The puzzles of sciences endure

FOR a senior writer from Scientific American, John Horgan ("Sun sets on science's glory days", HES, August 28) is dangerously misinformed about the frontiers of pure science.

Science, he claims, is at an end because everything that is knowable has been discovered and all that remains apart from applied science is "ironic science" based on unfalsifiable hypotheses. I would guess that such thinking stems from a desire to cash in on the fin-de-siecle malaise that made Francis Fukuyama's End of History such a popular best-seller.

When it comes to fundamental physics, however, Horgan is quite simply wrong. Physics is not complete - in particular, general relativity, which is our most accurate theory of gravity, breaks down in certain circumstances.

In the 1960s, Stephen Hawking and Roger Penrose showed that, given very reasonable physical assumptions, it is inevitable that "singularities" - infinite concentrations of matter and energy - develop and in this limit general relativity has no predictive power. As a result, physicists cannot yet fully explain what went on at the earliest moments of the beginning of the universe, nor do we fully understand black holes where singularities would invariably reside.

Such "strong field gravity" is believed to be the engine that powers the huge energy fluxes produced by distant quasars. However, without a detailed knowledge of black hole microphysics, any models of black hole engines for quasars can be tentative at best.

Models of the universe when it had an age of less than 0.01 seconds are similarly tentative. Many physicists believe that a quantum theory of gravity would solve the problem of the breakdown of general relativity posed by the singularity theorems. This would be a natural solution because we believe that quantum mechanics is universal.

Unfortunately, no one has been able to resolve quantum mechanics with general relativity despite decades of thought on the problem. Thus, in sharp contrast to the picture painted by Horgan, our two most basic theories are in fundamental crisis.

Given that all obvious attempts at reconciling the two theories have failed, it would be of no great surprise if the ultimate quantum theory of gravity shook our physical notions in a revolutionary way, in just the fashion that Horgan claims is impossible.

The distinguishing factor of much of what Horgan terms to be "ironic science" is that it is speculative simply because a lot of empirical data has not yet become available to decide between rival theories.

Sociology - one of Horgan's examples of "ironic science" - will probably look completely different after it has had more than 300 years of continuous development in which to mature, as physics has had. If we were ever to make radio contact with distant extraterrestrial civilisations, then fields such as comparative sociology would be completely revolutionised during the long time scales that such communication would take.

String theory, which describes a set of ideas that form an attempt to understand quantum gravity, is labelled by Horgan as "ironic science" because it has not yet delivered observational predictions. This overlooks the fact that a primary part of scientific method is the formation of a logically self-consistent hypothesis. In a mature science such as physics, whose language is highly mathematical, the gestation of a mathematically self-consistent hypothesis can take a much longer time than in other fields.

Attempts at finding a quantum theory of gravity should be compared with the gestation of general relativity: between 1905 and 1915, Einstein constructed his theory on the basis of pure thought, endeavouring to find a mathematical framework in which physically observable quantities could be identified while reconciling the principle of relativity with gravitation.

At the time, general relativity was viewed as a triumph of human intellect based on esoteric mathematics that very few people could understand and with little to say directly about the physical world, apart from predicting a minute bending of starlight near the surface of the sun and very tiny corrections to the orbital motion of Mercury. Today, however, technology has advanced to the point that the Global Positioning System relies on general relativity: our navigation systems would break down within a matter of hours if gravitational time dilation were not taken into account in determining the relative position of satellites.

General relativity has been transformed from an intellectual exercise to a hard-core, everyday phenomenological theory and there is no reason to suppose that this should not be the case for string theory, or any other theory that ultimately succeeds in describing quantum gravity.

The problem of quantum gravity is simply much harder than anything we have encountered before.

Because we have not yet solved the problems of mathematical consistency, we are not yet able to formulate a falsifiable hypothesis.

String theory, which is only a candidate, is itself still in fundamental flux. Most recently it has been realised that string theories which were previously thought to be distinct can be related to each other and should be thought of as part of a more fundamental theory ("M-theory" or "mystery theory") in which not only strings but other extended objects, such as membranes, enter on an equal footing.

The fact that it has taken so long to make progress in quantum gravity - at least 40 years if we go back to P.A.M. Dirac's original work - without yet arriving at a testable hypothesis, does not mean that the matter is unanswerable as Horgan implies.

There was more than two centuries between Newton's theory of mechanics and Einstein's relativity theory which radically modified it.

Science has progressed at a stupendous rate in the past century because more people have been working on it than ever before. But we should be careful about declaring that certain questions are unanswerable simply because of the impatience that such rapid progress engenders.

Brave souls will work on quantum gravity because of the intellectual thrill, despite the knowledge that a fully fledged theory may not be achieved in our lifetime.

There is good reason to believe, however, that answers to some presently unanswerable questions may not be that distant. The reason is that cosmology, which for much of this century lacked empirical data and was accorded the disdain inherent in Horgan's term "ironic science", is finally becoming a hard-core empirical science. Due to technological advances, we can now measure previously unmeasurable cosmological quantities.

The Hubble constant, which determines the present rate of expansion of the universe, will soon be known to within 10 per cent, as opposed to within a factor of two.

Large-scale red shift surveys, such as the two-degree field survey being undertaken by Australian and British astronomers at Coonabarabran, will enable us to determine other cosmological parameters that should definitively determine the structure and age of the universe.

Measurements of primordial fluctuations in the cosmic microwave background radiation on angular scales much finer than those of NASA's recent COBE experiment should allow us to determine the entire matter content of the universe: with enough data, we can untangle the sum of the characteristic spectra that various forms of matter would leave as relic "fingerprints" in the microwave radiation.

Gravitational wave detectors will enable us to "listen" to the collisions of black holes that are believed to lurk in the hearts of galaxies, thus enabling us to glean information about strong field gravity that has been denied to us up to the present.

In short, technology has finally reached the point that will enable the next 10 to 20 years to be a golden age for cosmology, just as atomic experiments led to a golden age for quantum mechanics in the 1920s and 30s. The excitement of practitioners in cosmology is palpable: we are very lucky to be alive right now!

The empirical power of future astronomical observations will not simply stop with a description of the large-scale structure of the universe, however. As ever-increasing energies are available, the closer one gets to the initial "moment" of creation, the very early universe provides the ultimate laboratory for not only our current theories but any conceivable theory of physics.

Particle physicists have thus far had the good fortune that governments have been willing to finance their large particle accelerators.

However, the energy scales have increased and it appears we are approaching the point where reaching higher energies may be politically impossible, as the demise of the Superconducting Supercollider shows. Observational data is much harder to interpret than experimental data, as one has no control over the physical conditions. However, with a bit of effort, all sorts of information are conceivably sitting out there in the universe and, if we are to make testable hypotheses in quantum gravity, this may well be the route we may have to take.

Physics and science are far from dead. Very fundamental issues remain to be resolved and these will involve answerable questions.

The only force that we have truly mastered to date is electromagnetism. When we can engineer the strong nuclear force to the extent that we can engineer the electromagnetic one, building useful everyday devices more sophisticated than medical tracers and clumsy nuclear bombs, when we understand quantum gravity so intimately that we could imagine how to turn it into an applied science also, then I think it would be fair to claim that physics, at least, has been "finished".

Fortunately, I know that that is not likely to be the case for a good while yet and many generations of mathematical physicists still have a chance to share in the thrill of discovery.

David Wiltshire is a research fellow in mathematical physics at the University of Adelaide.