Why Time Might Not Be an Illusion

Most everyday people like to think of the universe as something that evolves. It started simple and small, and a set of rules governed how it got bigger, and older, and more complex. It’s a nice picture. Instinctive. Natural. Yet, thanks to Einstein, many physicists would argue that it’s wrong.

Instead, the mainstream physics view is that the past, present, and future all co-exist in one static spacetime block. But quantum physicist Tony Short at the University of Bristol, in the UK, is making a pitch to re-awaken the older, more intuitive picture in which the universe evolves, as time passes.

The block universe picture came about after Einstein developed his special theory of relativity. This says that different observers in motion relative to one another experience different things. Someone on a moving train and someone on a platform may disagree about when the train’s whistle blows, for example. Crucially, Einstein tells us that each observer’s perspective is equally true: there’s no preferred reference frame giving any one observer the correct viewpoint, and no unique moment that can be picked out as "now." So there’s no single ’current state’ of the universe to apply a nice set of rules to, in order to see how it evolves.

This conundrum has led many physicists to think of the universe instead as a giant box of spacetime, pre-filled with all that ever was and all that ever will be. Any one person’s view of this box can be described by looking at a slice through it. But while this radical upheaval of our notions of space and time fits nicely with relativity, for Short, it seems a step too far.

"One reason I don’t like spacetime is that the whole thing has to leap into being at some point," says Short—and it seems odd for such a complex thing to exist without some mechanism for its construction. "It just suddenly Is True. And it seems to me that’s a quite complex thing. It seems unnatural." The old-fashioned evolving state theory, on the other hand, in which a complex universe gradually grows from simple initial conditions, he says, "seems a more fair way to generate complexity."

Short, who specializes in quantum physics, has spent the last few years tackling this conceptual problem from a host of different angles, with the help of an FQXi grant of almost $60,000. The idea is not necessarily to prove that one view is right and the other wrong, but more to explore the consequences of each theory both for relativity and quantum theory, which governs nature at the smallest scales. His recent FQXi work was inspired particularly by thinking about causality, and whether the ’evolving state’ model, or the ’spacetime box’ model, is better suited to describing it.

Loops, Whorls, and Paradoxes

One way to interrogate the spacetime box idea was to look at whether it might support crazy loops and whorls—like the classic grandfather paradox of science fiction, where someone goes back in time and kills their grandfather, preventing themselves from ever having been born, and thus preventing them from having gone back in time, etc. "We still don’t know how to deal with that one," Short laughs. Short and his colleagues investigated what might happen if you create a spacetime box that rules out the worst of these loops: those in which an individual directly receives a message from their future self. Even in that case, they found, the box as a whole can still have other kinds of time loops in it, allowing influences from the future to affect the past (Y. Guryanova et al, arXiv:1708.00669 (2017)). "Locally there’s nothing troubling, but globally there’s still something weird going on," Short says.

That provides a new perspective on the spacetime box model. "It gives non-trivial restrictions on our theories, from the assumption that one cannot signal into the past," explains quantum physicist Caslav Brukner of the University of Vienna, in Austria, who has independently investigated causality at the quantum scale. (See "Blurring Causal Lines.")

Renato Renner, of ETH Zürich, describes Short’s work as exploring the possibility that time-ordering is not equivalent with causality; that just because A causes B doesn’t always mean that A comes before B. "You have to give up something you took for granted, something that’s almost hard-coded in our brains," says Renner. Once this connection is blown open, notes Renner, we have a better chance of incorporating an evolving state picture into a relativistic universe.

Short has also been investigating another speculative model, in which the universe is made of tiny discrete bits of time and space: basically pixelated, like a computer screen, that ticks forwards in time, rather than sweeping forwards smoothly. He has seen some signs, for very simple particles, that it might hold true, and naturally lead to relativistic-phenomena on larger scales (T.C. Farrelly & A. J. Short, Phys. Rev. A 89, 062109 (2014).) And while Short admits that this model isn’t widely popular, analyzing it is still instructive. Looking at the world this way undermines the importance of relativity to the universe, Short notes, and so weakens the whole rationale behind moving to the block universe. "It takes away from the motivation to make this big shift," he says.

There are many, many variations on evolving state and spacetime box models of the universe, adds Renner; as many theories as there are physicists who study them. "This is why it’s so important to compare the consequences of each," he says. "This is the only way to get a clearer picture. I think it’s very important work."