The previous sections included transport methods for which there is a scientific or engineering consensus they are possible to build, even if not built yet. For completeness, this section includes theoretical transport methods for which: a consensus does not exist, there is no known method to implement it, or even contradicts established physics. They are sorted alphabetically, since there is no reliable way to rank or organize these methods. Although there is not a firm technical basis for these methods, to be listed here, they need at least some theoretical support in the form of published papers or other documentation. Ideas which only appear in fictional works or have no theoretical support can be found in Appendix 1: Fictional Methods Alternate Names: - Subspace, Hyperspace, Alternate Dimensions Type: Theoretical Description: - Spacetime in relativity theory is the 4 dimensional environment of three physical dimensions and one time dimension. Travel in ordinary spacetime is limited to the speed of light as far as we know. This method uses the idea that there is some other spacetime which can be reached from ours. If it has different properties than ours, it could allow transport or communication faster or more efficiently. If an alternate spacetime is in relative motion to ours, which is not constrained by the speed of light, then rapid travel could be possible by translating to it, and then translating back at the destination. There are theories that our Universe actually consists of more than 4 dimensions, such as string or M-theory, but the other dimensions are compacted to the quantum scale, or unreachable. As of 2012, there is no evidence for these theories, although a considerable number of scientific papers have been written on the subject, and some searches for observable effects are underway. Status: Theoretical as of 2012 Variations References Drosher and Houser, Space Propulsion Device Based on Heim's Quantum Theory, 2004. AIAA Paper 2004-3700. Assumes an extension of General Relativity using quantized higher dimensional space. Alternate Names: Type: Theoretical Description: Antigravity is the reduction or opposition to the normal force of gravity, which is attractive under most conditions. One method of producing it would be with a negative mass. If such existed, the formula for gravitational force would produce a repulsion rather than attraction. No material with negative mass is known to exist. By Einstein's mass-energy relation ( E=mc2 ), negative mass would also represent negative energy. Other methods of producing repulsion have been proposed, but suffer a similar lack of observable support except for one item - Dark Energy. The Universe as a whole appears to be expanding at an accelerating rate. The cause of this is hypothesized to be a cosmological constant, a pressure that exists throughout the Universe tending to expand spacetime. Since there is no known way to change the pressure caused by Dark Energy, which is distributed evenly in all directions, it is not useful as a transport method. Status: Theoretical as of 2012 Variations References Alternate Names: Type: Theoretical Description: This method assumes some violation of Newton's laws of motion are possible. Either an action without an equal and opposite reaction, which produces a reactionless thruster, or higher order terms in the motion equations that would allow an unbalanced force. While such formulas are easy to write, they do not have support from actual observations. A resonant extraction of Casimir forces from the quantum background has been proposed as a way to produce thrust. While the Casimir force is well observed, using it in a way that generates reactionless thrust is not. Status: Theoretical as of 2012 Variations References Alternate Names: Type: Theoretical Description: In theory, all black holes will emit particles as if it were a black body of a certain temperature. This is known as Hawking Radiation after physicist Stephen Hawking, who first described it. The temperature varies inversely with the size of the event horizon, so smaller black holes are hotter and emit higher energy particles. The emission of a black body changes as the 4th power of temperature, while the area of an event horizon changes more slowly. Therefore small black holes emit more energy, and ones small enough to emit useful amounts of energy are themselves particle sized, and thus called Quantum Black Holes. If new matter is added to the black hole at a rate sufficient to offset the emission losses, effectively 100% conversion of matter to energy can be achieved. The particles or gamma rays thus emitted are directed for thrust or used for power generation. Black holes, quantum or otherwise, are very massive, so the utility of such for propulsion is questionable for anything smaller than an asteroid sized spaceship. Although there is a good amount of theory about quantum black holes, there is not a consensus that they actually exist beyond theory. The difficulty is in how to form sub-stellar mass holes. Holes can be manipulated by adding a net charge and then using electrostatic or magnetic fields. Status: Stellar and larger mass highly condensed objects have been observed, and are presumed to be black holes. Quantum mass black holes have not been observed. Variations: References: Wikipedia article: Hawking Radiation Alternate Names: Type: Theoretical Description: Tachyons are hypothesized particles which travel faster than the speed of light. They would either allow higher exhaust velocity for an engine, or by some sort of conversion, possibly by quantum tunneling, convert an entire vehicle into tachyons so it would travel faster than light. Some searches for tachyons have been made, but they have not been observed in nature. Status: Theoretical as of 2012. Variations References Alternate Names: Type: Theoretical Description: Travel through spacetime is restricted by current theory to the speed of light. Spacetime itself is not limited in this way, and in fact current Inflationary Cosmology theory assumes a faster than light expansion in the early history of the Universe. This method assumes that spacetime itself is distorted locally around a vehicle in such a way that apparent travel to outside observers is faster than to internal passengers. An example of such is the Alcubierre drive, but current theory does not indicate how to actually generate such a space warp. Note that in General Relativity theory, gravity is caused by a warp of spacetime, and passengers appear to themselves to not accelerate, while outsiders see them in accelerated motion. The difficulty is gravitational fields are not mobile, being attached to the large masses which cause them, so their utility for space transport is limited to methods like Gravity Assist, where you can make use of the difference in motion between two large objects. Status: Theoretical as of 2012. Variations References Alternate Names: Type: Theoretical Description: A Wormhole is a hypothetical region of spacetime shaped to connect two distant points. If the connection is shorter than the non-wormhole path, traversing it would save time. Creation of wormholes in theoretical papers usually involves black holes or Exotic Matter, matter with unusual properties such as negative mass. While many such papers have been written about wormholes, it is not known if the theory matches reality. Therefore we do not know whether wormholes are possible or what their properties might be. Status: Theoretical as of 2012. Variations References Kanti, Panagiota et. al. Stable Lorentzian Wormholes in Dilatonic Einstein-Gauss-Bonnet Theory posted at arXiv.org 6 Dec 2011.
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