Quantum mechanics is able to effectively explain three of the four fundamental forces of the Universe - electromagnetism, weak interaction and strong interaction - but it does not explain gravity, which is currently only accounted for by general relativity, which is classical physics.

Identifying a plausible model of quantum gravity - a description of gravity within a quantum physics framework - is one of the major challenges physics is facing today. Despite many, many, many hypothetical models proposed to date, none has proved satisfactory or, more importantly, amenable to empirical investigation.

Because of the lack of need for empirical investigation before writing a paper, there is a lot of speculation in theoretical physics. So a group have asked, what if spacetime were a kind of fluid?

Some models attempting to reconcile gravity and quantum mechanics speculate that spacetime at the Planck scale (10-33cm) is no longer continuous – as held by classical physics – but discrete in nature. Just like the solids or fluids we come into contact with every day, which can be seen as made up of atoms and molecules when observed at sufficient resolution. A structure of this kind generally implies, at very high energies, violations of Einstein's special relativity (a integral part of general relativity).

In this hypothetical framework, it has been suggested that spacetime should be treated as a fluid. In this sense, general relativity would be the analogue to fluid hydrodynamics, which describes the behaviour of fluids at a macroscopic level but tells us nothing about the atoms/molecules that compose them.

Likewise, according to some models, general relativity says nothing about the "atoms" that make up spacetime but describes the dynamics of spacetime as if it were a "classical" object. Spacetime would therefore be a phenomenon "emerging" from more fundamental constituents, just as water is what we perceive of the mass of H2O molecules that form it.

What does it all mean? Well, not much. There are a lot of qualifiers, but Stefano Liberati, professor at the International School for Advanced Studies (SISSA) in Trieste, and Luca Maccione, a research scientist at the Ludwig-Maximilian University in Munich, created a framework using the tolls of elementary particle physics and high energy astrophysics to describe the effects that should be observed if spacetime were a fluid. Liberati and Maccione also propose observational tests of these phenomena, to take them out of the realm of speculation and into phenomenology.

**More in detail...**

In the past, models considering spacetime as emerging, like a fluid, from more fundamental entities assumed and studied effects that imply changes in the propagation of photons, which would travel at different speeds depending on their energy.

But there's more to it.

"If we follow up the analogy with fluids it doesn't make sense to expect these types of changes only" explains Liberati. "If spacetime is a kind of fluid, then we must also take into account its viscosity and other dissipative effects, which had never been considered in detail."

Liberati and Maccione cataloged these effects and showed that viscosity tends to rapidly dissipate photons and other particles along their path, "And yet we can see photons travelling from astrophysical objects located millions of light years away!" says Liberati. "If spacetime is a fluid, then according to our calculations it must necessarily be a superfluid. This means that its viscosity value is extremely low, close to zero.

"We also predicted other weaker dissipative effects, which we might be able to see with future astrophysical observations. Should this happen, we would have a strong clue to support the emergent models of spacetime. With modern astrophysics technology the time has come to bring quantum gravity from a merely speculative view point to a more phenomenological one. One cannot imagine a more exciting time to be working on gravity".

*Physical Review Letters*.

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