Tractor beams, a staple of science fiction, may be moving closer to science fact. In a paper published earlier this spring, physicists have proposed a structure that may enable light to pull objects.

Normally, light pushes on objects, albeit weakly. In the field of optical manipulation optical tweezers employ this pushing force to move microscopic objects from atoms to bacteria. The ability to pull as well would increase the precision and scope of optical manipulation. For spaceflight, engineers have proposed sails to capture the force exerted by light.

Rather than towing space vessels, the newly proposed tractor beam might be more useful in biology or medicine. "If you want to pull something towards you, you just reduce the pressure," says Mordechai Segev, a physicist at Technion–Israel Institute of Technology, who describes his team's idea in an April Optics Express paper. "You make a little bit of vacuum," he adds. The problem is that in sensitive medical applications, such as lung surgery, it is important not to change the pressure or introduce any new gases. "Here, the light will be the suction device," he says, "so the pressure would not change at all. It is just the light."

Previous ideas for a "tractor beam" have often focused on creating new gravitational fields to drag objects, heating air to create pressure differences or inducing electric and magnetic charges in objects so that they move against the direction of an incoming laser beam.

The latest proposal takes advantage of a phenomenon called negative radiation pressure. Russian physicist Victor Veselago first theorized its existence in his 1967 paper about materials with an unusual property called negative refraction index. An index of refraction is a number that describes the way light is bent when it goes into a glass lens or other medium, and at the time of the paper nobody knew if this number could be negative in any material. But in the past couple of decades several teams of researchers proved that negative refraction can occur in specially made substances called metamaterials, which have led to limited invisibility cloaks and distortion-free "super" lenses.

The mechanism of negative radiation pressure depends on two aspects of a light wave: its group and phase velocities. A light wave consists groups of smaller waves; the group velocity is the speed and direction of the overall wave group, the phase velocity refers to the speed and direction of a point on one of the smaller constituent waves. The electromagnetic energy of the light wave goes in the direction of the group velocity whereas the wave's effect on a particle goes in the direction of the phase velocity. If these two velocities point in different directions, then negative radiation pressure can result.

The use of metamaterials to move particles via negative radiation pressure has been hindered by the fact that most of these materials are solid, and introducing a gap for particles would eliminate the negative radiation pressure. Additionally, all current metamaterials contain metals, which absorb electromagnetic energy, rendering the pulling effect on particles negligible.

20 Comments

1. 1. JamesDavis 06:53 AM 6/20/12

   Shouldn't Tractor Beam be one word, "Tractorbeam". When I saw the headline, I thought it was going to be a farm tractor that could produce a beam. Now we know that the Russians have been spying on us. Where else could they have gotten that idea from other than the American movie, Star Trek? Why don't they just ask Captain Kirk to share the tractorbeam spects with them?

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2. 2. SigmaEyes 10:03 AM 6/20/12

   Loved the ending: "move toward the light!" -Brightened my day :)

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3. 3. Zexks 11:09 AM 6/20/12

   All he needs now is to find a nuclear wessel, and he's all set.

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4. 4. JamesDavis in reply to N a g n o s t i c 11:43 AM 6/20/12

   You are right, but Captain Kirk would have to authorize that information to leave the ship, wouldn't he?

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5. 5. Martin Wirth 02:41 PM 6/20/12

   Photons are massless but they do have momentum, which is a function of the phase velocity. The group velocity of the wave, being composed of a narrow spread of frequencies, determines the direction of propagation. The phase velocity is the property that transfers momentum.
   
   Using birefringent materials enables directional splitting of the wave. There doesn't seem to be any conflict in terms of conservation of momentum. While it wasn't explicitly mentioned, I would imagine that the part of the wave that increases the phase velocity in the direction of propagation is ignored. The interesting part that reverses the phase velocity is aligned with a channel where the particles to be pulled shall be located.
   
   Because both of these components originate from the same source, things could get interesting if we consider the consequences of quantum entanglement. If the ignored part of the wave isn't pushing something will negative radiation pressure be applied to particles in the pertinent structure?
   
   If I was a billionaire, I'd fund this experiment immediately because now I'm curious to know the answer.
   

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6. 6. MolBioPhys 03:25 AM 6/21/12

   This is brilliant! I am amazed by the creativity of Segev and his group. He's not asking for any materials that are too out of the ordinary. I, too, hope he gets funding soon so we can find out of this theory works in practice.
   
   In reference to Jack Ng's doubts about whether the force will be in the direction of energy transfer, can't that be ironed out within the same equations used to find the direction of energy transfer?

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7. 7. diandted 11:37 AM 6/21/12

   As a freshman physics honors student at Princeton in the 60's, I quickly realized that there were 2 kinds of fairly or very smart people. First, there were the folks like Ng, Segev, et al, and then there were folks like me who could understand the article above, but who could never contribute to the laboratory or theoretical work described above and who were better suited to a career in Medicine or Engineering. I still read every S.A. article with great comprehension, but never will regret my 35 years as a Pediatric Oncologist.
   

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8. 8. Wayne Williamson 04:51 PM 6/22/12

   Very Cool Discovery...and thanks for the graphics;-)

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9. 9. gmartfin 08:15 PM 6/22/12

   This will move what a dust mote? An atom?
   
   We need something capable of ripping a Klingon ship to shreds......
   
   But otherwise very cool!

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10. 10. kiteman 10:48 AM 6/23/12

    Can you attract something with a large electrostatic field, and will it work in a vacuum?

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11. 11. hamidsadeghipour 01:32 PM 6/23/12

    if we construct a surface made of ceramics, we might move a lubricated piece, like a membrane along a cylinderby with light( laser) . It could be used to focus a telescope by the light from a star, for example, by amplification. or for photography in the night and adjustment of focality by amplification of received light. or a series of membrane to have a stronger force

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12. 12. Dr. Strangelove 03:34 AM 6/26/12

    Jack Ng has a point. Negative refractive index has been proven but not negative radiation pressure. The latter does not theoretically follow from the former. The net force and pressure will follow the direction of the photon's velocity, the same direction as the phase velocity. The group velocity is in the opposite direction. So what?
    
    Segev says the transfer of energy follows the group velocity. There seems to be a confusion here. Energy is a scalar quantity. It doesn't have a direction in space. Force and velocity are vectors. They have both magnitude and direction in space. It doesn't make sense to equate the "direction" of energy transfer to a direction in space. They are not the same thing.

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13. 13. Dr. Strangelove 03:34 AM 6/26/12

    Jack Ng has a point. Negative refractive index has been proven but not negative radiation pressure. The latter does not theoretically follow from the former. The net force and pressure will follow the direction of the photon's velocity, the same direction as the phase velocity. The group velocity is in the opposite direction. So what?
    
    Segev says the transfer of energy follows the group velocity. There seems to be a confusion here. Energy is a scalar quantity. It doesn't have a direction in space. Force and velocity are vectors. They have both magnitude and direction in space. It doesn't make sense to equate the "direction" of energy transfer to a direction in space. They are not the same thing.

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14. 14. Dr. Strangelove 04:31 AM 6/26/12

    Correction: The photon's velocity follows group velocity. Hence, also the direction of force and pressure.
    
    Phase velocity is a misnomer. Phase speed is the appropriate term. It's not a vector. It's actually a product of scalar quantities: angular frequency, wavelength, energy and momentum.
    
    I agree with Jack Ng. It will not produce a net negative pressure because to create negative pressure you have to beam the electromagnetic wave to a material with negative refraction index. That creates positive pressure and then a negative pressure when the group velocity moves in the opposite direction. They will cancel each other. The net pressure is zero.

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15. 15. Massingale 06:10 PM 6/27/12

    This is the first time I could read information and make out the posting...it simple and basic, the formatted posting is good..Looks like another adventure is at hand..

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16. 16. Massingale in reply to JamesDavis 06:13 PM 6/27/12

    When you type in tractorbeam the red line shows up and when you write tractor beam it does not and I love Star Trek

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17. 17. Buffy52 in reply to SigmaEyes 10:35 PM 6/27/12

    I found the article fascinating(ahem...)but the comments have all been of equal worth and have cheered me up,to be reminded that smart witty people like yourselves are still part of or connected to the scientific community gives me hope for the future! :)

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18. 18. jaydax 08:04 AM 6/28/12

    The proof will be when we see a Crookes Radiometer running backwards.

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19. 19. dB333 in reply to jaydax 02:04 PM 6/28/12

    Except that light isn't causing the Crookes Radiometer to move (not directly anyway).

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20. 20. justyntoo 04:00 PM 6/28/12

    if you were to consider magnetism to be polarized gravitation would there not be a functionality?

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