CAU-3: Two devices make a time machine

Wormholes and warp drives are not merely fast: they are time machines with an assembly step.

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

The result

A wormhole that can be created and maintained for interstellar travel can be converted into a time machine: take one mouth on a relativistic round trip. Everett shows a simple modification of Alcubierre's model, with no extra assumptions, in which two warp bubbles give closed causal loops

What it forbids or costs

Wormholes and warp drives are not merely fast: they are time machines with an assembly step

Escape hatches

Chronology protection (CAU-2) if it holds

Who it bites

Physics proposals

• VIOLATES: Alcubierre 1994: the warp drive metric. Everett (1996) shows that a simple modification of this exact model with two bubbles gives closed causal loops with no extra assumptions. Alcubierre himself anticipated that a spacetime with closed causal curves could be built on the same idea.
• VIOLATES: Everett and Roman 1997: the superluminal subway. Section 4 constructs the time machine from two non-overlapping tubes explicitly, with the ship leaving Earth at $t = 2D$ and returning at $t \approx 0$, and notes the analogy to Everett's two bubbles and the two-wormhole system.
• VIOLATES: Fell and Heisenberg 2021: positive energy from hidden geometric structure. Everett's two-bubble loop drops exactly the global hyperbolicity the paper leans on, and the Shoshany–Snodgrass gluing of two non-unit-lapse drives into a closed timelike geodesic reaches this class; the paper does not consider two devices
• VIOLATES: Krasnikov 1998: the tube laid on the way out. The paper says the vehicles are square roots of time machines and describes the two-curve construction that closes a timelike loop; Everett and Roman carry it out with two non-overlapping tubes.
• VIOLATES: Lentz 2021: hyper-fast positive-energy solitons. The soliton is a unit-lapse flat-slice metric of exactly the kind Everett modifies to close a causal loop with two bubbles; Celmaster and Rubin note that Shoshany has shown a Lentz drive, if built, "could be configured for time travel"
• VIOLATES: Morris, Thorne and Yurtsever: the wormhole time machine (1988). This is the source paper for CAU-3: a maintainable wormhole plus one relativistic round trip of a mouth is a time machine.
• VIOLATES: Morris–Thorne traversable wormhole (1988). This is the wormhole CAU-3 was written about: take one mouth on a relativistic round trip and the shortcut becomes a time machine (Morris, Thorne and Yurtsever 1988).
• VIOLATES: Natário 2002: warp drive with zero expansion. Everett's two-bubble loop uses only superluminal transport between flat regions plus a boost of the second bubble's exterior, which every member of the class provides; the profile of $X$ plays no role.
• VIOLATES: Van Den Broeck 1999: the pocket geometry. The exterior is Alcubierre's, so Everett's two-bubble construction applies verbatim.
• VIOLATES: Visser's thin-shell wormholes (1989). The Morris–Thorne–Yurtsever round trip applies unchanged; the paper names the causal constraints as an open question rather than answering them.
• VIOLATES: Warp drives in modified gravity. Everett's two-bubble construction is a statement about the metric and a boost, and the metric is unchanged.
• SATISFIES: Einstein–Rosen bridge (1935). The conversion to a time machine needs a wormhole that can be maintained and traversed; this one pinches off before one crossing, so the assembly step never starts.
• SATISFIES: ER = EPR (Maldacena and Susskind 2013). The conversion needs a wormhole that can be maintained and crossed; the bridge is non-traversable by assumption, and if it were traversable the conjecture would be wrong by its own statement.
• SATISFIES: Fuchs et al. 2024: the constant-velocity physical warp shell. Everett's construction needs two superluminal bubbles; two of these shells are two slow spaceships
• SATISFIES: Gao, Jafferis and Wall: traversable wormholes via a double-trace deformation (2017). The conversion by boosting one mouth is explicitly excluded because the deformation breaks the $H_L - H_R$ symmetry; this is a wormhole that cannot be made a time machine.
• SATISFIES: Maldacena, Milekhin and Popov: traversable wormholes in four dimensions (2018), and the humanly traversable sequel (2021). The boost-one-mouth conversion fails because the through-time already exceeds the around-time by construction; the register's assembly step cannot close a loop.
• SATISFIES: Wheeler's geons and quantum foam (1955–1962). The conversion to a time machine needs a wormhole that can be maintained and crossed; these cannot be, so the assembly step is never reached.
• SILENT: Bobrick and Martire 2021: physical warp drives. Cites Everett 1996 in passing and never examines two-device loops; its superluminal classes would fall to Everett's construction and its subluminal class would not
• SILENT: NASA Eagleworks warp field interferometer. Never mentioned
• SILENT: White et al. 2021: the worldline-numerics Casimir pattern. Two devices are never considered, though the proposed experiment is an array of many

Fictional drives

• SILENT: 2001: A Space Odyssey (1968): the Star Gate. A network of fixed gates between moving worlds is the moving-mouth configuration the entry turns into a time machine; unaddressed in any instalment, including the sequels.
• SILENT: Asimov's Foundation: the Jump through hyperspace (1942 onward). Two Jump ships are the register's time machine with an assembly step; the Galaxy has millions and the question is not raised.
• SILENT: Babylon 5: hyperspace, jump gates and beacons (1993 to 1998). Every capital ship carries jump engines; two of them are the register's time machine, and the show does not raise it.
• SILENT: Banks's Culture: the energy grid, infraspace and ultraspace (1987 onward). Every Culture ship is one half of the register's two-device time machine; the fleet is enormous and the question is not asked.
• SILENT: Battlestar Galactica: the FTL jump (2004 series). The drive makes no wormhole with mouths to move; the time-machine construction never arises.
• SILENT: Campbell's Islands of Space (1931): where the word came from. Two of Arcot's ships would be Everett's two bubbles; the novel builds one and never asks.
• SILENT: Commonwealth: CST wormholes with trains through them (Hamilton 2004). The Second Chance is a wormhole mouth carried on a relativistic round trip, which is exactly the CAU-3 construction, and the text does not raise it.
• SILENT: Contact: the Machine and the tunnel system (Sagan 1985, Zemeckis 1997). The tunnel mouths are fixed at the stars they serve, but no rule says they are immovable, and nobody in the work considers what moving one would do. The one-mouth round trip that makes a wormhole a time machine is never raised.
• SILENT: Cordwainer Smith: planoforming, Space² and Space³. Two planoform ships in different states of motion are never set against each other; no time machine is built or feared.
• SILENT: Cowboy Bebop (1998): the astral gates. Fixed gates on planets orbiting at different speeds is the moving-mouth configuration of the entry; never raised.
• SILENT: Dune: the Guild navigators and folding space (Herbert 1965). The work does not describe a wormhole with mouths, so nothing is said about moving one; the time-machine construction is never in view.
• SILENT: Elite Dangerous: the frame shift drive (Frontier Developments, 2014). Thousands of player bubbles in motion at once is the many-bubble case in which Everett's construction gives closed loops; never considered.
• SILENT: EVE Online: the warp drive (CCP Games, 2003). A stargate network of fixed pairs is the case in which one boosted mouth makes a time machine; never considered.
• SILENT: Event Horizon (1997): the gravity drive. The drive is a wormhole made on demand, the exact object the entry turns into a time machine, and no instalment considers a second gateway or a moved mouth.
• SILENT: FTL: Faster Than Light, the jump drive as a game mechanic. Two drives in relative motion are every encounter, and the row's construction is never noticed.
• SILENT: Futurama: the Planet Express ship's dark matter engines (1999 onward). Every ship in the year 3000 is a moving bubble on Cubert's account, which is the many-bubble time machine of Everett; never noticed.
• SILENT: Guardians of the Galaxy Vol. 2 (2017): jump points. A network of fixed points between orbiting worlds is the moving-mouth configuration that becomes a time machine; the franchise's time travel is elsewhere and by another mechanism.
• SILENT: Halo: slipspace and the Shaw-Fujikawa translight engine (2001 onward). Two translight engines are the register's time machine with an assembly step; the war has thousands and the question never comes up.
• SILENT: Homeworld: hyperspace cores and the Far Jumpers. Three cores in relative motion are the sequel's whole plot and the question of a time machine is not put.
• SILENT: Hyperion: the farcasters (Simmons 1989). Portals are installed and fixed, but no rule says one cannot be moved and the time-machine construction is never raised.
• SILENT: Interstellar: the wormhole near Saturn (2014). The wormhole's two mouths stay where they were placed; nothing in the film moves a mouth or asks what a boosted mouth would do, and the time loop is routed through Gargantua instead.
• SILENT: Mass Effect: element zero and the FTL drive (BioWare, 2007). Relays are a fixed network of shortcuts and the drive is a ship; the two-device time machine of CAU-3 would apply to relay pairs, and no instalment considers it.
• SILENT: Mass Effect: the mass relays (BioWare 2007 onward). Relays are fixed and enormous, but no rule says they cannot be moved and one is destroyed in "Arrival"; the time-machine construction is never raised.
• SILENT: Niven's Known Space: the Outsider hyperdrive and the blind spot (1966 onward). Two hyperdrive ships are the register's time machine; Known Space has thousands and never assembles the loop.
• SILENT: No Man's Sky: the hyperdrive, warp cells and range. Ships in relative motion warp constantly and nothing is made of it.
• SILENT: Space Battleship Yamato: warp by wave motion engine. Gamilas ships warp too, in relative motion to Yamato, and no episode makes a time machine of it or notices that it could.
• SILENT: Star Citizen: quantum drive and jump points. Paired points in systems moving relative to one another is the moving-mouth configuration of the entry; never raised.
• SILENT: Star Trek: transwarp, quantum slipstream and the other beyond-warp drives. The Borg hub is a network of fixed shortcuts, which is the wormhole-network case in which CAU-3 says one boosted mouth makes a time machine; never considered.
• SILENT: Star Wars: hyperspace (1977 onward). Two ships with hyperdrives are the register's time machine with an assembly step; the films fly thousands of them and never notice.
• SILENT: Starfield (2023): the grav drive. Ships jumping between systems in relative motion is the moving-mouth configuration of the entry; never raised.
• SILENT: Stargate: the ships' hyperdrives. Two hyperdrive ships in relative motion are a routine event and are never used as a time machine; the row's construction is never noticed.
• SILENT: Starman Jones: the congruencies (Heinlein 1953). The anomalies are fixed features of space and are not moved; the time-machine construction is never raised.
• SILENT: Stellaris: warp, hyperlane and wormhole (Paradox Development Studio, 2016). Natural wormhole pairs and gateway networks are the fixed-shortcut case in which one boosted mouth makes a time machine; never considered.
• SILENT: Super Dimension Fortress Macross (1982): the space fold. Folding ships and fleets in relative motion is the moving-mouth configuration of the entry; never raised in any instalment.
• SILENT: The Alderson drive (Niven and Pournelle 1974). The tramline endpoints are fixed by the stars and are not moved, but no rule says what happens if the stars move relative to each other, and the time-machine construction is never raised.
• SILENT: The Expanse: the ring gates and the slow zone (Corey 2013 onward). The rings are fixed at their systems and the hub is fixed, but no rule says a ring cannot be moved and the time-machine construction is never raised.
• SILENT: The Forever War: collapsar jumps (Haldeman 1974). The collapsars are fixed stars and nobody moves one; the two-mouth time machine never arises, and the book does not ask whether two collapsars in relative motion would give it.
• SILENT: The Hitchhiker's Guide to the Galaxy: the Infinite Improbability Drive and bistromathics. Two improbability drives are never run against each other, and no instalment builds a time machine from the drive; the time travel in the series is by other means (Milliways, the Bistromath, the sofa).
• SILENT: The Orville: quantum drive (2017 onward). A fleet of Alcubierre bubbles is exactly the two-bubble time machine of Everett; the show has a fleet and does not raise it.
• SILENT: Thor (2011): the Bifrost. The Bifrost is a wormhole by the film's own naming, with one mouth on Asgard and one wherever Heimdall aims, which is the moving-mouth configuration of the entry; the time-machine consequence is never raised in any instalment.
• SILENT: Vinge's Zones of Thought: FTL only where physics permits it (1992). Two ultradrive ships are the register's time machine; the Beyond has millions and never assembles the loop.
• SILENT: Warhammer 40,000: the Warp (1987 onward). Two warp-capable ships are the register's time machine; the setting has millions and never assembles the loop.
• SILENT: Weber's Honorverse: hyper bands and Warshawski sails (1993 onward). Two hyper-capable ships are the register's time machine; the navies have thousands and never assemble the loop.
• SILENT: Wing Commander (1990) and the 1999 film: jump points. Fixed points between orbiting, moving systems is the moving-mouth configuration of the entry; never raised.
• SILENT: Xenocide: travel through the Outside. Two Outside-capable ships in relative motion are never considered; the fleet that matters is sublight.
• ENGAGES: Andromeda: the slipstream. One drive, one transit, one time machine: the sixth episode does with a single slipstream jump what CAU-3 needs two devices for, and the work accepts the result.
• ENGAGES: Doctor Who: the TARDIS and the time vortex. Two TARDISes in one place give a "time ram" and a temporal void ("The Time Monster", 1972); the closed loop of "Blink" (2007) is a time machine used to build its own instructions. The work does not need two devices to make a time machine because the one device already is one.
• ENGAGES: Farscape (1999): wormholes, and starburst as a separate thing. "Kansas" (4.12): a wormhole delivers the crew to Earth in the 1980s and history has already changed; the wormhole-as-time-machine is enacted, with the consequences shown and repaired.
• ENGAGES: Outer Wilds (2019): Nomai warp cores. The Ash Twin Project is the entry's construction, a wormhole converted into a time machine, built deliberately by amplifying the warp's negative interval with supernova energy; the assembly step is the plot.
• ENGAGES: Star Trek: warp drive (1966 onward). The slingshot effect is a warp drive plus a gravity well used as a time machine, on screen in "Tomorrow Is Yesterday", "Assignment: Earth" and Star Trek IV; the register's two-bubble construction has the same shape and the show depicts the consequence.
• ENGAGES: Stargate: the gate network (1994 film, SG-1 1997 onward). The show turns a wormhole into a time machine on screen, repeatedly, and builds episodes on it. The mechanism (a flare intersecting the wormhole) is not the register's boosted mouth, but the phenomenon is the one CAU-3 describes: a wormhole that can be created and maintained for travel can be converted into a time machine. Gates are also moved by ship in the series, with no comment on the consequence.

Sources

• Morris, Thorne and Yurtsever, Phys. Rev. Lett. 61, 1446 (1988); Everett, Phys. Rev. D 53, 7365 (1996) ✓