Unconditional quantum teleportation?

[sol invictus]

Really? I thought that it was the fact that the information is random and non-deterministic is why it didn't violate causality.

If the "information" is "random and non-deterministic", then it's not information.

Maybe I'm mixing up measuring the state of one of an entangled pair with actually "teleporting" the state. But the former case seems to involve information transfer at more than the speed of light.

Depending on your interpretation, measuring one particle of the pair either instantly affects the other - but in a way that conveys zero information - or simply doesn't affect it at all. I prefer the latter interpretation, but in any case there is no information transfer. Teleporting a state is transporting information, and it cannot and does not happen faster than light.

Why? If in my frame A precedes B , but from yours B precedes A, there must be some frame in which A and B are simultaneous?
Why is that more paradoxical than B preceding A? (Which I'm jolly sure it didn't!)

If in some frame A and B are simultaneous, then there exist some frames where A precedes B, and some other frames where B precedes A. That follows from the Lorentz transformation (the defining equations of special relativity), and it means that if you can transmit information instantaneously, you can also send it back in time. Are you asking how the Lorentz transforms work, and why the above is true?

What is NOT (necessarily) true is the converse: that if in some frame A precedes B, there is another where B precedes A (or where they are simultaneous).

 

[Soapy Sam]

If in some frame A and B are simultaneous, then there exist some frames where A precedes B, and some other frames where B precedes A. That follows from the Lorentz transformation (the defining equations of special relativity), and it means that if you can transmit information instantaneously, you can also send it back in time. Are you asking how the Lorentz transforms work, and why the above is true?

This is one of these things I think I understand until suddenly I realise I don't. Just at the moment (and I submit drink has been taken) it seems to me that if I switch on a lamp (A precedes B) and someone in an appropriately different frame sees the lamp go on before he sees me press the switch, his POV is simply wrong. But you (I strongly suspect) have reason to see things differently. So yes, a brief reminder about frames of reference is probably called for. (I may postpone reading it till tomorrow though, thanks).

What is NOT (necessarily) true is the converse: that if in some frame A precedes B, there is another where B precedes A (or where they are simultaneous).

 

[xtifr]

If the "information" is "random and non-deterministic", then it's not information.

Maybe "non-deterministic" is not the right word?

Depending on your interpretation, measuring one particle of the pair either instantly affects the other - but in a way that conveys zero information - or simply doesn't affect it at all. I prefer the latter interpretation, but in any case there is no information transfer.

But if I measure the particle, I now know what the state of the other particle is/was/will be when/if it's measured, even though I'm assured that there are no hidden variables storing that "information", and the state really doesn't exist until it's measured. That seems like a transfer of knowledge at least. I know a result of a physical action (measurement) potentially before that...I don't know what else to call it but information...could have reached me at the speed of light.

But perhaps this is a question for another thread.

Teleporting a state is transporting information, and it cannot and does not happen faster than light.

Yes, thanks. That part actually makes sense to me, which is a lot more than I can say for a lot of this stuff!

 

[Roboramma]

This is one of these things I think I understand until suddenly I realise I don't. Just at the moment (and I submit drink has been taken) it seems to me that if I switch on a lamp (A precedes B) and someone in an appropriately different frame sees the lamp go on before he sees me press the switch, his POV is simply wrong. But you (I strongly suspect) have reason to see things differently. So yes, a brief reminder about frames of reference is probably called for. (I may postpone reading it till tomorrow though, thanks).

If you switch on a light you switching it on and the light turning on are not simultaneous in your frame. A happened before B. All observers in all frames will agree that A preceded B. This is true of all causal events.

Basically if in your frame you can send a signal at the speed of light (or slower) from A before B happens, then all observers in all frames will agree that A happened before B.

If a signal sent at the speed of light can't get from event A (you flipping the switch) to event B (the light turning on) then different observers will disagree about the ordering of events. But then in that case clearly your flipping of the switch couldn't have been what caused the light to turn on.

 

[Soapy Sam]

If you switch on a light you switching it on and the light turning on are not simultaneous in your frame. A happened before B. All observers in all frames will agree that A preceded B. This is true of all causal events.

Basically if in your frame you can send a signal at the speed of light (or slower) from A before B happens, then all observers in all frames will agree that A happened before B.

If a signal sent at the speed of light can't get from event A (you flipping the switch) to event B (the light turning on) then different observers will disagree about the ordering of events. But then in that case clearly your flipping of the switch couldn't have been what caused the light to turn on.

Thanks.
Lesson learned:-Booze Good. Physics Good. Booze + physics, not so good.

 

[sol invictus]

Thanks.
Lesson learned:-Booze Good. Physics Good. Booze + physics, not so good.

Drunk deriving is not recommended.

 

[sol invictus]

But if I measure the particle, I now know what the state of the other particle is/was/will be when/if it's measured, even though I'm assured that there are no hidden variables storing that "information", and the state really doesn't exist until it's measured.

Whether or not the state "exists" prior to the measurement is a very tricky question. Anyway, it's true that you gain information about the other measurement, but...

That seems like a transfer of knowledge at least.

...there's no "transfer" of anything. Your counterpart learns nothing.

Perhaps you are imagining that information was transferred to you from the other particle, but that is not the case. You are simply learning something about the other particle by measuring your particle, and then combining the result with some prior knowledge you had of the correlated state of the two particles. There is no "transfer" of anything.

 

[Farsight]

Sounds good to me, sol.

But sadly IMHO, some people peddle quantum mysticism, and suggest that there is indeed some magical mysterious spooky action-at-a-distance going on. I'm with Einstein on saying there isn't. And Newton:

"That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an absurdity that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it”

That was in a letter to Dr Richard Bentley on 25 February 1692. Yes he was talking about gravity, but IMHO the same general principle applies to "quantum teleportation".

 

[Soapy Sam]

Drunk deriving is not recommended.

Hisssssssssssssss