Thank you very much for your comments!

Yes indeed, I've thought a lot about retrocausality. Basically, I think there are two different `types' of retrocausality - either you can have both a forward and backward arrow of time, or you can just have global laws (e.g. optimizing some constraint over the whole of history) which implies that what happens at one moment affects what happens at all other moments, past and future. I agree that the first type leads to contradictions of the type you mention, but I think the second type avoids that problem because the global equations will necessarily be solved in a consistent way.

Yes, I enjoyed Dr Hossenfelder's essay very much. It's a very interesting question to determine exactly which problems of pure maths translate over into physical reality - I'm sure there are many applications which we are yet to discover!

Hmmm, interesting - my intuition is that undecidable problems would become decidable if you allow yourself infinite time to solve them, but I'm not aware of any results one way or another on this question!

Thank you, this sounds very interesting? I presume it's discussed in your essay? I will take a look!

7 days later

Emily I loved your essay. To me it is "on stream". Probably because it describes what I did in developing the Successful Self Creation theory described in my essay. I got rid of all of the laws, rules, impossibles, etc. of current physics, started from a very different beginning and developed a theory with causes, retro causes and global causes in every SSC progression. In doing so I was able to derive a complete, mathematically consistent, explanation of the creation process that provides measurements that match the "generally accepted as true" measurements of the visible universe as well as the Planck measurements and the measurements of the major components solar systems, galaxies, etc.. In my derivation, I did not assume the laws and constants of physics were wrong - they just were not in existence before the beginning and are not in effect everywhere, all of the time and never change. The are created by the process as a component of the observed results. If you want to see your ideas put into actions and results read my paper. "Clarification of Physics: A Derivation of a Complete, Computable, Predictive Model of "Our" Multiverse". While the paper may seem simplistic, the differences from current theories are profound. For example the process makes its own mathematics as a component of the processing, gets rid of infinities and zero which allows for a complete mathematical explanation. I would appreciate your comments. John

    9 days later

    Emily I have "revised" my essay. In that revision I develop in more detail how the SSC processing produces its own mathematics and algorithms. The system in its encapsulation step also solves its own "halting problem". It instills it's own limits and boundaries. It solves the problems we are trying to impose on it. It creates its own temporal progression. In the revision I also extend the SSC fundamentals and their role in the other disciplines of science, philosophy, mathematics and religion. If you have already read the first edition of my essay, I suggest you read the revision and base your comments on it. John

    8 days later

    Dear Emily Adlam,

    You might be surprised: My essay tries putting the pre into mandatory physics. That's why I carefully read your essay. If I understand you correctly, you are hoping that accepting the weird retrocausal theory may yield better predictions by removing a constraint. Is there any evidence?

    I suggest calculating for convenience as usual, as if Einstein was correct and the now that does distinct between pre and after, past and future was an illusion. While I like backpropagation NNs, my point is that the map, the theory including the laws of nature is not the territory, not the reality. My stance has unwelcome consequences. Do you feel in position to defend?

    Regards,

    Eckard Blumschein

      Dear Emily,

      I think your essay digs around some questions that have become crucial in physics this century, and can be seen as part of a self-searching process that science is now engaged in. To some, it's an identity crisis, but however one takes it, it's very helpful to shine a searchlight on these questions, as you do.

      I also tend to take an overview kind of position, standing further back, as you do - to me two of the strands you mention need to go hand in hand: conceptual understanding, and the empirical side, mathematical theory/experiment. I agree that there's more to conceptual understanding that just a reverse form of prediction, though it's a good point nonetheless (you mention in a footnote that identifying emergence is another way we sometimes reach understanding).

      To me, with a jigsaw for a loose analogy, theory and experiment tells us how the pieces fit together, but conceptual understanding - seeing the picture - is also essential. They need to work together, each informing the other.

      I think the reason we're having to question our aims now is that one of these strands got left behind during the 20th century - theory/experiment got ahead. So the conceptual side needs to catch up, and we need to interpret the mathematical theory that we have.

      The main point in my essay is that to interpret QM, missing concepts must be found. Several things suggest missing concepts, including the lack of consensus on how to interpret the theory. But also the physics itself - apparently fundamental unpredictability, non-locality, superposition. If there are missing concepts that encompass these, we'll need to find them.

      Anyway, congratulations on another good essay - I hope you'll read mine, which is here. As well as looking at the overview and the different strands, it summarises an entirely new interpretation for QM, which anyone interested in quantum foundations might find interesting. A documentary was made partly about the interpretation last year, with some well-known physicists discussing it. I'd very much like to hear any thoughts you might have on my essay.

      Thank you, best regards,

      Jonathan

        Emily

        Great Job. It's almost as if you're giving a conceptual analysis of my essay! I can't then help but to agree just about all. And so well analysed and expressed. But I would like you to examine my concrete proposals for what you suggest.

        But let deal with the one discordant thing; your apparent agreement that we now "definitely can't have locality" AT ALL!

        Could you answer these questions about a row of 20 spinning balls with random orientations of axis. Wear a blindfold and try an 'exchange of momentum' with your finger tip, (representing polariser electron 'absorption'), then answering each time;

        1. Is the surface moving UP? or DOWN?

        2. Is it rotating clockwise? (PLUS) or anti clockwise? (MINUS).

        Fundamentally easy Yes? ..Or is it!?

        You encounter one with perfectly vertical motion, easy. BUT how certain is you +/- answer?

        You then touch one on a pole. +/- is easy Yes. So how certain are you then about UP/DOWN?!

        That is the natural physical PROPENSITY, and the more you test the more divergence from certainty.

        I gave the full Stern-Gerlach A/B 'measurement' sequence for that last year, giving Cos2Theta & Diracs QM equation, verified by Trails essays computer plot, but few even understand the problem, & most that do run a mile in the blindfold! (as Jochen Sz). I suspect you may see be less scared of it's consequences Emma?

        I touch on it again this year, but mostly on the consequences of updating our most fundamental ageing 'Laws'. I look forward to discussing.

        Very well done for yours. I have to down for a well earned top score.

        Very best

        Peter

          9 days later

          Dear Emily Christine Adlam,

          A very interesting essay. John Schultz's essay suggests that the limitations on knowability posed by algorithmic patterns are not applicable to non-algorithmic patterns. As a consequence, this escape from algorithmic limitations on knowability would have the side effect of limiting predictions, seemingly in agreement with your essay. A further consequence is that knowledge of ontology might become feasible, as opposed to the position of one commenter who claimed it is hubris to claim to know ontology. Feynman: "more can be known than can be proven."

          My updated essay proposes a means of understanding that is not based on predictions and I think you might find it interesting and worth thinking about.

          Deciding on the nature of time and space

          Best regards,

          Edwin Eugene Klingman

            6 days later

            Hi Emily,

            Thanks again for another really well written essay! You make an excellent point when you write that if we take the 'pre' literally in prediction, then we appear to be bound to two perspectives; either a fundamentally probabilistic theory, or hidden variables. Given we know local hidden variables seem to be bogus from Bell's theorem and further results experiments, it's pretty clear we need a global theory as you beautifully articulated.

            You mentioned several proposed undecidable physical systems i.e gapped vs ungapped energy bands. It would seem to me that these experiments would necessarily need to be built as local subsystems of the universe in a lab. By construction, they would be immersed in a thermal environment. Unless your experiment is perfectly decoupled from the world, it would necessarily be subject to the laws of thermodynamics which may induce a symmetry breaking in the system and force a solution and making it unpredictable simply because you're ignorant of the state of environment.

            Thermodynamics and statistical mechanics are certainly emergent theories, but do you think this is only the case because if how we constructed modern science? I guess my question would then be, do you think thermodynamics in itself is a consequence of 'pre' way of thinking about nature?

            Again, thanks for the great essay! If you get chance, I'd love to get your take on mine if you get a chance :)

            Cheers,

            Michael

              12 days later

              Dear Emily,

              I enjoyed reading your essay! I find it well-argued and well-written, and you point to several important insights and questions. For example, I really liked your point about "objective chance": it's much more than just having "arbitrary" outcomes, and one may wonder about the structural peculiarities of such a claim. In particular, how come we can navigate the world successfully by betting on those probabilities *in the long run* if the events are assumed to somehow happen *locally*? Given this, then global constraints of the kind you imagine seem like a plausible idea to pursue.

              However, let me ask you about a question that you formulate: "How are the laws of nature enforced?" Are you really sure that in a "universe [...] made up of tiny objects undergoing various sorts of local interactions, it's natural not to worry too much about how the laws of nature get enforced"?

              Even in this case, one could imagine asking: what is *actually, in all detail* happening when two particles (or billiard balls) collide? Why don't they just move across each other? Or, perhaps with a more religious ancient mindset: what kind of god is actually enforcing the collision laws, by making the balls turn when they come close?

              It seems to me that the "how-enforced"-question can indifferently be asked in all settings whatsoever. And that many physicists refrain from asking it not because it would be a hard question, but because they see it as an all-too-human non-question (like: which god enforces the collision laws?) which we have learned to give up.

              How would you argue against such a view? I'd be curious about your opinion.

              Best,

              Markus

                Thank you very much for reading the essay, and for your comments!

                I actually agree entirely with what you say - I think the 'how-enforced' questions is just as difficult for the billiard ball case as for the global constraint case. However, the point I was trying to make is that for many people, the 'how-enforced' problem looks worse in the global case, because we can't resort to the intuitive picture of small particles colliding that we have all internalised. Thus global theories are often viewed as less plausible than local theories, even though the problems of principle are really the same in each case.

                I also have a lot of sympathy with the idea that the 'how-enforced' question isn't actually well-formed and that physicists should learn not to ask it. My concern, however, is that even if physicists don't raise such questions explicitly, nonetheless we may still have ideas of this type in the back of our minds when we evaluate the relative plausibility of various theories, and this can potentially lead to unjustified prejudice against certain types of theories (such as global ones).

                Thank you for reading, and for your comments!

                I'm glad you raise the subject of thermodynamics, because I think it's a really interesting example of a theory which mixes physical laws with perspectival elements. The Boltzmann entropy, for example, is in some sense a summary of how much information we have (or rather, lack) about the microstate of a system, and so the second law of thermodynamics can in some sense be thought of as a reflection of the fact that if we prepare a system in a known state and then let it evolve, we will inevitably lose information about it. (I am grossly oversimplifying of course!). So the fact that it takes the form it does is certainly a consequence of our interest in taking initial states and projecting them forward in time.

                Thank you very much for reading and for your comments! I'm excited to hear that you're working on ideas similar to this and I'm looking forward to reading your essay.

                Thank you very much for reading! I definitely agree with your comments on the importance of conceptual understanding - as you say, we never fully came to grips with the conceptual elements of quantum theory, and this has affected all the physics that has been done since. So studying the interpretation of quantum mechanics isn't just a matter of intellectual curiosity, it's a promising route toward making new progress in physics.

                I look forward to reading about your new approach to the interpretation problem!

                Thank you for reading, and for your comments!

                I'm very interested in what you have to say about locality. I should clarify that I don't necessarily believe physics definitely can't be local - what I intended to convey in the essay was that if we accept Bell's theorem then we can't have locality (regardless of whether we have determinism) - but of course there are various ways of getting around Bell's theorem, such as the Everett interpretation or superdeterminism. For various reasons I'm a bit sceptical about both of those, but I'm always interested in new takes on this issue and I look forward to reading your essay!

                Thank you for your comments!

                At present I don't know of any examples of retrocausal theories actually yielding new predictions. However, there seems to be quite a lot of evidence that quantum mechanics may actually be retrocausal in character (e.g. see the recent paper by Leifer and Pusey). And if the laws of nature are in fact retrocausal in character, then it seems to me likely that the best way to make progress is to allow ourselves to write down laws which are retrocausal and try to understand what their consequences would be for the world as we observe it.

                Thank you for reading! I'm interested to see how limitations on knowability affect predictability, and I'm intrigued by the suggestion that there could be some way to have definite knowledge of ontology, so I look forward to reading your essay!

                Dear Emily,

                I greatly appreciated your work and discussion. I am very glad that you are not thinking in abstract patterns.

                "Insisting that all theories should takea predictive form from the start places an unnecessary and unjustified constraint on thespace of possible theories, and by removing that constraint we will probably be able todo better physics, no matter what the purpose of physics might turn out to be".

                While the discussion lasted, I wrote an article: "Practical guidance on calculating resonant frequencies at four levels of diagnosis and inactivation of COVID-19 coronavirus", due to the high relevance of this topic. The work is based on the practical solution of problems in quantum mechanics, presented in the essay FQXi 2019-2020 "Universal quantum laws of the universe to solve the problems of unsolvability, computability and unpredictability".

                I hope that my modest results of work will provide you with information for thought.

                Warm Regards, `

                Vladimir

                Dear Emily,

                As you note, the very concept of probability is tricky and unclear. Yet we "find" it and have to work with it anyway. I find the issue of correlated quantum measurements even more baffling than just the measurements by themselves.

                7 days later

                Ah, I see! Yes, I entirely agree with this! It *looks* worse in the global case, but it isn't really.

                Good luck for the contest!

                Best, Markus

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