Quantum Entanglement

Spooky Action at a Distance, Explained

clay shentrup
3 min readJul 22, 2020

Quantum entanglement and the resulting “spooky action at a distance” is weirder than most people ever realize. In my view, it’s extremely poorly explained in popular media. So here’s a simplified analogy.

Imagine you have two tiny handheld computers, each with two buttons, labeled + and -, and each having a small LED screen that can either show a + or -. Whenever you each hold down a button, you see a + or - appear on your screen.

At first, the pattern seems random, but then you realize that you always see the same symbol on your screen if you press the same button, and you see a different symbol on your screens if you’ve pressed a different button. For instance, if you both press + then either you both see a + or you both see a - on your screens. Whereas if you each press a different button, then your symbols are also opposite. The particular symbols you see are statistically random from your point of view: you’ll see + or - on your screen in a completely 50–50 random way, but the correlation between your two screens is perfectly consistent.

Okay, this is a cute little technology, but you figure these computers must simply be communicating via radio waves, just like your cell phone. Not exactly magic, right? But now suppose you and your friend travel to two different locations several light years apart, and repeat the experiment, writing down both your random button presses and the corresponding images that instantly appear. And suppose that when you later rendezvous, you find that the same pattern holds, even though no signal could have possibly traveled that fast—indeed, it would have required faster-than-light travel. Yet at the same time, there’s no way to use these devices to communicate faster than the speed of light, because you have no idea what image appeared on your friend’s screen (and vice versa) until you rendezvous.

Sound like science fiction? Nope! This is exactly the kind of thing we see in quantum experiments. Here’s a short Youtube video that demonstrates this with entangled photos, and the + and - buttons replaced by polarizing filters turned to random orientations.

Bell’s Theorem: The Quantum Venn Diagram Paradox

Unfortunately, these online explanations are typically too focused on esoteric specifics of the underlying physics that are orthogonal to the fundamental point about coincident behavior and how it proves necessity for faster than light communication.

Einstein refuted

But the physics aspect is definitely useful. Prior to these photon experiments proposed by physicist John Stewart Bell, Einstein had speculated that the “entangled” photons had some underlying state that dictated their ultimate observed polarization. He called this hidden variables. While it might seem “spooky” that the photons appear to only choose a definite state at the last minute, this theory says that they really had a state the whole time, and we simply hadn’t observed it yet.

Bell’s test brilliantly refuted this idea. To use the analogy of our computers, imagine we had always pressed the same buttons and merely observed what appeared to be random images. Hidden variables would be like saying that the computers had already been pre-programmed with a sequence of images that might superficially seem random, but were based on underlying states that were known the whole time. But by randomly choosing which button to press, we prevent that possibility, just like the Bell experiment randomly chooses the orientation of the polarizing filters.

At that point, the only remaining possibility was that information was somehow making it from one filter to another, but not faster than the speed of light. But this “loophole” was eventually disproved as well, when a version of Bell’s experiment was conducted over a distance of 144 km, and the results proven to have arrived faster than light could have covered that distance. To make the analogy with our computers, imagine we press our button at the exact same time, and observe that our images appear faster than light could have traversed the 144 km between us.

Another alternative is that our button presses aren’t actually random, but instead the universe is deterministic and it knew the whole time which buttons we’d press, so it could actually make up its mind ahead of time whether we would get + or -.

Regardless of which explanation we go with, the implications are unsettling to say the least.



clay shentrup

advocate of score voting and approval voting. software engineer.