Does Quantum Entanglement Act Faster Than Light?

TL;DR

Quantum mechanics predicts non-local correlations between entangled particles, requiring the state of a distant partner to adjust instantly when one particle is measured. Einstein argued that this apparent faster-than-light influence exposed an incomplete theory and favored local hidden variables, while John Bell later devised a way to distinguish Einstein’s local explanation from quantum mechanics experimentally.

Transcript

• In 1935. Einstein came up with a thought experiment that showed quantum mechanics breaks one of the most sacred principles in physics, that nothing can go faster They thought that at 56 because it was too radical. But 30 years later, one man stumbled across realized something, the prediction Quantum physics really does break the universal speed l... Read More

Key Insights

• Locality is the principle that physical effects spread between neighboring regions rather than acting instantaneously across arbitrary distances. Einstein’s theory of gravity preserved locality by describing spacetime disturbances that propagate outward at light speed instead of having gravity change everywhere at once.
• Instantaneous gravity is incompatible with relativity because observers moving differently can disagree about which distant event happened first. If gravity acted immediately, one observer could see Earth respond before the sun disappeared, reversing the expected order of cause and effect.
• A quantum wave function assigns multiple possible outcomes and their probabilities before measurement. When an electron is detected at one point, the probability of finding that same electron elsewhere becomes zero, making the measurement appear to influence the wave function across space instantly.
• Entanglement is a shared quantum state in which two particles have linked properties. In the EPR example, an electron and positron must have opposite spins, yet quantum mechanics does not assign either particle one definite spin result before a measurement occurs.
• The EPR argument presents two possible explanations for entangled measurements. Either wave-function collapse produces non-local action between distant particles, or the particles carry predetermined local information, called hidden variables, that tells each particle which result to produce.
• The Copenhagen interpretation treats the wave function as the complete description needed to predict measurements. Bohr considered questions about what particles are doing independently of measurement unnecessary, while Einstein believed the interpretation avoided rather than solved the underlying locality problem.
• Bohr’s authority helped marginalize the EPR challenge even though his published response was famously obscure. Because Copenhagen quantum mechanics and local hidden-variable theories initially produced the same predictions for the basic EPR setup, many physicists adopted a practical attitude and stopped debating interpretation.
• Bell’s central contribution was to search for a modified entanglement experiment in which quantum mechanics and local hidden-variable theories would predict different results. His work reopened a dispute widely treated as settled and transformed entanglement from a neglected conceptual issue into an experimentally testable question.

Questions & Answers

Q: Why did Einstein conclude that gravity cannot act instantly?

Einstein’s relativity allows observers moving differently to disagree about the order of distant events. If the sun disappeared and Earth responded to the gravitational change immediately, another observer could describe Earth’s response as happening before the sun vanished. Einstein avoided this contradiction by describing gravity as local spacetime curvature whose changes spread outward at light speed.

Q: How long would Earth keep orbiting if the sun disappeared?

A change caused by the sun’s disappearance would take about eight minutes to reach Earth, according to the account presented. Einstein’s theory describes gravity through local changes in curved spacetime. A disturbance spreads from one nearby region to another at light speed, so Earth would not respond to the gravitational change instantaneously.

Q: Why did Einstein think quantum mechanics violated locality?

Quantum mechanics describes an electron with a wave function spread across multiple possible positions. Once the electron is detected at one point, the probability of detecting it anywhere else becomes zero. Einstein argued that this measurement therefore appears to affect distant parts of the wave function immediately, regardless of their separation, which conflicts with locality.

Q: What is the Copenhagen interpretation of quantum mechanics?

The Copenhagen interpretation treats the wave function as the complete description required for predicting laboratory measurements. Before measurement, the wave function represents the possible outcomes and their probabilities. Bohr regarded further questions about what a particle is doing independently of measurement as unnecessary, while Einstein believed this approach concealed a serious non-locality problem.

Q: What is quantum entanglement in the EPR experiment?

Entanglement is a shared state connecting the possible measurement outcomes of two particles. In the simplified EPR example, an electron and positron must have opposite spins. Neither particle has one definite result before measurement under the quantum description, but measuring one particle fixes the opposite result required for its distant partner.

Q: What are local hidden variables?

Local hidden variables are predetermined instructions carried by particles that specify their later measurement outcomes without requiring distant communication. In Einstein’s preferred account of the EPR experiment, each particle already knows whether it will produce a plus or minus result. The apparent coordination then comes from their shared preparation rather than instantaneous wave-function collapse.

Q: Why was the Einstein-Bohr debate considered settled?

Bohr defended the Copenhagen interpretation, and his standing within physics strongly influenced how the dispute was remembered. The basic EPR experiment could not distinguish Copenhagen quantum mechanics from Einstein’s local hidden-variable explanation because both predicted the same observed results. Many physicists therefore regarded further debate as unproductive and adopted a practical, calculation-focused attitude.

Q: How did John Bell reopen the debate about quantum locality?

John Bell revisited the historical arguments after becoming dissatisfied with vague explanations of quantum mechanics. He recognized that the basic EPR setup could not decide between non-local quantum mechanics and local hidden variables. Bell therefore searched for a modified entanglement experiment in which the two accounts would make different predictions that experimenters could compare.

Summary & Key Takeaways

• Einstein rejected instantaneous gravity because observers moving differently could disagree about the order of cause and effect. General relativity replaced action at a distance with local changes in spacetime that spread outward at light speed. If the sun disappeared, its gravitational effect would therefore take about eight minutes to reach Earth.

• Quantum mechanics created a similar locality problem. Measuring an electron at one position changes its wave function everywhere, while measuring one member of an entangled pair fixes the corresponding state of its distant partner. Einstein, Podolsky, and Rosen argued that predetermined local hidden variables offered a more sensible explanation than instantaneous collapse.

• Bohr defended the Copenhagen interpretation, which treats the wave function as the complete tool for predicting laboratory measurements without specifying an underlying reality. Most physicists accepted that the Einstein-Bohr dispute was settled because both interpretations predicted the same EPR results, but John Bell later sought an experiment where their predictions would differ.