Tuesday, March 10, 2015

Pulling Us Toward Tractor Beams

It was futile to resist the Borg tractor beam. They
would draw a ship in and the beam would force the
ship to drain its energy reserves on deflectors and
propulsion trying to escape. I like how it looks like a
light beam, because we are using light to make
tractor beams today.
Star Trek lost an original crew member recently, Leonard Nimoy passed away on February 27, 2014, just one year before the original series celebrates its 50th anniversary. As the science officer character on ST:TOS (The original series), Mr. Spock got to play and explain much of the technology aboard the Enterprise.

Our series of posts on the coming reality of Star Trek technology is a way of showing that the series, and Nimoy’s character as the first cool scientist, still have relevance and a place in our world. The toys that Spock, Kirk, and Gene Roddenberry showed us have become the inspiration for doing real science. We continue in that vein today – the tractor beam.

In general terms, a tractor beam (short for attractor) is a traveling wave that can move a targeted object along its length back to the source, or hold it in place at a designated distance from the source. In Star Trek, the beam was an “attenuated linear graviton beam,” produced by a graviton generator.

In physics, the graviton is a hypothetical elementary particle that mediates the force of gravity in a quantum field. Dr. Sheldon Cooper’s beloved string theory predicts the existence of gravitons as closed strings while classic quantum field theory makes them a massless spinning particle. An observation in March 2014 from the BICEP2 telescope on Antarctica detected primordial gravitational waves from the Big Bang. If confirmed, they would strongly suggest that gravitons do indeed exist.

This would be cool – learn how to manipulate these and a piece of Star Trek technology would come into being just as it was suggested by the show. But that nasty little phrase, if confirmed, reared its head in late 2014 when results from the Planck satellite suggested that results from BICEP2 might have been from local contamination, not a remnant of the Big Bang. It doesn’t mean gravitons don’t exist, we probably just have found them yet.

Ripples on water exert a force away from the source, but not here. 
The part that moves up and down has teeth to
produce many small overlapping wavelets. Their
interference cause them to hit the ball on the side
away from the source, pushing it back toward
the source.
The good news – we don’t need gravitons – we have tractor beams right now! The bad news, they only pull pretty small things, and only a couple of ways have the potential to work in space. Just as we have described before, before we find one good way to do something, we usually come up with several “meh” ways to do it. Let’s look at our infant tractor beams.

Aquatic tractor beam – The National University of Australia demonstrated a tractor beam that can draw a plastic ball – it’s a ping pong ball I think – back towards a wave generator in 2014. Usually waves push things in the direction of the wave, like Tom Hanks trying to escape from his island in the movie Castaway. But if the waves are very small and overlap, they can produce small vortices whose direction is toward the source, even as the individual waves propagate away.

This idea goes back to the 1800’s and George Stokes’ idea of “Stokes drift,” but was first observed recently in the way particles of dust or such move in certain laser beams. By generating a wave front with many small waves, three-dimensional vortices interact with a force that meets the ball on the sides instead of in the front. It pushes the ball with an additive effect toward the wave generator. Remember this sideways approach – we’ll see it again.

Acoustic tractor beam – Scientists in the UK and the States are working together to lasso things in with sound. The system uses a medical ultrasound ablation system for destroying tumors, but applies the forces to moving attracting objects. I’ve heard that a whisper works for bringing someone closer, but ultrasound might work too.

The left cartoon shows how a flat wave generator
(like light) bounces off the target and pushes it away,
while the cone-shaped generator (like sound) pushes
it away even more. By using two sources and a
triangular target, you can make it hit the back side
and draw it in (right).
The members of the UK/US collaboration were trying to show how negative radiation forces act in the opposite direction of a propagating wave. Two sound waves are fired straight up, but the target is placed 51˚ from the vertical (to one or the other side of the wave if it traveled straight up). As the sound waves propagate out in all directions, some hit the sides of the triangular target (about 1 cm long) and are bent up. This transmits momentum to the target in the opposite direction - down.

By doing this in water, they could measure the weight of the triangular target. When the sound waves were turned on, the target got heavier, meaning that there was an additional force pulling it down – the tractor beam. The proponents of this technique say they can pull bigger objects than our remaining technique – light – but you can't use sound waves in space - like the Alien tagline - in space no one can hear you scream. I think we should move on.

This is a Crookes radiometer. It can’t measure or
demonstrate radiation force, but light will turn it. The
light is absorbed by the black side and heats it up. The
air on that side gains more energy and rushes to the
cooler (white side). As it rushes there, it pushes the
fan a bit. Since inside is ALMOST a vacuum, it
pushes easily.
Photonic tractor beams use light to move objects. Yes, you can move things with light. Even though photons are massless, they have momentum, and some of that momentum can be transferred to objects. This is called radiation pressure – use a high enough energy light and you can push things with it.

Radiation pressure actually works in two directions when you shine a laser on an object. One, there is momentum in the direction of the beam will push an object if it strikes it directly through its center of mass, or off to one side or the other if it bounce off the object at an angle (diffraction or scattering).

Two, a laser beam is more intense in its center than on the edges, so there is a gaussian gradient of pressure that pushes things toward the center of the beam. These two forces have been used for two decades in the life sciences in microscope-based instruments called optical traps or optical tweezers.

Optical Tweezers – Basically (very basically) if you shine a laser through a microscope lens and focus it on the lace of the slide, you can put a small molecule or a cell in the center of a laser beam. The transfer of momentum of the laser photons to the target work to keep the target in the center of the beam. If you slowly move the beam, the target will move as the momentum changes to push it back to the center of the beam.

This demonstrates the radiation pressure of a laser
beam. See how it is more intense in the middle, so a
movement out of the center will produce a larger force
by scattering because there is more light there. Since
the resulting momentum is equal and opposite, it pushes
the object back to the center.
You can drag molecules and cells around to put them where you want them. This is a true tractor beam because it would work in space just as easily as in a lab – in this case, moving an object with light doesn’t depend on moving air or water like the acoustic or aquatic beams do.

Using the optical tweezers, you can even measure the force exerted by one molecule on another when you bring them close together. This has been used to measure binding forces of DNA to enzymes and the force that kinesins generate when they walk on microtubules in intraflagellar transport systems.

Tunable Bipolar Tractor Beam – Yale scientists have come up with a completely different way to use light to move objects. Instead of using the radiation pressure of the laser, they have found a way to push an object using the phase of the wave. Their 2009 paper showed how the laser can be repulsive by separating one beam through two optical fibers. If the two “wave guides” are of different length, the light will beams will end up out of phase.

Optical tweezers are very close to an optical vortex,
demonstrated here. Here you can see the forces at work
that keep the target in the center of the vortex. It is still
due to radiation pressure.
Unlike how ions of opposite charge attract each other, photons out of phase are repulsive. You can use that repulsive force to move things. If you reintroduce two beams that are in phase, they will have an attractive force. The scientists want to use them to make optical computer switches in microchips and nanodevices, but in the strict sense, they are light exerting a force to move objects – a tractor beam. In this case, the problem is that it isn’t a beam in free space, it only works in the silica wire waveguides. I can’t see Spock ordering Sulu to engage the attractor microchip.

Hollow Optical Tractor Beam – A team from Australian National University has made a laser that is circular around the edge but hollow in the middle, a veritable light donut. A demonstration in 2014 pulled a gold plated hollow glass sphere over 20 cm. This tractor beam doesn’t work by radiation pressure, but by heating the air within the laser pipe. By polarizing the light in the beam in different directions, the air right next to different parts of the sphere can be heated.

Hot air has more energy and will bounce harder off the sphere in that area, so it will impart a force n the opposite direction. Using different polarization schemes, the air can be made to push the object either forward or backward. By altering the intensity of the beam, you can slow down the object or speed it up.

A Bessel Beam (named for Friedrich Bessel who proposed
them) has a bright center, which is reinforced by the
spinning of the outside rings, Even if you put an object in
the center dot, the dot will still be apparent behind the
beam since it is reformed by the outer rings. One, they
never disperse, and it would take an infinite amount of
energy to create a true Bessel beam.
Unfortunately, because it works by heating the air and letting that air push the object (like the Crookes radiometer in the animation above), it can’t be used in space – no air there. But there’s hope – we have another possibility. The hollow laser and the acoustic tractor are just attempts to get to a different type of beam – a Bessel beam (see picture).

Bessel Beam Tractor Beam - A Bessel beam is a concentric set of laser rings. We can’t make one yet, but if and when we can, the interactions of the different rings will provide changes in photon directions that will allow for striking the object on the front to push it forward, or on the back to push it toward the source of the beam. Since the interactions of the rings form the inner dot of the laser, that center spot can reform even if there is an object in its path. That's cool.

In 2012, a group at NYU used a pair of pseudo-Bessel beams that overlapped, since they couldn’t produce a single true Bessel beam. This apparatus was able to move 30 µm silica spheres in water toward the laser source. While a big step forward, it isn’t like a Klingon war bird is similar to a 30 µm particle, so we’ve got a ways to go.

Next week, Geordi’s visor gives him the ability to sense visible light – and much more.

Contributed by Mark E. Lasbury, MS, MSEd, PhD

Shvedov, V., Davoyan, A., Hnatovsky, C., Engheta, N., & Krolikowski, W. (2014). A long-range polarization-controlled optical tractor beam Nature Photonics, 8 (11), 846-850 DOI: 10.1038/nphoton.2014.242

Mo Li, W. H. P. Pernice, & H. X. Tang (2009). Tunable bipolar optical interactions between guided lightwaves Nature Photonics arXiv: 0903.5117v1

Démoré CE, Dahl PM, Yang Z, Glynne-Jones P, Melzer A, Cochran S, MacDonald MP, & Spalding GC (2014). Acoustic tractor beam. Physical review letters, 112 (17) PMID: 24836252

Punzmann, H., Francois, N., Xia, H., Falkovich, G., & Shats, M. (2014). Generation and reversal of surface flows by propagating waves Nature Physics, 10 (9), 658-663 DOI: 10.1038/nphys3041

Ruffner DB, & Grier DG (2012). Optical conveyors: a class of active tractor beams. Physical review letters, 109 (16) PMID: 23215079

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