Optical trapping! ball oflight stuck mid-air!

jwc said:

Optical trapping is based around an interesting property of light. Namely, there is something called the wave-particle duality which applies to, among other things, light. It basically states that light can be represented as having both "wavelike" phenomena and "particlelike" phenomena. For example, diffraction patterns are basically a wave phenomena, but if you were able to control the light such that only a few photons were to be incident onto a surface, you would see that the diffraction pattern turns into a bunch of tiny single photons. (Fun fact: the eye can see down to something like 6 photons).

Anyways, when representing light as a particle (photon), it becomes apparent that these particles have momentum. And because they have momentum, they have some kind of energy. And because they have an energy associated with them, they can impart that energy onto other particles, yielding a force.

So, if you have a tiny little object like a microscopic piece of burned sharpie, the force vector from the surrounding light tries to push this particle into an equilibrium position, which is right in the center of the beam. A while back, someone posted a diagram of how the force vectors look; it would be helpful in visualizing what's going on.

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Benm said:

For some reason this works extremely well with sharpies, numerous forum members have reported to get similar results. At first i was amazed it actually worked, but for some reason the sharpies seem to release particles that are small enough to be trapped yet big enough to produce a visible reflection.

There is even a video of someone moving the laser around keeping the particle trapped in the beam.

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