Entanglement and teleportation

In summary: Entanglement is a very strong connection between particles, but it doesn't mean that information is always transmitted instantly. Information can take some time to propagate.
  • #1
daytripper
108
1
I read up on wikipedia.com about entanglement and teleportation but it left me with a few questions. If you go to This Link. You'll see that they give the analogy "Bob has created two atoms called I and II which are maximally entangled". Now obviously, bob can't create two atoms at will so how do two particles become entangled? Other texts in the article suggest that due to the fact that they're identicle particles in the same instant of time, they're basically one particle so what happens to one will happen to the other. But then it says the transmition of information can not go faster than the speed of light. If this is true then I would assume that the communication between the particles is transmitted through some sort of EM wave. There's a lot of confusion right now, could someone clear this up for me? I just realized that my questions might not be obvious from the text so I will list them.
1) What defines which identicle particles in the same time period are entangled and which are not? Is there an "entanglement process" or are two identical particles that exist in the same point in time automatically entangled?
2) Does the transfer of particle state information happen instataneously no matter what the distance?
3) If not, is the information transmitted through some sort of EM wave?
 
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  • #2
An entangeled pair can be created by a technique called parametric down conversion. i suggest you google for that. Besides, two entangeled particles can indeed be seen as one 'bigger' particle with twice as much energy (suppose that each particle has the same amount of energy) or twice as much shorter wavelength. This is how entangled pairs can beat the diffraction limit. i refer to my journal for more info. Just look at the faster then light communication entry and the for qubit-lovers entry

marlon
ps this is nice : http://marcus.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf
 
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  • #3
Marlon, I read your faster than light article. Just to verify, you said that the communication between the two particles themselves is instataneous but the ability to read these particles has to be enabled through a classical communication platform? Do physicsists know how the particles communicate faster than the speed of light or is there not an explanation behind that?
 
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  • #4
daytripper said:
Do physicsists know how the particles communicate faster than the speed of light or is there not an explanation behind that?

The faster then light aspect directly comes from the entaglement. If you have an entangled pair of two atoms (one has spin up along the x-axis and the other has spin down, for example) and you measure the spin of one atom along the x-asis, then you automatically know the spin value of the other atom because it has to be the opposite direction. First of all, communication like that is impossible because of the necessary classical phonecall that is required between the two observers. Secondly, both observers need to measure along the very same axis but who say they will do that ?

regards
marlon
 
  • #5
I must have my concept of entanglement wrong then. I thought entanglement meant that what was done to one entangled particle was automatically done to the other. But this is just a matter of "If this particle is moving this way, the other particle has to be moving that way". No communication is done between the particles themselves. right?
 
  • #6
Lets say; two observers, separated a light year from each other, have come to the agreement that spin up refers to 1 and spin down to 0 (assuming they live quite long...). (They have come to this agreement trough the conventional ways of communication, in this example taking multiple light years, discerning the side effects such as signal-loss and such). When they agreed this, they also agreed to prepare two isolated photons (one here and one there) and to bring them into a 'long-lasting' state of entanglement.
Lets assume they have the possibility (by chance or agreed before) to each measure the photon the exact same time or in a time frame allowing FTL.

Then, Observer A puts the photon into a forced 'spin up' state, which will, due to entanglement, be instantaneously sent to the other photon, thus forcing that one at observer B into an immediate spin down state. Via this way, observer B will be able to read 0 from his photon.

Apart from the fact that measuring the spin state requires classical communication, the whole procedure will highly exceed light speed.
 
  • #7
daytripper said:
I must have my concept of entanglement wrong then. I thought entanglement meant that what was done to one entangled particle was automatically done to the other. But this is just a matter of "If this particle is moving this way, the other particle has to be moving that way". No communication is done between the particles themselves. right?

In fact it is neither !
If it were "what is done to one, is also done to the other", you obviously would have a faster-than-light communication channel. Some fools even thought that you could build a rocket motor that way, by having entangled atoms, one in the rocket, and one on earth, and then accelerate those on earth, so that those in the rocket would also accelerate :-)

On the other hand, it is not just learning about an unknown parameter of the other atom either. Bell's theorem tells us that that is not the case.

Let us separate two issues: one is discussions on the *mechanism* that is responsible for entanglement: there are many discussions about it, people have different views on what is actually going on (I have my own view which I don't stop defending over here :-). The other issue is about what is actually observed: here, most people agree (there's still a "local realist" crowd who denies all experimental results and claims it are all tricked, or badly analysed, or oversold results, but they are, by most others, seen as kind of cranky).

I won't go into the mechanism explanations. I will just try to state what is actually predicted by quantum theory, no matter what interpretational flavor. It is about 2 observers, Alice and Bob, who receive each one of the two entangled particles (photons, atoms, whatever).
Now, they can do only one measurement on the particle, but they have a choice of WHICH experiment they can do, which is parametrised by a variable, theta-Alice, and theta-Bob. (usually a polarisation angle).
So Alice makes a choice of theta-Alice, and then gets a result (up or down) for the measurement at hand.
Bob on his side makes a choice of theta-Bob, and then gets a result (up or down) for the measurement at hand.

Alice has a certain probability of getting "up", P(a_up, theta_Alice), which is only a function of theta_Alice.
Bob has a certain probability of getting "up", P(b_up, theta_Bob), which is only a function of theta_Bob.
So far, so good: this is what people mean by 'there is no information transfer': Bob, with his measurement, cannot learn anything about Alice's choice of theta-Alice and vice versa.

BUT, but...
If Alice and Bob COME TOGETHER, AND COMPARE NOTES, then they observe something strange: there is a correlation: the probability
P(a_up, b_up, {theta_Alice, theta_Bob} ) is such that it does not satisfy a property which is called Bell locality.
In order to explain this in detail, you should study a bit Bell's theorem. In short, it comes down to the following point. Bell worked out what would be the requirement on the joint probability P(a_up,b_up,{theta_Alice,theta_Bob}) when we assume that the two particles share some common "hidden variables", and then have to generate the probability of "up" or "down" at Bob and Alice, INDEPENDENTLY.
So Bell assumed that there is a common variable lambda, and that P(a_up, theta_Alice) is in fact given by P(a_up,lambda,theta_Alice), and that at Bob's the probability is given by P(b_up,lambda,theta_Bob) ; and that these probabilities are independently generated, once we know lambda.
This means then that the joint probability is a product:
P(a_up,b_up,lambda,{theta_Alice,theta_Bob}) = P(a_up,lambda,theta_Alice) x P(b_up,lambda,theta_Bob).
But we don't know anything about lambda, is just has an unknown probability distribution, P(lambda), so our observed correlation is then, according to Bell:

P(a_up,b_up,{theta_Alice,theta_Bob}) =
Integral P(lambda) P(a_up,b_up,lambda,{theta_Alice,theta_Bob}) d lambda

Of course, there is still a lot of freedom, because of the choice of P(lambda), and P(a_up,lambda,theta_alice) and so, but Bell succeeded nevertheless in writing down some INEGALITIES which the joint probability needs to satisfy.

Well, it turns out that the joint probabilities for entangled particles in quantum theory DO NOT ALWAYS SATISFY those Bell inequalities.

What does this mean, statistically ? Well, it just means that one of Bell's hypotheses are not satisfied.
And Bell's hypotheses are that the probabilities of Alice and Bob observing "up" for their chosen angles are generated INDEPENDENTLY as a function of a COMMON SET OF (HIDDEN) VARIABLES.
This is a very reasonable hypothesis when "statistical" things happen and when a correlation is observed. If somehow you arrange that there cannot be any DIRECT influence (because there's a big distance, a concrete wall etc.. between Alice and Bob), then if you observe a correlation, you normally assume a COMMON CAUSE (the hidden variable).
So this is somehow not true in quantum theory: you can have correlations without having a "common cause".
But it is also true that Bob cannot learn anything from Alice's CHOICE from his local measurement, nor can Alice learn anything from Bob's choice. So this thing doesn't allow you to send information from Alice to Bob.

cheers,
Patrick.
 
  • #8
daytripper said:
I must have my concept of entanglement wrong then. I thought entanglement meant that what was done to one entangled particle was automatically done to the other. But this is just a matter of "If this particle is moving this way, the other particle has to be moving that way". No communication is done between the particles themselves. right?

Well your definition is correct but the communication part is just the fact that if you measure one spin, you automatically know what the other observer will have as outcome when he measures the other atom of the entangled pair

regards
marlon
 
  • #9
daytripper said:
.
1) What defines which identicle particles in the same time period are entangled and which are not?
Their behavior wrt some detection scheme or other is
correlated. For example, different, separated parts of
the (same) television signal (wave) are entangled.

daytripper said:
Is there an "entanglement process" or are
two identical particles that exist in the same point in time
automatically entangled?
"Entanglement processes" produce the entangled phenomena
observed experimentally. The entangled phenomena have a
common cause (including, but not necessarily requiring, that
they've interacted prior to detection). Marlon mentioned PDC.
There are also other experimental processes that produce
entanglement.

daytripper said:
2) Does the transfer of particle state information happen
instantaneously no matter what the distance?
"Particle state information" is something that *we*
generate via theory and observation. Are the separated,
entangled physical phenomena *causing* each other
(instantaneously or superluminally)? There's no direct
evidence of that. But some interpretations have it that
that's what's happening. My personal opinion is that that
sort of *causation-at-a-distance* probably isn't what's
happening.

The correlations are a function of analyzing (even
via spacelike separated events) motional properties that the
entangled phenomena have in common due to their having
interacted in the past, or being created at the same time
and place (eg., a wave moving omnidirectionally away from
its source and rotating parallel to some plane-- separate,
individual points on the wave are entangled wrt the
rotation). Separated objects in any *system* of objects
moving together as a group are entangled wrt the movement
of the system as a whole.

daytripper said:
3)If not, is the information transmitted through
some sort of EM wave?
Information, in the sense of something being communicated
from one place to another, is transmitted electromagnetically.
There might be other waves in nature moving faster than EM
waves, but nobody has detected that yet. So, as far as
anybody knows, the speed of electromagnetic radiation in
a vacuum is the upper limit.

Nothing *needs* to be being transferred instantaneously or
superluminally to understand why the correlations of entangled
phenomena are what they are. For example, in the case of photons
entangled in polarization, light waves emitted (presumably by
the same atom) during the same interval are analyzed by
crossed linear polarizers. No nonlocal causation needs to be
happening -- the polarizers are simply, in effect, analyzing
the same light at the same time, and a cos^2 theta correlation
for coincidental detection emerges (which is what would be
expected if the same light is being analyzed by crossed linear
polarizers).

Now, I'm aware of analyses of this that conclude
that the light incident on the polarizers can't have been
made the same by the emission process, that it must happen
at the instant the detection that initiates a coincidence
interval occurs. But, these analyses are flawed, imho.
One way to approach it is by considering where the qm
projection along the plane of transmission (by the polarizer
at the initially detecting end) comes from. There's, imo,
a sound physical basis for it. Anyway, what results is
a probability of 1 for the initiating detection, and
a cos^2 theta probability at the other end for the same
interval. So, the joint probability of detection
(the probability of coincidental detection)
wrt any interval is 1(cos^2 theta). And, experiments
support this prediction.

The assumption of the causal independence of spacelike
separated individual results holds as long as one is
careful to modify the probabilistic picture following
the initiating detection. Maybe current 'pictures'
of spin and polarization are inadequate to describe
exactly what is happening. But, the plane of polarization,
and the intensity, of the light transmitted by the first
polarizer (associated with the start of the coincidence
interval) is a subset of the emitted light incident
on each polarizer for the common interval. This
light produced a photon, which represents maximal
intensity for that coincidence interval, at the first
detector. So, it follows from standard optics that
the probability that it will produce a photon at the
second detector (via analyzing the light from the
same emission, or set of emissions) is cos^2 theta,
where theta is the angular difference between the
settings of the two polarizers.
 
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  • #10
Sherlock said:
The correlations are a function of analyzing (even
via spacelike separated events) motional properties that the
entangled phenomena have in common due to their having
interacted in the past, or being created at the same time
and place (eg., a wave moving omnidirectionally away from
its source and rotating parallel to some plane-- separate,
individual points on the wave are entangled wrt the
rotation). Separated objects in any *system* of objects
moving together as a group are entangled wrt the movement
of the system as a whole.
This pretty much sums up my conception of the process. There just can't be any impossible or mysterious factors involved. We just haven't identified all the properties and restrictions on their motion. Unless I miss the point, communication at a distance is merely speculation, right?
:shy:
 
  • #11
LindaGarrette said:
This pretty much sums up my conception of the process. There just can't be any impossible or mysterious factors involved. We just haven't identified all the properties and restrictions on their motion. Unless I miss the point, communication at a distance is merely speculation, right?
Well, there can't be any *impossible* factors involved. :)

But, there are mysterious factors involved, and they have,
imo, as much (maybe more in the case of entanglement) to do
with the way competing formulations are analysed as with
the entangled phenomena themselves.

The (speculative) inference of instantaneous or
superluminal *causal* relationships between the separated
phenomena is allowed, logically, given certain assumptions
(or, more strictly, the experimental negation of certain
*interpretations* of certain assumptions via the formulation
of probability statements regarding joint detection, and
the restriction of alternatives).

I've outlined the reasons why I don't think that experimental
violations of Bell inequalities are telling us what some people
seem to think they're telling us. Was Bell wrong? No, he
said his formulation regarding probability of joint detection is
incompatible with qm. It is. It's also incompatible with
experimental results, which support the qm formulation.
The problem is that the usual lhv formulation, following Bell,
doesn't take into account that the probabilities for individual
detection have changed once a detection is registered and
a coincidence interval is initiated. If you give the qm
projection operator the correct, imo, physical interpretation
in these experiments, then the qm formulation can be
seen as a sort of lhv theory itself.

It seems like a good bet that all the properties of light, electricity,
etc. haven't been identified yet -- at least not precisely enough
to give a clear picture of the physical details of what's happening
in the entanglement experiments.
 
  • #12
Oh, I see. So it's more saying that because the photons were produced at the same exact time, any reading of the particles will be probably the same depending on the point in time the photon was "read". (seeing the same light at the same time). I was thinking like the idiots that were going to use it for rocket propulsion. Haha. Thank you for clearing that up for me. I thought that the actions of Alice would produce an effect to Bob's photon.
 
  • #13
Sherlock said:
The (speculative) inference of instantaneous or
superluminal *causal* relationships between the separated
phenomena is allowed, logically

But entanglement isn't one of those causual relationships, right?
 
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  • #14
daytripper said:
But entanglement isn't one of those causual relationships, right?

I don't think so, but some pretty smart people do.

The problems arise because of the way some lhv formulas
are done. If you describe joint detection in terms of
the product of the *initial* (prior to detection) individual
probabilities, then you get some predictions that don't agree
with qm (or experiment). But, the probability of individual
detection changes upon a detection being registered at one
end or the other. When that's taken into account, then
the idea that the filters are analyzing a common property
(or properties) imparted at emission is ok.
 
  • #15
daytripper said:
Oh, I see. So it's more saying that because the photons were produced at the same exact time, any reading of the particles will be probably the same depending on the point in time the photon was "read".
(seeing the same light at the same time).

More like, because the photons were produced at the same exact
time *and place* (like from the same atomic 'burp'), subsequent
analysis of the photons by the same sort of device will produce
results that are correlated.

There's a lot of great stuff written about this sort of thing.
If you're really interested, then you should read all of Bell's
work on this (and check out all of the citations, including the
EPR paper, the Aspect papers, etc). That should set you back
at least a few months, but it will give you a much better
understanding of the difficulties involved -- and the
considerations that led to the belief by some that there
are superluminal 'influences' (or whatever you want to
call the nonlocal stuff) in nature.
 
  • #16
Ok. I'll check out those articles. Thank you for helpin me out even through my confusion. I must go now.
 
  • #17
Sherlock said:
The problem is that the usual lhv formulation, following Bell,
doesn't take into account that the probabilities for individual
detection have changed once a detection is registered and
a coincidence interval is initiated. If you give the qm
projection operator the correct, imo, physical interpretation
in these experiments, then the qm formulation can be
seen as a sort of lhv theory itself.

I read, and re-read this several times, and I can't make up what you mean. I am reasonably well acquainted (or so I think) with Bell's reasoning.
What do you mean by "the probabilities for individual detection have changed once a detection is registered" ??

cheers,
Patrick.

EDIT: btw, this has probably already been cited, but I just found a very very thorough reference on all things Bell:

http://plato.stanford.edu/entries/bell-theorem/
 
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  • #18
vanesch said:
What do you mean by "the probabilities for individual
detection have changed once a detection is registered" ??
Prior to detection the probability of individual detection at each
end is .5. A detection at one end or the other starts the
coincidence circuitry. A 'coincidence interval' is electronically
defined and, for this interval, the probability of detection at
the detecting end is no longer .5. It's 1. The probability of
detection at the other end for this interval is no longer .5, but
cos^2 theta (where theta is the angular difference of the
polarizer settings). So, the probability of joint detection is
1(cos^2 theta).

The transmission axis of the polarizer at the initially detecting
end is taken or projected as the global emission parameter,
because:
(1) the intensity of the detected light is a subset of the
emitted light.
(2) the transmission axis of the polarizer at the initially
detecting end represents the or 'a' plane of maximal
transmission by the polarizer(s) wrt the light emitted
during the interval (a photon *was* produced out of
light that was filtered from the emitted light).
(3) PMT response is directly proportional to the intensity
of the light transmitted by the polarizer.
(4) the intensities of the light between the polarizers and
their respective PMT's are related by cos^2 theta, which
therefore represents the probability of joint detection
for any coincidence interval.
 
  • #19
daytripper said:
I thought that the actions of Alice would produce an effect to Bob's photon.

As far as anyone can tell, that is EXACTLY what happens. Of course it is just as likely that it is Bob's actions that affect Alice's results. These scenarios end up being indistinguishable, which is of course a bit puzzling.
 
  • #20
Sherlock said:
The problem is that the usual lhv formulation, following Bell,
doesn't take into account that the probabilities for individual
detection have changed once a detection is registered and
a coincidence interval is initiated. If you give the qm
projection operator the correct, imo, physical interpretation
in these experiments, then the qm formulation can be
seen as a sort of lhv theory itself.

That is certainly an unconventional description of the situation. Since the results change upon the "first" detection, as you also point out in other posts, and that "causes" the results at the other detector to immediately change, you are saying that the results ARE dependent on space-like separated observer settings. That is the opposite of a LHV interpretation.
 
  • #21
Sherlock said:
Prior to detection the probability of individual detection at each
end is .5. A detection at one end or the other starts the
coincidence circuitry. A 'coincidence interval' is electronically
defined and, for this interval, the probability of detection at
the detecting end is no longer .5. It's 1. The probability of
detection at the other end for this interval is no longer .5, but
cos^2 theta (where theta is the angular difference of the
polarizer settings). So, the probability of joint detection is
1(cos^2 theta).

Ah, I see :-)

But now you have another difficulty. I have a vague "deja vu" feeling, when I've been through this with a person whose nickname was nightlight.
Ok, you're absolutely right that EPR experiments, in a completely classical wave setting, are explainable using Maxwell's theory and detectors whose triggering is proportional to the incident intensity.
But,...

Now you have a serious problem with the "photon" concept! Because if what you write is correct, then a single-photon state, incident on a beamsplitter, and detected by two photodetectors on the two arms, should then click in perfect coincidence, no ? (it is as in your case, but with theta= 0 degrees)
After all the intensities on both detectors are identical (the beamsplitter splits the intensity 50-50).
So, do you expect coincidence or not ?


cheers,
Patrick.
 
  • #22
DrChinese said:
As far as anyone can tell, that is EXACTLY what happens. Of course it is just as likely that it is Bob's actions that affect Alice's results. These scenarios end up being indistinguishable, which is of course a bit puzzling.

As you might know by now, I don't think that Alice's actions "have some effect" at Bob's, because that would imply, in one way or another, a non-local dynamical interaction. Of course, the possibility is not excluded. It is possible that there is such a "spooky action at a distance". But then you should admit that it is very strange that, given that spooky actions at a distance are possible, that nevertheless there is a conspiration that forbids us to use it to make a faster-than-light telephone.
But it is also possible that all what happens is strictly local, and that the correlation probability only has a meaning AFTER the results have been brought together, by applying the Born rule only at the point where messengers (which are entangled with the different outcomes at Alice and Bob) come together and can allow for a physical implementation of a correlation measurement (a coincidence circuit with counter, say). It is only when we observe THAT circuit that we apply the Born rule. It is only by inference that we then suppose that we know what happened at Bob's or at Alice's place.
You can, or you cannot, agree with that explanation. If you feel like the observed world is "really out there" then you have no option but to accept "spooky action at a distance". But I would like to stress that this is *NOT* the only logical possibility. You can still "save locality" by accepting what quantum theory cries out: macroscopic superposition.

cheers,
Patrick.
 
  • #23
vanesch said:
As you might know by now, I don't think that Alice's actions "have some effect" at Bob's, ...

Yes, MWI (and a few others) is a possibility. What I was trying to stress is that Daytripper's idea that there is NOT a causal connection cannot be supported by the evidence. In other words, the evidence is compatible with a causal connection between Alice and Bob's observations.
 
  • #24
Q:
daytripper said:
I thought that the actions of Alice would produce an effect to Bob's photon.
A's:
vanesch said:
As you might know by now, I don't think that Alice's actions "have some effect" at Bob's, because that would imply, in one way or another, a non-local dynamical interaction.
DrChinese said:
As far as anyone can tell, that is EXACTLY what happens.

It seems that there is a difference in opinion as to what is actually happening. Is this something I have to get my own opinion on through research or, to put it bluntly, are one of you wrong?
 
  • #25
daytripper said:
It seems that there is a difference in opinion as to what is actually happening. Is this something I have to get my own opinion on through research or, to put it bluntly, are one of you wrong?

Both!

The "consensus" within the physics community is that either there are a) no hidden variables; or b) there are non-local ("spooky action at a distance") effects. There are also c) variant interpretations such as Many Worlds (MWI) that try to solve the paradox with other assumptions (as Vanesch himself has recently pointed out in a very nice original paper).

The main thing is to discard the "naive" view that the situation results from some lack of knowledge - even though there is unquestionably much more to learn. After all, a), b) and c) are all quite different!
 
  • #26
daytripper said:
But entanglement isn't one of those causual relationships, right?
Many scientists (including Sherlock) may disagree, but I think everything is causally determined, even at the quantum level. Only because there isn't enough evidence to the contrary. But, the effect of any quantum interaction on space time reality would be irrelevant. (Oops, Looks like my post is out of place in the thread. I'm not accustomed to finding most recent posts last.)
 
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  • #27
DrChinese said:
Yes, MWI (and a few others) is a possibility. What I was trying to stress is that Daytripper's idea that there is NOT a causal connection cannot be supported by the evidence. In other words, the evidence is compatible with a causal connection between Alice and Bob's observations.

Ok, I can agree with that :approve:

The only thing experimental evidence suggests strongly is that the individual probabilities, and the correlations, as calculated by quantum theory (according to your favorite scheme, they all give the same result of course) are strongly supported, and that this implies that some inequalities a la Bell are violated.
As such the total set of hypotheses that were used (locality, reality, independence of probabilities at remote places...) is falsified. But it is an error to jump directly to the throat of locality. This is a possibility, but it doesn't follow from any evidence. Just as the denial of hidden variables is a possibility, but not necessary.

The way I see it (even if you do not want to go explicitly in an MWI scheme) is that for Alice, it does not make sense to consider Bob's "outcomes" until she observes them (and calculates a correlation), in the same way as it doesn't make sense to talk about the position of a particle until it is observed.
If you use the quantity (Bob's outcome, or the position of a particle) without having observed it, it leads you to bizarre results, and I think that's simply what is happening here.
In many cases, you can get away with that (for instance if the particle can be considered classical, you can talk about its position without punishment), but in certain cases (double slit experiment) you get paradoxal situations.
In the same way, talking about the "result a remote observer had" before observing it yourself is something you can get away with most of the time, but sometimes you get paradoxal results (EPR).
This view is of course inspired by MWI, because, from Alice's point of view Bob didn't get one single outcome: he went into a superposition of states depending on the outcome (so talking about his outcome doesn't make sense yet). It is only upon interaction with Alice that a specific outcome state for Bob is chosen. But at that point, the hypotheses that go into Bell's inequality don't make sense anymore because information from both sides IS present.

So in a way, EPR is yet another example of a paradoxal result one can obtain when one talks about quantities that do not (yet) have an existence ; in this case, Bob's results before Alice saw them.

cheers,
Patrick.
 
  • #28
daytripper said:
It seems that there is a difference in opinion as to what is actually happening. Is this something I have to get my own opinion on through research or, to put it bluntly, are one of you wrong?

I think it is fine to have different opinions, as long as you understand the other's opinion. The factual information, however shouldn't be matter of opinion, and the factual information is that there is a strong indication that the quantum predictions are right. I say "strong indication" because (as is often underlined by local realists) in all cases, some experimental corrections are needed before the results come out.

I have been in favor of MWI, but I also know that there are other explanations. For instance in Bohmian mechanics, there is an explicit non-local interaction (the quantum potential). So there the issue is solved: locality is gone (from the start), and apart from that, the universe is (almost) classical. The predictions (at least in non-relativistic QM) are equivalent to standard quantum theory. So this is a clear settling of the issue (and when you look at Bohm's theory, you wouldn't even consider Bell's inequalities: the non-locality is so evident that it is clear that they will be violated).

I just wanted to point out that this is _not the only solution to the riddle_ and that locality can be saved at the expense, I agree, of some weirdness.

I could even say that if you let go locality, then Bohm' s mechanics is really the solution to all of your problems :-) The non-local mechanism is clear (it is the quantum potential).

The most ambiguous view is Copenhagen, with some quantum/classical transition. If you stick to it, you're in deep sh**! And that's what most people then have: they switched too early from quantum to classical behavior, and then they find themselves with "impossible" classical results: correlations without a mechanism !

cheers,
Patrick.
 
  • #29
vanesch said:
Ah, I see :-)

But now you have another difficulty. I have a vague "deja vu" feeling, when I've been through this with a person whose nickname was nightlight.
Ok, you're absolutely right that EPR experiments, in a completely classical wave setting, are explainable using Maxwell's theory and detectors whose triggering is proportional to the incident intensity.
But,...

Now you have a serious problem with the "photon" concept! Because if what you write is correct, then a single-photon state, incident on a beamsplitter, and detected by two photodetectors on the two arms, should then click in perfect coincidence, no ? (it is as in your case, but with theta= 0 degrees)
After all the intensities on both detectors are identical (the beamsplitter splits the intensity 50-50).
So, do you expect coincidence or not ?


cheers,
Patrick.

I'll get to this question below, but first:

I don't think there's any conflict with the
photon concept in the way I've learned to
look at the EPR/Bell experiments.

The photon is associated with a detection
event, which is associated with an emission
event. A detection event is a detection event.
It's not .5 a detection or 1.5 a detection -- it's
1 detection. Photons are, by definition, indivisible.

The emission models have been built from -- at
least in part, and certainly in the sense that
they must accord with -- the experimental
*results* (the detections).

What's happening in between (which is what
we're talking about) is anybody's guess. :-)

The way I learned about photons doesn't
*require* that I think of them as indivisible
wave trains, or energy packets, or point
particles when I'm thinking about light
in terms that I want to correspond to what's
physically happening prior to detection.

So, I don't have a problem with the photon
concept -- just looking for deeper (ie., real
physical) explanations for some experimental
results.

The result of beamsplitter situation you describe
is certainly resistant to explanation using wave
picture.

However, even the best beam splitters
don't produce an exact 50-50 split of the beam.
This is demonstrated experimentally. But the
difference is small, and in conjunction with PMT
calibration considerations seems not adequate to
explain the result of detection at one arm or the
other, but never both, per single
emission/detection interval.

Regarding your question, I would have to know
exactly the type of beamsplitter and detector
and calibrations to say what I would expect.
I already know the results of some of these
sorts of experiments. But, like the dot by dot
interference results, have so far no satisfactory
way to explain why, if there is some wave activity
incident on locations where it isn't detected, then
why isn't it detected?

The argument for the notion that the light wave(s)
itself (which correspond to photon detection event)
is *indivisible* is, I think, based on that question
pointing to the inadequacy of the wave picture
in some situations.

However, I think that we *have* successfully
refuted at least one set of arguments for the
existence of superluminal causal connections.

Whether or not these do exist in some medium that's
inaccessible to us remains an open question -- just
maybe not a necessary one at the moment.

The question of whether or not light is propagating
and interacting with other media, as, fundamentally,
*indivisible* wavetrains or bundles seems like a
better one.
 
  • #30
DrChinese said:
That is certainly an unconventional description of the situation. Since the results change upon the "first" detection, as you also point out in other posts, and that "causes" the results at the other detector to immediately change, you are saying that the results ARE dependent on space-like separated observer settings. That is the opposite of a LHV interpretation.

Yes, of course the joint results depend on the joint
settings. But, that doesn't mean that what happens
at one end is affecting what happens at the other.

I didn't say that the results change upon first
detection. The probability of detection changes,
because the result was that a detection occured.
This initiates a coincidence interval. And, during
this interval, the probability of detection at the
other end is different than it was prior to the
detection that initiated the interval.

What I offered was local interpretation in that
it requires no superluminal effects, no causal
connection between Alice and Bob to understand
why the probability of coincidental detection is
cos^2 theta.

You're right that it isn't, strictly speaking, a hidden
*variable* description. This is because the *variability*
of the global parameter isn't relevant to the
variability of coincidental detection. (The variability
of this parameter is, however, relevant to the variability
of individual detection. As Bell pointed out, if you
augmented qm with this value, then it would certainly
improve the precision of individual result predictions.
Such a formulation, for individual results, is not in conflict
with qm.)

The description I offered is certainly not unconventional.
What's unconventional is saying that spacelike separated
events are causally affecting each other, that what Alice
does has some influence on what Bob does via some sort
of superluminal 'transmission' or whatever. There's simply
no good reason to adopt that belief ... yet. :-)
 
Last edited:
  • #31
DrChinese said:
The "consensus" within the physics community is that either there are a) no hidden variables; or b) there are non-local ("spooky action at a distance") effects. There are also c) variant interpretations such as Many Worlds (MWI) that try to solve the paradox with other assumptions (as Vanesch himself has recently pointed out in a very nice original paper).

The main thing is to discard the "naive" view that the situation results from some lack of knowledge - even though there is unquestionably much more to learn. After all, a), b) and c) are all quite different!
I disagree with you here.

a) Hidden variables.

The *existence* of hidden variables is not in question. Bell's
point is that the addition of a *variable* global parameter will
not only not enhance qm predictions wrt joint detection, it
will give different predictions for some settings. That qm is
correct is confirmed by experiment.

Nobody knows how the emitted light is behaving prior
to detection. This is the hidden variable(s) -- and as
long as it's behaving pretty much the same at both
ends during any given coincidence interval (which
is the condition that the applicability of the cos^2 theta
formula depends on), this hidden variable(s) is *irrelevant*
to the determination of coincidental detection.

b) non-local ("spooky action at a distance") effects

This is an unnecessary option (given an understanding
of Bell's analysis, optics, and the probability calculus).

I suspect that if one suggests this as a serious possibility
to working physicists one will get a non-serious reply in
most cases.

c) variant interpretations such as MWI

I think that most physicists would put this
sort of stuff in the "not even wrong" category.
______

Regarding the naive view, if the "situation" you're referring
to is the *debate* about the meaning of experimental
violations of Bell inequalities, then "lack of knowledge"
would certainly seem to have something to do with it.

If the "situation" you're referring to is the data
produced in the experiments, then there is enough
knowledge to explain this via local transmissions.

The inference of nonlocal effects via violations of
Bell inequalities rests primarily on the assumption that
the general lhv formulation proposed by Bell is the
*only* way to formulate a local description of the
probabilities in the joint-detection context. The
problem is that this *general* lhv formulation is
flawed (ie., inapplicable) -- for reasons that I've
pointed out in other posts in this thread.

So, imho, what should be discarded are MWI, Bohmian
mechanics, and visions of events in New York instantaneously
affecting events in, say, Los Angeles -- even though events
happening at more or less the same time (8pm Eastern/5pm Pacific)
in each place might well be correlated wrt some context or
some phenomenon (like, say, a giant storm system covering
the entire continental United States). :)
 
  • #32
Sherlock said:
The photon is associated with a detection
event, which is associated with an emission
event. A detection event is a detection event.
It's not .5 a detection or 1.5 a detection -- it's
1 detection. Photons are, by definition, indivisible.

You are, by any coincidence, not the schizofrenic alter-ego of nightlight, are you ? :-)

What you describe is the so-called semi-classical model: we quantify "matter" but we treat the EM field as classical (Maxwell). It is true that many properties of light-matter interaction can be dealt with appropriately with this semiclassical model, and it is true that the "star" phenomena usually invoqued to point to the existence of "photons" (photo-electric effect, compton effect) are in fact also explainable by this semiclassical view, in that the *apparent* lumpiness of the EM interaction is due to the quantisation of matter, and not so much due to the quantization of the EM field itself.
But the quantum field view assigns a real existence to photons themselves, independently of their detection, and there are situations such as the anti-correlation detections which are not explainable in the frame of a semi-classical model, but follow quite nicely from a full quantum-field theoretic treatment.


The way I learned about photons doesn't
*require* that I think of them as indivisible
wave trains, or energy packets, or point
particles when I'm thinking about light
in terms that I want to correspond to what's
physically happening prior to detection.

Photons are not simply "point particles" or "wave trains": they have in fact no genuine existence in a classical field theory like Maxwell's. The quantum state of the EM field cannot, in most cases, be fully described by a classical field E(x,y,z) and B(x,y,z), and photons are specific quantum states of the field.
All imaging of photons as wave trains or classical point particles will, at a certain point, lead to paradoxes.


However, even the best beam splitters
don't produce an exact 50-50 split of the beam.
This is demonstrated experimentally. But the
difference is small, and in conjunction with PMT
calibration considerations seems not adequate to
explain the result of detection at one arm or the
other, but never both, per single
emission/detection interval.

Regarding your question, I would have to know
exactly the type of beamsplitter and detector
and calibrations to say what I would expect.
I already know the results of some of these
sorts of experiments. But, like the dot by dot
interference results, have so far no satisfactory
way to explain why, if there is some wave activity
incident on locations where it isn't detected, then
why isn't it detected?

Ha, but that's exactly where a true QED photon description differs from any Maxwellian picture: the point is that the initial state (an incoming single-photon state on a beam splitter) evolves into a quantum state which is a superposition of two quantum states you CAN measure. You CAN measure a photon in the reflexion arm, and you can measure a photon in the transmission arm. So somehow your "measurement states" of the EM quantum state (reflexion, transmission) are in a superposition of the incoming state (after the beam splitter), and the typical quantum measurement procedure takes place: with a certain probability you detect the first term (photon reflected) and with another probability you detect the second term (photon transmitted). However, you cannot detect both of them, because that corresponds not to a superposition, but to a product-state (a 2-photon state).

The "activity" at both detectors is only there in a Maxwellian picture. In the QED picture, there is a quantum mechanical superposition of "activity at T detector" and "activity at the R detector". As our measurement makes us apply the Born rule in this basis, nature has to choose, event by event, which of both eigenstates will be realized, following the Born rule.

The point usually made by people who want to stick to the semiclassical model is: I need a detailled description of the detector, the beam splitter etc...
I think that this is missing the point, and even "trying to confuse the ennemi" :-)
The reason is this: if QED makes the CORRECT predictions without this detailled knowledge, that means that this detailled knowledge doesn't matter. For instance, you say that the beamsplitter is never exactly 50-50. Granted. So say that it can vary between 40-60 and 60-40. What does this change ? QED doesn't need these details to give you a gross outcome which is verified. I didn't ask you if you expected 13.6% or 18.5% correlation. I asked if you expected about 100% correlation (after taking finite efficiencies into account), or about 0% correlation. This gross estimation should be independent about the details of the beamsplitter or the detector and its calibration, because QED can tell you this result in an "ideal" situation: 0% correlation. In an ACTUAL experiment (as certain have been conducted), you don't find 0% of course: you find something like 0.4%. But you don't find something like 85%.

The argument for the notion that the light wave(s)
itself (which correspond to photon detection event)
is *indivisible* is, I think, based on that question
pointing to the inadequacy of the wave picture
in some situations.

Well, the photon picture as given by QED is a bit more involved than "indivisible" light waves :-)

However, I think that we *have* successfully
refuted at least one set of arguments for the
existence of superluminal causal connections.

The problem is that you need a picture which is globally explaining results. You cannot switch to a Maxwellian picture which give you classical intensities to explain EPR results, and then switch to a particle view to explain anti-correlations. You have to explain both at once, with the same picture. QED can do that, but the price to pay is that you have to accept a full quantum view of the EM field. Once you do that, you cannot use the intensity explanation of classical fields anymore to explain the EPR correlations, in the sense that the photon passes, or doesn't pass, the polarizer, and not that its intensity passes "a bit".

cheers,
Patrick.
 
  • #33
Sherlock said:
I didn't say that the results change upon first
detection. The probability of detection changes,
because the result was that a detection occured.
This initiates a coincidence interval. And, during
this interval, the probability of detection at the
other end is different than it was prior to the
detection that initiated the interval.

I would say that that is the usual definition of conditional probability :-) I think that this is not the resolution of the EPR riddle.


What I offered was local interpretation in that
it requires no superluminal effects, no causal
connection between Alice and Bob to understand
why the probability of coincidental detection is
cos^2 theta.

Yes, but exactly the same classical intensity explanation DOES NOT WORK for anti-coincidence experiments.

cheers,
Patrick.
 
  • #34
Sherlock said:
This is an unnecessary option (given an understanding
of Bell's analysis, optics, and the probability calculus).

Let's not forget that optics is not necessary. If you take quantum mechanics for granted, you get exactly the same Bell violations with electrons. It is only that the experiments are easier to carry out with light than with electrons.

c) variant interpretations such as MWI

I think that most physicists would put this
sort of stuff in the "not even wrong" category.

As I pointed out before, this is a misconception. It would mean that all people working on subjects like quantum gravity, string theory or decoherence are working in the "not even wrong" category.

You cannot seriously calculate the quantum states (and its entropy) of a black hole without taking the superposition principle seriously on the scale of several solar masses. Also all the work on decoherence only makes sense in a MWI setting.

The reason to prefer MWI is not that it is somehow fancy or that the mystery part of it has some strange attraction. MWI is the natural consequence of two principles:
- the quantum-mechanical superposition principle
- locality (in the relativistic sense)

These two principles have been the major guiding ideas in the development of all of current modern physics, and at not one single instant have they had consequences which have been explicitly contradicted by experiment. Each time where the superposition principle could be tested, it won, from the exchange terms in molecular spectroscopy, over phonon and other collective quantum phenomena in solids, bose-einstein condensates and all that. It has never been put in failure.
In the same way, Lorentz invariance and its associated requirement of locality has been a major guiding principle which did withstand many experimental challenges. The price to pay was a major revision of our notion of time, which could have been classified in the "not even wrong" category too if intuition was the only judge.
At this point, there is absolutely no indication that we should limit the applicability, nor of the superposition principle, nor of locality. And when you take these two "good soldiers" seriously all the way, you have no choice but to end up in a MWI view.
That doesn't mean that tomorrow, we will not find the limits of applicability of these principles - gravity might be such a limit, although current indications go in the opposite direction. But as of now, they are to be considered universally valid, because they have never undergone any experimental contradiction what so ever. You are of course allowed to have personal preferences - based upon intuition - that make you dislike the apparent weirdness of MWI. However, you cannot say that MWI is "not even wrong". Remember that the weirdness of MWI is only on a philosophical level: concerning hard predictions of observation, it is in FULL agreement with all that has ever been observed, on the same level as Copenhagen QM or Bohmian mechanics. It does so however, without violating the two basic principles which led to the rest of the theory in the first place, a claim which Copenhagen or Bohmian mechanics cannot make.

cheers,
Patrick.
 
  • #35
vanesch said:
You are, by any coincidence, not the schizofrenic alter-ego of nightlight, are you ? :-)

I'm not nightlight, no ... or schizofrenic,
as far as I know. :) It was an interesting
discussion you had there. Lots of messages.
I still haven't read most of them.

I think that one could construct a semi-classical
model to account for the anti-correlation (beamsplitter)
situations. Not sure if *I* can do it. It would be an
interesting exercise.

About these photon fields that exist independent of
detection -- how do we know that they exist?

I don't think of photons using the images you (I) mentioned.

If you think of light in terms of photons, what sort of
imagery do you associate with this? Is your imagery wrt photons
strictly mathematical/symbolic? Or, do photons correspond
to some 'natural' physical form or phenomenon, and, if so, what?
That is, just what sort of picture do you get from
the quantum theoretical picture of photons and
quantized EM fields.

For me, the quantum theoretical 'picture' is, devoid of any
sort of real imagery (that is, imagery analogous to my sensory
experience of natural phenomena). When I'm (trying) to do a
calculation using quantum theory, I'm not thinking in pictures.
But, I *want* to think in pictures wrt this stuff. :) And, I
don't see any reason why that should be absolutely
impossible. Well, for the foreseeable future it's
impossible wrt some experiments. :)

The problem is that you need a picture which is globally explaining results. You cannot switch to a Maxwellian picture which give you classical intensities to explain EPR results, and then switch to a particle view to explain anti-correlations.

Why not? People do this all the time. In the case of the EPR
results it gives the simplest explanation, requiring no exotic
natural phenomenon.

You have to explain both at once, with the same picture. QED can do that, but the price to pay is that you have to accept a full quantum view of the EM field. Once you do that, you cannot use the intensity explanation of classical fields anymore to explain the EPR correlations, in the sense that the photon passes, or doesn't pass, the polarizer, and not that its intensity passes "a bit".

You're thinking of the qm ability to reproduce
the results of measurements as an explanation.
But, what actual understanding does it provide?
Isn't this the conundrum that is quantum theory
itself? Physical details of the processes that
produce the results of the beamsplitter and
double-slit experiments with photons aren't provided
by the theory. In that sense it is, following
Einstein's appraisal, an incomplete description
of the physical reality. Bohr said it is impossible
to visualize what's happening at the quantum level.
I'm not so sure that Bohr was correct about that.

It's true that if you characterize photons as indivisible
particles of light, then you can, after a fashion, 'account'
for the results of experiments. But, I wouldn't call these
accounts explanations in the usual way that we use the word
explanation.

Resorting to this characterization goes to the foundation
of quantum theory. No imagery, no real-world 3D details
of pre-detection behavior of the light. There are good
reasons for this approach of course. Just experimental
results. Quantum statistics. A consistent mathematical
structure. A method for calculating the probable results
of any experimental setup, and an associated abstract
'picture' that gives little insight into the actual
behavior of the 'phenomena' in question.
(There are no half-photons because there are no
half-detections.)

Anyhow, since I see no reason why the behavior of
waves in undetectable media should necessarily
be fundamentally different than the behavior of waves
in detectable media, I use the wave analogy when possible
and speculate about the details. I think that this
approach will eventually provide a better understanding
of nature than the more strictly instrumental approach
that characterizes quantum theory. I also think that
quantum theory will be around pretty much forever. It
does, after all, 'work'. In fact, I don't see how
you could *possibly* get incorrect predictions if you
use it correctly. There does seem to be something not
entirely arbitrary about the idea of a fundamental
quantum of action. Light might well be quantized
independent of detection -- I just don't know exactly
what that might mean in physically descriptive
terms.

To offer as an 'explanation' for the results
of the beamsplitter experiments that the emitted
light exists in a superposition of photon states
and that mother nature 'chooses' (quite randomly,
with equal passion for both detectors) which path
will be taken by all of the light emitted in a single
emission/detection interval ... well, forgive me
if I don't find that a compelling 'description' of
what's happening at the quantum level of interactions. :)

By the way, what does "Warnings" mean where it
says "View so-and-so's Warnings"?
 

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