we find ourselves in a bewildering world. we want to makesense of what we see around us and to ask: what is thenature of the universe? what is our place in it and where didit and we come from? why is it the way it is?

to try to answer these questions we adopt some “worldpicture.” just as an infinite tower of tortoises supporting the fiatearth is such a picture, so is the theory of superstrings. bothare theories of the universe, though the latter is much moremathematical and precise than the former. both theories lackobservational evidence: no one has ever seen a giant tortoisewith the earth on its back, but then, no one has seen asuperstring either. however, the tortoise theory fails to be agood scientific theory because it predicts that people should beable to fall off the edge of the world. this has not been foundto agree with experience, unless that turns out to be theexplanation for the people who are supposed to havedisappeared in the bermuda triangle!

the earliest theoretical attempts to describe and explain theuniverse involved the idea that events and natural phenomenawere controlled by spirits with human emotions who acted in avery humanlike and unpredictable manner. these spiritsinhabited natural objects, like rivers and mountains, includingcelestial bodies, like the sun and moon. they had to beplacated and their favor sought in order to ensure the fertilityof the soil and the rotation of the seasons. gradually, however,it must have been noticed that there were certain regularities:

the sun always rose in the east and set in the west, whetheror not a sacrifice had been made to the sun god. further, thesun, the moon, and the planets followed precise paths acrossthe sky that could be predicted in advance with considerableaccuracy. the sun and the moon might still be gods, but theywere gods who obeyed strict laws, apparently without anyexceptions, if one discounts stories like that of the sun stoppingfor joshua.

at first, these regularities and laws were obvious only inastronomy and a few other situations. however, as civilizationdeveloped, and particularly in the last 300 years, more andmore regularities and laws were discovered. the success ofthese laws led laplace at the beginning of the nineteenthcentury to postulate scientific determinism; that is, he suggestedthat there would be a set of laws that would determine theevolution of the universe precisely, given its configuration at onetime.

laplace’s determinism was incomplete in two ways. it did notsay how the laws should be chosen and it did not specify theinitial configuration of the universe. these were left to god.

god would choose how the universe began and what laws itobeyed, but he would not intervene in the universe once it hadstarted. in effect, god was confined to the areas thatnineteenth-century science did not under-stand.

we now know that laplace’s hopes of determinism cannotbe realized, at least in the terms he had in mind. theuncertainty principle of quantum mechanics implies that certainpairs of quantities, such as the position and velocity of aparticle, cannot both be predicted with complete accuracy.

quantum mechanics deals with this situation via a class ofquantum theories in which particles don’t have well-definedpositions and velocities but are represented by a wave. thesequantum theories are deterministic in the sense that they givelaws for the evolution of the wave with time. thus if oneknows the wave at one time, one can calculate it at any othertime. the unpredictable, random element comes in only whenwe try to interpret the wave in terms of the positions andvelocities of particles. but maybe that is our mistake: maybethere are no particle positions and velocities, but only waves. itis just that we try to fit the waves to our preconceived ideasof positions and velocities. the resulting mismatch is the causeof the apparent unpredictability.

in effect, we have redefined the task of science to be thediscovery of laws that will enable us to predict events up to thelimits set by the uncertainty principle. the question remains,however: how or why were the laws and the initial state of theuniverse chosen?

in this book i have given special prominence to the lawsthat govern gravity, because it is gravity that shapes thelarge-scale structure of the universe, even though it is theweakest of the four categories of forces. the laws of gravitywere incompatible with the view held until quite recently thatthe universe is unchanging in time: the fact that gravity isalways attractive implies that the universe must be eitherexpanding or contracting. according to the general theory ofrelativity, there must have been a state of infinite density in thepast, the big bang, which would have been an effectivebeginning of time. similarly, if the whole universe recollapsed,there must be another state of infinite density in the future, thebig crunch, which would be an end of time. even if the wholeuniverse did not recollapse, there would be singularities in anylocalized regions that collapsed to form black holes. thesesingularities would be an end of time for anyone who fell intothe black hole. at the big bang and other singularities, all thelaws would have broken down, so god would still have hadcomplete freedom to choose what happened and how theuniverse began.

when we combine quantum mechanics with general relativity,there seems to be a new possibility that did not arise before:

that space and time together might form a finite,four-dimensional space without singularities or boundaries, likethe surface of the earth but with more dimensions. it seemsthat this idea could explain many of the observed features ofthe universe, such as its large-scale uniformity and also thesmaller-scale departures from homogeneity, like galaxies, stars,and even human beings. it could even account for the arrowof time that we observe. but if the universe is completelyself-contained, with no singularities or boundaries, andcompletely described by a unified theory, that has profoundimplications for the role of god as creator.

einstein once asked the question: “how much choice didgod have in constructing the universe?” if the no boundaryproposal is correct, he had no freedom at all to choose initialconditions. he would, of course, still have had the freedom tochoose the laws that the universe obeyed. this, however, maynot really have been all that much of a choice; there may wellbe only one, or a small number, of complete unified theories,such as the heterotic string theory, that are self-consistent andallow the existence of structures as complicated as humanbeings who can investigate the laws of the universe and askabout the nature of god.

even if there is only one possible unified theory, it is just aset of rules and equations. what is it that breathes fire into theequations and makes a universe for them to describe? theusual approach of science of constructing a mathematical modelcannot answer the questions of why there should be a universefor the model to describe. why does the universe go to all thebother of existing? is the unified theory so compelling that itbrings about its own existence? or does it need a creator, and,if so, does he have any other effect on the universe? and whocreated him?

up to now, most scientists have been too occupied with thedevelopment of new theories that describe what the universe isto ask the question why. on the other hand, the people whosebusiness it is to ask why, the philosophers, have not been ableto keep up with the advance of scientific theories. in theeighteenth century, philosophers considered the whole of humanknowledge, including science, to be their field and discussedquestions such as: did the universe have a beginning?

however, in the nineteenth and twentieth centuries, sciencebecame too technical and mathematical for the philosophers, oranyone else except a few specialists. philosophers reduced thescope of their inquiries so much that wittgenstein, the mostfamous philosopher of this century, said, “the sole remainingtask for philosophy is the analysis of language.” what acomedown from the great tradition of philosophy from aristotleto kant!

however, if we do discover a complete theory, it should intime be understandable in broad principle by everyone, not justa few scientists. then we shall all, philosophers, scientists, andjust ordinary people, be able to take part in the discussion ofthe question of why it is that we and the universe exist. if wefind the answer to that, it would be the ultimate triumph ofhuman reason - for then we would know the mind of god.

albert einsteineinstein’s connection with the politics of the nuclear bomb iswell known: he signed the famous letter to president franklinroosevelt that persuaded the united states to take the ideaseriously, and he engaged in postwar efforts to prevent nuclearwar. but these were not just the isolated actions of a scientistdragged into the world of politics. einstein’s life was, in fact, touse his own words, “divided between politics and equations.”

einstein’s earliest political activity came during the first worldwar, when he was a professor in berlin. sickened by what hesaw as the waste of human lives, he became involved inantiwar demonstrations. his advocacy of civil disobedience andpublic encouragement of people to refuse conscription did littleto endear him to his colleagues. then, following the war, hedirected his efforts toward reconciliation and improvinginternational relations. this too did not make him popular, andsoon his politics were making it difficult for him to visit theunited states, even to give lectures.

einstein’s second great cause was zionism. although he wasjewish by descent, einstein rejected the biblical idea of god.

however, a growing awareness of anti-semitism, both beforeand during the first world war, led him gradually to identifywith the jewish community, and later to become an outspokensupporter of zionism. once more unpopularity did not stop himfrom speaking his mind. his theories came under attack; ananti-einstein organization was even set up. one man wasconvicted of inciting others to murder einstein (and fined amere six dollars). but einstein was phlegmatic. when a bookwas published entitled 100 authors against einstein, heretorted, “if i were wrong, then one would have been enough!”

in 1933, hitler came to power. einstein was in america, anddeclared he would not return to germany. then, while nazimilitia raided his house and confiscated his bank account, aberlin newspaper displayed the headline “good news fromeinstein - he’s not coming back.” in the face of the nazithreat, einstein renounced pacifism, and eventually, fearing thatgerman scientists would build a nuclear bomb, proposed thatthe united states should develop its own. but even before thefirst atomic bomb had been detonated, he was publicly warningof the dangers of nuclear war and proposing internationalcontrol of nuclear weaponry.

throughout his life, einstein’s efforts toward peace probablyachieved little that would last - and certainly won him fewfriends. his vocal support of the zionist cause, however, wasduly recognized in 1952, when he was offered the presidency ofisrael. he declined, saying he thought he was too naive inpolitics. but perhaps his real reason was different: to quote himagain, “equations are more important to me, because politics isfor the present, but an equation is something for eternity.”

galileo galileigalileo, perhaps more than any other single person, wasresponsible for the birth of modern science. his renownedconflict with the catholic church was central to his philosophy,for galileo was one of the first to argue that man could hopeto understand how the world works, and, moreover, that wecould do this by observing the real world.

galileo had believed copernican theory (that the planetsorbited the sun) since early on, but it was only when he foundthe evidence needed to support the idea that he started topublicly support it. he wrote about copernicus’s theory initalian (not the usual academic latin), and soon his viewsbecame widely supported outside the universities. this annoyedthe aristotelian professors, who united against him seeking topersuade the catholic church to ban copernicanism.

galileo, worried by this, traveled to rome to speak to theecclesiastical authorities. he argued that the bible was notintended to tell us anything about scientific theories, and that itwas usual to assume that, where the bible conflicted withcommon sense, it was being allegorical. but the church wasafraid of a scandal that might undermine its fight againstprotestantism, and so took repressive measures. it declaredcopernicanism “false and erroneous” in 1616, and commandedgalileo never again to “defend or hold” the doctrine. galileoacquiesced.

in 1623, a longtime friend of galileo’s became the pope.

immediately galileo tried to get the 1616 decree revoked. hefailed, but he did manage to get permission to write a bookdiscussing both aristotelian and copernican theories, on twoconditions: he would not take sides and would come to theconclusion that man could in any case not determine how theworld worked because god could bring about the same effectsin ways unimagined by man, who could not place restrictionson god’s omnipotence.

the book, dialogue concerning the two chief worldsystems, was completed and published in 1632, with the fullbacking of the censors - and was immediately greetedthroughout europe as a literary and philosophical masterpiece.

soon the pope, realizing that people were seeing the book as aconvincing argument in favor of copernicanism, regretted havingallowed its publication. the pope argued that although the bookhad the official blessing of the censors, galileo had neverthelesscontravened the 1616 decree. he brought galileo before theinquisition, who sentenced him to house arrest for life andcommanded him to publicly renounce copernicanism. for asecond time, galileo acquiesced.

galileo remained a faithful catholic, but his belief in theindependence of science had not been crushed. four yearsbefore his death in 1642, while he was still under house arrest,the manuscript of his second major book was smuggled to apublisher in holland. it was this work, referred to as two newsciences, even more than his support for copernicus, that wasto be the genesis of modern physics.

isaac newtonisaac newton was not a pleasant man. his relations withother academics were notorious, with most of his later life spentembroiled in heated disputes. following publication of principiamathematica - surely the most influential book ever written inphysics - newton had risen rapidly into public prominence. hewas appointed president of the royal society and became thefirst scientist ever to be knighted.

newton soon clashed with the astronomer royal, johnflamsteed, who had earlier provided newton with much-neededdata for principia, but was now withholding information thatnewton wanted. new-ton would not take no for an answer: hehad himself appointed to the governing body of the royalobservatory and then tried to force immediate publication ofthe data. eventually he arranged for flamsteed’s work to beseized and prepared for publication by flamsteed’s mortalenemy, edmond halley. but flamsteed took the case to courtand, in the nick of time, won a court order preventingdistribution of the stolen work. newton was incensed andsought his revenge by systematically deleting all references toflamsteed in later editions of principia.

a more serious dispute arose with the german philosophergottfried leibniz. both leibniz and newton had independentlydeveloped a branch of mathematics called calculus, whichunderlies most of modern physics. although we now know thatnewton discovered calculus years before leibniz, he publishedhis work much later. a major row ensued over who had beenfirst, with scientists vigorously defending both contenders. it isremarkable, however, that most of the articles appearing indefense of newton were originally written by his own hand -and only published in the name of friends! as the row grew,leibniz made the mistake of appealing to the royal society toresolve the dispute. newton, as president, appointed an“impartial” committee to investigate, coincidentally consistingentirely of newton’s friends! but that was not all: newton thenwrote the committee’s report himself and had the royal societypublish it, officially accusing leibniz of plagiarism. still unsatisfied,he then wrote an anonymous review of the report in the royalsociety’s own periodical. following the death of leibniz, newtonis reported to have declared that he had taken greatsatisfaction in “breaking leibniz’s heart.”

during the period of these two disputes, newton had alreadyleft cambridge and academe. he had been active inanti-catholic politics at cambridge, and later in parliament, andwas rewarded eventually with the lucrative post of warden ofthe royal mint. here he used his talents for deviousness andvitriol in a more socially acceptable way, successfully conductinga major campaign against counterfeiting, even sending severalmen to their death on the gallows.

glossaryabsolute zero: the lowest possible temperature, at whichsubstances contain no heat energy.

acceleration: the rate at which the speed of an object ischanging.

anthropic principle: we see the universe the way it isbecause if it were different we would not be here to observe it.

antiparticle: each type of matter particle has a correspondingantiparticle. when a particle collides with its antiparticle, theyannihilate, leaving only energy.

atom: the basic unit of ordinary matter, made up of a tinynucleus (consisting of protons and neutrons) surrounded byorbiting electrons.

big bang: the singularity at the beginning of the universe.

big crunch: the singularity at the end of the universe.

black hole: a region of space-time from which nothing, noteven light, can escape, because gravity is so strong.

casimir effect: the attractive pressure between two flat,parallel metal plates placed very near to each other in avacuum. the pressure is due to a reduction in the usualnumber of virtual particles in the space between the plates.

chandrasekhar limit: the maximum possible mass of a stablecold star, above which it must collapse into a black hole.

conservation of energy: the law of science that states thatenergy (or its equiva-lent in mass) can neither be created nordestroyed.

coordinates: numbers that specify the position of a point inspace and time.

cosmological constant: a mathematical device used byeinstein to give space-time an inbuilt tendency to expand.

cosmology: the study of the universe as a whole.

dark matter: matter in galaxies, clusters, and possiblybetween clusters, that can not be observed directly but can bedetected by its gravitational effect. as much as 90 percent ofthe mass of the universe may be in the form of dark matter.

duality: a correspondence between apparently differenttheories that lead to the same physical results.

einstein-rosen bridge: a thin tube of space-time linking twoblack holes. also see wormhole.

electric charge: a property of a particle by which it mayrepel (or attract) other particles that have a charge of similar(or opposite) sign.

electromagnetic force: the force that arises between particleswith electric charge; the second strongest of the fourfundamental forces.

electron: a particle with negative electric charge that orbitsthe nucleus of an atom.

electroweak unification energy: the energy (around 100gev) above which the distinction between the electromagneticforce and the weak force disappears.

elementary particle: a particle that, it is believed, cannot besubdivided.

event: a point in space-time, specified by its time and place.

event horizon: the boundary of a black hole.

exclusion principle: the idea that two identical spin-1/2particles cannot have (within the limits set by the uncertaintyprinciple) both the same position and the same velocity.

field: something that exists throughout space and time, asopposed to a particle that exists at only one point at a time.

frequency: for a wave, the number of complete cycles persecond.

gamma rays: electromagnetic rays of very short wavelength,produced in radio-active decay or by collisions of elementaryparticles.

general relativity: einstein’s theory based on the idea thatthe laws of science should be the same for all observers, nomatter how they are moving. it explains the force of gravity interms of the curvature of a four-dimensional space-time.

geodesic: the shortest (or longest) path between two points.

grand unification energy: the energy above which, it isbelieved, the electro-magnetic force, weak force, and strongforce become indistinguishable from each other.

grand unified theory (gut): a theory which unifies theelectromagnetic, strong, and weak forces.

imaginary time: time measured using imaginary numbers.

light cone: a surface in space-time that marks out thepossible directions for light rays passing through a given event.

light-second (light-year): the distance traveled by light inone second (year).

magnetic field: the field responsible for magnetic forces, nowincorporated along with the electric field, into theelectromagnetic field.

mass: the quantity of matter in a body; its inertia, orresistance to acceleration.

microwave background radiation: the radiation from theglowing of the hot early universe, now so greatly red-shiftedthat it appears not as light but as microwaves (radio waveswith a wavelength of a few centimeters). also see cobe, onpage 145.

naked singularity: a space-time singularity not surrounded bya black hole.

neutrino: an extremely light (possibly massless) particle thatis affected only by the weak force and gravity.

neutron: an uncharged particle, very similar to the proton,which accounts for roughly half the particles in an atomicnucleus.

neutron star: a cold star, supported by the exclusionprinciple repulsion between neutrons.

no boundary condition: the idea that the universe is finitebut has no boundary (in imaginary time).

nuclear fusion: the process by which two nuclei collide andcoalesce to form a single, heavier nucleus.

nucleus: the central part of an atom, consisting only ofprotons and neutrons, held together by the strong force.

particle accelerator: a machine that, using electromagnets, canaccelerate moving charged particles, giving them more energy.

phase: for a wave, the position in its cycle at a specifiedtime: a measure of whether it is at a crest, a trough, orsomewhere in between.

photon: a quantum of light.

planck’s quantum principle: the idea that light (or any otherclassical waves) can be emitted or absorbed only in discretequanta, whose energy is proportional to their wavelength.

positron: the (positively charged) antiparticle of the electron.

primordial black hole: a black hole created in the very earlyuniverse.

proportional: ‘x is proportional to y’ means that when y ismultiplied by any number, so is x. ‘x is inversely proportionalto y’ means that when y is multiplied by any number, x isdivided by that number.

proton: a positively charged particle, very similar to theneutron, that accounts for roughly half the particles in thenucleus of most atoms.

pulsar: a rotating neutron star that emits regular pulses ofradio waves.

quantum: the indivisible unit in which waves may beemitted or absorbed.

quantum chromodynamics (qcd): the theory that describesthe interactions of quarks and gluons.

quantum mechanics: the theory developed from planck’squantum principle and heisenberg’s uncertainty principle.

quark: a (charged) elementary particle that feels the strongforce. protons and neutrons are each composed of threequarks.

radar: a system using pulsed radio waves to detect theposition of objects by measuring the time it takes a single pulseto reach the object and be reflected back.

radioactivity: the spontaneous breakdown of one type ofatomic nucleus into another.

red shift: the reddening of light from a star that is movingaway from us, due to the doppler effect.

singularity: a point in space-time at which the space-timecurvature becomes infinite.

singularity theorem: a theorem that shows that a singularitymust exist under certain circumstances - in particular, that theuniverse must have started with a singularity.

space-time: the four-dimensional space whose points areevents.

spatial dimension: any of the three dimensions that arespacelike - that is, any except the time dimension.

special relativity: einstein’s theory based on the idea that thelaws of science should be the same for all observers, no matterhow they are moving, in the absence of gravitationalphenomena.

spectrum: the component frequencies that make up a wave.

the visible part of the sun’s spectrum can be seen in arainbow.

spin: an internal property of elementary particles, related to,but not identical to, the everyday concept of spin.

stationary state: one that is not changing with time: asphere spinning at a constant rate is stationary because it looksidentical at any given instant.

string theory: a theory of physics in which particles aredescribed as waves on strings. strings have length but no otherdimension.

strong force: the strongest of the four fundamental forces,with the shortest range of all. it holds the quarks togetherwithin protons and neutrons, and holds the protons andneutrons together to form atoms.

uncertainty principle: the principle, formulated by heisenberg,that one can never be exactly sure of both the position andthe velocity of a particle; the more accurately one knows theone, the less accurately one can know the other.

virtual particle: in quantum mechanics, a particle that cannever be directly detected, but whose existence does havemeasurable effects.

wave/particle duality: the concept in quantum mechanicsthat there is no distinction between waves and particles;particles may sometimes behave like waves, and waves likeparticles.

wavelength: for a wave, the distance between two adjacenttroughs or two adjacent crests.

weak force: the second weakest of the four fundamentalforces, with a very short range. it affects all matter particles,but not force-carrying particles.

weight: the force exerted on a body by a gravitational field.

it is proportional to, but not the same as, its mass.

white dwarf: a stable cold star, supported by the exclusionprinciple repulsion between electrons.

wormhole: a thin tube of space-time connecting distantregions of the universe. wormholes might also link to parallelor baby universes and could provide the possibility of timetravel.

acknowledgementsmany people have helped me in writing this book. myscientific colleagues have without exception been inspiring. overthe years my principal associates and collaborators were rogerpenrose, robert geroch, brandon carter, george ellis, garygibbons, don page, and jim hartle. i owe a lot to them, andto my research students, who have always given me help whenneeded.

one of my students, brian whitt, gave me a lot of helpwriting the first edition of this book. my editor at bantambooks, peter guzzardi, made innumerable comments whichimproved the book considerably. in addition, for this edition, iwould like to thank andrew dunn, who helped me revise thetext.

i could not have written this book without mycommunication system. the software, called equalizer, wasdonated by walt waltosz of words plus inc., in lancaster,california. my speech synthesizer was donated by speech plus,of sunnyvale, california. the synthesizer and laptop computerwere mounted on my wheelchair by david mason, ofcambridge adaptive communication ltd. with this system i cancommunicate better now than before i lost my voice.

i have had a number of secretaries and assistants over theyears in which i wrote and revised this book. on thesecretarial side, i’m very grateful to judy fella, ann ralph,laura gentry, cheryl billington, and sue masey. my assistantshave been colin williams, david thomas, and raymondlaflamme, nick phillips, andrew dunn, stuart jamieson,jonathan brenchley, tim hunt, simon gill, jon rogers, andtom kendall. they, my nurses, colleagues, friends, and familyhave enabled me to live a very full life and to pursue myresearch despite my disability.