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 nonconservative 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.

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Through discussion of our sonic screwdriver and tractor beam experiments, we aim to highlight the respective advantages of conservative and non-conservative forces. Commonly in acoustic trapping, conservative, gradient-induced mechanisms (e.g., standing waves) are used to manipulate matter. Such situations are reasonably described in terms of potential energy landscapes, an approach also applied to optics, for applications such as cell sorting [MacDonald et al, Nature 426 (2003)]. No such description is possible for radiation pressure, which is non-conservative, a distinction that is sometimes muddled in the literature, although it was made clear even in early work [e.g., King, Proc. R. Soc. Lond. A 147 (1934); Gor’kov, Sov. Phys. Doklady 6, 773 (1962)]. Our “sonic screwdriver” makes use of two non-conservative mechanisms: levitation by radiation pressure and rotation by transfer of azimuthal momentum components [Demore et al., PRL 108 (2012)]. We also note that the term “tractor beam” has often been reserved to describe an effect involving non-conservative forces, and demonstrate an attractive force produced in such an arrangement, even against a net momentum flux [Demore et al., PRL 112 (2014)].

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