The goal of theoretical physics - 2 Werner Heisenberg, discoverer of the famous Uncertainty Principle. Physics becomes even more uncertain when black holes are involved. (Photo Pfeiffer) Last month we published the first part prediction of singularities presuma­ of Stephen Hawking's inaugural lec­ bly indicates that the Classical The­ ture * as Lucasian Professor of Mathe­ ory will break down. However, there matics in the University of Cambridge. seems to be no reason why it should In the second part of the lecture, break down until the gravitational which follows here. Hawking turned field becomes strong enough so that to the subject of gravity. After Ein­ quantum gravitational effects are stein's monumental work on gravita­ important. Thus a quantum theory of tion earlier this century, efforts to gravity is essential if we are to des­ mite this force with the other cribe the early Universe and to give mechanisms found in Nature have some explanation for the initial con­ been consistently unfruitful. But there ditions beyond merely appealing to are indications that this is beginning to the Anthropic Principle, which can change. be paraphrased as 'things are as they are because we are'. Such a theory is also required if we 'So far most of the effort has been are to answer the question 'Does devoted to unifying the first three time really have a beginning and, categories of physical interactions, possibly, an end as is predicted by the strong and weak nuclear forces Classical General Relativity or are and electromagnetism. The fourth the singularities in the Big Bang and and last, gravity, has been neglected. the Big Crunch smeared out in some One justification for this is that grav­ way by quantum effects?' ity is so weak that quantum gravita­ This is a difficult question to give a tional effects would be large only at well-defined meaning to when the particle energies way beyond those very structure of space and time in any particle accelerator. Another themselves are subject to the Uncer­ Interactions' represented by inte­ is that gravity does not seem to be tainty Principle. My personal feeling ger-spin particles. It also has the enormalizable: in order to obtain is that singularities are probably still great advantage that many of the Jnite answers it seems that one may present though one can continue infinities which arise in quantum the­ have to make an infinite number of time past them in a certain mathe­ ory cancel each other out. Whether infinite subtractions with a corres­ matical sense. However any subjec­ or not they all cancel out to give a pondingly infinite number of unde­ tive concept of time that was related theory which is finite without any termined finite remainders. to consciousness or the ability to infinite subtractions is not yet Yet one must include gravity if one perform measurements would come known. It is hoped that they do is to obtain a fully unified theory. to an end. because it can be shown that theo­ Furthermore the Classical Theory of What are the prospects of obtain­ ries which include gravity are either General Relativity predicts that ing a quantum theory of gravity and finite or non-renormalizable, that is, there should be space-time singular­ of unifying it with the other three if one has to make any infinite sub­ ities at which the gravitational field categories of interactions? The best tractions, then one will have to make would become infinitely strong. hope seems to lie in an extension of an infinite number of them with a These singularities would occur in general relativity called supergravity. corresponding infinite number of the past at the beginning of the In this the graviton, the spin-2 par­ undetermined remainders. Thus if all present expansion of the Universe ticle that carries the gravitational the infinities in supergravity cancel (the Big Bang) and in the future inthe interaction, is related to a number of each other out, we could have a gravitational collapse of stars and, other fields of lower spin by so-call­ theory which not only fully unified all possibly, of the Universe itself. The ed supersymmetry transformations. the matter particles and interactions, Such a theory has the greater merit but which was also complete in the that it does away with the old dicho­ *' Is the end in sight for theoretical physics ?' sense that it did not have any unde­ by Stephen Hawking, published by Cam­ tomy between 'matter' represented termined renormalization para­ bridge University Press. by particles of half-integer spin and meters. CERN Courier, March 1981 71 The main computer centre at CERN. According to Stephen Hawking, the recent rapid rate of development of computers could mean that machines will take over theoretical physics! (Photo CERN 186.12.80) Although we do not yet have a ties that particles will be emitted in distribution. This might explain why proper quantum theory of gravity, let certain modes. the Universe started off in the Big alone one which unifies it with the It seems therefore that, even if we Bang in almost perfect thermal other physical interactions, we have find a unified theory, we may be able equilibrium because thermal equili­ an idea of some of the features it to make only statistical predictions. brium would correspond to the lar­ should have. One of these is con­ We would also have to abandon the gest number of microscopic configu­ nected with the fact that gravity view that there is a unique universe rations and hence the greatest pro­ affects the causal structure of that we observe. Instead we would bability. To echo Voltaire's philoso­ space-time, that is, gravity deter­ have to adopt a picture in which pher Panglossf'we live in the most mines which events can be causally there was an ensemble of all possi­ probable of all possible worlds.' related to each other. An example of ble universes with some probability What are the prospects that we this in the classical theory of General will find a complete unified theory in Relativity is provided by a black hole, the not too distant future ? Each time which is a region of space-time in The Lucasian we have extended our observations which the gravitational field is so Chair to smaller length scales and higher strong that any light or other signal is energies, we have discovered new dragged back into the region and at Cambridge layers of structure. At the beginning cannot escape to the outside world. Gravity specialist Stephen of the century the discovery of Brow- The intense gravitational field near Hawking is the present Luca­ nian Motion with a typical energy 2 the black hole causes the creation of sian Professor of Mathematics particle of 3 x 10 eV showed that pairs of particles and antiparticles, at the University of Cam­ matter was not continuous but was one of which falls into the black hole bridge. His immediate prede­ made up of atoms. Shortly thereafter and the other of which escapes to cessor was Sir James Light- it was discovered that these sup­ infinity. The particle that escapes hill and before that Paul Di- posedly indivisible atoms were appears to have been emitted by the rac One of the earlier holders made up of electrons with energies black hole. of this prestigious Chair was of the order of a few electronvolts An observer at a distance from the revolving about a nucleus. The Isaac Newtonf the founder of black hole can measure only the the theory of gravitation, nucleus in turn was found to be outgoing particles and he cannot which makes Hawking's ap­ composed of so-called elementary correlate them with those that fell pointment especially apt. Back particles, protons and neutrons, held into the hole because he cannot in the 1660s, Newton fs pred­ together by nuclear bonds of the 6 observe them. This means that the ecessor as Lucasian Professor order of 10 eV. The latest episode outgoing particles have an extra was Isaac Barrow, a remark­ in this story is that we have found degree of randomness or unpredic­ able character who was also that the proton and the neutron are tability over and above that usually Regius Professor of Greek made up of quarks held together by 9 associated with the Uncertainty and Gresham Professor of bonds of order 10 eV. It is a tribute Principle. In normal situations the Geometry. While Newton was to how far we have come already in Uncertainty Principle implies that withdrawn and introvert Bar­ theoretical physics that it now takes one can definitely predict either the row was a swashbuckler who enormous machines and a great deal position or the velocity of a particle among other things was a of money to perform an experiment or one combination of position and fighter of some repute. Again whose results we cannot predict. velocity. Thus, roughly speaking, in contrast to Newton who Our past experience might sug­ one's ability to make definite predic­ seldom ventured far, Barrow gest that there is an infinite tions is halved. However, in the case was a seasoned traveller who sequence of layers of structure at of particles emitted from a black once successfully defended higher and higher energies. Indeed, hole, the fact that one cannot his ship against attack by pi­ such a view of an infinite regress of observe what is going on inside the rates. In these days of tight boxes within boxes was official black hole means that one can defin­ budgets, this particular attri­ dogma in China under the Gang of itely predict neither the position nor bute remains valuable in any Four.