Duda Jarek

Yes I claim But instant communication seems to be much less against our intuition than retrocausality ... which is clearly seen in Wheeler's experiment ... CPT conservation suggests that causality should be able to go in both causality cones ... If we accept it, instant communication is a piece of cake - send information back and forward or oppositely... This doesn't mean that physics is 'nonlocal' as Bell's inequalities' enthusiasts claim - if we think about physics as in any field theory (QED, standard model, general relativity): fourdimesionally - it's absolutely local. Merged post follows: Consecutive posts mergedIf something is interested, there are two more interpretations of QM in which we try to understand QM fourdimensionally as in CPT conserving field theories - 'transactional interpretation' and 'theory of elementary waves'. I believe here starts new large discussion about it: http://groups.google.com/group/sci.physics.electromag/browse_thread/thread/749d5a06be67485f/eac28a1f73a81aab?lnk=raot#eac28a1f73a81aab

Why are You so sure that it's 'violating reality' and its 'self-consistency'? I will cite myself, from the link I've already attached, in which I'm trying to show that it doesn't have to lead to inconsistencies like time paradoxes: "But let's think about a possibility of a channel in which we could send information back in time on which we cannot fully rely - sometimes it's easier for physics to make it lie. Observe that quantum mechanics gives physics large freedom in choosing effects below our recognition to stabilize such causal time-loops to avoid time paradoxes: like choosing what spin will be measured, or make some small influences on statistically looking behaviors ... or eventually make that this backtime channel would lie. Assume that there are allowed causal time-loops, but they have to be stable. So if there would be such 'device' to transmit information back in time and I for example would send to me a picture of the girl who will be my wife ... and just before sending it back in time I would change my mind - this timeline would never appeared - there would be completely different situation (without this knowledge). So if I'd really get this picture from myself - I will have no choice but to really send it. Like in (a good...) SF movie - they go back in time to repair something ... and finally it occurs that the situation is exactly as it was ... How to use such 'device'? Imagine we take some real random number generator - for example measuring spin of photon 45 degrees from its polarization. Now the procedure is: (1) make a choice according to this generator, (2) if from future there is a message that it was a wrong choice - take a different one (3) wait for results of this choice (4) finally if it was wrong choice - send this information back in time to (2) So if there was a satisfying choice - it has created stable time loop - so in fact the possibility of using this device made that our random number generator (quantum mechanics) has already chosen properly before the loop. It can be imagined that each of choice starts a different timeline - in our language they are all entangled and physics will choose one of them in what we call future, such that there for example are no time paradoxes (like that on a string is allowed some discrete set of wavelengths). What if there wasn't any satisfying choice? Would it create time paradox? Not necessary - most probably the physics would destroy the weakest link of such loop - make that this 'device' had lied. Observe that even without this real random number generator, such 'device' could work without actually being used: if for example there had to be a successful terrorist attack, there would be sent information to prevent it ... and finally in a stable timeline this attack would never happen (because of e.g. proper quantum choices below our recognition)." What is wrong with such point of view, especially that the channel we are talking about (with cat and telescope) works probabilistically?

But in this article there is only the same argument - because it would violate the causality... Ok - there is reference to a paper in which is 'proven' that it's impossible. If it can be generally proven - shouldn't showing a problem with with given experiment be a piece of cake? So please tell me - why exactly this specific experiment won't work? Unfortunately 'proofs' in physics are extremely subtle things. To prove something we have to some complete information about the situations - assumptions. In physics we still don't have it - so we use some strange assumptions, approximations and proudly say that something is proven... No it isn't! For example - in phenomenological thermodynamics we 'prove' 2nd law of thermodynamics. But this 'proof' uses continuous functions of density, pressure, etc ... silently ignoring microscopic structure... All serious theories we use conserve CPT symmetry, so let us assume that there is a mathematical proof that in given model there is statistical property which always increase with time (entropy). Now use the same proof after CPT symmetry - the same entropy has to work - we get contradiction. Returning to the experiment - in this discussion on wikipedia is written: But what does it mean when photons goes in opposite directions? For example gets to us and the colony 'in the same moment'... What does it even mean for two points of spacetime 'not connected causally'? Special relativity says that we could change a frame of reference - make a boost: in one we get photons earlier, in a second - the colony. So from one frame of reference the experiment would work, but from the second not? Ok simpler question - still: what do You think about Wheeler's delayed choice experiment?

Hi, Do You mean experiment like on: http://www.newscientist.com/article/mg19125710.900-whats-done-is-done-or-is-it.html If yes -the problem is that it will always give wavelike behavior. If the photon will be measured on the longer path, this measurement meant that it has chosen this path - won't be seen on the shorter path. There is needed some additional split of photon: look at 6th post in http://www.scienceforums.net/forum/showthread.php?t=39787

So please show me what is wrong with it? So what is causality? Do You believe in CPT conservation? How You understand Wheeler's experiment: http://en.wikipedia.org/wiki/Wheeler%27s_delayed_choice_experiment that by choosing telescope, we choose behavior of photon a million years ago...?

Exactly! Choosing observation tool we can choose if probability of that is large or small - doesn't we have instant communication (a light year) here? And we get even more interesting effects if optic length to the cat is much smaller than to us... ... especially if we are near the cat ... (photons could be e.g. reflected back) If You are afraid that it's not physical, please look here: http://www.scienceforums.net/forum/showthread.php?t=39296

Look at: http://en.wikipedia.org/wiki/File:Kim_EtAl_Quantum_Eraser.svg Now place the upper detector in minimum of interference and connect to a poison and so the cat. On the lower part instead of these mirrors just choose to be able to distinguish paths or not - choosing probability of cat's death...

You've started with quantum eraser, but I see that this analogy was too far But ... let us take experimental setup like in the middle of: http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser A photon goes through both slits. Now using 'spontaneous parametric down conversion' (a photon is converted into pair of entangled photons having twice smaller energy) - we get 'entangled two pairs of photons'. Now as on the picture - one pair is sent to a detector in the colony. But this detector is placed in some (the first) minimum of eventual interference ... and causes death of poor car if it catches photon. The second pair is sent to us, but we don't allow beam splitters to choose if we can determine the slit or not, but as in Wheeler's experiment - just measure these photons with selected tool: - if we would use a telescope which can distinguish both slits - photons observed in the colony should behave corpuscularly - thee is no interference - the probability of killing the cat is large, - if we would use telescope with smaller resolution than required to distinguish the slits - we should get wavelike behavior - the probability of killing the cat would be smaller. What is wrong with this picture?

Perfect isolation denotes that inside and outside of this box are causally separated - there are two separate environments (e.g. by a light year). The cat/the interior of the box are already environments - thanks of detector+poison we amplify the quantum measurement. But this measurement doesn't inflict the outside of the box (or inflict a year later) - from this point of view we have entanglement of life and death cat. Ok - imagine that in the middle we would create two entangled photons, send them in opposite directions and in the colony measure its spin and kill the cat if and only if the spin is up. Wouldn't the photon we received be entangled with life/death of the cat? I wanted this picture to show that entangled means - connected causally in the past. And that saying about entanglement we have to specify the point of view. Yes, but in quantum erasure we usually have a coincidence counter. To make this 'coincidence measurement', we have to gather information from at least two sources - in presented experiment it takes at least half a year...

This experiment requires a 'perfectly separated box' - that there is no interactions with its outside. It can be realized by spatial separation - using that interactions can travel with at most speed of light. So imagine we've found a planet let say a light year from us - that we've analyzed it, can simulate it as perfectly as possible. We can send some 'reports' between us and this planet with a speed of light - they are received a year after sending. Let there be a cat which life depends on a spontaneous emission - we will get a report a year later, so before that we will be out of causal cone of this episode - from our perspective - this cat is in entangled state of life and death... More sophisticated example - imagine we've settled a colony there, such that some of their essential decisions are made using 'a real random generator', like measuring spin of photon 45deg from its polarization ('blind justice'). Each of such choices (like choosing the leader) starts a different timeline - before we get the report, from our perspective this colony is in entanglement of many timelines. So from our perspective - receiving the report is the measurement act - it 'chooses' one of timelines for the colony. If this report was 'good enough' - we could precisely track their real timeline - it's like getting corpuscular behavior in double slit experiment in which we determine the slit - we get classical behavior. But if there were some information which couldn't be determined - like in double slit experiment without determining the slit - shouldn't we observe kind of interference between timelines? Understanding this thought experiment should give good understanding of quantum entanglement. There is large discussion about QM interpretation: http://www.advancedphysics.org/forum/showthread.php?p=52905 What do You think about it?

I've just accidentally found out that very similar picture gives transactional interpretation of QM by John G. Cramer. You can find very good links here: http://en.wikipedia.org/wiki/Transactional_interpretation It's being taught on the University of Washington. If someone is interested in this discussion, here it has expanded: http://www.advancedphysics.org/forum/showthread.php?p=52593

You are overestimating our possibilities The universe just optimizes some fourdimensional action of some field and we are only a product of its internal reason-result relations ... somewhere in this spacetime ... it already knows all stupid things we will try to do and prepared for them such that we cannot do anything against its internal integrity ... So maybe we could use its protectiveness ... make it choose for us such that we will be satisfied with the results - because if not we will try to make a time paradox! This post is in fact about acceptance of CPT conservation ... against our 'evolving 3D' intuition... There is nice picture to upgrade this intuition here: http://arxiv.org/html/physics/9812021v2 The author suggests interesting view on the hypothetical picture that our universe will finally collapse and then there will be new big bang - evolution - collapse ... and so on ... Namely look at such collapse from the perspective of CPT conservation - from singularity of relatively small entropy it slowly evolves ... After this symmetry it's no longer collapse, but 'anti big bang' which creates universe built of antiparticles which evolves in opposite direction to our time arrow... which will finally (in maaany billions of years) crash with our reality...

Quantum electrodynamics (relativistic quantum field theory) is believed to be better approximation of physics than quantum mechanics - for example because it allows for extremely accurate predictions of Lamb shift. http://en.wikipedia.org/wiki/Quantum_electrodynamics One of formulations of field theories is due to Lagrangian density - physics finds the field which minimizes integral of this density over four dimensional space - so called action. Now using Euler-Lagrange equations, we can find necessity condition for such minimization, which is in form of time evolution - we get 'evolving 3D' picture. These equations are completely deterministic - we don't have a problem with for example probabilities, wavefunction collapses ... So why its predecessor which we commonly use - quantum mechanics is completely undeterministic - we can usually talk only about probabilities???? For me it clearly shows that QM is forgetting about something - kind of 'subquantum noise' ... which determines quantum choices. We cannot fully measure QM ... so it's probably even worse with this 'subquantum information'. We can say only about probabilities of quantum choices - but in fact they should be deterministic - they are stored somewhere there! What we can talk about is that events have been causally connected in the past - we call it entanglement: if two photons have been created together we cannot know what spin they have, but we know that it's the same one. So from our point of view - the future will decide which of entangled events will be chosen ... but in fact it's written in some subquantum information, but we cannot even think about measuring it. Bohm's interpretation uses 'pilot-wave' which goes into future to choose how to elongate trajectory. But maybe it would be better to use CPT conservation as in QED and try to interpret QM fully fourdimensionally. Now everything is clear: - probability is proportional to the square of amplitude, because it has to agree in both past and future halfplanes, - knowing only the past we can predict only probabilities, - entanglements means that events are causally connected in the past. One of them will be chosen in the future (as in Wheeler's experiment). Here is expanded this topic: http://www.advancedphysics.org/forum/showthread.php?t=11844 What do You think about it?

If we want to understand physics, first of all we have to understand, create a coherent picture of the nature of time. So in this thread I would like to discuss about consequences of hypothetical assumption that there is a possibility to influence the past - does it have to lead to contradictions (time paradoxes)? Observe that CPT conservation suggests that causality can go in both time directions... in general relativity we want more or less smooth 4D manifold - to minimize tensions from both time directions... http://www.advancedphysics.org/forum/showthread.php?p=52348 First of all, let's assume some interpretation of quantum mechanics ... physics. Some people believes that physics isn't deterministic (because of wavefunction collapses), some in many-worlds interpretations (that such collapses splits worlds - creates alternative realities). In many world interpretation - time travels doesn't seem to be forbidden - such travel would just create a new alternative timeline - there is no problem with time paradoxes. For now I'll assume the most probably (and physical) for me looking point of view: full determinism - there is some 'wavefunction of the universe' which evolves in an unique way. It's like in the general relativity theory: there is some already created fourdimensional spacetime and we 'travel' through its time direction - so the future is already there (eternalism). I'm afraid that full determinism/eternalism doesn't allow for perfect time traveling, like made because of that our spacetime would be bent so much that it would create stable 'wormholes' with endings in light cones of each other - such stable time-loop would allow for paradoxes physics couldn't handle with - like a machine which sends a beam to itself iff it doesn't do it. It's one of reasons I don't like Einstein's view: http://www.thescienceforum.com/viewtopic.php?t=15841 But let's think about a possibility of a channel in which we could send information back in time on which we cannot fully rely - sometimes it's easier for physics to make it lie. Observe that quantum mechanics gives physics large freedom in choosing effects below our recognition to stabilize such causal time-loops to avoid time paradoxes: like choosing what spin will be measured, or make some small influences on statistically looking behaviors ... or eventually make that this backtime channel would lie. Assume that there are allowed causal time-loops, but they have to be stable. So if there would be such 'device' to transmit information back in time and I for example would send to me a picture of the girl who will be my wife ... and just before sending it back in time I would change my mind - this timeline would never appeared - there would be completely different situation (without this knowledge). So if I'd really get this picture from myself - I will have no choice but to really send it. Like in (a good...) SF movie - they go back in time to repair something ... and finally it occurs that the situation is exactly as it was ... How to use such 'device'? Imagine we take some real random number generator - for example measuring spin of photon 45 degrees from its polarization. Now the procedure is: (1) make a choice according to this generator, (2) if from future there is a message that it was a wrong choice - take a different one (3) wait for results of this choice (4) finally if it was wrong choice - send this information back in time to (2) So if there was a satisfying choice - it has created stable time loop - so in fact the possibility of using this device made that our random number generator (quantum mechanics) has already chosen properly before the loop. It can be imagined that each of choice starts a different timeline - in our language they are all entangled and physics will choose one of them in what we call future, such that there for example are no time paradoxes (like that on a string is allowed some discrete set of wavelengths). What if there wasn't any satisfying choice? Would it create time paradox? Not necessary - most probably the physics would destroy the weakest link of such loop - make that this 'device' had lied. Observe that even without this real random number generator, such 'device' could work without actually being used: if for example there had to be a successful terrorist attack, there would be sent information to prevent it ... and finally in a stable timeline this attack would never happen (because of e.g. proper quantum choices below our recognition). It's great exercise to imagine the world with such devices. If it would allow to send message only some very small time back, we could use two such devices cyclically to increase this time as much as we want. It would have great use in science - choose a random amino acid sequence/reaction parameters/... crypotokey ... and use above procedure to ask if they are somehow optimal. One would say - we could get technologies from the future ... but I don't think it would be good choice for us in the future, because we are not prepared for most of them ... like artificial intelligence - for it 'we' in future would have to still want to send it, knowing the consequences. I don't like a picture that only government/rich would have access it ... but if it could be cheap, it would quickly spread: Imagine our world with commonly available such 'devices'... If someone would make a wrong choice - he could send this information back to prevent it - so finally we wouldn't consciously make choices which results wouldn't satisfy us! ... like choosing a school/job/... politicians... there would be no random accidents ... There would be also no hazard ... so what about economy? If some papers would have to drop ... they would drop to zero... So economy would have to completely change - that finally all is worth as much as it is really worth ... there would no risk management ... finally it should be extremely stable. The same with persons - no one would longer rely on illusionary values - we would have to work on our real values instead ... not depending on luck, frauds ... We could concentrate on studying/working/having fun/... building our world without any worry... Is such world without bad choices/risks perfect? Could we in this way really send some information back in time? That means it would make a stable timeline - that knowing this information, after a few years we would really like to send it ... reveling own future/destiny - taking away own free will ... But people are not perfect ... one would from the beginning been told that he has to do in life exactly this and this ... but I believe if it's properly used - such timelines shouldn't be stable - never happen ... for example even saying someone about his real future would be considered a crime against his free will... in stable timeline never happen (it would be prevented). ... I want to believe that in mature society it would be marginal cases ... and the main use would be just existing of such possibility - creating timeline without 'bad' choices ... ????? So in this picture we cannot send information back to change the past. Possibility of sending information is already enough to make that to prevent time paradoxes, physics would have to to prevent making 'bad' choices: if we would make such choice - we would try to send this information creating a paradox. So using a proper choice of what we call uncertainty, physics should make that we make a 'good' choice. What do You think about all of it?

In free electron laser(FEL) is used magnetic field to make electron move on sinus-like curve - thanks of synchrotron radiation and bosonic nature of photons is created coherent 'light' beam. http://en.wikipedia.org/wiki/Free_electron_laser All fundamental equations of physics we use, conserve CPT symmetry, so imagine above picture from the point of view of such symmetry - we get position moving on this sinus-like curve in reverse direction, producing light beam ... photons traveling back in time! To understand it, imagine 'standard' free positon laser - it produces photons which goes forward in time and space and finally hit into something, for example excitating its electrons. If we look at it after CPT transformation - excited (anti)something produces photon (returning to ground state) - this photon goes to free electron laser and is absorbed. To summarize - we see that usual free electron not only makes stimulated emission, but should also make stimulated absorption (be lasar). In FEL Such effect looks clear, but such effect should be observed also in cheaper lasers. Usual photons will be most probably absorbed in some point of time, but photons 'traveling back in time' cannot hit ground state atoms (conservation of energy) - they have to hit already excited matter - we would have to prepare to allow for such emission (energy drain from future). So to observe such effect, we would have to prepare the target by exciting it correspondingly (and continuously) - normally it should spontaneously emit photons in all directions, but if it would be hit by such photons 'traveling back in time', they should stimulate production of these photons in this direction. We could cover the whole spherical angle but this direction by detectors and if this laser/lasar will be turned on, these detectors should observe decreased emission ... earlier then turning the laser on (by the length of optical way). I've previously thought about using larger particles according to Feynman Stueckelberg, but using photons is much simpler and cheaper: http://groups.google.com/group/sci.physics.research/browse_thread/thread/9d10b4e5cbda1108 ------------------------ This picture is against our intuition - that we live in 'dynamic 3D' world - evolving 3D world with only pact->future causality relations... Imagine throwing a rock into water - it creates waves running outside the middle ... but in fact these waves are resultants of statistics of microscopic movements which goes in both directions! Big Bang can be imagined as such rock, which created 'waves of reality' which generally goes in one time arrow, what is observed in statistics as 2nd law of thermodynamics ... but it doesn't forbid future->past causality relations... 2nd law looks to contradict CPT conservation: assume we could proof that for assumed model of physics, there is mathematical property (entropy) which statistically always increase - assumed models of physics conserve CPT, so taking this transformation we get contradiction. So this law isn't fundamental equation of our physics, but is property of its solution we are living in - artifact of Bigbang which created spacetime with relatively small entropy. Our intuition is based on classical physics, but we know well that the world is more complicated. I'll show briefly some (in fact - well known) arguments that physics is four dimensional - there are causality relations in both time directions: - general relativity theory naturally operates on 4D spacetime and says that time flow is locally chosen by solutions. Einstein's point of view even allows to bend spacetime such that we create macroscopic time-loop - that means the future is already there... - all fundamental equations of physics conserves CPT - that means that past and future are somehow similar... - we are using equations of evolution of physics ... but didn't they come from Euler-Lagrange equations - optimizing some fourdimensional action ... ? - other approaches of introducing physics uses usually path integral formulations: assumes that particles 'see' all possible trajectories... - there is so called Wheeler's experiment: http://en.wikipedia.org/wiki/Wheeler%27s_delayed_choice_experiment In which after a photon passes two slits, we remove the barrier and place telescope far away to observe through which hole particle passed ... changing (later) its nature from wavelike into corpuscular... - Bell's inequalities says that we cannot construct complete past->future causality theory with some hidden variables. There is also new argument from statistical physics: Bolzman distribution is some fundamental mathematical law of statistics - QM should somehow correspond to it. If we would use it to choose statistics in 3D, we would get something like p(x)~e^-V(x). If we would use it to choose among paths going from past to this moment, we would get p(x)~psi(x), where psi is the ground state of hamiltonian. If we would take Bolzman statistics among full paths, we would get p(x)~psi^2(x) as expected. The square means that both past and future 'halfpaths' has to agree in this moment. http://www.thescienceforum.com/viewtopic.php?p=151272 For me these arguments clearly says that QM is just the result of fourdimensional nature of our world. So as in Wheeler's experiment - particles in fact travels through one trajectory - entanglement means that they haven't yet chosen - this choice will be specified in what we call future and if we know only past we can talk only about its probabilities. When we make a measurement - we ask about the current value - it has to agree with both past and future path, which are the same - it's where the square comes from. What do You think about it?

As I've argumented - if there is a possibility of quickly solving NP problems without restrictions - such cryptosystem could be still easily broken. If physics allow for that - the only safe will be based on one-time pad. If not - there will be for example restriction for time and number of entangled qbits - in that case classical ciphers (preinitialized) can be already made safe.

Quantum cryptography was believed to be ultimately safe, but in fact if someone for example could connect himself in the middle of such optic cable and can intercept the classical communication, he could say A that he is B and use their protocol to receive the massage. Then he can say B that he is A and using the same protocol to send some own message. So in fact safeness of quantum cryptography relies on safeness of classical cryptography - channels and authentication. I believe You mean something different - use for coding a hypothetical computer which can quickly solve some NP problems - which in each step chooses behavior analyzing some exponential number of possibilities. But if physics allows to quickly solve NP problems without restrictions, we could add to the set of these possibilities all possible keys (to find such that gives nontrivial correlations) - we still could break it. If there are restrictions for time and memory need of verification - we can easily exceed any limit using preinitialized cipher like based on ANS. About public key cryptography - You are right, but if there will be such computer (with restrictions), to make it safe we would have to ensure that every step of encoding requires large number of computations (much more than for RSA).

Ok - I was too pessimistic - we should be able to make protected and practical hybrid systems - public key cipher for very short message like a key for a secret-key cipher or a hash value for authorization. Most generally, public key is a parameter of some transformation which is extremely difficult to reverse. But there is the private key - some kind of 'clues' which make this reverse easy. So if someone could solve quickly NP problems: - he could try all possible 'clues' and for example check if for some block encrypting and then decrypting gives the same block. If yes - he could try a few more different blocks to be sure it's the correct private key, but there is also more dangerous attack: - searching not for these 'clues' but straightforward for the reverse function: having encrypted message in a form of independent blocks, for each block he could try to encode all possible input blocks with the public key to get the given block. So to protect it in analogy to secret-key ciphers, we rather have to make that encoding already require extremely large amount of calculations. The problem is that this time these huge calculations cannot be just made while initialization like before, but has to be made for each block - it could be practically used only for extremely short messages, like the key for a private key cipher or a hash value.

I see that You believe that the only real algorithmic advantage of QC is the Shor's algorithm. Probably You are right, but ... QC can theoretically make all calculations at once (is almost nondeterministic Turing machine) and the only problem is with the extraction. I'll show how to enhance it, but it would be strange if basic QC wouldn't already allow for more, maybe even solving NP in polynomial time. Observe that if someone would find a way, he could not necessary tell it loud, but for example try to became rich... In such case 1024 bit key would be a piece of cake - elongating key may be not enough. Especially that I believe that we could easily protect private key cryptosystems against such eventualities by using preinitialized ones. But designing public key cipher is much more difficult and I have no idea how to make a protected one??? The next argument that we should take such scenarios seriously is that maybe basic QC is not the only possibility for massive parallel computation physics gives us. First of all there is so called Feynman-Stueckelberg effect http://groups.google.com/group/sci.physics.research/browse_thread/thread/9d10b4e5cbda1108 which hasn't been taken seriously, but maybe it will change in a few months in LHC ... but such computer would require (huge?) accelerator. The other option can be (quantum) loop computers http://forums.devshed.com/security-and-cryptography-17/new-computation-method-which-could-endanger-used-cryptosystems-580926.html I'm strongly confused about this idea, especially for classical computers. But ... if used for quantum computation, such feedback should amplify the correct solution (wavefunction), making the rest of them vanish. It couldn't be standard approach to QC in which we use some sequence of for example external fields on some lattice of atoms. We would need a circuit which allows to sustain entanglement of many calculations. Observe that similarly to benzene, (-CH=CH-) sequence can be in quantum superposition with shifted one (=CH-CH=) - we could use such molecule as a wire for qbits. Unfortunately it has some resistance, but there are know such superconductors also. We know also transistors made of single molecule - they are irreversible so would destroy entanglement, but there should be possible also quantum gates made this way. The question is if such molecular quantum computers could sustain entanglement for practically long time ... There is also problem with auxiliary variables - we need a lot of them because in QC all calculations has to be reversible. They cannot be sent in the loop - they should be treated in some special way to not destroy the entanglement... ... but maybe ? Probably physics doesn't allow to solve NP in polynomial time, but I'm far from being sure of it. And I believe that preinitialized cryptosytems should be practically protected against all presented hypothetical possibilities. And this protection is achieved practically for free.

Physics most probably allows for non-classical computation methods, which makes parallel computation of all possible inputs - like becoming more and more realistic quantum computers. I think we should widely discuss if it's a real threat and if yes - how to design cryptosystems resistant to such eventualities. Especially that we wouldn't know if someone would use it... Here is one discussion of this type: http://www.ecrypt.eu.org/stream/phorum/read.php?1,1021,page=1 I wanted to start here one. Please share Your opinions and links to interesting discussions on this topic. There is know Shor's algorithm which breaks RSA. Generally cryptosystems have always a 'weakness' that make them prone to brute force attacks - that there is a key which properly decrypts the message. To make such attack we can use that there is practically only one key which makes that message decrypted with it has some significant correlation. So using a quantum computer we could use entangled all possible keys to decrypt the message, check for correlations and somehow extract some information about the correct key. This extraction is more difficult than it looks, but assuming that it's impossible could be dangerous. Quantum computers are extremely difficult to make and they most probably will be limited to relatively small number of qbits and time they can sustain entanglement - so I think that we can protect against imaginable QC by enforcing large amount of required computation. To make such cryptosystem practical, this computations should be done only once - specifically for given key. The next advantage of such preintialized cryptosystem, like based on asymmetric numeral systems, is that now the processing of the data can be extremely fast. Do You think it's a real threat? How strong it is - can be used only for some algebraic attacks or even for brute force? How to protect against such eventualities? What about public key cryptography???

I've just finished large paper about ANS. There is added some deeper analysis, gathered rethinked information I've placed on different forums... There is also shown that presented data correction approach can really allow to reach theoretical Shannon's limit and looks to have expected linear time, so should be much better than the only used such practical method - Low density Parity Check (LDPC) http://arxiv.org/abs/0902.0271

So please motivate, why You think it cannot be a realistic scenario? We have our subjective perception of direction of time, but it is only the result of the boundary conditions with relatively small entropy (big bang). What is less subjective is the causality: reason-result chains. And CPT conservation, which allows to switch past and future, suggests that such relations should be possible in both time directions.

In many worlds interpretation such causality loop doesn't have to bother to close. We have plenty of mystical QM interpretation and all of them wants to look at reality from the perspective of constant time plane - maybe here is the problem! GR, field theories strongly suggest that we have to think about physics four-dimensionally - we live in some already created spacetime and time arrow we percept is only some local solution (eternalism). As I've shown - choosing this view there shouldn't be a problem with QM interpretation. Even better - QM should naturally occur there! Making this assumption - eternalism - that 4D spacetime is already there and is stabilized, causality loops/question we will ask are already stabilized/answered. If physics couldn't do it, it can easily break the weakest link - prediction. For example by choosing statistics of the scattering so that we didn't spot these 'virtual' particles. ------------------------- If the idea could really work, transferring many bits would require much more resources than one. Especially that in practical problems the first algorithm would require hundreds/thousands of them. So I though about the second algorithm, let's remind it (we transfer one bit: B) - if B then 'input' -> next possible 'input' (cyclically) - if 'input' verify the problem -- then transfer back in time B=false -- else transfer back in time B=true. If it could it should stabilize on B=false and some solution. The problem is that 'input' have to be initialized somehow before. The solution could be using a really good random number generator, for example measuring photon 45deg from its polarization (before the loop). So the causality loop should cause that this generator has already chosen a good solution - cause something even in its own past. But because of it, this algorithm looks to be less stable - physics should make the prediction more difficult by for example making statistics of scattering worse.

I don't see a problem with some architecture? It would make calculations only once, but physics would ensure that it would be the correct one. But from practical reason (prediction) it should be extremely fast - so it even doesn't have to use clock. For many problems, like universal 3SAT, we could do this verifier using some number of layers (O(log N)) of basic logic gate -------------------------------------- Physicists usually believe in CPT conservation, that means that in small scale past and future should be symmetric. So if we make high energy scattering in some accelerator, there should be also created some amount of particles which travels into the past and hit e.g. some detector. From the perspective of out time perception, such particle was created in this detector and goes straight into the scattering point - we should be able to detect the scattering BEFORE it actually happen. So using accelerator as a 'part' of time-loop computer, we should be able to close the causality loop. http://www.scienceforums.net/forum/showthread.php?p=455246

I cannot agree - there is qualitative difference. Light 'thinks' that geometry changes/the world is curved when it goes through different materials. If it would be really geometry, X-rays would also 'think' so People going in parallel direction can be getting closer, because of positive curvature of the space OR because solutions of the field they are build of would have energetic tendency to bend so.