If you push an ordinary ball, it moves away from your hand. No surprise there. But if you pushed a ball with negative mass, it would actually accelerate backwards, moving towards you instead. It might be hard to picture how this could be possible, but according to Newtonian physics it should work in theory. Now, a team at Washington State University has demonstrated the phenomenon in practice, creating a fluid that has the properties of negative mass.

Part of the difficulty we have in imagining an object with negative mass might come from our use of the word. In casual conversation, the word "mass" is often thrown around interchangeably with "weight," but there's a difference: mass essentially describes how much matter makes up an object, while weight is the amount of force (usually gravity) that's acting on that object. In regular use here on Earth the two are closely linked, but take an object with a mass of 1 kg to the Moon and it will weigh just one sixth of that, due to the weaker gravity.

An object with a mass of -1 kg won't just float off the ground into space, but it will exhibit other strange behavior. Newton's Second Law of Motion says that a force on an object is equal to its mass multiplied by its acceleration (F = ma), so if that mass is a negative number, the force will also be negative. That's what causes the bizarre fluid to accelerate backwards and press itself up against whatever's pushing it, instead of moving in the direction of the force.

"That's what most things that we're used to do," says Michael Forbes, an author of the study. "With negative mass, if you push something, it accelerates toward you."

The Washington State researchers created the negative mass fluid using what's known as a Bose-Einstein condensate, a quirky state of matter that acts like a superfluid, where particles move in waves and can flow without losing energy. These condensates allow scientists to study quantum mechanics, and were recently used to create a previously-hypothesized form of matter known as a supersolid.

The team made the Bose-Einstein condensate by slowing down rubidium atoms with lasers, which cools them to just slightly above absolute zero and keeps them confined to a bowl-shaped area of about 100 microns across. Next, the scientists hit those atoms with another set of lasers that changed how they spin, a phenomenon known as "spin orbit coupling." That gives the rubidium the properties of a substance with negative mass when it's allowed to flow out of the bowl shape, which, according to the researchers, makes it look like it's hitting an invisible wall.

"What's a first here is the exquisite control we have over the nature of this negative mass, without any other complications," says Forbes. "It provides another environment to study a fundamental phenomenon that is very peculiar."

The strange substance could allow scientists to study some of the most mysterious forces in the universe, like black holes and dark energy.

The research was published in the journal Physical Review Letters.