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Editorial Note

This is a canonocal question meant to cover all possible specific questions that fit the schema, as well as the fully general “Can quantum entanglement be used for FTL communications?”

If a question fits the schema, close it as a duplicate of this one.

Generic Question

So I came up with this concept for FTL communication with quantum entanglement / EPR / spooky action at a distance:

I have one or more pairs of entangled particles, with Alice and Bob holding one of each pair.

My Very Clever Idea™ is to…

Blah blah blah, the details are irrelevant. This might be a well crafted highly speficic scenario, or it might be rather muddy and unclear. It may involve any or all of

  • changing one particle and expecting its partner to change too
  • measuring a chosen observation one one particle and expecting any measurement on its partner to yield any information about that without comparing notes via normal communication
  • assuming that the actions of Alice and Bob, who are outside of each other’s light cones, have an absolute time ordering

Constraints

This applies to real-universe physics, though may require unobtainium or engineering so advanced as to seem like magic.

If you want to postulate a change to the laws of QM, it would take a question to ask someone to work out the details to see if these results still apply. But, a question can be posted to simply assess whether the postulated change will break all of physics or might conceivably result in a universe like ours but with FTL communication

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  • $\begingroup$ FTL communication can't work under our current understanding of reality, quantum or not. And I'm pretty sure this was already asked and answered here, but I can't find it now. $\endgroup$ – Mołot Feb 25 '17 at 23:06
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    $\begingroup$ As a good starting point, I suggest you read this answer on Physics SE and follow-up with this. From there, there are lots of related questions on Physics about this, or if you don't want to trawl through them or ask your own, you can always ask on chat (here or physics), although entanglement is one of the really, really confusing things in quantum physics that no-one truly understands, so getting a good explanation of what you're looking for might be difficult $\endgroup$ – Mithrandir24601 Feb 26 '17 at 12:13
  • $\begingroup$ @JDługosz if you feel there is a need for canonical question about this, then why can't it be this one:worldbuilding.stackexchange.com/q/22243/809 for example? Or worldbuilding.stackexchange.com/q/34683/809 ? First already is a good canonical about FTL communication, and second looks like good canonical about entanglement. $\endgroup$ – Mołot Feb 27 '17 at 22:15
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    $\begingroup$ @HDE226868 Check the revision history; it was JDługosz who added the "canonocal question" part, as well as (judging by the Markdown diff) pretty much rewrote the whole thing. Not appropriate, IMO, and probably makes for good Worldbuilding Meta fodder (but I don't have the time at the moment). $\endgroup$ – a CVn Feb 28 '17 at 14:25
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    $\begingroup$ Redacted Redacted, you can roll back any edits which have been made, if you so desire. There should be a button in the edit history. Additionally, you should be able to comment on your questions and ping anyone else who has commented or edited; just put @ before their username. $\endgroup$ – HDE 226868 Feb 28 '17 at 16:19
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No.

The Speed Of Light is Really the Speed Of Information Transfer

The speed of light isn't the speed of light, it's the fastest that information can be transferred across spacetime. Light travels at that speed because it has no mass. No information can be transferred faster than the speed of light. Note that there can be causes and effects which appear to violate this, but when you look at them closely you'll find that there is no information transferred.

Watch PBS Spacetime's video The Speed of Light is NOT About Light.

The Result Of Observing Quantum Entanglement Is Random

For simplicity's sake let's say our entangled particles can be spin up or spin down. If you observe one is spin up, the other must be spin down, and vice versa. But which spin a particle will be is random. You cannot predict, before you observe it, what spin your particle will be.

You also cannot influence the result. You cannot, for example, make your particle comes out spin-down to ensure the other particle comes out spin-up.

Observing One Does Not Collapse The Other

I believe the thinking is that when one particle is observed and collapses, the other particle simultaneously collapses, and Observer B can know when Observer A looked at their particle.

Again, no information can be transferred this way because Observer B cannot know their particle is collapsed without observing it which collapses their own particle's waveform. An observer cannot tell the difference between "the waveform collapsed because the other particle was observed" and "the waveform collapsed because I observed it". One observer cannot use entangled particles to signal to the other that they've observed their particle.

Quantum Entanglement Conveys No (New) Information

Quantum entanglement only appears to violate this constraint, but in reality no information can be gleaned from an entangled particle faster than the speed of light. You can't even know if the other particle has been observed.

For example, let's say you move two entangled particles and observers far apart. Then Observer A observes their particle A, and it is spin-up. When Observer B observes their particle B, it must be spin-down. Ah ha! The two particles must have communicated with each other! The jury is out the mechanism, see Bell's Inequality, but no new information was transferred between Observer A and Observer B. They already had it all, and it was transferred at or slower than light, because the observers had to carry the entangled particles with them.

Since the result of the observation cannot be predicted, Observer A cannot influence their particle to come up a particular spin and thus alter what Observer B sees. All they know is Observer A saw the opposite of what they saw. But even that conveys no new information, Observer A already knew Observer B would get the opposite result. Since the result is random, Observer A didn't influence Observer B's result either. Observer A doesn't know anything they didn't already know at the start, they've been carrying that information with them at conventional speeds.

Consider Two Envelopes

To see why, replace particle A and particle B with two sealed envelopes. Observers A and B are handed the envelopes and told that one says "1", the other says "0". Then they move apart from each other at (or near) the speed of light. Then Observer A opens their envelope and sees "0". Now they know Observer B got "1". Nothing special happened. The pair of envelopes, and information about their entanglement, were transported conventionally. Even if Observers A and B are light years apart, the envelopes, and the information about their entanglement, still had to start together and travel apart at less than the speed of light.

Most importantly, Observer A can not use the envelopes to send any information to Observer B. They can't predict what's in Observer B's envelope without opening their own, information which was contained in the envelope all along and carried with them conventionally.

They cannot influence what's in Observer B's envelope, that information was already set at the start (in reality it wasn't, read on).

They can't even use their timing of their opening to send a message. Observer B doesn't know that Observer A opened their envelope, just as with the particles Observer B doesn't know that Observer A observed their particle.

Yes, Spooky Action at a Distance. No, Hidden Variables.

The envelope analogy is good to understand why no information has been transferred, but it implies that the spin of the particles is predetermined. It turns out this is probably not true due to something called Bell's Inequality which states:

No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.

Local hidden variables are one of the ways to get around the "spooky action at a distance" in things like quantum entanglement. The claim is that the entangled particles have hidden variables, like their spin, which are only revealed by observations.

But experiments have shown this isn't true over longer and longer distances. The recent QUESS mission tested quantum entanglement, quantum encryption, and Bell's inequality in space for the first time over 1200 km. The results are still being analyzed.

So yes, it seems there is spooky action at a distance between quantum entangled particles. It seems they do influence each other's outcome instantly. But that's ok because no information is transferred. The universe appears to be weird like that, and the speed of light only applies to information transferred across space.

Veritasim explains Bell's Inequality and why it is most likely true in Quantum Entanglement & Spooky Action at a Distance.

A Note About "The Observer"

Just to be very clear, no, the "observer" isn't a person, unless it's these guys. There's no quantum woo here about consciousness.

Quantum "observation" means that two quantum systems have interacted causing the wave function to collapse. You are a quantum system, but so is a laser, a microscope, and a coffee table.

No Schrodinger's Observer

Let's try to transfer information. Observers A and B are in a suicide pact. They come together and generate a pair of entangled particles. One year from now the one whose particle is spin up will kill themselves.

One year later, and far distant, Observer A observes their particle and sees it's spin-down. They instantly know Observer B is supposed to kill themselves! Doesn't that transfer information instantly?

No, Observer A had the information with them all along. They had to communicate conventionally with Observer B to make the pact and get their entangled particle. They had to carry it with them, conventionally, the whole year. They could have, at any time, observed their particle. This is just like the pair of sealed envelopes.

Summary

  • The entangled particles must be together when they're created.
  • The entangled particles must be transported apart at conventional speeds.
  • The result of the observations are random, they cannot be influenced.
  • You cannot know whether the other particle has been observed.

The only information an entangled pair transmits is "we are paired" and that information is transferred at conventional speeds. Just because they happen to be observed when far apart doesn't convey any new information to the observers.

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  • $\begingroup$ Good answer in general, but it should probably be clarified somehow that apart from "speed of information transfer", "speed of light" here also means "speed of light in a vacuum". Also, the fact that the entangled particles must be together at creation and transported at regular speeds are of course constraints on any semi-practical application of QE as communication, but does not actually in itself mean that FTL communication through QE is impossible. $\endgroup$ – andejons Mar 15 '17 at 10:34
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I don't understand what you are describing.

When working, the systems correlates A's change for 1

Are you saying that particle A is manipulated and then A1 is inspected to see if it changed?

Entanglement does not work that way, and this is wrong in many ways.

If you manipulate A, you lose the entanglement with A1. That's the opposite of what you think.

But I can confidently answer “no” even without understanding your proposed scheme or even reading it, provided you are referring to real-universe physics.

The only thing that entanglement will do for you is: measurements of A and of A1 will be correlated. So, in the case where the same observable is used such as a polarization direction, A and A1 will both read random values, but both will know that the other read the opposite value. A cannot send a message to A1.

The fact that entanglement cannot be used for communication FTL is a theorem; that is, a mathematically proven result. This means that you will not find some clever way to exploit the effect, any more than you will find integers such that A×(B+C) ≠ (A×B)+(A×C).

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  • $\begingroup$ Do you mean "if you measure A" as opposed to "manipulate A"? $\endgroup$ – Mithrandir24601 Feb 25 '17 at 23:54
  • $\begingroup$ No, I figure the OP is thinking that purposfully changing one causes the entangled partner to reflect the change. But it doesn’t matter; without waiting for clarification I can say that it can’t work. $\endgroup$ – JDługosz Feb 25 '17 at 23:58
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    $\begingroup$ So, the spin values of the particle determined randomly when breaking the superposition? $\endgroup$ – Mephistopheles Feb 26 '17 at 9:45
  • $\begingroup$ (reads some information about how fragile entanglement is) Dammit! I have to search for an alternative solution. $\endgroup$ – Mephistopheles Feb 26 '17 at 18:16
  • $\begingroup$ There is no real-world solution involving entanglement. There are existing questions (bearing lots of good answers) on in-story FTL communications and/or travel ideas that will be helpful in selecting something. $\endgroup$ – JDługosz Feb 27 '17 at 1:57
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I think one thing that may be missing in this discussion is String Theory. We are presuposing a restriction to the transfer of information faster than light using entangled pairs based on its restrictions in our dimension. It is fully possible that the connection between pairs passes through one of the other 9 dimensions and thus ignores that restriction. Also the observation hypothesis is only relevant to a first person observer who exist in the destination state. If the observation is in fact observed transitionally by a third person "non-human" observer and that information is translated to the same state first person observer you do not violate any rules.

Of course this is dependent on string theory and multiple dimensions where the Spookiness happens which allows for entangled particles to violate any rules with regard to FTL transmission.

One thing that may be a big factor in this is resonance. As resonance begins to break down we see less reliability in same state results between pairs. Therefore many different things could cause a break down in reliability in a Quantum Communications system. The result being that AI's interpreting results might have a sliding scale regarding a messages reliability based on spacial positioning at the time of communication.

The real truth is that from a writers standpoint it leaves you with a lot of potential.

When it comes to this stuff even the most brilliant Theoretical Physicist disagree. We are supposed to be exploring the possibilities of science not necessarily its conclusion. Thus the Fiction part. In this case a great deal of the science is completly unknown until we have a better grasp of the physics involved. Any number of discoveries being worked on could open up this entire possibility or shut it down.

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