Recently, there have been a few posts on LaserPointerForums with regards to a curious little experiment. A laser is focused as close to the aperture as possible and is pointed straight up at the ceiling. Then, a piece of very soft black plastic or felt marker tip is burned. With some luck, a small particle will burn off, and then seem to float in mid-air. In fact, it looks something like this:
The marker is pointing roughly to the particle in question. It is not smoke and this fact can be verified because you can actually move the laser around gently and the particle will follow it! I made a brief write-up in one post on why this works, but I decided to play around a bit and get some better quality pictures and a few videos.
Here’s the essence of why this phenomenon occurs. A principle called wave-particle duality states that light (but not only light) can be represented as both a wave and a particle; that is, it can have both wavelike and particlelike characteristics. When you observe Airy disks and other diffraction patterns, you are observing wavelike phenomena; the light behaves just as if you were to take a tank of water and perform the same experiments. However, light can also behave like a particle–the term for a particle of light being a photon. Particles have an energy and momentum associated with them and it was shown that photons do indeed have this property. For more information on wave-particle duality, I recommend the HyperPhysics slides on the subject.
Because photons have an associated energy and momentum, they can impart a force onto other objects. This is exactly what is occurring in the picture above! The particle is experiencing a set of force vectors from the laser light which cause it to find an equilibrium position very close to the waist of the beam. Here is an interesting explanation of this phenomenon, known as optical trapping. To borrow a diagram from Wikipedia’s excellent optical tweezers article:
When a particle moves out of its equilibrium position, a restoring force will push it back just above the waist.
To illustrate further, I’ve taken a video (720p :D) and a few pictures:
Edit: It seems as though my video has made it to Reddit and LiveLeak: