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A tractor beam reels in objects with sound

  1. Apr 20, 2014 #1

    Bobbywhy

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    “A tractor beam reels in objects with sound”

    “A newly constructed device generates a beam of concentrated sound that, for the first time, exerts a continuous, perceptible tug on objects large enough to see.”
    https://www.sciencenews.org/article/tractor-beam-reels-objects-sound
     
  2. jcsd
  3. Apr 20, 2014 #2

    Nugatory

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    That's a rather perverse redefinition of the word "tug". If I were exerting the pressure on the target object by pushing on it with a stick instead of sound waves, no one would be impressed.

    (sciencenews.org tends to bring out my inner curmudgeon)
     
  4. Apr 20, 2014 #3

    sophiecentaur

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    Hmmmm. This all seems to hang on the angles involved. The resultant force 'towards' the line of the sources of the sounds will only be positive if the force on each face of the target is at an appropriate angle. No one would be surprised if two sources from opposite sides of the wedge produced a net force on the wedge in the wanted direction (squeezing an orange pip effect). In the picture, the sources appear just a bit 'behind' a line parallel to the x axis. It seems to me that what is needed here is that the interaction of the sonic beam and the faces of the wedge needs to be, essentially, Normal to the faces - i.e a smooth enough surface on the wedge. Beyond that. it's more or less geometry and resolution of forces at work.
    I'm sure that a demonstration would be impressive but it isn't magic. (Also, I don't like the diagram in the link; it's misleading imo)
     
  5. Apr 20, 2014 #4

    Bobbywhy

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    There is no mention in the post or in the referenced article regarding pushing the target object with a stick.

    Upon careful examination of the text, the definition of a “tug” appears to exactly describe the experimental results, and not a perverse redefinition.

    From the Science News article: “Although the suspended target couldn't go very far, it visibly inched downward and tugged on the thread. A scale measured that pull and confirmed that the net force acting on the target was directed straight down, toward the ultrasound machine. A paper detailing the tractor beam will appear in Physical Review Letters.

    Since you claim “Sciencenews.org tends to bring out my inner curmudgeon” may readers here surmise that you consider that publication intellectually beneath you?
    Would you consider accepting the below paper by the experimenters without derisiveness?

    “Acoustic tractor beam”, Phys. Rev. Lett., in press, 2014
    Christine E. M. Démoré, Patrick M. Dahl, Zhengyi Yang, Peter Glynne-Jones, Andreas Melzer, Sandy Cochran, Michael P. MacDonald, and Gabriel C. Spalding
    Accepted: 27 February 2014
    Abstract
    “Negative radiation forces act opposite to the direction of propagation, or net momentum, of a beam but have previously been challenging to definitively demonstrate. We report an experimental acoustic tractor beam generated by an ultrasonic array operating on macroscopic targets (> 1 cm) to demonstrate the negative radiation forces, and to map out regimes over which they dominate, which we compare to simulations. The result and the geometrically simple configuration show that the effect is due to non-conservative forces, produced by redirection of a momentum flux from the angled sides of a target, and not by conservative forces from a potential energy gradient. Use of a simple acoustic setup provides an easily understood illustration of the negative radiation pressure concept for tractor beams, and demonstrates continuous attraction towards the source, against a net momentum flux in the system.”

    Cheers. Bobbywhy
     
  6. Apr 20, 2014 #5

    Bobbywhy

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  7. Apr 20, 2014 #6

    AlephZero

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    Figure 1 in your PDF shows the object is being pushed with two sticks, not one. It's a neat trick to show this "squeezing an orange pip" effect using a triangular shaped object, but there is no way this so-called tractor beam would pull a rectangular block towards the source, unless you are allowed to use mirrors to reflect the beam(s) onto the back surface of the block.

    I guess three people who have posted on this thread are less easily impressed than you are.
     
  8. Apr 20, 2014 #7

    Bobbywhy

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    As I look at Figure 1 in the pdf I see two acoustic beams impinging on a triangular shaped target object...not sticks.

    Fig. 1. Forward scattering of an acoustic or optical beam producing a net attraction force on a target.

    Quotation from the article: "The experiment presented here demonstrates an acoustic negative radiation pressure directed towards the source, without the need for an additional reflecting surface or refractive interface. Moreover, since the region of F– extends from the source, providing a continuous attraction against a net momentum flux in the system, it is compatible with bringing samples in, from a distance, to docking contact with a source. The acoustic tractor beam is demonstrated with macroscopic samples (here > 1 cm) since acoustic devices can generate significantly larger forces (mN) than optical tweezers (pN) over larger length scales [25]."
     
  9. Apr 20, 2014 #8

    Nugatory

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    That's right, there is not. But there should be, as it would clarify what's going on here. Sophiecentaur makes a similar point when he says "it's more or less geometry and resolution of forces at work".

    Note that the word "tug" does not appear in the abstract for the paper. The problem here is not this quite interesting paper and the work behind it, the problem is that sciencenews.org is misrepresenting it. If everyone who reads the sciencenews.org article were to follow up by reading and understanding the real thing, then the misrepresentation might not be so bad - but that's not happening.

    Look carefully at that setup, and consider all the radiation pressures. What is the direction of the net force on the source of the beam, and how does the magnitude of that force compare with the magnitude of the downwards force on the target? There's no attractive force between the source and the target here - they're both experiencing net downward forces, and the force on the source is necessarily greater than the force on the target. Thus, whether they move towards one another or away will depend on their relative masses... And describing an interaction that behaves that way as two objects being tugged towards one another is, if not perverse, at least rather non-standard.
     
  10. Apr 21, 2014 #9

    sophiecentaur

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    It strikes me that you could do the same thing with two streams of ball bearings. The only problem would be with the existence of friction forces, which would reduce the effectiveness of the process, except for very shallow angles. I think the target wedge would need to be very smooth / shiny to avoid significant tangential forces.
    Bottom line is that there really doesn't seem to be anything revolutionary here - just a useful application of a known bit of Physics. Using the Ultrasound seems to produce a result that is sufficiently different from expectation / experience that it impresses observers. Illusionists are always impressing us that way.
     
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