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John Cramer’s transactional interpretation of quantum mechanics (TIQM) is billed as resolving the fuzzy agnosticism of the Copenhagen interpretation while avoiding the alleged ontological excesses of the Many Worlds Interpretation. Yet it has a low profile.

Is this because no-one care anymore about ontology in physics, or is there something about TIQM which undermines belief in it?

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in the middle ages, the idea of a whole universe almost entirely devoid of matter, where the earth was just one speckle orbiting another, would probably have sounded a bit of a ontological excess too. in any case, good question – lurscher Apr 18 '12 at 14:58

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Nobody has explained to me how Shor's quantum factorization algorithm works under the transactional interpretation, and I expect this is because the transactional interpretation cannot actually explain this algorithm. If it can't, then chances are the transactional interpretation doesn't actually work. (I have looked at some of the papers that purport to explain the transactional interpretation, and have found them exceedingly vague about the details of this interpretation, but assuming this interpretation is actually valid, maybe somebody else with more determination could figure these details out.)

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In mathematical logic, an interpretation would be a particular model of the equations with certain structure and features: not all models of a theory need to be isomorphic. I guess the trouble is people don't formalise what they mean by their interpretations! – Nigel Seel Jan 25 '11 at 12:53
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But if from the description of the transactional interpretation you can't actually predict what will happen given a set of initial conditions (which I think is the case -- I believe the descriptions given are not precise enough), then I don't think even in mathematics it would be called an interpretation. – Peter Shor Jan 25 '11 at 14:11
Surely not - even especially in mathematics (or logic). – Nigel Seel Jan 25 '11 at 17:30
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The Afshar experiment only disproves Copenhagen for certain values of "Copenhagen." What it shows is that a naive interpretation of the complementarity principle (a photon either behaves like a wave or a particle, but not both) is incorrect. The complementarity principle was certainly one of Bohr's main tenets of quantum mechanics, so in this sense the Copenhagen interpretation is disproved. However, nobody nowadays argues that quantum mechanics predicts anything but the actual outcome of the Afshar experiment, so in that sense these are all interpretations of the same theory. – Peter Shor Jan 27 '11 at 3:01
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@Roy: Thanks for the link. My guess is that this may be the only well-formulated description of the transactional interpretation. I wonder if Cramer approves of it. – Peter Shor Feb 2 '11 at 18:50
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It's a combination of all these things and more. Most importantly, the TIQM interpretation is nonsense and all the positive words you hear about it are just unjustifiable hype promoted purely by John Cramer himself.

Ontology - or "realism", as it is technically called in quantum mechanics - has been falsified in physics in the mid 1920s and it can never be "unfalsified" again. It's established that objects don't possess well-defined properties before they're measured. This insight continues to be hard to swallow for many people - however it doesn't mean that there is anything questionable about it.

The TIQM merges all the usual misconceptions about the "real wave function" with some very special inconsistencies such as retrocausality - the influence of the future on the past - that is obtained by a bizarre interpretation of the Feynman-Wheeler theory, a theory that turned out to be incorrect by itself (although it helped to stimulate Feynman and others to find the right rules of quantum field theory). Concerning the Feynman-Wheeler theory, its historical role, and some of its problems, see

Wheeler-Feynman theory, QED without fields, vacuum polarization

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So a couple of points. (1) Why is retrocausality a problem? Isn't it just an interpretation of the equations, like positrons being interpreted as "electrons travelling backwards in time"? (2) If TIQM is just an interpretation of the equations of QM that everyone's signed up to in any case, isn't the criterion simply whether it's heuristically useful as a tool to think with? I'm not sure whether interpretations can be right or wrong rather than just useful or misleading? – Nigel Seel Jan 25 '11 at 12:48
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Dear Nigel, if something were "just" an innocent interpretation, it couldn't be (even) wrong. But the point is that TIQM is making many specific statements that simply are incorrect. So maybe one should say that it shouldn't be called an interpretation of quantum mechanics - it's a different (wrong) theory. Retrocausality is deadly for the logical consistency of any world because it introduces closed time-like curves - influences in both directions of time. Because $A_i$ implies $B_j$ and $B_j$ implies $A_k$ where $i\neq k$, we get a contradiction about what happened at $A$ - or anywhere. – Luboš Motl Jan 25 '11 at 17:51
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I have come across a very simple question or critique of the Transactional Interpretation in an article "Nine formulations of Quantum Mechanics". The multiple authors of this 2002 AMJ paper are QM physics teachers.

They ask how "two particle" transactional handshakes work: are there "two handshakes across spacetime" or "one handshake across Configuration Space"? Without an answer to this question they are unable to "report on how the TI differentiates between bosons and fermions".

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It has to be one handshake across configuration space, obviously, or else nothing works at all. This is not handled by Cramer, but it was probably understood implicitly by other proponents. The emphasis on relativistic wave equations is also kind of silly, he should use past-propagators for Schrodinger equation. I am not sure TI makes sense, but if you use past-propagators on configuration space, then it might be possible to turn it into a consistent-histories. – Ron Maimon Nov 20 '11 at 9:09
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I have a new approach to TI which is discussed in my forthcoming book for CUP. It will be available in fall 2012. I don't think the basic transactional picture has any problem accounting for quantum phenomena including multiparticle states, but the ontology of my version is different from Cramer's. I will look into Shor's algorithm, but I don't see why this would constitute a challenge for TI because TI has no problem with quantum computation that I know of. Any quantum system can be modeled in the transactional picture. The only situations regarded as challenges are contingent absorber experiments such as Maudlins; this issue is addressed and resolved in the book and a preview is here: http://philsci-archive.pitt.edu/8963/

Some recent papers on TI are on my website: rekastner.wordpress.com; some of this material is in the book.

Thanks for your interest.

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Hi, welcome to Physics: just a heads up on culture, in physics, people generally use free software, and so prefer pdf to word. – Ron Maimon Apr 18 '12 at 13:13
I would upvote, but the argument in the linked paper is no good. THe reason people object to TI with delayed choice is fundamentally different than the two-telescope double slit. In the two-telescope double slit nobody is confused regarding ordinary QM. In the two absorber idea, the reflected wave from the non absorption of the wave by the first barrier must reflect back to kill the half of the wavefunction, before the second barrier moves into place. The reflection must be from the first barrier, not the second, and this is not included in TI (but must be in order to make it consistent). – Ron Maimon Apr 18 '12 at 13:22
Ok, upvoted anyway, because it contributes to the discussion, but the paper is wrong, and should be rethought. – Ron Maimon Apr 18 '12 at 13:23
I think you misunderstand the argument in my referenced paper. It compares the delayed choice experiment as analyzed under 'standard QM' with the contingent absorber experiments raised as challenges for TI. People certainly are confused wrt the delayed choice if they think that this is compatible with the kind of 'block world' view they wish to impose on TI in order to argue that it can't handle contingent absorber experiments. The point is that the block world picture is problematic for both cases: delayed choice experiment as in Wheeler's presentation, and TI in contingent absorber experimen – Ruth Kastner Apr 26 '12 at 5:28
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@RuthKastner Please [register your account](physics.stackexchange.com/users/login) and don't use answers for comments. For larger discussions, you can use chat. – mbq♦ Apr 26 '12 at 10:27
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I suspect a problem people have with TIQM is that Wheeler-Feynman absorber is woven into this. The advanced potentials and the rest are a bit much for many to swallow.

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The real problem is that quantum mechanics is only mysterious when there is entanglement, and that requires many particles, and TIQM is formulated for single-particle qm only, which is a ludicrously uncontroversial and unproblematic sector. When I heard about TI, I automatically thought it was something else, where future measurements back-propagate information into the past to cull the wavefunction, and you can do something like this, and this is why TI got so much traction. But Cramers didn't do this in his paper, nor in followups. – Ron Maimon Nov 22 '11 at 5:54

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