Note that this is the effective mass. The whole concept of "negative effective mass" isn't new. Those of us who took intro Solid State Physics classes have seen these due to the way effective mass is defined.

What is new here is that they detected this in a superfluid system. But considering that there is now a smooth connection between BE condensate and BCS system (thanks to the work of the late Deborah Jin), it is not unexpected that one can have a dispersion in a BE condensate that can produce such negative mass.

That means that a force between this fluid an object with a corresponding positive mass would accelerate both in the same direction. I'm sceptical until I see such an experiment.

An air bubble is water has negative mass. One can easily renormalize the mass so that the water surrounding it has zero mass. This is no different than what is done in QFT, resulting in positive holes in semiconductors.

One must pay attention to the background information here. These are many-body effects, the same effect that gives electrons in solids an effective mass that can be hundred of times more than the bare mass! In other words, such effects are already extremely common in the modern electronics that you are currently using!

Does it behave exactly like "negative mass"? Or is the term just for news?

You can't make hoverboards with it, can you? because If Newton's gravitational law still holds, Then it would accelerate downward as any positive mass object.

Interestingly, What would happen if you place two identical objects with the same magnitude of mass but with opposite signs in space. They will just keep accelerating forever(Newtonian). It won't violate conservation of energy or momentum. I think my mind has just gone Kaboom. Can't you use that to reach nearby stars faster?

"Effective" is the third word in the PRL abstract: "A negative effective mass..."
Anyone want to find the first appearance of the word "effective" in the phys.org piece?

Could this negative mass have anything to do with dark energy? Or the vacuum temperature considerable above of absolute zero is already an indicative that it's not the case?

does the equation "k=1/2mv^2 " still apply to negative mass? If it did, does that equal to negative energy? If it does not lead to negative energy, does that mean absolute value bars need to be added to the equation?

I'm using "k=1/2mv^2" for an example, of course my question apply to all energy equations that use mass.

When i saw the news article i wondered how conservation of momentum worked out? For example if you push something away from you and it accelerates towards you then to maintain conservation of momentum don't you also have to accelerate towards the object? Its as if the negative mass of the object somehow gives you negative mass as well?

There are two kinds of momentum, "true" momentum and effective momentum, the latter one being also called "crystal momentum" for the more specific case of particles moving in a crystal.
E.g. for an electron moving in a crystal, the effective mass can be negative as upon acceleration, the electron is reflected more strongly from the crystal lattice. While the effective momentum may also be negative, the total momentum is conserved, as momentum is transferred to and carried by the crystal.