CHICAGO — Albert Einstein said his biggest blunder was changing his equation describing space to add a term for repulsive energy. Such energy was needed, he thought, to keep the universe from collapsing.
But then in 1929 Edwin Hubble discovered that the universe is expanding. So Einstein renounced his repulsive energy. He shouldn’t have, though, because seven decades later astronomers found evidence that repulsive energy permeated space after all.
“Einstein blew it,” says cosmologist Rocky Kolb. “He could have been famous if he had stuck to his guns.”
Still, neither predicting repulsive energy nor retracting it was really Einstein’s biggest blunder, says physicist Charles Bennett. It was confusing people about quantum entanglement.
Entanglement is one of the weirdest features of quantum physics. It refers to a situation in which two “particles” (a term that must be construed loosely in the quantum world) share a common history that makes their futures intertwined.
In the standard explanation, two entangled particles are sent to the labs of physicists named Alice and Bob. When Alice performs a measurement on her entangled particle, the fate of Bob’s particle is sealed. So if Alice tells Bob the result of her measurement, he instantly knows what the result would be of measuring his particle. He doesn’t need to bother doing the actual measurement.
“This worst thing that happens here is when people describe entanglement — and I’m not going to say who is the highly reputed scientist who did this first — as ‘spooky action at a distance,’” Bennett said last week at the annual meeting of the American Association for the Advancement of Science. “That has generated an enormous amount of confusion that people in this field are trying to undo all the time.”
At first glance, it does seem that entanglement permits instantaneous messaging from one lab to another, no matter how far apart they are. That’s what Einstein didn’t like. But actually, no signal is sent. Alice’s measurement merely alters the description of the “quantum state,” the math describing the entangled system containing the two particles. Alice’s measurement changes the universe in a way that gives Bob’s particle a definite property to measure that it did not previously possess.
“It’s spooky,” says Bennett, “but it’s not action at a distance.”
There is no instant at-a-distance communication, because Bob does not know what his result will be until Alice tells him her result by ordinary means, like e-mail or perhaps a text. (No tweets! Alice and Bob would never get any of their experiments done if they wasted time on Twitter.)
Entanglement remains a strange phenomenon. But it’s not magic.
“When you observe one part of a quantum system, you change the state of the universe and you change it in a way that we understand,” Bennett says. “It cannot send information faster than the speed of light. It’s entanglement. Learn about it.”