One reason physics is so exciting is that whenever you think we know a fair bit about the universe, you can find a new phenomenon where no one has a clue about what’s going on. Some of these you don’t even hear about much, because there are only so many ways journalists can cover the “scientists still scratching their heads” angle of a story. That's is a shame because enduring mysteries can be the most fascinating.
One of the mysteries that fascinates me is a phenomenon called sonoluminesence. Imagine you take a vial of liquid and run sound waves through it intense enough to create a bubble, a process called cavitation. Since the newly formed bubble is a very low-pressure area in the middle of the higher pressure of the liquid, it will quickly collapse. As the bubble bursts, crazy, inexplicable stuff happens. First, the interior of the bubble gets insanely hot. Laboratories have measured the center of the bubble at thousands of degrees, with recorded temperatures going as high as 20,000° Celsius. (To compare, the surface of the sun is only 6,000 degrees Celsius.) At the same time, you can detect a flash of light in the center of the bubble—usually bluish in color—lasting for just a few trillionths of a second. This flash of heat and light is sonoluminesence.
Here's what it looks like in a video by the UCLA Putterman Research Group. Wait for the flash.
Physicists have been observing sonoluminesence for over 80 years now, in various liquids from water to sulfuric acid. You can even find instructions online on how to do it. But what actually causes sonoluminesence? No one knows. I find this incredible—we live in an era where we can explain the first moments of the universe and how matter is formed, but can’t explain why you get superheated light flashes when you run sound through a water bottle.
This isn’t to imply scientists haven’t been working the problem, of course. Laboratories are now capable of keeping a single bubble stable for some time in fluid in order to study it, and know that adding small amounts of noble gases to the mixture, such as neon, argon, and xenon, will make the flash of light particularly bright. And finally, we know sonoluminesence exists in nature—the fascinating mantis shrimp hunts by snapping its forelimbs so quickly during hunting that it creates cavitation bubbles. The sonoluminesence that follows sends out shock waves that can kill prey.
There are various theories of so-called bubble dynamics aiming to explain sonoluminesence. These all rely on various assumptions, namely that the bubble is spherical and uniform in most respects, and the key to sonoluminesence relies on the fluid dynamics around the bubble. Various thermal and electrical processes have been proposed and tested, but not proven. There are reasons to understand this phenomenon beyond mere curiosity. For example, could sonoluminescent bubbles be made even hotter, to achieve the millions of degrees needed to start a of nuclear fusion reaction? Right now, no one really knows what the limitations are.
It’s a fascinating topic, and one that will almost certainly merit a Nobel Prize when the solution is found.
Until then, we're stuck simply wondering.