CAU-5: The no-communication theorem

Every "entanglement radio" and quantum ansible.

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

The result

No-communication theorem: measurements on one half of an entangled pair cannot transmit information to the other half. The statistics of one party's results are unaffected by anything the other party does

What it forbids or costs

Every "entanglement radio" and quantum ansible

Escape hatches

None within quantum mechanics. Nonlinear modifications of quantum mechanics would allow signalling and are constrained for that reason (Gisin 1990, Polchinski 1991, not yet verified here)

Who it bites

Physics proposals

• VIOLATES: Entanglement signalling and the no-communication theorem. This is the entry that closes the proposal. The radio requires Bob's statistics to depend on Alice's operation; the partial-trace argument shows they cannot. Nonlinear quantum mechanics would reopen it (Gisin 1990, Polchinski 1991, both verified here) and is excluded for that reason (Simon, Bužek and Gisin 2001)
• VIOLATES: Retrocausal signalling proposals. The scheme requires Bob's singles statistics to depend on Alice's choice. The partial trace shows they do not; Cramer and Herbert's case-by-case analysis finds the fringes and anti-fringes always cancel
• SATISFIES: ER = EPR (Maldacena and Susskind 2013). The paper invokes the no-communication theorem directly ("no local operation on one member of a entangled pair can influence the other") and builds the identification on it; the bridge is the geometric face of a correlation that carries no information.
• SATISFIES: Gao, Jafferis and Wall: traversable wormholes via a double-trace deformation (2017). The teleportation reading needs the coupling as its classical channel; entanglement alone (the undeformed thermofield double) transmits nothing, in agreement with the no-communication theorem.
• SATISFIES: Maldacena, Milekhin and Popov: traversable wormholes in four dimensions (2018), and the humanly traversable sequel (2021). The entangled-black-hole reading transmits nothing faster than light; the fermion exchange that generates the coupling is itself an ordinary field propagating through the outside.

Fictional drives

• SILENT: Ender's philotic ansible. Paired particles whose partner forms and deforms as the sender does is the entanglement-radio claim under another name; the theorem that closes it is never touched, and Card disclaims any real-physics basis
• SILENT: Le Guin's ansible. The texts name no quantum mechanism, so the theorem has nothing to close on the page; every later attempt to give the ansible one is the entanglement radio and is closed by it
• SILENT: Mass Effect: element zero and the FTL drive (BioWare, 2007). The quantum entanglement communicator of Mass Effect 2 sends messages instantly by entanglement; the Codex entry does not address the no-communication theorem. Scored here because the device is part of the same setting, not the drive.
• SILENT: Revelation Space lighthuggers. The Exordium is described as a quantum, superposition-based link to other selves; the no-communication theorem is not mentioned and the text does not say how the channel evades it
• ENGAGES: Avatar's Venture Star. The official companion asserts a "Superluminal Communications" link "using McKinney quantum entanglement encoding" at a few bits an hour; the no-communication theorem forbids exactly this at any bit rate
• ENGAGES: Xenocide: travel through the Outside. The philote is the novel's stated mechanism and it does on the page what the no-communication theorem forbids: correlated parts carry information (the ansible) and, in Xenocide, matter. The work addresses the subject of the row directly and asserts the opposite of the theorem, which is what the entanglement signalling dossier prices.

Sources

• Eberhard, Nuovo Cimento B 46, 392 (1978); Ghirardi, Rimini and Weber, Lett. Nuovo Cimento 27, 293 (1980); Peres and Terno, Rev. Mod. Phys. 76, 93 (2004), arXiv:quant-ph/0212023, §II.E ✓ (escape-hatch sources ⚠ v1)