Page 40 - Nature Of Space And Time

3. Quantum Cosmology
S. W. Hawking

In my third lecture I shall turn to cosmology. Cosmology used to be considered a
pseudo-science and the preserve of physicists who may have done useful work in their

earlier years but who had gone mystic in their dotage. There were two reasons for this.
The rst was that there was an almost total absence of reliable observations. Indeed,
until the 1920s about the only important cosmological observation was that the sky at

night is dark. But people didn't appreciate the signi cance of this. However, in recent
years the range and quality of cosmological observations has improved enormously with
developments in technology. So this objection against regarding cosmology as a science,
that it doesn't have an observational basis is no longer valid.
There is, however, a second and more serious objection. Cosmology can not predict

anything about the universe unless it makes some assumption about the initial conditions.
Without such an assumption, all one can say is that things are as they are now because
they were as they were at an earlier stage. Yet many people believe that science should be

concerned only with the local laws which govern how the universe evolves in time. They
would feel that the boundary conditions for the universe that determine how the universe
began were a question for metaphysics or religion rather than science.
The situation was made worse by the theorems that Roger and I proved. These
showed that according to general relativity there should be a singularity in our past. At

this singularity the eld equations could not be de ned. Thus classical general relativity
brings about its own downfall: it predicts that it can't predict the universe.
Although many people welcomed this conclusion, it has always profoundly disturbed
me. If the laws of physics could break down at the begining of the universe, why couldn't

they break down any where. In quantum theory it is a principle that anything can happen if
it is not absolutely forbidden. Once one allows that singular histories could take part in the
path integral they could occur any where and predictability would disappear completely.
If the laws of physics break down at singularities, they could break down any where.

The only way to have a scienti c theory is if the laws of physics hold everywhere
including at the begining of the universe. One can regard this as a triumph for the
principles of democracy: Why should the begining of the universe be exempt from the
laws that apply to other points. If all points are equal one can't allow some to be more

equal than others.
To implement the idea that the laws of physics hold everywhere, one should take the
path integral only over non-singular metrics. One knows in the ordinary path integral case

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