Why you can’t use quantum entanglement for faster than light communication

Albert Einstein, with his theory of special relativity, established that the speed of light is the absolute speed limit of the universe.  A rocket ship attempting to accelerate to the speed of light encounters some well known effects: time dilation, mass increase, and length contraction.  The closer to the speed of light it gets, the higher its mass climbs, the slower its passage of time, and the shorter its length.  To actually reach the speed of light, it would need to acquire infinite mass, zero passage of time, and zero length, which would require infinite energy.  (Translation: you can’t do it.)

Things are marginally more hopeful for photons, which have no mass.  They always travel at the speed of light.  As the unit of all electromagnetic radiation, they enable communication at the speed of light.  But that’s the fastest they enable it at.

Often when these facts come up in discussions, someone raises the possibility of using quantum entanglement for communication.  Entanglement, we are told, is a non-local effect.  Doesn’t this, as a fair amount of science fiction implies, mean that there might be some effect we could use in the future for faster than light communication?

Unfortunately, the answer is no.  This doesn’t come from a pessimistic view of the possibilities, but from an understanding of what entanglement actually is, an understanding I have to admit I’ve only recently fully come to appreciate.  I first got it when reading Adam Becker’s What Is Real?, but wanted to wait to discuss it until I’d gotten some confirmation from Sean Carroll’s Something Deeply Hidden, which I’m currently reading.

That understanding is that entanglement is inherently about information.  When two quantum objects interact, they become entangled with each, meaning that they’re described by an overall common wave function.  But that description, for most people, isn’t very enlightening.   So let’s do an analogy.

Imagine both Alice and Bob, living far away from each other, each have a subscription to the New York Times, and each of them knows about the other’s subscription.  Let’s further suppose they both have very reliable and timely delivery of their paper.  When Alice gets a particular issue of the Times and looks at it, she knows that Bob is getting the same issue with the same information.  You could say that each copy of the same issue of the Times is entangled with every other copy, including Alice’s and Bob’s, which is to say, they share a causal history that enables information about one to provide information on the other.

So far this isn’t any big deal.  Alice and Bob each know what the other is seeing, but can’t use that information in any way to communicate with each other.  If Bob alters his copy of the Times, it doesn’t effect Alice’s.  All it really does is break the entanglement between them, that is, erase his ability to use his paper to know what’s in Alice’s copy.  (Technically since information is always conserved, it spreads the entanglement around, but let’s not get sidetracked.)

So what’s the big deal with entanglement?  Well, let’s say that a very special issue of the Times comes out, a quantum version of the paper, one that is in a superposition of possible states until a reader actually looks at it.  One branch of the superposition says the stock market went up yesterday, the other says it crashed.  Under standard interpretations of quantum mechanics, it is meaningless to talk about what the paper actually says until someone looks at it.

But, as soon as Bob or Alice actually look at their paper, the wave function of the quantum copy collapses into a definite value.  When Alice looks at her copy, she knows what Bob will see, even though Bob hasn’t looked at his yet.  This is true even if Alice and Bob are separated by light years.  In other words, what the paper says isn’t a definite value, until either Bob or Alice (or some other subscriber) looks at theirs, but as soon as either does, the other’s copy instantly becomes definite too, with the same values.  But if both copies were in an undefined state prior to their collapses, how do those copies “know” which one to collapse to so they agree with the other?

This is the aspect of quantum theory that bothered Einstein enough to co-author a paper with Boris Podolsky and Nathan Rosen in 1935, the famous EPR paradox paper.  In their view, it indicated that quantum theory could not be complete.  Einstein famously called it “spooky action at a distance”.  Bell’s theorem would eventually prove he and his co-authors wrong, at least if everything is happening in one consistent universe.

But just like our classical edition of the paper, there’s nothing Alice or Bob can do to their quantum copies that would allow them to communicate.  Again, if Bob alters his copy, all he does is break the entanglement (technically spread it around).

Bringing this back home to particles, there’s nothing you can do with one particle of an entangled pair of particles that will control the state of the other particle.  (Other then bring them back together and have them interact again.)  Yes, the act of measuring the first particle causes the other to assume a definite value, but there’s no way either party can know ahead of time what those values will be.  And attempting to control them, alters the particle’s state, breaking (spreading) the entanglement.

This might be frustrating, because we seem so close.  But of course, that closeness is an illusion, borne of a misunderstanding of what actually happens with entanglement.

To be clear, quantum entanglement, under most interpretations of quantum mechanics, violates the spirit of special relativity.  It allows communication of a sort between the entangled items, but it doesn’t violate the letter of relativity, since it’s not communication we’re able to actually do anything with.

Unless of course, I’m missing something?

I would draw a similar graph for FTL and knowledge of physics

Click through for full sized version and the red button caption.

via Saturday Morning Breakfast Cereal.

Sometimes, learning science means discovering the constraints reality puts on us, and that part isn’t always fun.