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The goal of theoretical - 2

Werner Heisenberg, discoverer of the famous . Physics becomes even more uncertain when black holes are involved.

(Photo Pfeiffer)

Last month we published the first part prediction of singularities presuma­ of '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 becomes strong enough so that to the subject of . After Ein­ quantum gravitational effects are stein's monumental work on gravita­ important. Thus a quantum of tion earlier this century, efforts to gravity is essential if we are to des­ mite this force with the other cribe the early 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 , 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 or are and . The fourth the singularities in the 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 way beyond those very structure of 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 . corresponding infinite number of the past at the beginning of the In this the , 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 transformations. the 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 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 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 with a typical 2 the black hole causes the creation of sian Professor of 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 . 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 . 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 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 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. However it seems that gravity the velocities of the emitted parti­ senior position in . should provide a limit but only at the cles. All one can give are probabili­ very short length scale of 10~33 cm

72 CERN Courier, March 1981 or the very high energy of 1028 eV. particles that carry the weak interac­ special. It seems that it may be the On length scales shorter than this, tions, but not the other two. One only theory: one would expect that space-time would therefore have to believe that 1. which is in four dimensions, would cease to behave like a smooth many or most of the observed parti­ 2. incorporates gravity, continuum and that it would acquire cles such as gluons or quarks are not 3. which is finite without any infinite a foam-like structure because of really elementary as they seem at subtractions. quantum fluctuations of the gravita­ the moment but that they are bound I have already pointed out that the tional field. states of the fundamental N = 8 third property is necessary if we are There is a very large unexplored particles. It is not likely that we shall to have a complete theory without region between our present experi­ have accelerators powerful enough parameters. But it is difficult to mental limit of about 1010 eV and to probe these composite structures account for properties 1 and 2 with­ the gravitational cut-off at 1028 eV. within the foreseeable future, or out appealing to the 'Anthropic Prin­ It might seem naive to assume, as indeed ever, especially if one makes ciple'. There seems to be a consist­ Grand Unified do, that a projection based on the current ent theory which satisfies proper­ there are only one or two layers of economic trends. Nevertheless the ties 1 and 3 but which does not structure in this enormous interval. fact that these bound states arose include gravity. However, in such a However, there are grounds for from the well-defined N = 8 theory universe there would probably not optimism: at the moment at least it should enable us to make a number be sufficient in the way of attractive seems that gravity can be unified of predictions that could be tested at forces to gather together matter into with the other physical interactions energies that are accessible now or the large aggregates which are prob­ only in some supergravity theory. will be in the near future. The situa­ ably necessary for the development There appear to be only a finite tion might thus be similar to that for of complicated structures. Why number of such theories. In particu­ the Salam-Weinberg Theory unify­ space-time should be four-dimen­ lar, there is a large such theory, the ing electromagnetism and weak sional is a question that is normally so-called N= 8 extended supergrav­ interactions. The low energy predic­ considered to be outside the realm of ity. This contains one graviton, eight tions of this theory are in such good physics. However there is a good spin-3/2 particles called , agreement with observation that the Anthropic Principle argument for twenty-eight spin-1 particles, fifty- theory is now generally accepted that too. Three space-time dimen­ six spin-1/2 particles and seventy par­ even though we have not yet sions, that is, two space and one ticles of spin 0. Large as these num­ reached the energy at which the time, are clearly insufficient for any bers are they are not large enough to unification should take place. complicated organism. On the other account for all the particles that we There ought to be something very hand, if there were more than three seem to observe in strong and weak distinctive about the theory that des­ spatial dimensions, the orbits of pla­ interactions. For instance the twen­ cribes the Universe. Why does this nets round the Sun or electrons ty-eight spin-1 particles in the N = 8 theory come to life while other theo­ round a nucleus would be unstable theory are sufficient to account for ries exist only in the minds of their and they would tend to spiral the gluons that carry the strong inventors? The N = 8 supergravity inwards. There remains the possibil­ interactions and two of the four theory does have some claims to be ity of more than one time dimension

CERN Courier, March 1981 73 but I, for one, find such a universe laws which would curtail the free­ is no good. If the theory survives very hard to imagine. dom of God to change his mind?' these tests, it will probably be some So far I have implicitly assumed Nevertheless they did, and got away years more before we develop com­ that there is an ultimate theory. But with it. In modern times we have putational methods that will enable is there? There are at least three effectively eliminated possibility 3 us to make predictions and before possibilities: by incorporating it within our we can account for the initial con­ 1. There is a complete unified the­ scheme: quantum is ditions of the Universe as well as the ory. essentially a theory of what we do local physical laws. These will be the 2. There is no ultimate theory, but not know and cannot predict. outstanding problems for theoretical there is an infinite sequence of Possibility 2 would amount to the in the next twenty years c theories which are such that any picture of an infinite sequence of so. particular class of observations structures at higher and higher ener­ But, to end on a slightly alarmist can be predicted by taking a gies. As I said before, this seems note, they may not have much more theory sufficiently far down the unlikely because one would expect time than that. At present computers chain. that there would be a cut-off at the are a useful aid in research but they 3. There is no theory. Observations Planck energy of 1028 eV. This have to be directed by human minds. cannot be described or predicted leaves us with possibility 1. However, if one extrapolates their beyond a certain point but are just At the moment the N = 8 super- recent rapid rate of development, it arbitrary. gravity theory is the only candidate would seem quite possible that they The third view was advanced as an in sight. There are likely to be a will take over altogether in theoreti­ argument against the of number of crucial calculations within cal physics. So maybe the end is in the seventeenth and eighteenth cen­ the next few years which have the sight for theoretical physicists if not turies. 'How could they formulate possibility of showing that the theory for theoretical physics.' People and things

Policy Committee. The award was He is an experimentalist working On people made in recognition of his 'out­ mainly in the fields of photoproduc- standing contribution to the organ­ tion and electron-positron annihila­ At the Annual Meeting of the Amer­ ization of experimental high energy tion, and at present is involved in ican Physical Society and the Amer­ physics, particularly through the the NA1 andNA7 at ican Association of Physics Teach­ direction of the Rutherford Labo­ the CERN SPS. ers, held in New York in January, ratory/ retiring APS President Herman Frank Solmitz, from the Feshbach awarded the Heinemann R. Scrimaglio has been reappointed Lawrence Berkeley Laboratory, died Prize to Jeffrey Go/dstone of MIT for another period of three years on 28 August last year following for his work in , as Director of the Frascati National five years of incapacity after a and quan­ Laboratory of INFN (Italy). Professor tragic accident. He made major tum field theory. Scrimaglio is a well-known experi­ contributions in the early days of mentalist in intermediate energy bubble chamber physics in develop­ The UK Institute of Physics has and nuclear physics. He is at pre­ ing the associated computational announced the award of the Glaze- sent collaborating in an experiment techniques. These techniques were brook Medal and Prize for 1981 to being prepared for the LEAR low crucial to the many particle discov­ Godfrey Stafford, Director General energy antiproton ring at CERN. eries made by Luis Alvarez' group of the Rutherford and Appleton P. Spillantini has been nominated at Berkeley. Frank Solmitz had suc­ Laboratories and until recently as successor to A. Reale as Leader ceeded Alvarez as group leader in Chairman of the CERN Scientific of the Research Division at Frascati. 1970.

74 CERN Courier, March 1981