Physicists Observe Negative Mass in Bose-Einstein Condensate

Washington State University physicists have created a fluid with negative mass. Push it, and unlike every physical object in the known world, it doesn’t accelerate in the direction it was pushed. It accelerates backward. Led by Peter Engels, WSU professor of physics and astronomy, the researchers generated the conditions for negative mass by cooling rubidium atoms to near absolute zero, creating a Bose-Einstein condensate.

Hypothetically, matter can have negative mass in the same sense that an electric charge can be either negative or positive. People rarely think in these terms, and our everyday world sees only the positive aspects of Isaac Newton’s Second Law of Motion, in which a force is equal to the mass of an object times its acceleration, or F=ma.

In other words, if you push an object, it will accelerate in the direction you’re pushing it. Mass will accelerate in the direction of the force. “That’s what most things that we’re used to do,” says Michael Forbes, an assistant professor of physics and astronomy at WSU and member of the research team. “With negative mass, if you push something, it accelerates toward you.”

In the Bose-Einstein state, predicted by Satyendra Nath Bose and Albert Einstein, particles move slowly and, following the principles of quantum mechanics, behave like waves. They also synchronize and move in unison as what is known as a superfluid, which flows without losing energy.

The team used lasers to slow the particles, making them colder and allowing hot, high energy particles to escape like steam, cooling the material further. The lasers also trapped the atoms in a space less than a hundred microns across. At this point, the rubidium superfluid has regular mass, but breaking the trap allows the rubidium to rush out, expanding as the rubidium in the center pushes outward.

To create negative mass, the researchers applied a second set of lasers that kicked the atoms back and forth and changed the spin so that when the rubidium rushed out it behaved as if it had negative mass. “Once you push, it accelerates backward,” says Forbes. “It looks like the rubidium hits an invisible wall.”

The WSU research team described in technique in Physical Review Letters. According to the researchers, the technique avoided some of the underlying defects encountered in previous attempts to understand negative mass. “What’s a first here is the exquisite control we have over the nature of this negative mass, without any other complications,” says Forbes. The research clarified, in terms of negative mass, similar behavior seen in other systems.

The heightened control gives researchers a new tool to engineer experiments to study analogous physics in astrophysics, like neutron stars, and cosmological phenomena like black holes and dark energy, where experiments are impossible.