Essay

LAWS OF PHYSICS

Adapted from P. Coles, 1999, The Routledge Critical Dictionary of the New Cosmology, Routledge Inc., New York. Reprinted with the author's permission. To order this book click here: http://www.routledge-ny.com/books.cfm?isbn=0415923549

The basic tools of physical science, sometimes called the laws of nature, comprising mathematical equations that govern the behaviour of matter (in the form of elementary particles) and energy according to various fundamental interactions. Experimental results obtained in the laboratory or through observations of natural physical processes can be used to infer mathematical rules which describe these data. Alternatively, a theory may be created first as the result of a hypothesis or physical principle, which receives experimental confirmation only at a later stage.

As our understanding evolves, seemingly disparate physical laws become unified in a single overarching theory. The tendency of apples to fall to the ground and the tendency of the Moon to orbit the Earth were thought to be different things before the emergence of Isaac Newton's laws of motion and his theory of gravity. This theory was thought to be complete until the work of Albert Einstein, who showed that it was lacking in many aspects. A more complete (and much more mathematically intricate) theory of general relativity took the place of Newton's theory in 1915. In modern times, physicists are trying to unify general relativity with the rest of the theory of fundamental interactions into a theory of everything, a single mathematical formula from which all of physics can be derived (see also grand unified theory, string theory, supersymmetry).

Although this ambitious programme is far from complete, similar developments have occurred throughout the history of science, to the extent that the exact form of laws of physics available to working scientists changes significantly with time. Nevertheless, the task of a physical cosmologist remains the same: to take whatever laws are known (or whichever hypotheses one is prepared to accept) and work out their consequences for the evolution of the Universe at large. This is what cosmologists have done all down the ages, from Aristotle to the present generation of early-Universe cosmologists.

But there are deep philosophical questions below the surface of all this activity. For example, what if the laws of physics were different in the early Universe - could we still carry out meaningful research? The answer to this is that modern physical theories actually predict that the laws of physics do change, because of the effects of spontaneous symmetry-breaking. At earlier and earlier stages in the Big Bang theory, for example, the nature of the electromagnetic and weak interactions changes so that they become indistinguishable at sufficiently high energies. But this change in the law is itself described by another law: the so-called electroweak theory. Perhaps this law itself is modified at scales on which grand unified theories take precedence, and so on right back to the very beginning of the Universe.

Whatever the fundamental rules may be, however, physicists have to assume that they apply for all times since the Big Bang. It is merely the low-energy outcomes of these fundamental rules that change with time. By making this assumption they are able to build a coherent picture of the thermal history of the Universe which does not seem to be in major makes the assumption reasonable, but does not prove it to be correct.

Another set of important questions revolves around the role of mathematics in physical theory. Is nature really mathematical, or are the rules we devise merely a kind of shorthand to enable us to describe the Universe on as few pieces of paper as possible? Do we discover laws of physics, or do we invent them? Is physics simply a map, or is it the territory itself?

There is also another deep issue connected with the laws of physics pertaining to the very beginning of space and time. In some versions of quantum cosmology, for example, we have to posit the existence of physical laws in advance of the physical universe they are supposed to describe. This has led many early-Universe physicists to embrace a neo-Platonist philosophy in which what really exists is the mathematical equations of the (as yet unknown) theory of everything, rather than the physical world of matter and energy. But not all cosmologists get carried away in this manner. To those of a more pragmatic disposition the laws of physics are simply a useful description of our Universe, whose significance lies simply in their very usefulness.