

Thinking of space and time as a liquid might help reconcile quantum mechanics and relativity
Crab Nebula

Light from the Crab Nebula (shown here in a Hubble Space Telescope photo) limits the possibilities for fluid spacetime.

If spacetime is like a liquid—a concept some physicists say could help resolve a confounding disagreement between two dominant theories in physics—it must be a very special liquid indeed. A recent study compared astrophysical observations with predictions based on the notion of fluid spacetime, and found the idea only works if spacetime is incredibly smooth and freely flowing—in other words, a superfluid.
Thinking of spacetime as a liquid may be a helpful analogy. We often picture space and time as fundamental backdrops to the universe. But what if they are not fundamental, and built instead of smaller ingredients that exist on a deeper layer of reality that we cannot sense? If that were the case, spacetime’s properties would “emerge” from the underlying physics of its constituents, just as water’s properties emerge from the particles that comprise it. “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.”
Physicists have been considering this possibility since the 1990s in an attempt to reconcile the dominant theory of gravity on a large scale—general relativity—with the theory governing the very smallest bits of the universe—quantum mechanics. Both theories appear to work perfectly within their respective domains, but conflict with one another in situations that combine the large and small, such as black holes (extremely large mass, extremely small volume). Many physicists have tried to solve the problem by “quantizing” gravity—dividing it into smaller bits, just as quantum mechanics breaks down many quantities, such as particles’ energy levels, into discrete packets. “There are many attempts to quantize gravity—string theory and loop quantum gravity are alternative approaches that can both claim to have gone a good leg forward,” says Stefano Liberati, a physicist at the International School for Advanced Studies (SISSA) in Trieste, Italy. “But maybe you don’t need to quantize gravity; you need to quantize this fundamental object that makes spacetime.”
Liberati, along with his colleague Luca Maccione of Ludwig Maximilian University in Munich, recently explored how that idea would affect light traveling through the universe. An emergent spacetime, one that acted like a fluid, would not be immediately distinguishable from the spacetime of any other theory. But in extreme situations, such as for very energetic light particles, Liberati and Maccione found that some differences would be noticeable. In fact, by examining observations of high-energy photons flying across the universe from the Crab Nebula, the physicists were able to rule out certain versions of emergent spacetime, finding that if it is a fluid at all, it must be a superfluid. The researchers published their results in the April 14 Physical Review Letters.
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.
One real-world example is the Crab Nebula, a supernova remnant about 6,500 light-years from Earth that emits high-energy x-ray and gamma-ray light. By the time this light reaches our telescopes, its energy should have dissipated somewhat if spacetime has liquid properties. Observations of the Crab Nebula, however, show no sign of such an effect. “We show the spectrum would be severely affected by this energy loss, even if it’s a very tiny effect, because it travels for so long,” Liberati says. The lack of a dissipation signal allowed the researchers to put strong constraints on the liquid effects that could be present in spacetime, showing they must be extremely small if they are present at all. “This is not telling you that this idea is completely ruled out,” Liberati says. The findings do, however, narrow the possibilities for liquidlike spacetime to only liquids with very low viscosities that cause almost no dampening—superfluids.
Even supporters of the fluid spacetime idea say the concept is not very popular, and perhaps unlikely. But might it be true? “I have absolutely no idea,” says Renaud Parentani, a physicist at the University of Paris–Sud who originally suggested the idea of considering dissipation effects. “My frank opinion is that nobody has any idea. All we can do is model the various possibilities.”
If it is true that spacetime is a superfluid and that photons of different energies travel at different speeds or dissipate over time, that means relativity does not hold in all situations. One of the main tenets of relativity, the Lorentz invariance, states that the speed of light is unchanging, regardless of an observer’s frame of reference. “The possibility that spacetime as we know it emerges from something that violates relativity is a fairly radical one,” Jacobson says. It does, however, clear a potential pathway toward rectifying some of the problems that arise when trying to combine relativity and quantum mechanics. “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.”
So if spacetime is a superfluid, then it’s surf’s up for theoretical physicists.

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