Liquified Physics

To the disappointment of physicists everywhere, you can't fit a black hole in a lab. But you can create a similar system that operates under similar rules -- like fluid dynamics. This week's Workshop on Effective Gravity in Fluids and Superfluids at ICTP is focused on exactly that analogy: How the strange physics of flowing liquids emulate the strange physics that take place among black holes and even quantum mechanics.

Physicist William Unruh of the University of British Colombia was among those presenting this week. His studies have focused on how sound waves in fluid bodies emulate the effects of light and black holes. "What I found in 1981 was that, if you look at sound waves in a flowing fluid, you can make them look just like flowing fields on a background spacetime," said Unruh. "The velocity of the flowing fluid creates the metric, and by appropriately choosing it you can make it look exactly like a black hole. So the mathematical equations for sound waves are exactly the same as the equations for these fields travelling on the black hole metrics."

The analogies don't work perfectly. Unruh was quick to point out that the rules are not exactly the same in both models, but enough about the two systems are similar that insights are possible. One realm in which the analogy works is Stephen Hawking's well-known work on black hole radiation and evaporation.

"Step by step, you can get exactly the same kind of arguments Hawking used," said Unruh. "As a result, if you have a place where the velocity of fluid goes faster than the velocity of sound, that's a horizon because the sound can't climb out of that region."

Unruh said that analogies have become a powerful theme in physics the past few years, as similarities between fields such as general relativity and fluid dynamics can use analogies to build on each other. "There are limitations of course," he said. "But often just having this new tool allows you to ask new questions and answer it in different ways than you would otherwise."

For more details about the Workshop, visit the website.