What is gravity?
For you and me, the question is relatively straightforward—it’s what keeps our feet on the ground, computers on our laps, and water in our glass. But for physicists, it’s not that simple.
Gravity forms the foundation of general relativity, the theory that much of modern physics is built on. It warps spacetime and allows galaxies, stars, and planets to form. It’s helped us make sense of the universe, but it’s also hard to reconcile with quantum mechanics, the leading theory that describes most of what general relativity doesn’t—the really small stuff.
Physicists have been searching for years for ways to get gravity to agree with quantum mechanics. That search has produced string theory, causal dynamical triangulation, and others which seek to break gravity down into its component parts. Now, Stefano Liberati of the International School for Advanced Studies and Luca Maccione of Ludwig Maximilian University think they have a better approach.
Spacetime, they say, can be understood as a liquid. A superfluid, really, composed of fundamental objects we may not have discovered yet. Spacetime’s properties then emerge, like water, which has emergent properties like fluidity and cohesion when H2O molecules are grouped together at the right temperature and pressure.
Clara Moskowitz, reporting for Scientific American:
In this analogy particles would travel through spacetime like waves in an ocean, and the laws of fluid mechanics—condensed matter physics—would apply. Previously physicists considered how particles of different energies would disperse in spacetime, just as waves of different wavelengths disperse, or travel at different speeds, in water. In the latest study Liberati and Maccione took into account another fluid effect: dissipation. As waves travel through a medium, they lose energy over time. This dampening effect would also happen to photons traveling through spacetime, the researchers found. Although the effect is small, high-energy photons traveling very long distances should lose a noticeable amount of energy, the researchers say.
Liberati and Maccione have their eyes and instruments trained on the Crab Nebula, a source of high-energy X-rays and gamma rays. They’re hoping that changes in that radiation as it travels to Earth will help test their superfluid theory.
Currently, the superfluid theory doesn’t seem to have many supporters. But then again, neither did Einstein at first. If Liberati and Maccione are proven correct, they could finally unite the two theories of physics into one.