Hannah Devlin in The Guardian:
Scientists have created a fluid that exhibits the bizarre property of “negative mass” in an experiment that appears to defy the everyday laws of motion.
Push an object and Newton’s laws (and common experience) dictate that it will accelerate in the direction in which it was shoved.
“That’s what most things that we’re used to do,” said Michael Forbes, a physicist at Washington State University and co-author of the paper, which shows that normal intuitions do not always apply to physics experiments. “With negative mass, if you push something, it accelerates toward you.”
Negative mass has previously cropped up in speculative theories, including those suggesting the existence of wormholes, a form of cosmological shortcut between two points in the universe. Just as electric charge can be either positive or negative, matter could, hypothetically, have either positive or negative mass.
For an object with negative mass, Newton’s second law of motion, in which a force is equal to the mass of an object multiplied by its acceleration (F=ma) would be experienced in reverse.
Theoretically, this sounds straightforward, but picturing how this behaviour would work in the real world is bewildering, even for experts.