Exotic matter
There are several proposed types of exotic matter: Negative mass. Negative mass would possess some strange properties, such as accelerating in the direction opposite of applied force. Despite being inconsistent with the expected behavior of "normal" matter, negative mass is mathematically consistent and introduces no violation of conservation of momentum or energy. It is used in certain speculative theories, such as on the construction of artificial wormholes and the Alcubierre drive. The closest known real representative of such exotic matter is the region of pseudo-negative-pressure density produced by the Casimir effect. Complex mass. A hypothetical particle  with complex rest mass would always travel faster than the speed of light. Such particles are called tachyons. There is no confirmed existence of tachyons. If the rest mass formula_2 is complex this implies that the denominator is complex because the total energy is observable and thus must be real. Therefore, the quantity under the square root must be negative, which can only happen if "v" is greater than "c". As noted by Gregory Benford "et al.," special relativity implies that tachyons, if they existed, could be used to communicate backwards in time (see tachyonic antitelephone). Because time travel is  considered to be non-physical, tachyons are believed by physicists either not to exist, or else to be incapable of interacting with normal matter. In quantum field theory, complex mass would induce tachyon condensation. Materials at high pressure. At high pressure, materials such as sodium chloride (NaCl) in the presence of an excess of either chlorine or sodium were transformed into compounds "forbidden" by classical chemistry, such as and . Quantum mechanical calculations predict the possibility of other compounds, such as , and . The materials are thermodynamically stable at high pressures. Such compounds may exist in natural environments that exist at high  pressure, such as the deep ocean or inside planetary cores. The materials have potentially useful properties. For instance, is a two-dimensional metal, made of layers of pure sodium and salt that can conduct electricity. The salt layers act as insulators while the sodium layers act as conductors.