# Instantaneous info exchange between particles that doesn't violate FTL limits. How? **URL:** \ **Category:** Factual Questions\ **Created:** [January 15, 2006, 7:38pm UTC](https://boards.straightdope.com/t/instantaneous-info-exchange-between-particles-that-doesnt-violate-ftl-limits-how/340116 "2006-01-15T19:38:51Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![astro](https://avatars.discourse-cdn.com/v4/letter/a/9dc877/32.png) [@astro](https://boards.straightdope.com/u/astro)\ **Post date:** [January 15, 2006, 7:38pm UTC](https://boards.straightdope.com/t/instantaneous-info-exchange-between-particles-that-doesnt-violate-ftl-limits-how/340116/1 "2006-01-15T19:38:51Z")
Stumbled over this via Fark What’s the catch? There’s always a catch. How can truly “instantaneous” information exchange occur between “entangled” particles without violating the limits of the physical universe that proscribe faster than light anything? [Can two particles become linked, so that a change in one is instantly reflected in its counterpart, even if a universe separates them?](http://www.quantumbiocommunication.com/media/quantum-entanglement-the-future-of-teleportation.html) > [@](#): > > Normally the photons exit the crystal such that one is aligned in a horizontally polarized light cone, the other aligned vertically. By adjusting the experiment, the horizontal and vertical light cones can be made to overlap. Even though the polarization of the individual photons is unknown, the nature of quantum mechanics predicts they differ. > > To illustrate, if an entangled photon meets a vertical polarizing filter the photon may or may not pass through. If it does, then its entangled partner will not because the instant that the first photon’s polarization is known, the second photon’s polarization will be the exact opposite. > > It is this instant communication between the entangled photons to indicate each other’s polarization that lies at the very heart of quantum entanglement. This is the “spooky action at a distance” that Einstein believed was theoretically implausible. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [January 15, 2006, 9:43pm UTC](https://boards.straightdope.com/t/instantaneous-info-exchange-between-particles-that-doesnt-violate-ftl-limits-how/340116/2 "2006-01-15T21:43:33Z")
An important point to understand is that you cannot in fact _ **change** _ one particle and have it’s entangled partner instantly change. You can perform measurements on one (even measurements where your measurement determines just what factor is being tested), but all that does is tell you that if you get result A on one particle then the other must be -A. But you can’t actually alter one particle in a way which would encode information and have that instantly transmitted to the other. In a sense, you can say that as long as the two particles remain entangled, you don’t actually have two separate pieces of information, state of particle P1 and state of particle P2. You have ONE piece of information, with the two particles being either P1=P2 or more commonly P1 = NOT P2. In fact you could even define entanglement as the state in which the particles haven’t truly gained separate identities yet. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [January 16, 2006, 5:46am UTC](https://boards.straightdope.com/t/instantaneous-info-exchange-between-particles-that-doesnt-violate-ftl-limits-how/340116/3 "2006-01-16T05:46:24Z")
This is based on a famous experiment by J.S. Bell. You may want to look it up. ---
**Author:** ![ambushed](https://avatars.discourse-cdn.com/v4/letter/a/3bc359/32.png) [@ambushed](https://boards.straightdope.com/u/ambushed)\ **Post date:** [January 16, 2006, 10:02am UTC](https://boards.straightdope.com/t/instantaneous-info-exchange-between-particles-that-doesnt-violate-ftl-limits-how/340116/4 "2006-01-16T10:02:50Z")
Bell didn’t do the experiment, he was the first to come up with his famous “inequality” (other mathematical physicists later joined with other, similar inequalities) which mathematically revealed (many would say “proved”, I say “strongly suggested”) that “local variable” theories of QM are inadequate. Bell and others thought that a physical experiment might one day be constructed which would confirm this, and this was later done by Alain Aspect. An important note is that not even Aspect (no anyone else) even tested this theory in an experiment in which the two coupled particles were outside each others’ light cones. In other words, we still do not know with anything approaching certainty that the state of the two particles remain correlated beyond a distance that would require superluminal correspondences. ---
**Author:** ![Snooooopy](https://avatars.discourse-cdn.com/v4/letter/s/ee7513/32.png) [@Snooooopy](https://boards.straightdope.com/u/Snooooopy)\ **Post date:** [January 16, 2006, 3:42pm UTC](https://boards.straightdope.com/t/instantaneous-info-exchange-between-particles-that-doesnt-violate-ftl-limits-how/340116/5 "2006-01-16T15:42:16Z")
> [@SaintCad](#): > > This is based on a famous experiment by J.S. Bell. You may want to look it up. You could check the Dilbert strip where it was mentioned! ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [January 16, 2006, 3:46pm UTC](https://boards.straightdope.com/t/instantaneous-info-exchange-between-particles-that-doesnt-violate-ftl-limits-how/340116/6 "2006-01-16T15:46:26Z")
I don’t see how this translates to the instantaneous exchange of anything. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [January 17, 2006, 11:26am UTC](https://boards.straightdope.com/t/instantaneous-info-exchange-between-particles-that-doesnt-violate-ftl-limits-how/340116/7 "2006-01-17T11:26:18Z")
To elaborate on what **Lumpy** said. Suppose you had two pennies, penny A and penny B. And when you flip them individually, they each give you a random stream of heads and tails. But when you flip them together, penny B always gives the same result as penny A, by “spooky action at a distance”. So you mail penny A to me and start flipping penny B yourself. And it gives you a random stream of heads and tails. Then I start flipping penny A, and penny B over at your house magically starts matching penny A. Trouble is, before I started flipping, you get a random stream of heads and tails, and after I start flipping, you _still_ get a random steam of heads and tails, albeit the same random stream as I’m getting. As far as you’re concerned, there’s no change. I can’t use the “spooky action at a distance” to signal you in any way. So no actual information is transmitted, and causality is preserved. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [January 17, 2006, 4:18pm UTC](https://boards.straightdope.com/t/instantaneous-info-exchange-between-particles-that-doesnt-violate-ftl-limits-how/340116/8 "2006-01-17T16:18:23Z")
> [@matt](#): > > So you mail penny A to me and start flipping penny B yourself. And it gives you a random stream of heads and tails. Then I start flipping penny A, and penny B over at your house magically starts matching penny A. Trouble is, before I started flipping, you get a random stream of heads and tails, and after I start flipping, you still get a random steam of heads and tails, albeit the same random stream as I’m getting. As far as you’re concerned, there’s no change. I can’t use the “spooky action at a distance” to signal you in any way. So no actual information is transmitted, and causality is preserved. But you’ve got yourself an excellent encryption system! ---
**Author:** ![DarrenS](https://avatars.discourse-cdn.com/v4/letter/d/977dab/32.png) [@DarrenS](https://boards.straightdope.com/u/DarrenS)\ **Post date:** [January 17, 2006, 5:38pm UTC](https://boards.straightdope.com/t/instantaneous-info-exchange-between-particles-that-doesnt-violate-ftl-limits-how/340116/9 "2006-01-17T17:38:26Z")
**astro** - in his latest book, “The Fabric of the Cosmos”, Brian Greene goes into great (layman) detail about this phenomenon, including explaining in language that someone without a PhD in physics can understand the meaning of Bell’s paper, EPR, Aspect’s experiment and entanglement in general. Highly recommended. It cleared up a lot of similar questions I had.