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    Physics Psychics? How Alice And Bob Talk, Without Saying A Word
    By News Staff | April 18th 2013 12:06 PM | 38 comments | Print | E-mail | Track Comments
    In quantum physics, objects can be in more than one place at a time and future events can change the past - don't get caught up too much in that or you won't sleep at night.

    A new paper says quantum physics can be even spookier. Using the “chained” quantum Zeno effect, the researchers write that they have discovered a form of "almost psychic communication" in which information can be exchanged between two parties without photons, or any physical particles, traveling between them.

    Let's invoke "Alice" and "Bob", the most famous couple in physics, to illustrate what they mean. The basic set-up works like this: A photon leaving Alice's station can, on Bob's end, have its polarization state switched or kept the same. In other words, Bob has the choice of either absorbing the bit of information or not absorbing it. Depending on what Bob chooses, different detectors are set off on Alice's end. So Alice can figure out Bob's choice by checking her own detectors. But these researchers created a counterintuitive result: A communication occurred between Alice and Bob, but the photon never left Alice's station. It's long been assumed in physics that for information to travel in empty space between two parties, Alice and Bob, physical particles have to travel between them. But by using a series of beam splitters, the researchers created a setup in which information can be exchanged between Alice and Bob without any photons, tiny units of electromagnetic light, actually passing through the communication channel, introducing what Texas A&M physics professor M. Suhail Zubairy calls a "new paradigm" in quantum communication.

    "In technical terms, we call this 'counterfactual,'" Zubairy said. "It's been believed that if two parties want to communicate, something needs to be sent, and something is allowing for its passage. That's been an underlying assumption in every communications system. This is the first possible setup where that doesn't exist. That's what I mean by 'almost psychic.'"

     Here BS stands for beam splitter and SW stands for ideal switches. In the transmission channel, the photon is accessible to Eve. (a) The BSs have large reflectivity, R = cos2 θ=cos2 (π/2N) with N being the total number of beam splitters. (b) By using a chained version of the setup shown in (a), we can achieve direct counterfactual quantum communication. Two kinds of BSs are used. One is BSM for M big cycles. The other is BSN for N small cycles within each M cycle. There are a total of M × N cycles for one signal. As discussed in the text, the probability of finding a signal photon in the transmission channel is nearly zero. Clicks at D1 or D2 reveal to Alice Bob’s bit choices. Note that in this case, the photon is accessible to "Eve," the group's token name for the possible third party in the "Alice" and "Bob" scenario -- an eavesdropper. (Credit: Salih et al., arXiv:1206.2042

    Zubairy says they were inspired by the development in recent years of the branch of quantum cryptography called quantum key distribution, a system that allows secure communication between two parties by the exchange of "keys" that allow for decoding of messages sent through a public channel.

    "What we are proposing goes a couple steps beyond that," Zubairy said. "We are talking about direct communication, not through a key. Now we have a protocol for eliminating the middle man. From a fundamental point of view, this is amazing. It sort of brings up these old questions, such as what is a photon?"

    "The concept of two parties communicating without exchanging particles is mind-boggling and highly counterintuitive," said co-author Mohammad Al-Amri of the National Center for Mathematics and Physics at the King Abdulaziz City for Science and Technology in Saudi Arabia. "It raises interesting questions: How could Alice sense Bob's moves when her photon never left her station?"

    Citation: Hatim Salih, Zheng-Hong Li, M. Al-Amri, and M. Suhail Zubairy, 'Protocol for direct counterfactual quantum communication', arXiv:1206.2042, upcoming in Physical Review Letters.

    Comments

    vongehr
    Depending on what Bob chooses, different detectors are set off on Alice's end. So Alice can figure out Bob's choice by checking her own detectors.
    What. The. Fuck???
    This is so wrong, I am speechless. And after that it gets worse. This whole article is just complete nonsense.  Please no more of these completely idiotic press releases without at least a disclaimer warning about it being such.
    Are you insensed by the quality of the reporting or by the claim itself?

    If the latter, perhaps you could explain to those of us who don't understand this stuff where the idea differs from the Elitzur–Vaidman bomb tester?  (The latter can be enhanced by making sure the beam splitter sends a much higher amplitude along the "safe" path and repeating the test coherently.) In both cases, the observation depends on amplitude but the unwanted effect depends on amplitude squared. So where does Bob, signalling with his switches, differ from the bomb mechanism? If Alice can tell whether a bomb is a dud with negligible probability of setting it off if it's not, why can she not tell how Bob has set the switches with negligible probability of Eve being able to intercept a photon?
     
    Puzzled. 

      
    vongehr
    The press release is just too misleading right from the start. You should write them; yes, bomb testing is what should have started the article, not time travel nonsense.

    The paper is more or less fine, although the claim is somewhat sensationalist. It writes "no photons pass through the transmission channel", however, it is only that none end up on the other side of the channel at D4 (or on Alice's side in D3) in case all works out fine. Actually, the photon "passes through" (as usually as its 'potentiality wave' or QM wavepacket before it is detected) ~2000 times (for the whole to work ~95% of the time). The interferometer has to have a precise, unobstructed length on Bob's end in order for bit=0 to be transmitted. Eve simply trying to grab a photon is as if Bob blocks the mirror (bit=1), so Eve can disturb the communication. It certainly is not so that she can do nothing on grounds of that there is "nothing physical" (something that can be physically interfered with) in the channel. This method may be worse than what has come before, because the photon's QM wavepacket goes forth and back 2000 times (rather than never, as one may be mislead to conclude), leaving a lot of time for Eve. She can divert the paths from the channel and talk to Alice as if she is Bob and to Bob as if she is Alice or perhaps even check out Bob's Pockels cell settings (the secret information) with other wavelengths somehow. Thus, their claim to offer secure direct communication (rather than just a key distribution) is something that should be very carefully checked out.

    The comparison is not too fitting but somewhat applies: I do not send you a particle if it rains tomorrow, and afterward I go "look people, I told a guy around the world that it rains without exchanging anything physical, so Eve could do nothing; I found the perfect secret communication". Looking into the details reduces the substance of the claim considerably.
    You should write them; yes, bomb testing is what should have started the article, not time travel nonsense.
    For a scary moment I mis-read that to mean *my* time-travel nonsense :)
     

    One of things I dislike about the term "bomb testing" is that the counterfactual observation does not depend on a counterfactual explosion, it just depends on the absorbtion or non-absorbtion of the photon.

    "One percent of a photon" doing the observation and sneaking away without setting off the bomb is lot easier to swallow than a bomb going off in Counterfactualand and making its presence felt in Actualand. Of course if you've gone down the rabbit hole with relative-state MW you won't have a lot of sympathy for any such desperate clinging to the shattered remnants of direct reality!


     
    vongehr
    1) To your charge against MW: If you want to get the probability of the bomb exploding to exact zero, you must wait forever (here for example N,M go to infinity). So, as long as you get finished with Zeno-effect procedures, the bomb's exploding is relatively actualized. It is somewhat silly to say that the probing works because of the explosion in a parallel world, but on the other hand, the exploding belongs crucially to the whole (if there is no cutoff on small probabilities, i.e. QM is truly unitary).

    2) Your alternative description using 0.01 of a photon interacting goes against the notion of interaction quantum. Your position on weak QM interaction via counterfactuality seems to be that quantization is in the emergence of the classical, while the unitarity of QM, i.e. the continuity of the phase, corresponds to interaction being fundamentally not quantized - the 'bit from it' direction of the fundamental description. I think such can be defended via the "it" being nothing traditionally ontological.
    the exploding belongs crucially to the whole (if there is no cutoff on small probabilities, i.e. QM is truly unitary).
    Well, let's not worry about non-linearities and the like. Yes I agree that the explosion is actualized in a counterfactual world - CF in the sense that it is one of the minority of worlds and which, if we are careful and not too unlucky, we are not the ones to observe it. But I don't think the explosion is crucial. Of course an explosion demolishes the detector thus making the absorbtion of the photon somewhat "irreversible", but, as you well know, this is merely a dramatic device designed to distract from the fact that the absorbtion is, in principle, still reversible - because the zillions of counterfactual worlds preserve the phase in their entanglements. Admittedly, getting a half-silvered mirror for the entire environment, explosion and all, may pose a few experimental difficulties, but quantum erasure in simple systems is well-established. So any recording device, not necessarily a bomb, is enough. 
    Your alternative description using 0.01 of a photon interacting goes against the notion of interaction quantum.
    Of course it does. That's why I said it represents "desperate clinging to the shattered remnants of direct reality". On the other hand, in worlds where the bomb goes off, the wavefunction must normalize itself, which is even worse unless, like a magician, you hold a black cloth over the process of "universe splitting".  
    Your position on weak QM interaction via counterfactuality seems to be that quantization is in the emergence of the classical
    Well yes, that is my "position" (as I made clear to Scott at one point) but I am far from confident that I understand enough of what is going on. This would seem to involve getting to grips with the entire observation process, that is to say combining the observed system, the observing system and environmental decoherence into one. It may all be there in the Mott problem but I haven't gone over it yet. It would be nice to get rid of the Born rule.
    I think such can be defended via the "it" being nothing traditionally ontological.
    I think so too. Relative states is one thing but degrees of reality too... out of the frying pan into the fire.
    vongehr
    But I don't think the explosion is crucial.
    I will be convinced if you have a proceedure where it goes to P=0 without us waiting for an infinity.
    Of course it does. That's why I said it represents "desperate clinging to the shattered remnants of direct reality".
    Not if it never leaks into the emergent classical phenomena. In fact, direct reality is nowadays often seen as a sort of cellular automaton rather than a continuum, which can never be operationally meaningful.
    I will be convinced if you have a proceedure where it goes to P=0 without us waiting for an infinity.
    All I said is that is doesn't have to be a bomb. Black paper or letting the photon escape would do just as well.
    vongehr
    Come on - you should know I don't care about whether stuff explodes. The explosive issue is whether one can/should causally blame the relative actualization of the alternative outcome for counterfactual results. I agree that such is somewhat silly and misleading towards the misinterpretation of QM as being due to a statistical ensemble of directly real worlds.  However, if the counterfactuality never occurs when Zeno procedures are taken all the way to P=0 (no measure for alternative outcome worlds), one might as well claim such necessity of the bomb exploding in a parallel world. If so, the weak measurement and Zeno procedures would then count as ways to more effectively couple many more worlds to the exploding bomb.
    Come on - you should know I don't care about whether stuff explodes.
    :)
    However, if the counterfactuality never occurs when Zeno procedures are taken all the way to P=0
    Umm, isn't that a bit "counterfactual" in itself? P=0 is surely a limit that cannot be obtained in finite time or imperfect apparatus? Why tack an arbitrary conclusion on a impossible premise? "If the sky falls we shall catch larks".

    What would be more interesting - and somewhat beyond my ability right now - would be to see what happens as one approaches that limit. Perhaps it becomes impossible to maintain coherence - not because of some non-unitariness in QM itself, but because every system must couple to some extent with the environment so there is always a non-zero limit. Just a thought.
    If so, the weak measurement and Zeno procedures would then count as ways to more effectively couple many more worlds to the exploding bomb.
    ... to a bomb that never explodes in any world.

    Oh I forgot. You love multiplying braches of a splitting universe to accomodate entanglement. May as well multiply a branch that doesn't exist by infinity to get it back. :)

     
    vongehr
    couple many more worlds to the exploding bomb. ... to a bomb that never explodes in any world
    ??? The whole point is that "if so ...", the bomb crucially does explode in at least one world.
    Also, that P=0 cannot be fully gotten is due to the nature of the Zeno proceedures. This is precisely my point. I want to see counterfactuality with proceedures that do not have this problem, where P=0 is atainable with finite effort. Of course, now you say that even at a 90 degree polarizer, one photon may go through because you never get 90 degree exactly. But that is different from having to wait for the end of infinity due to the nature of the proceedure.
    The whole point is that "if so ...", the bomb crucially does explode in at least one world.
    A world which is only present at zero amplitude... ;p
     
    vongehr
    ??? What the hell are you talking about? Our discussion is about that P does not go to zero. If the amplitude were zero, P would be zero. Are you going nuts or is it me?
    Our discussion is about that P does not go to zero.
    No, we are discussing hypothetrical procedures "where P=0 is atainable with finite effort". 
    Are you going nuts or is it me?
    Hard to tell. My money is on you being as mad as a box of frogs but that I mistook your intended meaning where you spoke about "coupling many more worlds to the exploding bomb".
     
    vongehr
    I clearly, without leaving any doubt whatsoever, wrote "if/under the assumption that P=0 only with infinite effort ... then ..." constructions.  I thought you may have some insight I am missing, but if it merely comes down to your not being able to read, it is just too silly. I'm out'a here.
    If you want to go off in a huff, that's your prerogative.  But the problem isn't my not being able to read, it's your not being able to write. Stuff like this: 
    one might as well claim such necessity of the bomb exploding in a parallel world
    is illogical nonsense, while word-heaps like this
    couple many more worlds to the exploding bomb
    must have Everett rolling in his grave.
    Hank
    He's a German living in China writing in English so the form needn't be perfect. But does he have a point or not is the issue?
    Fair enough, Hank. But as to whether he has a point, that is what I was trying to find out. There is a proposed experiment - the one demonstrating "counterfactual communication" which was sloppily reported - and a related, much simpler, one nicknamed a "bomb tester", which has been constructed, admittedly without actual bombs. It behaves as quantum mechanics predicts, so we are confident that the counterfactual communicator will also work. This is not just a tweak in some correlation statistics - as with the Alain Aspect EPR experiments where the sheer impossibility of a common-sense interpretation is not immediately obvious. In the bomb-tester case, quantum mechanics allows measurements to be made on a bomb without the bomb experiencing the interaction (and therefore being set off). The system returns the result that *would have* been obtained if a photon had been detected by the bomb trigger, but *without* such a detection and explosion occurring. So it's a lot more in-your-face than the correlation between polarizations of two entangled photons. It once again confirms that quantum mechanics "does what it says on the tin" as the (UK?) advert says.

    On all this, Sascha and I have no disagreement, though he might well put things differently as I'm learning the stuff as I go along whilst to him it's second nature. The little altercation here started over my comment that I don't like the term "bomb tester" as it doesn't really test the bomb, it tests whether the bomb's trigger absorbs the photon. So the same result would and does obtain without having to blow the apparatus up.  This is why I say the bomb is not crucial.

    Does Sascha have a point? Well, in addition, the system is not 100% effective so he was speculating on there being a different procedure which would make it perfect. Now *that* is interesting! Unfortunately he also said that such hypothetical procedures would couple many more worlds to the exploding bomb. That doesn't make a lot of sense to me, indeed I misuderstood one aspect of it. However it's rare for Sascha to write without having a serious point and I am looking forwards to seeing his ideas about whether a totally interaction-free measurement can ever be achieved rather than speculations about my ability to read. 
    Bonny Bonobo alias Brat
    Wow, nicely put Derek! It all makes a lot more sense now ;)
    My latest forum article 'Australian Researchers Discover Potential Blue Green Algae Cause & Treatment of Motor Neuron Disease (MND)&(ALS)' Parkinsons's and Alzheimer's can be found at http://www.science20.com/forums/medicine
    logicman
    (UK?) advert

    I remember it well - Ronseal.
    vongehr
    Unfortunately he also said that such hypothetical procedures would couple many more worlds to the exploding bomb.
    NOOOOO! I clearly said that IFF there is NO such proceedure that can be completed in finite time, THEN, in that case, we would have to grudgingly admit the seemingly silly descriptions which are based on at least one world where the bomb necessarily must explode [in other words, the non-exploding worlds are coupled efficiently to, but cannot be without (= must be so coupled) to one exploding bomb world (Assuming the IFF!, OK? IFF!!!)]. Somebody please tell me how that is not entirely clear because of my German Chinglish???
    And don't distort the issue. Your point is not the bomb being unnecessary. You toss again and again quibs claiming zero amplitude (no actualization) of the exploding.  Look: If Amplitude = 0, then P = 0, because P = Amplitude squared.
    The bomb being there is not an issue - in fact, you know well that I would have at least strapped a few babies to the bomb in order to get the point across.
    Arguments please Mr Knowitall!!

    I may be a layman, but I call BS on this article too. To me, nothing is explained at all. just claimed. strange conjectures that are not followed up on, and statements like '...we can achieve direct counterfactual quantum communication.' sound really fancy, but are not backed up in any way. complete waste of my time, and very VERY annoying to see this kind of article on Science2.0. please stop writing.

    LOL, love that first comment. And when I read the paper, every time I saw the BS standing for "beam splitter" I thought of something else. Then what with the mirrors and the photons, I couldn't work out how they arrived at their claim.

    Besides, couldn't one communicate in a particle-free fashion via the "Aharonov-Bohm" effect? In figure 2 of Ehrenberg and Siday's 1949 classical paper "The Refractive Index in Electron Optics and the Principles of Dynamics you can see how A gets skewed, reminiscent of the way a hose reel rotates a little when you turn the water on. There's no electromagnetic field outside a long solenoid, and no particles flying out of the wire, but you can detect the potential.

    That article was written in 1949, back in the good old days when a field was a field and a particle was a particle. Alas, things have moved on now and the fields have disappeared, to be replaced by infinite collections of virtual particles... So is there a field involved?
    There's no electromagnetic field outside a long solenoid
    say Ehrenberg and Siday. Well yes and no. The classical electric and magnetic fields are indeed zero but they are not the whole story if one includes the potentials as part of the full electromagnetic field, as they are. That is the point Ehrenberg and Siday are trying to make: NOT that there is no field at all. So with today's understanding, the Aharonov-Bohm effect is just as dependent on particles as plain old radio. On the other hand, this report is about using real particles for communication: a single binary digit per photon. So it's hardly particle-free. 
    I don't think things have moved on Derek. It's supposed to be Quantum Field Theory. And see http://prd.aps.org/abstract/PRD/v7/i6/p1668_1 where "the identity of these evanescent waves with virtual photons is established". That was 1973, and there are no actual photons flitting back and forth between an electron and a proton. Hydrogen atoms don't twinkle. Magnets don't shine. I thought Matt Strassler gave a fair description of virtual particles here: http://profmattstrassler.com/articles-and-posts/particle-physics-basics/... . It's like you divide your field up into little squares, then you shut up and calculate. But what you don't do is start thinking that virtual photons aren't virtual. Draw a photon as a one-wavelength sine wave, take the integral to get a hump of potential, and note that it's highest in the middle where the sine wave goes to zero. The potential changes because the photon is a field-variation, not because it's surrounded by a cloud of photons. And have you seen the given explanation for two photon physics? See http://en.wikipedia.org/wiki/Two-photon_physics . Pair production allegedly occurs because one of the photons turns into an electron-positron pair. Spontaneously. Like worms from mud. Then magically turns back into ONE photon. And all the while it manages to keep on travelling at c. So pair production occurs... because pair production occurs. If Feynman was still around, I'd ask him where he thought QED morphed into cargo-cult science. And where Alice and Bob started dabbling in magick.

    Whatever. I was merely explaining why replacing individual real photons with hoards of virtual ones doesn't have the same significance for counterfactual communication. I was not intending to turn the blog into an Arab Bazaar for ontological haggling.
    It isn't ontological haggling. It's a refusal to believe in magic. Given the subject of this thread and given that your name is Potter, I was rather hoping you'd appreciate that.

    Suit yourself. You said
    I couldn't work out how they arrived at their claim
    but I have no interest in convincing you if you don't wish to learn.



     
       
    I'm always willing to learn Derek. So please, given that a wave or wavefunction or field or field variation or change in potential or interaction can be considered to comprise virtual particles, and no particles whatsoever are allegedly exchanged here, please do explain. I'd like to understand how Alice and Bob can communicate without interaction and when nothing changes at all. Let's have it. I'm all ears. Note the reference to "physical particles" in the article above.

    Note the reference to "physical particles" in the article above.
    Yes - one wonders what other sorts of particle there are. The fact that the authors were very careless, or possibly sensationalist, in their use of words should be warning enough not to take them (the words, not the authors) at face value. So if you are merely objecting to the original claim:
    In this paper we propose a surprising mode of communication whereby no physical particles travel between sender and receiver.
    there is no more to be said. One must always apply the Principle of Charity and try to work out what the authors are getting at when they seem to be talking nonsense. Of course it could be they are really talking nonsense but in most cases it's just a poor choice of words. That's why the A-B effect is not a good rebuttal to Salih et al. It is more useful to ask why they say that no particles travel between sender and receiver. In fact the experiment requires n photons per bit where, with perfect optics and detectors, n can be reduced arbitrarily close to unity but never to precisely unity. Similarly, the probability of an absorbing detector (Bob or Eve) intercepting a photon can be reduced arbitrarily close to zero but never to precisely zero.  All this is loosely expressed as "no physical particles travel between sender and receiver" but the paper itself explains the matter adequately for anyone to get started.
     
    Your point is not the bomb being unnecessary.
    Of course it is. I had said
    One of things I dislike about the term "bomb testing" is that the counterfactual observation does not depend on a counterfactual explosion, it just depends on the absorbtion or non-absorbtion of the photon.
    I then said 
    But I don't think the explosion is crucial.
    and then
    All I said is that it doesn't have to be a bomb. Black paper or letting the photon escape would do just as well.
    How could my point be anything else?
    You toss again and again quibs claiming zero amplitude (no actualization) of the exploding.
    Yes, in response to you talking about how the worlds might couple together.  You had said
    couple many more worlds to the exploding bomb
    Last time I looked, worlds couple to other worlds not to bombs. Indeed you have now said
    (= must be so coupled) to one exploding bomb world  
    Putting in the missing word certainly helps.

    However your loose language did confuse me at first and I thought that "exploding bomb" referred to an outcome where the bomb explodes, not to one where it would have exploded without the Zeno procedure. It would have been helpful if you'd focused on that instead of saying (twice, once with added nuts)
    Look: If Amplitude = 0, then P = 0, because P = Amplitude squared.
    I did actually know that.
    The bomb being there is not an issue
    Whoo-hoo! At last! 
    vongehr
    ?!?
    Well, I give you one thing. It requires guts to actually insist on that one's own main point is so completely irrelevant. Fine then, so that it is not necessarily a bomb but could be a pointer instead is important to you (I hope we agree it must be some macroscopic irreversibility, i.e. decoherence), rather than the potential necessity of quantum-correlation coupling to at least one exploding bomb (and thus to its world - what is the point to split that hair? The "world" is not meant to be the directly real universe box with the bomb inside! The "world" denotes the exploding bomb result.).

    You quote me out of context every time as if i did not clearly put the phrases X, the ones you criticise so much, inside constructs like
    Under the assumption that ... , then the seemingly silly X.
    It would be by this point much more productive if you could tell me how it is that my Chinglish makes my "if..., then ..." constructions invisible to you.
    For your reference from above:
    However, if the counterfactuality never occurs when Zeno procedures are taken all the way to P=0 (no measure for alternative outcome worlds), one might as well claim such necessity of the bomb exploding in a parallel world. If so, the weak measurement and Zeno procedures would then count as ways to more effectively couple many more worlds to the exploding bomb.
    Well, I give you one thing. It requires guts to actually insist on that one's own main point is so completely irrelevant.
    Har har. If you think I'm going to argue about *that*, you have another think coming.
    It would be by this point much more productive if you could tell me how it is that my Chinglish makes my "if..., then ..." constructions invisible to you.
    That was Hank being charitable, not me. I don't blame you for being foreign, I blame you for being muddled :)

    No, it's nothing to do with the IF or IFF condition. Whether the condition is met or not, the dependent proposition is unclear and ambiguous. It now seems that what you meant was "if a bomb is used then it will necessarily explode in some world" which is quite different from saying that "it is necessary to have a bomb (which explodes in some world) in the system for it to work". Unfortunately your actual wording was  "one might as well claim such necessity of the bomb exploding in some world" which could have meant either. 
    I hope we agree it must be some macroscopic irreversibility, i.e. decoherence
    Yes, but you surely know I don't believe in decoherence except as a statistical thing! That's why I said at the beginning
    Admittedly, getting a half-silvered mirror for the entire environment, explosion and all, may pose a few experimental difficulties, but quantum erasure in simple systems is well-established. So any recording device, not necessarily a bomb, is enough.  
    edited at 22:52 local time

    vongehr
    It now seems that what you meant was "if a bomb is used then it will necessarily explode in some world"
    This is not what I mean, due to the curious focus on the bomb, which, again, is just an example for a macroscopic pointer, as you know and as I have explicitly said early on, by now more times than I care to count.
    "it is necessary to have a bomb (which explodes in some world) in the system for it to work".
    This is precisely the X (again, the bomb being here an example for some pointer, and only as the X after the "if ...").
    Unfortunately your actual wording was  "one might as well claim such necessity of the bomb exploding in some world" which could have meant either.
    I don't agree, because I told you early on that whether bomb or baby killing pointer is irrelevant. So what is the problem? Would it be better with "necessity for ..."? Do I need to put commas ("necessity, namely that of ... in at least one world, ...")? I don't see it.

    Look - charity of interpretation in this case should mean that you ask yourself: Even if one could possibly understand it that way, am I really likely to mean that quantum mechanics needs bombs in order to work (= Sascha must have totally lost it), or isn't it much more likely that this is about whether the proceedures will ever allow P=0 for the exploding bomb outcome in finite time. If not, then P>0 for ever, which in turn means precisely that the exploding of the bomb is actualized relative to an observer (= there is such a world in any many-world model that I may construct around the issue, which one can actually reasonably do for some issues, like EPR).
    No you are not going to repair this:
    However, if the counterfactuality never occurs when Zeno procedures are taken all the way to P=0 (no measure for alternative outcome worlds), one might as well claim such necessity of the bomb exploding in a parallel world. If so, the weak measurement and Zeno procedures would then count as ways to more effectively couple many more worlds to the exploding bomb.
    with a comma. It needs to be re-written completely so you clearly say that it is the photon absorbtion that is necessary and not the bomb. If you then want to justify the claim that the explosion in a parallel world is necessary, that is up to you, but I think it confuses the issue. I can't help with the second sentence at all. At the very least you need to make it clear how "coupling many more worlds to the exploding bomb" is consistent with there being a zero measure of those worlds. Sometimes it seems that MWI is more trouble than it's worth :)
     
    Look - charity of interpretation...
    I almost agree with this. However I genuinely looked for a plausible meaning and could not find one.
     
    vongehr
    it is the photon absorbtion that is necessary and not the bomb.
    There is no photon absorption without a measurement of it. You need a pointer in order to talk about measurements!
    need to make it clear how "coupling many more worlds to the exploding bomb" is consistent with there being a zero measure of those worlds.
    Wow - I mean just wow - absolutely wow wow wow. Now I really give up - that is it. If you have STILL not understood that I wrote "(!)IFF(!) P>0, then A>0", there is no point communicating.
    <sigh> I was actually answering your question.