The Trouble with Physics

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