LOR-2: Tachyons are an instability

L1 by particle physics is closed: the tachyon is an instability, and if it were a particle it would be a time machine.

Kind: constraint · Family: LOR (Kinematics) · Last reviewed: 2026-09-12

The result

Tachyons. Bilaniuk, Deshpande and Sudarshan re-examine faster-than-light particles within classical special relativity: such a particle has spacelike four-momentum, is always faster than light and can never slow below it. Feinberg builds a Lorentz-invariant field theory for them. Aharonov, Komar and Susskind show that causal superluminal group velocities arise as small oscillations about an unstable configuration, so the field-theory tachyon is an instability, not a courier. Tachyonic particles would in any case give the antitelephone (CAU-1)

What it forbids or costs

L1 by particle physics is closed: the tachyon is an instability, and if it were a particle it would be a time machine

Escape hatches

None found in nearly sixty years of searching; Lorentz-violating frameworks

Who it bites

Physics proposals

• VIOLATES: Alcubierre 1994: the warp drive metric. For $v_s > 1$ the world line of the leading edge of the wall matter is spacelike (Krasnikov), so a bubble created on demand is sourced by tachyonic matter, which LOR-2 says is an instability rather than a courier. The only way out is CON-2's pre-laid route.
• VIOLATES: Meta relativity (Bilaniuk, Deshpande and Sudarshan 1962). This is the paper the register cites first under LOR-2; the classical kinematics are correct and the quantised version was shown by Aharonov, Komar and Susskind to describe an instability, while every search since 1968 has been null.
• VIOLATES: Natário 2002: warp drive with zero expansion. For $v_s > 1$ the wall matter ahead of the ship lies outside its light cone and moves on spacelike world lines in the exterior frame, so creating the bubble on demand needs tachyonic matter; the class inherits Krasnikov's objection to Alcubierre.
• VIOLATES: Tachyons (Feinberg 1967). This is the entry that closes the proposal: Aharonov, Komar and Susskind show the causal superluminal group velocities are small oscillations about an unstable configuration, and nearly sixty years of searches (Alväger and Kreisler, Baltay et al., Danburg et al., the tritium endpoint, OPERA) have found nothing.
• VIOLATES: Van Den Broeck 1999: the pocket geometry. The paper states that part of the energy in the region moves tachyonically, which LOR-2 says is not a courier; this is the problem it defers.
• VIOLATES: Warp drives in modified gravity. The spacelike leading edge of the wall matter for $v_s > 1$ is a statement about the metric; in Einstein-Cartan theory that matter carries the spin, so it is the spin fluid itself that would have to move tachyonically on a first flight.
• DODGES: Gott's cosmic-string time machine (1991). No tachyon is present, yet the two-string system has the holonomy, and so the total energy-momentum, of a tachyon (Deser, Jackiw and 't Hooft); the construction obtains a spacelike total momentum from subluminal parts, which Carroll et al. show cannot be reached from timelike initial data in an open universe.
• DODGES: Lorentz-violating frameworks (the Standard-Model Extension and Hořava-Lifshitz gravity). The register lists "Lorentz-violating frameworks" as the escape hatch; these frameworks use positive $m^2$ and modified maximal velocities, never a tachyonic mass term, so the instability argument does not arise.
• SATISFIES: Everett and Roman 1997: the superluminal subway. The construction excludes tachyonic signals by design; the forward light cone of the launch contains every modification.
• SATISFIES: Krasnikov 1998: the tube laid on the way out. The whole construction is designed to avoid tachyons: utter causality is imposed precisely so that nothing, matter or metric, acts outside the light cone, and Proposition 1 is proved under that assumption.
• SATISFIES: The OPERA superluminal neutrino (2011). The tachyonic-neutrino reading predicts $(v - c)/c \propto 1/E^2$; the observed energy independence of $\delta t$ disfavours a special-relativistic tachyon (Amelino-Camelia et al. 2011) and the corrected null result removes the need for one.
• SILENT: Bobrick and Martire 2021: physical warp drives. Class II "superluminal matter", at rest in a spacelike frame and violating the dominant energy condition, is tachyonic matter in all but name; the paper neither engages the instability result nor claims such matter exists

Fictional drives

• SILENT: Babylon 5: hyperspace, jump gates and beacons (1993 to 1998). The beacon and long-range communications are called tachyon signals, which is the register's instability used as a courier; the show uses the word and never addresses what it implies.
• SILENT: Flight of the Navigator (1986). A ship that starts on a Florida lawn and reaches two million c has crossed the barrier, which tachyon kinematics forbids; the film never shows or discusses the crossing.
• SILENT: Mass Effect: element zero and the FTL drive (BioWare, 2007). Ships cross c from below with the field on; tachyon kinematics forbids the crossing for any particle and the games never examine the moment of crossing.
• SILENT: Space Battleship Yamato: warp by wave motion engine. The reference gloss says the engine runs on tachyons; if that is in the scripts, the series names LOR-2's particle as a fuel and says nothing about what a tachyon is. Recorded as silent pending verification against the 1974 dialogue.
• SILENT: The Lensman inertialess drive (E. E. Smith, 1934 onward). A ship that starts at rest and ends above c crosses the barrier, which tachyon kinematics forbids for any particle; the crossing is instantaneous in the fiction and never examined.
• SILENT: The Skylark of Space (E. E. Smith, 1928). The ship crosses c from below, which is what tachyon kinematics forbids for any particle; the book does not raise the crossing.
• ENGAGES: Revelation Space lighthuggers. The state-four "tardyonic to tachyonic" attempt is a tachyon drive in all but name, and it fails catastrophically; the series' verdict agrees with the register's
• ENGAGES: Star Trek subspace radio. Tachyons are named and used weekly, and Sisko states their defining property on screen; the franchise asserts as a working tool the particle the register records as an instability
• ENGAGES: Timescape's tachyon message. Feinberg and Sudarshan are named, imaginary mass and no rest frame are stated; the novel asserts the particle the register says is an instability, and the author has said he thought it unlikely by the time he wrote

Sources

• Bilaniuk, Deshpande and Sudarshan, Am. J. Phys. 30, 718 (1962); Feinberg, Phys. Rev. 159, 1089 (1967); Aharonov, Komar and Susskind, Phys. Rev. 182, 1400 (1969) ✓