Spooky action communication

[PeterPendragon]

If entangled pairs can be used to communicate instantaneously, does that mean one of the paired particles has to be physically delivered to the reception point in order to establish communication?

 

[jfizzix]

If entangled pairs can be used to communicate instantaneously, does that mean one of the paired particles has to be physically delivered to the reception point in order to establish communication?

If entangled pairs could be used to communicate instantaneously, you would still need to deliver half of each pair to the reception point.

However, entangled pairs cannot be used for instantaneous communication.
The reason is simple.
If you have a group of particles and no outside info...
(so you're trying to figure out what's going on based on what you yourself can measure)
...then there is no way of telling if these particles are halves of entangled pairs or not.

I believe it's called the no communication theorem, if you want to look it up.

 

[mfb]

...then there is no way of telling if these particles are halves of entangled pairs or not.

Even if you would know that, you cannot use it for instantaneous communication. While a proper quantum mechanical description is a bit more complicated, here is a classical analogy that captures the relevant features: Write "yes" on one paper and "no" on another, put them in separate identical-looking envelopes and mix them, then give the envelopes to people far away. Before you open the envelopes you don't know which answer you will find. You know whenever someone opens the envelope and sees "yes" or "no", then the other envelope will have the opposite answer. But you cannot influence that answer, so you cannot use it for communication.

 

[PeterPendragon]

Even if you would know that, you cannot use it for instantaneous communication. While a proper quantum mechanical description is a bit more complicated, here is a classical analogy that captures the relevant features: Write "yes" on one paper and "no" on another, put them in separate identical-looking envelopes and mix them, then give the envelopes to people far away. Before you open the envelopes you don't know which answer you will find. You know whenever someone opens the envelope and sees "yes" or "no", then the other envelope will have the opposite answer. But you cannot influence that answer, so you cannot use it for communication.

Thanks. I get that, but I thought if you altered the state of one pair (eg spin), you changed the other, hence communication?

 

[DrewD]

Thanks. I get that, but I thought if you altered the state of one pair (eg spin), you changed the other, hence communication?

You are not alone in this thought, but you are not correct. The problem in your intuition is that the entangled particles are not separately in fixed states that change. They are in a shared state and you cannot control which one ends up in which outcome. It is only guaranteed that they will be "opposite" states.

 

[mfb]

Thanks. I get that, but I thought if you altered the state of one pair (eg spin), you changed the other, hence communication?

In some interpretations you change the other, but you still cannot predict in which way you change it - you change it randomly (simplified description). You can measure that afterwards, but then you cannot change it any more.

 

[DrChinese]

Thanks. I get that, but I thought if you altered the state of one pair (eg spin), you changed the other, hence communication?

And just to add to DrewD and mfb's excellent replies: You cannot actually determine (or distinguish) whether A influences B (where measurement of A occurs before measurement of B) or the reverse (also where measurement of A occurs before measurement of B). That certainly takes the heart out of the argument that communication is occurring.

 

[PeterPendragon]

Okay, all this is great, I appreciate all the answers. Using mfb's paper analogy, it seems that since there are only two possibilities, of course when you open one, the other is the opposite. And, why can't you "change it anymore"? I thought the whole point was that if you changed the state of a member of an entangled pair, it's partner also changes. Could anyone recommend a good but relatively simple article or book I could read on this? Thanks.

 

[mfb]

And, why can't you "change it anymore"?

Entanglement is broken once you measure the entangled property.

I thought the whole point was that if you changed the state of a member of an entangled pair, it's partner also changes.

In some interpretations of quantum mechanics, not in all. That alone shows that you cannot transmit information.

 

[Nugatory]

And, why can't you "change it anymore"? I thought the whole point was that if you changed the state of a member of an entangled pair, it's partner also changes.

That's not how it works. If you change the spin of either member, the entanglement is broken. All entanglement says is that if you measure one member of the pair, then you know what the value of the the corresponding measurement on the other will be, when and if such a measurement is made - for all you know, that other measurement has already been made. Furthermore, it only works for the first measurement on each member - after that the entanglement is broken.

Could anyone recommend a good but relatively simple article or book I could read on this?

"Sneaking a look at God's cards" by GianCarlo Girardi covers a lot more than just entanglement, but it's good and relatively simple.

 

[PeterPendragon]

Thanks all, especially mfb and Nugatory who really got me to understand. Okay, so maybe not for this forum, but why this, I think, common misconception about faster than light communication?

 

[mfb]

It makes great news headlines. Same with many other things - some oversimplified or misinterpreted statement produces more clicks than the truth. See also how often the Higgs boson is called "god particle" in the news. No scientists uses that name, that's a pop-science-only name.

 

[PeterPendragon]

Yeah, I used to work at NASA. You've seen the mermaid, human head, rat etc we've found on Mars!