Questions re Cramer's Transactional Interpretation?

rkastner

Hello Forum Users,

I am currently writing up a general discussion of John Cramer's Transactional
Interpretation (TI) , intended to address questions, confusions, or concerns about TI.

I have noticed occasional comments here and there regarding TI on this forum,
both pro and con. I would be interested in hearing from you (in this thread) if you have
such questions, which will be helpful to me in addressing them and in clearing up any
misconceptions about TI. I can't promise to answer all (or even most) posts but will still
appreciate the input.

Many thanks,
Ruth E. Kastner
UMCP Foundations of Physics Group

Demystifier

Is the wave function everything that exists?
Does wave function allways obey the Schrodinger equation?
If not, what causes the breakdown of the Schrodinger equation? How this breakdown is described mathematically?
If yes, then why there is an appearance of the wave function collapse?

Dmitry67

While TI makes understanding of some things (like Bell) easier, TI is one of the collapse interpretations. And I guess collapse is like Luminiferous Ether now.

cesiumfrog

I never fully understood TI: it seemed to be fixing realism at the beginning and end of the universe/experiment, but still leaving everything between in a giant wave-superposition (as to still want MWI there anyway). Is this mistaken?

Has TI proven a fruitful way of looking at the world, in terms of new research? (The pop' science journalism hasn't picked up on much other than that attempt at superluminal communication..)

rkastner

My reply:
(1) Not exactly. In TI, what exists are (i) offer waves (OW) and (ii) confirmation waves (CW). OW are represented mathematically by state vectors in Hilbert Space ('kets'). CW are represented by dual vectors ('bras') in a dual H. S. Under Possibilist TI ('PTI'), these are viewed as physically real possibilities. When a transaction occurs between a particular OW and CW, the result is an actualized outcome. So there are two levels of reality: possible and actual.

(2) Not in the sense that there is nothing going on but unitary evolution. An OW is described by the Sch Eqn until it is absorbed. The absorber(s) emit CW which obey the complex conjugate Sch Eqn. A transaction may then form between the emitter and one of the absorbers. The formation of the transaction is an a-spatiotemporal 'collapse' and is not desribed by the Sch Eqn: it is discontinuous and stochastic. There is no 'breakdown' of the Sch Eqn, it's just that it applies to the propagation of the OW and CW (the latter in c.c. form) and not to the formation of the transaction which, by its nature, is discontinuous. The probabilistic weight of the transaction is described by the Born Rule which describes the amplitude of the CW at the locus of the emitter. I sense in your question a desire to have a complete mathematical description, so if you think this is not complete enough even given the discontinuous nature of the transaction, I'd be interested in knowing what else you're looking for in a mathematical description.

(3) there is a collapse but it does not occur within spacetime. In PTI (my possibilist variant of TI), spacetime is viewed as emergent and not as a substantive 'container' for events. It is an epiphenomenon of dynamical process (OW, CW, and transactions).

rkastner

Not at all. The problem with collapse has been that is has traditionally been seen as observer-dependent, which brings in "consciousness" and ill-defined use of the term 'measurement'; at what point is the 'measurement' really finished? etc.etc. TI avoids all of this because the collapses are not observer-dependent, they are only absorber-dependent. You don't need a 'conscious observer' or a 'rational observer' or any of those things. All you need is an emitter and one or more absorbers and stuff can happen.

There is currently an interpretation getting serious consideration in the literature which employs an ad hoc modification of qm, called the 'GRW theory,' which puts in collapse 'by hand' by adding a nonlinear term to the Sch Eq. So collapse is very much with us, and it's important to pick an interpretation that describes collapse in a natural way, not in terms of an ad hoc modification to a perfectly good theory (qm).

rkastner

See my other answers; this may address your first question. In particular, it is thoroughly realist but it's realism about possibilities. Also, it greatly differs from MWI in that, as noted above, there are transactions, and therefore determinate states of affairs in a single universe, any time an emitter has one or more absorbers available.

An emphatic YES to your second question, although TI is an interpretation of QM, not a new theory, and as such should not be required to generate novel predictions. However, it can shed light on conceptually confusing experiments. See, e.g. my discussion of the bizarre "quantum liar experiment": http://arxiv.org/abs/0906.1626

Also, to all of you with questions, I have posted some recent papers comparing TI with Many-Worlds (Everettian) interps. and with the GRW ad hoc 'collapse' theory, at
arxiv.org (they appear as items 2 and 3 in this list):
http://arxiv.org/find/quant-ph/1/au:.../0/1/0/all/0/1

rkastner

Thanks, I appreciate your questions! Feel free to follow up if you're still skeptical or confused.

Demystifier

So basically, TI replaces observers by emitters and absorbers. However, the physical nature of emitters and absorbers remains obscure. Is that correct? Or is there a mathematical theory/model/description of emitters and absorbers?

Frame Dragger

I think Dirac was right, and QM is a provisional theory (as Roger Penrose also believes), useful for (possibly) a very long time, but by no means a correct description of nature. I really believe that trying to find an Interpretation is pointless. We have two partial descriptions of nature (GR/QM), and both are useful. I think that a lot of what is happening now, ESPECIALLY with Renormalization, is just mathematical tinkering with no hope of experimental evidence. That isn't physics, which is an exploration of the physical world. Instrumentalism is the only sane response (as we can hardly discount many of the predictions of QM or GR), and having been on these forums has only hardened my stance.

I think the TI is fascinating, but in the end I think it's an attempt to solve a problem that isn't there. I don't know what the solution to apparant wavefunction collapse will be, but I'd bet it will be with a new theory that is novel in the manner of SR/GR. I don't claim that this is anything but my opinion, for the record.

Dmitry67

'emitters' and 'absorbers' in TI are equivalent to measurement devices. For example, metal which reflects light (mirror) is NOT an absorber while the same metal in photoelectric effect IS an absorber :) The same scent of 'pure magic' as in CI :)

Fredrik

Unfortunately different people seem to mean different things when they say "interpretation of QM". I started out assuming that it always refers to an attempt to turn the theory defined by the standard axioms of QM into a "description of what actually happens" to the physical system (instead of just a set of rules that tells us how to calculate probabilities of possible results of experiments). But when I started reading articles about the MWI, I found that those guys have something else in mind. They are actually trying to change the axioms (by removing the Born rule), so they're not trying to interpret QM (as defined by those axioms) at all. They're trying to reinterpret the underlying mathematical structure. (I consider a theory to be defined by its axioms, so I would say that they were trying to find a different but equivalent theory. I would also say that they did it really badly and failed miserably. I believe there is an MWI that makes sense, but it's an interpretation of the first kind I mentioned, an interpretation of the theory rather than a reinterpretation of the mathematics).

I have also found that people who talk about interpretations of QM very often don't bother to make their "interpretations" well-defined. They don't write down a set of axioms that are meant to turn the "set of rules that tells us how to calculate probabilities of possibilities" into a description of the real world. Instead they just make a bunch of loosely stated remarks about what sort of things they think are actually happening.

And occasionally you run into someone who wants to tell you their "interpretation of QM" and then proceeds to tell you their ideas about what sort of theory might describe the reality underlying QM.

Of these four different things that people can mean by "interpretation of QM", I think only the first two deserve to be taken seriously. Of course, an interpretation of the third kind ("a bunch of loosely stated remarks") can sometimes be an intermediate step towards an actual interpretation.

My concern is that the "transactional interpretation" is still just an interpretation in that third sense, i.e. that it's not really an interpretation at all, but just a collection of ideas about how to make an interpretation. Now, I'll be the first to admit that this could be due to my ignorance about the TI. I haven't even read Cramer's original paper. But a quick glance at it reveals that it's really short and contains very little mathematics. So I find it hard to believe that it successfully defines something that I would consider an interpretation.

I'd be interested in hearing your thoughts on this. If I choose to read articles on the TI, will I find a bunch of comments about how there might exist some sort of waves going this way and that way, or will I find precise statements that identify the specific pieces of mathematics in standard QM that represent these waves? Does it include an axiom that tells us how to make that identification in all experiments, or is the identification done for one experiment at a time? Does the TI change any of the standard axioms of QM?

Demystifier

I agree with that.

Yes, that's my impression too.

Frame Dragger

Yes.

No.


And that's that!

Dmitry67

One interesting issue with TI is that you can point light beam into black depth of cosmos, and as our Universe is expanding and is less and less dense, that light would never find an absorber!

rkastner

An emitter is just another dynamical object, and as such is also described by quantum theory. E.g., you can have an offer wave as an emitter: an OW for an electron can emit an OW for a photon, and another electron OW can act as an absorber for that photon. So it would be incorrect to think that emitters and absorbers are undefined or primitive, as is the case with an "observer".

rkastner

What would be the basis for such a belief?

Might this be because the current mainstream interpretations are inadequate?

One can agree that there is a lot of suspect ad hoc tinkering without resorting
to instrumentalism, which is an abdication of the motivating spirit of scientific inquiry. One can just as easily try to find a deeper, more valid interpretation that vitiates the need for any ad hoc tinkering.

You have already alluded to a problem in your concern about the tinkering, and there is also the demonstrably unsatisfactory nature of most prevailing interpretations. Why reject
a possible solution?

TI is a good solution for wf collapse, since it describes all physical processes by qm and provides for collapse without reference to an observer. Until TI has been given fair consideration, there is no reason for us to throw up our hands and give up on understanding what qm might be telling us about reality.