The now delusion
IMAGINE standing outside the universe. Not just outside space, but outside time too. From this spectacular vantage point, you gaze down upon the universe. At one end you see its beginning: the big bang. At the other, you see鈥 whatever it is that happens there. Somewhere in the middle is you, a minuscule worm: at one end a baby, the other end a corpse. From this impossible perspective, time does not flow, and there is no 鈥渘ow鈥. Time is static. Immutable. Frozen.
Fantastical as it seems, for most physicists today the universe is just like that. We might think of time flowing from a real past into a not-yet-real future, but our current theories of space and time teach us that past, present and future are all equally real 鈥 and fundamentally indistinguishable. Any sense that our 鈥渘ow鈥 is somehow special, or that time flows past it, is an illusion we create in our heads (see diagram).
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鈥淥ur current theories of space and time teach us that past, present and future are all equally real 鈥 and fundamentally indistinguishable鈥
Physics, in fact, has killed time as we know it. The question is: do we need it back?
It was Newton who began to stick the knife into now. His laws of motion, formulated late in the 17th century, were the first to capture time in mathematical equations. Soon it was natural to depict motion on a graph with time on a spatial axis. Once that was done, any special, unique point of 鈥渘ow鈥 started to look as subjective as a 鈥渉ere鈥 on a map of space.
Einstein landed the fatal blow at the turn of the 20th century. According to his special theory of relativity, there is no way to specify events that everyone can agree happen simultaneously. Two events that are both 鈥渘ow鈥 to you will happen at different times for anyone moving at another speed. Other people will see a different now that might contain elements of yours 鈥 but equally might not. 鈥淵ou can define it, but people won鈥檛 necessarily agree,鈥 says physicist of the California Institute of Technology in Pasadena.
The result is a picture known as the block universe: the universe seen from that impossible vantage point outside space and time. You can by all means mark what you think is 鈥渘ow鈥 with a red dot, but there is nothing that distinguishes that place from any other, except that you are there. Past and future are no more physically distinguished than left and right. There are things that are closer to you in time, and things that are further away, just as there are things that are near or far away in space. But the idea that time flows past you is just as absurd as the suggestion that space does.
Contradicting reality
, a cosmologist at the University of Cape Town, South Africa, does not buy any of that. The block universe contradicts every single experience we have, he says. 鈥淚t doesn鈥檛 represent the passage of time, and that鈥檚 one of the most fundamental features of daily life. So it鈥檚 a bad model of reality.鈥 What鈥檚 more, accept it and any attempt to understand anything about anything becomes meaningless. 鈥淭he whole process of doing science depends on time rolling on. We make a hypothesis, test it, accept it or reject it 鈥 the process rolls on in time.鈥
Back in 2006, Ellis . His starting point was not relativity, but quantum physics. A strange fact about quantum physics is that indeterminate future outcomes are seemingly governed by probabilities in the present. Quantum objects exist in 鈥渟uperpositions鈥 of more than one state until such time as we measure them, when they adopt one or other of their possible forms. The most notorious illustration is Schr枚dinger鈥檚 cat: locked in a box with a vial of poison whose seal may or may not be intact, it is simultaneously dead and alive 鈥 until you open the box, when it is most definitely one or the other.
Such quantum oddities are a blow to the block-universe conception of an equally real past, present and future, says Ellis. 鈥淓ven if you know everything about the state of the universe today, you can鈥檛 predict what will be tomorrow. The future can鈥檛 be real because it鈥檚 not even fixed yet.鈥
For Ellis, quantum physics supplies an objectively defined present moment: it is the boundary between what our experiments have determined and what remains to be determined. We live on the leading edge of a 鈥済rowing block鈥 universe, on a surface we call the present that shimmers into existence one moment at a time as quantum measurements are made.
鈥淲e live on an edge of the universe, on a surface we call the present that shimmers into existence one moment at a time鈥
Ellis thinks it is perfectly possible to define this 鈥渘ow鈥 within relativity, too. Einstein鈥檚 general theory of relativity, published a decade after his special theory, is a full picture of space and time, describing how a combined space-time is warped by the presence of matter to produce the force we call gravity. If we gathered enough data and had a big enough computer, we might take account of all the space-time distortions of all the galaxies, black holes and other matter in the universe to calculate a 3D surface on which each point is exactly the same age as the point where we are. 鈥淪pace-time is defined up to then and not beyond,鈥 says Ellis.
This present is still not 鈥渘ow鈥 as we know it, because not everything on this 3D surface happens simultaneously: as demanded by special relativity, if you and I are moving at different speeds on it, we will still disagree on what is happening now. But that doesn鈥檛 necessarily matter. Within relativity, things that are causally related to one another happen in the same order from all perspectives, even if individual observers can鈥檛 agree on exactly when they happened. 鈥淭hat鈥檚 just psychology,鈥 says Ellis. 鈥淚t makes you feel happy to think this is simultaneous with that, but it doesn鈥檛 mean anything for physics.鈥
There are still wrinkles in the growing block. Quantum theory reveals that the future is indeterminate, and that aspects of the present are, too: Schr枚dinger鈥檚 poor feline is an example, if we don鈥檛 bother to check whether it is alive or dead. Ellis鈥檚 solution, formulated with his colleague Tony Rothman in 2009, is that the present is not a solid surface, but one pitted with indeterminacies that gradually solidify into certainties (). These gaps in the present are not something we鈥檇 notice: the cat notwithstanding, quantum indeterminacies can only occur for very small things, and over small timescales. 鈥淭he holes in the present aren鈥檛 going to be big enough to fall through,鈥 says Ellis.
He is still working to fill in gaps in the theory, most recently how the creation of the future cascades down from the cosmological to the quantum scale (). Not everyone is convinced he is on the right track. , a philosopher of physics at the University of Cambridge, isn鈥檛 persuaded by the premise of the present as a dividing line between a real past and an unreal future. Even if the future is indeterminate, he says, it still can be real: you might not be able to determine what鈥檚 on the other side of a mountain from your current location, but that doesn鈥檛 mean it doesn鈥檛 exist.
Carroll鈥檚 beef is that Ellis鈥檚 argument depends on the truth of the 鈥淐openhagen鈥 interpretation of quantum mechanics. This is the idea that acts of measurement determine the world鈥檚 future trajectory, and it to square quantum theory鈥檚 indeterminacy with the decidedly determined world around us. Carroll prefers the 鈥渕any worlds鈥 scenario, in which every quantum possibility occurs in different universes: the present Schr枚dinger鈥檚 cat is dead in some universes, but alive in others. The cat鈥檚 future is just as defined as its past; it just has many possible futures. If this interpretation 鈥 or any other of the many interpretations of quantum theory 鈥 is correct, Ellis鈥檚 way of defining the present vanishes.
That鈥檚 one reason why theoretical physicist Lee Smolin of the Perimeter Institute in Waterloo, Ontario, Canada, thinks that we must be more radical to rescue time. In his recent book , he argues that if we want to square how we perceive time with what physics tells us about it, it鈥檚 no good adapting the block universe: we must throw it out altogether.
Smolin鈥檚 starting point is a reformulation of general relativity known as , developed by the independent physicist Julian Barbour and others. Whereas in relativity space and time stretch and condense for observers travelling at different speeds, in shape dynamics only sizes change. Two distant observers will always agree on what鈥檚 happening 鈥渘ow鈥 in a galaxy regardless of their relative motions; they just won鈥檛 be able to agree how big things in that galaxy are.
That might seem like a zero-sum game, replacing one uncomfortable principle with another. For Smolin, though, bending only space, rather than space and time, neatly recreates a conception of time of the sort quantum physics uses, one in which a single external clock provides a beat that distinguishes one moment from the next. The great prize on offer is the possibility of unifying our understanding of quantum theory with that of gravity, the only one of the fundamental forces of nature to have no quantum description. The route to a 鈥theory of everything鈥, Smolin thinks, is through a better understanding of time.
A unique present
Once simultaneity is regained, it becomes possible to describe the entire universe as a series of layered moments 鈥 a succession of objectively identified times in which all events are simultaneous. 鈥淎ll that exists is this moment,鈥 says Smolin. This is unlike the block universe, where past, present and future are equally real, or Ellis鈥檚 conception, where only the past and the present are. Instead, the only things that are real about the past or future in Smolin鈥檚 world are signs of them in the present: records of the past and indicators of what is to come in the future. Smolin is working with from the University of Edinburgh, UK, to flesh out the idea with mathematics, and to explore which of the many theoretical approaches to quantum gravity it is compatible with.
Price is unmoved. Even if Smolin鈥檚 or Ellis鈥檚 approach can provide an objective way of defining the present, he says, there is still a big logical hole. On the one hand, such arguments demand that the present moment be unique; on the other, they demand that every other moment also acquire that unique property. 鈥淭he whole idea of a privileged present moment is incoherent,鈥 he says.
, a philosopher and mathematician at New York University, has a different objection. Even if Ellis鈥檚 or Smolin鈥檚 theory provides a physical basis for our intuitive conception of here and now, neither explains the fact that we see time flowing, whereas physics suggests it is stationary. This is a fundamental omission, says Maudlin. 鈥淭he notion that time passes is absolutely commonplace; it is not a bit of technical jargon invented by philosophers.鈥 Without flowing time, he says, nothing would move at all. Things like rivers appear to flow in space, but 鈥渋t鈥檚 the fundamental direction in time that underlies all of these other directionalities鈥.
For the past five years, Maudlin has been working on what he calls the theory of linear structures, which he hopes will allow him to reincorporate a flowing time into physics. The idea is rooted in mathematics rather than physics: unlike shape dynamics, it doesn鈥檛 provide a rival physical basis for the warped space-time geometry introduced by relativity. 鈥淚t is the language in which to write a physical theory, not a physical theory itself,鈥 says Maudlin, who is aiming to publish the details in a .
The principal addition to this language鈥檚 vocabulary is an object called a directed line. In any conventional geometry, lines between two points in space and time do not come with a natural direction: we have to define a line in terms of a coordinate system, specifying that it passes from me to you rather than you to me, or drawing an arrowhead on the line to make things clear. In Maudlin鈥檚 geometrical language, however, that arrowhead is implicit in the definition of any line. Once this is built into the fundamental language of geometry, time can naturally acquire a direction.
Ellis thinks Maudlin鈥檚 work is interesting, and also compatible with his growing block picture, explaining in more detail how the flow of time can be fundamental to physics. 鈥淚n the end, you have to base your theories on some fundamental givens. Time, it鈥檚 just kind of a given, which everything else flows around,鈥 he says.
Carroll is more sceptical. Rather than attempting to change the block universe to explain our experience of time flowing, he says we should concentrate on explaining human experience in light of what our very successful physics tells us about the block universe. That task, he says, is quite achievable. 鈥淭hat doesn鈥檛 mean that we鈥檝e done it yet, but I see no obstacle to doing it.鈥
Going with the flow
Craig Callender, a philosopher from the University of California, San Diego, agrees. Explaining our apparently aberrant perception of time does not mean we have to overturn physics or invent a whole new way of doing geometry. When, he says, we 鈥渆mbed critters like us鈥 in a universe like ours, it makes sense that we should see a flowing time and distinguish past, present and future 鈥 even when the reality is something different.
To explain why, we can return to that vantage point gazing down on the entire block universe, and zoom in on that tiny human speck: the four-dimensional worm with a baby at one end and a corpse at the other. This worm鈥檚 perception of time differs from 鈥渞eality鈥 first in that it remembers the past but does not see the future. That can be explained as a consequence of thermodynamics. The universe started off in a highly ordered, taut state after the big bang, and has been expanding into an ever more disordered, flaccid state ever since. There is an infinitude of paths in which the universe can evolve forward in time, but only one path back into its history. Why the universe works like that is another, fundamentally unanswered question 鈥 but it means that, purely statistically, we are only ever likely to have a clear view backwards in time.
Even then, you would expect we worms to feel as if we are stationary in time with a view in only one direction, rather what we experience: moving backwards into the future with no clear view of where we are heading. For Callender, the key to this illusion is an important psychological fact about ourselves: we have a sense of identity. According to physics, your life is described by a series of slices of your worm 鈥 you as a baby, you as you ate breakfast this morning, you as you started reading this sentence and so on, with each slice existing motionless in its respective time. We generate time鈥檚 flow by thinking that the same self that ate breakfast this morning also started reading this sentence. 鈥淩eally there鈥檚 all these different mes at all these different times,鈥 says Callender. 鈥淏ut because I think that I鈥檓 identical over time, that鈥檚 why time seems to flow, even though it doesn鈥檛.鈥
鈥淧erhaps the key to the illusion of time is an important psychological fact about ourselves: we have a sense of identity鈥
So do we really need to mourn time鈥檚 passing? Einstein, for one, drew solace from the view of the timeless universe he had helped to create, consoling the family of a recently deceased friend: 鈥淣ow he has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion.鈥 .
This article appeared in print under the headline 鈥淭he now delusion鈥
