A Renaissance woodcut shows a man breaking through the “crystal spheres'” part of classical cosmology, to a new concept of the universe (© The Bridgeman Art Library)
The new book by by Lee Smolin, Time Reborn: From the Crisis in Physics to the Future of the Universe, we are told, “will no doubt be remembered as one of the essential books of the 21st century.” Such commercials invite the reaction: “Oh no it won’t.” For a start I am not convinced that there is a crisis in physics.
But beyond the hype is an interesting if controversial tale. In a nutshell: are the laws of nature timeless, as science through the ages has implicitly assumed, or are they evolving, as Smolin suggests? According to the former view, time is an illusion, and the universe will eventually degrade into disorder and die, like we ourselves. Smolin, by contrast, argues that time has some essential reality, and the universe is a machine that creates ever more complex structures, of which the present is but the start of a rich and exciting future.
Newton’s laws of motion, and their generalisation by Einstein in the equations of relativistic mechanics, or by Dirac and others in quantum mechanics, do not distinguish future and past for individual particles or atoms. Our orbit around the Sun maps out an ellipse in space, but the laws of mechanics say nothing about whether we voyage clockwise or anticlockwise: they are reversible in time.
Yet in practice we experience time’s arrow. We can travel backwards or forwards in space. However, travelling through time is a one-way voyage.
Years ago at school, there was a film club. Someone, for a dare, unwound one of the reels and rewound it in reverse, creating much mirth as we watched a dead cowboy spontaneously come to life and leap backwards up onto his horse, which then ran off hind first. It is usually easy to tell when a film is being played in reverse, as events in the real world take place in a specific sequence. The natural progression is of order turning to disorder, of ageing and decay. The bits and pieces of matter are more likely to become disordered simply because there are more options available; it is this sequence of events, going from order to disorder, that gives our perception of the natural passage of time.
This forward march of time is intuitively obvious, and yet profoundly mysterious. We remember the past and imagine the future. Smolin’s fundamental question, as noted earlier, is whether time is an illusion—emerging as the springs of clocks unwind—or real, giving a fundamental distinction between past and future. Smolin votes for reality. This might seem like common sense, but his position goes against generations of thinkers, including Newton and Einstein.
Does this matter? For philosophers such issues may be meat and drink; but for most scientists, little of this seems to affect the practical problems of our craft. Smolin argues that this is because science focuses on isolated systems, ignoring all of the universe but for the specific situation of interest. Thus although astrologers might insist that wild forces might erupt if Uranus is in Leo, practical physics is able to predict tides and eclipses by focusing on the motion of Earth and Moon around the Sun, ignoring all else. It is this focusing on isolated systems that leads to successful, timeless description of motion. For example, ignoring all but the Earth and Sun, the Earth’s annual orbit—an ellipse—is timeless. It encodes the positions in space that the Earth may occupy; there is nothing in this example to distinguish the orbit of 2013 from that of 1066 or any other year.
The laws do not care in which direction we orbit. Play the movie of the orbiting Earth in reverse, and so long as we don’t look at events taking place on the planet’s surface, we would have no clue that 2013 was unwinding towards 1066. That in the real universe the Earth is orbiting in one direction—“forward” from 1066 to 2013 in the sense of our movie—and not the other is due to what physicists call “initial conditions.” And there’s the rub.
The word “initial” hints at time having set the stage for the play, and then having retired when the curtain went up. This is not a fundamental problem in practice. The processes at work leading to the birth of the solar system happened by chance to set the Earth travelling in its chosen direction. This state of affairs can be explained by events in the epoch when the Earth itself was being formed, and can then be ignored; the motion subsequent to this is timeless.
However, when one extends one’s ambition to describing the dynamics of the universe in its entirety, such issues come home to roost: what determined the initial conditions of the universe? If the universe is degrading towards equilibrium—in the jargon, a state of high entropy—in accord with the second law of thermodynamics, what set it in a state of low entropy at the start? One may plead an anthropic accident—“if it hadn’t been like that then we wouldn’t be here”—thereby dismissing this “crisis” of the initial conditions, and getting on with practical calculations. Yet this strikes many as avoiding a profound question. It is the desire to escape from such special pleading that enthuses some who believe in multiple universes. In a multiverse, one can imagine our universe as “isolated” from the grand landscape of the whole. The timeless descriptions of isolated systems then can apply to our universe. However, if, like Smolin, one prefers economy, our universe is everything; one cannot meaningfully “isolate” it. Smolin argues that a way out of this “crisis” is to do away with a timeless view and accept that time is real.
The first half of the book, deconstructing the foundations of timeless physics, is very readable; reconstructing the reality of time in the second half, is less so. Take a deep breath. Concepts such as “causal dynamical triangulations” appear, for which the notes add: “technically speaking the dual of a triangulation is a three manifold.” Enthusiastic readers may delve further into technical papers with even more daunting titles: “non-perturbative quantum gravity.”
Perhaps in the course of time I shall become convinced, but there is a feature that to me seems important, and which lurks behind the scenes without ever quite bursting through. It is this.
The properties of individual atomic particles, in isolation, can be described by “timeless,” “inert” laws. However the illusion of time passing, and consciousness, which is needed to experience that illusion, have a common feature: large numbers of atoms are present. Macroscopic systems, made of billions of atoms, can exhibit properties that differ radically from those of one or two such particles, and in so doing can mask the underlying timeless laws.
For example, an individual atom follows paths dictated by quantum mechanics, which in effect says it can go almost anywhere. But when enough atoms act collectively, their individual options mix in such a way that only the motion consistent with Newton’s laws survives. In effect, Newton’s laws emerge as a phenomenological description for large objects, out of the more fundamental quantum laws applicable to systems of one or two particles.
Paradoxes of the quantum world, such as whether cats are alive or dead, can arise when this essential truth is ignored. A single atom cannot think, yet a large number, when configured in a highly ordered and unlikely combination, can form a conscious mind. The same atoms configured in countless other ways will have no consciousness, and be defined as dead.
There are many examples where large numbers of atoms exhibit phenomena that individual ones do not. The “crisis” that Smolin sees when we try to describe the entire universe with timeless physics, may be due to large numbers of conscious atoms—us—needing to be included in the equations, rather than believing that at a fundamental level, time itself is to blame.
In any event, Smolin highlights the fundamental difference between isolated, or “closed” systems, and those that are open to external influences. Decay ensues for “closed systems”, which is how we perceive the passage of time, but add energy to the mix and novelties can occur: the refrigerator being but one example. We ourselves are highly ordered systems of atoms, which are the result of Sunlight impacting on the biosphere. For every high-energy photon that arrives from the Sun, the Earth radiates into space vast numbers of low energy photons. Incoming is a highly ordered state—a single photon; the outgoing multitude has a large number of options. The second law of thermodynamics works. But do not confuse the behaviour of closed systems, isolated from their surroundings, with those that are intimately coupled. This is the message that Smolin makes eloquently, whether or not you agree with his conclusion.
Physics has been successful when dealing with systems that can be isolated from their surroundings. To describe the universe as a whole—a goal of modern cosmology—either we must isolate it, which is an oxymoron, or decide whether our inability leads us into what Smolin describes as a crisis.
This is not a book to be read in a couple of weeks. It merits either several months of close study, or none at all. Time will tell.