Physicists have performed an experiment that suggests time in our Universe may be directed by gravity, not thermodynamics, and that the Big Bang could have created two parallel universes - our own, in which time runs forwards, and a mirror one where time runs backwards.

Although the idea sounds pretty out there, the new hypothesis could help physicists solve some of their biggest issues with time - mainly the fact that they still can't work out why it runs in only one direction.

In fact, this single "arrow of time" is one of the biggest conceptual problems of modern physics and has puzzled physicists for more than a century. 

The root of the dilemma is the fact that all of the fundamental laws of physics - such as Einstein's special and general relativity and Newton's gravitation - work just as well if time is flowing forwards or backwards.

In models of physical systems, which physicists use to mimic our Universe, a preferred direction of time does sometimes arise, but this typically only happens when researchers tinker with the system and set specific starting conditions. 

So why then does our Universe only move forwards in time? Why are stars emitting light rather than sucking it up and why do we remember the past and not the future?

Currently, the leading theory is that the direction of time's arrow is controlled by the laws of thermodynamics - more specifically, entropy

Entropy is a measure of disorder within a thermodynamic system - a system with low entropy is extremely organised and predictable, whereas a high entropy system is more random. Thermodynamic law states that the entropy of an isolated system, such as our Universe, will only ever move from a state of low entropy to a state of high entropy.

Most physicists generally accept that this is why time moves forward - because at the birth of our Universe everything was extremely ordered, and so the direction of time is the same as the direction of increasing entropy.

As Lee Billings explains for Scientific American, this is "a product of the universal tendency for all things to settle toward equilibrium with one another".

But this hypothesis relies on those highly organised, low entropy conditions being present at the start of the Universe in order to give time a direction. Which is something we simply can't prove, much to the frustration of many physicists.

Now new theories are emerging that suggest this idea of time being governed by entropy isn't the only possibility. 

And the new work, led by Julian Barbour from the University of Oxford in the UK, suggests it may in fact be gravity, not thermodynamics, that controls the direction of time's arrow. 

The research, which also involved Tim Koslowski from the University of New Brunswick and Flavio Mercati of the Perimeter Institute for Theoretical Physics, both in Canada, and was published in October in Physical Review Letters.

Their model suggests that the Universe doesn't need a special, low-entropy initial state in order for it to define an arrow of time - instead, the flow of time is just the inevitable result of gravity.

They came to this conclusion after studying a very simple model of our Universe comprised of just 1,000 particles. Using computer simulations, they tested how these particles interacted under nothing but the influence of the laws of Newtonian gravity.

What they found was that, no matter how the system was originally arranged, the particles would all eventually end up in these tightly packed, low-complexity states without any tinkering, simply through the sheer force of gravity.

This means that in order to set the direction of time's arrow, we don't need any perfect low-entropy conditions, we just need gravity.

But perhaps most interesting is what happened next in their model. From that highly condensed place, the system expanded outwards - but in two separate directions, each with their own time arrow travelling in a different direction.

Along both time pathways the particles were pulled by gravity into larger, more ordered and complex structures. As Billings writes for Scientific American, these are our equivalent of the Universe forming galaxies, stars and planetary systems.

Of course, we are a long way from knowing if this is what occurred in our own Universe - the system that Barbour and his team tested was extremely simple, and didn't factor in general relativity or the effects of quantum mechanics.

But if it's true, it means that what we perceive as the future would really be the distant past for any life that exists in the parallel universe.

"This two-futures situation would exhibit a single, chaotic past in both directions, meaning that there would be essentially two universes, one on either side of this central state," Barbour told Billings for Scientific American

"If they were complicated enough, both sides could sustain observers who would perceive time going in opposite directions. Any intelligent beings there would define their arrow of time as moving away from this central state. They would think we now live in their deepest past."

Which is extremely trippy to think about. Even more mind-blowing is how Tim de Chant from PBS NOVA puts it:

"From that perspective, maybe George Lucas's Star Wars didn't take place a long time ago in a galaxy far, far away, but in the far future - our deepest past - of our mirror universe."

However, devotees to the idea that entropy controls the direction of time, such as cosmologist Sean Caroll from California Institute of technology, will need much more proof before they subscribe to the hypothesis.

"This paper by Barbour, Koslowski and Mercati is good because they roll up their sleeves and do the calculations for their specific model of particles interacting via gravity, but I don't think it's the model that is interesting—it's the model's behaviour being analysed carefully," Caroll, who wasn't involved in the research, told Scientific American.

"I think basically any time you have a finite collection of particles in a really big space you'll get this kind of generic behaviour they describe. The real question is, is our Universe like that? That's the hard part."

If we could answer that question, then not only would it change our entire perspective on the Universe, but, importantly, it could help us properly explain the expansion and growth of the Universe that we observe - which is something we still struggle with.

If you're as fascinated by all of this as we are, read the amazing account of how this hypothesis emerged and other theories that are out there in Billings' piece for Scientific American.

Source: Scientific American, PBS NOVA, APS Physics