Weird science helps push back the boundaries

If you push things, generally they move in the direction you push. The harder you push them, the more they tend to accelerate in that direction. That is, as it is commonly understood, Newton’s second law of motion.

Now scientists have created a fluid with the opposite effect. If you push it and it is moving away from you, it gets slower rather than faster. It is not Newton’s law that has changed but something even stranger - the fluid has negative mass.

Just as particles can be positively or negatively charged a collection of them can also, in theory, have a form of negative mass.

One of the strangest consequences of that is that a substance would try to accelerate towards you when you push, rather than away.

Creating it is far from simple. The scientists, from Washington State University, cooled rubidium atoms to a fraction above absolute zero, which at -273.15C is the lowest it is possible to be.

At this temperature, atoms barely move and form a BoseEinstein condensate. Because of quantum interactions, and because they all have almost no energy, in a Bose-Einstein condensate the collection of atoms behaves in effect like one single atom. They act as a superfluid, able to flow without losing energy - and even climb the walls of vessels containing it.

Producing a Bose-Einstein condensate is relatively routine, but what was different about this time was the scientists also used lasers to trap the atoms in a tiny tube. Then they used another set of lasers to precisely tune the properties of the atoms - before being released from either end of the tube.

And, instead of behaving like normal particles, they behaved like they had negative mass. They had been tweaked so that the collective cloud of atoms accelerated in the wrong direction.

‘‘Once you push, it accelerates backwards,’’ said Michael Forbes, of his work published in the journal Physical Review Letters. ‘‘It looks like the rubidium hits an invisible wall.’’

Andrew Fisher, from University College London, who was not involved in the research, said it was impressive for the ‘‘exquisite control’’ they had. ‘‘They knew this kind of condensate was predicted to have funny relationships,’’ he said. ‘‘They have cleverly engineered it by tinkering, in such a way that if you shove it, it starts going slower the more you push.’’

Jon Butterworth, also from UCL, said no applications were yet clear.

‘‘The main motivation is just to demonstrate strange quantum effects and see if they understand them. But playing around with weird condensed matter systems in all probability ends up with applications.’’ He also said we were unlikely to see objects at the human scale that you could push yourself which would behave in this way. "For one thing, your hand would get very cold."