Author Topic: Almost an ansible? entanglement without classical particles or EM Comms  (Read 47703 times)

Offline Stormbringer

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http://phys.org/news/2015-03-quantum-scheme-states-transmitting-physical.html

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What's key is that during the entire process, the photon did not enter the channel between sender and receiver: As long as the photon passes through the channel, it will be absorbed by the ensemble if the control atom is in the ground state – but if the atom is in the Rydberg state, the photon will be absorbed by the detector. In this way, an unknown quantum state can be transferred between two distant participants without any physical particles traveling between them.


It looks to me like it says you can send information without sending particles or energy the classical way. So far all quantum communications had to have a classical component and it meant that no information exchange could actually take place faster than FTL. But this seems to say that is not needed.

But it also looks like they take pains to say (at least twice) that it applies to unknown information or an unknown state... So i am confused about whether this is FTL communications or not. What are they saying?

« Last Edit: 03/11/2015 02:28 am by Stormbringer »
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Offline Robotbeat

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Nope, you still can't send classical information in this manner faster than light.
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Offline JPLeRouzic

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http://phys.org/news/2015-03-quantum-scheme-states-transmitting-physical.html

Quote
What's key is that during the entire process, the photon did not enter the channel between sender and receiver: As long as the photon passes through the channel, it will be absorbed by the ensemble if the control atom is in the ground state – but if the atom is in the Rydberg state, the photon will be absorbed by the detector. In this way, an unknown quantum state can be transferred between two distant participants without any physical particles traveling between them.


It looks to me like it says you can send information without sending particles or energy the classical way. So far all quantum communications had to have a classical component and it meant that no information exchange could actually take place faster than FTL. But this seems to say that is not needed.

But it also looks like they take pains to say (at least twice) that it applies to unknown information or an unknown state... So i am confused about whether this is FTL communications or not. What are they saying?
My understanding (and I am not a scientist) is that a photon goes several times from Alice's side to Bob's side as it is shown in the pictures. So it is not FTL per definition.
I think what they meant is that contrary to quantum entanglement they don't need to transport states from the place of entanglement to the measurement place.
* In quantum entanglement at the moment one side makes a measurement, it can deduce the measurement at the other side, without looking at the other side but having painstakingly transported in a classical manner both entangled states from the place of entanglement to their present locations.
* Here the quantum state on Bob's side is determined by Bob influence on the atoms but this state information is not transported "classically" on the Alice's side. However photons are going from Alice's side to Bob' side and vice versa. The authors argue that as the photons don't really read the state on Bob's side, then the wavefunction doesn't collapse at the first travel and state information is transfered on Alice side without time being spent.

To give an analogy:
- in normal entanglement, one breaks a set of cards in two and transport them in two different locations. In one of them Alice browse her own subset and therefore knows instantly what Bob's subset is composed of.
- In the article, Bob breaks a set of cards in two and authorizes Alice to read classically in one of the two subsets, but only one card at a time. At the end Alice knows what is in the first subset and instantly knows what is in the second subset.
What Shakespeare said about EMDrive or blackholes is also valid for entanglement "much ado for nothing".
 8)
« Last Edit: 03/11/2015 09:02 am by JPLeRouzic »

Offline Stormbringer

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Offline KelvinZero

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The real problem is not how to build an instantaneous communicator. The problem is that two events at different locations that are simultaneous to one observer are not in general simultaneous to another, and you could trivially create a time paradox.


Offline Paul451

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This has always bugged me. Wouldn't the time paradox not matter, because the second observer could not communicate any information about the time paradox he witnessed back to the first observer before the paradox happens in the first observer's frame of reference?

Apparently there's always a frame of reference that would allow this. No matter how the FTL system worked, there's a way to design a paradox around it.

[I can occasionally get my head around specific cases, like communication using wormholes, but I don't understand the general case. You might have better luck.]

Offline ddunham

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Wouldn't the time paradox not matter, because the second observer could not communicate any information about the time paradox he witnessed back to the first observer before the paradox happens in the first observer's frame of reference?

Just because it's not a problem in one frame doesn't keep it from being a problem in another frame. 

If you keep speeds below c, then no matter how you shift things around, all observers agree on causality.  Either A happened before B, B happened before A (both time like), or A and B are not connected (space like).  All inertial frames get the same result.

As soon as information can exceed c, then causality of some events changes in the view of some observers.  They will not agree on whether A caused B or B caused A, something that was well-defined before.  Because of this you can get a set of observers that see sequentially that A causes B, B causes C, and C causes A.  That's a problem.

Offline Bob Shaw

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Information cannot exceed C; wavefronts may do so, but can't carry information.

We're trying to use English here to describe subtle mathematics, and our innate perception of causality etc interferes with any understanding of the outer universe - we are simply one of the Great Apes, looking out into a Universe which doesn't fit in with our mental predispositions, and attempting to erect a narrative.

Go and read Jastrow.

Offline QuantumG

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As soon as information can exceed c, then causality of some events changes in the view of some observers.  They will not agree on whether A caused B or B caused A, something that was well-defined before.  Because of this you can get a set of observers that see sequentially that A causes B, B causes C, and C causes A.  That's a problem.

Why is that a problem?

Who cares if two observers disagree about what they see? That happens all the time now and we all continue to exist.
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Offline Nilof

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As soon as information can exceed c, then causality of some events changes in the view of some observers.  They will not agree on whether A caused B or B caused A, something that was well-defined before.  Because of this you can get a set of observers that see sequentially that A causes B, B causes C, and C causes A.  That's a problem.

Why is that a problem?

Who cares if two observers disagree about what they see? That happens all the time now and we all continue to exist.

Because if you have an ansible that can transmit information faster than light, you can send information back into your past light cone, something that all observers will agree on, and you get things like grandfather paradoxes.
« Last Edit: 04/04/2015 01:00 pm by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline QuantumG

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Because if you have an ansible that can transmit information faster than light, you can send information back into your past light cone, something that all observers will agree on, and you get things like grandfather paradoxes.

Again.. who cares? While I think we can all agree that it would be strange, it's hardly much stranger than, say, Schrödinger's cat and - last I checked - there's little to support the notion that quantum mechanics should be rejected because it's so weird and wonderful. All the arguments against the possibility of faster than light travel, instantaneous communications or, heck, even time travel, amount to this same argument. We're supposed to cross our arms and scream "impossible!" because it'd ruin our neat little ideas of how causality works. Well boo hoo. We've known for a very long time that the real world is weirder than we can imagine.
Human spaceflight is basically just LARPing now.

Offline Nilof

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Sure. But it is useful to point out that the consequences of even the slightest form of FTL effect are very drastic - it'd imply that the universe would be strongly acausal. For example, if you could use entanglement to transmit information, you could mess with events in billion-year old faraway stars or galaxies just by looking at them.

At some point the consequences become so strong that you'd have to start wondering why we haven't seen any time travellers from the future.
« Last Edit: 04/04/2015 02:51 pm by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Star One

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Sure. But it is useful to point out that the consequences of even the slightest form of FTL effect are very drastic - it'd imply that the universe would be strongly acausal. For example, if you could use entanglement to transmit information, you could mess with events in billion-year old faraway stars or galaxies just by looking at them.

At some point the consequences become so strong that you'd have to start wondering why we haven't seen any time travellers from the future.

How would we know that we haven't, if you're intent on going down that particular metaphorical rabbit hole.

Offline KelvinZero

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Sure. But it is useful to point out that the consequences of even the slightest form of FTL effect are very drastic -
I think you might be confusing them if you say "sure". The implication is that FTL gives you timetravel and we can just get used to that. What it really gives you is a paradox. FTL is probably no more a route to time travel than 1+1=3 is a route to free energy.

Anyone who thinks they can live with it should at least attempt to decipher this new world and explain it to us. What sort of world is it where one observer at one velocity sees a man dying from a poisoned kool-aid while his phone rings beside him, while another observer at a different velocity sees him answer his phone and pour out the poison?

Offline MichaelBlackbourn

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Sure. But it is useful to point out that the consequences of even the slightest form of FTL effect are very drastic -

Anyone who thinks they can live with it should at least attempt to decipher this new world and explain it to us. What sort of world is it where one observer at one velocity sees a man dying from a poisoned kool-aid while his phone rings beside him, while another observer at a different velocity sees him answer his phone and pour out the poison?

Manyworlds interpretation would.

Offline KelvinZero

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Manyworlds interpretation would.
I don't see how in this particular case. The two observers at different velocities could still come back together and compare results. In fact they could both meet up at the crime scene.

Offline Stormbringer

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I think we are in trouble if our scientific analysis consists of "This violates some fudged principle created to make physicists feel better about the universe on philosophical grounds."

There is absolutely no evidence of which I am aware that any of these postulates about preserving order have any basis in physical facts let alone that they are universally true.

Furthermore physicists will evoke time travel when convenient. Feynman and wheeler posited advanced waves and retarded waves traveling into the future and the past to solve certain otherwise intractible problems.

Recent entanglement experiments strongly supported future measurements affecting the state of particles that had already ceased to exist.

Relativity allows (so far) limited time travel but it is time travel. all of it allowed by the rules; given a traversible wormhole which is not yet ruled out. Relativity allows it; physicists have *faith* however that something in Quantum Gravity theory which hasn't been discovered yet will forbid that.

*Faith* that some how "theory X" will forbid it. LOL. Better get out the robes, incense and ritual fetishes.

EDIT: Unless i am mistaken any relativistic travel allows time travel and we have observed physical objects with mass that are traveling at relativistic speed with our astronomical instruments. Certain celestial objects that got booted by a massive gravitational partner at some point. If one of these objects were to return to it's starting point at speed it would amount to time travel. 
« Last Edit: 04/05/2015 09:15 am by Stormbringer »
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Offline Nilof

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Furthermore physicists will evoke time travel when convenient. Feynman and wheeler posited advanced waves and retarded waves traveling into the future and the past to solve certain otherwise intractible problems.


The Feynman propagator does indeed have a part that can seem to violate causality, but the existence of antimatter automatically ensures that these terms always exactly cancelled out so that all local observables commute(are independent) if they are separated by a spacelike distance.

What one really needs to appreciate is that for every form of physics discovered so far has not just been causal, but explicitly checking whether a theory is causal has been a very useful tool to better understand it.

First of all, any form of time-travel would have to rely on entirely new and undiscovered physics. There are theorems in both General Relativity and quantum field theory that show that the theories are causal under very general assumptions (CPT invariance, averaged null-energy condition).


With that said, you could split up the possibillities into five classes, in increasing order of unlikeliness:

1) "Boring physics" - Faster than light travel and information transfer is not possible. Causality holds.
2) Hawking's chronology protection conjecture - FTL possible, but no causality violation: Limited forms of faster than light travel including some types of wormholes are possible, but closed timelike curves are explicitly prohibited.
3 & 4) No causality but reasonable spacetimes: Time travel is possible, and paradoxes are handled by either a foliation of spacetime, or through some prescription of Novikov's self-consistency principle. In some viewpoints (usually many-worlds inspired) the two may be equivalent.
5) The universe is not a Hausdorff topological space. Really pedantic and difficult to construct any natural examples of, but occasionally included in such lists as it is a common axiom in GR theorems that all manifolds are Hausdorff.
« Last Edit: 04/05/2015 01:55 pm by Nilof »
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline Stormbringer

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Even time travel where you could not effect your own timeline would be very very useful for information gathering and for stealing acquiring artifacts (to preserve them for posterity) and grabbing specimens say for botany, biology or zoology. if there are  "many worlds" then some would be so close to identical that the only difference could be some detail so minor as to be unnoticeable.
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Offline Paul451

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I think we are in trouble if our scientific analysis consists of "This violates some fudged principle created to make physicists feel better about the universe on philosophical grounds."
There is absolutely no evidence of which I am aware that any of these postulates about preserving order have any basis in physical facts let alone that they are universally true.

Like conservation of energy, etc, these core assumptions are tested every time we test a theory which has it as an assumption.

More so, every time someone comes up with a mathematical trick that appears to allow violation of causality, speed of light, conservations of energy/charge/etc, it always turns out that the effect can't be used to do so. Every single frickin' time. At some point you have to accept what the universe is telling you.

EDIT: Unless i am mistaken any relativistic travel allows time travel

But only forwards. There's no way for sub-FTL relativistic travel to result in travel back in time.

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