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The Goal of Theoretical Physics The goal of theoretical physics Paul Dirac, who discovered the equation which describes the behaviour of the electron. He has pointed out that this equation is the basis of 'most of physics and all of chemistry'. 75 the end in sight for theoretical of the century it was believed that physics ?'This was the question posed everything could be understood in by Stephen Hawking in his inaugural terms of continuum mechanics. All lecture * as incumbent of the presti­ that was needed was to measure a gious Lucasian Chair of Mathematics certain number of coefficients of in the University of Cambridge. elasticity, viscosity, conductivity etc. Among his many other accomplish­ This hope was shattered by the dis­ ments, Hawking has successfully covery of atomic structure and quan­ brought together ideas from particle tum mechanics. Again, in the late physics, from general relativity and 1920s Max Born told a group of from thermodynamics (see October scientists visiting Gottingen that 1977 edition, page 334) which have 'Physics, as we know it, will be over greatly influenced thinking on the in six months.' This was shortly after origin of the Universe. the discovery by Paul Dirac of the We are publishing the text of the equation which governs the behav­ lecture in two parts, the first dealing iour of the electron. It was expected with the motivations for constructing that a similar equation would govern unified theories in physics, and going the proton, the only other sup­ on to describe current efforts to unify posedly elementary particle known the electroweak picture with that of at that time. However, the discovery quark dynamics. As an influential phy­ of the neutron and of nuclear forces sicist not directly involved with parti­ disappointed these hopes. We now cle research, Hawking s views on par­ know that neither the proton nor the ticle theory make interesting reading. neutron are elementary but that they In the second part, to be published in are made up of smaller particles. our March issue, Hawking goes on to Nevertheless, we have made a lot of cover the subject of gravity, where his progress in recent years and there statistical methods and abandon any pronouncements always provide sub­ are some grounds for cautious pretence of solving the equations stantial food for thought. optimism that we may soon see a exactly. Although in principle we complete theory. know the equations that govern the Even if we do achieve a complete whole of biology, we have not been unified theory, we shall not be able to able to reduce the study of human make detailed predictions in any but behaviour to a branch of applied the simplest situations. For example, mathematics. 'I want to discuss the possibility that we already know the physical laws What would we mean by a com­ the goal of theoretical physics might that govern everything that we plete and unified theory of physics? be achieved in the not too distant experience in everyday life: as Dirac Our attempts at modelling physical future, say, by the end of the century. pointed out, his equation was the reality normally consist of two By this I mean that we might have a basis of 'most of physics and all of parts: complete, consistent and unified chemistry'. However we have been 1. A set of local laws that are obeyed theory of the physical interactions able to solve the equation only for by the various physical quantities. which would describe all possible the very simplest system, the These are usually formulated in observations. hydrogen atom consisting of one terms of differential equations. Of course one has to be very proton and one electron. For more 2. Sets of boundary conditions that cautious about making such predic­ complicated atoms with more elec­ tell us the state of some regions of tions : we have thought that we were trons, let alone for molecules with the Universe at a certain time and on the brink of the final synthesis at more than one nucleus, we have to what effects propagate into it sub­ least twice before. At the beginning resort to approximations and intui­ sequently from the rest of the Uni­ tive guesses of doubtful validity. For verse. * 'Is the end in sight for theoretical physics?' macroscopic systems consisting of by Stephen Hawking, published by Cam­ Many people would claim that the 23 bridge University Press. 1 0 particles or so, we have to use role of science was confined to the CERN Courier, January/February 1981 3 first of these and that theoretical that stars' would never have been makes it very difficult to hold an physics will have achieved its goal formed by gravitational condensa­ anthropocentric view or to believe when we have obtained a complete tion. that the structure of the Universe set of local physical laws. They Some people have gone so far as is determined by anything so would regard the question of the to elevate these restrictions on the peripheral as some complicated initial conditions for the Universe as initial conditions and the parameters molecular structures on a minor pla­ belonging to the realm of metaphy­ to the status of a principle, the net orbiting a very average star in the sics or religion. In a way this attitude Anthropic Principle, which can be outer suburbs of a fairly typical spiral is similar to that of those who in paraphrased as 'Things are as they galaxy. earlier centuries discouraged scien­ are because we are'. According to If we are not going to appeal to the tific investigation by saying that all one version of the principle there is a Anthropic Principle, we need some natural phenomena were the work of very large number of different sepa­ unifying theory to account for the God and should not be inquired into. rate universes with different values initial conditions of the Universe and I think that the initial conditions of of the physical parameters and dif­ the values of the various physical the Universe are as suitable a sub­ ferent initial conditions. Most of parameters. However, it is too diffi­ ject for scientific study and theory as these universes will not provide the cult to think up a complete theory of are the local physical laws. We shall right conditions for the development everything all at one go (though this not have a complete theory until we of the complicated structures does not seem to stop some people; can do more than merely say that needed for intelligent life. Only in a I get two or three unified theories in 'Things are as they are because they small number, with conditions and the mail each week). What we do were as they were.' parameters like our own universe, instead is to look for partial theories The question of the uniqueness of will it be possible for intelligent life to that will describe situations in which the initial conditions is closely develop and to ask the question certain interactions can be ignored related to that of the arbitrariness of 'Why is the Universe as we observe or approximated in a simple manner. the local physical laws: one would it ?' The answer is, of course, that if it We first divide the material content not regard a theory as complete if it were otherwise there would not be of the Universe into two parts, 'mat­ contained a number of adjustable anyone to ask the question. ter' particles such as quarks, elec­ parameters such as masses or cou­ The Anthropic Principle does pro­ trons, muons etc., and 'interactions' pling constants which could be given vide some sort of explanation of such as gravity, electromagnetism any values one liked. In fact it seems many of the remarkable numerical etc. that neither the initial conditions nor relations that are observed between The matter particles are described the values of the parameters in the the values of different physical para­ by fields of half-integer spin and theory are arbitrary but that they are meters. However, it is not com­ obey the Pauli Exclusion Principle somehow chosen or picked out very pletely satisfactory: one cannot help which prevents more than one parti­ carefully. For example, if the proton- feeling that there is some deeper cle of a given kind from being in any neutron mass difference were not explanation. Also, it cannot account state. This is the reason that we can about twice the mass of the electron, for all the regions of the Universe. have solid bodies that do not col­ one would not obtain the couple of For example, our solar system is lapse to a point or radiate away to hundred or so stable nuclides that certainly a prerequisite for our exis­ infinity. The matter particles are div­ make up the elements and are the tence as is an earlier generation of ided into two groups, the hadrons, basis of chemistry and biology. Sim­ nearby stars in which heavy ele­ which are composed of quarks, and ilarly if the gravitational mass of the ments could have been formed by the leptons, which comprise the proton were significantly different, nuclear synthesis. It might even be remainder. one would not have had stars in that the whole of our galaxy was The interactions are divided phe- which these nuclides could have required. But there does not seem nomenologically into four catego­ been built up and if the initial expan­ any necessity for other galaxies to ries. In order of strength they are: the sion of the Universe had been exist, let alone the million million or strong nuclear forces which interact slightly smaller or greater, the Uni­ so of them that we see, distributed only with hadrons, electromagne­ verse would either have collapsed roughly uniformly throughout the tism which interacts with charged before such stars could have evolved observable Universe.
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