Objects with “negative mass” react to the application of force in exactly the opposite way from what you would expect.
All the matter you’ve ever interacted with has mass, and as such it obeys the standard laws of motion as enunciated by Newton centuries ago. If you push something, it moves in the direction you push it. However, matter with negative mass would do the opposite. It sounds like wacky science fiction, but it’s close to becoming reality. Researchers at the University of Rochester have worked out a way to create negative mass particles using, what else, lasers. Is there anything lasers can’t do?
Physicists have been chasing real-world examples of negative mass for years, but it’s all been theoretical until recently. The math predicted negative mass was possible, though. In the classic physics equation for force (F = ma), all three variables are positive. However, if you make mass a negative number, the resulting force is negative as well. Thus, pushing an object with negative mass causes it to accelerate toward you. Try to pull it toward you and it’ll move away. It’s a real mind-bender.
The University of Rochester team says the new experiment published in Nature Optics is the first example of creating particles that exhibit negative mass. In the experiment, a laser bounces off mirrors within a small optical cavity. The key to generating negative mass particles was the use of an ultra-thin semiconductor made of molybdenum diselenide. The photos on the laser and excitons in the semiconductor then interact to produce the negative mass effects.
This alone is “interesting and exciting from a physics perspective,” says Nick Vamivakas, an associate professor of quantum optics and quantum physics at the University of Rochester’s Institute of Optics. “But it also turns out the device we’ve created presents a way to generate laser light with an incrementally small amount of power.”
We’re getting into serious condensed matter physics here, but the gist is that an exciton is a bound quantum state of an electron and an “electron hole” where an electron could exist in the semiconductor. The end result of this interaction is a new quasiparticle called a polariton that has negative mass. The researchers verified negative mass qualities in the experiment, but we’re a long way from harnessing that power to actually do something.
Lead author Nick Vamivakas describes a way negative mass particles could be employed in, you guessed it, lasers. Applying an electrical field across a device with negative mass particles could allow researchers to apply push and pull forces with much more efficiency. With polaritons, it’s possible to generate a laser with much lower energy input. Taking things to a more sci-fi place, negative mass is also one of the requirements for the theoretical Alcubierre warp drive. Of course, we’re a long way from figuring out how to make that much negative mass.
Researchers in Vamivakas’ lab, including co-lead authors Sajal Dhara (now with the Indian Institute of Technology) and PhD student Chitraleema Chakraborty, embedded an atomically thin molybdenum diselenide semiconductor in the microcavity.
The semiconductor was placed in such a way that its interaction with the confined light resulted in small particles from the semiconductor—called excitons—combining with photons from the confined light to form polaritons.
“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,” Vamivakas explains. “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.”
Other research groups have been experimenting with similar devices, Vamivakas says, but this is the first device to produce particles with negative mass.
Though applications are “still down the road,” Vamivakas adds, his lab will continue to explore:
How the device might serve as a substrate for producing lasers. “With the polaritons we’ve created with this device, the prescription for getting a laser to operate is completely different,” Vamivakas says. “The system starts lasing at a much lower energy input” than traditional lasers now in use.
The physical implications of creating negative mass in the device. “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.”