All people – and many animals – have an intuitive understanding of how objects work; we push them, and they begin to move (accelerate) in that direction. This behaviour is described by Newton’s Second Law of Motion, a pillar of classical physics, which states that a force is equal to the product of an object’s mass and acceleration.

“That’s what most things that we’re used to do,” said Dr Michael Forbes, an assistant professor at the University of Washington, who was involved with the research. “[But] with negative mass, if you push something, it accelerates toward you.”

This behaviour is unlike that of any known physical object. But in theory, matter can have a negative mass in the same way that things can have a negative electrical charge. In 2014, Canadian physicists based at the Université de Montréal announced that it could be possible for negative mass to exist without violating Einstein’s theory of general relativity.

Creating and studying a fluid that demonstrates this behaviour requires highly specific laboratory conditions.

The researchers used lasers to cool rubidium atoms to just above absolute zero (-273°C). This formed a Bose-Einstein condensate. A Bose-Einstein condensate is a state of matter predicted by Satyendra Nath Bose and Albert Einstein; particles move very slowly and combine into a single wave-like entity.

These rubidium atoms also acted as a superfluid – a type of liquid which can keep flowing without losing any energy. Superfluids are popularly known for being able to creep up the edges of their containers.

The atoms were trapped in place by the lasers, as though they were contained in a tiny bowl a tenth of a millimetre across. A second set of lasers “kick” the atoms back and forth; changing how they spin. As the virtual bowl is broken, the rubidium expands and rushes out. If it rushes out fast enough, it demonstrates some unusual behaviour.

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

Previous attempts to capture negative mass in the laboratory have met with logistical problems. This research, which is featured in Physical Review Letters, clarifies the behaviour of an object with negative mass.

“What’s a first here is the exquisite control we have over the nature of this negative mass, without any other complications,” said Dr Forbes.

“It provides another environment to study a fundamental phenomenon that is very peculiar.”

This control gives researchers a new tool to engineer experiments which probe analogous areas in physics. Many of these are impossible to explore otherwise; such as neutron stars, black holes and dark energy, a proposed form of energy causing the universe to expand more quickly. Negative mass is also incorporated into speculative concepts such as the construction of wormholes.