Entangled particles used for communication

[Michael Kieth Adams]

Why radio telescopes fail to encounter signs of civilization.   Using radio signals for communication does not make sense.   Long distance conversations become next to impossible using radio, but using some form of communication based on entangled particles should be instantaneous, and would quickly displace radio communications. We hear nothing because we are listening for the wrong things.  Just a thought.

[George Jones]

Entangled particles cannot be used to transmit information faster than the speed of light.

[andrew s]

Entanglement requires a local interaction to entangle the particles and they would then need to be separated. In addition you would have to stop them interacting with anything else on route to prevent destroying the entanglement.  The CMB is very efficient  in this regard.

Regards 

Andrew 

[robin_astro]

22 hours ago, andrew s said:

Entanglement requires a local interaction to entangle the particles and they would then need to be separated. In addition you would have to stop them interacting with anything else on route to prevent destroying the entanglement

As I understand it, even if you did take one of the pair with you on your intergalactic journey, you could not use it for communication when you arrive as you cannot chose the state when you "open the box" 

[andrew s]

10 minutes ago, robin_astro said:

As I understand it, even if you did take one of the pair with you on your intergalactic journey, you could not use it for communication when you arrive as you cannot chose the state when you "open the box"

You can measure the state when you open the box and from that in some circumstances (measured orientation v the entangle state orientation) deduce what the other state would be exactly. So if you measured "up"" the other would be "down". You can't  choose which of up or down you will get.

This is as close as you get to classical correlation where a red sock is put in one box and blue in the other. Open one and you instantly know what the other one is however far apart they are.

More generally you would only know the probability of what would be measured at the other end.

Neither amounts to communication as you say, just correlation. 

To me the mind boggling fact is that the two measurements can be space like separated (I.e. you can't even say which took place first) and at any chosen measurenent angles, decided just before the measurements are made,  the correlations will still persist.

Regards Andrew 

 

 

[Corncrake]

I blame Einstein with his "spooky action at a distance", which implies cause and then effect.

Not so in this case.

 

[Corncrake]

The boggling can be reduced if we think of the entangled thing (pair) as a single entity, not as two particles.

In a similar way to how a photon exists everywhere in the universe (with a varying probability) until it is observed (collapsed) so the entangled 'thing' does not become two isolated particles until after it is interfered with and after its attributes have been determined.

Well, it helps me to reduce my bogglement ! ymmv

 

Edited March 25, 2021 by Corncrake

[vlaiv]

13 minutes ago, andrew s said:

To me the mind boggling fact is that the two measurements can be space like separated (I.e. you can't even say which took place first) and at any chosen measurenent angles, decided just before the measurements are made,  the correlations will still persist.

I find that also exceptionally interesting as it can have several implications.

One would be that "collapse" of wave function is not random at all and is predetermined - so it really does not matter "which one is measured first".

I'm just wondering if we explain the loss of entanglement with decoherence - or interactions of particles with the environment - then particle gets weakly entangled to bunch of other particles and that destroys the phase of original strong entanglement. That implies that all particles are in constant state of entanglement with environment on some level - what if we simply can't perceive all that complexity and it leads to apparent randomness, but in reality - it is all very deterministic?

Maybe there is universal reference frame after all and chronology of collapse is tracked in that frame?

[andrew s]

37 minutes ago, vlaiv said:

I find that also exceptionally interesting as it can have several implications.

One would be that "collapse" of wave function is not random at all and is predetermined - so it really does not matter "which one is measured first".

I'm just wondering if we explain the loss of entanglement with decoherence - or interactions of particles with the environment - then particle gets weakly entangled to bunch of other particles and that destroys the phase of original strong entanglement. That implies that all particles are in constant state of entanglement with environment on some level - what if we simply can't perceive all that complexity and it leads to apparent randomness, but in reality - it is all very deterministic?

Maybe there is universal reference frame after all and chronology of collapse is tracked in that frame?

Loss of entanglement is due to decoherence.  Evidence points against a universal reference frame difficult to know what it could be in curved spacetime with no universal coordinates possible.

The closest you can get to predetermined is the Block Universe where all spacetime exists all at once and is unchanging. It is just that we are embedded in it and perceive it as dynamic.

Tought stuff.

Regards Andrew 

[Michael Kieth Adams]

Thanks to everyone.   Just because we can’t figure out how to do this does not mean it cannot be done.   Eventually we should encounter a radio using civilization, but it may be that entanglement based communication may be the rule with radio being the rarity.   We are human.  Impossible just means we haven’t figured it out yet, give us a century or two.

[Sunshine]

Oh it works great! the particles in my brain are right entangled at this moment!

[Ouroboros]

Even if there are better ways than radio waves to communicate, an alien civilisation  would almost certainly advertise their presence by setting up a radio wave beacon because they would know that most/all technological civilisations will at some point have discovered radio communication. This assumes that they would want to advertise their presence of course. 

[Corncrake]

5 hours ago, Ouroboros said:

 This assumes that they would want to advertise their presence of course. 

Exactly so.

The fact that we appear to be alone in the universe means one of three things :-
(1) the top predator out there has not yet found us.
(2) the civil civilisations out there have already found us and (sensibly?) want nothing to do with us.**
(3) we really are alone.

**Looking at the world in general,,  and the way we cant even manage something as simple as distributing vaccines amongst us, do you blame them ?

[andrew s]

14 minutes ago, Corncrake said:

Exactly so.

The fact that we appear to be alone in the universe means one of three things :-
(1) the top predator out there has not yet found us.
(2) the civil civilisations out there have already found us and (sensibly?) want nothing to do with us.**
(3) we really are alone.

**Looking at the world in general,,  and the way we cant even manage something as simple as distributing vaccines amongst us, do you blame them ?

Maybe our and their technology is not up to it yet. Strength of signal  and sensitivity of detectors. Regards Andrew 

[robin_astro]

Only indirectly on topic  for this thread but for a dose scientifically rigorous but distinctly outside the box thinking I periodically dive into exoplanet astronomer Prof Greg Laughlin's  fascinating blog 

https://oklo.org/

The latest talks about "grabby" civilisations which grab a sphere of influence and how it is all described in Depeche Mode's "Everything Counts" 

[Tiny Clanger]

The Drake Equation has been round for , what, sixty years ? It has been the subject of much discussion , academic and popular (I have a book by the SF writer Asimov on the topic, written in 1979) and the original equation includes in addition to the parameters mentioned above

"L = the length of time for which such civilizations release detectable signals into space"

It's all guesswork,  and has a perhaps dubious skewed viewpoint simply because we are humans, and make unwarranted assumptions about our own importance and the single , life evolving and supporting planet we know  of.   Recent interesting debate around the Drake Equation has included the  mediocrity principle which essentially adds the attitude 'why not ?' rather than the more strict approaches which seem to me to have echoes of , um, human exceptionalism* , rather than, say, evolution building us to fit the conditions on Earth . My training is in geology , and my view on this topic is influenced by having learned about the Cambrian explosion and read about the incredible range of  fascinating and bizarre life forms discovered in the Burgess Shale .

An interesting very recent publication   : https://arxiv.org/ftp/arxiv/papers/2004/2004.03968.pdf

Heather

 

* I'm carefully stepping around religion here ... philosophy is quite bad enough ...

[JamesF]

This thread calls to mind Cixin Liu's "The Three Body Problem" :)

https://en.wikipedia.org/wiki/Faster-than-light_communication#Quantum_entanglement

Perhaps also relevant regarding the radio silence is his "Dark Forest Theory" from the same series of books, though I'm fairly sure the idea doesn't originate with him.  I can't find a particularly good explanation online so perhaps this will have to do to start with:

https://thoughtcatalog.com/christine-stockton/2021/02/heres-what-the-dark-forest-theory-is-and-why-its-so-terrifying/

I think the underlying principle however is that any given civilisation can't be sure of knowing how an alien civilisation might behave towards them before that alien civilisation has the potential to know of their own existence.

James

[JamesF]

I'm not sure I'd recommend the books, by the way.  I suspect they'd have been improved if he'd had an editor who told him to keep the plot, but get rid of half the words :)

James

[Zermelo]

This is a thread that is certainly going places (probably several at once).

Einstein's "spooky action at a distance" was an unfortunate phrase, but the essential constraint of special relativity - forbidding faster-than-light communication - has been tested at the boundaries several times by physicists in a variety of experiments. In each case, exceeding c has been "permitted" by Reality only if the circumstances prohibit superluminal messaging. Any attempt to facilitate a breach invariably results in the "effect" disappearing. I read about one such effort where the experimenters tried to "sidle up" on Mother Nature, quite subtly and in stages, nudging a prism or tweaking a half-silvered mirror, but the moment they did something that would have enabled >c messaging - even if they weren't attempting to do so at the time - pooof.

I saw a piece about some of the latest quantum entanglement chicanery just yesterday  (precis:  it's possible to arrange for two events to occur such that each is plausibly the cause of the other and, simultaneously, the effect).

For me, the really mind-bending aspect of all this is the violation of the Bell inequalities and the answer in the negative to the EPR paradox. This appears to force us to concede one of the following: (i) there is no such thing as objective reality, (ii) locality is broken (also appearing to violate special relativity, though that needn't be so), or (iii) the universe is superdeterministic (usually interpreted as meaning that the outcomes of all experiments are determined in advance, and that we have no free will at all).

On the point about the (probable) absence of alien contact, a.k.a. the 'Fermi Paradox', there was a nice series here recently exploring a range of possible solutions.

 

 

[vlaiv]

19 minutes ago, Zermelo said:

For me, the really mind-bending aspect of all this is the violation of the Bell inequalities and the answer in the negative to the EPR paradox.

Why do you think that Bell inequalities are violated?

I think that Bell inequalities are compatible with deterministic universe. They just state that simple logic does not work for QM phenomena.

To us, following statements are nonsense:

There is a ball in the box and at the same time there are no balls in the box.

Or this one:

I'm moving forward with respect to you and at the same time I'm moving backward with respect to you.

This does not mean indeterminacy - like it is not determined what is happening and both things are possibility - it means that it is precisely determined what is happening - although incomprehensible to us - how can something move both forward and backward at the same time or be in the box and not be in the box at the same time.

[andrew s]

Classical physics follows both classical logic and the simplest probability rule. For probability it means if the are n exclusive options the sum of the probabilities of each n individually sum to one. This is the simplest probability theory but not the only possible one.

Quantum theory does not follow classical logic nor classical probability. The failure to follow classical logic is exemplified by the violation if the Bell inequality.  Quantum probabilities follows the second  simplest probability theory which is that the sum of squares of the "n options" adds to one. Both are related to the quantum state being a vector (strictly  a ray) in a suitable complex space and not a real number space as in classical physics. 

The violation of Bells inequality also shows there can be no local classical theory underlying QM (no hidden variables). 

Regards Andrew

PS Schrodinger's cat is never in a superposition of dead and alive it is in an entangled state with the trigger/poison being either trigger fire and dead or trigger not fired and alive.  All entangled means is that measuring (e.g. looking at) one element gives information  about the other. 

Edited March 27, 2021 by andrew s

[andrew s]

47 minutes ago, vlaiv said:

 it means that it is precisely determined what is happening - although incomprehensible to us 

This is not true in QM. The outcome of some measurements on a given prepared state can only be given as probabilities. Any one measurement is indeterminate in principle given the no local hidden variables consequence of the violation of the Bell inequality. You have to do many identical experiments on identically prepared states to get the results to match the predicted probabilities.

Regards Andrew 

 

[vlaiv]

26 minutes ago, andrew s said:

This is not true in QM. The outcome of some measurements on a given prepared state can only be given as probabilities. Any one measurement is indeterminate in principle given the no local hidden variables consequence of the violation of the Bell inequality. You have to do many identical experiments on identically prepared states to get the results to match the predicted probabilities.

Regards Andrew 

 

Outcome of the measurements can only be given as probabilities but that does not mean that quantum state that is in superposition of states - say electron that is in superposition of spin up and spin down is not in exact quantum state. There is nothing probabilistic about that state. Being in superposition of spin up and spin down - does not mean "maybe it's spin up and maybe it's spin down" nor does it mean "spin state is not determined / it is "fuzzy" until we measure it". It means that electron is in exactly that state - being linear combination of two eigenstates a*|up> + b*|down>.

It is due to decoherence that above state evolves into either up or down state once measurement is made.

If I'm not mistaken - Bell's inequality does not imply that such evolution can't be deterministic. It implies that there is measurable difference between electron being in a*|up> + b*|down> rather than being in up or down state with us not knowing which one until we measure.

I've always found glove explanation with pair of entangled electrons to be good representation of all of this. "Classical" thinking would be - after we separate two electrons - one of them is carrying left glove and other right glove but we simply can't tell which one has which until we do actual measurement - and then, lo and behold - we get anticorrelated measurement on them and it does not really matter which one is made first - in all reference frames it will make sense. This is hidden variable part - electron has hidden glove handedness from us until we measure it (no way of determining it apart from directly measuring it).

Bell's inequality just says, no, actual state of both electrons is spin up and spin down at the same time and it will not "materialize" into being either up or down until we do the measurement and there is actual numerical difference between in prediction between these two reasonings.

 

 

[andrew s]

1 hour ago, vlaiv said:

Outcome of the measurements can only be given as probabilities but that does not mean that quantum state that is in superposition of states - say electron that is in superposition of spin up and spin down is not in exact quantum state. There is nothing probabilistic about that state. Being in superposition of spin up and spin down - does not mean "maybe it's spin up and maybe it's spin down" nor does it mean "spin state is not determined / it is "fuzzy" until we measure it". It means that electron is in exactly that state - being linear combination of two eigenstates a*|up> + b*|down>.

I agree the state is as you say but the measurement will give spin up a^2 of the time and b^2 there rest of the time. Any given result is random. If you changed your mind and  did it at a different angle you would get different results depending on the a and b. There is no determined answer  to be revealed on a measurement by measurement basis. 

What Bell's inequality says is that there can be no classical local variable that could code the outcome of a given measurement (e.g. no hidden gloves) or give rise to the probabilities of multiple measurements on repeated experiments. 

Regards Andrew 

PS also the states are not fixed in a single basis. Say you have |up> in the z direction you could express this as a|+45> + a|-45> in a basis at 45° to the z axis.

Where the "a" are such to normalise the state.

This can be easily shown with 3 linear polarising sheets. Take a randomly polarised light source and pass it through the first vertical polariser and 50% gets through . Add one at 90° and none gets through. Place the third at 45° in the middle and 25% gets through. 

Edited March 27, 2021 by andrew s

[vlaiv]

26 minutes ago, andrew s said:

I agree the state is as you say but the measurement will give spin up a^2 of the time and b^2 there rest of the time. Any given result is random. If you changed your mind and  did it at a different angle you would get different results depending on the a and b. There is no determined answer  to be revealed on a measurement by measurement basis. 

What Bell's inequality says is that there can be no classical local variable that could code the outcome of a given measurement (e.g. no hidden gloves) or give rise to the probabilities of multiple measurements on repeated experiments. 

Regards Andrew 

 

We agree on almost everything except measurement part (root of all evil in QM? )

I'll try to explain what I mean and if you can, you'll provide an answer to the question I'll be posing if you know one. I don't know the answer to it - I just suspect it - so it is not classical disagreement in the sense that I'm firmly standing behind what I'm saying.

I agree that if we say prepare electron in up+down state and measure it - it will randomly 50% of the time be detected as up and 50% of the time be detected as down. There is no arguing with that.

What I want to know if:

1) process is truly random

2) if answer to 1) is no if it contradicts Bell's theorem

Now I shall proceed to explain how it can be deterministic. If we take a particle that is in superposition of states and we let it interact with another single particle - we can easily evolve their state in deterministic way. Only when a particle interacts with complex system that is in thermal chaos that we see raise to "random" - I'll use that expression but don't subscribe to associated interpretation - wavefunction collapse.

What if in reality following happens:

1/2 * |up> + 1/2 * |down> evolves in deterministic way by entangling itself with the environment on so many levels that we simply can't calculate to produce one of two resulting states:

a * |up> + b * |down> or c * |up> + d * |down> where either a >> b and d >> c

which one will depend on configuration of macroscopic instrument particles at the time of measurement

In fact I believe that decoherence is actually telling us that entanglement with environment puts things out of phase so there no longer can be superposition - I'm just extending this to say what if it also gives priority to certain coefficient.

This would both explain why we perceive things as random - too much information so it is in essence pseudorandom, and why there is notion of wavefunction collapse - because there will be definite QM state that is "aligned" to eigenstate almost 100%.

I believe that above does not contradict Bell's theorem as it does not contain hidden variables - it is still all the same QM except too complex when macroscopic systems are involved and hence appears random.