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- What causes sonoluminescence? Sonoluminescence is the generation of small light bursts in liquids caused by sound. Bubbles form in the liquid at low pressure points of the sound wave, then collapse again as a high pressure wave passes. At the point of collapse a small flash of light is produced. The exact cause has been the subject of intense speculation and research.
For more details, try this:

- What causes high temperature superconductivity? Is it possible to make a material that is a superconductor at room temperature? Superconductivity at very low temperatures has been understood since 1957 in terms of the BCS theory, but high temperature superconductors discovered in 1986 are still unexplained.
To learn more about superconductivity, see this web page and its many links:

- How can turbulence be understood and its effects calculated? One of the oldest problems of them all. A vast amount is known about turbulence, and we can simulate it on a computer, but much about it remains mysterious.
- The Navier-Stokes equations are the basic equations describing fluid flow. Do these equations have solutions that last for all time, given arbitrary sufficiently nice initial data? Or do singularities develop in the fluid flow, which prevent the solution from continuing?

This is more of a question of mathematical physics than physics per se—but it's related to the previous question, since (one might argue) how can we deeply understand turbulence if we don't even know that the equations for fluid motion have solutions? At the turn of the millennium, the Clay Mathematics Institute offered a $1,000,000 prize for solving this problem. For details, see:

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