Physics Fact for Fiction: Tractor Beams

In my day job as a physicist, I come across a lot of fun physics facts that inspire or enhance my writing.  Now I'd  like to share a few of those with you!

Ah, the tractor beam.  What would science fiction be without that essential of all tools?  But the tractor beam is more than just fun fiction.  It's fun science, too.

Scientists have been able to manipulate tiny objects over short distances for quite a while.  Think:  optical tweezers, which can move a single bacterium fractions of an inch in a liquid.  But, recently, researchers have been able to move larger particles distances of about a yard using only the power of light.

So, how do they do it?

First they use a hollow laser.  As you can imagine, this is a laser where all the light is on the outside of the beam and the inside is dark.  Tiny glass particles are placed in the hollow of the beam.  The light particles (called photons) on the outside of the laser beam heat the air around the beam, but the inside of the beam (where there are no photons) remains cold.  When the glass particles start to drift away from the cold center of the beam, the  hot air molecules around the outside of the laser, which move faster because they're warmer, push the glass particles back to the center.

But wait, there's more!  A small amount of light also seeps toward the center of the beam and heats the air on one side of the glass particles, pushing it along the length of the beam.  If another hollow laser is placed on the opposite side of the beam, the speed and direction of the glass can be controlled by changing the temperatures of the laser beams.  And viola!  You have a tractor beam.

As you may have guessed, this isn't going to work in space since there's no air to heat, but here on Earth, this technology can be used to move chemical agents or dangerous substances, advance optical tweezers, and even be used as chemical containers.