CAU-4: The preferred-frame escape

Any FTL scheme that keeps full Lorentz invariance with no fixed FTL speed is a time machine.

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

The result

The preferred-frame escape. Faster-than-c signals are kinematically compatible with special relativity, which requires an invariant speed rather than a maximum one, and they avoid causal paradox when a soft breaking of Lorentz invariance (a property of the vacuum state, not of the fundamental laws) fixes the FTL speed in one frame. Paradoxes arise only from tachyons whose speed has no fixed value in any frame. The authors argue this for the Scharnhorst case and say statements of general validity are impossible without specifying the propagation

What it forbids or costs

Any FTL scheme that keeps full Lorentz invariance with no fixed FTL speed is a time machine. Any that avoids the time machine has picked a frame, and must say what picks it

Escape hatches

This is the escape hatch. Its cost is that special relativity is no longer the whole story for the FTL sector

Who it bites

Physics proposals

• VIOLATES: Meta relativity (Bilaniuk, Deshpande and Sudarshan 1962). The paper insists that the postulates of special relativity are strictly adhered to and gives class III particles no fixed velocity in any frame, which is exactly the configuration the register says is a time machine; no frame is picked and the paper does not consider picking one.
• VIOLATES: Tachyons (Feinberg 1967). Feinberg's theory keeps full Lorentz invariance with no fixed tachyon speed, which the register names as the one case that is a time machine; the proposal never picks a frame and its whole point is not to.
• DODGES: Morris–Thorne traversable wormhole (1988). No preferred frame is chosen by the theory; the fixed mouths supply one by accident, and nothing fixes it once mouths move or a second wormhole exists.
• DODGES: Visser's thin-shell wormholes (1989). No preferred frame is chosen by the construction; the fixed mouths supply one by accident.
• SATISFIES: Lorentz-violating frameworks (the Standard-Model Extension and Hořava-Lifshitz gravity). The entry demands that any scheme avoiding the time machine say what picks the frame; the SME says the vacuum expectation values of tensor fields (spontaneous breaking) and Hořava says the foliation, and Kostelecký and Lehnert give the conditions (small coefficients, a nonlocal completion above the breaking scale) under which the answer is consistent.
• SATISFIES: The Scharnhorst effect. This is the worked example CAU-4 was written from: the Casimir vacuum picks a frame, the FTL speed is fixed in it, and the authors argue the propagation is benign
• SILENT: Alcubierre 1994: the warp drive metric. No preferred frame is proposed. The metric is written in the rest frame of the stars, but nothing in the construction says what picks that frame, and full general covariance is kept; CAU-4 says such a proposal must either pick a frame or accept the time machine, and the letter does neither.
• SILENT: Bobrick and Martire 2021: physical warp drives. Mentions that causality "may be recovered at the expense of Lorentz invariance" and neither picks a frame nor says what would
• SILENT: Brane-bulk shortcuts (Chung and Freese 2000, Ishihara 2001, Caldwell and Langlois 2001). The papers never address what picks the frame; the register infers that the bulk supplies one, and the literature has not settled it for general brane configurations (Csáki, Erlich and Grojean and Greene, Levin and Parikh show brane motion and asymmetric warping generate genuine Lorentz violation on the brane, which is the same question from the other side).
• SILENT: Entanglement signalling and the no-communication theorem. A working radio would need the preferred-frame escape (the frame in which collapse is simultaneous), and the proposals never say what picks it; relativistic quantum mechanics has no such frame and Peres and Terno show none is needed. The one worked case of the escape, the Scharnhorst effect, has plates to pick the frame; a radio has nothing
• SILENT: Everett and Roman 1997: the superluminal subway. The authors state that closed timelike curves could be avoided only by a preferred axis fixing the orientation of every tube, which is a CAU-4 frame; they propose no mechanism that would pick it.
• SILENT: Fell and Heisenberg 2021: positive energy from hidden geometric structure. No preferred frame is chosen or discussed
• SILENT: Heim theory and the Dröscher-Häuser propulsion proposal (2004 to 2005). The papers claim Lorentz invariance under their dilatation and give the parallel space a light speed $nc$, but never say what frame the speed $nc$ is fixed in; the register's demand that a scheme avoiding the time machine say what picks the frame goes unanswered.
• SILENT: Krasnikov 1998: the tube laid on the way out. Everett and Roman note that closed timelike curves could be avoided only by a preferred axis along which every tube is oriented, which is a CAU-4 frame choice; Krasnikov proposes no such thing and says nothing about what would pick it.
• SILENT: Lentz 2021: hyper-fast positive-energy solitons. No preferred frame is chosen and the possibility is not discussed
• SILENT: NASA Eagleworks warp field interferometer. No frame discussed, though the boost-on-initial-velocity picture implicitly privileges the frame in which $v_i$ is measured
• SILENT: Natário 2002: warp drive with zero expansion. No preferred frame is proposed and general covariance is kept; the constraint asks what picks the frame that avoids the time machine, and the paper does not say.
• SILENT: Retrocausal signalling proposals. A working retrocausal signal could not be rescued by a preferred frame, since the ordering is reversed in the laboratory frame itself; the proposals do not address the point
• SILENT: The EM drive. Not for FTL, but White's proposed mechanism, pushing against the quantum vacuum as a medium, would require the vacuum to have a rest frame. The paper does not say what picks it, and standard quantum field theory says nothing does
• SILENT: The Hartman effect and the Nimtz tunnelling claims. If the signal were real it would need the preferred-frame escape; the claim never says which frame the barrier picks, and the Casimir-type argument of Liberati, Sonego and Visser is not invoked
• SILENT: The Mach effect thruster. Momentum is said to be exchanged with the distant universe through the gravitational field, which means the universe's rest frame enters the dynamics; Woodward does not say whether this defines a preferred frame in the sense of CAU-4
• SILENT: The OPERA superluminal neutrino (2011). The paper says any deviation beyond $10^{-19}$ "would point to Lorentz invariance violation in the neutrino sector" and stops there; it never says what would pick the frame, leaving that to the theoretical responses.
• SILENT: Van Den Broeck 1999: the pocket geometry. No preferred frame is proposed or discussed.
• SILENT: Warp drives in modified gravity. No preferred frame is proposed in either theory; torsion is localised in matter and conformal gravity is generally covariant.
• SILENT: White et al. 2021: the worldline-numerics Casimir pattern. No frame is discussed

Fictional drives

• SILENT: 2001: A Space Odyssey (1968): the Star Gate. No preferred frame is offered; the makers' network is simply assumed to have a single "now" and the work never says so.
• SILENT: Andromeda: the slipstream. No preferred frame is ever stated; the setting's Commonwealth Year calendar is used across galaxies without anyone asking how it is synchronised.
• SILENT: Babylon 5: hyperspace, jump gates and beacons (1993 to 1998). The setting behaves as if it had a preferred frame but no rule states one.
• SILENT: Banks's Culture: the energy grid, infraspace and ultraspace (1987 onward). No preferred frame is stated; the shared clock is a convention of the narration, not a rule of the fiction.
• SILENT: Battlestar Galactica: the FTL jump (2004 series). Fleet jumps are simultaneous in some frame and the series never names it; there is no stated preferred frame.
• SILENT: Commonwealth: CST wormholes with trains through them (Hamilton 2004). No preferred frame is stated for the gateways or the ship.
• SILENT: Contact: the Machine and the tunnel system (Sagan 1985, Zemeckis 1997). No preferred frame is stated. The clock mismatch (a day inside against twenty minutes outside) is reported as fact and never explained by a frame choice or anything else.
• SILENT: Cordwainer Smith: planoforming, Space² and Space³. A universal galactic calendar is used throughout the future history, which would be a preferred frame if anyone said so; nobody does.
• SILENT: Cowboy Bebop (1998): the astral gates. No preferred frame; the solar system shares a "now" on screen without anyone saying so.
• SILENT: Doctor Who: the TARDIS and the time vortex. No preferred frame is ever stated; the vortex is said to be outside spacetime, which is not a frame but the absence of one, and no story asks how "now" on Gallifrey is defined relative to Earth.
• SILENT: Dune: the Guild navigators and folding space (Herbert 1965). No preferred frame is stated for Guild travel; the film's "without moving" implies simultaneity in some frame and names none.
• SILENT: Elite Dangerous: the frame shift drive (Frontier Developments, 2014). The universal galactic clock is the preferred frame in all but name; the game never states it as a rule of its physics.
• SILENT: Ender's philotic ansible. "Instantaneous" needs a frame and the philotic ontology of Xenocide supplies none; the preferred-frame escape is neither taken nor refused
• SILENT: EVE Online: the warp drive (CCP Games, 2003). EVE time is a single clock for the whole cluster and is never named as a rule of the fiction's physics.
• SILENT: Event Horizon (1997): the gravity drive. No preferred frame is stated. Weir's "we can't break relativity, we can go around it" is the opposite claim: full relativity kept, with a geometric exception.
• SILENT: Farscape (1999): wormholes, and starburst as a separate thing. No preferred frame is offered; the show lets the paradox happen rather than picking a frame to avoid it. The one rule, that the traveller holds the destination in mind, is about navigation, not about frames.
• SILENT: Flight of the Navigator (1986). No preferred frame is stated; Faraday's Earth clock is treated as the clock, without saying so.
• SILENT: FTL: Faster Than Light, the jump drive as a game mechanic. The fleet's fixed rate of advance is an implicit universal clock and is never named as a frame.
• SILENT: Futurama: the Planet Express ship's dark matter engines (1999 onward). One clock for the universe, assumed; if scientists raised c in 2208 they raised it in some frame, and the joke does not say which.
• SILENT: Guardians of the Galaxy Vol. 2 (2017): jump points. No preferred frame; the galaxy simply shares a "now" on screen without anyone saying so.
• SILENT: Halo: slipspace and the Shaw-Fujikawa translight engine (2001 onward). No preferred frame is stated; the inconsistency of slipspace time is left as a mystery rather than used as a rule.
• SILENT: Homeworld: hyperspace cores and the Far Jumpers. A shared galactic calendar is used across both games and never named as a frame.
• SILENT: Hyperion: the farcasters (Simmons 1989). No preferred frame is stated for farcaster transit or fatline messages.
• SILENT: Interstellar: the wormhole near Saturn (2014). No preferred frame is stated for the wormhole; the time loop is constructed by beings for whom time is a spatial direction, which is a different escape and is not argued as a frame choice.
• SILENT: Mass Effect: element zero and the FTL drive (BioWare, 2007). The Codex's locally raised light speed is exactly the kind of sector-specific maximal velocity CAU-4 prices, and the setting never says what frame the field's speed is fixed in; the Citadel's single clock is assumed, not stated.
• SILENT: Mass Effect: the mass relays (BioWare 2007 onward). No preferred frame is named for relay transit or for the drive core's raised light speed.
• SILENT: Niven's Known Space: the Outsider hyperdrive and the blind spot (1966 onward). No preferred frame is stated.
• SILENT: No Man's Sky: the hyperdrive, warp cells and range. The shared server clock is an unexamined universal frame; the game never names it.
• SILENT: Outer Wilds (2019): Nomai warp cores. No preferred frame is stated; the interval is a bare fact about departure and arrival with no observer named, and the game never asks what a moving observer would see.
• SILENT: Revelation Space lighthuggers. The state-four transition and the Exordium are described without a frame; whether the unravelling is Lorentz-invariant is never asked
• SILENT: Space Battleship Yamato: warp by wave motion engine. A shared Earth calendar is the implicit frame and is never named as one.
• SILENT: Spaceballs: ludicrous speed (1987). The film has an implicit galaxy-wide "now" (the videocassette gag depends on it) and never states a frame.
• SILENT: Star Citizen: quantum drive and jump points. No preferred frame; the UEE runs one calendar across all systems without saying why.
• SILENT: Star Trek subspace radio. A signal with a definite speed through a medium implies the medium's rest frame; the shows never say what it is or whether subspace has one
• SILENT: Star Trek: transwarp, quantum slipstream and the other beyond-warp drives. A ship at warp 10 occupies all points "simultaneously", which presupposes a frame; "Threshold" does not say which.
• SILENT: Star Trek: warp drive (1966 onward). The single Starfleet clock is the preferred frame the drive would need, and no episode or manual names it as such.
• SILENT: Star Wars: hyperspace (1977 onward). The galaxy behaves as if it had a preferred frame (instant HoloNet, simultaneous arrivals) but no film says so, and the escape is never claimed.
• SILENT: Starfield (2023): the grav drive. No preferred frame is offered; the Settled Systems share a calendar on screen without anyone saying why.
• SILENT: Stargate: the gate network (1994 film, SG-1 1997 onward). No preferred frame is stated; transit is treated as effectively simultaneous everywhere and nobody says what frame that is in.
• SILENT: Stargate: the ships' hyperdrives. Real-time subspace communication and same-day arrivals imply a universal frame that is never named.
• SILENT: Starman Jones: the congruencies (Heinlein 1953). No preferred frame is stated for a transition.
• SILENT: Stellaris: warp, hyperlane and wormhole (Paradox Development Studio, 2016). The galactic calendar is a single clock and is never stated as a rule of the fiction's physics.
• SILENT: Super Dimension Fortress Macross (1982): the space fold. No preferred frame is offered.
• SILENT: The Alderson drive (Niven and Pournelle 1974). No preferred frame is stated; the jump is simultaneous in some frame the text never names. Secondary accounts say Dan Alderson chose limits to avoid paradox, unsourced in the text.
• SILENT: The Expanse: the ring gates and the slow zone (Corey 2013 onward). The hub is a pocket outside ordinary space and would serve as a preferred frame; the text never uses it that way or names any frame.
• SILENT: The Hitchhiker's Guide to the Galaxy: the Infinite Improbability Drive and bistromathics. No preferred frame is stated or implied. The drive has no frame at all, being everywhere, which is a joke, not a rule.
• SILENT: The Lensman inertialess drive (E. E. Smith, 1934 onward). The series assumes one galactic clock and one "now" without ever stating a preferred frame; the escape hatch is used unknowingly and never named.
• SILENT: The Orville: quantum drive (2017 onward). The Union's single clock is the preferred frame the drive would need and is never named as such.
• SILENT: The Skylark of Space (E. E. Smith, 1928). The book has an implicit preferred frame, the frame in which stopwatch distances are "facts", but never names it or notices that it needs one.
• SILENT: Thor (2011): the Bifrost. No preferred frame is offered. Heimdall's real-time sight across the realms implies a universal "now" but the film never says so.
• SILENT: Warhammer 40,000: the Warp (1987 onward). No preferred frame is stated; the Warp has no clock rather than a different one, and the setting does not use that to avoid anything.
• SILENT: Weber's Honorverse: hyper bands and Warshawski sails (1993 onward). No preferred frame is stated; "relativistic physics applies at any given point" is local and says nothing about frames for the FTL sector.
• SILENT: Wing Commander (1990) and the 1999 film: jump points. No preferred frame; the war has one calendar and nobody says why.
• SILENT: Xenocide: travel through the Outside. An implicit universal now runs through the series, which would be a preferred frame if named; it never is.
• RULE: Asimov's Foundation: the Jump through hyperspace (1942 onward). The Jump takes no time at all in the Galaxy's one calendar ("between two neighboring instants", Foundation I.1), which fixes the FTL speed in one frame; the stated rule is the preferred-frame escape, though the series never says that is what it is for.
• RULE: Campbell's Islands of Space (1931): where the word came from. Chapter 9's rule that the outside universe sees an instantaneous displacement while the crew age fixes the FTL motion in Earth's frame; the stated rule is the preferred-frame escape, unrecognised as such.
• RULE: Le Guin's ansible. The "constant of simultaneity" and the one-end-fixed-on-a-massive-planet rule (Left Hand, Word for World) together fix a frame for the signal, which is the register's escape hatch taken without being named
• RULE: The Forever War: collapsar jumps (Haldeman 1974). "Redefine simultaneity" is, read literally, the preferred-frame escape: the fiction asserts that simultaneity was re-fixed to make the jump consistent. The rule is stated in one clause and never elaborated, but it is the work's own answer.
• RULE: Time for the Stars' telepathy. Babcock's assertion in ch. VIII that simultaneity is real and physics wrong to discard it is the preferred-frame escape stated as a rule; the book pays the price the register names by rewriting physics into "irrelevance" in chs. XIII and XVI
• RULE: Timescape's tachyon message. Both transmitter and receiver sit on Earth, so the channel has fixed endpoints in one frame, which is one of the escape hatches the register lists; the novel takes it without naming it
• RULE: Vinge's Zones of Thought: FTL only where physics permits it (1992). The maximal speed is set by position in the Zone structure, which is at rest with respect to the galaxy; the stated rule fixes the FTL speed in one frame as a property of the medium, which is the entry's escape, though the novel never says that is what it does.

Sources

• Liberati, Sonego and Visser, Ann. Phys. 298, 167 (2002), arXiv:gr-qc/0107091 ✓