one needs matter that warps space–time in the opposite way, like the surface of a saddle. The same is true of any other way of warping space–time to allow travel to the past if the universe didn’t begin so warped that it allowed time travel. What one would need would be matter with negative mass and negative energy density to make space–time warp in the way required. Energy is rather like money. If you have a positive bank balance, you can distribute it in various ways. But, according to the classical laws that were believed until quite recently, you weren’t allowed to have an energy overdraft. So these classical laws would have ruled out us being able to warp the universe in the way required to allow time travel. However, the classical laws were overthrown by quantum theory, which is the other great revolution in our picture of the universe apart from general relativity. Quantum theory is more relaxed and allows you to have an overdraft on one or two accounts. If only the banks were as accommodating. In other words, quantum theory allows the energy density to be negative in some places provided it is positive in others. The reason quantum theory can allow the energy density to be negative is that it is based on the Uncertainty Principle...

These particle–antiparticle pairs are said to be virtual because one cannot measure them directly with a particle detector. However, one can observe their effects indirectly. One way of doing this is by what is called the Casimir effect. Imagine that you have two parallel metal plates a short distance apart. The plates act like mirrors for the virtual particles and anti-particles. This means that the region between the plates is a bit like an organ pipe and will only admit light waves of certain resonant frequencies. The result is that there are a slightly different number of vacuum fluctuations or virtual particles between the plates than there are outside them, where vacuum fluctuations can have any wavelength. The difference in the number of virtual particles between the plates compared with outside the plates means that they don’t exert as much pressure on one side of the plates compared with the other. There is thus a slight force pushing the plates together. This force has been measured experimentally. So, virtual particles actually exist and produce real effects.

At no time in the 10,000 years or so since the last Ice Age has the human race been in a state of constant knowledge and fixed technology. There have been a few setbacks, like what we used to call the Dark Ages after the fall of the Roman Empire. But the world’s population, which is a measure of our technological ability to preserve life and feed ourselves, has risen steadily, with a few hiccups like the Black Death. In the last 200 years the growth has at times been exponential—and the world population has jumped from 1 billion to about 7.6 billion. Other measures of technological development in recent times are electricity consumption, or the number of scientific articles. They too show near-exponential growth.

At the beginning of the twentieth century we understood the workings of nature on the scales of classical physics that are good down to about a hundredth of a millimetre. The work on atomic physics in the first thirty years of the century took our understanding down to lengths of a millionth of a millimetre. Since then, research on nuclear and high-energy physics has taken us to length scales that are smaller by a further factor of a billion. It might seem that we could go on forever discovering structures on smaller and smaller length scales. However, there is a limit to this series as with a series of nested Russian dolls. Eventually one gets down to a smallest doll, which can’t be taken apart any more. In physics the smallest doll is called the Planck length and is a millimetre divided by a 100,000 billion billion billion. We are not about to build particle accelerators that can probe to distances that small. They would have to be larger than the solar system and they are not likely to be approved in the present financial climate. However, there are consequences of our theories that can be tested by much more modest machines.

It won’t be possible to probe down to the Planck length in the laboratory, though we can study the Big Bang to get observational evidence at higher energies and shorter length scales than we can achieve on Earth. However, to a large extent we shall have to rely on mathematical beauty and consistency to find the ultimate theory of everything.

The Star Trek vision of the future in which we achieve an advanced but essentially static level may come true in respect of our knowledge of the basic laws that govern the universe. But I don’t think we will ever reach a steady state in the uses we make of these laws. The ultimate theory will place no limit on the complexity of systems that we can produce, and it is...

Lincoln Steffens once said, “I have seen the future and it works.” He was actually talking about the Soviet Union, which we now know didn’t work very well.

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.