CAU-2: Chronology protection

Any scheme that turns FTL into a time machine, which by CAU-1 and CAU-3 is all of them given two devices.

Kind: constraint · Family: CAU (Causality) · Last reviewed: 2026-09-12

The result

Chronology protection conjecture: the laws of physics do not allow the appearance of closed timelike curves, because the expectation value of the stress-energy tensor grows very large as timelike curves become almost closed and the back reaction prevents them closing

What it forbids or costs

Any scheme that turns FTL into a time machine, which by CAU-1 and CAU-3 is all of them given two devices

Escape hatches

A conjecture, not a theorem; semiclassical, so a full quantum gravity might not obey it

Who it bites

Physics proposals

• VIOLATES: Morris, Thorne and Yurtsever: the wormhole time machine (1988). Chronology protection is the conjecture that the closed timelike curves this device produces cannot appear; Hawking's 1992 paper is the reply to this construction, and the two cannot both hold.
• VIOLATES: Morris–Thorne traversable wormhole (1988). A maintainable wormhole of this class is convertible into a device that produces closed timelike curves; if chronology protection holds, it is the maintainable wormhole that fails, since the conversion step uses nothing beyond the wormhole itself.
• VIOLATES: Visser's thin-shell wormholes (1989). The wormhole is maintainable and traversable, hence convertible into a device producing closed timelike curves; chronology protection and a maintainable thin-shell wormhole cannot both hold.
• DODGES: Fell and Heisenberg 2021: positive energy from hidden geometric structure. Section 3.3 argues that global hyperbolicity is a precondition of the construction, so the slices are Cauchy surfaces and closed timelike curves cannot form, and that horizons prevent manipulating the distribution past light speed; both are assumptions about a single drive, which is the entry's escape hatch rather than an answer
• DODGES: Gott's cosmic-string time machine (1991). Hawking's theorem assumes a compactly generated chronology horizon; Gott's horizon extends infinitely into the past because the strings are infinite, so the theorem does not apply, though Grant's divergence shows the conjecture still bites.
• DODGES: Passage through the Kerr interior (Carter 1968). Carter's closed timelike curves are eternal features of the analytic extension rather than curves that appear, so chronology protection, which concerns their formation, is not engaged; the extension containing them is in any case not the interior of a real collapse.
• SATISFIES: Brane-bulk shortcuts (Chung and Freese 2000, Ishihara 2001, Caldwell and Langlois 2001). Every shortcut is a null geodesic of the bulk, so the full five-dimensional spacetime has an ordinary causal structure with no closed causal curves for the conjecture to act on.
• SATISFIES: Einstein–Rosen bridge (1935). The maximal extension contains no closed timelike curves; chronology protection has nothing to protect against.
• SATISFIES: ER = EPR (Maldacena and Susskind 2013). The eternal black hole contains no closed timelike curves and the conjecture creates none; chronology protection has nothing to act on.
• SATISFIES: Fuchs et al. 2024: the constant-velocity physical warp shell. No closed timelike curves can be built from a subluminal shell; there is nothing for chronology protection to forbid
• SATISFIES: Gao, Jafferis and Wall: traversable wormholes via a double-trace deformation (2017). No closed timelike curves form; the coupling fixes the relative time coordinate of the two boundaries and the authors show the boost trick is unavailable.
• SATISFIES: Kaluza-Klein and the hyperspace reading (Kaluza 1921, Klein 1926, ADD 1998, Randall-Sundrum 1999). The compactified and warped solutions have ordinary global causal structure with no closed timelike curves.
• SATISFIES: Lorentz-violating frameworks (the Standard-Model Extension and Hořava-Lifshitz gravity). Hořava's preferred foliation is a global time function, which excludes closed timelike curves outright; in the flat-space SME the causal structure is fixed by the concordant-frame light cones of the fastest species and closes on nothing.
• SATISFIES: Maldacena, Milekhin and Popov: traversable wormholes in four dimensions (2018), and the humanly traversable sequel (2021). No closed timelike curves form; the authors state the long wormhole cannot be converted into a time machine.
• SATISFIES: Wheeler's geons and quantum foam (1955–1962). None of the geon, charged-throat or foam constructions produces closed timelike curves; nothing appears for chronology protection to forbid.
• SILENT: Alcubierre 1994: the warp drive metric. The letter does not mention chronology protection. Hawking's conjecture would forbid the two-bubble time machine of CAU-3 by semiclassical back reaction, and whether it does is unsettled; no analysis of the Alcubierre chronology horizon exists.
• SILENT: Bobrick and Martire 2021: physical warp drives. Notes in the introduction that superluminal motion "allows for closed timelike loops" and never discusses chronology protection; the subluminal class does not raise it
• SILENT: Everett and Roman 1997: the superluminal subway. Hawking's result that closed timelike curves require WEC violation is cited, but whether chronology protection forbids the two-tube machine is not adjudicated; the authors say it forces one "to confront all the associated problems".
• SILENT: Krasnikov 1998: the tube laid on the way out. Krasnikov cites Hawking 1992 for the energy condition consequence of compactly generated Cauchy horizons but does not adjudicate whether chronology protection forbids the two-tube machine; the question is open.
• SILENT: Lentz 2021: hyper-fast positive-energy solitons. Chronology protection is not raised in the paper or in the conference reply
• SILENT: NASA Eagleworks warp field interferometer. Never mentioned
• SILENT: Natário 2002: warp drive with zero expansion. Chronology protection is not discussed; whether Hawking's back reaction forbids the two-bubble loop for this class is unsettled and uncomputed.
• SILENT: Van Den Broeck 1999: the pocket geometry. Not discussed; the two-pocket time machine's semiclassical fate is unsettled.
• SILENT: Warp drives in modified gravity. Neither paper mentions chronology protection; whether semiclassical back reaction in a theory with torsion or fourth-order gravity would forbid the two-bubble loop is unexamined.
• SILENT: White et al. 2021: the worldline-numerics Casimir pattern. Chronology protection is never mentioned

Fictional drives

• SILENT: 2001: A Space Odyssey (1968): the Star Gate. No closed causal curves are contemplated; chronology protection is not in the work's vocabulary and predates the conjecture by decades.
• SILENT: Andromeda: the slipstream. No chronology protection; the loop of the sixth episode is self-consistent by construction; nothing in the setting enforces it.
• SILENT: Asimov's Foundation: the Jump through hyperspace (1942 onward). No time machine is built or forbidden; the series has no closed loops and never says why not.
• SILENT: Babylon 5: hyperspace, jump gates and beacons (1993 to 1998). No loop built, none forbidden; "Babylon Squared" and "War Without End" send a station through time by a different device, not by hyperspace.
• SILENT: Banks's Culture: the energy grid, infraspace and ultraspace (1987 onward). No loop built, none forbidden; the Culture has no time travel and never says why.
• SILENT: Battlestar Galactica: the FTL jump (2004 series). No chronology protection and no time loop to protect against.
• SILENT: Campbell's Islands of Space (1931): where the word came from. No loop is built or forbidden.
• SILENT: Commonwealth: CST wormholes with trains through them (Hamilton 2004). No chronology protection; no time loop exists in the novels.
• SILENT: Contact: the Machine and the tunnel system (Sagan 1985, Zemeckis 1997). Chronology protection never arises; the novel has no time loop to protect against and the film has none either.
• SILENT: Cordwainer Smith: planoforming, Space² and Space³. No chronology protection is stated; nothing in the stories stops a loop because no story notices one is possible.
• SILENT: Cowboy Bebop (1998): the astral gates. No closed causal curves are contemplated; chronology protection is not in the show's vocabulary.
• SILENT: Dune: the Guild navigators and folding space (Herbert 1965). No chronology protection; there is no time loop in the 1965 book to protect against.
• SILENT: Elite Dangerous: the frame shift drive (Frontier Developments, 2014). No causal loops are built; the conjecture is not engaged.
• SILENT: EVE Online: the warp drive (CCP Games, 2003). No causal loops are built; the conjecture is not engaged.
• SILENT: Event Horizon (1997): the gravity drive. No closed timelike curves are contemplated, so nothing for chronology protection to forbid; the film does not go near it.
• SILENT: Flight of the Navigator (1986). David lands minutes after he left, within his own past light cone; the film never asks whether the loop is consistent or protected.
• SILENT: FTL: Faster Than Light, the jump drive as a game mechanic. No chronology protection; no loop is possible in a game with no time axis.
• SILENT: Guardians of the Galaxy Vol. 2 (2017): jump points. No closed causal curves are contemplated; chronology protection is not in the film's vocabulary.
• SILENT: Halo: slipspace and the Shaw-Fujikawa translight engine (2001 onward). No loop is built or forbidden.
• SILENT: Homeworld: hyperspace cores and the Far Jumpers. No chronology protection is stated.
• SILENT: Hyperion: the farcasters (Simmons 1989). No chronology protection is invoked; the Time Tombs' backward motion is a separate device and no loop through a farcaster is built or forbidden.
• SILENT: Interstellar: the wormhole near Saturn (2014). No chronology protection is invoked; the loop through the tesseract is allowed to close without comment on why nothing prevents it.
• SILENT: Mass Effect: element zero and the FTL drive (BioWare, 2007). No closed causal loops are constructed; the question does not arise.
• SILENT: Mass Effect: the mass relays (BioWare 2007 onward). No chronology protection; no time loop exists in the games.
• SILENT: Niven's Known Space: the Outsider hyperdrive and the blind spot (1966 onward). No loop built, none forbidden.
• SILENT: No Man's Sky: the hyperdrive, warp cells and range. No chronology protection is stated; the lore's talk of iterations and simulation is not a rule about loops.
• SILENT: Outer Wilds (2019): Nomai warp cores. The Nomai build the closed loop and nothing in the physics of the game resists it; the loop fails for want of a supernova, not because timelike curves refuse to close. Chronology protection is not in the game's vocabulary.
• SILENT: Space Battleship Yamato: warp by wave motion engine. No chronology protection is stated.
• SILENT: Star Citizen: quantum drive and jump points. No closed causal curves are contemplated; chronology protection is not in the lore's vocabulary.
• SILENT: Star Trek subspace radio. No rule of the fiction prevents the loops it stages; temporal paradoxes are resolved by plot, and Janeway's "Don't even try" is the nearest thing to a statement
• SILENT: Star Trek: transwarp, quantum slipstream and the other beyond-warp drives. "Timeless" builds a causal loop by a temporal transmitter and the franchise has no back-reaction; Hawking's conjecture is never engaged.
• SILENT: Star Trek: warp drive (1966 onward). The slingshot builds closed loops and the franchise polices them with a Department of Temporal Investigations (DS9 "Trials and Tribble-ations"), which is bureaucracy, not back-reaction; Hawking's conjecture is never engaged.
• SILENT: Star Wars: hyperspace (1977 onward). No instalment turns two hyperdrives into a time machine or forbids it; chronology protection has nothing to protect because the question is never posed.
• SILENT: Starfield (2023): the grav drive. No closed causal curves are contemplated; chronology protection is not in the game's vocabulary.
• SILENT: Stargate: the gate network (1994 film, SG-1 1997 onward). Chronology protection is never invoked; the time loops of "1969", "2010" and "Moebius" close or rewrite without any principle forbidding them.
• SILENT: Stargate: the ships' hyperdrives. No chronology protection is stated for the drive.
• SILENT: Starman Jones: the congruencies (Heinlein 1953). No chronology protection; no loop exists in the book.
• SILENT: Stellaris: warp, hyperlane and wormhole (Paradox Development Studio, 2016). No causal loops are built; the conjecture is not engaged.
• SILENT: Super Dimension Fortress Macross (1982): the space fold. No closed causal curves are contemplated; chronology protection is not in the show's vocabulary.
• SILENT: The Alderson drive (Niven and Pournelle 1974). No chronology protection; no loop exists in the novels.
• SILENT: The Expanse: the ring gates and the slow zone (Corey 2013 onward). No chronology protection; no time loop exists to be protected against.
• SILENT: The Forever War: collapsar jumps (Haldeman 1974). No chronology protection is named and no loop is built; the rebuilt relativity is asserted, not described.
• SILENT: The Hitchhiker's Guide to the Galaxy: the Infinite Improbability Drive and bistromathics. No chronology protection; the universe of the Guide has a restaurant at its end and a handbook of tense formations. Nothing in the work stops a loop.
• SILENT: The Lensman inertialess drive (E. E. Smith, 1934 onward). No closed causal loops are built; the books never reach the question chronology protection answers.
• SILENT: The Orville: quantum drive (2017 onward). No causal loops are built by the drive; the show's time travel uses another device and does not engage the conjecture.
• SILENT: The Skylark of Space (E. E. Smith, 1928). No closed timelike curves are built or contemplated; chronology protection has nothing to protect against, and the question is not put.
• SILENT: Thor (2011): the Bifrost. No closed causal curves are contemplated, so chronology protection has nothing to act on and is not mentioned.
• SILENT: Vinge's Zones of Thought: FTL only where physics permits it (1992). No loop built, none forbidden; the shift of the Zones is not a chronology-protection mechanism and is never described as one.
• SILENT: Warhammer 40,000: the Warp (1987 onward). Nothing prevents a closed loop except that no story writes one; chronology protection is neither invoked nor contradicted.
• SILENT: Weber's Honorverse: hyper bands and Warshawski sails (1993 onward). No loop built, none forbidden.
• SILENT: Wing Commander (1990) and the 1999 film: jump points. No closed causal curves are contemplated; chronology protection is not in the franchise's vocabulary.
• SILENT: Xenocide: travel through the Outside. No chronology protection is stated; no loop is ever attempted, so nothing forbids one.
• ENGAGES: Doctor Who: the TARDIS and the time vortex. The show has its own chronology protection: Reapers that sterilise a paradox ("Father's Day"), the Blinovitch Limitation Effect ("Day of the Daleks", 1972) and fixed points that reassert themselves ("The Waters of Mars"). Hawking's conjecture in story form, with the back reaction played by monsters.
• ENGAGES: Farscape (1999): wormholes, and starburst as a separate thing. The Ancients' policy that wormhole knowledge must be suppressed, and Einstein's judgement of whether Crichton should live, is chronology protection by enforcement rather than by physics; "Unrealized Reality" makes it the episode's stake.
• ENGAGES: Timescape's tachyon message. Markham's stuck-switch rule is a chronology-protection argument in miniature: the loop that would close on a contradiction hangs in an intermediate state instead. The ending's branch is the alternative outcome
• RULE: Revelation Space lighthuggers. The in-story fact that a causality-breaking transition unwrites its own agents from history acts as a chronology-protection rule by fiat: the loop is not permitted to close

Sources

• Hawking, Phys. Rev. D 46, 603 (1992) ✓