Physicists in Australia have become the first researchers to levitate a macroscopic object using lasers. The physicists used three lasers to form a levitational “tripod” that could hold a small mirror in free space. Beyond the inherent awesomeness of optical levitation, the physicists believe that the setup could be used as an incredibly accurate sensor for fickle forces such as gravity, and perhaps ameliorating the greatest contradiction of them all: quantum mechanics vs. general relativity.

As you can probably imagine, levitating a physical object with lasers (a stream of massless photons) is rather difficult. “Imagine you’ve got something above you and you are throwing ping pong balls at it to keep it in the air,” is the simple (and not wholly accurate) analogy given by Ben Buchler, one of the physicists involved with the study. The key to the levitation is a force known as radiation pressure, and a branch of physics called optomechanics.

The macroscopic object to be levitated — a two-millimeter mirror in this case — has three optical cavities fashioned into it. These cavities (also called optical resonators) are essentially an arrangement of mirrors that trap light inside them, causing a standing wave. A standing wave is where radiation (sound, light, radio) is held in place by interfering radiation coming from the opposite direction. In this case, a light wave (photon) enters the cavity, bounces off a mirror, and then hits the next photon behind it, holding it perfectly in place. This effect creates enough upwards momentum to levitate the object.

Optical levitation has been performed before, but usually with just a single laser on a nano-scale target (pictured below). By using three lasers, the physicists at The Australian National University can harness enough power and stability to levitate a larger object. Curiously, this is similar to the approach used by Swiss researchers earlier in the year, who used carefully manipulated standing acoustic waves to levitate arbitrarily shaped objects.

As for applications of the tri-laser optical levitation, the physicists aren’t entirely sure. Because lasers and mirrors are being used, the setup can intrinsically be used as a highly precise sensor — if the position of the mirror changes (due to external forces such as gravity), the lasers can be combined with time-of-flight tech to measure the movements exactly. Because optical levitation bridges the divide between quantum (photons) and classical physics (the macroscopic mirror), this research may also provide valuable insight into one of the greatest conflicts in modern science: the contradiction of quantum mechanics and general relativity.

Now read: Japanese maglev train begins public testing, buzzes peaceful countryside at 313 mph

Research paper: DOI: 10.1103/PhysRevLett.111.183001 – “Scattering-Free Optical Levitation of a Cavity Mirror” [Free pre-print PDF]

[Image credit]