Negative mass

The equations of Newtonian and relativistic gravitation do not exclude the possibility of negative mass. Counterintuitively, in one model, given a pair of bodies with equal magnitude but opposite signs of mass, a positive mass would attract a negative mass, while a negative mass would repel a positive mass (so-called ‘runaway motion’, which the model’s proposer described as ‘preposterous’). A negative gravitational mass would fall vertically downwards on the Earth, while on a negative mass planet both positive and negative mass objects would fall upwards. Researchers at Washington State University have observed a ‘negative mass’ phenomenon in a Bose-Einstein condensate. The team cooled rubidium atoms to close to absolute zero and noticed that, unlike normal materials, which accelerate in the direction of the applied force, the atoms of the condensate accelerated in the opposite direction. One of the researchers, Michael Forbes, commented that: ‘It looks like the rubidium hits an invisible wall’.