Hello, friends, and welcome to a special Halloween edition of Sciency Words! Today, we’re talking about the spookiest of scientific terms. And that super spooky term is:

**QUANTUM ENTANGLEMENT**

Quantum mechanics is the study of the tiniest of tiny things in our universe: things like atoms and quarks and electrons. And these super tiny things do some pretty weird stuff, if our current mathematical models are to be believed. Stuff that seems to defy our human notions of common sense.

In the 1930’s, when quantum theory was still brand new, Albert Einstein did not approve of all that common-sense-defying stuff that quantum mechanical models were predicting. So in 1935, Einstein and two of his colleagues, Boris Podolsky and Nathan Rosen, published a paper that was supposed to prove quantum theory was incorrect, or at least that it was woefully incomplete.

The Einstein-Podolsky-Rosen paper (or E.P.R. paper, as it’s now commonly known) didn’t quite get the job done. Quantum theory survived the attack. In response to the E.P.R. paper, Erwin Schrödinger (of Schrödinger’s cat fame) wrote a letter to Einstein. It was in this letter, from Schrödinger to Einstein, that the word “entanglement” was first used in reference to quantum theory. Well, actually, Schrödinger used the word *Verschränkung*, a German word which translates into English as “entanglement.” (The relevant section of Schrödinger’s letter is quoted in this article from *The Stanford Encyclopedia of Philosophy*.)

Entanglement refers to the way a pair of quantum particles can interact with each other and then remain “entangled” with each other after their interaction is over. If you measure the quantum state of one entangled particle, the other will instantaneously change to match. This implies that entangled particles can somehow exchange information at faster-than-light speeds. As Schrödinger wrote in his letter, this is not just a weird quirk of quantum theory; it’s the “characteristic trait” that makes quantum mechanics so radically different from classical physics.

Einstein was still not happy. Neither was Schrödinger; however, as I’ve come to understand the story, Schrödinger was able to set his personal feelings about quantum theory aside and continue his research. Einstein, meanwhile, kept trying to prove quantum theory was wrong until the day he died.

You might even say the idea of quantum entanglement haunted Einstein for the rest of his life. In 1947, in a letter to another physicist named Max Born, Einstein referred to entanglement as *spukhafte Fernwirkung*, a phrase which is commonly translated into English as “spooky action at a distance.” (The relevant section of Einstein’s letter is quoted in this book.)

Thus, quantum entanglement is the spookiest scientific term.