Is gravity a new type of force that arises from cosmic entropy?

There are some things in life that just sort of happen. Desks get covered in dust and scraps of paper. Clothes get dirty and the laundry basket fills up. Weeds slowly creep across an untended flowerbed. Things, in other words, tend to get messier unless we step in and tidy up.

Now here’s an idea: what if gravity itself works like that? It would certainly be a different way of looking at the force that keeps our feet on the ground and conducts the twirling dance of the planets. Most physicists see it as one of the four forces of nature, about as fundamental as you can get. But back in 2010, physicist Erik Verlinde suggested that it wasn’t a force at all, but simply a byproduct of the universe’s natural inclination to become more disordered. “For me, gravity doesn’t exist,” he told reporters at the time.

The reaction, to put it kindly, was mixed. But over the years, things have changed. We now have a clearer picture of how this idea, known as entropic gravity, could work. There are hints that it could explain some huge mysteries, including the nature of dark energy and dark matter. And most recently, proposals have even been put forward for how we might subject this idea to experimental testing. All of this is prompting a few physicists to ask: is gravity really a force at all?

For a subject known for its precision, physics has had a surprisingly long fascination with disorder, going back to the sparks and steam of the industrial revolution. In the mid-19th century, physicist Rudolf Clausius resigned himself to the fact that engines could never convert all their heat into useful work. For that, he blamed a new quantity he called entropy. In the case of an engine, heat naturally flows from hot to cold – and in doing so, some of that heat is inevitably lost to the surroundings. This leakage was so universal, argued Clausius, that it could be enshrined as a fundamental rule, which we now know as the second law of thermodynamics.

A proper definition of entropy came a decade later from Ludwig Boltzmann, who considered the number of possible arrangements of atoms and molecules in a given space. His insight was that some arrangements are more orderly than others. For instance, a scenario with zillions of fast molecules on one side of a piston and zillions of slow ones on the other is tidier than one with fast and slow molecules mixed together. Entropy, said Boltzmann, is a measure of this disorder – and overall, it always rises.

About a century later, the physics of entropy took an astronomical leap. Theorist Jacob Bekenstein at Princeton University had been studying the disorder contained inside a black hole. He worked out that a black hole’s entropy must be proportional to the surface area of its event horizon, the boundary beyond which nothing can escape its gravitational pull.

Physicist Ted Jacobson went on to show that, in certain circumstances, the gravity in space itself behaves like a thermodynamic system. This was a startling finding: gravity is believed to be a force that applies to all objects, even single elementary particles, whereas thermodynamics usually only emerges from things made up of a great many small parts. Still, Jacobson’s insight was widely taken as a coincidence, rather than any deep statement about nature.

Then came a moment in the summer of 2009, when Verlinde was stranded on holiday, his passport having been stolen. He had read Jacobson’s paper many times and couldn’t shake off the idea that this equivalence between entropy and gravity was more than a coincidence. Now, stuck with little else to do, he turned it over in his mind again and arrived at an interpretation he would outline in a curiously simple paper the following year.

In essence, he argued that gravity is just an artefact of the deeper, truly fundamental law that entropy must always rise. “I emphasised more than others that if you take these laws of thermodynamics and black holes seriously, you should take the emergent perspective of gravity seriously,” he says.

Emergent gravity

That word “emergent” is important. Physicists distinguish between fundamental ingredients of reality, which can’t be broken down into simpler parts, and emergent phenomena, which are a result of many simpler things acting together. Take something like the air pressure in a tyre. Pressure is a useful concept, but it isn’t fundamental to reality – we understand that it is the result of many air particles bouncing around inside the tyre. Verlinde was arguing that gravity is emergent in a similar sense. His peers didn’t know what to make of it. “Some people have said it can’t be right, others that it’s right and we already knew it,” said Harvard University theorist Andrew Strominger at the time.

The trouble was, physicists already thought they knew they were on the right track with gravity. Our best understanding of this force comes from Albert Einstein’s theory of general relativity. But for most of the past century, physicists had been trying to find ways to describe it in the language of quantum theory – not an easy task, as the two theories start from totally different assumptions.

The most promising way to mend the division has been string theory, which reconstructs particles and space-time from one-dimensional entities known as strings, which are coiled and spread over 10 dimensions. After decades of effort, string theorists haven’t been able to describe a universe similar to ours, but their hopes are undiminished.

At first glance, entropic gravity might seem like a completely different approach, but in fact, Verlinde’s original work leaned heavily on an idea in string theory known as holography. The gist is that, mathematically speaking, there are ways to perfectly translate what goes on in one reality into another reality that has fewer dimensions. It means that the three-dimensional world we move through could be no more real than a ghostly hologram, a mere projection from a deeper and altogether flatter two-dimensional reality.

Thinking along these lines, Verlinde imagined a mass placed just outside a two-dimensional screen in the shape of a sphere, enclosing another mass within. He worked out that the outer mass experiences a gentle push inwards – not due to any physical pull, but because the total entropy of the system increases if the two masses get closer. This was Verlinde’s epiphany: to see gravity not as a force, but simply as the result of nature’s tendency towards greater entropy in a secret, lower-dimensional realm.

What is this realm, though? “A major question that I think was left open by Verlinde’s work is understanding what the underlying microscopic system is and what [model] of entropy one should be using,” says Grant Remmen, a theorist at New York University who has previously worked on entropic gravity. In 2010, Verlinde offered no clear answer. But that changed a few years later when he drew once more on parallel developments in quantum gravity. In quantum physics, particles can become entangled, such that their behaviours appear to instantly affect each other, even when separated by vast distances. A modern trend among some quantum gravity theorists is to proclaim that an information network based on entanglement itself is the primary stuff of existence. In other words, deep down, reality is information.

Information and reality

This view may not be for the fainthearted, but for Verlinde it made plenty of sense. What is the fundamental part of reality that is becoming more disordered? The entanglement information network. And there were hints that Verlinde was on to something. As he worked through the equations, he found that the entanglement underlying the regions around galaxies ought to be more disrupted than the bare distribution of matter would otherwise suggest, resulting in more entropy and “extra” gravity. Incredibly, he had landed on a solution to a major problem in physics – that galaxies are observed to rotate too fast for the amount of visible matter, and hence gravity, in them. In other words, they should be tearing themselves apart. Astrophysicists are usually forced to invoke a mysterious and invisible “dark matter” to explain this, but Verlinde’s approach worked without it.

Even so, most experimental physicists paid little heed, because Verlinde’s hypothesis lacked any specific predictions that would enable anyone to test it. “We just wanted to know what we had to look for,” says Dan Carney, a physicist at the Lawrence Berkeley National Laboratory in California. Like Verlinde, Carney was also captivated by Jacobson’s early paper on entropy and gravity. One line in particular haunted him. It suggested that gravity may be no more fundamental than a passing sound wave made up of ebbing and flowing air molecules. Perhaps both are equally ephemeral, equally dependent on simpler things, with no need of a description in the primal tongue of quantum mechanics.

Carney has been musing on this for years, but only recently did he find a possible way to test it in the lab. His idea is remarkably simple. Rather than be tied to the metaphysical “information is reality” stance favoured by Verlinde, he and his colleagues posit a more generic background system – a collection of microscopic entities whose exact nature doesn’t matter. Like the molecules in a piston, this background system is thermodynamic, and conserves energy while striving to maximise entropy. Remarkably, they find that when test masses interact with it, the masses attract according to Isaac Newton’s law of gravitation, even though the force of gravity isn’t explicitly included. “It shows there are other ways of thinking about gravity,” says Carney.

The researchers actually considered two different models. One, which was very simple, predicted a gravity that was highly erratic, unlike anything we observe. By contrast, the other model included quantum effects, namely an ability of the bedrock ingredients of reality to be in more than one energy state at the same time, and to be entangled with one another. In this model, gravity was much more realistic – but crucially, not exactly. Since the force arises from a background system that follows the rules of thermodynamics, it would necessarily exhibit tiny jitters. In other words, if the model is right, we ought to see small irregularities in the otherwise-smooth gravitational attraction between objects.

This means that, finally, the doors are open to test entropic gravity. All physicists would have to do is seek those telltale gravitational blips. The kinds of device that would be needed already exist. For example, they could be tiny, weighted levers that would move – smoothly or in fits and starts – as a tiny mass was brought close to them. These devices are usually designed to explore other topics, such as gravitational waves or the limits of quantum behaviour. Repurposing them to detect entropic gravity would take time, but it is possible.

Carney and his colleagues are already devising an experiment consisting of a weight on a twisting pendulum next to a cloud of atoms in a quantum state. As the weight moves to and fro, traditional gravity would generate well-behaved changes in the cloud’s quantum state. But if there are any random jiggles due to entropic effects, they should be detectable. All this seems interesting, says Remmen, “especially that they find an experimental signature”. He points out, however, that Carney’s work so far only recreates Newton’s laws of gravitation, not the more advanced nuances of general relativity.

For his part, Verlinde would have preferred the model to include holography, which he believes is necessary for a truly emergent gravity picture. But he calls it a “really nice” development and cherishes the possibility of experiments. “Theorists as well as experimentalists need inspiration,” he says. “They need to talk to each other – and that’s where this paper is really useful. Dan connects these two worlds.”

Meanwhile, other physicists are discovering the attraction of entropic gravity. Returning to Verlinde’s original paper as inspiration, Kazem Rezazadeh at the Institute for Research in Fundamental Sciences in Iran wanted to refine the description of the entities on the two-dimensional holographic screen that generates gravity through rising disorder. In thermodynamics, it is known that entropy doesn’t always scale in exact proportion to the energy of the microscopic components in a system. This year, applying suitable corrections to Verlinde’s entropy equations, Rezazadeh found that entropic gravity over the largest scales in the universe ought to result in an accelerated expansion of space-time – an observed phenomenon cosmologists have been at a loss to explain for nearly three decades, referring to it as some vague dark energy.

Amazingly, Rezazadeh’s approach matches the observations that signify dark energy better than our leading description of the universe – the standard cosmological model – does. Once again, an entropic view of gravity has suggested that another great mystery of physics could be nothing more than a mirage. It is a great result, but “we need to wait for more precise observational data to be able to comment more definitively on its acceptability”, says Rezazadeh.

There remains the greater question of what entropic gravity really means. No one can confidently identify the disorderly microscopic entities that supposedly produce our sensation of gravity, nor explain whether their world – their spaceless, two-dimensional screen – is truly more real than ours. But for some physicists, the question isn’t so pressing. If one adopts the view that everything ultimately consists of information, then debates over what that information belongs to are more or less irrelevant. Verlinde himself believes this is fitting for our era. We use the language of information now, he says, “because that’s the technological age we live in”.

That might seem an oddly sociological view for a theoretical physicist, but perhaps it was ever thus. Carney says that Einstein was obsessed by relativity partly because of a problem widespread in his day: that of synchronising train times between distant cities. “We’re all trying to find answers in the language of the world we find ourselves in,” he says.