should therefore not be surprised if they observe that their locality in the universe satisfies the conditions that are
necessary for their existence. It is a bit like a rich person living in a wealthy neighborhood not seeing any poverty.
One example of the use of the weak anthropic principle is to “explain” why the big bang occurred about ten thousand
million years ago – it takes about that long for intelligent beings to evolve. As explained above, an early generation of
stars first had to form. These stars converted some of the original hydrogen and helium into elements like carbon and
oxygen, out of which we are made. The stars then exploded as supernovas, and their debris went to form other stars
and planets, among them those of our Solar System, which is about five thousand million years old. The first one or
two thousand million years of the earth’s existence were too hot for the development of anything complicated. The
remaining three thousand million years or so have been taken up by the slow process of biological evolution, which
has led from the simplest organisms to beings who are capable of measuring time back to the big bang.
Few people would quarrel with the validity or utility of the weak anthropic principle. Some, however, go much further
and propose a strong version of the principle. According to this theory, there are either many different universes or
many different regions of a single universe, each with its own initial configuration and, perhaps, with its own set of
laws of science. In most of these universes the conditions would not be right for the development of complicated
organisms; only in the few universes that are like ours would intelligent beings develop and ask the question, “Why is
the universe the way we see it?” The answer is then simple: if it had been different, we would not be here!
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. We cannot, at the moment at least,
predict the values of these numbers from theory – we have to find them by observation. It may be that one day we
shall discover a complete unified theory that predicts them all, but it is also possible that some or all of them vary
from universe to universe or within a single universe. The remarkable fact is that the values of these numbers seem
to have been very finely adjusted to make possible the development of life. For example, if the electric charge of the
electron had been only slightly different, stars either would have been unable to burn hydrogen and helium, or else
they would not have exploded. Of course, there might be other forms of intelligent life, not dreamed of even by
writers of science fiction, that did not require the light of a star like the sun or the heavier chemical elements that are
made in stars and are flung back into space when the stars explode. Nevertheless, it seems clear that there are
relatively few ranges of values for the numbers that would allow the development of any form of intelligent life. Most
sets of values would give rise to universes that, although they might be very beautiful, would contain no one able to
wonder at that beauty. One can take this either as evidence of a divine purpose in Creation and the choice of the
laws of science or as support for the strong anthropic principle.
There are a number of objections that one can raise to the strong anthropic principle as an explanation of the
observed state of the universe. First, in what sense can all these different universes be said to exist? If they are
really separate from each other, what happens in another universe can have no observable consequences in our
own universe. We should therefore use the principle of economy and cut them out of the theory. If, on the other
hand, they are just different regions of a single universe, the laws of science would have to be the same in each
region, because otherwise one could not move continuously from one region to another. In this case the only
difference between the regions would be their initial configurations and so the strong anthropic principle would
reduce to the weak one.
A second objection to the strong anthropic principle is that it runs against the tide of the whole history of science. We
have developed from the geocentric cosmologies of Ptolemy and his forebears, through the heliocentric cosmology
of Copernicus and Galileo, to the modern picture in which the earth is a medium-sized planet orbiting around an
average star in the outer suburbs of an ordinary spiral galaxy, which is itself only one of about a million million
galaxies in the observable universe. Yet the strong anthropic principle would claim that this whole vast construction
exists simply for our sake. This is very hard to believe. Our Solar System is certainly a prerequisite for our existence,
hand one might extend this to the whole of our galaxy to allow for an earlier generation of stars that created the
heavier elements. But there does not seem to be any need for all those other galaxies, nor for the universe to be so
uniform and similar in every direction on the large scale.
One would feel happier about the anthropic principle, at least in its weak version, if one could show that quite a
number of different initial configurations for the universe would have evolved to produce a universe like the one we
observe. If this is the case, a universe that developed from some sort of random initial conditions should contain a
number of regions that are smooth and uniform and are suitable for the evolution of intelligent life. On the other hand,
if the initial state of the universe had to be chosen extremely carefully to lead to something like what we see around
us, the universe would be unlikely to contain any region in which life would appear. In the hot big bang model
described above, there was not enough time in the early universe for heat to have flowed from one region to another.
This means that the initial state of the universe would have to have had exactly the same temperature everywhere in
order to account for the fact that the microwave back-ground has the same temperature in every direction we look.
The initial rate of expansion also would have had to be chosen very precisely for the rate of expansion still to be so
close to the critical rate needed to avoid recollapse. This means that the initial state of the universe must have been
very carefully chosen indeed if the hot big bang model was correct right back to the beginning of time. It would be
very difficult to explain why the universe should have begun in just this way, except as the act of a God who intended
to create beings like us.
In an attempt to find a model of the universe in which many different initial configurations could have evolved to
something like the present universe, a scientist at the Massachusetts Institute of Technology, Alan Guth, suggested
that the early universe might have gone through a period of very rapid expansion. This expansion is said to be
“inflationary,” meaning that the universe at one time expanded at an increasing rate rather than the decreasing rate
that it does today. According to Guth, the radius of the universe increased by a million million million million million (1
with thirty zeros after it) times in only a tiny fraction of a second.
Guth suggested that the universe started out from the big bang in a very hot, but rather chaotic, state. These high
temperatures would have meant that the particles in the universe would be moving very fast and would have high
energies. As we discussed earlier, one would expect that at such high temperatures the strong and weak nuclear
forces and the electromagnetic force would all be unified into a single force. As the universe expanded, it would cool,
and particle energies would go down. Eventually there would be what is called a phase transition and the symmetry
between the forces would be broken: the strong force would become different from the weak and electromagnetic
forces. One common example of a phase transition is the freezing of water when you cool it down. Liquid water is
symmetrical, the same at every point and in every direction. However, when ice crystals form, they will have definite
positions and will be lined up in some direction. This breaks water’s symmetry.
In the case of water, if one is careful, one can “supercool” it: that is, one can reduce the temperature below the
freezing point (OoC) without ice forming. Guth suggested that the universe might behave in a similar way: the
temperature might drop below the critical value without the symmetry between the forces being broken. If this
happened, the universe would be in an unstable state, with more energy than if the symmetry had been broken. This
special extra energy can be shown to have an antigravitational effect: it would have acted just like the cosmological
constant that Einstein introduced into general relativity when he was trying to construct a static model of the
universe. Since the universe would already be expanding just as in the hot big bang model, the repulsive effect of
this cosmological constant would therefore have made the universe expand at an ever-increasing rate. Even in
regions where there were more matter particles than average, the gravitational attraction of the matter would have
been outweighed by the repulsion of the effective cosmological constant. Thus these regions would also expand in
an accelerating inflationary manner. As they expanded and the matter particles got farther apart, one would be left
with an expanding universe that contained hardly any particles and was still in the supercooled state. Any
irregularities in the universe would simply have been smoothed out by the expansion, as the wrinkles in a balloon are
smoothed away when you blow it up. Thus the present smooth and uniform state of the universe could have evolved
from many different non-uniform initial states.
In such a universe, in which the expansion was accelerated by a cosmological constant rather than slowed down by
the gravitational attraction of matter, there would be enough time for light to travel from one region to another in the
early universe. This could provide a solution to the problem, raised earlier, of why different regions in the early
universe have the same properties. Moreover, the rate of expansion of the universe would automatically become
very close to the critical rate determined by the energy density of the universe. This could then explain why the rate
of expansion is still so close to the critical rate, without having to assume that the initial rate of expansion of the
universe was very carefully chosen.
The idea of inflation could also explain why there is so much matter in the universe. There are something like ten
million million million million million million million million million million million million million million (1 with eighty
zeros after it) particles in the region of the universe that we can observe. Where did they all come from? The answer
is that, in quantum theory, particles can be created out of energy in the form of particle/antiparticle pairs. But that just
raises the question of where the energy came from. The answer is that the total energy of the universe is exactly
zero. The matter in the universe is made out of positive energy. However, the matter is all attracting itself by gravity.
Two pieces of matter that are close to each other have less energy than the same two pieces a long way apart,
because you have to expend energy to separate them against the gravitational force that is pulling them together.
Thus, in a sense, the gravitational field has negative energy. In the case of a universe that is approximately uniform
in space, one can show that this negative gravitational energy exactly cancels the positive energy represented by the
matter. So the total energy of the universe is zero.
Now twice zero is also zero. Thus the universe can double the amount of positive matter energy and also double the
negative gravitational energy without violation of the conservation of energy. This does not happen in the normal
expansion of the universe in which the matter energy density goes down as the universe gets bigger. It does happen,
however, in the inflationary expansion because the energy density of the supercooled state remains constant while
the universe expands: when the universe doubles in size, the positive matter energy and the negative gravitational
energy both double, so the total energy remains zero. During the inflationary phase, the universe increases its size
by a very large amount. Thus the total amount of energy available to make particles becomes very large. As Guth
has remarked, “It is said that there’s no such thing as a free lunch. But the universe is the ultimate free lunch.”
The universe is not expanding in an inflationary way today. Thus there has to be some mechanism that would
eliminate the very large effective cosmological constant and so change the rate of expansion from an accelerated
one to one that is slowed down by gravity, as we have today. In the inflationary expansion one might expect that
eventually the symmetry between the forces would be broken, just as super-cooled water always freezes in the end.
The extra energy of the unbroken symmetry state would then be released and would reheat the universe to a
temperature just below the critical temperature for symmetry between the forces. The universe would then go on to
expand and cool just like the hot big bang model, but there would now be an explanation of why the universe was
expanding at exactly the critical rate and why different regions had the same temperature.
In Guth’s original proposal the phase transition was supposed to occur suddenly, rather like the appearance of ice
crystals in very cold water. The idea was that “bubbles” of the new phase of broken symmetry would have formed in
the old phase, like bubbles of steam surrounded by boiling water. The bubbles were supposed to expand and meet
up with each other until the whole universe was in the new phase. The trouble was, as I and several other people
pointed out, that the universe was expanding so fast that even if the bubbles grew at the speed of light, they would
be moving away from each other and so could not join up. The universe would be left in a very non-uniform state,
