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
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.