Bessel Beam 'Tractor Beam' Concept Theoretically Demonstrated 54
cylonlover writes "Last year, NASA revealed it was evaluating three potential 'tractor beam' technologies to deliver planetary or atmospheric particles to a robotic rover or orbiting spacecraft. At the time, the third of these, which involved the use of a Bessel beam, only existed on paper. Researchers at Singapore's Agency for Science, Technology and Research (A*STAR) have now proven the theory behind the concept, demonstrating how a tractor beam can be realized in the real world – albeit on a very small scale (abstract)."
Re:"theoretically demonstrated" (Score:3, Informative)
Better (and longer) form:
Physical property demonstrated that (if extrapolated infinitely) would do cool things.
Apparently a "Bessel Beam" is a perfectly parallel laser. They demonstrated tiny amounts of net pull from hitting very small things with a sufficiently parallel laser. So if you could perfect the parallel aspect and put about 6 giga-universes of energy behind one, you'd get a Star Trek style tractor beam.
The actual paper, freely accessible (Score:5, Informative)
Re:Any experts out there? (Score:4, Informative)
The article isn't very helpful and the paper is only available for sale.
In the article it states that Bessel beams are unlike laser beams which "diffract or spread out as they propagate". I know laser beams diffract but I didn't think they spread out (and that was the whole point of them).
Diffract is the same as spread out over distance. The main thing about laser beams is that they are monochromatic (mostly), and with the proper construction, the beam that leaves the oscillation chamber is mostly in phase (other than laser "speckle"). The xy profile of the beam is another matter.
Most laser beams are constructed to have a mostly a gaussian profile which is really close to the diffraction-less bessel profile (both of which are unfortunately of infinite extent). The gaussian profile is a solution of the Helmholz harmonic oscillator pde in cylindrical coordinates using a paraxial (ray-tracing) approximation, and the Bessel profile is a solution that is a true plane wave (basically a better approximation). Of course you can't make either an gaussian or a bessel (they have infinite support), so it's only an approximation, and since it's an approximation, it will diverge (but slowly and bessel more slowly as it's a better approximation). Think of it as picking the goldilocks profile for the beam that's not too sharp on the edge (causing it to diffract away), and not too blurry and that the exact shape so that math works out so it has a constant envelope over time and distance.
Can anybody explain exactly what's going on here and why are the Bessel beams imparting force/energy on the objects toward the beam source?
From what I can tell it seems to be a bit complicated, but as I understand it the gist is that the laser beam creates an electromagnetic wave that interacts with the object's dielectric/magnetic permitivity and creates what is called a Poynting vector S = E x H (a cross product) which is a flux energy in a specific direction. By manipulating the relative polarlization of this laser beam (the Transverse Electric vs Transverse Magnetic components), you can create a situation where this cross-product vector is mostly pointing back to the source. So basically you set up a dipole in the object itself to help you. This effect is small, but if the time averaged beam profile is constant over distance so it doesn't have a z-gradient where it diverges so that the electric field is lower at greater distance (because you used a bessel beam profile), this small flux energy effect will have the tendency to drag the object towards the beam. If there was a z-gradient where the field was lower at greater distance because of diffraction, then that will probably counteract this effect and cause the object to be pushed out instead of tractor-ed in.