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BIG BANG
The day
time began
Box with feature, 27 April 1996, page one
In my view it is the job of physics to explain
the world based on lawlike principles. Scientists
adopt differing attitudes to the metaphysical
problem of how to explain the principles
themselves. Some simply shrug and say we must
just accept the laws as a brute fact. Others
suggest that the laws must be what they are
from logical necessity. Yet others propose that
there exist many worlds, each with differing
laws, and that only a small subset of these
universes possess the rather special laws needed
if life and reflective beings like ourselves are to
emerge. Some sceptics rubbish the entire
discussion by claiming that the laws of physics
have no real existence anyway--they are
merely human inventions designed to help us
make sense of the physical world. It is hard to
see how the origin of the Universe could ever be
explained with a view like this.
In my experience, almost all physicists who
work on fundamental problems accept that the
laws of physics have some kind of independent
reality. With that view, it is possible to argue
that the laws of physics are logically prior to the
Universe they describe. That is, the laws of
physics stand at the base of a rational
explanatory chain, in the same way that the
axioms of Euclid stand at the base of the logical
scheme we call geometry. Of course, one
cannot prove that the laws of physics have to be
the starting point of an explanatory scheme,
but any attempt to explain the world rationally
has to have some starting point, and for most
scientists the laws of physics seem a very
satisfactory one. In the same way, one need not
accept Euclid's axioms as the starting point of
geometry; a set of theorems like Pythagoras's
would do equally well. But the purpose of
science (and mathematics) is to explain the
world in as simple and economic a fashion as
possible, and Euclid's axioms and the laws of
physics are attempts to do just that.
In fact, it is possible to quantify the degree of
compactness and utility of these explanatory
schemes using a branch of mathematics called
algorithmic information theory. Obviously, a
law of physics is a more compact description of
the world than the phenomena that it
describes. For example, compare the
succinctness of Newton's laws with the
complexity of a set of astronomical tables for
the positions of the planets. Although as a
consequence of Godel's famous incompleteness
theorem of logic, one cannot prove a given set
of laws, or mathematical axioms, to be the
most compact set possible, one can investigate
mathematically whether other logically
self-consistent sets of laws exist. One can also
determine whether there is anything unusual or
special about the set that characterises the
observed Universe as opposed to other possible
universes. Perhaps the observed laws are in
some sense an optimal set, producing maximal
richness and diversity of physical forms. It may
even be that the existence of life or mind
relates in some way to this specialness. These
are open questions, but I believe they form a
more fruitful meeting ground for science and
theology than dwelling on the discredited
notion of what happened before the big bang.
Box with feature, 27 April 1996, page two
So how does this relate to the Universe?
Well, say you start with nothing at all--not
even space or time. Presumably the total energy
of this system would be zero. Is it possible to
make a Universe of space, time and matter
whose total energy is still zero? The answer is
yes. "You can't create something out of
nothing," says Vilenkin. "But the Universe is an
exception. Gravitational energy is negative and
matter energy is positive. In a closed
Universe--one where if you keep going in one
direction you come back to the same
point--the negative energy of gravity exactly
cancels the positive energy of matter, so the
total energy is zero."
In the classical picture, the Universe cannot
appear out of nothing because it is forbidden to
adopt a certain range of sizes. But in quantum
theory, the Universe can tunnel through this
size barrier, and appear spontaneously with a
size greater than the critical value.
Can we ever know if the Universe began at a
single point, or has simply been going on for
ever? There is yet another complication, which
may make the whole question academic. It
stems from an idea called inflation, first
developed in the early 1980s to solve some
vexing problems with the standard big bang
model. In its earliest versions, inflation theory
stipulated that, immediately after the big bang,
the Universe suddenly ballooned, increasing its
diameter by more than a trillion trillion times
in just a tiny fraction of a second. After this,
the Universe switched to a non-inflationary
phase, and expanded at a more sedate rate. But
in the mid-1980s, cosmologist Andrei Linde at
Stanford University realised that such a system
would be self-replicating. Once you kicked it
off with a big bang, it would go on forever.
Even when most of the Universe had moved
out of the inflationary phase, Linde reasoned,
tiny fluctuating regions would still be capable of
undergoing inflation. These would then go from
being infinitesimal regions to sizeable chunks of
Universe in a split second, and would
themselves go on to spawn new patches of
Universe and so on. In each case, once inflation
was over, the patch would evolve according to
standard big bang theory.
If this is true, the whole Universe could be made
up of a huge number of expanding patches,
which could be quite different from our own.
The problem is that we can never know. "We
are removed by a tremendous distance from
regions that underwent a different history,"
says Steinhardt. "Inflation casts a pall on things
because it makes the part of the Universe we
see so infinitesimal compared to the entire
Universe, and perhaps not even representative.
We will never be able to see the edge of the
patch we live in, and this puts us beyond the
ability to be able to probe things through
observations."
What's more, an eternal, self-replicating
Universe may not even need a big bang.
Vilenkin says he has proved in a theorem that
the inflationary Universe must have had an
origin, but Linde is skeptical. He thinks it likely
but not proved that there was an initial big
bang from which all of the "pretty big bangs"
came. However, he adds that the question is so
far removed from our experience that it is
irrelevant: "Say you have an infinite number of
bubbles, all producing new ones. You live in one
of these bubbles and you look at the point the
bubble was formed. For all practical purposes
that's the beginning of your Universe." Because
there are infinitely many such bubbles, we have
no reason to believe that ours is the first, or
even the hundredth. It's more likely, says Linde,
that our own personal big bang is actually a
pretty insignificant one, way down the line
from the one that set the whole Universe going.
by Gabrielle Walker