"Going Negative"

Josh Kessler, COSI's Program Manager, had a quick question for me as we were driving to OSU to talk with the Astronomy Department about solar eclipse plans (more on that later). He heard in the news "something something discovering negative mass", and that seemed like a big deal.

Indeed, discovering negative mass would be a big deal, since it doesn't exist in our universe. Particles with negative mass would repel positive-mass particles, which means you could put one next to a positive-mass particle and watch as they spontaneously accelerate off to infinity. That seems...wrong.

So imagine my surprise when the headlines came rolling in recently about a "breakthrough" that is "turning physics upside down". For once, the blame isn't fully on the press release or the media. The scientists flat-out put it in the title of their paper accepted by Physical Review Letters:

"Negative mass hydrodynamics in a Spin-Orbit--Coupled Bose-Einstein Condensate"

How profound! But the very first sentence of their abstract gives the game away: "A negative effective mass can be realized in quantum systems..."

A negative *effective* mass is an entirely different beast than a negative mass. In fact, the term is really a historical artifact that doesn't mean what you think it means. It means that in ultra-cold quantum systems, there are internal forces that make a fluid move in surprising ways. In this case, when they shut off their trapping laser, the fluid expanded a little bit then stopped. That's it.

It's a totally routine operation in this branch of physics, not at all surprising, and not at all negative.