Light is a liquid.
"No its not!" I hear you shout, and normally you'd be right, but under certain conditions it can be made to act like a liquid.
When we talk about the three states of matter, Gas, Liquid and Solid we understand that these only apply to atomic matter, matter that is made up of atoms. Light is made up of photons so how can refering to it as a Gas, a Liquid or a Solid make any sense? But the fact is that light does have some properties that are normally only applied to atomic matter. Within a beam of light photons move around randomly and they can exert a force on other objects (like with solar sails) just like a gas. In fact some researchers do talk about a beam of light as being like a gas.
If its a gas then it can be condensed into a liquid.
All materials slow down a beam of light by a fixed amount (the speed of light c isn't a constant, the speed that you are normally quoted is its speed in a vaccum). This is why you get a refraction effect when light passes through a pane of glass, and why things look closer if they are underwater. Recently researchers have been working on a special type of materials called "non-linear" materials. Basicly with a non-linear material the amount that light is slowed down isn't a constant but instead depends on the intensity of the light, the greater the intensity, the more the light is slowed. So the inside of a beam of light would slow down more than the outside, almost as if the light had been passed through a convex lens
But you can also have materials which do the reverse, the greater the intensity of the light the less it slows down. This would have the effect of concentrating all the light into a very tight column. Given a powerfull laser you could then create a super-concentrated column of light. This would then act like a liquid.
Computer simulations carried out by a team led by Humberto Michinel at the University of Vigo in Ourense, suggest that this column would act like a liquid. It would have a form of surface tension and it would shatter into drops when it hit a surface (these results have been published in Physical Review E vol. 65, p. 066604).
The only problem is that it can't be done in reality because the right sorts of non-linear materials are not yet in existence. But it looks like they might be in the near future as there are research teams working on creating such materials.
If this is all proven to be true then liquid light might well form the basis of optical computers as they would be able to move around corners in circuits more effectivly than bouncing pulses of light around them. But this may not be with out its own problems, one researcher was quoted as saying "A liquid can end up anywhere and be quite unpredictable".
This information came from several sources, but the main one was New Scienist 6th July 2002, p. 16.