Pick Peat!

That’s a heck of a word. “Sono” means sound, and “luminescence” means emitting light without the application of heat.   And, as it turns out, sonoluminescence happens under water.  Sound, light, water, what?

If you’re confused, you’re in good company: this phenomenon had scientists baffled until the late 1980s.

Simply put, sonoluminescence is a fancy term for how loud sounds can cause tiny flashes of light to appear in water.  The basic mechanism is that an intense sound wave can crush tiny bubbles in water, and when those bubbles get crushed, they (sometimes) produce a flash.

What’s fun about sonoluminescence is how extreme the conditions are:

• The light flashes are measured in trillionths of a second.  That’s millionths of millionths of a second — so small it’s almost impossible to put in understandable terms.  For example, you could get six billion flashes in the time it takes you to blink your eye.

• While the bubble is being crushed, the walls are moving fast enough to create a supersonic shockwave inside of the bubble.

• By the time it starts emiting light, the bubble is 1/100th the width of a human hair, and the gas in the bubble has reached a temperature of around 20,000 degrees Kelvin.   That’s three times hotter than the surface of the sun.

Although the density of the energy in the center of a sonoluminescing bubble is incredibly high, it’s so small in size that the only thing you’ll experience is a very, very brief flash of light.

In fact, the flashes of light are relatively difficult for us to see with the naked eye, happen only in tiny bubbles (which are difficult to track), and so short that they’re practically impossible to observe and analyze.

That’s why it took almost fifty years to figure out what was really happening.

The trick was trapping a single bubble in a “standing” sound wave.

A sound wave has “peaks” of high pressure and “valleys” of low pressure.   You can hear because the pressure changes in a sound wave cause microscopic hairs in your ears to vibrate, which tells your brain that you’re hearing something.

A standing sound wave is just a sound wave that stays in a constant position, where the pressure rises and falls without pushing in any particular direction.   This is a great environment for studying sonoluminescence, because a bubble in a standing wave stays in one spot and undergoes sonoluminescence with each pressure peak: you know where it is, and you can watch it flash as often as you like to gather information about what’s happening.

Cool.

So, where does the light come from?

Due to the heat in the bubble, some of atoms “ionize” and loose an electron.   That electron shoots off at tremendous speed, and if it whacks into another ionized atom, the interaction gives off energy — in this case, light.

After figuring out the standing wave trick, scientists have been able to tune their bubbles with different gasses and sizes to alter what kind of light is produced.

So, that’s what’s up with sonoluminescence.

If you’d like to read the account of a guy who’s actually produced and measured sonoluminescence, check out this page.

I hope he forgives me for including this photo:

It’s amazing to think that the little white dot is three times hotter than the surface of the sun.