In theoretical physics, **negative mass** is a hypothetical concept of matter whose mass has a negative sign relative to ordinary (positive) matter, e.g.: -2 kg. Such type of matter violates one or more conditions of energy and exhibits special properties, arising from the ambiguity of how attraction should refer to force or acceleration oriented opposite to negative mass. It is used in certain speculative theories, for the construction of Einstein-Rosen bridges. Originally, the most realistic known representation of this type of exotic matter was the density with pseudo negative pressure produced by the Casimir effect.

The concept of negative mass arises in the first instance by analogy with electric charges, of which there are both positive and negative varieties. Just as a positive electric charge can be canceled by a negative charge, thus giving rise to the possibility of screening against electric forces, so we can envisage the possibility of “gravity screens,” if negative mass existed to neutralize ordinary, positive mass.

Although it is not known if negative mass exists, or even if its existence is theoretically possible, several scientists have speculated on its properties. Among these are Hermann Bondi in the 1950s, Banesh Hoffman (1906-1986), of the City University, New York, in the 1960s and ’70s, and Robert Forward, in the context of spacecraft propulsion, in the 1980s. In both Newton’s and Einstein’s theories of gravity, negative mass is a requirement for antigravity to exist.

General relativity describes gravity and Newton’s laws of motion as both positive and negative particles and therefore also with negative mass but not including the other fundamental interactions. On the other hand, the standard model describes elementary particles and other interactions but does not include gravity. A new unifying theory that can make the concept of negative mass better understood would be appropriate.

Several scientists have speculated on its properties. Among these are Hermann Bondi in the 1950s, Banesh Hoffman (1906-1986), of the City University, New York, in the 1960s and ’70s, and Robert Forward, in the context of spacecraft propulsion, in the 1980s. In both Newton’s and Einstein’s theories of gravity, negative mass is a requirement for antigravity to exist. Since April 2017, researchers at Washington State University in the United States have experimentally demonstrated the existence of negative mass by cooling Rubidium atoms with lasers. Professor Peter Engels and his team at Washington State University say they observed a negative mass on April 10, 2017 by reducing the temperature of Rubidium atoms near absolute zero and generating a Bose-Einstein condensate with these atoms. Using a laser trap, the team was able to reverse the spin of some of the atoms in the condensate and observed that once released from the trap the atoms expanded and exhibited negative mass properties, and in particular accelerating in the direction from which the force was coming instead of away from it.

See: *Negative-Mass Hydrodynamics in a Spin-Orbit–Coupled Bose-Einstein Condensate*. M. A. Khamehchi, Khalid Hossain, M. E. Mossman, Yongping Zhang, Th. Busch, Michael McNeil Forbes, and P. Engels. Phys. Rev. Lett. 118, 155301 – Published 10 April 2017 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.155301

## References

- Bondi, H. “Negative Mass in General Relativity,”
*Reviews of Modern Physics*, Vol. 29, No.3, July 1957, pp. 423-428. - Forward, R. L. “Negative Matter Propulsion,”
*Journal of Propulsion and Power*(AIAA), Vol. 6, No. 1, Jan.-Feb. 1990, pp. 28-37.