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# Critical Review by **John Barrow**

SOURCE: "Battle of the Giants," in *New Scientist*, Vol. 149, No. 2021, March 16, 1996, pp. 48-9.

*In the following review of* The Nature of Time and Space, *Barrow states that the book's debate format works well and the author's theories are clearly explained*.

General relativity and quantum theory have always held a special fascination for physicists. They govern empires that appear superficially disjoint and rule their separate dominions with a precision unmatched by any other products of the human mind. The accuracy of Einstein's general theory of relativity, for example, is demonstrated by the spectacular observations of a pulsar engaged in a gravitational pas de deux with a dead star. Einstein's expectations are born out by observations—accurate to one part in 10 14. Almost as impressive is the accuracy of the quantum theory: agreeing with experiment at one part in 10 11.

The quantum world deviates strongly from those of Newton when things are very small. By contrast, general relativity only changes Newton's predictions when gravitational fields are strong and masses are very large. These conditions rarely overlap except in the cosmological problem of the Universe's first expansive moments.

Over the past thirty years, Stephen Hawking and Roger Penrose have done more than anyone to further our understanding of the nature of gravitation and cosmology. Both have developed new approaches to these problems that differ from the mainstream work by particle physicists (and from each other). ** The Nature of Space and Time** is the result of their attempt to stage a structured dialogue about these problems, to isolate points of disagreement, and stimulate further investigation of these problems. Alternate lectures are presented by the two protagonists, culminating in a final debate where they summarise their points of agreement and disagreement. The level of argument is highly technical, but you can skip the equations and still get a feel for what is going on.

Generally, great debates in science don't work. Science is not a democratic activity in which the idea that gains the most popular votes wins. Politicians need not apply. Nonetheless, this volume shows that this adversarial style can be extremely valuable—at least at a textual level. Both authors are well acquainted with each others' ideas and write with great clarity. They agree on much and have to struggle a bit to play up points of disagreement over the interpretation of quantum mechanics, irreversibility, violation of CPT in gravitational collapse, the equivalence of black and white holes.

The opening two lectures introduce the minimum collection of ideas from differential topology needed to understand what a singularity is (an edge of space-time found, for example in a black hole, where all laws of physics break down), and the conditions under which it would be inevitable in our past. Then they move on to quantum effects and gravity with Hawking discussing black hole thermodynamics and introducing Euclidean methods. Penrose lets the cat out of the bag (and into a box) introducing the problems of interpreting quantum measurement, even proposing a simple formula for the time duration of wave function collapse.

The final pair of lectures is about quantum cosmology. Hawking argues for the inevitability of the Hartle-Hawking "no-boundary" condition as a way of describing the initial state of the Universe which uses quantum mechanics to explain how time originates at the big bang. Penrose, on the other hand, argues for some measure of gravitational entropy. In this picture the second law of thermodynamics implies that a low value for gravitational entropy at the initial state would be natural. So the Universe would be almost isotropic and homogeneous initially, but chaotically irregular at any final singularity.

If you cast a critical cosmological eye over the proceedings, then several things are evident. Neither author is very impressed by superstring theory (to the exasperation of at least one questioner at the end of the lecture), both have wonderful geometrical intuitions and their taste in theories is strongly influenced by that penchant. Neither believes that inflation—the fashionable idea that the Universe underwent a phase of accelerated expansion in the first moment of its existence—is the whole answer to the problem of why the Universe is so isotropic and homogeneous today. And neither adequately considers the impact of cosmological inflation upon their viewpoint.

Hawking argues that superstring theories make no observable predictions that are not those of general relativity, to which superstrings reduce when gravity is weak. By contrast, he also claims that the no-boundary condition of quantum cosmology makes two successful predictions: the amplitude and spectrum of fluctuations in the microwave background. This claim is surely a piece of gamesmanship. These two predictions come from inflation, not from the no-boundary condition. The no-boundary condition allows inflation to occur but only by adding extra matter called a scalar field. This leads to the observed spectral slope of the fluctuations in microwave background radiations, but I could just as well have inserted a different scalar field into the early Universe which would give fluctuations in conflict with the observations, even though the no-boundary condition still holds.

The "correct" fluctuations come in all cases from an arbitrary choice of the scalar field, rather than from any prediction of the no-boundary condition itself.

Another important aspect of the picture of an inflationary Universe that both authors ignore is that observation requires only the "beginning" of the Universe be uniform over a tiny region. Inflation can enlarge that uniform region so that now it is almost uniform over a region larger than our entire visible Universe. Beyond our horizon, however, the Universe could be quite different. The global structure of the Universe today may be extremely irregular: parts may be collapsing, rotating or possess huge variations in density. This possibility arises naturally from general initial conditions.

The possibility of such initial conditions cuts through many of the assumptions made by both protagonists in this debate. They both maintain that current observations require a high level of uniformity in the initial stages of the Universe, using this to justify their own strong theories about cosmological initial conditions. But the observations do not require this and the initial state may have been globally highly irregular, contrary to Penrose's claim that the initial Weyl curvature was very small or Hawking's claim that the Universe began in a ground state defined by the no-boundary condition.

Considering this possibility is vital because, if allowed, itchanges the entire nature of the Universe. It removes the evidence for any initial state of low "gravitational entropy" or the need to distinguish fundamentally between initial and final singularities. Indeed, there need be neither a global initial singularity nor any quantum tunnelling of the Universe out of nothing. It shows how cosmology is unlike any other physical problem: the causal horizon structure of the Universe forbids us access to the information that we require to test a theory of cosmological initial conditions.

The debate between Hawking and Penrose is a live one between brilliant scientists that covers far more ground than their seven cameo lectures can encompass. This elegant little volume provides a clear account of two approaches to some of the greatest unsolved problems of gravitation and cosmology. It is recommended to critical readers who should not forget that there are other more widely supported views about these cosmological problems. Which, if any, view is true? At present only God knows—or maybe not.

This section contains 1,215 words(approx. 5 pages at 300 words per page) |