If Spacetime Is a Superfluid, It Could Explain Everything
Thinking of spacetime as a liquid could be the holy grail of theoretical physics: a theory of everything.
Several different theories (string theory, M-theory, etc.) have been hailed as a potential "theory of everything," or a theory that unifies the seemingly disparate prevailing theories of physics: quantum mechanics and general relativity. A recent study found that liquid spacetime could bridge gaps between these two theories, but only if it is a superfluid, or a liquid that flows without friction.
Quantum mechanics and general relativity are theories of physics that both consistently work in their respective domains (quantum mechanics for subatomic particles and general relativity for larger objects), but are not reconcilable in certain extreme physical circumstances, such as black holes, which combine large mass with infinitesimally small volume. Other theories, such as string theory, have attempted to solve this problem by quantizing gravity, or dividing gravity into its component parts. The superfluid theory, by contrast, attempts to quantize spacetime.
Spacetime as a fluid would solve many problems regarding the unification of quantum mechanics and general relativity as a result of liquids' emergent properties. "Water is made of discrete, individual molecules, which interact with each other according to the laws of quantum mechanics, but liquid water appears continuous and flowing and transparent and refracting," explains Ted Jacobson, a physicist at the University of Maryland, College Park. "These are all 'emergent' properties that cannot be found in the individual molecules, even though they ultimately derive from the properties of those molecules." If spacetime were comparable to a liquid, then it would also have emergent properties that are not observable in its constituent parts, but are present in spacetime itself. These emergent properties would explain the gaps between the two dominant theories, such as the behavior of black holes.
While the liquid theory has been circulating the theoretical physics community for quite some time, this recent study found that, if spacetime is a liquid, it necessarily must be a superfluid. If spacetime does behave as a liquid, then fluid mechanics must apply to its behavior in all circumstances. Luca Maccione of Ludwig Maximilian University and Stefano Liberati, a physicist at the International School for Advanced Studies, tested whether spacetime shared the property of dissipation with liquids. As waves travel through a medium, they gradually lose energy. So if spacetime is a fluid, then photons should lose energy while traveling through it, as they would a liquid. But they observed that the photons emitted from the Crab Nebula (below) did not experience any such dissipation. "This is not telling you that this idea is completely ruled out," said Liberati. But it does mean that spacetime could only be a liquid that has extremely low viscosity, or a superfluid, so the effects of the dissipation would be miniscule and difficult to observe.
In addition to its behavior surrounding and inside black holes, superfluidity is also consistent with other properties of spacetime; for example, when a superfluid is stirred, it creates many vortices in which the superfluidity breaks down, but everywhere else the fluid retains its superfluid properties. Similarly, spacetime breaks down in limited spaces near rotating cosmic strings.
That being said, even proponents of the idea admit that the conception of spacetime as a superfluid is not especially popular. But that doesn't necessarily mean that it is incorrect. When asked if the theory could be true, Renaud Parentani, a physicist at the University of Paris–Sud who originally suggested studying dissipation effects said, "I have absolutely no idea. My frank opinion is that nobody has any idea. All we can do is model the various possibilities."
One of the major objections to the superfluidity theory contends that it is inconsistent with general relativity in certain situations. If photons dissipate over time, then the speed of light is not constant regardless of frame of reference, which violates one of the basic tenets of general relativity. Although even supporters of the theory admit that disposing of general relativity is extremely radical, the superfluidity theory could then render some of the problems associated with general relativity moot. Jacobsen says, "Violating relativity would open up the possibility of eliminating infinite quantities that arise in present theory and which seem to some unlikely to be physically correct."