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Top Comments: Progress on Developing a Tractor Beam? [1]

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Date: 2023-02-02  

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Those of us who were, at least at one time, obsessed with Star Trek in one or all of its forms are familiar with the concept of a tractor beam. A beam of… something (Light? Some other kind of elementary particles? “Energy”?) emerges from the U. S. S. Enterprise, makes contact with a space probe or an asteroid, and the ship can then pull the object in whatever direction it wants. Might such a device be possible. Currently it is possible to control the motions of molecule-sized objects and nanoparticles using lasers in a configuration called “optical tweezers,” but nothing like this has promised to work on macroscopic objects. Now, a report from China claims to have used a beam of laser light to move a macroscopic object. The video below shows the evidence.

x YouTube Video

So, what’s happening here? Here’s a description:

The scientists vapor-coated a sliver of glass with reflective gold, and then stuck a flake of cross-linked graphene to the other side. Then, they pointed blue, cyan, and green lasers at the flake of graphene. Lo and behold, it moved toward the laser emitter. ... The device works partly by way of graphene’s unique properties. Graphene is optically absorptive, meaning it retains some percent of the energy when photons hit it. It’s also a semiconductor and an effective heat pipe. So effective, the paper concludes, that when the scientists pointed the laser at the graphene sandwich, the graphene carried that energy right to the far side of the piece. Thermodynamics says that hot things emit more energy than cold things, all else being equal. In the lab environment, that differential heating was enough to make the object move.

To slow things down a little: graphene absorbs light readily. A flake of it is placed on the back-side of a sliver of glass whose front side has been coated with gold. (I haven’t read the paper, but I’m guessing the laser wavelength is one not efficiently reflected by the gold). The light passes through the gold-coated glass sliver and is absorbed by the graphene. The graphene then re-emits the light (fluorescence), but at a longer wavelength, a wavelength that the gold will reflect. The momentum of the fluorescent light (both the emission away from the back surface, and the emission toward the front surface then reflected off the gold), in accord with Newton’s 3rd law of motion, causes the glass sliver to move in the opposite direction, toward the laser light source.

This is an ingenious strategy to give the desired apparently attractive motion of the glass sliver, but it’s not an actual attractive force between the glass sliver and the laser beam. Pursuing this strategy will not result in the sorts of technology we all saw on Star Trek, unless the objects caught by the tractor beam are all transparent and have graphene stuck on the back. On the other hand, you have to start somewhere, and as I said, the set-up is pretty ingenious. We’ll have to see what other ideas might be developed to give us our tractor beams.

Comments are below the fold.

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