Why quantum entanglement doesn't allow faster-than-light communication (2016)

Use of Entanglement and FTL in Fiction

• Several comments critique “3 Body Problem” for using quantum entanglement as real-time FTL communication, contrasting it with the more “plausible” tech in The Expanse.
• Some argue the story could mostly work with a trusted local AI representative instead of actual FTL comms, at least in the early Netflix season. Others insist that real‑time information from Earth is plot‑critical later.

Why Entanglement Can’t Send Messages

• Repeated point: you cannot distinguish a measured from an unmeasured or entangled from a non‑entangled particle locally.
• Any interaction counts as a measurement. There is no experiment on one side alone that reveals whether a remote measurement has occurred.
• Outcomes at each end are always random (typically 50/50); only correlations emerge when both sides later compare data via classical communication.
• Thus no bit (“measured/not measured” or “0/1”) can be reliably encoded or decoded FTL, even though correlations are “instantaneous”.

Bell’s Theorem, CHSH, and Hidden Variables

• Multiple comments reference Bell’s theorem and the CHSH inequality: local hidden‑variable models predict weaker, linear correlations than quantum mechanics.
• Experiments violate these inequalities, ruling out simple “pre‑set values in a box” or glove/coin analogies as complete explanations.
• Some mention superdeterminism or global hidden variables as logical loopholes, but these are treated as exotic and unsatisfying.

Experiments and “Spookiness”

• Delayed‑choice and quantum eraser experiments are cited as especially mind‑bending; others stress they don’t allow retrocausal signalling, only “spooky correlation”.
• Debate over terminology: “spooky action” suggests a physical influence; some prefer “spooky correlation without causation” to avoid implying a propagating signal.

Interpretations and Ontology

• Many‑Worlds is highlighted as local and realist, removing objective “collapse” but introducing branching worlds.
• Others see the wavefunction as just a prediction tool, prompting pushback: if it’s mere bookkeeping, what is it bookkeeping for?
• Decoherence is described as what turns entanglement into ordinary classical correlation by destroying phase relationships.

Broader Reflections

• Several comments lament that entanglement doesn’t rescue us from the light‑speed limit for communication, reinforcing how hard interstellar coordination and exploration will be.
• There is recurring confusion between “speed of light” and “speed of information/causality”; commenters emphasize that quantum theory blocks FTL signalling without explicitly invoking relativity.