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FQXI ARTICLE

January 22, 2019

Why Time Might Not Be an Illusion

Einstein’s relativity pushes physicists towards a picture of the universe as a block, in which the past, present, and future all exist on the same footing; but maybe that shift in thinking has gone too far.

FQXi Awardees: Tony Short

February 28, 2018

Tony Short

University of Bristol

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.

You have to give up

something you took for

granted, something that’s

almost hard-coded in

our brains.

something you took for

granted, something that’s

almost hard-coded in

our brains.

- Renato Renner

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.

All time like the present.

According to Einstein’s relativity, the past, present, and future co-exist in a

static block universe.

Credit: istockphoto, jokerpro

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,

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."

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ANONYMOUS wrote on January 17, 2019

On the side of the ledger for "pixelated", globally discrete quantum spacetime(s), is the the classic rhetorical 'error of omission' in the logic of BlockTime. It is dependent on assuming that the scale of space and time are the same universally, and on not posing the question of what rate of passage of time is the prime referrence when it can only be constrained as theoretically being anywhere between light velocity and nil.

The problem for global pixelation of spacetime, is how to...

On the side of the ledger for "pixelated", globally discrete quantum spacetime(s), is the the classic rhetorical 'error of omission' in the logic of BlockTime. It is dependent on assuming that the scale of space and time are the same universally, and on not posing the question of what rate of passage of time is the prime referrence when it can only be constrained as theoretically being anywhere between light velocity and nil.

The problem for global pixelation of spacetime, is how to...

ECKARD BLUMSCHEIN wrote on January 16, 2019

You mentioned a paper "Zeh 2010". I cannot find it in the references you gave.

By the way, I wonder why didn't you mention elapsed time alias age. Didn't you read FQXi essays of mine?

EB

You mentioned a paper "Zeh 2010". I cannot find it in the references you gave.

By the way, I wonder why didn't you mention elapsed time alias age. Didn't you read FQXi essays of mine?

EB

AMRIT SRECKO SORLI wrote on January 15, 2019

Time has merely the mathematical existence.

Time has merely the mathematical existence.

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