LOR-3: Front velocity is the signal velocity

Every L5 claim built on a group velocity or a tunnelling time.

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

The result

Phase velocity and group velocity may exceed c; the front velocity of a signal cannot. Sommerfeld and Brillouin showed the wavefront in any causal medium travels at exactly c. Tunnelling "superluminality" (the Hartman effect, the Nimtz experiments, the single-photon measurement of Steinberg, Kwiat and Chiao at 1.7c) is not a genuine signal velocity: Winful shows the group delay in tunnelling is a lifetime of stored energy, not a transit time. The gain-assisted pulses of Wang, Kuzmich and Dogariu are classical interference between frequency components in anomalous dispersion. Neither is a signal beating its own front

What it forbids or costs

Every L5 claim built on a group velocity or a tunnelling time

Escape hatches

None; this is settled

Who it bites

Physics proposals

• VIOLATES: The Hartman effect and the Nimtz tunnelling claims. The claim requires the front-velocity theorem to fail for evanescent modes. Winful shows the group delay is a cavity lifetime, that no point of the transmitted pulse leaves the light cone of the incident pulse and that the experiments contain no evidence of superluminality. LOR-3 holds the verified account
• DODGES: Lorentz-violating frameworks (the Standard-Model Extension and Hořava-Lifshitz gravity). Front velocity equals the species' maximal velocity $c_a$, and the Sommerfeld-Brillouin argument goes through with $c$ replaced by $c_a$; the settled result is respected in form and evaded in value.
• DODGES: The Scharnhorst effect. LOR-3's front-velocity argument assumes a causal medium with n → 1 at high frequency. The Casimir vacuum has n < 1 at all frequencies in the effective theory, and Barton and Scharnhorst argue the signal velocity is modified too. The register should note this as the one principled exception, of size $10^{-32}$
• SATISFIES: Brane-bulk shortcuts (Chung and Freese 2000, Ishihara 2001, Caldwell and Langlois 2001). The front velocity of a bulk signal is the bulk $c$; the apparent brane superluminality is a path-length effect, not a front outrunning its own light cone.
• SATISFIES: Gain-assisted superluminal light propagation. This is the entry's own worked example. The paper claims a group velocity above c and a front velocity of c, and says so; the FTL reading would violate LOR-3 and is refuted by the same analysis (K0)
• SATISFIES: Kaluza-Klein and the hyperspace reading (Kaluza 1921, Klein 1926, ADD 1998, Randall-Sundrum 1999). Bulk and brane fields have hyperbolic equations with the bulk light cone as characteristic; front velocities are $c$.
• SATISFIES: Tachyons (Feinberg 1967). The tachyonic Klein-Gordon equation is hyperbolic with characteristics at c, so the front velocity of any disturbance is c even though the group velocity exceeds it; this agreement with LOR-3 is precisely why the field cannot carry a signal faster than light.
• SATISFIES: The OPERA superluminal neutrino (2011). The bunched-beam runs of OPERA and ICARUS measure arrival of individual interactions against individual proton bunches, a front measurement not a group-velocity inference, and they agree with c.
• SILENT: NASA Eagleworks warp field interferometer. The planned time-of-flight experiment would time pulses through a test article; the entry's front-velocity result is what any such advance would have to answer, and it is never mentioned
• SILENT: White et al. 2021: the worldline-numerics Casimir pattern. The proposed test is a transit-time comparison for a pulse through a micron structure, exactly the measurement the entry warns about: group delay in a structure is not a signal velocity, and the paper does not distinguish them

Fictional drives

• SILENT: Alien's Nostromo. The distress beacon, Ripley's log and the colony's two-week answer are signals with no stated speed; a signal limit applies and is never addressed
• SILENT: Firefly's Verse. Waves are conversational across the system with no stated delay; a signal-speed limit applies and is never addressed
• SILENT: Tau Zero's ramjet. Contact with Earth is lost as the ship accelerates, but no fetched source shows the text treating the speed or cost of a signal home; the constraint applies and the work does not touch it
• ENGAGES: Passengers' Avalon. The message booth's nineteen years out and fifty-five for a reply is a light-speed signal limit stated on screen and accepted by the plot; the figures do not match the stated speed, and the film does not address the gap
• RULE: Aurora's generation ship. The only link home is beamed light and light-speed messages, under the author's stated rule of no FTL communication; the late laser is a consequence of that rule
• RULE: Avatar's Venture Star. The companion's "Lightspeed Communications" are the stated rule for the ship's ordinary links; the film never shows a message between the stars
• RULE: Revelation Space lighthuggers. The author's stated rule of no FTL travel or communication keeps every signal at or below c; the decades of light-lag that structure Redemption Ark follow from it
• RULE: Star Trek subspace radio. The signal has a stated finite speed through a stated other medium, with range limits and relays; the rule answers the front-velocity constraint by leaving ordinary space rather than by beating it

Sources

• Brillouin, Wave Propagation and Group Velocity (1960); Steinberg, Kwiat and Chiao, Phys. Rev. Lett. 71, 708 (1993); Wang, Kuzmich and Dogariu, Nature 406, 277 (2000); Winful, Phys. Rep. 436, 1 (2006) ✓