Physicists observe 'negative mass'

I think it's a wrong attitude for this article. I prefer to use "beware that the naïve layman explanation may be misleading and wrong". The research article is good, the layman explanation is impossible to write.

This is a very technical subject. I don't understand all the details, so I'll mix some information with guess.

First a similar experiment: Inside a semiconductor, the conduction electrons are not isolated. You can image that an electron has a cloud of perturbations around it. So the effective mass of an electron inside a semiconductor is not the mass of an isolated electron.

Moreover, in some cases the effective mass can be negative. If you push them with an electric or magnetic field, they will move in the wrong direction.

So it's easier to understand their behavior thinking that they are positively charged particles with a positive mass, rather than negatively charged particles with a negative mass. The usual name for them is "holes" and any Engineer that use semiconductors will use them as if they were real particles. (A Physicist will be totally convinced that they are real particles, but real has here not the usual definition.)

So, there are some examples of "things" with negative mass. This case is different because it's a fluid (a bose condensate?) instead of a solid, and they are using rubidium atoms that are bosons instead of electrons that are fermions. So I'm not sure about the details ...

But this is probably a fluid with negative mass that only can live inside some weird material. It's not a fluid that can be isolated and put in a balance and get a negative result.

[And it can't be used to build an Alcubierre Drive.]