- Penrose, Roger.
Fashion, Faith, and Fantasy.
Princeton: Princeton University Press, 2016.
ISBN 978-0-691-11979-3.
-
Sir Roger Penrose
is one of the most distinguished theoretical physicists and
mathematicians working today. He is known for his work on
general relativity,
including the
Penrose-Hawking
Singularity Theorems,
which were a central part of the renaissance of general relativity
and the acceptance of the physical reality of black holes in the 1960s
and 1970s. Penrose has contributed to cosmology, argued that
consciousness is not a computational process, speculated that
quantum mechanical processes are
involved
in consciousness, proposed experimental tests to determine whether
gravitation is involved in the apparent mysteries of quantum
mechanics, explored the extraordinarily special conditions which appear
to have obtained at the time of the Big Bang and suggested a model which
might explain them, and, in mathematics, discovered
Penrose tiling,
a non-periodic tessellation of the plane which exhibits five-fold symmetry,
which was used (without his permission) in the
design of
toilet paper.
“Fashion, Faith, and Fantasy” seems an odd title for
a book about the fundamental physics of the universe by one of the
most eminent researchers in the field. But, as the author describes
in mathematical detail (which some readers may find forbidding), these
all-too-human characteristics play a part in what researchers may
present to the public as a dispassionate, entirely rational, search
for truth, unsullied by such enthusiasms. While researchers in
fundamental physics are rarely blinded to experimental evidence by
fashion, faith, and fantasy, their choice of areas to explore,
willingness to pursue intellectual topics far from any mooring in
experiment, tendency to indulge in flights of theoretical fancy
(for which there is no direct evidence whatsoever and which may not
be possible to test, even in principle) do, the author contends, affect
the direction of research, to its detriment.
To illustrate the power of fashion, Penrose discusses
string theory,
which has occupied the attentions of theorists for four decades
and been described by some of its practitioners as
“the only game in town”. (This is a
piñata which has been
whacked by others, including Peter Woit in
Not Even Wrong [June 2006]
and Lee Smolin in
The Trouble with Physics [September 2006].)
Unlike other critiques, which concentrate mostly on the failure of
string theory to produce a single testable prediction, and the failure
of experimentalists to find any evidence supporting its claims
(for example, the existence of
supersymmetric
particles), Penrose concentrates on what he argues is a
mathematical flaw in the foundations of string theory, which
those pursuing it sweep under the rug, assuming that when a final
theory is formulated (when?), its solution will be evident.
Central to Penrose's argument is that string theories are formulated
in a space with more dimensions than the three
we perceive ourselves to inhabit. Depending upon the version
of string theory, it may invoke 10, 11, or 26 dimensions. Why don't
we observe these extra dimensions? Well, the string theorists argue
that they're all rolled up into a size so tiny that none of our experiments
can detect any of their effects. To which Penrose responds, “not
so fast”: these extra dimensions, however many, will vastly
increase the functional freedom of the theory and lead to a mathematical
instability which will cause the theory to blow up much like the
ultraviolet
catastrophe which was a key motivation for the creation of the
original version of quantum theory. String theorists put forward
arguments why quantum effects may similarly avoid the catastrophe
Penrose describes, but he dismisses them as no more than arm waving.
If you want to understand the functional freedom argument in detail,
you're just going to have to read the book. Explaining it here would
require a ten kiloword review, so I shall not attempt it.
As an example of faith, Penrose cites
quantum mechanics
(and its extension, compatible with
special relativity,
quantum field theory),
and in particular the notion that the theory applies to all interactions in
the universe (excepting gravitation), regardless of scale. Quantum mechanics
is a towering achievement of twentieth century physics, and no theory has
been tested in so many ways over so many years, without the discovery of the
slightest discrepancy between its predictions and experimental results. But all
of these tests have been in the world of the very small: from subatomic particles
to molecules of modest size. Quantum theory, however, prescribes no limit on the
scale of systems to which it is applicable. Taking it to its logical limit,
we arrive at apparent absurdities such as
Schrödinger's cat,
which is both alive and dead until somebody opens the box and looks
inside. This then leads to further speculations such as the
many-worlds interpretation,
where the universe splits every time a quantum event happens, with every possible
outcome occurring in a multitude of parallel universes.
Penrose suggests we take a deep breath, step back, and look at what's going
on in quantum mechanics at the mathematical level. We have two very different
processes: one, which he calls U, is the linear evolution of the wave
function “when nobody's looking”. The other is R, the
reduction of the wave function into one of a number of discrete states
when a measurement is made. What's a measurement? Well, there's another ten
thousand papers to read. The author suggests that extrapolating a theory of the
very small (only tested on tiny objects under very special conditions) to
cats, human observers, planets, and the universe, is an unwarranted leap of
faith. Sure, quantum mechanics makes
exquisitely precise
predictions confirmed by experiment, but why should we assume it is correct
when applied to domains which are dozens of orders of magnitude larger and
more complicated? It's not physics, but faith.
Finally we come to cosmology: the origin of the universe we inhabit,
and in particular the theory of the big bang and
cosmic inflation,
which Penrose considers an example of fantasy. Again, he turns to
the mathematical underpinnings of the theory. Why is there an
arrow of time?
Why, if all of the laws of microscopic physics are reversible in time, can we
so easily detect when a film of some real-world process (for example, scrambling
an egg) is run backward? He argues (with mathematical rigour I shall gloss over here)
that this is due to the extraordinarily improbable state in which our universe
began at the time of the big bang. While the
cosmic background
radiation appears to be thermalised and thus in a state of very
high
entropy,
the smoothness of the radiation (uniformity of temperature, which
corresponds to a uniform distribution of mass-energy) is, when gravity
is taken into account, a state of very low entropy which is
the starting point that explains the arrow of time we observe.
When the first precision measurements of the background radiation were
made, several deep mysteries became immediately apparent. How could
regions which, given their observed separation on the sky and the finite
speed of light, have arrived at such a uniform temperature? Why was the
global
curvature of the universe
so close to flat? (If you run time
backward, this appeared to require a fine-tuning of mind-boggling precision
in the early universe.) And finally, why weren't there primordial
magnetic monopoles
everywhere? The most commonly accepted view is that these problems are
resolved by cosmic inflation: a process which occurred just after the moment
of creation and before what we usually call the big bang, which expanded the
universe by a breathtaking factor and, by that expansion, smoothed out any
irregularities in the initial state of the universe and yielded the uniformity
we observe wherever we look. Again: “not so fast.”
As Penrose describes, inflation (which he finds dubious due to the lack of
a plausible theory of what caused it and resulted in the state we observe
today) explains what we observe in the cosmic background radiation, but
it does nothing to solve the mystery of why the distribution of mass-energy
in the universe was so uniform or, in other words, why the gravitational
degrees of freedom in the universe were not excited. He then goes on to examine
what he argues are even more fantastic theories including an infinite number
of parallel universes, forever beyond our ability to observe.
In a final chapter, Penrose presents his own speculations on how
fashion, faith, and fantasy might be replaced by physics: theories
which, although they may be completely wrong, can at least be tested
in the foreseeable future and discarded if they
disagree with experiment or investigated further if not excluded
by the results. He suggests that a small effort investigating
twistor theory
might be a prudent hedge against the fashionable pursuit of string
theory, that experimental tests of
objective
reduction of the wave function due to gravitational effects be
investigated as an alternative to the faith that quantum mechanics
applies at all scales, and that his
conformal
cyclic cosmology might provide clues to the special conditions at the
big bang which the fantasy of inflation theory cannot. (Penrose's
cosmological theory is discussed in detail in
Cycles of Time [October 2011]). Eleven mathematical
appendices provide an introduction to concepts used in the main text which
may be unfamiliar to some readers.
A special treat is the author's hand-drawn illustrations. In addition
to being a mathematician, physicist, and master of scientific
explanation and the English language, he is an inspired artist.
The Kindle edition is excellent, with the table
of contents, notes, cross-references, and index linked just as they
should be.
October 2016 