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 50^th 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

As Many Exceptions As Rules

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

Star Trek Phasers, Coming To Us Sooner or Laser

The Star Trek phaser could look like a small car

key on a fob, a handgun, a rifle, or a huge bank

mounted on the ship. It could be set for stun or

kill, but this is the only time I saw it set for

make their head explode.

Did you know that the original Star Trek series featured both lasers and phasers? It turns out that more thought went into their fictional design than one might have imagined. And as time has passed, real technologies have moved closer to their fictional counterparts.

Gene Roddenberry gave Kirk and his cohorts palm-held phasers as well as hand-held laser guns early in the first season. But as time went on, the laser was phased out as a weapon – get it? Phased out. Roddenberry worried that the more people learned about the early lasers of the 1960’s, they would lose faith in his laser weapons.

Lasers of the mid-1960’s weren’t very strong. There were more things they couldn’t do than things they could; laser pointers for your public seminar were the height of technology and weighed a ton. So Roddenberry dropped the laser and focused on the phaser.

The word phaser is a portmanteau of the word photon and the acronym maser. What’s a maser? It’s the same thing as a laser except that instead is using visible light, it generates microwaves. In the acronym, they just switch an “M” in for the “L” but the rest is the same – Amplification by Stimulated Emission of Radiation.

Masers were invented in the 1950’s, and microwaves were more mysterious than light rays, so a microwave-based weapon was believable. In truth though, the “photon maser = phaser” was really just another name for a laser. Before the acronym “laser” took hold, the device using visible light was called an “optical maser.”

James T. Kirk’s phaser was scalable; it could stun, kill, or mounted on the ship it could destroy continents. It did its job by emitting a stream of subatomic particles called “rapid nadions” whatever those are. As such, Star Trek referred to phasers as directed energy weapons. These types of weapons are fundamentally different than anything the real world has seen.

The photon torpedo was a traditional bomb or

missile that used a matter/antimatter reaction

as the warhead. Don’t worry, we won’t have

them. In the history of the world, we have

produced about one hundred millionth of one

gram of antimatter. A true antimatter weapon

would need at least 10 million times this

amount (0.1 gram).

In the history of the Earth, weapons have been of two types. There are those that hit the target with a projectile and those that use a huge explosion to deliver heat, pressure, and projectiles. Both are basically kinetic energy weapons; the destructive force is produced by moving particles and pressure.

A directed energy weapon emits a highly focused beam of energy, delivered directly as energy and at the speed of light (or nearly so). The beam can be made of atomic or subatomic particles, or of energy waves, but they have to be of negligible mass. Until not too long ago, these were theoretical weapons, but we’ve made significant progress – if you want to call it progress. Here are some coming directed energy weapons:

Particle beams –

Particle beam weapons are like atomic sand blasters. They use extremely small particles (on the order of electrons, protons, or neutrons) that are accelerated to nearly the speed of light. We already have machines that can do that. Basically, an old-fashioned television cathode ray tube is a particle accelerator, we’ve just learned how to make them bigger, more powerful and use things other than electrons.

But accelerate subatomic particles to near the speed of light, and they become destructive. Neutral particles are the easiest to work with, since they don’t repel each other and spread the beam out (diverge). The US had a neutral particle beam based in space from 1998 to 2006 for testing. It was recovered in 2006 and is now in the Smithsonian.

Lately we have been able to expand from neutrons to accelerating protons. A 2011 study showed that lasers can be used to focus and accelerate protons for a new kind of particle beam. Many applications are possible for this type of beam, from producing new states of matter, to medical uses, and space research. But weapons might be possible as well; however, problems would have to be overcome.

We’re not close to having any hand held directed

energy weapons, even if we can make the

weapons themselves. One the left is a microwave

weapon that was deployed, but never used, in

Afghanistan. One the right is a naval high energy

laser mounted on a battleship.

Particle beams use very small particles; therefore, the holes they make in ships or planes or tanks or people would also be very small. To be effective weapons, they would have to spread the beam out, but this causes a loss of energy. So you would have to increase the amount of energy you put into a system that already requires millions of volts just to generate a beam of any size. Good luck making that into a hand-held weapon.

High energy masers (HEM) –

The original phaser was a photon maser, but our masers are usually very low energy. Masers are used as timers in atomic clocks and for space research, but they have a disadvantage in that they must be cooled extensively. Does your microwave oven have a refrigerant? No, that’s because it isn’t a maser, it’s just a microwave emitter.

However, starting in 2012, a room temperature/solid state maser was demonstrated, and a few countries have moved forward in developing maser weapons even if they are only non-lethal weapons. Since microwaves are lower energy than light waves, they don’t penetrate the body very far. But boy, they can heat your skin up and hurt like heck. As such, they are used for crowd control, rather than human destruction. In other words, our current phasers can’t be set to stun or kill, just sunburn.

High energy lasers (HEL) –

Plane based high energy lasers were developed in the 1970’s by the US Air Force. By the mid 1980’s, these lasers had been abandoned as weapons but remained as targeting mechanisms. You can use a low energy laser to sight and distance a target, and even have other weapons follow a laser path to a target.

Deformable mirrors are most used in astronomy.

They can take distorted waves and correct them

by bouncing them off deformed mirrors. This

gives much clearer images of deep space objects.

But they can go the other way too. Take a laser

beam that would distort the mirror and the

beam would then be distorted. Change the

shape of the mirror and this will correct for

the distortion and refocus the beam.

The reason that high energy lasers were not feasible was just that – they were high energy. Lasers are focused by mirrors and mirrors aren't indestructible. The higher energy lasers would deform the mirrors being used to focus them. This led to a loss of focus and intensity.

In the late 2000’s, deformable mirrors, or adaptive mirrors, were developed that could change shape to accommodate the deformation induced by high energy lasers. First they were liquid mirrors, but now there are solid versions. Using these mirrors, the US Navy shot down a drone with a high energy laser in 2011 and destroyed a small ship with a ship-based laser in 2013.  This naval experiment was particularly useful in learning how to power the massive laser while still powering the ship.

The US Air Force has an air borne laser program (ABL) which uses three different types of lasers on the same instrument; one for firing, one for targeting, and two for illumination. The ABL has been tested for years, and now the Air Force wants it implemented on the upcoming AC-130J gunships in 2017.

Usually lasers “lase” (use as a substrate) a solid or gas material in order to produce the radiation. Unfortunately, these substrates are expensive, finite, and hard to produce. But what if that wasn’t necessary? There are now free electron lasers (FEL). These feature electrons, just like in your TV tube, only they are sped up with supermagnets. Electrons of sufficient energy and speed will then produce photons of laser light all on their own.

This is a toroid plasma loop produced a couple

of years ago. Alone it is harmless, although right

at its surface it’s the temperature of the Sun. This

ring could be used to focus an electron beam –

basically a lightning bolt. You’ve made toroids –

remember tapping on the bottom of your oatmeal

can after putting hole in it and using it to extinguish
a candle?

A 100 kW FEL has been developed by the US Navy and will deployed on many ships by 2018, according to the Office of Naval Research. These are still very large, but they offer the potential for being miniaturized.

Light-induced plasma channel weapon (LIPC) -

One final potential directed energy weapon is worth discussing. The LIPC is basically a lightning bolt fired down a laser beam. The laser turns the air into a plasma (all air molecules are stripped of their electrons), and this provides a tract of least resistance for high energy electrons to be directed.

Sound like science fiction? Well, the US Army fired one successfully in 2012. This was not a very focused test, but several advancements have been made since then. In 2013, scientists figured how to fire a toroid plasma beam in free air instead of a vacuum. This might someday allow for a plasma tunnel to direct a lightning bolt without the need of a high energy laser.

Another 2013 study showed that lasers can be used to accelerate electrons on their own and can focus them with the magnetic waves it produces. This might make a LIPC possible without the need for a plasma tunnel. The main reason for this research right now is energy production. The scientists want to develop fusion energy for consumer use, but how many times have we seen commercial products weaponized or weapon technology that becomes commercialized.

Next week - a tractor beam (from “attractor”) is a classic science fiction tool, and Star Trek made use of them as well. Science fact is now just now starting to catch up to science fiction – is this topic pulling you in?

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

As Many Exceptions As Rules

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Oxborrow, M., Breeze, J., & Alford, N. (2012). Room-temperature solid-state maser Nature, 488 (7411), 353-356 DOI: 10.1038/nature11339

Peralta, E., Soong, K., England, R., Colby, E., Wu, Z., Montazeri, B., McGuinness, C., McNeur, J., Leedle, K., Walz, D., Sozer, E., Cowan, B., Schwartz, B., Travish, G., & Byer, R. (2013). Demonstration of electron acceleration in a laser-driven dielectric microstructure Nature, 503 (7474), 91-94 DOI: 10.1038/nature12664

R.D. Curry (2012). Investigation of a toroidal air plasma under atmospheric conditions Plasma Science (ICOPS) DOI: 10.1109/PLASMA.2012.6383564