A tractor beam can be loosely defined as a device that uses a beam of energy to attract an object from a distance.
For many years, these devices have been largely hypothetical and confined to the realms of Sci-Fi television.
However, a large amount of research into creating a real tractor beam has been conducted over the last few decades and recently researchers have achieved a major breakthrough in this area.
Past Attempts at Tractor Beams
Over the years, several attempts have been made to create a tractor beam with varying levels of success:
- Gravity beam experiments were were reported to conducted independently by Guy Obolensky and Eric Dollard during the 1980s, inspired by observations by Nikola Tesla.
- In 1992, Russian Chemistry professor Yevgeny Podkletnov, and Nieminen of Tampere University of Technology discovered weight fluctuations in objects above a huge, electromagnetically levitated composite superconducting disk. After three years, Podkletnov published results from further experiments with a toroidal disk superconductor. Italian physicist, Giovanni Modanese, attempted to offer a theoretical explanation of Podkletnov’s observations arguing that the slight expulsive force and the shielding effect at the shielded zone border can be explained in terms of responses to modifications to the local cosmological constant in the superconductor. A number of groups worldwide tried imitating Podkletnov’s gravity shielding observations.
- In 2001, Modanese and Podkeltnov reported that a beam of gravity-like impulses was generated. Their paper showed that a high voltage discharge device had been built that emitted a collimated, horizontal beam that could penetrate different bodies without any significant energy loss. The apparatus was described as an impulse gravity generator.
- A research team led by Professor Andrei Rode at the Australian National University created a device similar to a tractor beam that can move tiny particles around 1.5 m through the air. This device used a doughnut-shaped Laguerre-Gaussian laser beam that has a high intensity light ring surrounding a dark core along the axis of the beam.
- Dr Clifford Schlecht and Prof. John Sinko studied a form of reversed-thrust laser propulsion as a macroscopic laser tractor beam. Applications include remote manipulation of space objects at distances up to 100 km, removing space debris and retrieving tools on-orbit.
- In March 2011, Chinese scientists reported that a particular type of Bessel beam, which is a special kind of laser that does not diffract at the centre, can create a pull-like effect on particular microscopic particles, pushing them towards the beam source.
- Physicists at New York University demonstrated functional tractor beams based on solenoidal beams of light. The non-diffracting beams create a spiralling intensity distribution that traps illuminated objects and helps overcome radiation pressure that would normally drive them down the optical axis. Orbital angular momentum is transferred from the helical wavefronts of the solenoid beam then the trapped objects are driven upstream along the spiral. Both solenoidal and Bessel-beam tractor beams are under consideration for NASA applications.
Theory Behind New Breakthrough
Dr Tomas Cizmar, research fellow in the School of Medicine at the University of St Andrews, has successfully turned a laser beam into a tractor beam that works at the microscopic level.
According to him, the practical applications could be truly great and exciting. The tractor beam is highly sensitive to the properties of the particle that it acts on, so potentially specific particles could be picked up in a mixture.
Normally when microscopic objects are struck by a light beam, they are forced along the beam direction by light photons. German astronomer Johannes Kepler first identified this radiation force when he observed that comet tails always point away from the sun. The technique adopted by Cizmar’s team allows that force to be reversed.
Previously, scientists used a technique known as an optical vortex for the movement of individual particles with light beams; however, this new approach works in a vacuum and liquids.
In the St. Andrews experiment, a VERDI V5 laser generates a Gaussian beam, which is a light beam where the profile is described by a Gaussian mathematical formula. This beam is then led through a lens and then passes through a suspension of dielectric spheres that are arranged between two coverslips.
The lower slip is a half-silvered reflecting mirror that reflects part of the beam back to the source. A standing wave is produced by the interference of the incoming and reflecting beams with each other. Based on the sphere properties their location, the standing wave is shaped such that the light forces the spheres back toward the laser source.
In simple terms, the standing wave at a specific spot near the light source interacts with sphere having a specific mass and size such that light waves push in the wrong direction.
The team findings were published in Nature Photonics.
Future Practical Applications
It is anticipated that the tractor beam could be used in photonics and other disciplines, such as the easy and economical optical sorting of organelles, macromolecules, or cells.
This novel technique has medical applications, e.g., by targeting and attracting certain cell types, components of blood samples could be sorted.
NASA has funded a study of tractor beams for gathering samples for analysis in future missions.
The team identified three options for the capture and gathering of sample materials either in future orbiting spacecraft or on planetary rovers. One is adapting a well known effect known as optical tweezers wherein objects are trapped within the focus of one or two laser beams. However, this would need an atmosphere for operation.
The other two methods are based on specially shaped laser beams - instead of a beam where the intensity increases at the centre and then gradually tails off, the team is studying Bessel and solenoid beams. In a solenoid beam, the intensity peaks are found in a spiral round the line of the beam whereas in a Bessel beam, the intensity rises and falls in peaks and troughs at increased distances from the beam line. In all three cases, the effect is small; however, it could in certain instances outperform present methods of sample gathering.
Sources and Further Reading
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