In our everyday experience, if you push something, it moves away from you. But objects with negative mass would turn that basic principle on its head and accelerate towards you instead. It sounds like science fiction but the idea is theoretically possible, and its effects have been observed in recent experiments. Now, scientists from the University of Rochester have developed a device that can create particles that exhibit negative mass.

Newton's Second Law of Motion says that the force on an object is equal to its inertial mass multiplied by its acceleration (F = ma). Normally all of those values are positive: applying positive force on an object with positive mass will result in positive acceleration, pushing the object forwards. But if an object has negative mass, the force becomes negative too, meaning the object will move in the opposite direction – essentially, pressing itself against your hand if you try to push it.

At least, that's the idea. Negative mass has mostly only been demonstrated in theoretical analyses, although an experiment last year manipulated rubidium atoms with lasers to create a fluid that acted like it had negative inertial mass. Now, the University of Rochester researchers say they've developed a device that can create particles exhibiting negative mass, by combining photons from laser light and excitons in a semiconductor.

The device is based on a laser, with a core difference. Normally, light is bounced between a pair of mirrors facing each other, and the space where that light is confined is called the optical cavity, or microcavity. In this device's optical microcavity, the team placed an atomically-thin semiconductor made of molybdenum diselenide, where it could interact with the confined light. Excitons in the semiconductor combined with photons in the confined laser light to form new particles called polaritons, which have negative mass.

"By causing an exciton to give up some of its identity to a photon to create a polariton, we end up with an object that has a negative mass associated with it," says Nick Vamivakas, lead author of a study describing the device. "That's kind of a mind-bending thing to think about, because if you try to push or pull it, it will go in the opposite direction from what your intuition would tell you."

The researchers are still working to explore the physics of negative mass particles in the device, and although practical applications are still a long way off, one of the first improvements could be more efficient lasers.

"We're dreaming up ways to apply pushes and pulls – maybe by applying an electrical field across the device – and then studying how these polaritons move around in the device under application of external force," says Vamivakas. "But it also turns out the device we've created presents a way to generate laser light with an incrementally small amount of power. With the polaritons we've created with this device, the prescription for getting a laser to operate is completely different. The system starts lasing at a much lower energy input."

The research was published in the journal Nature Optics.