Review: The Trouble with Physics, by Lee Smolin

[Russ Allbery]

The Trouble with Physics
by Lee Smolin

Publisher: Mariner
Copyright: 2006
Printing:  2007
ISBN:      0-618-91868-X
Format:    Trade paperback
Pages:     355

A brief recap of the state of theoretical physics: Quantum mechanics
and particle physics have settled on the standard model, which provides
an apparently complete inventory of fundamental particles and explains
three of the four fundamental forces. This has been very experimentally
successful up to and including the recent tentative observation of the
Higgs boson, one of the few predictions of the standard model that had
yet to be confirmed by experiment. Meanwhile, Einstein's theory of
general relativity continues as the accepted explanation of gravity,
experimentally verified once again by LIGO and Virgo detection of
gravitational waves.

However, there are problems. Perhaps the largest is the independence of
these two branches of theoretical physics: quantum mechanics does not
include or explain gravity, and general relativity does not sit easily
alongside current quantum theory. This causes theoretical understanding
to break down in situations where both theories need to be in play
simultaneously, such as the very early universe or event horizons of
black holes.

There are other problems within both theories as well. Astronomy shows
that objects in the universe behave as if there is considerably more
mass in galaxies than we've been able to observe (the dark matter
problem), but we don't have a satisfying theory of what would make up
that mass. Worse, the universe is expanding more rapidly than it
should, requiring introduction of a "dark energy" concept with no good
theoretical basis. And, on the particle physics side, the standard
model requires a large number (around 20, depending on how you measure
them) of apparently arbitrary free constants: numbers whose values
don't appear to be predicted by any basic laws and therefore could
theoretically be set to any value. Worse, if those values are set even
very slightly differently than we observe in our universe, the nature
of the universe would change beyond recognition. This is an extremely
unsatisfying property for an apparently fundamental theory of nature.

Enter string theory, which is the dominant candidate for a deeper,
unifying theory behind the standard model and general relativity that
tries to account for at least some of these problems. And enter this
book, which is a critique of string theory as both a scientific theory
and a sociological force within the theoretical physics community.

I should admit up-front that Smolin's goal in writing this book is not
the same as my goal in reading it. His primary concern is the hold that
string theory has on theoretical physics and the possibility that it is
stifling other productive avenues, instead spinning off more and more
untestable theories that can be tweaked to explain any experimental
result. It may even be leading people to argue against the principles
of experimental science itself (more on that in a moment). But to mount
his critique for the lay reader, he has to explain the foundations of
both accepted theoretical physics and string theory (and a few of the
competing alternative theories). That's what I was here for.

About a third of this book is a solid explanation of the history and
current problems of theoretical physics for the lay person who is
already familiar with basic quantum mechanics and general relativity.
Smolin is a faculty member at the Perimeter Institution for Theoretical
Physics and has done significant work in string theory, loop quantum
gravity (one of the competing attempts to unify quantum mechanics and
general relativity), and the (looking dubious) theory of doubly special
relativity, so this is an engaged and opinionated overview from an
active practitioner. He lays out the gaps in existing theories quite
clearly, conveys some of the excitement and disappointment of recent
(well, as of 2005) discoveries and unsolved problems, provides a solid
if succinct summary of string theory, and manages all of that without
relying on too much complex math. This is exactly the sort of thing I
was looking for after Brian Greene's The Elegant Universe.

Another third of this book is a detailed critique of string theory, and
specifically the assumption that string theory is correct despite its
lack of testable predictions and its introduction of new problems. I
noted in my review of Greene's book that I was baffled by his embrace
of a theory that appears to add even more free variables than the
standard model, an objection that he skipped over entirely. Smolin
tackles this head-on, along with other troublesome aspects of a theory
that is actually an almost infinitely flexible family of theories and
whose theorized unification (M-theory) is still just an outline of a
hoped-for idea.

The core of Smolin's technical objection to string theory is that it is
background-dependent. Like quantum mechanics, it assumes a static
space-time backdrop against which particle or string interactions
happen. However, general relativity is background-independent; indeed,
that's at the core of its theoretical beauty. It states that the shape
of space-time itself changes, and is a participant in the physical
effects we observe (such as gravity). Smolin argues passionately that
background independence is a core requirement for any theory that aims
to unify general relativity and quantum mechanics. As long as a theory
remains background-dependent, it is, in his view, missing Einstein's
key insight.

The core of his sociological objection is that he believes string
theory has lost its grounding in experimental verification and has
acquired far too much aura of certainty than it deserves given its
current state, and has done so partly because of the mundane but
pernicious effects of academic and research politics. On this topic, I
don't know nearly enough to referee the debate, but his firm dismissal
of attempts to justify string theory's weaknesses via the anthropic
principle rings true to me. (The anthropic principle, briefly, is the
idea that the large number of finely-tuned free constants in theories
of physics need not indicate a shortcoming in the theory, but may be
that way simply because, if they weren't, we wouldn't be here to
observe them.) Smolin's argument is that no other great breakthroughs
of physics have had to rely on that type of hand-waving, elegance of a
theory isn't sufficient justification to reach for this sort of
defense, and that to embrace the anthropic principle and its inherent
non-refutability is to turn one's back on the practice of science. I
suspect this ruffled some feathers, but Smolin put his finger squarely
on the discomfort I feel whenever the anthropic principle comes up in
scientific discussions.

The rest of the book lays out some alternatives to string theory and
some interesting lines of investigation that, as Smolin puts it, may
not pan out but at least are doing real science with falsifiable
predictions. This is the place where the book shows its age, and where
I frequently needed to do some fast Wikipedia searching. Most of the
experiments Smolin points out have proven to be dead ends: we haven't
found Lorentz violations, the Pioneer anomaly had an interesting but
mundane explanation, and the predictions of modified Newtonian dynamics
do not appear to be panning out. But I doubt this would trouble Smolin;
as he says in the book, the key to physics for him is to make bold
predictions that will often be proven wrong, but that can be
experimentally tested one way or another. Most of them will lead to
nothing but one can reach a definitive result, unlike theories with so
many tunable parameters that all of their observable effects can be
hidden.

Despite not having quite the focus I was looking for, I thoroughly
enjoyed this book and only wish it were more recent. The physics was
pitched at almost exactly the level I wanted. The sociology of
theoretical physics was unexpected but fascinating in a different way,
although I'm taking it with a grain of salt until I read some opposing
views. It's an odd mix of topics, so I'm not sure if it's what any
other reader would be looking for, but hopefully I've given enough of
an outline above for you to know if you'd be interested.

I'm still looking for the modern sequel to One Two Three... Infinity,
and I suspect I may be for my entire life. It's hard to find good
popularizations of theoretical physics that aren't just more examples
of watching people bounce balls on trains or stand on trampolines with
bowling balls. This isn't exactly that, but it's a piece of it, and I'm
glad I read it. And I wish Smolin the best of luck in his quest for
falsifiable theories and doable experiments.

Rating: 8 out of 10

Reviewed: 2018-06-23

-- 
Russ Allbery (eagle at eyrie.org)              <http://www.eyrie.org/~eagle/>