book reviews
## books by Roger Penrosereviewed by T. Nelson |

by Roger Penrose

Princeton, 2016, 501 pages

reviewed by T. Nelson

oger Penrose's earlier book, *The Road to Reality*, was didactic: I've recommended
some of his chapters as nice introductions to various topics. This one isn't.
It's an argument against some of the weird stuff in modern physics. His take:
it's not weird enough.

Criticizing string theory might seem pointless; it's been done to death, and the theory could change radically tomorrow. So what we want from Penrose is new ideas. (No more quantum consciousness, please!) New ideas are here, but this book is aimed at a general audience, so they're way in the back.

Part 1 (‘Fashion’) are his arguments against string theory. The main one is those extra useless dimensions that are curled up in every string. He says there's more than enough energy in the solar system to affect a string and disrupt them. But of course that's irrelevant: what we really want is high energy density, not just a lot of it. The cross section of such an interaction would be way too small. Moreover, not just any kind of energy would do; it would have to be gravitational energy. Maybe I'm misunderstanding the argument; not sure what he's getting at here.

Part 2 (‘Faith’) is an elementary nontechnical explanation of quantum mechanics starting with the two-slit experiment and the Mach-Zehnder interferometer. The idea here is that we take QM on faith, but there are problems, the main one being that decoherence might not be something that occurs automatically as everybody assumes; it only happens, he suggests, when a shift in mass occurs. This is wholly described in the last two pages of part 2; people knowledgeable about QM can skip the rest.

In part 3 (‘Fantasy’) he has two main points: first, he favors equating dark matter with Einstein's cosmological constant Λ. Second, he regards the theory of multiverses and cosmological inflation as sheer fantasy. Not, he says, that there's anything wrong with that, but we need to recognize that inflation is purely conjectural. All the ideas so far aren't fantastic enough: “Something more, perhaps with an even greater element of fantasy, is needed!”

Instead of inflation, Penrose favors the ekpyrotic / cyclic cosmology theory. And so it's not until page 328 that the book gets really interesting.

In Part 4 (‘A new physics for the universe?’) we finally find out why Penrose was arguing against fashion: he proposes taking his own invention of twistors, which are about as unfashionable as you can get (which could well mean they're closest to the truth), and applying them to cosmology.

What is the audience for this book? Not scientists, who would already know all this stuff; they should skip the first 327 pages and go straight to the good part. Not people looking for a debunking of ideas of modern physics: Penrose's critiques are pretty darn gentle. I'd say the typical reader would be a layman who wants a basic orientation to QM, QFT and cosmology but has little physics or math background. These readers should start with the math appendices, which are a very light introduction to complex numbers and vectors.

It could be a roller coaster ride. Judging from the reviews on Amazon, many laymen seem to be struggling with even the little math that's here. I'd say, though, that it's really not that bad.

* nov 21, 2016*

by Roger Penrose

Vintage, 2010, 288 pages

reviewed by T. Nelson

n this pop-physics book, Penrose talks about the theory that he calls Conformal Cyclic Cosmology, or CCC. A conformal theory simply means that two different things are essentially the same except for a scale factor, called a conformal factor. In this case the two ‘things’ are the present universe and the newer, shinier, and cuter baby universe, which magically springs out of the old one.

In this highly speculative theory, the universe's scale factor, called Ω,
gradually changes over time and turns into a reciprocal of itself:
Ω↦Ω^{−1}.
Penrose is not the first to propose basic physical parameters changing over time,
but usually they're imagined to happen at extremely high energies like at the
Big Bang. Such a change, if it could happen, would transition the universe
from colossally big and sparse to small and highly dense, causing a Big Bang,
which is just what we need for eternal identical universes, but Penrose is a bit
fuzzy on exactly how that's supposed to happen.

The CCC is quite different from the cosmology in string theory, which proposes a collision of colossal brane structures, and the big bounce theory (where the universe collapses in on itself), which Penrose dismisses. Penrose also is skeptical of the cosmic inflation theory, which was invented to explain how the universe came to be so uniform.

One of the main characteristics of a universe is that its entropy increases over time. So to conserve entropy, the CCC requires the initial entropy to match the final entropy in the old universe. Getting rid of the entropy would be like stuffing billions of tons of Silly String back into a single can and having it happen spontaneously. Short of running time backwards, it is hard to imagine it happening.

Current thinking is that black holes, which have enormous entropy, will be the last remaining objects in our universe. Then they will evaporate, ultimately leaving only subatomic particles and photons. Conversely, at the Big Bang, entropy is supposed to be zero (or nearly zero). Thus, Penrose says we must accept huge entropy (i.e. information) losses in black holes. As he recognizes, this would violate our physical laws as well. How that works is a detail that still needs to be worked out.

This book should be easy for laymen to read, but they should bear in mind that the CCC faces an uphill battle against a shedload of competitors, most of which hope to explain spacetime as well, which makes them far more ambitious. CCC tries to expain one thing—an eternally recycling universe—which so far is just speculation.

* nov 08, 2020*