Wednesday, October 05, 2011

The future of science: Will models usurp theories?

This year's Nobel Prize for physics was awarded to Saul Perlmutter, Brian Schmidt and Adam Riess for their discovery of an accelerating universe, a finding leading to the startling postulate that 75% of our universe contains a hitherto unknown entity called dark energy. All three were considered favorite candidates for a long time so this is not surprising at all. The prize also underscores the continuing importance of cosmology since it had been awarded in 2o06 to George Smoot and John Mather, again for confirming the Big Bang and the universe's expansion.

This is an important discovery which stands on the shoulders of august minds and an exciting history. It continues a grand narrative that starts from Henrietta Swan Leavitt (who established a standard reference for calculating astronomical distances) through Albert Einstein (whose despised cosmological constant was resurrected by these findings) and Edwin Hubble, continuing through George Lemaitre and George Gamow (with their ideas about the Big Bang) and finally culminating in our current sophisticated understanding of the expanding universe. Anyone who wants to know more about the personalities and developments leading to today's event should read Richard Panek's excellent book "The 4 Percent Universe".

But what is equally interesting is the ignorance that the prizewinning discovery reveals. The prize was really awarded for the observation of an accelerating universe, not the explanation. Nobody really knows why the universe is accelerating. The current explanation for the acceleration consists of a set of different models, none of which has been definitively proven to explain the facts well enough. And this makes me wonder if such a proliferation of models without accompanying concrete theories is going to embody science in the future.

The twentieth century saw theoretical advances in physics that agreed with experiment to an astonishing degree of accuracy. The culmination of achievement in modern physics was surely quantum electrodynamics (QED) which is supposed to be the most accurate theory of physics we have. Since then we have had some successes in quantitatively correlating theory to experiment, most notably in the work on validating the Big Bang and the development of the standard model of particle physics. But dark energy- there's no theory for it that remotely approaches the rigor of QED when it comes to comparison with experiment.

Of course it's unfair to criticize dark energy since we are just getting started on tackling its mysteries. Maybe someday a comprehensive theory will be found, but given the complexity of what we are trying to achieve (essentially explain the nature of all the matter and energy in the universe) it seems likely that we may always be stuck with models, not actual theories. And this may be the case not just with cosmology but with other sciences. The fact is that the kinds of phenomena that science has been dealing with recently have been multifactorial, complex and emergent. The kind of mechanical, reductionist approaches that worked so well for atomic physics and molecular biology may turn out to be too impoverished for taking apart these phenomena. Take biology for instance. Do you think we could have a complete "theory" for the human brain that can quantitatively calculate all brain states leading to consciousness and our reaction to the external world? How about trying to build a "theory" for signal transduction that would allow us to not just predict but truly understand (in a holistic way) all the interactions with drugs and biomolecules that living organisms undergo? And then there's other complex phenomena like the economy, the weather and social networks. It seems wise to say that we don't anticipate real overarching theories for these phenomena anytime soon.

On the other hand, I think it's a sign of things to come that most of these fields are rife with explanatory
models of varying accuracy and validity. Most importantly, modeling and simulation are starting to be considered as a respectable "third leg" of science, in addition to theory and experiment. One simple reason for this is the recognition that many of science's greatest current challenges may not be amenable to quantitative theorizing, and we may have to treat models of phenomena as independent, authoritative explanatory entities in their own right. We are already seeing this happen in chemistry, biology, climate science and social science, and I have been told that even cosmologists are now extensively relying on computational models of the universe. Admittedly these models are still far behind theory and experiment which have had head starts of about a thousand years. But there can be little doubt that such models can only become more accurate with increasing computational firepower. How accurate remains to be seen, but it's worth noting that there are already books that make a case for an independent, study-worthy philosophy of modeling and simulation. These books extol philosophers of science to treat models not just as convenient applications and representations of theories (which are then the only fundamental things worth studying) but as ultimate independent explanatory devices in themselves that deserve separate philosophical consideration.

Could this then be at least part of the future of science? A future where robust experimental observations are encompassed not by beautifully rigorous and complete theories like general relativity or QED but only by different models which are patched together through a combination of rigor, empirical data, fudge factors and plain old intuition? This would be a new kind of science, as useful in its applications as its old counterpart but rooting itself only in models and not in complete theories. Given the history of theoretical science, such a future may seem dark and depressing. That is because as the statistician George Box famously quipped, although some models are useful, all models are wrong. What Box meant was that models often feature unrealistic assumptions about all kinds of details that nonetheless allow us to reproduce the essential features of reality. Thus they can never provide the sure connection to "reality" that theories seem to. This is especially a problem when disparate models give the same answer to a question. In the absence of discriminating ideas, which model is then the "correct" one? The usual answer is "none of them", since they all do an equally good job of explaining the facts. But this view of science, where models that can be judged only on the basis of their utility are the ultimate arbiters of reality and where there is thus no sense of a unified theoretical framework, feels deeply unsettling. In this universe the "real" theory will always remain hidden behind a facade of models, much as reality is always hidden behind the event horizon of a black hole. Such a universe can hardly warm the cockles of the heart of those who are used to crafting grand narratives for life and the universe. However it may be the price we pay for more comprehensive understanding. In the future, Nobel Prizes may be frequently awarded for important observations for which there are no real theories, only models. The discovery of dark matter and energy and our current attempts to understand the brain and signal transduction could well be the harbingers of this new kind of science.

Should we worry about such a world rife with models and devoid of theories? Not necessarily. If there's one thing about science that we know, it's that it evolves. Grand explanatory theories have traditionally been supposed to be a key part- probably
the key part- of the scientific enterprise. But this is mostly because of historical precedent as well a psychological urge for seeking elegance and unification. Such belief has been resoundingly validated in the past but it's utility may well have plateaued. I am not advocating some "end of science" scenario here - far from it - but as the recent history of string theory and theoretical physics in general demonstrates, even the most mathematically elegant and psychologically pleasing theories may have scant connection to reality. Because of the sheer scale and complexity of what we are trying to currently explain, we may have hit a roadblock in the application of the largely reductionist traditional scientific thinking which has served us so well for half a millennium

Ultimately what matters though is whether our constructs- theories, models, rules of thumb or heuristic pattern recognition- are up to the task of constructing consistent explanations of complex phenomena. The business of science is explanation, whether through unified narratives or piecemeal explanation is secondary. Although the former sounds more psychologically satisfying, science does not really care about stoking our egos. What is out there exists, and we do whatever's necessary and sufficient to unravel it.

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Sunday, October 02, 2011

Book Review: Robert Laughlin's "Powering the Future"

In the tradition of physicists writing for the layman, Robert Laughlin has emerged as a writer who pens unusually insightful and thought-provoking books. In his "A Different Universe" he explored the consequences and limitations of reductionism-based physics for our world. In this book he takes an equally fresh look at the future of energy. The book is not meant to be a comprehensive survey of existing and upcoming technologies; instead it's more like an assortment of appetizers designed to stimulate our thinking. For those who want to know more, it offers an impressive bibliography and list of calculations which is almost as long as the book itself.

Laughlin's thinking is predicated on two main premises. The first is that carbon sources are going to eventually run out or become inaccessible (either because of availability or because of legislation). However we will still largely depend on carbon because of its extraordinarily fortuitous properties like high energy density, safety and ease of transportation. But even in this scenario, simple rules of economics will trump most other considerations for a variety of different energy sources. The second premise which I found very intriguing is that we need to uncouple our thinking on climate change from that on energy instead of letting concerns about the former dictate policy about the latter. The reason is that planetary-level changes in the environment are so vast and beyond the ability of humans to control that driving a few more hybrids or curbing carbon emissions will have little effect on millennial events like the freezing or flooding of major continents. It's worth noting here that Laughlin (who has been called a climate change skeptic lately) is not denying global warming or its consequences here; it's just that he thinks that it's sort of beside the point when it comes to thinking about future energy, which will be mainly dictated by economics and prices more than anything else. I found this to be a commonsense approach based on an appreciation of human nature.

With this background Laughlin takes a sweeping and eclectic look at several interesting technologies and energy sources including nuclear energy, biofuels, energy from trash, wind and solar power and energy stored beneath the sea. In each case Laughlin explores a variety of problems and promises associated with these sources.

Because of dwindling uranium resources, the truly useful form of nuclear energy for instance will come from fast breeder reactors which produce their own plutonium fuel. However these reactors are more susceptible to concerns about proliferation and theft. Laughlin thinks that a worldwide, tightly controlled system of providing fuel rods to nations would allow us to fruitfully deploy nuclear power. One of his startling predictions is the possibility that we may put up with occasional Chernobyl-like events if nuclear power truly becomes cheap and we don't have any other alternatives.

Laughlin also finds promises and pitfalls in solar energy. The basic problem with solar energy is its irregular availability and problems with storage. Backup power inevitably depends on fossil fuel sources which sort of defeats the purpose. Laughlin sees a bright future for molten salt tanks which can very efficiently store solar energy as heat and which can be used when the sun is not shining. These salts are simple eutectic mixtures of potassium and sodium nitrates with melting points that are conveniently lowered even more by the salts' decomposition products. Biofuels also get an interesting treatment in the book. One big advantage of biofuels is that they are both sources and sinks of carbon. Laughlin talks about some recent promising work with algae but cautions that meeting the sheer worldwide demand for energy with biofuels that don't divert resources away from food is very challenging. Further on there's a very intriguing chapter on energy stored under the sea. The sea provides a stupendous amount of land beneath it and could be used for energy storage through novel sources like high-density brine pools and compressed natural gas tanks. Finally, burning trash which has a lot of carbon might appear like a useful source of energy but as Laughlin explains, the actual energy in trash will provide only a fraction of our needs.

Overall the book presents a very thought-provoking treatment of the nature and economics of possible future energy sources in a carbon-strapped world. In these discussions Laughlin wisely avoids taking sides, realizing how fraught with complexity and ambiguity future energy production is. Instead he simply offers his own eclectic thoughts on the pros and cons of energy-related topics which may (or may not) prove important in the future. Of the minor gripes I have with the volume is the lack of discussion of promising recent advances in solar cell design, thorium-based fuels and next generation nuclear reactor technology. Laughlin's focus is also sometimes a little odd and meandering; for instance at one point he spends an inordinate amount of time talking about interesting aspects of robotic technology that may make deep sea energy sequestration possible. But these gripes detract little from the volume which is not really supposed to be an exhaustive survey of alternative energy technologies.

Instead it offers us a very smart scientist's miscellaneous musings on energy dictated by commonsense assumptions based on the simple laws of demand and supply and of human nature. As responsible citizens we need to be informed on our energy choices which are almost certainly going to become more difficult and constrained in the future. Laughlin's book along with others will stimulate our thinking and help us pick our options and chart our direction.

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