The wonder of universal symmetry

The Comprehensible Cosmos
April 13, 2007

John Gribbin welcomes a fascinating whistlestop tour of physics

It is a rare delight for a reviewer to be asked to comment on a book that spells out an idea that he has been promoting himself. I therefore have to confess to feeling a warm appreciation for Victor Stenger's work, even before I plunged in to the text. The fact that the text does not entirely do justice to the idea is mildly disappointing, but what Stenger has to say is so important that it should at least be discussed everywhere that physics is taught.

The essence of the argument is that the task of physics is not to uncover "the truth" about how the universe works, but to find models that enable us to make accurate predictions about the outcome of experiments. If a model makes accurate predictions, it is a good one; if two models both make accurate predictions, they are both good, and there is no need to choose which one is "right."

This is easy to see using the example of atoms. The model of atoms as hard tiny spheres rattling around inside a box is a good one if you want to calculate how the pressure of the gas in the box changes with temperature.

The model of an atom as a central nucleus "orbited" by electrons is a good one if you want to calculate the position of lines in the spectrum of hydrogen. Both are "right" in the context in which they are used, but neither is "the truth" about atoms. However, Stenger goes much further, arguing that even concepts such as space and time are "only" models that enable us to carry out calculations and predict the outcome of experiments.

It is important to emphasise that these are not the views of some disreputable crank but of a physicist who worked for many years at the University of Hawaii and who, in retirement, is an adjunct professor of philosophy at the University of Colorado at Boulder. Now in his early seventies, he has held visiting positions as an experimental physicist at the University of Heidelberg, the Rutherford Laboratory, the National Nuclear Physics Laboratory in Frascati and the University of Florence.

Stenger was involved in experiments that helped establish the standard model of particle physics, and in his last project before retiring he collaborated on the underground experiment in Japan that showed definitively that the neutrino has mass. As you would expect with this background, he is by no means knocking standard science. "Science," he says, "knows a lot more than most people, even many scientists, realise."

Backing this statement up, in The Comprehensible Cosmos he offers, within the context of his own world-view, an otherwise conventional whistle-stop tour of physics, from Newton's laws to supersymmetry theory, but (disappointingly) stopping short of any real discussion of membranes. The tour really is very short: the main text of the book ends on page 189 and it is followed by eight "Mathematical supplements" that pull no punches - a little over the top for the average reader but potentially very useful for physics undergraduates. The best, and most readable, sections are the first and last chapters of the main text, where the author first sets out and later summarises his main argument.

Apart from Stenger's enthusiasm for the idea of model-based realism, the Big Idea in his book is the Big Idea of modern theoretical physics, that the world we see around us has been produced from a more uniform state by a process known as symmetry-breaking. The familiar analogy is with the way the structure seen in frozen water in the form of a snowflake can be produced from unstructured water vapour as the vapour cools. It is generally true that more energetic systems are more symmetric than systems with lower energy and that structure emerges as systems lose energy, or cool.

The best models we have of the behaviour of the early universe suggest that there was no distinction between the four fundamental forces of nature in the earliest moments after "time zero", just one superforce. Gravity, the strong and weak nuclear forces and electromagnetism then appeared as separate entities as the energy density in the universe fell and symmetry was broken. Similar arguments produce very successful models (in the sense that they make predictions that match the outcome of experiments) to account for the appearance of the variety of known particles built up from quarks and leptons. The ultimate version of this idea, supersymmetry, requires that every kind of particle we know about has, in principle, a "supersymmetric partner" that does not exist today but was present in the early universe and might be created in high-energy events.

Stenger is good at explaining the big issues, but there are a few irritating minor errors in the book. For example, he falls into the all-too-common trap of saying that Einstein's general theory of relativity implies that light is bent when passing near a gravitating object such as the Sun without explaining that Newtonian theory also predicts light-bending, but not to the same extent as Einstein's prediction. And several of my cosmologist friends will be niggled by the repetition of the canard that the discovery that the expansion of the universe is accelerating was "unanticipated". It wasn't anticipated by the people who made the discovery, but then they weren't cosmologists. Stenger could, in fact, have used this example to highlight his point about models, since cosmologists have models that will account for almost any discovery that might be made about the behaviour of the universe. The discovery of accelerated expansion simply promoted one of these ideas from the status of speculation to that of a good model.

But these minor irritations are more than compensated for by Stenger's dismissal of the widely quoted view of Thomas Kuhn, spelt out in his unfortunately influential book The Structure of Scientific Revolutions , that science does not progress gradually but instead through a series of revolutionary "paradigm shifts". Any half-decent working scientist will tell you this is utter rubbish and would endorse Steven Weinberg's view that the only real revolution in scientific thinking was the one with which Isaac Newton started the ball rolling in the 17th century. Personally, I even have my doubts about just how revolutionary Newton was, but this is not the place to go into that.

A subtheme of The Comprehensible Cosmos is the role of symmetry in physics and in our model-building, which leads Stenger to his own as yet unproven cosmological speculation. The expansion of the universe away from the big bang is a decidedly asymmetric process, which gives us a well-defined arrow of time - the past is when the universe is more compact. To restore symmetry, Stenger suggests that there should be a mirror universe expanding in the opposite direction of time on the other side of the Big Bang.

Because this will have the opposite arrow of time from us, overall symmetry is restored and our observation of time as having a preferred direction is merely a local effect.

It may also, says Stenger, be merely a macroscopic effect. One of the recurring themes of his work is that at a quantum level there is no arrow of time, and interactions can be described just as well "backwards" as "forwards" in time. In the classic example, all interactions involving positrons moving forward in time can be explained equally well in terms of electrons moving backwards in time - or one single electron zigzagging forwards and backwards in time. The idea was introduced by Richard Feynman more than half a century ago and remains an entirely valid description of "reality." According to Stenger, "many, if not all, of the so-called paradoxes of quantum mechanics can be understood as consequences of forcing the familiar time direction of ordinary experience on the description of quantum events".

Stenger also approves of Feynman's contention that there are no such things as quantum waves, quoting with approval his remark that "light comes in this form - particles. It is very important to know that light behaves like particles". Which is rather ironic, given that in some circumstances the wave model is an entirely adequate way to make calculations and predictions that correspond with the outcome of experiments and is therefore a good model by Stenger's own criterion.

John Gribbin is a visiting fellow in astronomy at Sussex University and author of The Universe: A Biography .

The Comprehensible Cosmos

Author - Victor J. Stenger
Publisher - Prometheus
Pages - 340
Price - £19.99
ISBN - 1 59 102424 2

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