Stephen Hawking - The Beginning of Time

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The Beginning of Time

In this lecture, I would like to discuss whether time itself has a beginning, and whether it will

have an end. All the evidence seems to indicate, that the universe has not existed forever, but

that it had a beginning, about 15 billion years ago. This is probably the most remarkable

discovery of modern cosmology. Yet it is now taken for granted. We are not yet certain

whether the universe will have an end. When I gave a lecture in Japan, I was asked not to

mention the possible re-

collapse of the universe, because it might affect the stock market.

However, I can re-

assure anyone who is nervous about their investments that it is a bit early

to sell: even if the universe does come to an end, it won't be for at least twenty billion years.

By that time, maybe the GATT trade agreement will have come into effect.

The time scale of the universe is very long compared to that for human life. It was therefore

not surprising that until recently, the universe was thought to be essentially static, and

unchanging in time. On the other hand, it must have been obvious, that society is evolving in

culture and technology. This indicates that the present phase of human history can not have

been going for more than a few thousand years. Otherwise, we would be more advanced than

we are. It was therefore natural to believe that the human race, and maybe the whole

universe, had a beginning in the fairly recent past. However, many people were unhappy with

the idea that the universe had a beginning, because it seemed to imply the existence of a

supernatural being who created the universe. They preferred to believe that the universe, and

the human race, had existed forever. Their explanation for human progress was that there had

been periodic floods, or other natural disasters, which repeatedly set back the human race to a

primitive state.

This argument about whether or not the universe had a beginning, persisted into the 19th and

20th centuries. It was conducted mainly on the basis of theology and philosophy, with little

consideration of observational evidence. This may have been reasonable, given the notoriously

unreliable character of cosmological observations, until fairly recently. The cosmologist, Sir

Arthur Eddington, once said, 'Don't worry if your theory doesn't agree with the observations,

because they are probably wrong.' But if your theory disagrees with the Second Law of

Thermodynamics, it is in bad trouble. In fact, the theory that the universe has existed forever

is in serious difficulty with the Second Law of Thermodynamics. The Second Law, states that

disorder always increases with time. Like the argument about human progress, it indicates that

there must have been a beginning. Otherwise, the universe would be in a state of complete

disorder by now, and everything would be at the same temperature. In an infinite and

everlasting universe, every line of sight would end on the surface of a star. This would mean

that the night sky would have been as bright as the surface of the Sun. The only way of

avoiding this problem would be if, for some reason, the stars did not shine before a certain

time.

In a universe that was essentially static, there would not have been any dynamical reason,

why the stars should have suddenly turned on, at some time. Any such "lighting up time" would

have to be imposed by an intervention from outside the universe. The situation was different,

however, when it was realised that the universe is not static, but expanding. Galaxies are

moving steadily apart from each other. This means that they were closer together in the past.

One can plot the separation of two galaxies, as a function of time. If there were no

acceleration due to gravity, the graph would be a straight line. It would go down to zero

separation, about twenty billion years ago. One would expect gravity, to cause the galaxies to

accelerate towards each other. This will mean that the graph of the separation of two galaxies

will bend downwards, below the

straight line. So the time of zero separation, would have been

less than twenty billion years ago.

At this time, the Big Bang, all the matter in the universe, would have been on top of itself. The

density would have been infinite. It would have been what is called, a singularity. At a

singularity, all the laws of physics would have broken down. This means that the state of the

universe, after the Big Bang, will not depend on anything that may have happened before,

because the deterministic laws that govern the universe will break down in the Big Bang. The

universe will evolve from the Big Bang, completely independently of what it was like before.

Even the amount of matter in the universe, can be different to what it was before the Big Bang,

as the Law of Conservation of Matter, will break down at the Big Bang.

Since events before the Big Bang have no observational consequences, one may as well cut

them out of the theory, and say that time began at the Big Bang. Events before the Big Bang,

are simply not defined, because there's no way one could measure what happened at them.

This kind of beginning to the universe, and of time itself, is very different to the beginnings that

had been considered earlier. These had to be imposed on the universe by some external

agency. There is no dynamical reason why the motion of bodies in the solar system can not be

extrapolated back in time, far beyond four thousand and four BC, the date for the creation of

the universe, according to the book of Genesis. Thus it would require the direct intervention of

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Stephen Hawking - The Beginning of Time

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