Scientists have created a sonic tractor beam. And it's not simply an amazing proof of concept: This beam pulls with a billion times more force and can tow objects a million times larger than previous schemes for tractor beams.

Tractor beams are staples of science fiction, projected from the Death Star in Star Wars and the Enterprise in Star Trek. The term was coined by sci-fi author and chemical engineer E.E. "Doc" Smith in 1931. In reality, scientists have previously succeeded in using light and sound waves to push and pull objects around on the microscopic level. For example, take laser-based optical tweezers, in which light imparts a slight push or pull on an item. Since the forces involved are small, these devices are used for moving things of correspondingly tiny size, including cells.

Now acoustical engineers, led by Christine Démoré and Patrick Dahl at the University of Dundee in Scotland, have developed an acoustic tractor beam that can reel in centimeter-size objects. "The forces involved with this tractor beam are really huge compared to what had been seen before," says study coauthor Gabriel Spalding, a physicist at Illinois Wesleyan University in Bloomington.

The device is a square array of about a thousand ultrasound emitters placed at the bottom of a water-filled chamber. In their experiments, detailed in Physical Review Letters, the researchers used the array to pull in hollow triangular objects. To pull the objects, the array generates a low-pressure zone in front of the targets. For the array to work, it must have sound emitters spaced far enough apart to bounce sound waves off the backside of a target. When it does, sound waves can ricochet off the rear part of an item—after reflecting off the target, the waves continue forward in the direction they were traveling, helping to push the object they hit toward the array. For example, if the device faced the flat side of a triangular object, emitters facing the item would emit sound waves against its sloping backsides.

While the acoustic tractor beam blows away its predecessors, it pulls with just millinewtons of force—thousandths of the force exerted by a falling, medium-size apple. Furthermore, the acoustic tractor beam would have problems with objects that do not have a backside sticking out—for example, if the tractor beam had to deal with a flat board face-on. Alas, the acoustic tractor beam also will not work in space, as vacuum does not carry sound. "The goal isn't something the Borg can pull you in with," Spalding says.

Credit: APS/Alan Stonebraker

But you can do a lot with a little bit of force. "You can imagine medical diagnostics much like the tomato-freshness test, squeezing cells to tell if they're cancerous or metastatic or normal," Spalding says. In addition, you could also imagine sculpting beams to do surgery without cutting people. "Ultrasound has been used for some time to break up kidney stones, but if you want to work in the rib cage or skull, you really have to make sure you can sculpt the beams you use to compensate for all the scattering they might experience off various parts of the body."

Electrical engineer Andrea Alu of the University of Texas, Austin, who did not take part in this research, says it will take some time to scale up to that kind of work. But it will be worth it. "I believe that making this device practical enough for the effect to be used in a biomedical lab will still take significant development, but the authors have made important progress."

Acoustic tractor beams could also be used in conjunction with acoustic levitation devices, which use sound waves to suspend objects in midair. "Acoustic levitation is used in chemistry where you might expect reactions between chemicals and the surfaces used to hold them," Spalding says. Acoustic tractor beams could help pull those materials around during experiments.

This is not the first time Spalding and his colleagues have dabbled with technology out of science fiction. For instance, they have developed sonic screwdrivers that spin objects with sound beams, and Spalding notes that holographic arrays that can physically manipulate objects are, essentially, "holodecks."

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