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Is Spacetime Like a Very Slippery Superfluid? Posted by Guy Pirro on 4/29/2014 9:07 AM

Stefano Liberati, professor at the International School for Advanced Studies (SISSA) in Trieste, Italy and Luca Maccione, a research scientist at the Ludwig Maximilian University in Munich, Germany have devised innovative ways of using the tools of elementary particle physics and high energy astrophysics to describe the effects that should be observed if spacetime was a fluid. (Image Credit: Departement de Mecanique, Ecole Polytechnique)

What if spacetime was a kind of fluid? This is the question tackled by theoretical physicists working on quantum gravity by creating models attempting to reconcile gravity and quantum mechanics.

If spacetime was a fluid, it would have very low viscosity, just like a "superfluid." A study carried out jointly by the International School for Advanced Studies (SISSA) in Trieste, Italy and the Ludwig Maximilian University in Munich, Germany shows how the "atoms" making up the fluid of spacetime should behave, according to models of quantum gravity. The considerations suggested in this study impose very strong constraints on the occurrence of effects related to this possible "fluid" nature of spacetime, showing that it is possible to discriminate between the quantum gravity models developed so far in order to go beyond Einstein's general relativity mechanics.

Some of these models predict that spacetime at the Planck scale (10 ^-33 cm) 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.

In this theoretical 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 behavior of fluids at a macroscopic level but tells us nothing about the atoms and 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 was 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.

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, have devised innovative ways of using the tools of elementary particle physics and high energy astrophysics to describe the effects that should be observed if spacetime was a fluid. Liberati and Maccione also proposed the first observational tests of these phenomena.

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, a theory developed in the realm of classical physics.

Electromagnetism - The electromagnetic force causes electric and magnetic effects such as the repulsion between like electrical charges or the interaction of bar magnets. It is long-ranged, but much weaker than the strong force. It can be attractive or repulsive, and acts only between pieces of matter carrying electrical charge.

Weak Interaction - The weak force is responsible for radioactive decay and neutrino interactions. It has a very short range and, as its name indicates, it is very weak.

Strong Interaction - The strong interaction is very strong, but very short-ranged. It acts only over ranges of order 10 ^-13 centimeters and is responsible for holding the nuclei of atoms together. It is basically attractive, but can be effectively repulsive in some circumstances.

Gravity - The gravitational force is weak, but very long ranged. Furthermore, it is always attractive, and acts between any two pieces of matter in the Universe since mass is its source.

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Identifying a plausible model of quantum gravity (that is, a description of gravity within a quantum physics framework) is one of the major challenges physics is facing today.

However, despite the many models proposed to date, none has proved satisfactory or, more importantly, amenable to empirical investigation. Studies like the one carried out by Liberati and Maccione provide new instruments for assessing the value of possible scenarios for quantum gravity.

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 catalogued 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," he continues. "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," concludes Liberati. "With modern astrophysics technology, the time has come to bring quantum gravity from a merely speculative viewpoint to a more phenomenological one. One cannot imagine a more exciting time to be working on gravity."