The Best Australian Science Writing 2014 (22 page)

This present is still not ‘now' 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't 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't agree on exactly when they happened. ‘That's just psychology,' says Ellis. ‘It makes you feel happy to think this is simultaneous with that, but it doesn't mean anything for physics.'

There are still wrinkles in the growing block. Quantum theory reveals that the future is indeterminate, and that aspects of the present are, too: Schrödinger's poor feline is an example, if we don't bother to check whether it is alive or dead. Ellis's solution, formulated with his colleague Tony Rothman and published in the
International Journal of Theoretical Physics
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'd notice: the cat notwithstanding, quantum indeterminacies can only occur for very small things, and over small timescales. ‘The holes in the present aren't going to be big enough to fall through,' 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. Huw Price, a philosopher of physics at the University of Cambridge, isn't 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's on the other side of a mountain from your current location, but that doesn't mean it doesn't exist.

Carroll's beef is that Ellis's argument depends on the truth of the ‘Copenhagen' interpretation of quantum mechanics. This is the idea that acts of measurement determine the world's future
trajectory, and it is the most popular way among physicists to square quantum theory's indeterminacy with the decidedly determined world around us. Carroll prefers the ‘many worlds' scenario, in which every quantum possibility occurs in different universes: the present Schrödinger's cat is dead in some universes, but alive in others. The cat's future is just as defined as its past; it just has many possible futures. If this interpretation – or any other of the many interpretations of quantum theory – is correct, Ellis's way of defining the present vanishes.

That's one reason why theoretical physicist Lee Smolin of the Perimeter Institute in Ontario, Canada, thinks that we must be more radical to rescue time. In his recent book
Time Reborn
, he argues that if we want to square how we perceive time with what physics tells us about it, it's no good adapting the block universe: we must throw it out altogether.

Smolin's starting point is a reformulation of general relativity known as shape dynamics, 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's happening ‘now' in a galaxy regardless of their relative motions; they just won't 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 ‘theory of everything', Smolin thinks, is through a better understanding of time.

* * * * *

Once simultaneity is regained, it becomes possible to describe the entire universe as a series of layered moments – a succession of objectively identified times in which all events are simultaneous. ‘All that exists is this moment,' says Smolin. This is unlike the block universe, where past, present and future are equally real, or Ellis's conception, where only the past and the present are. Instead, the only things that are real about the past or future in Smolin's 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 Marina Cortes from the University of Edinburgh 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's or Ellis's 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. ‘The whole idea of a privileged present moment is incoherent,' he says.

Tim Maudlin, a philosopher and mathematician at New York University, has a different objection. Even if Ellis's or Smolin's 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. ‘The notion that time passes is absolutely commonplace; it is not a bit of technical jargon invented by philosophers.' Without flowing time, he says, nothing would move at all. Things like rivers appear to flow in space, but ‘it's the fundamental direction in time that underlies all of these other directionalities.'

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't provide a rival physical basis for the warped space–time geometry introduced by relativity. ‘It is the language in which to write a physical theory, not a physical theory itself,' says Maudlin, who is aiming to publish the details in a book.

The principal addition to this language's 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's 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's 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. ‘In the end, you have to base your theories on some fundamental givens. Time, it's just kind of a given, which everything else flows around,' 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. ‘That doesn't mean that we've done it yet, but I see no obstacle to doing it.'

* * * * *

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 ‘embed critters like us' in a universe like ours, it makes sense that we should see a flowing time and distinguish past, present and future – 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's perception of time differs from ‘reality' 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 – 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 than 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 – 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's flow by thinking that the same self that ate breakfast this morning also started reading this sentence. ‘Really there's all these different mes at all these
different times,' says Callender. ‘But because I think that I'm identical over time, that's why time seems to flow, even though it doesn't.'

So do we really need to mourn time's 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: ‘Now 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.' So it goes.

* * * * *

Postscript
: Tim Maudlin's book,
New Foundations for Physical Geometry
, was published in May 2014.

The oldest known star

The quantum spinmeister

Reached by committee, nineteen eighty-three

Paul Magee

A metre is the length travelled by a ray of light

in a

vacuum

over

299,792,458
^ths

of a second.

They used to count

one million six hundred and fifty thousand

seven hundred and sixty-three point seventy-three

wave

lengths

of the radiation between the levels two p ten and five d five

of the Krypton eighty-six atom in the dark, old

seconds prior to accurate measurement, before we embraced

the light.

The now delusion

Liner notes,
Voyager
Golden Record

Material of the future: Sticky tape, honey and graphene

Lisa Clausen

Three clear bottles stand like trophies on an otherwise empty shelf in Professor Dan Li's office at Melbourne's Monash University. Two are filled with powder the colour of midnight, while the third contains a lump of silver-grey rock. They're all forms of graphite, a type of coal we all rely on somehow, whether it's in brake lining, batteries or pencils. But that's not why Li has the bottles displayed behind his desk. Among scientists like Li, graphite is now celebrated as the source of graphene, the phenomenal new material that researchers, governments and corporations the world over are betting could transform a multitude of industries, from electronics to renewable energy.

Scientists had long suspected graphite contained something interesting. But while they knew this smudgy, light rock was composed of stacks of graphene sheets, none of the brilliant minds working on it could figure out how to isolate a single sheet, let alone manipulate it. Then in 2004, University of Manchester physicists Andre Geim and Konstantin Novoselov had an inspired idea. Taking a block of graphite, the pair simply began stripping off flakes with sticky tape. They ended up with micro
flakes of a completely new material, each too thin to be seen by the naked eye, its carbon atoms arranged in a dazzlingly perfect honeycomb pattern. Their playfulness won them, just six years later, the Nobel Prize in Physics. ‘No one really thought [releasing graphene] was possible,' said the Royal Swedish Academy of Sciences. ‘Carbon, the basis of all known life on earth, has surprised us once again.'