book reviews
## Philosophically oriented books on quantum gravityreviewed by T. Nelson |

Which of our fundamental assumptions are wrong?

A. Aguirre, B. Foster, and Z. Merali, eds.

Springer, 2015, 274 pages

Reviewed by T. Nelson

The 21st century was when science got tough. We've done the easy stuff: we cured most of the easy diseases and figured out how light behaves. But cancer is caused by DNA mutations in cells. How do you cure a mutation? Even harder to cure are diseases like Parkinson's and Alzheimer's, which only occur in humans, which means we can't use experimentation to study them, so our ideas about what causes them are mainly educated guesses, and they're probably wrong.

But biologists have it easy. Some of the predictions made by the most powerful theory in physics, quantum field theory, are off by amusingly large factors,* and the new theories intended to fix them require equipment so big that they can never, even in principle, be tested. Certainly our equations are incredibly accurate for ordinary phenomena; but like ancient maps whose uncharted latitudes were decorated with dragons, the extremes of our equations are marked ‘here be singularities.’ And worst of all, the Higgs boson turned up at 125 GeV, exactly where it was predicted, earning Prof. Higgs a Nobel prize and giving us proof positive that something is very wrong with our view of the cosmos.

This means the next step will have to be a transition from merely describing
phenomena to actually explaining *why* things happen. It tells us very
little, for instance, to say that the reason a radioactive nucleus decays is
that it is unstable, or that the reason a particle has a certain mass is that
it has a particular amount of energy. When science describes things by slapping
a descriptive formula on it, it risks falling into what physicists
disparagingly call ‘botany.’

Of course, that would be unfair: the boundary of knowledge has to be descriptive, by definition, since we don't yet have an explanation.**

Theorists are constructing ever more imaginative theories, like
M theory, which proposes that the universe, in many ways, is like a gigantic tangled
ball of string; or
agravity, which proposes that length
and mass are illusions;
or holographic theories, which propose that the *entire* universe is an
illusion and we should, perhaps, just lie down and go to bed until it goes away.
Max Tegmark says the universe is composed of mathematics,
while Seth Lloyd swears
up and down that the universe is a computer. There are so many theories, each more
exciting than the last, that the average person can't keep track of them.

Of course, not all them will turn out to be correct. Hence the need for an examination of the assumptions that may be holding us back. The essays here are the “winners” in a competition among 270 entries. (The editors don't say what the prize was—I suspect the prize was having to proofread your essay in some Springer book—if so, not all of them seem to have claimed their prize.)

The essays are, generally, pretty interesting even if you're not a physicist. They show that physics is drifting back to its roots in philosophy. Philosophy itself seems unable to fill the necessary void: philosophy seems to be in even worse shape than science, having drifted of late toward political conformity and sterility (for whatever reason, epistemology seems hardest hit). These days physicists are the ones asking questions like whether there really such a thing as space and time and how we can know whether the world is real.

The real value of these brainstorming essays is as a jumping-off point. The authors talk about their ideas in mostly non-technical language and give citations to the literature so you can decide whether to follow up. For example, in Chapter 2 Robert W. Spekkens suggests that kinematics and dynamics are meaningful only when fused together. He introduces the idea of causal structure, which is a theory that statisticians have devised to distinguish whether things are causally connected or merely correlated, and says that only the effective causal structure of a theory should be considered physically relevant. He cites articles in places like Phys. Rev. A and Proceedings of the American Mathematical Society—all much more technical than the discussion here.

Another essay asks why small objects create interference patterns in the two slit experiment, but not large ones like tables. According to the authors, the equations say that if we could blast tables (let us assume a spherical table here) through a two slit apparatus, we should see evidence of superposition of states, that is to say an interference pattern. Why this doesn't happen may give us clues about the nature of quantum stuff.

In “The Universe Is Not a Computer” Ken Wharton takes issue with
the Newtonian Schema, where people use a mathematical representation of physical
reality to map events onto parameters. ~~No one~~ Almost no one takes the
idea that the universe is a computer literally, of course, but Comp. Sci. and
Newtonian physics use the same mode of thinking, and Wharton suggests that an
alternative schema, which he calls the Lagrangian Schema, might work better.

* The prediction of the vacuum energy density
differs from the measured value
by a factor of 10^{100} (= 1 googol, finally giving us a use for that
term), suggesting that there may be a need to tweak our equations.

** ‘Descriptive’ is sometimes a pejorative word in science, but the most fundamental cutting-edge theories must always be descriptive, since a description is only an explanation of a phenomenon in terms of something more fundamental.

*feb 09, 2017*

The foundations of quantum gravity

N. Huggett, K. Matsubara, and C. Wüthrich, eds.

Cambridge, 2020, 362 pages

Reviewed by T. Nelson

People tend to think that physics is all about complicated math. To some extent that's true, but the math is only useful if it corresponds to something real. To make that connection, theoretical physicists face not only technical barriers but also conceptual ones: it is now glaringly apparent that to achieve a theory of quantum gravity, physics will have to explain what spacetime is. Inventing Planck-scale entities and linking them together on an existing continuum is no longer an option. Discovering ways to overcome those conceptual barriers is the theme of this book.

In Part 1 (Spacetime Emergence), Daniele Oriti discusses how quantum field theory and relativity might somehow give us emergent spacetime. S. Brahma hypothesizes about how time might spontaneously emerge from holonomy corrections in loop quantum gravity and emphasizes how quantum fuzziness poses challenges for cosmology. R.H. Brandenberger speculates what might come next in inflationary cosmology, and Daniel Harlow talks about AdS/CFT duality and black holes.

In Part 2 (Time in quantum theories of gravity), the contributors discuss how their theories relate to the two traditional philosophical views of time: presentism, which says the present is fundamentally different from past and future—or, to say it another way, reality is becoming—and block time, in which the present, past, and future are merely names that depend on one's perspective. Carlo Rovelli says it is a false choice. Fay Dowker gives us the perspective of causal set theory, an alternative to loop quantum gravity and superstring theory that is compelling due to its simplicity. Lee Smolin proposes that time is the only irreducible phenomenon, while space is emergent. Henrique Gomes seems to agree with Julian Barbour that time is not real at all.

Part 3 (Issues of interpretation) is more philosophical. David Wallace talks
about the problems with information loss in black holes. Richard Dawid, whose
controversial suggestion that we can dispense with experimentation altogether
made waves a few years ago, takes a highly reductionist approach in trying to
decide whether physics is up to the task of dealing with things beyond the reach
of experimentation. S. de Haro speculates how quantum gravity might eliminate
spacetime as a fundamental structure. Radin Dardashti *et al.* discuss AdS/CFT,
a powerful holographic theory, emphasizing (as does the previous chapter) the
importance of duality. Tizania Vistarini sees potential connections between
David Lewis's possible worlds logic and the ambiguities in string theory,
hoping perhaps to drag metaphysical speculation kicking and screaming from
Lewis's innumerable hypothetical worlds into a theory of countable worlds
produced by spatiotemporal deformations in string theory. Ko Sanders
talks about the categories of Lorentzian manifolds and locally covariant
quantum field theory.

Despite the near-absence of math, familiarity with some aspects of quantum gravity is assumed. Some of the articles are easier to follow than others, and some will leave you wondering about how anyone could ever consider their idea plausible, but no one can read this book and think that physics is running out of ideas.

*oct 04, 2020*

Understanding emergence in effective field theory and quantum gravity

K. Crowther

Springer, 2016, 205 pages

Reviewed by T. Nelson

A new kind of philosophy of physics has the philosopher taking on the role of a Dian Fossey, seeking out the physicists in the mist. Instead of noting who has the most bananas, the philosopher studies how theories are constructed and the philosophical problems they face.

Quantum gravity is an area where philosophers are badly needed. To create a theory of quantum gravity, physicists will eventually have to figure out what space and time are. And here, our language, our thinking patterns, and even our math may be getting in the way. If we talk about two objects, the very concept of ‘two’ already assumes space and a distance between the two objects; to talk about two particles in a place where there is no distance and no time seems nonsensical. So have we unknowingly baked spacetime into all our theories? Maybe even mathematics itself implicitly assumes what we are trying to prove.

Here's a concrete example: in the pre-geometric (i.e. pre-big bang) phase of ‘quantum graphity’ theory (the name is a pun on quantum gravity), there are a bunch of nodes all connected to each other, which is said to mean there are no distances between any of them. Then in the low-energy graph, most of the nodes are unconnected, so distance, dimensionality, and geometry now exist. But they are the same nodes and they haven't moved! It's not a realistic model of anything, but it's an interesting demonstration of how space could be just an emergent phenomenon created just using time.*

This is the problem of emergence: how to create a theory where spacetime emerges from something that is not already a collection of objects in space. Quantum graphity has to assume that time is fundamental, otherwise nothing could ever happen. Or to put it another way, space occurs on a background of pre-existing time.

In these chapters the author remains very low-key and circumspect, a wise move since these theories are rapidly evolving. I'm not sure who the intended audience was. Certainly not physicists, who would find the discussion of the philosophy of field theories irrelevant. Not students, who would be disappointed to find that a working knowledge of quantum mechanics is needed in order to follow the text. And not anthropologists, since there is no social context for the theories, few bananas, and no poo-throwing at all. The lack of equations doesn't make this a pop science book.

The first few chapters discuss the merits of top-down vs. bottom-up theorizing, how theories can be fundamental, and some background on the physics. Finally on page 147 it gets to the interesting stuff: discrete approaches to quantum gravity, a quick discussion of causal set theory, and loop quantum gravity (LQG). LQG isn't background-independent at all. Like string theory, it merely imposes a structure on spacetime; it doesn't explain why it is a space. There is clearly a long way to go.

Neither of the books reviewed on this page has an index, which makes it just a little harder for people to cite them.

* It is actually a model of dimension reduction rather than creation of space, since the fully connected state is actually an n-dimensional hyperspace.

*feb 19, 2017; updated feb 20, 2017*