Thinking of space and time as a liquid might help reconcile quantum mechanics and relativity.
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 Physical Review Letters1 in April.
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.