Ages 8–12 · about 6 minutes
Is Space a Kind of Sea?
What if everything is a ripple on something we cannot see?
Do this first · Try this at home
Draw a sound horizon
Start with your hands. The reading makes far more sense afterwards.
You need: Paper and a pencil
0 of 3 steps done
What should I notice?
Notice: At flow 2, upstream sound stays in place in this simple model. Beyond it, that sound is carried right. These are invented speed units for a paper model of long-wavelength sound, not measurements or instructions for a water experiment.
Now read on to find out why it happens.
The big idea
Waves in a material can model parts of other physics. Matching one behavior does not make the two systems identical.
- wave
- A disturbance that travels and carries energy.
- excitation
- A disturbance above a system's lowest-energy state. In a quantum liquid, some excitations behave like particles.
- superfluid
- A quantum state of a fluid with a component that can flow without ordinary viscous resistance, within its operating limits.
- sound horizon
- A boundary beyond which the relevant sound waves cannot travel back upstream in a flowing-fluid model.
A wave carries energy across water. The water moves locally, often in loops; it does not only bob straight up and down. The traveling pattern and the movement of the material are different things.
Tiny disturbances
In a quantum liquid, a small disturbance is an excitation. A phonon is one quantum of a sound wave. Scientists can calculate how these excitations travel and interact, rather like particles within a larger system.
Very cold helium can become superfluid. Its superfluid component can flow without ordinary viscous resistance. Temperature and motion still matter: excitations and tangled vortices can cause drag. A superfluid is not a promise that any swirl keeps going forever.
When sound cannot move upstream
Imagine fluid flowing right while sound travels left relative to the fluid. In our paper model, sound moves 2 units left. A flow of 1 unit right leaves it moving 1 unit left overall. A flow of 3 units right carries it 1 unit right overall.
Where a smooth flow reaches the sound speed, upstream sound stops moving along the channel. Beyond that sound horizon, the flow carries these long-wavelength waves downstream. In 1981, William Unruh showed how a fluid model could reproduce parts of black-hole wave physics. The trapped waves are sound, not light; the analogy has limits at short wavelengths.
Jeff Steinhauer's 2016 paper reported correlations between sound excitations on opposite sides of a horizon in an ultracold atomic condensate. Those correlations were evidence for analogue Hawking radiation. The measured signal was correlated sound excitations, not a glow of escaping light.
What the model can tell us
Grigory Volovik studies how some low-energy excitations in superfluid helium resemble particles described by relativistic equations. A known material can then model selected physics. This does not establish that space is helium or that every particle is a water ripple.
Volovik also uses quantum-liquid thermodynamics to explore why vacuum energy could be small. Turning that proposal into a complete account of our expanding universe still requires more theory and measurements. A useful model helps us ask a precise question; it does not automatically supply the answer.
Sources: Unruh, 10.1103/PhysRevLett.46.1351; Steinhauer, 10.1038/nphys3863; Volovik, 10.1063/1.2354594; Hänninen and Schoepe, 10.1007/s10909-008-9834-2; NOAA, Why does the ocean have waves?.