What's the fuss over quantum entanglement about? Is it because people have misunderstood?

Question

The first video I've seen about quantum entanglement claimed that changing A's spin will immediately take effect on B's spin as well. The more I read into this topic the less it seems that this is true. Heading back to documentaries, many state this as being a fact but leaving pop science areas and reading into scientific papers and forums I can find almost nothing about this - the latter is rather full of people stating the opposite.

I'd appreciate clarification on the following:

• Generally speaking, who is right on this? Do spin changes affect the other's spin?

• My biggest hope in entanglement was the opportunity to speed up communication - apart from spin, are there other factors that, done to A can affect B immediately so e.g. binary data could be transferred?

• If above questions can be answered with no, what's left to explore then? Is there some extraordinarily interesting area to explore or is it worth exploring but nothing out of ordinary except for the fact that the media has made it blow up?

Answer 1

The thing about quantum mechanics is that it is counter-intuitive. In other words, it does not seem to behave in the way that we are used to in the macroscopic world. If one reads about the EPR paradox and about the violation of Bell's inequality one finds out that this counter-intuitive aspect is revealed by the observation that nature does not obey local-realism.

What does it mean? It means that nature is either nonlocal or that it does not have a unique reality (or both). As far as I know, we do not yet know which of these scenarios is actually valid. However, it seems that the general concensus is that nature is local but does not have a unique reality.

So what does this say about entanglement? If nature were nonlocal then it would have been possible to change the spin of an object far away by measuring the state of a local object that is entangled with the far away one. However, this interpretation runs into all sorts of problem when relativity is brought into the picture. So therefore we pick the other option, namely that there is no unique reality.

What does that say about entanglement? To understand this, it is useful to work with a particular interpretation of quantum mechanics. One can pick any one of the gazillions of interpretations, because none of then can be ruled out by experiments. So I'm going to pick the many-worlds interpretation. I don't necessarily believe that nature works that way, but it is useful to understand how quantum mechanics works.

According to the many-world interpretation, entanglement implies that there are different realities with different combinations for the states. For instance, in one reality particles A and B both have horizontal polarization (spin) and in another reality both have vertical polarization. By measuring the local particle, one fixes the reality in which you made the observation. In that reality the spin of the far away particle is then fixed to be that same as that of the local article. Note however that in this way one did not actually change the spin of the far away particle. One merely selected the reality in which it exists.

Answer 2

With quantum entanglement, the spins of A and B will be opposite if measured in the same orientation. If the entanglement is not broken, the particles can be moved arbitrarily far away, and their spins will still be opposite. (If you measure them in different orientations, things get weird. It's as if they don't actually have definite spins, but the spins they don't have are opposite. That's why I'm only talking about measurement in the same orientation.)

We can measure the spins on whatever particle we have handy, and if we do we'll know what the spin on the other particle is. If we change the spin on our particle, we've broken the entanglement. Nothing happens to the other particle. If someone measures it, and sends us the measurement, we'll find that they were opposite. (If they're separated in space, the words "immediately" or "at the same time" aren't well defined, but that's relativity for you.)

Let's take a non-quantum analogy. I have a red card, a black card, and two identical opaque envelopes. I put the cards in the envelopes and send one of them to you. When you open your envelope, you know what card I kept, but if you put a different card into your envelope it doesn't affect the card I kept, and there's no way to communicate this way. (This analogy works for a few special cases of quantum entanglement and measurements, and breaks down real fast if you start varying things.)

Answer 3

It is true. If A and B are entangled but are, for example, light years apart, then when A is observed B's spin will immediately hold the opposite spin. This is true regardless of distance. But if that information takes time (limited by the speed of light) to go from A to B after A's spin is measured, then that would allow A and B to be measured at the same time, potentially introducing contradictory spin values, which is impossible, so we know that the "transfer" cannot take time.

This is not a problem. When observed, quantum spin is random. You cannot intentionally send any meaningful information from A to B or assign any meaningful value to different spins before distancing A and B. Thus, this immediate transfer does not introduce any paradox of time. A cannot send a message to B.

For any information transfer to be meaningful, B would have to be able to send back information in consequential response to A's spin before A is observed, which introduces a variety of impossibilities. In any case, that we cannot define a particle's quantum spin, that we can only observe it, makes this immediate transfer useless. Trying to set the spin breaks the entanglement.