Physicists Create Fluid with ‘Negative Mass’

An international team of physicists from the United States, Japan, and China has created a fluid with negative mass, which is exactly what it sounds like. A paper describing the research is published by the journal Physical Review Letters.

Khamehchi et al created a fluid with negative effective mass. Image credit: Gerhard Boegner.

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.

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. With negative mass, if you push something, it accelerates toward you,” explained lead co-author Dr. Michael McNeil Forbes, a physicist at Washington State University in Pullman and the University of Washington in Seattle.

Dr. Forbes and his co-authors, from Washington State University, OIST Graduate University in Japan, and Shanghai University in China, created the conditions for negative mass by cooling rubidium atoms to almost absolute zero (minus 459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius), creating what is known as a Bose-Einstein condensate.

In this state of matter, particles move extremely 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 created these conditions by using lasers to slow the particles, making them colder, and allowing hot, high energy particles to escape like steam, cooling the material further.

The lasers trapped the atoms as if they were in a bowl measuring less than a hundred microns across.

At this point, the rubidium superfluid has regular mass.

Breaking the bowl will allow the rubidium to rush out, expanding as the rubidium in the center pushes outward.

To create negative mass, Dr. Forbes and his colleagues applied a second set of lasers that kicked the atoms back and forth and changed the way they spin.

Now when the rubidium rushes out fast enough, if behaves as if it has negative mass.

“Once you push, it accelerates backwards. It looks like the rubidium hits an invisible wall,” Dr. Forbes said.

“The phenomenon is rarely created in laboratory conditions and can be used to explore some of the more challenging concepts of the cosmos,” he added.

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M.A. Khamehchi et al. 2017. Negative-Mass Hydrodynamics in a Spin-Orbit–Coupled Bose-Einstein Condensate. Phys. Rev. Lett. 118 (15): 155301; doi: 10.1103/PhysRevLett.118.155301