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    Does Time Ever Stop?Can the PastBe Altered?It is impossible to meditate on time and the mystery of the creativepassage of nature without an overwhelming emotion at the limitations of hu-man intelligence.

    -ALFRED NORTHWHITEHEADThe oncept of Nature

    T ere has been a great deal of interest among physicists of latein whether or not there are events on the elementary-particlel e v e l hat cannot be time-reversed, that is, events for whichimagining a reversal in the direction of motion of all the par-ticles involved is imagining an event that cannot happen innature. Richard Feynman has suggested an approach toquantum mechanics in which antipart icles are viewed as par-ticles momentarily traveling backward in time. Cosmologistshave speculated about two universes for which all the eventsin one are reversed relative to the direction of time in theother: in each universe intelligent organisms would live nor-mally from past to future, but if the organisms in one universecould in some way observe events in the other which many

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    physicists consider an impossibility), they would find thoseevent ; going in the opposite direction. It has even been con-jectured that if our universe stops expanding and starts tocontract, there will be a time reversal, but it is far from clearwhat that would mean. Most of the speculations of this kindare quite recent, and interested readers will find many of themexamined in four chapters of my N ew Ambidextrous Universe.

    Iri this chapter I shall consider two bizarre questions abouttime 1.-hat are not discussed in the book. Indeed, these ques-tions are of so little concern to scientists that only philoso-phers and writers of fantasy and science fiction have had muchto say about them: I s it meaningful to speak of time stopping?Is it meaningful to speak of altering the past?

    Neither question should be confused with the familiarsubject of time's relativity. Newton believed the universe waspervaded by a single absolute time that could be symbolizedby an imaginary clock off somewhere in space (perhaps out-side the cosmos). By means of this clock the rates of all theevents in the universe could be measured. The notion workswell within a single inertial frame of reference such as thesurface of the earth, but it does not work for inertial systemsmoving in relation to each other a t high speeds. According tothe theory of relativity, if a spaceship were to travel from oursolar system to another solar system with a velocity close tothat of light, events would proceed much slower on the space-ship than they would on the earth. In a sense, then, such aspaceship is traveling through time into the future. Passen-gers on the spaceship might experience a round-trip voyageas taking only a few years, but they would return to find thatcentu.ries of earth-years had elapsed.The notion that different parts of the universe can changeat different rates of time is much older than the theory of rel-ativity. In the Scholastic theology of the Middle Ages angelswere considered to be nonmaterial intelligences living by atime different from that of earthly creatures; God himself wasthought to be entirely outside of time. In the first act of LordByron's play, Cain, Mystery, the fallen angel Lucifer says:

    W ith us acts are exempt from time.and weC a n crowd eternity into a n hourOr stretch a n h our into eternityWe breathe not by a m ortalmeasurement-But that s a mystery.

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    Doer Time Euer Stop? Can the Part Be ~ b m d ? 43

    In the 20th century hundreds of science-fiction stories haveplayed with the relativity of time in different inertial systems,but the view that time can speed up or slow down in differentparts of our universe is central to many older tales. A popularmedieval legend tells of a monk who is entranced for a minuteor two by the song of a magical bird. When the bird stopssinging, the monk discovers that several hundred years havepassed. In a Moslem legend Mohammed is carried by a mareinto the seventh heaven. After a long visit the prophet returnsto the earth just in time to catch a jar of water the horse hadkicked over before starting its ascent.

    Washington Irving's Rip Van Winkle is this country'sbest-known story about someone who sleeps for what seemsto him to be a normal time while two decades of earth-yearsrush by. King Arthur's daughter Gyneth slept for 500 yearsunder a spell cast by Merlin. Every culture has similar sleeperlegends. H. G. Wells used the device in When the Sleeper Wakesand it is a common practice in science fiction to put astro-nauts into a cryogenic sleep so they can survive interstellarvoyages that are longer than their normal life span. In Wells'sshort story The New Accelerator a scientist discovers a wayto speed up a person's biological time so that the world seemsto come almost to a halt. This device too is frequently en-countered in later science fiction.

    The issue under consideration here, however, is not howtime can vary but whether time can be said to stop entirely.It is clearly meaningful to speak of all motion ceasing in onepart of the universe, whether or not such a part exists. In thetheory of relativity the speed of light is an unattainable limitfor any object with mass. If a spaceship could attain the speedof light (which the theory of relativity rules out because themass of the ship would increase to infinity), then time on thespaceship would stop in the sense that all change on it wouldcease. In earth time it might take 100 years for the spaceshipto reach a destination, but to astronauts on the spaceship thedestination would be reached instantaneously. One can alsoimagine a piece of matter or even a human being reduced tosuch a low temperature (by some as yet unknown means) thateven all subatomic motions would be halted. For that pieceof matter, then, one could say that time had stopped. Actuallyit is hard to understand why the piece of matter would notvanish.The idea of time stopping creates no problems for writersof fantasy, who are not constrained by the real world. For

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    examlple, in L. Frank Baum's The Capture of Father Time,one of the stories in his American Fairy Tales (now back inprint in a Dover facsimile edition), a small boy lassoes Time,and for a while everything except the movements of the boyand Father Time stops completely. In Chapter of JamesBranch Cabell's Jurgen: A Comedy of Justice outside timesleeps while Jurgen enjoys a pleasurable stay in Cocaigne withQueein Anai'tis. Later in the novel Jurgen stares into the eyesof the God of his grandmother and is absolutely motionlessfor 35' days. In Jorge Luis Borges' story The Secret Miraclea writer is executed by a firing squad. Between the commandto fire and the writer's death God stops all time outside thewriter's brain, giving him a year to complete his masterpiece.

    Many similar examples from legend and literature showthat the notion of time stopping in some part of the universeis not logically inconsistent. But what about the idea of timestopping throughout the universe? Does the notion that every-thing stops moving for a while and then start s again have anymeaning?

    If it is assumed that there is an outside observer-per-haps a god-watching the universe from a region of hyper-time, then of course the notion of time stopping does havemeaning, just as imagining a god in hyperspace gives meaningto the notion of everything in the universe turning upside down.The history of our universe may be like a three-dimensionalmotioln picture a god is enjoying. When the god turns off theproje~ctor o do something else, a few millenniums may go bybefore he comes back and turns it on again. (After all, whatare a few millenniums to a god?) For all we can know a billioncenturies of hypertime may have elapsed between my typingthe first and the second word of this sentence.

    Suppose, however, all outside observers are ruled out anduniverse is taken to mean everything there is. Is there stilla way to give a meaning to the idea of all change stopping fora while? Although most philosophers and scientists would saythere is not, a few have argued for the other side. For ex-ample, in Time without Change Sydney S. Shoemaker, nowa philosopher at Cornell University, makes an unusual argu-ment in support of the possibility of change stopping.

    Slhoemaker is concerned not with the real world but withpossible worlds designed to prove that the notion of timestoppling everywhere can be given a reasonable meaning. Heproposes several worlds of this kind, all of them based on the

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    same idea. I shall describe only one such world here, in aslightly dramatized form.Imagine a universe divided into regions A, B and C. Innormal times inhabitants of each region can observe the in-habitants of the other two and communicate with them. Everynow and then, however, a mysterious purple glow permeatesone of the regions. The glow always lasts for a week and isinvariably followed by a year in which all change in the regionceases. In other words, for one year absolutely nothing hap-pens there. Shoemaker calls the phenomenon a local freeze.Since no events take place, light cannot leave the region, andso the region seems to vanish for a year. When it returns toview, its inhabitants are unaware of any passage of time, butthey learn from their neighbors that a year, as measured byclocks in the other two regions, has elapsed. To the inhabi-tants of the region that experienced the local freeze it seemsthat instantaneous changes have taken place in the other tworegions. As Shoemaker puts it: People and objects will ap-pear to have moved in a discontinuous manner or to have van-ished into thin air or to have

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