# B Faster than light communication via entanglement?

1. Mar 8, 2016

### Chrisjohnson

So quantum entanglement acts faster than light but it's not technically communication and not actually transferring "information"?

Can someone explain this to me?

2. Mar 8, 2016

### phinds

I'd suggest a forum search. It's been explained here dozens of times.

A good place to start is the list of "similar discussions" threads at the bottom of this thread.

3. Mar 8, 2016

### Strilanc

Even though a classical simulation would require communication behind-the-scenes, entities within the simulation can't bootstrap that into actual useful "Hey how are you doing I'm fine how are you?" communication.

Scott Aaronson puts it this way:

A simple example of a system with this requires-hidden-communication-but-can't-be-used-for-communication property is magic boxes that can be asked two yes/no questions, and the boxes' answers disagree if and only if both boxes are asked the second question. (These hypothetical boxes are even more strongly non-local than quantum mechanics.)

4. Mar 8, 2016

### Staff: Mentor

Suppose you have two entangled spin-1/2 particles with total spin zero. Then when you measure the individual spins along some axis, they must have opposite spins. So if you measure particle A to be "spin up", then someone else must measure particle B to be "spin down." However, you can't force particle A to be "spin up" (as opposed to "spin down"), so there's no way to use it as a "transmitter" for a signal to particle B's observer.

5. Mar 9, 2016

### Chrisjohnson

So it's not "classical communication" but there is a sort of "communication" in a manner of speaking between the two particles?

And if we could find a way to control the particles then we could use it for simple FTL communication?

6. Mar 9, 2016

### Strilanc

No. You can't use it for communication.

7. Mar 9, 2016

### Chrisjohnson

I thought if we could spin it then we could maybe do something like Morse code. Based on what that guy said

8. Mar 9, 2016

### phinds

You are missing the point. The spin is whatever it is when you measure it, not something you can set it to be. It is random. What is not random is that whatever it is when you measure it, the other particle will be the opposite. You have NO way of knowing whether the other person has already measure the other particle unless that information is communicated sub-FTL.

9. Mar 9, 2016

### DrChinese

Particles can be entangled in one or more observables (spin, momentum, etc). If they are entangled in some basis, they are in a superposition and do not have a well defined value on that basis. Vice versa: if they have a well defined value, they are not entangled on that basis.

So if they are entangled, you never know their value in that basis until you measure. And the result of such measurement is always a random value. Not much information to be had there.

10. Mar 9, 2016

### Chrisjohnson

Well it was just a hypothetical. "IF" we COULD change the spin, then we could use it, correct?

Just wondering. Idk why people are being so condescending.

11. Mar 9, 2016

### Strilanc

If you're just going to change the rules of reality, you can do whatever you want. This is a forum for asking about what happens when you use the actual rules.

12. Mar 9, 2016

### Chrisjohnson

More condescention...

Hypothetical questions can give real answers. Im simply trying to understand the nature of this reality and an answer to that specific question would help, even if only a little.

Like "if we could lift the earth and drop it when would it reach terminal velocity and how many pound of force would the impact be from a drop of 10 feet" (forgive my bad science, I'm not very adept) this question is something we can't do but still can give us insight.

Can't expect help here I guess. Not without criticism.

13. Mar 9, 2016

### StevieTNZ

Yes, if you could control whether you got spin up or down, then it would be possible. But changing it AFTER measurement would not allow such communication.

14. Mar 9, 2016

### DrChinese

Chris,

A number of people took time to provide you with some relevant information. I don't think there is any condescending evidenced. If you have more questions, you should ask them. But please realize that some people here are not speaking English as a first language, and many assume you will look around and understand the format and general practice of the forum. So I would focus on the answer, not the tone of the answer.

-DrC

15. Mar 9, 2016

### Chrisjohnson

I didn't get an answer, more or less a confirmation on that "it's not classical communication but there is some sort of "communication" between the two particles."

You know, Not in the literal sense, but like cause and effect. 1 affects the other.

Right? I'm noob

16. Mar 9, 2016

### phinds

"Communication" normally means some sense of "the transmission of something meaningful". In that sense, there is no information transmitted. There is simply a situation where you have what amounts to being two halves of a single construct that is such that when you know something about one half, you automatically know something about the other half, but that does not "communicate" anything meaningful between the two halves. You can verify after the fact of measuring one that measuring the other gave the expected result, but since you already knew that, it doesn't tell you anything.

As for the answers to you, as has been said, this is a forum about real physics. It is not to answer questions that amount to "if the laws of physics don't apply, what would the laws of physics say about <insert nonsense of your choice>.

17. Mar 9, 2016

### Chrisjohnson

Can energy be created or destroyed? That's the laws of physics right?

There's a debate everyday about "something from nothing" and universe from nothing. Against the current laws of physics but they are subject to change.

And it's a hypothetical, hypotheticals have a purpose. What don't you get exactly? I'd be happy to explain it to you.

18. Mar 9, 2016

### StevieTNZ

19. Mar 9, 2016

### Staff: Mentor

Closed pending moderation

20. Mar 11, 2016