Physicists observe 'negative mass:
Yeah that was a tantalizing headline Han, but the popular science press neglected the crucial word in the correct phrase from that paper: “negative
effective mass.” Negative effective mass has been around for at least a decade, but this is the first time I’ve heard about it in a fluid. Basically it’s a kind of physics parlor trick that emulates the inverse response to force that we think of when we think of “negative mass,” but without anything actually possessing negative mass. Here’s the paper that article is referencing:
“Negative-Mass Hydrodynamics in a Spin-Orbit–Coupled Bose-Einstein Condensate,” Khamehchi et al., Physical Review Letters, 2017
https://arxiv.org/pdf/1612.04055.pdf
Far more exciting are the appearances of actual negative energies in quantum field theory, and the negative gravitational effect attributed to the hypothetical “dark energy” driving the cosmological acceleration, because those are both physical evidence for the reality of negative gravitation – which is required for “warp field propulsion” aka gravitational field propulsion, as well as the more speculative concepts of wormholes and time machines.
A lot of people have somehow missed the significance of both of those discoveries, so you’ll still hear people with an out-dated high-school level of understanding of physics boldly proclaiming that negative mass-energy doesn’t exist in nature, and that negative gravitation is the stuff of science fiction. They’re dead wrong on both counts. The cosmological acceleration between galaxy clusters is proof of negative gravitation - which is fully consistent with general relativity by the way, and the Casimir effect is clear evidence of negative mass-energy in the lab. So if some rotten internet troll tries to tell you differently, here are some of the myriad citations in the academic literature to prove them wrong:
“We thus have experimental evidence from the bending of light, that space-time is curved, and confirmation from the Casimir effect, that we can warp it in the negative direction.”
"Space and Time Warps,” Hawking, Nd Public Lectures. Cambridge University, 2006
“Such a wormhole would tend to collapse with time, unless it were held up by the repulsive gravity of a negative-energy density. Classically, energy densities are always positive, but quantum field theory allows the energy density to be negative locally. An example is the Casimir effect.”
“Chronology protection conjecture,” Hawking, Physical Review D, 1992
http://thelifeofpsi.com/wp-content/uploads/2014/09/Hawking-1992.pdf
“In Gravitation, Astrophysics and Cosmology, the Casimir effect arises in space-times with non-trivial topology. The vacuum polarization resulting from the Casimir effect can drive the inflation process.”
“New Developments in the Casimir Effect,” Bordag, Mohideen, and Mostepanenko, Physics Reports, 2001
https://arxiv.org/pdf/quant-ph/0106045.pdf
“Nonetheless, we have also found that it is possible for the net energy density in the region between the plates to be negative, depending upon the plate separation and the plasma frequency of the material involved.”
“Contrary to the view expressed by Lamoreaux [11], the appearance of negative energy density in a quantum field theory is very natural. One can easily find quantum states of the free quantized electromagnetic field in empty space which have local negative energy densities. A squeezed vacuum state is an example [28, 29].”
"The Energy Density in the Casimir Effect,” Sopova and Ford, Physical Review D, 2002
https://arxiv.org/pdf/quant-ph/0204125.pdf
“Does this theorem mean that superluminal travel is impossible? No, because the weak energy condition is not obeyed by systems of quantum fields. The best example is the Casimir effect, and in fact, the Casimir effect does provide an example which satisfies condition 1.”
“Superluminal travel requires negative energies,” Olum, Physical Review Letters, 1998
https://arxiv.org/pdf/gr-qc/9805003.pdf