It might seem surprising as you look in the mirror and spot grey hairs and growing wrinkles, but the accepted view in physics is that our experience of time passing is just an accident of the Universe. In the laws of fundamental physics it's just as valid for time to pass backwards as forwards, and the accepted view is that we live in a *block universe*, a four-dimensional block of spacetime that contains all the space and all the time – the past, the present and the future – already set into this chunk of cosmological concrete. (You can read more about the block universe and the symmetry of time in previous articles in this collection.)

In Einstein's general theory of relativity time isn't even that special: it's just one of the four dimensions of spacetime, the fundamental fabric of the universe. Time as we experience it is an illusion, a byproduct of our Universe starting in the Big Bang. But Marina Cortês, a cosmologist from the Royal Observatory, Edinburgh, and a growing number of other physicists are challenging this view and developing alternative theories where time comes first.

### Time passes

"We're trying to find out the fundamental theory of nature, but we want to bring back into physics this natural basic aspect of our experience, which is that time only moves forward," says Cortês. "We're not happy with the explanation that time moving forward is an illusion, explained by this very unlikely Big Bang state." Cortês and her collaborator Lee Smolin have instead developed ideas that would mean time was fundamental, and that the passing of time was real and irreversible.

In their model, the most fundamental description of the Universe is as an *energetic causal set*. And rather than starting from a continuous spacetime, as in Einstein's general theory of relativity, Cortês and Smolin's picture starts from discrete "atoms" of time: "like the ticks of the clock." Think of the process of you reading this article and dramatically slow down time until you get to a series of individual photons of light from the screen interacting with your retina. Each of these interactions is an instance of time in Cortês' model, and each instant has an associated energy and momentum. These instances are discrete, but when you zoom out they appear to merge into a continuous passage of time. The future hasn't happened yet, and the past, those instances of time that no longer interact with any others, effectively doesn't exist.

These instances are linked together and ordered into a network by mathematical equations that describe the physics of the world around us. And for energetic causal sets these equations are fundamentally asymmetric in time – time only flows forward. "What has happened can't unhappen," says Cortês. This asymmetry comes from using *stochastic mechanics*, similar to the random outcomes in quantum mechanics. You can't definitively predict the future, but you also can't definitely reconstruct the past. "It's like when you throw a six, you can't reconstruct what side of the dice faced up when it was in your hand."

The equation *y= ^{2}* gives an answer of

*y=4*for both the values

*x=2*and

*x=-2*.

The equations also have a mathematical property that encodes this time asymmetry; they are *surjective*, where two or more distinct elements can evolve into the same outcome. Think of the simple equation *y=x ^{2}*, both the values

*x=2*and

*x=-2*give the answer of

*y=4*. If you you know the output of your equation was a four, you can't definitively say if the input was

*x=2*or

*x=-2*. Thinking in terms of our example of throwing a dice: it might be possible to throw a six, starting with any of the faces of the die facing up in you palm.

Cortês and Smolin have been able to show that when you zoom back out from the ticks of their clock to larger scales spacetime emerges, as well as aspects of quantum mechanics and gravity. They hope to eventually recover the full suite of physical laws – the standard model of particle physics, general relativity and newtonian mechanics – complete with their apparent symmetry of time. This would then be a much sought for theory of quantum gravity, and one where the forward flow of time is fundamental.

### The deepest clue

Theirs is just one of a number of potential approaches where time comes first. Forty of the key researchers from cosmology and philosophy, some of whom support the orthodox block universe picture and some working on alternative theories, were invited by Cortês and Smolin to the Time in cosmology conference in June this year at the Perimeter Institute for Theoretical Physics in Canada. The conference was designed to be intimate, with the sessions organised to encourage discussion and debate of the broad variety of opinions and ideas held by those there. But it turned out to be very popular, regularly over 80 people crowded into the room to hear the discussions, leading to those not registered for the conference being kicked out due to health and safety reasons! (You can read more about the conference in this watch the discussions on the Perimeter Institute website, including the welcome and opening remarks,which gives an accessible account of the motivations for the conference. )

The growing block universe - how does the present crystallise from the past?

One of the other alternatives discussed at the conference was the *growing block universe*, supported by George Ellis. In this theory we do live in a block universe, but the future hasn't happened yet. Instead, the present is the "surface" of the block universe, the block universe of the past growing ever larger. For Cortês this halfway version still feels a little odd, as it still gives the past the same status as the present. "They believe the past exists somewhere, but where is the past? Can I turn the corner and see it?"

Many other approaches were discussed at the conference, such as an alternative causal set model where causality, rather than time, is the fundamental principle (you can read more here), as well as interpretations of the orthodox block universe view. The heated debates and indepth discussions captured the excitement of this rising area of physics that aims to reconcile fundamental physics with our perception of time. "I believed [in the block universe] for a long time," says Cortês. "But gradually I realised it doesn't feel right. I'm convinced that the passage of time is the deepest clue that nature is giving us to how nature works. And physicists are turning their backs on it." Perhaps it's just a matter of time before we uncover the fundamental theory of nature.

### About this article

*This article is part of our Stuff happens: the physics of events project, run in collaboration with FQXi. Click here to see more articles and videos about the block universe and block time. *

Rachel Thomas is Editor of *Plus*. She interviewed Marina Cortês in April and August 2016.

## Comments

## article

Interesting

## Block time

I disagree profoundly with he traditional view of block time for two reasons.

Firstly, it is simply not true to say that 'In Einstein's general theory of relativity time isn't even that special: it's just one of the four dimensions of spacetime'. One of he most fundamental formulae in GR is the formula for what is called the 'interval' S between two events. ('Interval' is to spacetime what 'distance apart' is to space) The formula can be written like this:

S = sqrt(x^2 + y^2 + z^2 - (ct)^2)

The minus sign makes it immediately clear that the temporal dimension (t) is quite different from the spatial dimensions (x, y and z).

Secondly, it is not possible to say that, because of the symmetry of all the known laws of physics, 'it's just as valid for time to pass backwards as forwards'. I entirely agree with Cortes here that the randomness built in to quantum mechanics proves that the future is indetermimate i.e. it 'hasn't happened yet'. If you subscribe to the 'bock universe' view you have a serious problem explaining why some photons are reflected and some photons are transmitted at a half silvered mirror because, in a block universe, nothing happens at random because the future is already fixed. The conventional interpretation of QM (the Copenhagen interpretation) allows for quantum events to be truly random and is therefore incompatible with the Block universe'.