Observed bubble compression: By bouncing laser light off the bubble, physicists measure the size of the contracting bubble and then compute its temperature. The observed contraction of the bubble should heat the gas inside to temperatures of about 10,000^o C., which is higher than the temperature of the surface of the sun. Many physicists accept this prediction.

The tiny white spot in the center is the light emitted by the air bubble, which expands and collapses repeatedly, emitting a flash of light with each collapse. (photo courtesy of L. Crum)

Shock wave: Some physicists argue that the collapse of a bubble produces a shock wave, something like the one in a sonic boom but spherically symmetric. The wave crushes the gas in the bubble, heating it up as the wave passes. Then, when the shock wave reaches the center of the bubble and rebounds, it heats the gas again, and in the process can produce temperatures as large as 100,000^o C.