South of the German city of Koblenz, the river Rhine narrows over a stretch of 30 miles, forcing its already strong current to increase. Speckled with underwater rocks, this route was once a risky cruise. It is the subject of legends and folk tales. It plays a prominent role in Wagner’s operas. It’s also a black hole.
If your boat is upstream of the river’s high-speed section and doesn’t have an engine powerful enough, the passage where the river narrows and accelerates acts like an event horizon: once you cross it, there’s no return. No matter what you actions you take, you will inevitably be sucked downstream with the flow.
This analogy between gravity and fluids with varying speed is much more than a simple metaphor; it can be made mathematically precise. To derive a relation between gravity and fluids, physicists don’t study boats – which could move with arbitrary speed – but waves, whose speed depends only on the properties of the fluid itself. If the fluid’s speed exceeds the wave’s speed, then waves cannot travel upstream. It’s like being onboard one supersonic plane leading another: you can’t hear the engine noise of the second. Only for black holes, it’s the light that can’t escape, rather than the sound.
This analogy doesn’t only work for surface waves, but also for sound waves in flowing gases. If you push gas through a narrow channel, thereby increasing its speed so much that it exceeds the speed of sound, you create an acoustic horizon. No sound can cross the acoustic horizon because the gas is flowing too fast.
Sound-traps of this type have been coined “dumb holes” by Bill Unruh, who pioneered the idea that gravity can be mimicked by fluids in the mid 1980s. Since then, this field of “analogue gravity” has flourished. Physicists have found many other systems where waves travel like in strong gravitational fields, and they devised ways to simulate not only black holes, but also rapidly expanding spaces like that of the early universe. And all this can now be done in the laboratory just by observing how perturbations travel in fluids or gases.