Essay Abstract

I argue for the presence of indefinite spacetime causal structure in nature, and show how it removes predictability obstacles regarding 1) spacetime singularities, 2) ultra-high energy quantum field theory, and 3) quantum gravitational computations.

Author Bio

Ding Jia is a PhD student at Perimeter Institute for Theoretical Physics and University of Waterloo.

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9 days later

Hi Ding Jia,

Thanks for letting us in having a peak on your research of indefinite causal structure. I followed Lucien Hardy's work on operational approaches to physics a while and found it very interesting. But sadly I was never able to take the time to follow him in his roads to quantum gravity.

Can you make some qualitative phenomenological prediction that might follow from a indefinite causal structure? The causal structure seems to be somehow a priori in the sense that it is difficult (impossible?) to express scientific experience in a non definite causal structure. On the other hand it is surely good to critically question principles that might restrict the physical inquiry.

In my essay I argue that a causal description of events might be complementary to a realistic one (assignment of properties independently of a measurement).

During the work on my essay, where I tried study the conceptual structure of physics, I was speculating that gravity prevents the ability to define strictly separable free objects. And this might be the cause, why perturbative quantum field theoretical approaches might fail to renormalizable with a finite set of observable quantities.

Your essay surely deserves more attention a it seems to get.

Luca

    Hi Luca,

    Thanks for your interest!

    One opportunity for phenomenological prediction is the cosmological primordial spectrum. Maybe indefinite causal structure plays a role in generating the correlations in CMB fluctuations.

    Doing physics in the absence of definite spacetime structure belongs to the greater effort of doing physics in the absence of a preferred spacetime background. I think the contributions of generations of physicists to this pursuit of "background independence" has made the task with indefinite causal structure much easier than it could otherwise be.

    Your essay looks quite interesting. I will put a post there if I gather any thought or question worth sharing.

    Related to your thoughts on renormalizability, I think quantum gravity prevents an assignment of definite spacetime distance to events/objects due to the superposition of spacetimes in a gravitational path integral. Regarding the failure of perturbative quantum gravity, my guess is that it is due to using propagators which depend on the spacetime distance with respect to some fixed the background spacetime, which obscures the superposition of spacetime distances.

    Best,

    Ding

    Dear Ding,

    It looks to me as if a lot of work has gone into some lovely original ideas. Well done.

    I have taken a particular interest in Feynman's path integral method. Although I would guess you would find it unbelievable, I think that both Special and General Relativity can be mathematically derived using Feynman's method. Therefore, I one hundred percent agree with you about indefiniteness actually being an essential part of those theories. I also agree with your principal of causal neutrality, but I think it works better where laws of physics are replaced a simple algorithm that can be used to predict Nature, and it is that algorithm that produces indefiniteness.

    I've explained it in more detail in my essay. Old Seth clearly liked to look at things differently and it seems to me that that there is an alternative way Feynman's maths can be used to interpret Nature completely differently. Interesting if you like being unconventional!

    It must be great working at the Warterloo Institute! idea

    All the best,

    David

      7 days later

      Dear David,

      Thanks for your interest and sorry for the delayed response.

      The idea of using Feynman's path integral to derive Relativity sounds quite intriguing! Could you offer more details or point to a reference with more details?

      Best,

      Ding