- Bell's experiments proved quantum entanglement is real, disproving local hidden variables.
- Measuring one entangled particle instantly affects its partner at any distance.
- These instant correlations cannot transmit information, so relativity holds.
In 1935, Einstein tried to prove quantum mechanics was incomplete. He devised a thought experiment showing that if quantum theory was correct, measuring one particle would instantly affect another across any distance - violating his beloved speed limit.
Einstein was sure this absurdity proved quantum mechanics must be missing something. He was wrong about that. But his "spooky action at a distance" turned out to be very real.
In a video from Veritasium, the exploration begins with Einstein's famous thought experiment and its surprising vindication decades later. When two entangled particles are separated across vast distances, measuring one does instantly affect the other - no matter how far apart they are.
Key figure
25% vs 33%
Quantum mechanics predicted 25% disagreement between entangled particles; hidden-variable theories required at least 33%. Experiments matched quantum mechanics.
The EPR Paradox That Shocked Physics
Einstein, along with colleagues Podolsky and Rosen, imagined a high-energy photon splitting into an electron and positron. These particles become entangled - their spins must always be opposite to conserve energy.
Here's where it gets strange. Quantum mechanics says both particles exist in all possible spin states simultaneously until measured. But when you measure the electron and find it spinning up, the positron must instantly "know" to spin down - even if it's on the other side of the galaxy.
Einstein called this "spooky action at a distance." He argued this couldn't be real - there must be hidden variables determining the outcomes in advance. Otherwise, the measurement here would instantly affect the wave function over there, seemingly violating relativity.
Albert Einstein didn't believe entanglement could be real, calling it a 'spooky action'. Photograph by Orren Jack Turner, Princeton, N.J.
Bell's Brilliant Test
For thirty years, the debate remained philosophical. Then John Bell realised Einstein's argument could actually be tested.
Bell designed an experiment using entangled particles measured at different angles. Quantum mechanics predicted the particles would disagree 25% of the time when measured differently. Any local hidden-variable theory - where particles carry predetermined values and don't communicate faster than light - predicted at least 33% disagreement.
What is Bell's theorem?
In 1964, physicist John Bell devised a mathematical test to settle the debate over quantum entanglement. His theorem sets a limit on how often entangled particles can disagree when measured at different angles, if their outcomes are predetermined. Quantum mechanics predicts correlations below that limit, so any experiment matching the quantum prediction rules out hidden variables.
When scientists finally ran the experiment, quantum mechanics won. Einstein's hoped-for local hidden variables didn't exist.
We're obliged to invoke something like actions going faster than light from one place to another.
Derek Muller, Veritasium host
But here is a crucial point: while the correlations are instantaneous, you can't use them to send information faster than light. Relativity remains intact.
The Escape Route
There might be one way to restore a fully local picture: the many-worlds interpretation. Instead of wave function collapse causing instantaneous correlations, every measurement splits reality. Both outcomes happen in parallel universes.
This preserves locality in a different sense - the particles don't need to communicate because all possible outcomes already exist across branching realities. It's one way physicists have tried to reconcile quantum weirdness with Einstein's intuitions, though it remains one interpretation among several rather than an established solution.
Einstein lost the battle over hidden variables. But his instinct that something profound was at stake in quantum nonlocality?
That proved absolutely right.
Fact Check: Claim-by-Claim Verification Verified
The recap accurately summarizes the Veritasium video and aligns with established quantum mechanics history and Bell's theorem from peer-reviewed sources.
Commentary
- Video uses electron-positron (not protons, per correction); recap's simplification acceptable for popular science.
- Many-worlds interpretation presented as one possible local reconciliation, correctly noted as unestablished.
- Quote on faster-than-light actions matches video phrasing from physicist.
Sources used for verification
Fact-checked by Perplexity Sonar Pro on 2025-12-21
