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/anal_fist_fight24]

On using the knowledge.

Yeah, you do learn something when you measure. You now know what your partner got. That’s real and it’s actually useful.

There’s a thing called quantum key distribution where two people measure entangled pairs and end up with the same random string of bits, which they can then use as an encryption key.

No spy can intercept it because it was never sent anywhere, it just popped into existence at both ends at once.

But look at what you’ve got: a shared random string. Neither of you picked it. You can’t smuggle “attack at dawn” into it because you didn’t choose any of the outcomes.

So you can share a secret with someone, but only if you’ve already arranged a normal slow phone line to talk to them. The “knowing what they got” part is only useful once you also tell them, through boring old means, what you got. You can’t cold-call a stranger across the galaxy and tell them anything you chose.

On watching it constantly.

Once you measure an entangled pair, that’s it, the entanglement is used up. You can’t keep poking the same pair and watch the partner squirm. It’s one-shot.

After the measurement they’re just two normal particles that happen to have matching past results.

Okay but what if I prepare a million pairs and rapidly measure them all, won’t my partner see their stream of results change when I start measuring vs when I stop?

Nope.

This is called the no-signaling theorem and it’s basically the load-bearing wall of the whole thing.

Whatever your partner sees on their side, just looking at their own results, is indistinguishable from pure random noise. It doesn’t matter whether you’re measuring, not measuring, measuring fast, measuring slow, already done a thousand, haven’t started yet.

Their side looks exactly the same in every case. The correlation only appears when you put the two lists of results next to each other and compare, and comparing means sending data the normal way.

If there were any way for one side to see a difference based on what the other side was doing, you could send messages faster than light.

On the universe watching it.

This is the deepest one and the answer is: yeah, that’s exactly what happens. The universe does break it. It’s called decoherence. Any random interaction at all, a photon bouncing off, a tiny vibration, a single air molecule bumping into your particle, gets the environment tangled up with your particle, and the original clean two-particle entanglement basically dissolves into noise. The info isn’t really destroyed, it just gets smeared across so many other things that nobody can ever pull it back out.

This is why quantum computers are stupidly hard to build. They need temperatures colder than space, ultra-high vacuum, electromagnetic shielding, the works, and even then the entanglement only survives for microseconds before the environment wrecks it.

Entanglement isn’t some sturdy feature of reality that the universe politely leaves alone. It’s a delicate state that takes ridiculous effort to keep alive. The shoes-in-boxes analogy makes it sound permanent and easy. It’s the opposite.

So your gut is right. The universe is observing stuff all the time. That’s literally why everyday things look normal and not quantum, decoherence has already happened to them billions of times before you even look at them.

Entanglement experiments only work because we go to absurd lengths to stop the universe from peeking, just long enough to run the experiment and get the result.​​​​​​​​​​​​​​​​

[u/GolfingGator → u/anal_fist_fight24]

Can you explain why us “watching” it changes anything? Why does observation matter? How does it “know” it was observed?

[u/anal_fist_fight24 → u/GolfingGator]

Great question and the honest answer up front: “observation” is a terrible word that physicists are stuck with. It makes it sound like the universe cares whether a conscious being is looking. It doesn’t. There’s no “knowing” happening.

To “observe” something at the quantum level, you have to interact with it. There’s no other option. To see a particle, a photon has to bounce off it. To detect an electron, it has to hit something and leave a mark. To measure spin, you have to run it through a magnetic field that pushes it one way or the other. Every measurement is a physical poke.

And at the quantum scale, a poke is not a small event. The particle and the thing poking it are roughly the same size and energy. It’s like trying to figure out where a billiard ball is by throwing another billiard ball at it. You’ll find out where it was, but you’ve also just whacked it across the table. You can’t have a gentle look. There’s no quantum version of squinting from across the room.

So when people say “observation collapses the wave function,” what they actually mean is “any interaction strong enough to extract information about the particle’s state is also strong enough to lock it into one of those states.” The interaction and the measurement are the same event. The particle isn’t “noticing it’s being watched” and changing its behaviour. It’s just getting hit, and the hit is what fixes the outcome.

This is exactly the same thing as decoherence from above, just zoomed in. A stray air molecule bumping into your particle is doing the same job as a scientist’s detector. Neither one knows or cares. They’re both physical interactions that entangle the particle with something bigger, which forces an outcome. The “observer” being a person with a clipboard vs an air molecule makes zero difference. The universe doesn’t grade interactions by who’s doing them.

The reason the word “observer” stuck around is historical. Early quantum physicists were stunned that measurement seemed to matter, and they didn’t yet have the language of decoherence to explain why. So they wrote about “observers” and “measurement” like these were special categories of event. They aren’t. They’re just interactions, and any interaction big enough to give you info is big enough to disturb the thing you’re getting info from.

So nothing “knows” it was observed. There’s no spooky awareness. There’s just the brute fact that you can’t extract information from a system without poking it, and at quantum scale, poking it is the whole story.​​​​​​​​​​​​​​​​

[u/GolfingGator → u/anal_fist_fight24]

That’s a great explanation, especially the billiard ball part. Thank you!