FROM ETERNITY TO HERE - Oneworld Publications

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FROM ETERNITY TO HEREThe Quest for the Ultimate Theor y of TimeSEAN CARROLLPrelims.indd 4191/12/2011 12:41:15 PM

A Oneworld BookFirst published in Great Britain and the Commonwealthby Oneworld Publications 2011Copyright Sean Carroll 2010Published by arrangement with Dutton, a member of the Penguin Group ( USA) , Inc.The moral right of Sean Carroll to be identified as the Authorof this work has been asserted by him in accordance withthe Copyright, Designs and Patents Act 1988All rights reservedCopyright under Berne ConventionA CIP record for this title is availablefrom the British LibraryPhotograph on page 37 by Martin Röll, licensed under the Creative Commons AttributionShareAlike 2.0 License, from Wikimedia Commons. Photograph on page 47 courtesyof the Huntington Library. Image on page 53 by the NASA/WMAP Science Team.Photograph on page 67 courtesy of Corbis Images. Image on page 119 courtesy of Getty Images.Figures on pages 147, 153, 177, 213, 270, 379, and 382 by Sean Carroll. Photograph onpage 204 courtesy of the Smithsonian Institution. Photograph on page 259 courtesy ofProfessor Stephen Hawking. Photograph on page 267 courtesy of Professor Jacob Bekenstein.Photograph on page 295 by Jerry Bauer, from Wikimedia Commons. Photograph on page 315courtesy of the Massachusetts Institute of Technology.All other images courtesy of Jason Torchinsky.ISBN: 978-1-85168-795-4 (Trade Paperback)ISBN: 978-1-85168-842-5 (Travel Edition)Typeset by Jayvee, Trivandrum, IndiaCover design by Richard GreenPrinted and bound byOneworld Publications185 Banbury RoadOxford, OX2 7AREnglandLearn more about Oneworld. Join our mailing list tofind out about our latest titles and special offers at:www.oneworld-publications.comOW 310 Prelims.indd ivPrelims.indd 4201/12/2011 12:38:57 PM1/12/2011 12:41:15 PM

CO N T E N T SPrologueThe nature of time, the importance of entropy, and the role of cosmology.1PART ONE: TIME, EXPERIENCE, AND THE UNIVERSE1. The Past Is Present MemoryTime has different meanings: a label on different moments, the durationbetween events, and a medium of change. We can think of the past,present, and future as equally real.92. The Heavy Hand of EntropyThe direction of time is governed by the Second Law of Thermodynamics:In a closed system, entropy only increases or stays the same. Entropymeasures the disorder of a system.283. The Beginning and End of TimeThe evolution of the universe through time, beginning with a hot,dense Big Bang (which may not be the true beginning), expanding andforming stars and galaxies, eventually accelerating into emptiness.49PART TWO: TIME IN EINSTEIN’S UNIVERSE4. Time Is PersonalEinstein’s Special Theory of Relativity. You can’t go faster than thespeed of light; you stay within a light cone in spacetime. Time measuresthe duration elapsed along different trajectories.755. Time Is FlexibleEinstein’s General Theory of Relativity. Spacetime is curved, which weexperience as gravity. The curvature of spacetime underlies black holesand the expansion of the universe.92Prelims.indd 4231/12/2011 12:41:15 PM

v iiiContents6. Looping Through TimeClosed timelike curves would allow you to visit the past without violatingthe rules of relativity. A time machine of this sort doesn’t necessarilylead to paradoxes, but might be impossible to create according to thelaws of physics.105PART THREE: ENTROPY AND TIME’S ARROW7. Running Time BackwardsThe fundamental laws of physics, as we understand them, conserveinformation: The future and past can be predicted and retrodictedfrom perfect knowledge of the present state. Microscopic processes arereversible.1358. Entropy and DisorderLudwig Boltzmann discovered our modern understanding of entropy:the number of ways microscopic constituents can be arranged to formthe same macroscopic system. It’s natural for entropy to increase, butonly if we assume a “Past Hypothesis” that entropy started very low.1629. Information and LifeThe growth of retch abit, we can imagine a particular collection of clocks (including molecularvibrations and other periodic processes) that all change in concert withone another, but apart from the rest of the world. Then we would have toask whether it was appropriate to say that the rate at which time passes hadreally changed within that collection.Consider an extreme example. Nicholson Baker’s novel The Fermata tells the story of a man, Arno Strine, with the ability to “stop time.”(Mostly he uses this miraculous power to go around undressing women.)It wouldn’t mean anything if time stopped everywhere; the point is thatArno keeps moving through time, while everything around him stops.We all know this is unrealistic, but it’s instructive to reflect upon the wayin which it flouts the laws of physics. What this approach to stopping timech1.indd 191/12/2011 11:38:21 AM

20FROM ETERNIT Y TO HEREentails is that every kind of motion and rhythm in Arno’s body continues as usual, while every kind of motion and rhythm in the outside worldfreezes absolutely still. Of course we have to imagine that time continues for all of the air and fluids within Arno, otherwise he would instantlydie. But if the air in the rest of the room has truly stopped experiencingtime, each molecule must remain suspended precisely in its location; consequently, Arno would be unable to move, trapped in a prison of rigidlystationary air molecules. Okay, let’s be generous and assume that timewould proceed normally for any air molecules that came sufficiently closeto Arno’s skin. (The book alludes to something of the sort.) But everything outside, by assumption, is not changing in any way. In particular,no sound or light would be able to travel to him from the outside world;Arno would be completely deaf and blind. It turns out not to be a promising environment for a Peeping Tom.9What if, despite all the physical and narrative obstacles, something likethis really could happen? Even if we can’t stop time around us, presumablywe can imagine speeding up the motion of some local clocks. If we trulymeasure time by synchronized repetition, and we arranged an ensemble ofclocks that were all running fast compared to the outside world while theyremained in synchrony with one another, wouldn’t that be something like“time running faster” within that arrangement?It depends. We’ve wandered far afield from what might actually happen in the real world, so let’s establish some rules. We’re fortunate enoughto live in a universe that features very reliable clocks. Without such clocks,we can’t use time to measure the duration between events. In the world ofThe Fermata, we could say that time slowed down for the universe outsideArno Strine—or, equivalently and perhaps more usefully, that time forhim sped up, while the rest of the world went on as usual. But just as well,we could say that “time” was completely unaffected, and what changedwere the laws of particle physics (masses, charges on different particles)within Arno’s sphere of influence. Concepts like “time” are not handed tous unambiguously by the outside world but are invented by human beingstrying to make sense of the universe. If the universe were very different, wemight have to make sense of it in a different way.Meanwhile, there is a very real way for one collection of clocks tomeasure time differently than another: have them move along differentch1.indd 201/12/2011 11:38:21 AM

The Past Is Present Memor y21paths through space-time. That’s completely compatible with our claimthat “good clocks” should measure time in the same way, because we can’treadily compare clocks unless they’re next to one another in space. Thetotal amount of time elapsed on two different trajectories can be different without leading to any inconsistencies. But it does lead to somethingimportant—the theory of relativity.Twisty paths through spacetimeThrough the miracle of synchronized repetition, time doesn’t simply putdifferent moments in the history of the universe into order; it also tells us“how far apart” they are (in time). We can say more than “1776 happenedbefore 2010”; we can say “1776 happened 234 years before 2010.”I should emphasize a crucial distinction between “dividing the universe into different moments” and “measuring the elapsed time betweenevents,” a distinction that will become enormously important when we getto relativity. Let’s imagine you are an ambitious temporal10 engineer, andyou’re not satisfied to just have your wristwatch keep accurate time; youwant to be able to know what time it is at every other event in spacetimeas well. You might be tempted to wonder: Couldn’t we (hypothetically)construct a time coordinate all throughout the universe, just by buildingan infinite number of clocks, synchronizing them to the same time, andscattering them throughout space? Then, wherever we went in spacetime,there would be a clock sitting at each point telling us what time it was, onceand for all.The real world, as we will see, doesn’t let us construct an absolute universal time coordinate. For a long time people thought it did, under no lessan authority than that of Sir Isaac Newton. In Newton’s view of the universe, there was one particular right way to slice up the universe into slicesof “space at a particular moment of time.” And we could indeed, at leastin a thought-experiment kind of way, send clocks all throughout the universe to set up a time coordinate that would uniquely specify when a certain event was taking place.But in 1905, along comes Einstein with his speci

Figures on pages 147, 153, 177, 213, 270, 379, and 382 by Sean Carroll. Photograph on page 204 courtesy of the Smithsonian Institution. Photograph on page 259 courtesy of Professor Stephen Hawking. Photograph on page 267 courtesy of Professor Jacob Bekenstein. Photograph on page 295 by Je

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