(Inside Science) -- Spacetime is a somewhat slippery concept -- Einstein described the universe in four dimensions, combining the well-known three dimensions of space with time. Physicists now suggest that spacetime may itself be a fluid, a very slippery type known as a superfluid.
These new findings could help scientists in their quest for a theory of everything that explains how the cosmos works in its entirety.
Scientists have long sought to develop a theory that can describe every aspect of how the universe operates. Currently, researchers have two disparate theories: quantum mechanics and general relativity. Respectively, these two theories can mostly explain the cosmos on its tiniest scales and its largest scales. Quantum mechanics can explain the behavior of all the known particles, while general relativity describes the nature of spacetime and gravity.
When it comes to "quantum gravity" theories that seek to reconcile quantum mechanics and general relativity, there are currently two main scenarios. One suggests the force of gravity can be described in terms of packets of energy known as gravitons, just as light is embodied by photons. The other suggests the fundamental constituents of spacetime essentially condense together like a fluid. The properties of gravity would emerge from the overall behavior of this fluid, rather than its individual parts, just like the flow of water is explained by fluid equations and not the properties of the individual molecules that make it up.
This analogy is not supposed to suggest that spacetime flows anywhere, but is meant to help envision the fabric of spacetime as emerging from more basic entities, said theoretical physicist Luca Maccione at the Ludwig-Maximilian University in Munich, Germany. These fundamental constituents of spacetime would be below the size at which space and time is smooth and continuous — a Planck length, or roughly 100 billion billion times smaller than the width of a proton.
Now researchers suggest that if spacetime is a fluid, it must be an extraordinary kind of fluid known as a superfluid. These findings could help test models of quantum gravity.
A superfluid is a fluid that flows with virtually zero friction or viscosity. In comparison, water might seem as slow as molasses. Liquid helium can behave like a superfluid when cooled to temperatures just a few degrees above absolute zero, the coldest possible temperature.
Scientists have looked for paradoxical or unlikely predictions in models that treat spacetime as a fluid in order to support or disprove these models. For instance, past research suggested that photons might travel at different speeds depending on their energy if spacetime is a fluid.
If spacetime is a fluid, it might have viscosity. This means it could impede anything traveling within it, reasoned Maccione and his colleague Stefano Liberati, a theoretical physicist at the International School for Advanced Studies in Trieste, Italy.
The researchers found that if spacetime was a viscous fluid, it would rapidly dissipate the energy of photons and other particles along their paths. Since astronomers can see photons traveling from stars and galaxies located billions of light years away, Liberati and Maccione's calculations revealed that if spacetime is a fluid, it must be a superfluid.
"This type of general knowledge about what properties spacetime and gravity can fundamentally have is very important to guide the development of the theory of quantum gravity," said theoretical physicist Sabine Hossenfelder at the Nordic Institute for Theoretical Physics in Stockholm, Sweden, who did not contribute to this paper.
The scientists do not rule out models where spacetime has some miniscule level of viscosity to it. They suggest analyzing gamma rays and high-energy neutrinos from deep space, and if any of their energy dissipated, that could reveal that spacetime is a fluid, greatly backing specific models of quantum gravity.
"This model provides a significant new probe of the nature of spacetime and possible quantum gravity theories by using high-energy astrophysical observations," said theoretical astrophysicist Floyd Stecker at NASA's Goddard Space Flight Center, in Greenbelt, Maryland, who did not participate in this study.
The fact that quantum gravity models often depend on physics at the remote, infinitesimal levels of the Planck scale has greatly discouraged researchers in the field. By introducing possible visible dissipative effects of spacetime, "effects analogous to the 'oomph' we exert in stirring honey that originate at the Planck scale, this result opens up a new observational window of possible quantum gravity effects," said theoretical physicist Seth Major at Hamilton College in Clinton, New York, who did not take part in this research.
Liberati and Maccione detailed their findings online April 14 in the journal Physical Review Letters.