No-Communication Theorem (why entanglement can't signal)
No-Communication Theorem (why entanglement can't signal)
One-line summary: Entanglement produces correlations but not a controllable channel — a local measurement can't change the other party's statistics, so no information (or matter) crosses faster than light.
The insight
This is the formal answer to the most common entanglement misconception — including the speculation in this thread's first source that a civilization might use entanglement for instantaneous communication or travel (see quantum-entanglement). The no-communication theorem proves that measuring one half of an entangled pair cannot transmit information to the holder of the other half, at any distance. Entanglement's correlations are real and non-local, but they are not a usable signal: extracting them requires comparing results over a classical channel limited by the speed of light.
Evidence
- From 2026-05-30-autoresearch-quantum-entanglement: For a joint state on H = H_A ⊗ H_B, whatever Alice does locally, "the relative state of Bob's system after Alice's operation" — the partial trace over Alice's system — is invariant; "Bob cannot in any way distinguish the pre-measurement state from the post-measurement state." (Wikipedia: No-communication theorem)
- From 2026-05-30-autoresearch-quantum-entanglement: each party's local outcomes are random; "the 'global' information about the correlation can only be established through a classical channel limited by the speed of light." The no-signaling principle is built into the entanglement framing itself (Grokipedia).
- Peer-reviewed handle (from 2026-05-30-academic-research-quantum-entanglement): no-signaling is load-bearing enough to ground cryptographic security — some device-independent QKD proofs "exploit only the no-signaling principle" (Pironio et al. 2009, NJP). See device-independent-quantum-key-distribution.
Why it matters
It resolves the open question carried on quantum-entanglement (from JRE #2506): entanglement is not a faster-than-light communication or travel mechanism. It also bounds what quantum networks can do — they distribute entanglement and correlations, but security/teleportation protocols always require an accompanying classical channel.
Contradictions / tensions
- A literature search (2026-05-30-academic-research-quantum-entanglement) surfaced fringe papers claiming FTL signaling is possible (Szabó 2024; Holton 2019; Smarandache 2025) — all with zero/near-zero citations in non-indexed venues, contradicting a textbook result with no replication. They are not credible and must not inform this page; recorded only so a future ingest doesn't mistake them for evidence. Treat with the limits-of-science posture: extraordinary, unreproduced.
Open questions
- None substantive. The theorem is settled; "circumvention" proposals have not survived scrutiny.