Quantum entanglement. What the hell is it? Perhaps you’ve read that,
Quantum entanglement is a bizarre, counterintuitive phenomenon that explains how two subatomic particles can be intimately linked to each other even if separated by billions of light-years of space. Despite their vast separation, a change induced in one will affect the other.
Not true. Maybe you’ve read that,
When two or more particles link up in a certain way, no matter how far apart they are in space, their states remain linked.
Nope. Even Wikipedia is wrong, stating:
Quantum entanglement is the physical phenomenon that occurs when a group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance.
The official YouTube account of Fermilab, which is hosted by an award-winning physicist and celebrated science communicator, states that quantum information can travel faster than light, which is certainly false.
The Royal Institution recently hosted one of my favourite and most lucid communicators of theoretical physics who said entanglement has to happen when two particles interact. Ok, that is kind of true. But, the whole point of the video is to shock you with some explanation of how entanglement is some spooky connection between distant objects.
The picture is like this.
The moral of any quantum entanglement story is that it arises when particles interact and create a “link,” which we call entanglement. Importantly, entanglement remains no matter how far apart they might be. The “state” of each individual particle is not well defined, but their joint (entangled) state is. Thus, the two particles must be considered as a single entity, spread across a potentially vast distance. If you believe that story, I agree it’s mystical, just as the internet told you.
The mechanics of what is going on when actually creating entangled states of quantum things seems to corroborate this story. We’ve been creating entangled states of photons (laser light) this way for decades. A single high-energy photon enters a special material that converts it into two lower-energy photons that fly off in different directions. Because of an ambiguity in their properties, quantum physics says they are entangled. Numerous experiments have verified the predictions of measuring the two entangled photons, with the latest separations being tens of kilometers. In fact, each new experiment has accompanying press releases staking their claim on the current quantum entanglement distance record.
But, here’s the thing. While the mathematics of quantum theory is correct enough to make accurate predictions, the model of entanglement as a physical connection between two distant objects is wrong. To see why, consider the fact that entanglement can be created between two particles without ever having them interact. They can be so far away from each other that not even light signals could reach one another.
The idea goes back to a quartet of Spaniards and Austrians in 1998, and it’s quite simple. First, imagine two atoms separated by a large distance, and both are in an excited state. When either atom decays, it will release a photon that is detected at a central station. Since the detector cannot distinguish which of the two atoms decayed, the state of the pair becomes correlated. In fact, quantum mechanics dictates they must be entangled.
The atoms never interacted or exchanged any information. In fact, they couldn’t have — the only signal that something had happened made it halfway between them at the moment the entanglement was created. Are we to believe a physical link of double that size was immediately generated?
A better way to think about entanglement is as only abstract information about correlations. In the above example, what immediately happens when the photon is detected is that we learn the two atoms are correlated, which is a statement only about our expectations, not the physical reality experienced by the distant atoms.
Imagine if, instead of atoms, there were two distant boxes, each with a ball in it. The ball might be removed and sent to you from either box. At some median location, you receive a ball in the post. Immediately, the boxes become correlated — one is empty, and the other is not — because you don’t know which box the ball came from. It is simply your ignorance and future expectations about what might be revealed that defines the correlation. There is certainly no mystical “link” that physically manifested between the boxes the moment the post arrived, and the same is true for atoms.
Our conventional understanding of correlation involves classical information — bits of data that could be stored on digital computers. Entanglement is just a correlation involving a different kind of information. In the end, it’s all just information, and that might be more profound than a fictitious story about a mystical link between stray particles.