Thanks to George Coyne who writes:
You might find this experiment of interest. Wikipedia states: Prof. Peter Engels and a team of colleagues at Washington State University observed negative mass on the 10th April 2017 when they created new negative mass by reducing the temperature of rubidium atoms to near absolute zero, generating a Bose-Einstein condensate. By using a laser-trap, the team were able to reverse the spin of some of the rubidium atoms in this state, and observed that once released from the trap, the atoms expanded and displayed properties of negative mass, in particular accelerating towards a pushing force instead of away from it.
From the BBC on April 19, 2017: Physicists observe 'negative mass'
[GB: This interpretation that “negative mass” has been produced is a wonderful illustration of regressive physics. It is true that the definition of mass is “the resistance to acceleration.” As you can see from the figure below, any impacts by supermicrocosms from the macrocosm will increase the mass of a microcosm. In other words, the momenta of the submicrocosms within the microcosm will increase. This is nothing more than Newton's Second Law of Motion acting across the microcosmic boundary. In other words, a “push” across that boundary will increase the motion of the insides of that microcosm before it accelerates the microcosm as a whole. As was explained in the neomechanics section of “The Scientific Worldview,” the microcosm will tend to expand as those submicrocosms within the microcosm impact the microcosmic boundary on all sides. The regressive physicists involved in this study have disingenuously misinterpreted this push-back as “negative mass.” It cannot possibly involve a decrease in mass, because any push-back by the submicrocosms within the microcosm only makes it even more difficult to accelerate the microcosm. Again, mass is the resistance to acceleration. Without acknowledging the submicrocosms producing this effect, regressive physicists are forced to invent yet another absurd ad hoc that makes no sense: “negative mass.”]
 Borchardt, Glenn, 2007, The Scientific Worldview: Beyond Newton and Einstein: Lincoln, NE, iUniverse, 411 p. [http://www.scientificphilosophy.com/].