Emergent gravity may be a dead idea, but it’s not a bad one

Gravity may not emerge, but some interesting ideas did.

Emergent gravity is a bold idea.

It claims that the force of gravity is a mere illusion, more akin to friction or heat—a property that emerges from some deeper physical interaction. This emergent gravity idea might hold the key to rewriting one of the fundamental forces of nature—and it could explain the mysterious nature of dark matter.

But in the years since its original proposal, it has not held up well to either experiment or further theoretical inquiry. Emergent gravity may not be a right answer. But it is a clever one, and it's still worth considering, as it may hold the seeds of a greater understanding.

Emergency situation

To understand what emergent gravity means, we first have to clarify what gravity is supposed to be emerging from and what the word “emergent” even means.

Emergence is an old concept that appears and reappears in many contexts, from physics to philosophy to art. In physics especially, emergence refers to a clear but slightly uncomfortable fact: Despite our deep understanding of the innermost workings of nature at the subatomic scale, we often can’t use that knowledge to describe most of the systems we actually care about.

One way to view nature is as a vast hierarchy. At the “bottom” of the hierarchy are the quantum fields, which we use quantum field theory to understand. On top of that are all the myriad subatomic and atomic interactions, also governed by quantum mechanics. Above that is chemistry, where the quantum starts mattering less. And on top of all that, far, far removed from quantum fields, are all the wonderful branches of science and their various tools that describe all manner of phenomena: astrophysics, oceanography, geology, sociology, and so on.

It’s technically true that “underneath” everything—say the circulation of a great ocean gyre or the formation of a newborn star or the panic that sets in when you come across a bear in the woods—is quantum field theory. But good luck using quantum field theory to describe those systems. That’s because these higher-order systems are emergent—they have new properties, new laws, and new behaviors that emerge from countless interactions operating at deeper levels.

Most of that time, we can't ever hope to make connections between lower and higher layers, and some physicists and philosophers argue that it might just be impossible to do so consistently. But in some cases, we can tie together low-level principles with higher-order emergent behavior. The best example of this is the relationship between thermodynamics and statistical physics.

Thermodynamics is the study of familiar everyday properties of systems like temperature, pressure, volume, entropy, and all their friends. We have examples of relationships between these properties, like the ideal gas law. Amazingly, you can derive, test, and use the ideal gas law while having no idea what a “gas” is made of (atoms and molecules) and what those components of the gas are doing (bouncing around a lot). So we don’t need a connection between those properties and any underlying rules.

But in an amazing feat of 19th-century physics that doesn’t get nearly enough airtime, we made exactly that connection. Through a set of techniques known as statistical mechanics, we can take the behaviors of individual gas molecules—their kinetic energy and momentum, for example—and use those to derive the emergent properties of temperature, pressure, and entropy of a gas that consists of a whole bunch of molecules working together.

Here, temperature is emergent. An individual molecule doesn’t have a temperature. It only has momentum and kinetic energy. But properties like temperature and relationships like the ideal gas law emerge out of all these fundamental properties.

From the abyss, gravity emerges

So what does gravity have to do with any of this?

The roots of the idea that gravity might be emergent go all the way back to the funky '70s, when physicists like the famed Stephen Hawking and the should-be-more-famed Jacob Bekenstein discovered that black holes aren’t entirely, totally, 100 percent black. Instead, they give off a minute amount of radiation. Specifically, this radiation is thermal, meaning that the radiation has the same properties as any other generic hot thing in the Universe. This means the rules of thermodynamics and statistical mechanics should apply to black holes.

But black holes are objects of pure gravity. They are a prediction of Einstein’s general theory of relativity. They are places where gravity becomes so extreme that nothing can escape. They are punctures in space itself. They are most definitely not like any other generic hot thing in the Universe.

So perhaps nature is giving us a clue. Maybe there is a deeper connection between the laws of thermodynamics and the laws of gravity. And it could be that if thermodynamics is really an emergent property of some deeper set of physics, then maybe (just maybe, but let’s roll with this and see where it goes) the fact that black holes look kind of like warm glowing objects is telling us that gravity is also an emergent property of some deeper physics.

The nature of that “deeper physics” was anybody’s guess. In 2009, Dutch physicist Erik Verlinde guessed that the deeper physics might be some quantum information encoded on the surface of the Universe. This isn’t just some random idea plucked out of the Hat of Magical Physics; it's grounded in the very real observation that a black hole’s surface is much more important than its volume.

Specifically, when information—which, for our purposes, we can take to be the total description of every property of matter and radiation—enters a black hole (in the form of infalling matter or radiation or your worst enemy), the surface of a black hole grows proportionally. To be clear, so does the volume, but not proportionally to the amount of information.

This simple relationship unlocked an entire line of research centered on holography, the idea that the physics of our Universe, especially that of gravity, might actually take place on the surface of the cosmos. Indeed, the surface might be all there is, with the four dimensions of spacetime manifesting from various quantum interactions happening on that boundary.

Verlinde combined this holographic approach with the concept of black hole thermodynamics to rewrite Newton’s laws, and eventually general relativity, in terms of statistical relationships that give rise to gravity, making it an emergent force.

Now, as my email inbox can attest, any random crank can reassemble the laws of gravity by substituting variables in the equations, and by that metric, Verlinde’s work wasn’t all that impressive. But it’s a completely different ball game to take your own ideas seriously and develop a complete theory of physics, one that matches up with known observations and is capable of saying something new and surprising about the Universe.

In Verlinde’s formulation, spacetime itself has thermodynamic properties like entropy. Normally, the entropy of spacetime is completely swamped by everything occupying spacetime. It’s only when you go far away from everything that the densities of matter drop enough that the entropic nature of spacetime, and hence of gravity, starts to become apparent. And in those conditions, Verlinde discovered a slight deviation from the predictions of normal gravity. Specifically, both Newton and Einstein predict that the strength of gravity diminished inversely proportional to the distance squared. Verlinde’s emergent gravity suggests that, in extremely low-density environments, gravity is merely inversely proportional to the distance.

That’s interesting. That’s something we can test.

Several years after his original formulation, Verlinde discovered that if the Universe is filled with dark energy (which all observations suggest it is), then this leaves behind a “residue” in the structure of spacetime that adds an additional attractive component to gravity. Most interestingly, in high-density environments like the Solar System, this residue goes away, meaning we won’t be able to detect it based on local observations. As before, only in low-density environments, like in interstellar space, does this additional attractive component become apparent. Since a wealth of observations suggest there are some extra gravitational interactions happening at galactic scales and above, this started to look like an enticing solution for the problem of dark matter.

If it’s interesting, it’s probably wrong

Ideas can be beautiful, elegant, captivating… and dead wrong. Nature is the ultimate arbiter of ideas in physics, so it’s always up to the evidence to determine which theories we embrace and which we discard.

Our currently accepted paradigm for explaining the large-scale behavior of the Universe is rooted in general relativity, which has been put through its paces with over a century of successful experimental tests. Yet mysteries abound in the Universe, such as the apparent need for matter that’s invisible to us, known as dark matter, to explain the behaviors of stars and galaxies.

Can emergent gravity do any better?

While the question is simple, answering it is not easy. It’s one thing to devise a new conception of gravity. It’s entirely another to calculate accurate predictions for complex situations such as the motions of stars in a galaxy. So accepting the predictive power of emergent gravity means (a) trusting the theoretical calculations that lead to a prediction and (b) devising strong observational tests of those calculations.

The results of these endeavors have been decidedly mixed. The first examples of dark matter were described in the rotation rates of stars within galaxies and the motion of galaxies within galaxy clusters—stars orbit far too quickly, and the galaxies have far too high velocities given the amount of visible matter that is available to keep everything together. Initial tests of emergent gravity found that it could explain galaxy rotation curves but didn’t do any better than the usual dark matter approach. In other tests, it just outright didn’t work. Further tests within galaxy cluster environments found that emergent gravity got the matter density wrong by up to a factor of six.

In defense of emergent gravity, however, those tests used predictions from the theory that are, at best, approximations of what gravity “actually” does. It could be that a more accurate, robust mathematical description would yield better predictions that would end up agreeing with these observations.

In the end, these are pretty limited tests. There’s a lot more to the Universe that dark matter can explain, like the growth of large structures over cosmic time or the fluctuations in the appearance of the cosmic microwave background. Any theory that hopes to replace dark matter must run the full gamut, not just limit itself to galaxies and clusters. So far, nobody has attempted to approach these larger questions through the lens of emergent gravity.

On the theoretical front, emergent gravity has run into issues as well. The journey from vanilla general relativity to emergent gravity takes more than a few assumptions, leaps of faith, detours, and approximations. We don’t know if the holographic approach to physics is valid. We don’t know if the relationship between thermodynamics and black holes is nothing more than a coincidence. We don’t know the underlying quantum physics that might give rise to gravity in this picture.

In this regard, other theorists have come forward, arguing that the assumptions that underly emergent gravity (namely that surfaces obey thermodynamics) are flawed and are not compatible with general relativity. And since the whole point of emergent gravity is to make a theory that recapitulates relativity except in regions where we haven’t tested it yet, this could grind the entire emergent program to a halt.

Is emergent gravity a dead idea? Right now, it seems likely that it is. But is emergent gravity a bad idea? Absolutely not.

Ideas like emergent gravity are at—or even beyond—the cutting edge of physics. We know that we lack a complete understanding of gravity, and we know that we are surrounded by tantalizing hints of what may come next. And all things being equal, emergent gravity is a worthy concept that combines several different innovative threads into a novel view of our Universe. Some of the criticisms it faces are valid, and its lack of ability to confront observations is troubling. But these tests and criticisms are good things. Experiments only fail when we don’t learn anything new and when we don’t challenge ourselves.

Like explorers facing an unknown land, we don’t know which directions lead to lush forests and which lead to barren deserts. We can only discover new physics by taking bold plunges into the dark. Even if emergent gravity turns out to be a blind alley, it may contain the seeds that will later germinate into a more fruitful theory.

The only way to push forward in our progress to understand the Universe is to create these ideas ourselves. In other words, we can’t just sit around waiting for new physics to… emerge.

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