In a nutshell, the measurement problem (MP) is this: given an interaction among quantum systems (such as an unstable atom, atoms comprising a Geiger Counter, atoms comprising a vial of gas, a cat, a friend of Wigner, etc.), which of those interactions constitutes ‘measurement,’ and why? During the past several decades, worries about the MP largely abated due to a popular sense that environmental decoherence took care of defining measurement in a unitary-only picture (even though there were numerous criticisms of that approach—e.g., Dugić and Jeknić-Dugić, 2012; Fields, 2010; Kastner, 2014c). However, there remains a marked lack of consensus, and recently there has been a resurgence of concern around this issue. Griffiths goes so far as to remark that:
“…the failure of quantum physicists to solve the measurement problem(s) is not only an intellectual embarrassment…but also a serious impediment to ongoing research in areas such as quantum information, where understanding microscopic quantum properties and how they depend on time is central to the enterprise.” (Griffiths, 2017)
However, perhaps the situation is not so dire. The present author would like to issue a gentle reminder that in fact there is a strong contender for solving the measurement problem in the Relativistic Transactional Interpretation (e.g., Kastner, 2012); which must be carefully distinguished from the original TI of Cramer (1986). Making that distinction clear is a major objective of the present work. First, however, it is well known that about a decade after Cramer’s original proposal, Maudlin (1996; 2nd ed. 2002) raised what appeared at the time to be a fatal objection to TI, and at that point a consensus developed that TI was not viable. What went largely unnoticed after Maudlin’s apparent disposal of TI were several publications demonstrating that the Maudlin objection was not in fact fatal (e.g., Marchildon, 2006; Kastner, 2006; Kastner 2012, Chapter 5). More importantly, however, is that the Maudlin objection is itself completely nonviable once the relativistic level of the transactional picture (RTI) is taken into account (Kastner 2017a).In view of the ongoing concern about the MP, this more recent nullification of the Maudlin objection is briefly reviewed herein, as well as the RTI solution to the measurement problem, including quantitative criteria for the processes of emission and absorption (Kastner 2012, Section 6.3.4). The latter were taken as primitive in the original Cramer account, apparently leading many researchers to discount it. The RTI development, which remedies these lacunae in the original TI, does not seem to have penetrated the community, since a recent review by L. Marchildon of Cramer’s latest book (Cramer 2016) completely omits it. Based only on the older version of TI presented in Cramer’s book, Marchildon expresses his worry that
“In an important sense, TI is not better defined than the the Copenhagen interpretation…in Cramer’s view, transactions play the part of collapse. True, they are somewhat immune to questions like “When does the collapse occur?,” but they require emitters and absorbers. These should be macroscopic (classical) objects if transactions are truly irreversible. The classical-quantum distinction or apparatus definition therefore plagues Cramer’s view just as it does Bohr’s or von Neumann’s.” (Marchildon 2017
)In fact, however, this is no longer the case. Emission and absorption are now quantitatively defined at the microscopic level, and the microscopic/macroscopic transition is quantitatively defined (although fundamentally indeterministic).1 So the issue leading to Marchildon’s assessment that TI fares no better than the Copenhagen Interpretation is precisely what has been resolved in the relativistic extension of TI (RTI). Since this is a serious misunderstanding of the present status of the transactional interpretation, I shall deal with that first (following a brief review of basic principles of TI), and shall subsequently review the nullification of the Maudlin challenge.
[For the full paper, click the link below. Agree? Disagree? Comments? Post them here.]
On the Status of the Measurement Problem Arxiv3
Transactional Interpretation 
Dear Dr. Kastner,
I’m afraid this paper on measurement is more technical than I can follow. But I have a question about TI and the measurement problem. It arises from this 29-minute video Eugene Kutoryansky “Measurement Is Entanglement.” https://www.youtube.com/watch?v=sFRdVj4rrAE (Yes, the title is weird, but Kutoryansky is very knowledgeable and is making an interesting point.) To paraphrase Kutoryansky: Quantum interactions can occur while the particles remain in superposition without collapsing into a single real particle in spacetime. For example, two electrons can be brought close together (thus, are entangled) so that they take on opposite spins, but they still remain in a superposition. Isn’t this interaction–an exchange of information (if not an exchange of spacetime energy)?
Have the particles emitted offer and confirmation waves in order to entangle in this way? I don’t know—I would guess not from what you write. But what is occurring when this kind of entanglement occurs? In my mind, I’ve added to TI that collapse is created by the creation of information in spacetime. Are there 2 kinds of information–one type that is created in Quantumland and one type that is created in spacetime? If so, what determines in which land the info is created?
Also, I want to thank you for “Understanding Our Unseen Reality.” It’s very, very explanatory, not just of TI, but also of several QM ideas that I couldn’t previously understand. I can see that you have put huge effort into communicating with a lay audience.
I wish TI were more well-known. I just answered the single Quora.com question about it. But there are tons of Quora questions on quantum mechanics. If there were answers stated in TI terms, linking to your website or mentioning your book, it might be a way to generate more understanding and interest.
I hope that you have time to reply to my question.
Thank you again for the book,
Alexandra Hopkins
http://www.QuantumSkylight.com
Thanks very much, Alexandra. And yes indeed, there are two main types of interactions. The kinds of interactions that can create entanglement (or can take place while retaining entanglement) are those involving only force-based connections, i.e., which occur at the virtual particle level. In contrast, the kinds of interactions that break entanglement and lead to ‘measurement’ involve energy transfer, via real (as opposed to virtual) particles. This is discussed in Chapters 4 and 5 of “Understanding Our Unseen Reality,” but I’m working on a follow-up book which hopefully will make this issue more clear.
Thanks for answering the Quora question. Do you have a link that would get me to the right place? I’ll see if I can provide some info there.
Thanks for your fast and clear reply, Dr. Kastner. This issue has been puzzling me for weeks. I read your early chapters several times, but have given Chapters 4 & 5 only a once over lightly. Hopefully, when I re-read them, I’ll get more out of them. But, I definitely look forward to reading your new book.
I answered a TI question on http://www.Quora.com. Here’s a link to my answer to the Quora question on TI: https://www.quora.com/I-have-lik-the-transaction-interpretation-and-the-new-variant-PTI-could-this-be-responsible-for-quantum-collapse/answer/Alexandra-Hopkins. (The question is probably written by a second language person.)
When I searched on Quora today, I saw that there were a few other questions on TI that I hadn’t noticed before. But putting “quantum mechanics” or “quantum physics” in the search bar yields tons of questions–a very popular topic. I’ve put in my Quora profile that I’m interested in such questions. So, they’re sent to me by e-mail.
I’ve gotten many views of my answers. One of my most viewed answers was “What is the difference between a photon and an electron?” It got 36,100 views! (I wrote it early-on in my journey to learn QM, and probably should see if it garbles things a bit!)
Thanks again,
Alexandra Hopkins
Thanks! I’ll take a look. Congrats on your work learning QM and on the website you’re building!
Ruth, I was intrigued by a recent column at Peter Woit’s blog, in which he raised the same question you raise here — the status of the measurement problem — and more specifically how “probability” gets introduced into quantum mechanics:
http://www.math.columbia.edu/~woit/wordpress/?p=10533&cpage=1#comments
Naively perhaps, I thought I might contribute something by posting the link to the arxiv paper associated with your notes here, while just briefly summarizing that your Relativistic Transactional Interpretation did provide a precise definition of measurement in terms of the interaction between emitters and absorbers, and that you quantitatively linked the probabilities to the coupling amplitude and fine structure constant.
I was disappointed to see that not only was my comment not posted, and thus the subject of RTI never broached, but Woit even took the opportunity to take a dig at another of your papers in the comments (search for “Kastner”).
This made me think about the cold shoulder TI and RTI have received over the years, and wondered if you might have something to say about the professional reaction to your work. Do you think your ideas have gotten a “fair shake”? If not, what do you attribute that to?
Thanks for your attempt to include RTI in the conversation. Yes there indeed has been a lot of resistance to TI and RTI, and I’m not surprised at the pre-emptively negative reaction you describe. I discuss the issue of ‘mainstream’ neglect of TI/RTI in the Epilog of my forthcoming book with WSP. RTI challenges the usual metaphysical assumptions about how fields work, and ‘mainstream’ researchers are loathe to seriously consider it, often based on worries about Wheeler and Feynman having earlier abandoned their absorber theory upon which TI is based. They probably don’t know that Wheeler was re-advocating his absorber theory in 2003, and that the Maudlin objection has been completely nullified at the relativistic level (the latter is often wrongly taken as a definitive refutation of TI). I’m not particularly worried about this situation, however; a truly good idea will not go away, and the best and most open-minded thinkers will figure that out. John Gribbin just said of my forthcoming book: ” …a good, readable exposition of a model which not only removes the mystery from quantum mechanics but offers an explanation of the underlying way quantum effects produce the structure of spacetime and give us the impression of an arrow of time, while allowing us genuine free will.”
Thank you for the response. Here’s to good ideas!