Forum Replies Created
August 10, 2019 at 12:00 pm #5619
Dear Ruth, thanks a lot . Still I don’t think that you are completely right : your view is like the one of Heisenberg for the passage from the potential to the actual and when you wrote ” “Collapse” in RTI is actualization of a single invariant spacetime interval from a set of non-spatiotemporal set of possibilities, represented by weighted projection operators (the weights being Born Rule probabilities).” that is also the way of thinking of Costa de Beauregard. All the machinery you use is only a way do deal with quantum causality even if following Bohr this is a more illusion.
For me your sub empirical level is like a hidden variable in disguise. In your neo-copenhagen ontology only detections events have a physical objective meaning so why do you want to introduce a mathematical formalism with offer and confirmation waves if you dont believe in some way in the possibility to describe this sub-empirical level?
A last point : a offer wave require a boundary into the past and a confirmation a boundary into the future or at least in the present. Every thing in between is hidden but you need any way two boundaries.August 8, 2019 at 7:28 am #5596
Dear Ruth, thank you for answering. I agree that an approach like the one of Sutherland requires two boundaries (one in the past and the other in the future) and that the foliation needed is arbitrary. However, the choice could have a cosmological meaning breaking the symmetry. Anyway, I think that this is the same for all retrocausal intepretations including TI. In the TI or in Costa de Beauregard interpretation wave functions also require foliations so that the problem is here aswell. This very clear in the interpretation of Aharonov and Gruss which is a form of TI where two states and thus two boundaries are needed. By the way you say that you dont want hidden variables but you include a sub-empirical level whihc is hidden so what is the difference?August 7, 2019 at 4:21 pm #5591August 7, 2019 at 4:19 pm #5589
Dear Ruth, I would like to add a comment concerning the role of preferred frames in a quantum ontology and in connection with retrocausality. It is indeed possible to develope Bohmian like model without preferred frames. There is one recent approach by Sutherland but I also recently proposed a different one [https://arxiv.org/abs/1904.08134] where backward in time motion plays a role and where preferred frames are not anymore problematic. May be a topic on that will be interesting.August 2, 2019 at 8:49 am #5552
I forgot the link to the paper: https://www.ijqf.org/archives/5401
I use this occasion to add a link to a related paper I recently submitted
(arxiv : https://arxiv.org/abs/1904.08134) with the title
‘Lorentz-invariant, retrocausal, and deterministic hidden variables’.
This work focuses on Gisin’s and Hardys’ claim purporting the impossibility of Lorentz-invariant deterministic hidden-variable model for explaining quantum nonlocality. Those theorems claim that the only known solution to escape the conclusions is either to accept a preferred reference frame or to abandon the hidden-variable program altogether. Here we present a different alternative based on a foliation dependent framework adapted to deterministic hidden variables. We analyse the impact of such an approach on Bohmian mechanics and show that retrocausation (that is future influencing the past) necessarily comes out without time-loop paradox.October 25, 2018 at 3:57 pm #5210
Dear Friends, I discovered the work by Frauchiger and Renner yesterday.
I didn’t read the literature on the topics but I wrote a kind of comment which I put on arxiv one hour ago. Being a Bohmian my view is that actually this is just a rephrasing of Hardy’s paradox involving nonlocality between agents. The problem is not more paradoxical that it was before when Hardy’s wrote his fantastic article. For me the nicest part of Hardy’s work is about Lorentz invariance and it pushed people like Durr and coworkers to develop a hypersurface Bohm dirac model which I simply love.
Now, involving massive thing is not a problem for a Bohmian you will have a lot of empty waves and nonlocal interactions but no paradox.
The preprint will be available on arxiv tomorrowJanuary 7, 2016 at 7:26 am #3267
Dear Shan dear all, I gave a kind of reply to Shan’s work concerning psycho physical parallelism in the topic ‘The End of the Many-Worlds? (or Could we save Everett’s interpretation)’
with best regards AurélienJanuary 7, 2016 at 7:23 am #3266
Dear Lev, don’t worry: my aim was only to recognize the influence that your view on MWI has on mine (your work on bohm with the ‘surrealistic’ bullet and the cat was very great as well). I will add a sentence that you dont share my view.
with best regards AurélienJuly 23, 2015 at 9:33 am #2958
Dear Nathan, I think that the model of Rod is very similar to the one proposed by Miller with negative probabilities in his Phys lett A 222 31-36 (1996) [butwith the real part in addition to get only real probability and not complex like in Miller’s work]. Of course Miller didn’t use hidden variables but the theory of Rod is really the cournterpart of Miller’s work in the same way as Bohm is the couterpart of usual QM. The main ingredient in the paper of Miller (like in a different paperof 2005 by Aharonov and Gruss) is to define a sum rule for going back to the usual QM probabilities. this is done by introducing a requirement acting like the postulate used by Bohm for justifying born rule. I think that the theory is self consisten if we accept this rule. However this also means that we have a statistical ensemble of systems with different final states.
with best regards
AurélienJuly 23, 2015 at 8:53 am #2957
Dear Bob, I Think we should may be continue this discussion by email since the page of the forum is sometimes blocked for unknown reasons.
First, concerning the ontology: BM uses also the Hilbert space for defining the guiding waves otherwise the theory will not work. The ontology is double : particles and waves. Both are necessary and are on an equal footing.
Now, in your CH interpretation the key ingredient is the counterfactual interpretation of the many-time correlation formula also known as ‘Wigner’ formula (for example trace [P2P1rhoP1P2]] where P2 and P1 are projectors associated with measurements at different times ). You want to use this formula to speak about things which were not observed but could have happened. Since you didn’t observe them I am pretty sure that a Bohrian like Zeilinger would class your theory in the ‘hidden variable’ drawer together with BM. They are some differences since the ontology are different but I am affraid that they are both metaphysical in the sense of Heisenberg. I am convinced that, in the present state of our knowledge, in order to solve the problem of which ontology is ‘better’ one would need ‘god eyes’. In other words, if we dont find new data in the future the game is over for making any progress. I can not believe in that of course but this can not be excluded. May be we have already reached the end of ontological science. If quantum mechanics is rigorously true for ever there is no hope to solve the dilemma.
Second, I have your fantastic book in front of my eyes and I went through the chapters you mentioned specially the chapter 14 and 20. What I do not understand is that if you think about an history you can update for sure your information and select was is necessary like for an historian analyzing the Bastille attack. However, the historian can not change the past : facts are facts and he can only put filters to select the good information from the full mess (which was already written in a block world picture to which I subscribe). Now, the choice of the projector families used to define your ontology is forced by your consistency condition, which is by the way not exactly the same as the one used by Gellmann and Hartle, and not all the sets of correlators are consistent if you want to find the usual classical sum rules for probabilities. However, by doing that you introduce some very strong constraints which can lead to contradiction like in the Wheeler delayed-choice experiment involving retrocausality (a point which is absent in BM at least in the preferred frame: each theory has is own surrealism). Your stochastic theory analyzes the interferometer with the two BSs using consistent histories which are not the same as the ones used without the second BS. This is a contradiction for me if you don’t involve retrocausality. Did I missed something? In the same vain I have also the feeling that even worst contradictions could come from Hardy’s experiment watched in different Lorentz frames since what is allowed for an observer in a reference frame is not necessarily possible for an observer in a different frame when the causal order of events is changed.
with best regards Aurélien Drezet
PS: I like very much this discussion and may be we will find a way to disagree or agree in a short text for this forum or the IJQF journal at the end.July 22, 2015 at 7:48 am #2945
Thank you for the different answers. My point concerning the precision of the path concerns the ontology used in the CH model. If your aim is to define counterfactuals associated with paths not observed but supposed to exist then it seems that your CH model is less precise than BM since you don’t have determinism, i.e no precise position x(t) and no trajectory. If you accept that of course this is OK otherwise the theory is incomplete.
Any way, by reading some of your previous papers and re-reading the book by Roland Omnes and one by Bernard Despagnat I realized that the usual Mach Zehnder interferometer experiment is analyzed in a very ‘orthodox’ way in your CH model. This leads to curious properties no so intuitive for a realist.
Indeed, consider a state ‘psi_0’ sent on a beam splitter 1 (BS1) and splits into two paths ‘left’ and ‘right’. These path are subsequently sent on a second beam splitter BS2 and all the state finishes its journey in the exit door 3 with probability P(3)=1; the exit door 4 being always empty. I convinced my self this morning in front of a good coffee (before the heat of the day start :)) that the set of histories ‘Psi0–>Left–> 3’, ‘Psi0–>Left–> 4’, ‘ Psi0–>right–> 3’, ‘Psi0–>right –> 4’, is not a consistent set in your interpretation (as you indeed wrote many times) because the different wave functions involved are not orthogonal (sorry if I repeat some obvious things for you but this theory is quite new for me). Now, the probability to observe 3, i.e. P(3)=1, is the sum of
P(3,Left)+P(3,right)+2Real[ <Psi0–>Left–> 3|Psi0–>right–> 3>]=1/4+1/4+2*1/4=1 and the probability to find ,the particle in the gate 4 P(4)=0 is similarly P(4,Left)+P(4,right)+2Real[ <Psi0–>Left–> 4|Psi0–>right–> 4>]=1/4+1/4-2*1/4=0. The non always positive diagonal terms induce interference and allow to obtain the good probability P(3)=1 and P(4)=0 from histories which have all a weight 1/4>0. I understand that you consistency condition prohibits such an history set by definition and that the question ‘which path was followed by a particle in an interferometer’ is a wrong question in the CH interpretation. But then what is good history set and what does it implies concerning realism?. A good set of histories in the CH interpretation is obviously associated with ‘Psi0–>sigma+–> 3′,’Psi0–>sigma+–> 4’, ‘Psi0–>sigma- –> 3′ ,’Psi0–>sigma- –> 4’ where the state sigma+= (right +i*Left)/sqrt2 and sigma-=(right -i*Left)/sqrt2 which are associated with cat states have been introduced.
Now, we have P(3)=P(3,sigma+)=1 and P(4)=P(4,sigma+)=0 since all other terms, including the non diagonal elements, vanish. What I find most striking here is that by allowing a second beam splitter BS2 you find, in full agreement with orthodox ‘copenhagenists’, that between the preparation and the final actual measurement at gate 3 or 4 the good histories can not include paths ‘left’ and ‘right’ while it can do it of course if only the first BS1 is present. How can that be in an ontological theory? Your reality is thus changing as a function of the context? This is what we indeed learn in text books but in the context of Bohr approach which refuse to speak about the nature of the system between measurements (counterfactuals have no meaning for Bohr). I was convinced that you tried to save realism while your theory seems better to destroy it unless you admit some non contextual features that your theory can not explain. This implies the paradoxical fact that the presence of the second BS2 which can be introduced after the wave packet crossed BS1 can change the nature of the object even retrodictively by acting into the past (see the Wheeler delayed choice experiment). So your theory is not so intuitive as I expected. BM do not need backward causation and the nonlocality is induced thanks to a preferred frame. I dont say that this is better but both approach are ‘surrealistic’ contrarily to your claim. Clearly it means for me that CH is not better than BM for explaining QM in an ‘intuitive’ way (this is not so surprising finally).
I hope that my explanation was not too detailed with best regards Aurelien.
PS: I agree with you that the disagreement with a ‘plausible answer’ in BM is in cm not in nanometer. I could even add in meters or kilometers since beam splitters and optical fibers are used on large scale nowadays. Bujt the same is now also true for the CH interpretation is wou can act retrodictively over kilometers it seems that the notion of past becomes anbiguous in your approach.July 21, 2015 at 9:10 am #2931
Dear Bob, I found your paper ‘Bohmiann mechanics and consistent histories’ fascinating. Before reading this paper I always missed the point(s) concerning your interpretation (even though I apparently downloaded your preprint 10 years ago…). Now it seems that things a getting more clear for me.
If I follow the reasoning of your paper CH (consistent history) paths have only an approximate meaning since projectors should not be too precise if we dont want to disturb the wave packets too much. For Bohmian mechanics of course the precision is instead infinite since the theory defines a dynamical trajectory for a point like object. In your CH model you can not preserve interference and path(s) at the same time since your precision is limited by the law of measurements (for example in the region J of the figure 1 we have fringes). Bohmian mechanics can explain both path and interference in one model and in the region J the particle must bounce in order to explain fringes and nodes. There is no contradiction between both models if we compare them ONLY with observables i.e. detectors outputs limited by the Heisenberg principle and the law of entanglement. Now, for me the question is the following : Are you ready to accept an approximate precision in the definition of an ontology? This is clearly not the case for Bohmian but it seems to be clearly the case in your CH interpretation.
Of course, I agree with you that Bohmian mechanics will contain a metaphysical part leading to surrealistic behavior but this will never contradict facts and even explain them (this is the aim of an ontological model after all). This is the price you have to pay in the Bohmian universe and I agree with you: it is fundamental to accept that limitation. Your model instead, is the best that we can keep by sticking to the strict laws of observables (with the Wigner correlation formula P(1–>2)=tr(P2P1rhoP1P2) etc..). The price you have to pay is that you definition is coarse grained and that the more precise you could define a path the more invasive you will be. Still my question ‘are you ready to accept such an imprecise ontology?’ remains. For me there is no contradictions as far as you accept to consider these paths as detector paths like in a bubble chamber or in the experiments used by Serge Haroche with Rydberg atoms. If instead you want to define path with an infinite precision and go back to an ontology without approximation a la Newton or Einstein I am afraid that you should use instead something like the de Broglie Bohm dynamics. Of course, I want to point out, that I have a very deep respect for both ways of looking at the problem and since I am mainly an experimentalist working with photon detectors and correlators I am very sensitive to your approach which is pragmatic and still keep a track of ontology (unlike the too positivist Bohrian do). Still I am convinced that the aim of physics is to describe the universe as it is not only as we see it (with imprecise eyes). By doing that I am necessarily weakening the correspondance between facts and theory which was admitted to be one to one in the Newton time (at least in theory of course: Newton or Laplace were not idiots and understood very well the diffference between dreams and real experiments with unperfect data and apparata). This is a philosophical aspect I found fascinating something like the end of Popper.
I would be very interested to know your point of view on my short analysis.
I will go through the phys Today debate and to the comments by Basil during this day if I can find the time.
With best regards
PS: I added a paper I wrote where my view was balanced between Bohr and Bohm (actually I prefer de Broglie 🙂 )July 20, 2015 at 3:05 pm #2915
Dear Robert Griffiths,
I am sorry I could not find the time to answer to your late remarks during the forum ( the temperature in Grenoble France was then reaching 40 ° Celsius and it was difficult for me to focus on science).
I would be very pleased however to try something in order ‘ to take up the cudgels” as you propose.
For me the main issue regarding your consistent history interpretation concerns the meaning of the paths defined using the ‘Wigner formula’ for correlations when there is no observation made (i.e. when discussing about the meaning of a path in an interferometer without path detector). For example, some authors like Y. Aharonov or D. Miller are not afraid to use retrodictively QM between measurements but what about the consistent history interpretation? Are you only considering actual experiments like in Bohr’s approach? If yes what is the difference with copenhagen? Also the consistency condition on the sum on histories (sum on non diagonal term=0) plays a fundamental role in your approach. Why is that since only one history will be actualized any way? If you give some value to all other histories which didn’t happen (but could have) using a consistency condition are you somehow making a new ontological axiom concerning what is allowed to be or not to be?
Any way I will read again your comments and papers in order to clarify those points.
On the same footing Basil Hiley in his book with Bohm discussed already the consistency interpretation and Sheldon Goldstein also commented it in his phys today paper. Do you think they missed the points concerning your theory?
It would be very interesting to have a discussion with you on these topics in order to compare those two interpretations.
PS: concerning surrealistic paths I wrote few papers on this subject which clarify (at least for my Bohmian eyes) some aspects of the problem
With best wishesJuly 19, 2015 at 4:16 pm #2901
Dear Jean, I like what you wrote. For me BM is indeed a rational theory. But we can reverse the problem. Are we sure that the so called orthodox interpretation is a rational theory? I have strong reasons to doubt about that since for Bohr everything should be necessarily described in term of macroscopic languages while at the same time detectors are also made of atoms which should be describd by the theory. It looks like the problem of the chicken of the egg but here touching the ontology of the things themselves. It is only with a clear ontology that problems vanish and this is the main advantage of Bohm over Bohr.July 18, 2015 at 8:49 am #2853
Dear Daniel, Thank you for your answer. The reason why I Invoke ontology is because as you just replied now ”I don’t prove that quantum mechanics is intrinsically retrocausal”. But if we dont prove and if you only suggest then you are going beyond facts? or not? Actually, my problem is the following David Hume already showed that causality can not be proven experimentally : you will never be sure that the succesion you observed once will come back again if you reproduce the experiment. You need a law and a postulate to fix that. Classical physics with Newton called that determinism and it relied on the study on an ontology describing the dynamics of the real world existing independently of the observer. If now you say that it is ‘natural’ to use retrocausality you somehow enter in the metaphysical universe of ontology like Bohmian do. This is beyond Bohr of course and I say that only because in my view you are hidding some (may be unconscious ) ontological preconceptions under your interpretation.July 18, 2015 at 8:46 am #2851
Thank you very much rod, I have still problems with the two-time boundary condition but I have to solve that myself
AurélienJuly 17, 2015 at 4:26 pm #2841
It is a bit late sorry, the server is making strange things like posts disscuned in mail but not appearing on this web page. I wrote you this answer a couple of days ago but it got lost :
Dear Miroljub Dugic, I have presently no idea how to give a clear answer to that question (Bohmian Mechanics is a program of research in this domain). What I can tell you for now is that the non relativistic version of BM exists with external potentials. This works very well. Then you have the model proposed by Bohm for quantum EM fields where bosons are described in a different ways as electrons. In principle so your answer is in that work (even if I dont like it too much because it is strange for me that photon are not playing the same role as electrons as particles). However, I dont think that anyone has developped a good model of QED in the context of bohmian mechanics so that your question has no good answer even if the maths will tell that indeed the electron do not always radiate in agreemeent with QM . In my view BM is sill waiting for something deeper in the relativistic domain and the answer to your question will come in this context. Probably BM is not the only guiltyguy and relativistic BM shares the complete responsability as well. This is one of my topic work but I have unfortunatelly not so much time devoted to Bohmian mechanics currently (students are demanding).
with best regards
AurélienJuly 17, 2015 at 11:55 am #2827
As a kind of comment to that I wrote here a quote By Bell I found in the paper by Daniel Rohrlich:
‘ For me, it is so reasonable to assume that the photons in those experiments carry with them programs, which have been correlated in advance, telling them how to behave. This is so rational thatI think that when Einstein saw that, and the others refused to see it, he was the rational man. The other people, although history has justified them, were burying their heads in the sand. I feel that Einstein’s intellectual superiority over Bohr, in this instance, was enormous; a vast gulf between the man who saw clearly what was needed, and the obscurantist. So for me, it is a pity that Einstein’s idea doesn’t work. The reasonable thing just doesn’t work.’
Well I hope that the destiny of Bohmian mechanics will be better but we are in good company any way.
best AurélienJuly 17, 2015 at 11:32 am #2826
Dear Dustin, The DGZ paper is not the only way to see Born’s rule (Chaos, H theorem etc…) and I think that it is fair to say that the derivation didnt convince every one including myself. It will be too long to discuss that today but the problem is still waiting for a good answer.
PS: the foundation of classical statistical mechanics is much better 🙂July 17, 2015 at 9:45 am #2823
Dear Rod, I realized That I didnt make justice to all you comments concerning my questions. Here a small list of points:
–You wrote :’If you’re puzzled about something with the final psi, ask yourself if you would still be puzzled if you were just talking about the initial psi instead.’
This is an important point it reminds me the double-standard objection of Huw Price. But what is puzzling for me is not that you are using the final psi or the initial psi but that you are using both. I am not convinced that this will not introduce some contradictions if you want to prove the equivalence with the usual Bohm formalism: that is with QM it self. Actually, I have the same problem with the two-time formalism of Aharonov et al..
— You wrote: ‘So the final boundary condition is fixed in an analogous way to the state preparation of an initial psi. ‘
This is also mysterious. The final state should equivalent to what is given by orthodox QM and represents therefore the evoving state of Psi in. How can we be sure that we dont introduce too many degrees of freedom in the model? Feynman used a trick like yours in QED but we can show that this is formally equivalent to usual Quantul Field with only one initial boundary condition. The equivalence is not obvious in your model.
— You wrote: ‘Unlike classical physics, there is no doubt that two boundary conditions are needed in this picture, since otherwise Bell’s nonlocality could not be explained in a Lorentz invariant fashion via a spacetime zigzag. ‘
That’s not obvious. Even if a time symmetric theory is better for explaining Bell results and nonlocality (on this I agree completely) this is not a proof that more usual way will not work. Again, this is the story of Feynman theory. May be the usual wave function contains already retrocausality like in some model by Nikolic (which I dont want to comment here since I do not agree with the details).
Any way your approach is one of most interesting presented to solve the difficulties of covariance. This is much better that the usual preferred foliation used by Durr et al.July 16, 2015 at 1:09 pm #2795
Dear Daniel, Your example remind me aslo the old paper by elitzur [http://arxiv.org/abs/quant-ph/0205182] where retrocausality is also involved. This is of course connected to the work by Pres. My question concerning your claim that QM is retrocausal goes like that: 1) what do you mean by QM 2) how can you prove that this version of QM is retrocausal.
To 1) I think you mean QM from the macroscopic observer point of view (i.e., Bohr?). such an observer dont care about the ontology of the real staff (like Bohmian could do instead). So, in this orthodox version of QM you will only collect clicks on your detectors and makes statistics. But then there is no proof of retrocausality since you can only observe the presence or absence of entanglement between Alice and Bob by making correlations with Jim. If Jim is missing data you will just observe a lot of uncorrelated points at A and B. We could thus claim that it is only your post analysis which showed that A and B were correlated even if it looks magical. Clearly, you cant use this scheme to transmit faster than light signal which could then be equivalent to backward causation and convince me. This is my answer to 2) It seems that you are thinking in a ontological way like Bohmian or Rod Sutherland do. Then it seems that you are going beyond the ‘macroscopic observer view’ of QM. Now if you take an ontological view like the one of Rod Sutherland you have a retrocausal theory from the ground (I still dont konw if the theory is correct or not but let admit it here). Still this retrocausality will stay hidden like in your example and I dont think that there is any example demonstrating retrocausality from the view of macroscopi observer otherwise this could look like a time machine?
I precise also that I am not convinced by the point of Travis Norsen. Bohmian mechanics neeed a special foliation to explain the delayed entanglement. This is not very elegant to go back to the old aether concept after 1 century of relativity isn’ it ? I would like to knwo the point of view of Travis on that point as well.July 15, 2015 at 4:00 pm #2774
Dear Reinhard, I was not apparently allowed by my computer to write you a comment directly (probably the bohmian demon in my Windowsystem). I Copy and paste a reply in the Max section on Bohmian concerning the value of Bohmian mechanics for physics: I changed a bit the structure.
First of all, BM offers a much better perspective than Copenhagen concerning describing what is reality. More precisely, if you focus your attention on the empirical contents of Bohm’s theory and say that it looks like the view of a god (: ‘only god knows where the particle is ‘ ) then you could conclude that BM is indeed metaphysical. However, I ask now what do you exactly mean by empirical contents? I think that when you try to answer a question like this one you should be quite prudent and modest. Like Heisenberg discussing with Einstein we should never forget: Theories always come before the experiments (I am both a theoretician and an experimentalist and I accept that very well). Now, BM is a theory it has a clear dynamical framework and it reproduces all data (at least in the non relativistic domain). What you want more?
Like you we could say that the paths predicted by BM are surrealistic (actually it was Scully after Heisenberg who used this language but it fits here as well) and not observable. Well, that’s a bit provocative but this is not true. Trajectories given by BM agree with facts and can be tested in that reduced sense (Weak measurements or Protective measurements even allow more see below). Indeed, BM will predicts the good probability which are associated with the trajectories and there is nowhere contradiction between BM and QM (by the way what is QM without a clear ontonlogy a set of rules for technicians ?). Of course, you can not measure a path like you could do it in classical mechanics because Heisenberg principle prohibits that but this is the price to pay here: If you want to reproduce QM predictions you must accept this limitation. You must abandon some aspects of Classical mechanics. If you reject that: no chance for you to explain QM. Heisenberg refused even to listen Bohm for that reason : BM for him was classical physics so it should be wrong. But Heisenberg was mistaken : We dont try to save the classical realm but its goal which is to give an interpretation of the world independently of the observer.
I want to say a bit more about that: If you anyway reject Bohm or Stochastic QM à la Nelson what should you propose instead? If you go back to Copenhagen then you are only hiding yourself under the quantum carpet since you dont have a definition of what is the reality anymore: you need an observer but you cant define it precisely. Do you need a PhD, like Bell suggested provocatively, an environment, an infinite number of Wigner’s friends (with cats)? This is wavy and the choice of Bohm is not. If you want to observe a path anyway I suggest to use protective measurements [Aharonov Vaidman, Phys Lett. A 178, 38 (1993).]. Indeed, the protective measurement protocol can be used to ‘detect’ the particle at points that the Bohmian particle never comes near. This is because the wave function is an active element in BM. This allows to record a velocity without disturbing the position. There is no paradox because you didn’t use a ‘destructive’ von Neumann protocol for velocity. You are still free to define the position of the particle after that so you will get both the velocity and the position. This is not yet a path but you are getting closer from it. Clearly, Of course the Bohmian program has some limitations. If you remember Popper and his falsificationism you realize that BM is not completely testable (This is necessary, I repeat it, for reproducing QM). This is a problem I agree but nowhere It has been written that Popper was right for ever. Additionally, the sciences do it in the same way like BM and nobody seems to be offended by that. Consider cosmology and Black holes ? Would you say that these objects are not for science? what about quarks if we can not separate them ? These theoretical objects have some consequences which can be tested but we can not observe all and some part will stay for ever may be outside the experimental deomain. Anyway, I agree with you on a another point BM is not unfortunately unique. I don’t speak here about Nelson theory which is no yet in the maturity of BM (despite years of efforts) but more about the problem of relativistic BM. Since for BM we need a privileged observable (like position in the non relativistic theory) we should define the same for quantum field and an univocal answer is not yet existing. Personally, I believe that BM is only a temporary expedient. One day we will get a much better theory which will explain why particles have such and such properties like mass and charge. BM is the best candidate for helping us if we can give a better foundation to the theory. Still, I think that Bohr view on reality is a dead end since it will only offer you the sleeping property of opium and will not allow you to, may be, discover something new (or not).July 15, 2015 at 10:25 am #2760
We agree, may be someone will try to develop a modified version of your protocol. This could be a nice exercice.July 15, 2015 at 6:44 am #2757
Thanks a lot Rod,
You understood my question quite well . Any way on such a subject anbiguities can come easily and this is certainly my fault not yours.
I will go back to your work soon I will recontact you with pleasure since I am currently writing a paper on the same subject with a different strategy.July 15, 2015 at 6:33 am #2756
Sorry Lev, My answer got lost in a parallel Universe (probably the bohmian demon).
I wanted only to say that entanglement will change all. If you use an additional BS then you are indeed in the regime that I am mentioning. But the main problem is now that Bohmian mechanics is strongly sensitive to contexts so that the time ordering of the operations and the way you are doing entanglement will make your dynamic even more counter-classical. In some case Alice will receive the particle coming from her mirror (like in your example) but in other the particle could come from Bob mirror. You mentioned already these problems in your cat paper years ago and Hardy did a lot in that direction as well. Bell was probably the first to see that of course.July 14, 2015 at 6:44 pm #2736
Dear All, I think that the problem here is not if Bohmian mechanics is the good theory but to know what will happen if you abandon the old Credo of physics concerning realism à la Einstein. For me and many the most interesting aspect of Bohmian theory is that it provides a framework for solving the problems accumulated by Copenhagen followers (Schrodinger cat’s, Wigner friends etc..). These are serious problems which can not be put under the carpet like that. Bohmian theory is a first step not the last one so it is not a waist of time to consider it as important. It is the opposite which could be risky for me since I am afraid that such an attitude could scleroses the future of microphysics.July 14, 2015 at 4:17 pm #2730
Dear Rod, Like Nathan Argaman I was impressed by your first manuscript. It is indeed a bohmian version of the two-time formalism of Aharonov et al. I have however several problems with this idea. I will only discuss briefly one here. My question is what is fixing your final boundary condition? In classical physics we have some freedom for choosing a Green function retarded advanced symmetric etc.. all of them are strictly equivalent if we use the good boundary conditions. For example you can write a field E like Ein +Eret= Eout + Eadv. Now in your theory you need two wave functions and this seems for me representing a breaking of this equivalence. What does it mean really to use such a future amplitude going backward in time? I have the feeling that your theory still needs a deeper justification.July 14, 2015 at 3:59 pm #2729
Dear Lev, Thank you once again for presenting such an interesting example. To answer to your question: ‘is this example counterfactual according to Bohmian mechanics ?’ I can say that factual means here classical. Your example is indeed counter intuitive on this you agree I think but by intuitive you only mean classical. And indeed due to the quantum potential or the empty wave Bohmian mechanics is not strictly equivalent to clasical physics. This is the same problem you mentionned in you previous examples discuss in ‘Can You Kill with an Empty Wave Bullet?’.
One point to add: your example is not fully quantum since Bob can select the output but can not superpose them in a coherent quantum way. To do that you will need entanglement with degrees of freedom of Bob. This is of course not very difficult to do but then you will certainly realize, like Hardy did in the 90’s with your famous interaction free protocol, that non locality and the time ordering of the differen processes can modify in even more counter classical ways the dynamics of the particles.July 13, 2015 at 7:24 am #2676
Dear Max, Here is my reply to your question and to the provocative comments by Reinhard Werner .
First of all, BM offers a much better perspective than Copenhagen concerning describing what is reality. Reinhard Werner gives strong criticisms about the Bohm program but I think that his view is somehow misleading because he is unable to clearly understand that point. More precisely, if you focus your attention on the empirical contents of Bohm’s theory and say that it looks like the view of a god (: ‘only god knows where the particle is ‘ ) then you you could conclude with him that it is metaphysical. However, I ask him now what does he means by empirical contents? I think that when you try to answer a question like this one you should be quite prudent and modest. Like Heisenberg discussing with Einstein we should never forger: Theories always come before the experiments (I am both a theoretician and an experimentalizing and I accept that very well). Now, BM is a theory it has a clear dynamical framework and it reproduces all data (at least in the non relativistic domain). What you want more?
Like Reinhard Werner we could say that the paths predicted by BM are surrealistic (actually it was Scully which used this language but it fits here as well) and not observable. Well, that’s a bit provocative but this is not true. Trajectories given by BM agree with facts and can be tested in that reduced sense (Weak measurements or Protective measurements even allow more see below). Of course, you can not measure a path like you could do it in classical mechanics because Heisenberg principle prohibits that but this is the price to pay here: If you want to reproduce QM predictions you must accept this limitation. You must abandon some aspects of Classical mechanics. If you reject that : no chance for you to explain QM. I want to say a bit more about that: If you anyway reject Bohm or Stochastic QM à la Nelson what should you propose instead? If you go back to Copenhagen then you are only hiding yourself under the quantum carpet since you dont have a definition of what is the reality anymore: you need an observer but you cant define it precisely. do you need a PhD an environment an infinite number of Wigner’s friends? This is wavy and the choice of Bohm is not. If you want to observe a path anyway I suggest to use protective measurements [Aharonov Vaidman, Phys Lett. A 178, 38 (1993).]. Indeed, the protective measurement protocol can be used to ‘detect’ the particle at points that the Bohmian particle never comes near. This is because the wave function is an active element in BM. This allows to record a velocity without disturbing the position. There is no paradox because you didn’t use a ‘destructive’ von Neumann protocol for velocity. You are still free to define the position of the particle after that so you will get both the velocity and the position. This is not yet a path but you are getting closer from it. Clearly, Of course the Bohmian program has some limitations. If you remember Popper and his falsificationism you realize that BM is not completely testable (This is necessary, I repeat it, for reproducing QM). This is a problem I agree but nowhere It has been written that Popper was right for ever. Additionally, the sciences do it in the same way like BM and nobody seems to be offended by that. Consider cosmology and Black holes ? Would you say that these objects are not for science? what about quarks if we can not separate them ? Anyway, I agree with Reinhard Werner on a another point BM is not unfortunately unique. I don’t speak here about Nelson theory which is no yet in the maturity of BM (despite years of efforts) but more about the problem of relativistic BM. Since for BM we need a privileged observable like position in the non relativistic theory we should define the same for quantum field and an univocal answer is not yet existing. Personally, I believe that BM is only a temporary expedient. One day we will get a much better theory which will explain why particles have such and such properties like masse and charge. BM is the best candidate for helping us if we can give a better foundation to the theory. Still, I think that Bohr view on reality is a dead end since it will only offer you the sleeping property of opium.July 12, 2015 at 10:30 am #2658
Dear Richard, I would like to comment briefly on your suggestion that the so called orthodox interpretation of QM (OIQM) is better than the de Broglie Bohm approach.
I think that the crucial element which is missing in the OIQM is a clear statement of what is ontology in QM. For Bohr the world must be described by something classical but he is not really able to say what does it really mean. The apparatus is at the end a quantum object and the point of Heisenberg and Bohr is that we must introduce a kind of cut or shifty split. Where does the split occurs this is the big issue in the OIQM and Heisenberg only pointed out that the position of the split is irrelevant. Of course, he was right in the sense that is now given by decoherence theory à la Zurek but still the ontology is not better that it was at the time of Bohr since the environment should be included as an observer in the interpretation. This is even more mysterious since the exact nature of the environment is illusive and also shifty. Do we need to say that the” Universe “ is an observer? For me the OIQM is paradoxical with or without decoherence. Consider the Schrodinger cat or the Wigner friend paradox ; these are examples showing that the border between what is described by the wave function and by the observer has an impact on the ontological nature of the object. The quantum system can’ t at the same time be and not here.! But remember: Following the OIQM the exact nature of an electron before a measurement by an observer is undefined. This is necessary for Bohr otherwise the electron could follow two paths at the same time (I.e. wave particle duality). So if this true for an electron is must also be true for an observer watching a second observer. I can not see how you can exit this circle in the context of OIQM (this was the point of Everett in his thesis). For me the role of the observer is too fragile its definition too mysterious to give a good basis for an interpretation of QM. We need something better something of clear and I think we should accept a realistic view to solve the measurement problem. Bohmian mechanics is the first step. It shows that this is possible. I do not think that the theory will survive in its current form but I believe that OIQM is a dead end for making progress in physics.July 8, 2015 at 1:32 pm #2526
Dear Ken Wharton, In my view there is no contradiction between your first impression concerning the single particle theory and the many-particles approach acting in the configuration space. Indeed, actually you are confusing the measurement problem with the ontological problem. The most basic part of Bohmian theory is of course the ontological problem and following this view the single particle indeed follow a simple path in the 3D space. As you mentionned this is the most intuitive part of the theory. However, when you need to consider interactions and measurements in most cases you get something non local due to entanglement. This makes sense in the configuration space since you need then a larger system. It is only with some very simple measurements that your intuition can be recovered and in most cases you have to be prudent, has it was pointed out many times by Bohm, Bell and others.