Sonoluminescence is where, when sound waves travel through a certain liquid (usually water), the liquid cavitates, or creates bubbles of gas. In somoluminescence, the bubble will collapse violently just after creation and release light for a few picoseconds. Theories of what creates the light include static electricity and, strangely, fusion (because, theoretically, the temperature inside a collapsing bubble can reach one million degrees celsius).

Sonoluminescence is the emission of light (lumin) from bubbles in a liquid that has been excited by sound (sono). It was first discovered in 1934 at the University of Cologne but deemed rather uninteresting.

Today, the question of how the energy of sound (rather low in the energy density) is concentrated in the space of a micron to make it emit light. To reach this, it requires the concentration of the energy by a factor of about one trillion.

To produce sonoluminescence, ultrasound pulses are aimed at an air bubble in a small water cylinder. The sound waves cause the air bubble to oscillate in size wildly (known as "cavitating") and expand to a maximum size of 50 microns. At this point, there is an almost perfect vacuum within the bubble which triggers a collapse of the bubble to somewhere between 0.1 microns and 1 micron.

Each flash emits approximately 1,000,000 (one million) photons. This was determined by averaging the photon count over a large number of flashes (more than 10,000).

The flash itself comes from a bubble that is about 1 micron and is collapsing with the wall velocity greater than mach 1 (the speed of sound). Much of the light emitted by the collapse is emitted in the UV which seems to indicate that the temperature is at least 10,000 K to 25,000 K (appoximated from the light distribution from black body radiation). Compare this to the temperature of the surface of the sun of 7,000 K. It is possible that shorter frequency light (such as x-ray) is emitted, however this would be absorbed by the water and so remains an unknown. It is possible that the temperature may exceed one million degrees K. The flash itself lasts for less than 12 picoseconds.

The cause of sonoluminescence is still unknown despite almost 70 years of study. There exist many reputable theories as to the cause:

shock waves
The collapse of the bubble at high speeds to a size less than a micron and pressures several times that of sea level atmosphere (simulations show that the pressure may reach millions of atmospheres) may cause temperatures of about 10,000 K. However, this fails to get the temperature high enough for significant amounts of ultraviolet light.

The less than a micron value comes from the pictures of the phenomenon and the size of the source of the light. It is possible that the bubble is compressed further to the billionths of a meter which could cause the temperature to reach much higher levels.

Quantum mechanics
At very small distances quantum mechanics begins to take a role. This possible explanation was made by Claudia Eberlein of the University of Cambridge. It is possible that the boundary layer between the bubble and the surrounding water is moving through space at the rate of an atomic radii in a trillionth of a second. With these extreme conditions it might be possible for virtual photons to be made into real photons that are then observed. This is highly speculative - some physicists argue that pulling virtual photons from quantum vacuum fluctuations may require the bubble to move faster than the speed of light.

No, this isn't made up - from http://www.susx.ac.uk/physics/faculty/cceberl.htm

Collapsing gas bubbles in fluids can emit substantial amounts of light the origin of which has so far been essentially unknown. Inspired by an idea by Schwinger I have proposed a theory of quantum vacuum radiation in order to explain the effect. The surface of the bubble is an interface of two dielectrics of different polarizability which radiates photon pairs when moved non-uniformly. The single-photon spectrum is shown to resemble a black-body spectrum which is a consequence of the correlations of the photons in a pair and the tracing over one photon during measurement.

Much of the interest now in sonoluminescence is the possibility of sonofusion. Although speculative at this point, it is believed that the high temperatures and pressures that create sonoluminescence may also be useful in triggering low level nuclear fusion reactions.

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