James

Personally, I have considerable difficulty with the concept of randomness in fundamental physics (even though it is an incredibly useful concept for many areas of applied physics). If you give me a bit string 010010....01 that you claim has been generated randomly, I will give you a deterministic rule for generating that same bit string. Now some might say that randomness is informationally incompressible determinism. Well I would say that in practice the two may well be indistinguishable. However, at a fundamental level the latter is generated by a deterministic rule and the former, presumably, is not. In my view the sooner we get back to thinking about physics deterministically (even though it may be computationally irreducible determinism) the better!!

Best wishes

Tim

This is a really exciting essay; I'm really intrigued by the connections you suggest between quantum mechanics and chaos theory, and am now keen to learn more about this area.

I did have one general question about the motivation for this approach. If I understand you correctly, the idea is that by constraining the state of the universe to evolve on some uncomputable fractal subset of state space, we get a natural way to violate statistical independence (without the denial of free will) and thus we can have violations of Bell's inequality without violations of locality.I wonder, though, why you consider it important to avoid violations of locality? As I understand it, the similarity of Schrodinger's equation to the Liouville equations leads you to consider underlying deterministic dynamics, and then since Bell's theorem rules out any underlying deterministic local dynamics, you turn to non-computability as a means of violating statistical independence and thus invalidating Bell's theorem. But an alternative possible route would have been to accept the existence of nonlocality and consider how Schrodinger's equation could arise from an underlying deterministic non-local dynamics - is there a specific reason you chose not to go down this route?

I also have some questions about the sense in which 'locality' is preserved by your model. First, consider applying the constraint of evolution on a fractal subset to an indeterministic model. Then if the evolution of the universe is constrained to remain on the fractal subset, it would seem that the (non-deterministic) evolution of the universe at any one spacetime point must depend on the (non-deterministic) evolution of the universe at all points spacelike related to that point, as if the points evolved independently and non-deterministically then it would be possible to go off the fractal subset. So constraining the universe to lie on the fractal subset does not, in the absence of determinism, seem to give us a local theory. So now consider a deterministic model as you propose. Here the evolution of the universe is fully determined by the initial state (I am assuming here that by 'deterministic' you are referring to initial-value determinism, as the term is commonly used), and so the constraint you suggest comes down to requiring that the universe has a fine-tuned initial state which ensures that its evolution always remains on the fractal subset. But surely if this sort of fine-tuning is allowed then we can quite easily explain nonlocality without needing to appeal to noncomputability or fractal subspaces - i.e. we can encode the choices of measurements and the measurement results for all Bell experiments which will ever be performed directly into the initial state, and thus produce experimental results which appear non-local even though they are in fact produced from local evolution from this fine-tuned initial state. I think most physicists are not keen to adopt this approach to eliminating nonlocality because it seems unreasonably conspiratorial and fine-tuned - do you think your fractal approach gets round this complaint in some way, and if so, how?

I was also interested in the approach you take to recovering 'free will.' The distinction you make between defining free will via counterfactuals vs defining free will as the absence of constraint clearly ties into long-standing arguments in philosophy about the nature of free will, and I think there are indeed good arguments in favour of the latter approach even before one comes to the specific theoretical model that you introduce here - indeed I would be fascinated to read a paper discussing the links between your proposal and the body of philosophical literature on this topic!

    Dear Emily

    Thank you for your interesting and important questions. In replying I need to make sure I don't end up writing another paper!

    As far as my motivation is concerned, I did my PhD many years ago in GR (under the cosmologist Dennis Sciama) and in truth the reason why I have got so interested in Bell's Theorem is not because I am interested in Bell's Theorem per se, but rather that I think sorting this out properly is crucial to finding a viable approach to the synthesis of quantum and gravitational physics. These issues of locality vs nonlocality are very subtle: I can violate Bell's Theorem with the postulates "Free Choice on the Invariant Set" and "Factorisation on the Invariant Set". Technically these violate "Free Choice" and "Factorisation" in Bell's Theorem, but I would argue that they do so in a way that is comprehensible from the perspective of space-time causality in GR. To be honest, instead of talking about locality and nonlocality, I would rather ask: Does a nonlinear deterministic fractal state-space approach to quantum physics lead to a better model for synthesising with GR than conventional quantum theory? Well there are certainly some hints that it might!

    Regarding fine-tuning, I sometimes use the analogy with BCC TV Doctor Who's Tardis. From the outside it is a tiny 1950s Police Box. From the inside it is an incredibly spacious space-ship. A Cantor Set has a similar property. From the outside it has measure zero with respect to the measure of the embedding Euclidean Space and therefore appears insignificant. However, from the inside it has the Cardinality of the continuum and therefore appears incredibly massive and spacious!!

    So it all depends how you define this notion of "fine-tuned". From the intrinsic (technically Haar) measure of the invariant set, it is not at all fine tuned. Moreover using the p-adic metric (discussed in my essay), points which do not lie on the invariant set are necessarily distant from points which do, even though from a Euclidean point of view the distance between such points may appear tiny.

    Finally, you are much more knowledgeable about the philosophy literature on free will than me. I would love to discuss with you how my ideas may or may not connect up to this literature. You will see in the essay that I flagged one of the key ideas in Lewis's theory of counterfactual causality as potentially being incorrect if the metric on state space is not Euclidean. I'm sure there is much to discuss here!!

    Best wishes

    Tim

    Dear Tim,

    It was a pleasure reading your essay. and the valuable insights it gives. Would you happen to know if Connes' non-commutative geometry formalism would classify as uncomputable?

    As regards quantum gravity, I beg to submit that I have made important progress recently, and proposed the theory of Spontaneous Quantum Gravity, described for example in my paper

    Nature does not play dice at the Planck scale

    Independent of anything to do with the present contest, I will value your critique of my theory. I would like to reach out to many physicists with a request to examine this theory.

    Many thanks,

    Tejinder

    Dear Tejinder

    Connes comments in his book that non-commutative differential geometry provides a way to represent the measure of fractal sets such as the Julia set. Since these fractionally dimensioned sets are non-computable, then non-commutative geometry and non-computable geometries are related.

    Hence, perhaps there are some interesting connections to be made between my invariant set model and Connes' non-commutative models of quantum physics.

    Having said that, non-commutative geometry is not an area of mathematics of which I have any great knowledge.

    I will take a look at your essay. The title sounds very appealing to me!!

    Best wishes

    Tim

      Thanks Tim! This connection between non-computable geometries and non-commutative geometries is extremely interesting! I did not know about it. My new quantum theory of gravity builds on Connes' non-commutative geometry and Adler's trace dynamics.

      Best,

      Tejinder

      Dear Tim,

      what a wonderful essay and it rings a bell.

      Now since 8 years I studied general fractals (better known as wild embeddings) to get a spacetime representation for quantum states. You also discussed this interesting relation and you are right, there is a direct link between such objects and general relativity as well. The space of leaves of a foliation is also a non-commutative geometry (a la Connes) and this space is related to wild embeddings (or fractals) as well.

      Amazingly, these connections are naturally related to the structure of spacetime (I mean the 4-manifold). If you choose another differential structure then you get automatically these connections. But enough for now.

      I enjoyed very much reading your essay and gave them the highest score.

      Maybe you are also interested to ream my essay

      Because of Corona, I was to late this year....

      Best wishes

      Torsten

      Hi Tim,

      I read your essay a while back and I have not seen any comment from you on mine. Since there is some overlap in our topic emphases; it would be nice to have your opinion.

      All the Best,

      Jonathan

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