Commentary

Complex, Not Peculiar

I'm am now reading Bill Bryson's A Short History of Everything, which is a popular survey of science ranging from cosmology to the evolution of man. Until this past year, I had not read popular writing on science since I was a teenager. Now I'm reading articles in popular magazines and in newspapers, and with this book, I have begun reading the popular books on astronomy and physics. A common belief threading Bryson's book and articles by other authors is that our universe is beyond comprehension. For instance, Bryson quotes a biologist named J. B. S. Haldane as saying “The universe is not only queerer than we suppose; it is queerer than we can suppose.”

There are two ways of taking this cliche: disagree with it, because it violates the spirit of science, or agree with it, because there are limits to our understanding of the universe. I myself disagree with this statement, even though I have argued for the related idea that there is a limit to our ability to understand the universe. In the last commentary I argued that we do not understand quantum mechanics, although the theory is an accurate description of what occurs in nature. But a limitation in our ability to understand nature is not necessarily the same as stating that the universe is stranger than we can suppose; I've seen astronomers come up with an amazing number of bizarre ideas to explain what turned out to be rather mundane processes. It is not the universe that is strange; rather, it is many of the speculative theories spun to explain what is seen that are strange.

Our mathematics give us unlimited opportunities to imagine strange universes. In principle, the number of mathematical theories is infinite, so physicists have no problem concocting new theories of fundamental physics when data contradict older theories. As long as we can place number on fundamental phenomena, we can develop mathematical equation that return those numbers. This becomes somewhat more difficult when explaining higher-level phenomena, because generally we must explain the phenomena with our limited set of established equations. But if the problem is too thorny to solve, perhaps changes in fundamental physics are justified.

When evidence that the galaxies in our universe are moving apart first surfaced, scientists developed two theories to explain the phenomena. The dominate theory was dubbed The Big Bang theory by its opponents. It simply states that matter in the universe is on average uniformly distributed, and this matter is spreading out, with gravity slowing the expansion. One conclusion from this theory is that the universe is a finite age, with the beginning of the universe defined by the limit of infinite density. A competing theory was the Steady-State cosmology, which hypothesized that a universe of average uniform density expands at a constant rate, and that matter and energy spontaneously come into existence to keep the density of the universe constant. The Big Bang cosmology explains the observations very well, while the Steady-State cosmology is unable to explain either the microwave radiation we see in all directions or the evolutionary changed seen when comparing distant galaxies to our neighboring galaxies. But are either of these theories strange? To me, the Steady-State cosmology is very strange, because it supposes that the universe does not conserve energy. The Big Bang cosmology, on the other hand, strikes me as very reasonable, for it is grounded in conventional physics.

Many of the most peculiar ideas in astronomy are not based on objects that have been observed, but on theories developed from simple mathematical principles. Black holes arise naturally in general relativity. Worm holes are hypothetical objects that are found in general relativity if matter is given some unphysical properties. Cosmic strings are hypothetical objects in some very speculative theories of particle physics. The existence of each of these entities is unproven, which is to say that they are things we have imagined rather than seen. Astronomy abounds with such speculative theories that give us an exotic universe. We have no problem in supposing the universe to be strange.

The real barrier to science is that the universe is more complex than we can comprehend. This is a particular problem for astronomy, because we receive so little information about astronomical sources. Many of the objects we study appear as points, so we have no direct information about their shapes. Can you imagine modeling Earth's atmosphere without being able to see the outlines of the continents and oceans? How does one model Earth's albedo without seeing where the clouds are?

Much of the art of theoretical astrophysics is in deciding what physics to neglect and what shapes to assume in simplifying a problem; the point is to look past the complexity, which cannot be observed, and capture the essential behavior of an astronomical object. This pervasive practice is a continued source of amusement to observers. One observer remarked to me that if a theorist had a dairy, all of his cows would be round.

An amusing consequence of this methodology is that astronomical theories fit best when the observations are primitive; when observational detail is absent, only the effects of the dominant physical processes are seen. This means that our theories are better at describing the distant stars than our Sun, which we observe in much greater detail. We understand the Sun better than the distant star, but phenomena these theories are describing are far more complex than what is seen in the distant stars.

It is this hidden complexity of the universe is the greatest impediment to progress in theory. When a simple theory fails to match the observations, how do we determine the reason? The theory is wrong, but is it dramatically wrong, so that a whole different theory must be developed, or is it wrong in the simplifying assumptions, so that the retention of a bit of neglected physics will solve the problem? In part, this problem is the reason so many exotic and speculative theories float among the theorists; the simplicity of new physics allows the theorist to avoid the complexity of our universe. We are faced not with a universe stranger than we can suppose, but rather with a universe more complex than we can sanction.