by In 2009, theoretical physicist Erik Verlinde proposed a radical reformulation of gravity. In his theory, gravity is not a basic force but an expression of deeper hidden processes. But in the past 15 years, little experimental support has been given to the idea. So where do we go next?
Emergence is common throughout physics. Temperature, for example, is not an intrinsic property of gases. Instead, it is the result of numerous microscopic collisions. We have the tools to match those microscopic collisions with temperature; in fact, there is an entire branch of physics, called statistical mechanics, that informs these connections.
In other areas, the links between microscopic behavior and emergent properties are less clear. For example, while we understand the simple mechanisms behind superconductivity, we do not know how high-temperature superconductors arise as a result of microscopic interactions.
Related: Why Einstein must be wrong: The search for a theory of gravity
Verlinde’s theory is based on what Stephen Hawk and Jacob Bekenstein observed in the 1970s: Many properties of black holes be expressed in terms of the laws of thermodynamics. However, the laws of thermodynamics themselves are emerging from microscopic processes. For Verlinde, this was just a coincidence and suggested that what we see as gravity may be arising from some deeper physical process.
In 2009, he published the the first version of his theory. Crucially, we don’t need to know what those deeper processes are, since we already have the toolkit — statistical mechanics — to describe emerging properties. So Verlinde applied these techniques to gravity and came up with an alternative formulation of gravity. And since gravity is also connected to our concepts of motion, inertia, space and time, this means our entire concepts. globe also emerging from the same deeper processes.
At first, not much came of this; While rewriting a known law of physics is interesting, it does not necessarily provide deeper insights. But in 2016, Verlinde expanded his theory discovering that there is a universe in which dark energy naturally as a result of a new emergent property of space, allowing it to push itself into low-density regions.
There was a lot of excitement at this discovery, because it gave another explanation dark dark. Currently, astronomers believe that dark matter is a mysterious, invisible substance that makes up most of the mass of all galaxy. Although that hypothesis has been able to explain an enormous wealth of observations, from the rotation rates of stars within galaxies to the evolution of the largest structures in the cosmos, we have yet to identify the mysterious particle.
In Verlinde’s picture of emergent gravity, once you get into low-density regions—basically, anything outside the solar system—gravity behaves differently than Einstein’s theory of general relativity would expect. On large scales, there is a natural pull in to space itself, forcing matter to be denser than it would otherwise be.
This idea was exciting because it gave astronomers a way to test this new theory. Observers could take this new theory of gravity and fit it into models of galaxy structure and evolution to find differences between it and models of dark matter.
Over the years, however, the experimental results have been mixed. Some early tests favored emergent gravity over dark matter when it came to the rotation rates of stars. But later observations found no advantage. And dark matter can also explain much more than galaxy rotation rates; tests within galaxy clusters have found that emergent gravity is short-lived.
Related stories:
—Is the origin of gravity dark matter itself?
—Why is gravity so weak? The nature of space-time may be the answer
—’Quantum gravity’ could finally help unify quantum mechanics with general relativity
This is not the end of emerging gravity. It’s still a new idea and requires a lot of assumptions in its calculations to make it work. Without a fully realized theory, it’s hard to say whether the predictions it makes about the behavior of galaxies and clusters accurately reflect what emergent gravity has told us. And astronomers are still trying to develop more rigorous tests, like using data from the cosmic microwave backgroundreally put the theory through its paces.
Emerging gravity is an interesting idea. If correct, we would have to radically reshape our understanding of the natural world and view gravity and motion — and even more fundamental concepts, such as time and space — through a lens that emerges from deeper and more complex interactions. . But for now, it’s just an interesting thought. Only extensive observational testing will tell us if we are on the right track.
