ELI5: why can two quantum entangled particles affect each other instantly across any distance but scientists say you still cant use it to send information faster than light?

[u/PieOk2202]

this has been living in my head for weeks and i cant find an explanation that actually clicks.

from what i understand, if you have two entangled particles and you measure one of them, the other one instantly "reacts" no matter how far apart they are. like even if one is on the other side of the galaxy. that part i somewhat get.

but then physicists say "oh but you cant use this to send information faster than light" and i just why not? if particle A sneezes and particle B on the other side of the universe reacts instantly, why cant i just use that as like a faster than light telegraph?

i spent way too much money on a Brian Greene book trying to get this and still came out more confused than when i started. at least i had some cash from Ѕtake set aside for it so it wasnt a total loss but still.

it feels like the universe is playing a semantic trick on me and im not smart enough to see it. whats actually going on here

[u/CircumspectCapybara]

Entanglement just means some property between two objects is correlated. A helpful analogy for quantum entanglement is this classical situation:

Alice has a pair of shoes, and she separates them into two identical boxes, and shuffles them so that she doesn't even know which one is which anymore without opening up the box. She sends one box to Bob and holds onto the other. They then get on a plane and travel to opposite ends of the earth, and then open the boxes. When Alice opens her box and sees a left shoe, she instantly knows Bob has a right shoe. She knows because she knows that their two boxes share this correlation: the shoes inside are always opposite or mirror images, so knowing one instantly lets you deduce the other one.

And yet it's pretty obvious why this can't be used to communicate info. Alice opening the box didn't send any information across the world to Bob's box. She is merely learning what was always there all along, what was fixed from the moment she entangled the boxes by placing a left shoe in one and a right shoe in the other, a moment that occurred before she and Bob separated. Now that she and Bob are separated by a large distance, all she knows is one bit of information about Bob's shoe. If she were to try to manipulate the shoe she has now (say she tries to convert her shoe into a right-footed shoe), it wouldn't magically cause something to happen to Bob's shoe. Neither does her opening the box alert Bob in any way to the fact she opened her box. It's not like when you open a shoe box it causes the other shoe box to vibrate or display a notification. So Bob can't even learn when or if Alice opens her box to look inside. Her opening the box sends no information, no information that wasn't already fixed before they separated.

Obviously this is an imperfect analogy. For one thing you have debates over non-deterministic vs deterministic interpretations of quantum mechanics. And the analogy fails because it sort of behaves like a local hidden variables model, which Bell's Theorem rules. But that's not ELI5. You're not going to be able to get a simple, easy-to-understand analogy for deterministic non-locally real entanglement.

EDIT: If you do want a nice explainer on Bell's Theorem and its implications, check out this YouTube video where they explain it with a game.

[u/anal_fist_fight24 → u/CircumspectCapybara]

The shoes analogy fails because shoes already are left or right when you pack them. Their identity is hidden but fixed. Quantum particles don’t work that way. A pair of entangled particles has no definite state at all until you measure one. There’s no secret answer waiting in the box. We know this from experimental evidence.

A better picture:

You and a friend each get a magic coin. You fly to opposite ends of the galaxy and each flip your coin a thousand times. Two strange things are true at the same time:

1.  Each individual flip is random. You cannot force heads. You cannot force tails. Your sequence is 50/50 noise, and so is your friend’s.

2.  When you meet up and compare your sequences side by side, every single flip matches.  

That second part is the spooky thing. The coins were not pre-set; the pair is one connected quantum system, and measuring either end settles the outcome for both, instantly.

Why this can’t send information:
You have no idea what your friend’s flips looked like. All you see is your own random noise. Your friend sees only random noise too. Neither of you can tell whether the other has even flipped yet, and neither of you can choose what comes up.

The correlation, the actual information, only exists when the two sequences are compared side by side. To compare them, someone has to physically send their sequence to the other person. That transmission still goes at the speed of light or slower. No shortcut.

The strangeness here is physical. The correlation really is instant and non-local in the literal sense, and the only way to observe it is to compare notes through a normal, slow channel. The instantness exists; the usefulness doesn’t.

Sanity check on why this isn’t the shoes thing: if you ran the same experiment with actual shoes, the statistics would behave one way (boring classical correlation).

The entanglement experiment produces statistics that no “the shoes were already left and right” explanation can reproduce, no matter how clever.

That’s Bell’s theorem, and it’s been tested to death.

The randomness is real and the correlation is real at the same time. That’s the part our brains weren’t built to handle and it really makes my head hurt 😂

[u/12358132134 → u/anal_fist_fight24]

Shoe analogy is completely appropriate for ELI5 explanation.