Time has an arrow: It only ever seems to move in one direction. The future is always unknown to us, while the past forever remains locked and inaccessible. And yet, the vast majority of the laws of physics don’t seem to care about the direction of time at all. The equations that govern everything—from subatomic particles to the orbits of planets—can’t discern between forward and backward motion in time.
So, where does the arrow of time come from? A growing group of physicists believe that it originates in the force of gravity itself. But to get there requires a radical reshaping of Einstein’s General Theory of Relativity. The new idea is called Shape Dynamics, and it just may be our future theory of gravity.
For over a century most physicists have believed that the key to time’s arrow lies in the concept of entropy, which very roughly is a measure of a system’s disorder. The second law of thermodynamics states that in closed systems entropy always goes up, meaning that systems always go from ordered to disordered. Not only does this law govern physical systems, but it also jibes with everyday experience: it’s much harder to clean a room than it is to make it messy.
But this connection raises an annoying problem. For entropy to generate an arrow of time, our universe had to start in an exceptionally low-entropy, highly ordered state. This strikes most physicists as a contrived, tacked-on assumption that doesn’t mesh with our understanding of a messy, chaotic Big Bang, our best theory for how our universe began.
In 2014, physicist Julian Barbour and his colleagues proposed an intriguing solution to this apparent paradox: gravity. But not gravity as we know it through the general theory of relativity, which says that space and time bend under the influence of matter and energy. Instead, he recasted Einstein’s famous equations using another language, called Shape Dynamics. Shape Dynamics focuses on the relationships between objects, rather than the spacetime, the unified fabric of space and time, that they sit in. Using this framework, Barbour found that if you take a random collection of particles and let them interact through their mutual gravity, an arrow of time naturally emerges.
As the particles interact, they build more complex arrangements with increasing entropy, but not before passing through a period of low-entropy, highly ordered organization. During the experiment, this all seemed to happen naturally. Even though gravity doesn’t care about the flow of time, an arrow organically emerged through the gravitational dynamics of the particles.
But Barbour’s model was highly simplistic. He pretended the universe was nothing more than a collection of particles that only interacted through the single force of gravity. The real universe is much richer than that, with many different kinds of particles interacting through the four forces of nature we know today.
Since Barbour’s initial publication a decade ago, it has met its fair share of criticism – and support. Physicists largely acknowledge that our current understanding of the flow of time and its connection to entropy is probably incomplete, but it will take a lot of work to convince them of an alternative idea. So the approach to an idea like Barbour’s is with skeptical curiosity: poking at it from all directions to see if it holds up to scrutiny.
The first step is to see if Shape Dynamics is a valid theory of physics. In science, you’re welcome to come up withany mathematical framework you want, but the real trick is to make sure it lines up with observations of the real universe. While Shape Dynamics largely agrees with General Relativity, it does have some important differences. While in some simple toy models there doesn’t seem to be any issue, when it comes to bigger problems like the nature of black holes, Shape Dynamicspredicts different mathematical behaviors. It’s not exactly clear yet if these differences invalidate Shape Dynamics. This work is ongoing, and so far it seems promising.
The next step is to see if this hypothesis can be broadened to include a more realistic portrait of the universe. Recently, researchers have found that the same general concept behind Shape Dynamics—that physical interactions naturally lead to an arrow without appealing to entropy—can be applied to subatomic systems. Others wereable to find behaviors that resemble our traditional understanding of the Big Bang.
But so far, nobody has been able to use Shape Dynamics to build a fully realistic portrait of the evolution of the universe to the same level of detail that we understand the cosmos through General Relativity. However, even though the theory is decades old, relatively few researchers around the world are working on it, so we shouldn’t expect huge advances—yet. Still, it’s a fascinating idea: we experience the flow of time because it’s a natural outcome of the basic laws of physics. And that makes the idea of Shape Dynamics worth pursuing further.
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Headshot of Paul M. Sutter
Paul M. Sutter is a science educator and a theoretical cosmologist at the Institute for Advanced Computational Science at Stony Brook University and the author of How to Die in Space: A Journey Through Dangerous Astrophysical Phenomena and Your Place in the Universe: Understanding Our Big, Messy Existence. Sutter is also the host of various science programs, and he’s on social media. Check out hisAsk a Spaceman podcast and hisYouTube page.