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Sign In | Register 0 Subscription Center Subscribe to All Access » Subscribe to Print » Give a Gift » View the Latest Issue » Subscribe News & Features Topics Blogs Videos & Podcasts Education Citizen Science SA Magazine SA Mind Books SA en español In the evolution of cosmic structure, is entropy or gravity the more dominant force? The answer to this question has deep implications for the universe's future, as well as its past. Credit: NASA; ESA; G. Illingworth, D. Magee, and P. Oesch, University of California, Santa Cruz; R. Bouwens, Leiden University; and the HUDF09 Team Space » News 2 Futures Can Explain Time's Mysterious Past New theories suggest the big bang was not the beginning, and that we may live in the past of a parallel universe December 8, 2014 | By Lee Billings Physicists have a problem with time. Whether through Newton’s gravitation, Maxwell’s electrodynamics, Einstein’s special and general relativity or quantum mechanics, all the equations that best describe our universe work perfectly if time flows forward or backward. Of course the world we experience is entirely different. The universe is expanding, not contracting. Stars emit light rather than absorb it, and radioactive atoms decay rather than reassemble. Omelets don’t transform back to unbroken eggs and cigarettes never coalesce from smoke and ashes. We remember the past, not the future, and we grow old and decrepit, not young and rejuvenated. For us, time has a clear and irreversible direction. It flies forward like a missile, equations be damned. For more than a century, the standard explanation for “time’s arrow,” as the astrophysicist Arthur Eddington first called it in 1927, has been that it is an emergent property of thermodynamics, as first laid out in the work of the 19th-century Austrian physicist Ludwig Boltzmann. In this view what we perceive as the arrow of time is really just the inexorable rearrangement of highly ordered states into random, useless configurations, a product of the universal tendency for all things to settle toward equilibrium with one another. Informally speaking, the crux of this idea is that “things fall apart,” but more formally, it is a consequence of the second law of thermodynamics, which Boltzmann helped devise. The law states that in any closed system (like the universe itself), entropy —disorder—can only increase. Increasing entropy is a cosmic certainty because there are always a great many more disordered states than orderly ones for any given system, similar to how there are many more ways to scatter papers across a desk than to stack them neatly in a single pile. 45 :: Email :: Print Follow Us: More from Scientific American ADVERTISEMENT Latest News Most Read What Was the Most Disappointing New Technology of 2014? Inclusion Illusion Lessens Racial Bias Future Smartphone Could Fall Smartly Too Obama Says Keystone Pipeline Will Have Little Impact on U.S. Gas Prices Coal Ash Is Not Hazardous Waste under U.S. Agency Rules Subscribe Home Search LATEST STORIES: What Was the Most Disappointing New Technology of 2014? Inclusion Illusion Lessens Racial Bias Future Smartphone Could Fall Smartly Too GGiivvee aa GGiifftt && GGeett aa GGiifftt -- FFrreeee!! Give a 1 year subscription as low as $14.99 Subscribe Now! > X 2 Futures Can Explain Time's Mysterious Past - Scientific Am... http://www.scientificamerican.com/article/2-futures-can-expla... 1 of 6 22/12/14 9:01 pm

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Page 1: 2 Futures Can Explain Time's Mysterious Past - Scientific American

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In the evolution of cosmic structure, is entropy orgravity the more dominant force? The answer tothis question has deep implications for theuniverse's future, as well as its past.Credit: NASA; ESA; G. Illingworth, D. Magee, andP. Oesch, University of California, Santa Cruz; R.Bouwens, Leiden University; and the HUDF09Team

Space » News

2 Futures Can Explain Time'sMysterious PastNew theories suggest the big bang was not the beginning, and that we may live in the past of aparallel universe

December 8, 2014 | By Lee Billings

Physicists have a problem with time. Whether through Newton’s gravitation,Maxwell’s electrodynamics, Einstein’sspecial and general relativity or quantummechanics, all the equations that bestdescribe our universe work perfectly if timeflows forward or backward. Of course the world we experience isentirely different. The universe isexpanding, not contracting. Stars emit lightrather than absorb it, and radioactive atomsdecay rather than reassemble. Omeletsdon’t transform back to unbroken eggs andcigarettes never coalesce from smoke andashes. We remember the past, not thefuture, and we grow old and decrepit, notyoung and rejuvenated. For us, time has a clear and irreversible direction. It fliesforward like a missile, equations be damned. For more than a century, the standard explanation for “time’s arrow,” as theastrophysicist Arthur Eddington first called it in 1927, has been that it is an emergentproperty of thermodynamics, as first laid out in the work of the 19th-century Austrianphysicist Ludwig Boltzmann. In this view what we perceive as the arrow of time isreally just the inexorable rearrangement of highly ordered states into random, uselessconfigurations, a product of the universal tendency for all things to settle towardequilibrium with one another. Informally speaking, the crux of this idea is that “things fall apart,” but more formally,it is a consequence of the second law of thermodynamics, which Boltzmann helpeddevise. The law states that in any closed system (like the universe itself), entropy—disorder—can only increase. Increasing entropy is a cosmic certainty because thereare always a great many more disordered states than orderly ones for any givensystem, similar to how there are many more ways to scatter papers across a desk thanto stack them neatly in a single pile.

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From Boltzmann’s era onward, scientists allergic to the notion of such an immaculateconception have been grappling with this conundrum. Boltzmann, believing the universe to be eternal in accordance with Newton’s laws,thought that eternity could explain a low-entropy origin for time’s arrow. Givenenough time—endless time, in fact—anything that can happen will happen, includingthe emergence of a large region of very low entropy as a statistical fluctuation from anageless, high-entropy universe in a state of near-equilibrium. Boltzmann mused thatwe might live in such an improbable region, with an arrow of time set by the region’slong, slow entropic slide back into equilibrium. Today’s cosmologists have a tougher task, because the universe as we now know itisn’t ageless and unmoving: They have to explain the emergence of time’s arrowwithin a dynamic, relativistic universe that apparently began some 14 billion years agoin the fiery conflagration of the big bang. More often than not the explanation involves‘fine-tuning’—the careful and arbitrary tweaking of a theory’s parameters to accordwith observations. Many of the modern explanations for a low-entropy arrow of time involve a theorycalled inflation—the idea that a strange burst of antigravity ballooned the primordialuniverse to an astronomically larger size, smoothing it out into what corresponds to avery low-entropy state from which subsequent cosmic structures could emerge. Butexplaining inflation itself seems to require even more fine-tuning. One of theproblems is that once begun, inflation tends to continue unstoppably. This “eternalinflation” would spawn infinitudes of baby universes about which predictions andobservations are, at best, elusive. Whether this is an undesirable bug or a wonderfulfeature of the theory is a matter of fierce debate; for the time being it seems thatinflation’s extreme flexibility and explanatory power are both its greatest strength andits greatest weakness. For all these reasons, some scientists seeking a low-entropy origin for time’s arrowfind explanations relying on inflation slightly unsatisfying. “There are manyresearchers now trying to show in some natural way why it’s reasonable to expect theinitial entropy of the universe to be very low,” says David Albert, a philosopher andphysicist at Columbia University. “There are even some who think that the entropybeing low at the beginning of the universe should just be added as a new law ofphysics.” That latter idea is tantamount to despairing cosmologists simply throwing in thetowel. Fortunately, there may be another way. Tentative new work from Julian Barbour of the University of Oxford, Tim Koslowskiof the University of New Brunswick and Flavio Mercati of the Perimeter Institute forTheoretical Physics suggests that perhaps the arrow of time doesn’t really require afine-tuned, low-entropy initial state at all but is instead the inevitable product of thefundamental laws of physics. Barbour and his colleagues argue that it is gravity, ratherthan thermodynamics, that draws the bowstring to let time’s arrow fly. Their findingswere published in October in Physical Review Letters. The team’s conclusions come from studying an exceedingly simple proxy for ouruniverse, a computer simulation of 1,000 pointlike particles interacting under theinfluence of Newtonian gravity. They investigated the dynamic behavior of the systemusing a measure of its "complexity," which corresponds to the ratio of the distancebetween the system’s closest pair of particles and the distance between the most

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would evolve into this low-complexity state. Thus, the sheer force of gravity sets thestage for the system’s expansion and the origin of time’s arrow, all without anydelicate fine-tuning to first establish a low-entropy initial condition. From that low-complexity state, the system of particles then expands outward in bothtemporal directions, creating two distinct, symmetric and opposite arrows of time.Along each of the two temporal paths, gravity then pulls the particles into larger, moreordered and complex structures—the model’s equivalent of galaxy clusters, stars andplanetary systems. From there, the standard thermodynamic passage of time canmanifest and unfold on each of the two divergent paths. In other words, the model hasone past but two futures. As hinted by the time-indifferent laws of physics, time’sarrow may in a sense move in two directions, although any observer can only see andexperience one. “It is the nature of gravity to pull the universe out of its primordialchaos and create structure, order and complexity,” Mercati says. “All the solutionsbreak into two epochs, which go on forever in the two time directions, divided by thiscentral state which has very characteristic properties.” Although the model is crude, and does not incorporate either quantum mechanics orgeneral relativity, its potential implications are vast. If it holds true for our actualuniverse, then the big bang could no longer be considered a cosmic beginning butrather only a phase in an effectively timeless and eternal universe. More prosaically, atwo-branched arrow of time would lead to curious incongruities for observers onopposite sides. “This two-futures situation would exhibit a single, chaotic past in bothdirections, meaning that there would be essentially two universes, one on either sideof this central state,” Barbour says. “If they were complicated enough, both sides couldsustain observers who would perceive time going in opposite directions. Anyintelligent beings there would define their arrow of time as moving away from thiscentral state. They would think we now live in their deepest past.” What’s more, Barbour says, if gravitation does prove to be fundamental to the arrowof time, this could sooner or later generate testable predictions and potentially lead toa less “ad hoc” explanation than inflation for the history and structure of ourobservable universe. This is not the first rigorous two-futures solution for time’s arrow. Most notably,California Institute of Technology cosmologist Sean Carroll and a graduate student,Jennifer Chen, produced their own branching model in 2004, one that sought toexplain the low-entropy origin of time’s arrow in the context of cosmic inflation andthe creation of baby universes. They attribute the arrow of time’s emergence in theirmodel not so much to entropy being very low in the past but rather to entropy being somuch higher in both futures, increased by the inflation-driven creation of babyuniverses. A decade on, Carroll is just as bullish about the prospect that increasing entropy aloneis the source for time’s arrow, rather than other influences such as gravity.“Everything that happens in the universe to distinguish the past from the future isultimately because the entropy is lower in one direction and higher in the other,”Carroll says. “This paper by Barbour, Koslowski and Mercati is good because they rollup their sleeves and do the calculations for their specific model of particles interactingvia gravity, but I don’t think it’s the model that is interesting—it’s the model’s behaviorbeing analyzed carefully…. I think basically any time you have a finite collection ofparticles in a really big space you’ll get this kind of generic behavior they describe. Thereal question is, is our universe like that? That’s the hard part.”

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time’s arrow, but without the addition of gravity or any other forces. Thethermodynamic secret to the model’s success, they say, is assuming that the universehas an unlimited capacity for entropy. “If we assume there is no maximum possible entropy for the universe, then any statecan be a state of low entropy,” Guth says. “That may sound dumb, but I think it reallyworks, and I also think it’s the secret of the Barbour et al construction. If there’s nolimit to how big the entropy can get, then you can start anywhere, and from thatstarting point you’d expect entropy to rise as the system moves to explore larger andlarger regions of phase space. Eternal inflation is a natural context in which to invokethis idea, since it looks like the maximum possible entropy is unlimited in an eternallyinflating universe.” The controversy over time’s arrow has come far since the 19th-century ideas ofBoltzmann and the 20th-century notions of Eddington, but in many ways, Barboursays, the debate at its core remains appropriately timeless. “This is opening up acompletely new way to think about a fundamental problem, the nature of the arrow oftime and the origin of the second law of thermodynamics,” Barbour says. “But reallywe’re just investigating a new aspect of Newton’s gravitation, which hadn’t beennoticed before. Who knows what might flow from this with further work andelaboration?” “Arthur Eddington coined the term ‘arrow of time,’ and famously said the shuffling ofmaterial and energy is the only thing which nature cannot undo,” Barbour adds. “Andhere we are, showing beyond any doubt really that this is in fact exactly what gravitydoes. It takes systems that look extraordinarily disordered and makes themwonderfully ordered. And this is what has happened in our universe. We are realizingthe ancient Greek dream of order out of chaos.”

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December 8, 2014, 2:32 PMedevere

My reasoned opinion is that both Sean Carroll and Julian Barbour have it wrong.

My argument (which is similar to Loschmidt's argument) is that you can't get a time-irreversiblemacroscopic law of physics (i.e. the Second Law of Thermodynamics) unless there is atime-irreversible microscopic law of physics (i.e. the weak nuclear force.)

As such, it is my reasoned opinion that the weak nuclear force is the explanation for what we callthe arrow of time. I've written a Platonic dialogue with Sean Carroll as a starring characterhopefully help convey my issues with deriving time-irreversible macroscopic law of physics (2ndLaw) from time-reversible laws of physics (i.e. gravity, E&M, and the strong nuclear force.

http://eddiesblogonenergyandphysics.blogspot.com/2014/03/what-is-cause-of-arrow-of-time.html

In general, the problem with Sean Carroll's approach is that, if entropy is real, then there needs tobe a microscopic law of physics that tells us how quickly it increases with time. Sean wants to havehis cake and eat it too. He wants entropy to be real, but then says that its rate of increase can becalculated using time-reversible laws of the physics. This is not possible. Boltzmann had to add alaw of physics (i.e. molecular chaos during collisions) in order to derive entropy generation.

At least Julian Barbour is consistent. He doesn't think that entropy exists, and hence he doesn'tneed to explain why it appears to increase. He thinks that entropy is a figment of our imagination.

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superfluid helium.) We only see entropy generation when there are Fermions (electrons, quarks,and neutrinos: all of which can interact via the weak nuclear force if they are close enough to eachother and have enough energy.)

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December 8, 2014, 7:14 PMTheGreatUnknown

Whimsically thinking, perhaps, the "other side" of black holes (if they do have one) is another,unique universe. Along this pondering, when a black hole is created in this universe, a Big Banghappens for a new universe. And so on, and so forth, creating many unique universes. A black holetherefore would be a one-way doorway to it's own, unique, and newer universe. Further, in ouruniverse, that Dark Matter and Dark Energy is extra-universal matter that has arrived through ouruniverse's "Big Bang" black hole significantly after its inception (our big bang) and therefore canonly emerge as a lower energy, lower density state of existence.

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December 8, 2014, 8:46 PManil rajvanshi

There are two interesting articles that try to explain this article but from a diffrent angle of ancientIndian concept of time and origin of Universe.

http://www.nariphaltan.org/time.pdf How time can be introduced and removed.

http://www.nariphaltan.org/origin.pdf How gravity came out first from Universal Consciousness.

Cheers.

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December 8, 2014, 11:35 PMLeathermonster TheGreatUnknown

I wonder if you were able to cross the event horizon and enter the black hole to be spit out theother side, if you would arrive in the alternate universe at the moment of that universe's Big Bang,no matter how long the black hole had existed in our universe.

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December 9, 2014, 7:01 AMreidhb

1. Time will come to a stop sometime in the future. 2. Time will not come to a stop some time inthe future.

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December 9, 2014, 8:51 AMjjm319

It makes sense that the particles come together under gravity. But can someone please explain whythey spread out again? Can we get the code for this simulation?

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December 9, 2014, 1:08 PMjayjacobus

Time must always surround now. The order is last, current, next. If time reverses it will be next,current, next but only initially. After that the order will be last, current, next again and the arrow oftime will still point into the future which is now the past. No matter what the arrow of time willpoint from last to next.

It is possible that next(-2) no longer exists. If that's the case in a reversal, then time will stop orcreate a brand new past unlinked to the previous past.

In manufacturing, products have a history but each product is made in the present and has no linkto products made days ago other than they were all made in a present using similar resources,processes and materials. The current universe has a similar relationship to past universes.

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December 9, 2014, 3:41 PMroyniles

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December 10, 2014, 2:00 AMgzuckier

Isn't the central issue our asymmetry regards time, in our consciousness? We can look at a video ofa cue ball breaking the racked balls and now immediately whether it's being run forwards orbackwards, when technically it could be reconstructed to run "in reverse" by carefully firing all theballs in from the edges in the precise velocities that they slam together and spit the cueball out thefront.

After we figure that out, then we can maybe tackle the question of whether the future already existsand we are just moving our consciousness along a stationary time axis instead of time expandingitself forwards into the undetermined future.

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December 10, 2014, 8:25 AMvonHasensprung

Schon seit langer Zeit behaupte ich, dass eine positive und negative Zeitachse existiert, die dieBasis für unser Universum bildet.

Grundlage dafür war zum einen die Erkenntnis, dass alles was es gibt, wenigstens zwei Seiten hat.

Und zum anderen erschien es mir logisch, dass die Vorgänge in einem Atomkern sich auf dernegativen Zeitachse bewegen müssen. Abgeleitet habe ich dies von der Tatsache, dass die Zeit füreinen langsamer vergeht, je schneller man sich bewegt. Ebenso gilt dies für Massezentren (je näherich mich an einem Massezentrum befinde, umso langsamer vergeht die Zeit für mich). Bewege ichmich fast mit Lichtgeschwindigkeit, steht die Zeit beinahe und befinde ich mich an der Schwelle zueinem Atomkern oder schwarzen Loch, ebenso. Würde ich - rein theoretisch - diese Schwellejeweils überschreiten müsste ich mich in der Zeit also rückwärts bewegen.

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