| TIME MACHINES
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1. Goedel universe (time machine)
Hypothetical universe that admits time travel into past. It is infinite, static (not expanding), rotating, with nonzero cosmological constant (our is expanding). Goedel was among first fascinated by the possible physical basis of time travel. In his invited essay on Einstein's work (1949), introduced "grandpa paradox."
2. Van Stokum time machine
A time machine based on an infinite rotating cylinder. Walking around such a cylinder can lead to a close timelike curve. Based on Lense-Thiring effect. Ordinary matter used, however of incredible density. A 100 km long cigar of radius could be a good physical implementation of TRS, it would require matter 10^8 times more dense than the nuclear matter.
3. Black hole with charge or rotating
Kerr black hole is formed by rotating matter. Possesses a ring singularity. Of direct interest for time travelers since it permits close timelike curves (trough the ring).
Reissner-Nordstrom black hole is formed by a non rotating but electrically charged matter. When collapsing, such arrangement forms a Cauchy horizon Formation of closed timelike curves is uncertain.
3. Wormhole time machine
Traversable wormhole
A wormhole with the tidal forces small enough to allow a save passage of a potential traveler. Typically, tidal forces would tear a traveler into pieces.
4. Gott time machine
Two cosmic strings that pass each other with the velocity near c produce time-like close curves, hence may be considered as a TM. Good sides: no cosmological cosmological constant, exotic matter, etc (besides the controversy of existing of the cosmic strings). Bad sides: no way to construct one. Use it only if you encounter one!
5. Ori-Soen time machine
A time machine with asymptotically flat space-time and a toroidal region of altered geometry. Does not require exotic matter, as it does not violate the weak energy condition.
5. Quantum time machine
As suggested by Aharomov, quantum mechanics admits in the microscale a rapid jumps to the future or the past. Unpredictability makes this principle rather useless as a controlled machine for time travel.
6. Warp drive
A region of space that "moves" with respect to the rest of space with arbitrary velocity. Discovered as a solution to GFT by Alcubierre (1994) and proposed as a time machine by Everett (1996). It requires exotic matter, like wormhole time machine, but forms a new family of time machines that do not change the ST topology.
Lense-Thiring effect
Discovered in 1918 by Joseph Lense and Hans Thiring. Rotation of matter may lead to closed time-like curves. Such time machines as Goedel universe, Kerr black hole, Tipler's rotating cylinder are examples of Lense-Thiring effect in action. Actually, one may add Gott's time machine, for a couple of cosmic strings passing each other has a rather enormous angular momentum as a system.
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