Where did time come from?

Re­searchers are un­rav­el­ling space-time to find out if one of its com­po­nents is an il­lu­sion

All About Space - - Contents - Writ­ten by Giles Spar­row

“It’s that rid­ing the wave of the in­creas­ing en­tropy of the uni­verse that gives us the per­cep­tion of the flow of time”

Prof Sean Car­roll

Time is a con­stant part of our ev­ery­day lives – even as you read this sen­tence, its first words have dis­ap­peared into the past while the next para­graph looms up from the fu­ture. We’re so used to the ex­pe­ri­ence of flow­ing time that we very rarely stop to think about what it re­ally is or why it works in the way that it does.

While most cos­mol­o­gists agree that time is an in­nate fea­ture of the uni­verse with its di­rec­tion de­fined only by other laws of physics, a con­tro­ver­sial new the­ory sug­gests that its flow could be driven by the fact that our uni­verse is ex­pand­ing – an idea which means that, at least in the­ory, time could one day be thrown into re­verse.

For­tu­nately, even though time is hard to vi­su­alise, sci­en­tists have a well-es­tab­lished way of treat­ing it as a fourth di­men­sion: a di­rec­tion in which phe­nom­ena can change their lo­ca­tion, sim­i­lar to the fa­mil­iar three di­men­sions that lo­cate ob­jects in space.

“Time is not that hard to un­der­stand,” says Pro­fes­sor Sean Car­roll, re­search pro­fes­sor at the Cal­i­for­nia In­sti­tute of Tech­nol­ogy and au­thor of From Eter­nity to Here: The Quest for the Ul­ti­mate The­ory of Time, op­ti­misti­cally. “I don’t think it’s a mystery, and I don’t think it’s been a mystery for a very long time. It was a bit sim­pler back when we had Isaac New­ton and time and space were both ab­so­lute. Then we would have con­sid­ered the uni­verse to be made of space and ev­ery­thing in it, and the uni­verse keeps hap­pen­ing over and over again – time is just the la­bel we put on those dif­fer­ent ver­sions that hap­pen one af­ter an­other with things in dif­fer­ent po­si­tions, a bit like the pages of a book.”

As he ex­plains, our modern view of time – su­per­sed­ing New­ton’s – was shaped by the break­throughs of Al­bert Ein­stein a lit­tle over a cen­tury ago: “In Ein­stein’s view there’s ac­tu­ally ‘space-time’ [a struc­ture with four di­men­sions that de­ter­mines how ob­jects are lo­cated in the uni­verse], and how an ob­server slices that space-time into time and space is a lit­tle ar­bi­trary – dif­fer­ent peo­ple can look at it in dif­fer­ent ways, and no one is right and no one is wrong. But still, in any one point of view there’s a se­quence of mo­ments. It’s a lit­tle more com­pli­cated, but re­ally it’s not that hard – it’s cer­tainly no more pro­found to ask what time is, than-to-ask-than, what space is.

“I guess I’m smug­gling in an eter­nal­ist point of view here, which treats ev­ery mo­ment of time as on an equal foot­ing, as op­posed to a ‘pre­sen­tist’ point of view that says only the world right now is real. In a post-Ein­stein world that’s re­ally not how many physi­cists think,” he con­tin­ues.

But if ev­ery mo­ment in time is equiv­a­lent, why can’t we move back and forth in time at will – why does time only ap­pear to flow in one di­rec­tion, and does it re­ally ‘flow’ at all?

“An im­por­tant sub­tlety is the dif­fer­ence be­tween time as a la­bel on dif­fer­ent points ver­sus the ‘ar­row of time’ – the fact that the past and fu­ture are dif­fer­ent for us in a way that is not true for space,” ex­plains Car­roll. “All di­rec­tions in space are the same (at least out­side the in­flu­ence of grav­ity), but in time the two di­rec­tions are very dif­fer­ent, and there’s just one di­rec­tion that we go in.”

Ask most physi­cists about the ar­row of time and they’ll al­most cer­tainly ex­plain it in terms of the se­cond law of ther­mo­dy­nam­ics (the sci­ence that re­lates heat and en­ergy, dis­cov­ered and ex­panded upon dur­ing the 19th cen­tury). The se­cond law is a sim­ple state­ment that says the amount of dis­or­der or ‘en­tropy’ in a closed sys­tem in­creases with time, un­less en­ergy is sup­plied to cre­ate more or­der.

The sys­tem can be any­thing, and its en­tropy can be thought of as the ‘use­less’ en­ergy within it. A clas­sic ex­am­ple is a pair of boxes con­tain­ing hot and cold gases with a con­nect­ing door be­tween them. At first this par­tic­u­lar ar­range­ment’s en­tropy is low be­cause most of its en­ergy is in use­ful form – locked up in hot, fast-mov­ing gas mol­e­cules in one box that could be used, for ex­am­ple, to pump an en­gine pis­ton.

Open the door be­tween the boxes, how­ever, and over time the gas mol­e­cules of dif­fer­ent tem­per­a­ture will in­evitably mix to­gether. Hot­ter gas mol­e­cules will col­lide with colder and slow­er­mov­ing ones, trans­fer­ring en­ergy un­til even­tu­ally both boxes are filled with gas of an in­ter­me­di­ate tem­per­a­ture. A once-or­derly sys­tem with low en­tropy has now be­come a disor­derly (high-en­tropy) sys­tem of jum­bled mol­e­cules with less ca­pac­ity to do use­ful work.

But what ex­actly does all this have to do with the di­rec­tion of time? Well, ac­cord­ing to cur­rent cos­mol­ogy, the uni­verse too can be thought of as a closed sys­tem – there’s only so much en­ergy to go around and no way of sup­ply­ing en­ergy from out­side to re­verse the rise of en­tropy. The uni­verse there­fore started out in a highly or­dered, low-en­tropy state (the Big Bang era where all mat­ter had uni­formly high tem­per­a­tures), and en­tropy

has been in­creas­ing ever since. Today, much of its mat­ter is still con­cen­trated in low-en­tropy sys­tems such as stars, but in the far fu­ture, as suc­ceed­ing gen­er­a­tions of stars burn out, the cos­mos will suc­cumb to ‘heat death’ in which mat­ter and en­ergy be­come more and more evenly and thinly spread.

“In my per­spec­tive, the ar­row of time just comes from the fact that the en­tropy of the uni­verse was smaller in ear­lier times and grows larger at later times. There’s noth­ing pro­pel­ling the uni­verse for­ward in time, it’s just you have all these dif­fer­ent mo­ments dis­tin­guished by the rule that ear­lier means lower en­tropy, later means higher en­tropy,’ says Car­roll. “That ex­plains why you can re­mem­ber the past and not the fu­ture, why you can make choices about the fu­ture but not the past and so forth. There are good rea­sons why a per­son, con­sid­ered as a se­ries of peo­ple at mo­ments in time with in­creas­ing en­tropy, feels that time flows in that di­rec­tion.

“It’s more of a psy­cho­log­i­cal ef­fect than any­thing else – we carry around in our minds a mov­ing im­age of what we were a se­cond ago, what we will be a se­cond from now, and we’re con­stantly up­dat­ing on the ba­sis of what we learn, how our sur­round­ings change and so on. It’s that rid­ing the wave of the in­creas­ing en­tropy of the uni­verse that gives us the per­cep­tion of the flow of time.

If, how­ever, the idea is that there’s some ac­tive el­e­ment of re­al­ity that pushes the uni­verse for­ward in time to bring about change, then that’s not re­ally part of physics as we un­der­stand it.”

Nev­er­the­less, that pos­si­bil­ity of a ‘driv­ing force’ be­hind the flow of time is where a con­tro­ver­sial new idea put for­ward by physi­cist Richard Muller of the Univer­sity of Cal­i­for­nia, Berke­ley could come into play. Muller’s own 2016 book, Now:

The Physics of Time, sug­gests that time is a real phe­nom­e­non, and that more time is cre­ated as space it­self grows.

While Car­roll and many other cos­mol­o­gists are doubt­ful about the need for a driv­ing force be­hind the ar­row of time, Muller does at least put for­ward a plau­si­ble way of test­ing his pro­posal. The idea comes from grav­i­ta­tional waves – the minute dis­tor­tions of space-time that rip­ple out across the uni­verse from cer­tain cat­a­clysmic events

in­volv­ing large asym­met­ric masses.

Although such waves were pre­dicted by Ein­stein in the early 20th cen­tury, they have only been de­tected at the su­per-sen­si­tive Laser In­ter­fer­om­e­ter Grav­i­ta­tional-Wave Ob­ser­va­tory (LIGO) in the past cou­ple of years. Muller and his Cal­tech col­lab­o­ra­tor Shaun Maguire ar­gue that be­cause the black hole col­li­sions that gen­er­ate grav­i­ta­tional waves cre­ate ‘new’ space, they should also cre­ate a small amount of new time. What’s more, the quan­ti­ties of time in­volved (around a mil­lisec­ond) are large enough to be mea­sur­able us­ing ex­ist­ing LIGO in­stru­ments, so Muller’s in­trigu­ing hy­poth­e­sis could ei­ther be proved wrong or pass its first ob­ser­va­tional test in the rel­a­tively near fu­ture.

While Muller’s model of time is a rad­i­cal de­par­ture from those sup­ported by the ma­jor­ity of cos­mol­o­gists, the way it makes the flow of time ’real’, rather than some­thing merely de­fined by the in­crease of en­tropy, gives it an ob­vi­ous in­tu­itive ap­peal. Tech­ni­cally, the dif­fer­ence be­tween the roles of time and those of other di­men­sions is its lack of ‘sym­me­try’. Physi­cists de­fine sym­me­try as the prop­erty of a sys­tem that re­mains un­al­tered by a ‘trans­for­ma­tion’ or move­ment in terms of one or more di­men­sion. A sys­tem can be trans­formed

Gen­eral rel­a­tiv­ity al­lows flex­i­bil­ity in space­time, per­haps in­clud­ing the pos­si­bil­ity of ‘worm­hole’ tun­nels be­tween one re­gion of space-time and an­other

In 1908, Her­mann Minkowski showed how space and time could be mod­elled as a four­di­men­sional space-time ‘man­i­fold’

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