May 18, 2018
Written by Ashley Hamer
You know you’ve made an impact on the world when people are still testing your ideas six decades after your death. Einstein was no fan of the concept known as quantum entanglement and believed that any explanation for it had to involve some other variables we just don’t know about yet. Well, Einstein wasn’t right about everything — and the biggest experiment of quantum entanglement yet adds his objection to that list.
Related Video: How Quantum Entanglement Solved a Long-Standing Debate
Made You Look
For the very, very small particles described by quantum mechanics, there is no definitive state of being. Quantum particles exist as a swarm of probabilities, basically existing in several states at once — say, spinning clockwise and counterclockwise — until they’re measured, when they seem to “choose” a state and stick with it. A particle that makes this “choice” is said to have collapsed as if all of its various potential states have fallen away to reveal the one state it settled on. Even weirder, it’s possible for two particles to become “entangled,” meaning they will retain a sort of causal relationship with each other, no matter their distance in time and space. If you measure one particle and it spins clockwise, for example, then its entangled companion would instantly collapse into a counterclockwise spin, even if it’s on the other side of the universe. That either means that one communicated with the other in an instant, or the state of each particle only popped into existence once one was measured.
We know what you’re thinking. For one thing, this whole idea is ridiculous: Things are what they are regardless of whether you’re looking at them. For another, nothing can go faster than light, so how can two particles communicate across the universe in an instant? Einstein thought the same thing, derisively calling the idea “spooky action at a distance.” Those in Einstein’s camp are in favor of a concept called “local realism.” “Locality” says that no signal can travel faster than light, and “realism” says that particles have definite states even before you measure them.
If, in fact, two particles “communicate” through a vast distance, then according to local realism, something else must be going on. Maybe the measurement tools are somehow in on it, or maybe the universe places some limits on the particles’ possible states that we just don’t know about yet. MIT’s Andrew Friedman told Quanta that it’s as if the universe is a restaurant with 10 dishes on the menu. “You think you can order any of the 10, but then they tell you, ‘We’re out of chicken,’ and it turns out only five of the things are really on the menu. You still have the freedom to choose from the remaining five, but you were overcounting your degrees of freedom.”
This idea that there might be less freedom of measurement than we think there is called the “freedom-of-choice” loophole. This loophole poses a challenge because even seemingly total randomness can’t close it; use a random number generator to decide which properties to measure, and there’s still the chance that the entangled particles have influenced the random number generator. That’s why in November of 2016, a global experiment used a method that can’t be influenced by puny quantum particles: free will. Specifically, the free will of a bunch of gamers.
Saved by the Bell
The experiment, dubbed the BIG Bell Test, was modeled on a classic experiment designed by physicist John Stewart Bell. In the traditional version of the experiment, pairs of entangled particles are generated, separated, and sent to different locations where various properties of those particles are measured. (Like we mentioned before, researchers will typically use random-number generators to decide which property to measure at any given time.) If the measurements match, then that’s evidence for quantum entanglement and against local realism.
The BIG Bell test happened with the help of 100,000 people around the world, who used an online video game to contribute their own random sequences of zeros and ones to 12 labs on five continents, including in Brisbane, Australia; Shanghai, China; Vienna, Austria; Rome, Italy; Munich, Germany; Zurich, Switzerland; Nice, France; Barcelona, Spain; Buenos Aires, Argentina; Concepción, Chile; and Boulder, Colorado. In many different individual experiments, those labs used the numbers, or “bits,” to set the angles of polarizers and other tools to determine how the entangled particles would be measured.
The results, published in May 2018 in the journal Nature, are robust: “The observed correlations strongly contradict local realism,” the authors write. Sorry, Einstein. You’ll always have relativity.