On the Foundational Assumptions of Modern Physics by Benjamin F. Dribus

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2. You make the very helpful analogy that "the causal links are the "wires" in the quantum circuit." If so, I don't see any disagreement on this point, because the directed graphs representing quantum circuits are not transitive graphs. Also, the classical causal networks in your arXiv paper seem not only intransitive, but almost "maximally so" in this sense.

3. Regarding my reasoning for not absolutely ruling out cycles, I actually think GR is very discouraging to fans of time travel, and I'm certainly not trying to rescue GR here. It's true that GR gives a sliver of hope to believers in causal cycles, but I don't take these solutions very seriously. My reasoning is partly caution and partly based on some potentially interesting or suggestive properties of graphs containing cycles. The models I have thought the most about are acyclic, however.

4. Of course you're correct that a binary relation doesn't determine a metric in general. Sorkin discusses this at length. His "order plus number equals geometry" motto is based on metric recovery theorems that take as input an appropriate binary relation together with some volume data. His choice of how to provide volume data is the simplest (counting), but there are other ways, defined by taking advantage of local data in the graphs. For a homogeneous graph, simple counting is probably the only option, but I don't prefer the assumption of homogeneity.

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5. I clearly did not explain my use of the sum over histories method adequately enough, and it is no wonder given the length constraints. First, in his 1948 paper Feynman discussed summing over particle trajectories in Euclidean spacetime and thereby recovered "standard" quantum theory, with its Hilbert spaces, operator algebras, Schrodinger equation, etc. Feynman was able to take all the trajectories to be in the same space because he was working with a background-dependent model; the ambient Euclidean space is unaffected by the particle moving in it. Now, if GR has taught us anything, it is that "spacetime" and "matter-energy" interact, so different particle trajectories mean different spacetimes. Hence, in a background-independent treatment, Feynman's sum over histories becomes a sum over "universes," with a different classical spacetime corresponding to each particle trajectory. His original version is a limiting case in which the effect of the particle on the spacetime is negligible.

What are the "classical spacetimes" in my approach? Well, they are directed graphs. However, it is not quite right to just sum over graphs. The reason why can be understood by looking at Feynman's method more carefully. He considered a region R of spacetime, and interpreted his path sum as the amplitude associated with measuring the particle somewhere on the upper (i.e., future) boundary given an initial measurement on the lower boundary. Hence, the path sum measures not the amplitude of a particular universe, but the amplitude of "transition" from one family of universes to another. A discrete approximation of this represents each particle trajectory as a sequence of directed segments in the corresponding configuration space, which inherits a partial order from the time-orders of the individual spacetimes. It is now clear how to generalize to the nonmanifold case: the appropriate sums are sums over paths in causal configuration space.

Take care,

Ben

Daniel,

I appreciate the feedback. I will itemize my reply:

1. The length requirement did limit my ability to explain my background thoughts. My emphasis on causality is principally motivated by two factors:

First, most of science, and particularly the experimental method, consists of establishing "causal" relationships between things ("whenever we do so-and-so, we observe such-and-such"). The success of this approach is obvious, and I feel that its full potential should be exploited. Remarkably, much of modern theoretical physics has relegated causality to a secondary and sometimes obscure role, and many recent theories reject its fundamental importance altogether. Instead, they are based on objects like continuum manifolds, which have undeniable mathematical advantages, but which are disturbingly idealistic and exhibit properties that are obviously physically irrelevant (like the least upper bound property, nonmeasurable subsets, etc.)

Second, it is startling how much of the structure of even such idealized theories can be recovered from causal relations, and how almost every "spacetime-related concept" you can think of has an analogous "causal interpretation" that is more natural and more general. For instance, a "light cone" in relativity is an abstract geometric locus of events in spacetime, and the rule that "information cannot escape its light cone" is viewed as secondary. In causal theory, the light cone is simply the scope of information flow, and the idealized view of a geometric locus is secondary. As another example, "frames of reference" in relativity are associated with abstract local coordinate systems on a manifold, and the relativity of simultaneity is viewed as secondary. In causal theory, frames of reference are different orderings of events compatible with the causal order, so the physical idea of relativity of simultaneity is direct, and the idealized view of a coordinate system is secondary. In both examples, I think it's obvious which is the more natural and physical point of view.

2. Regarding the recovery of established physics, I don't mean that every prediction of currently accepted models, including those that haven't been experimentally verified, must be reproduced; that would be pointless. What I mean is that a new theory must do at least as well as current theories in explaining and predicting what we actually see. For instance, general relativity modifies Newton's laws only very slightly for solar system dynamics; if general relativity had predicted an inverse cube force for the gravity between the earth and the moon, it would have been thrown out. Similarly, a new theory must look like general relativity and quantum theory in any situation where those theories have been proven to work.

3. Regarding the general requirements for a theory, yes, I agree completely. Experimental evidence is the final judge.

Take care,

Ben

Dear Pentcho,

Thanks for the feedback! By "recovering" relativity, I don't mean that I believe relativity in Einstein's original form is absolutely valid (otherwise, why would it need to be replaced or superseded?) What I mean is that in any case in which relativity makes good predictions, any theory that supersedes it must do at at least as well. Hence, a new theory must be able to describe/predict anything that relativity can describe/predict.

Regarding the constancy of the speed of light, my guess would be that a concept like this only makes sense at sufficiently large scales. "Speed" requires a notion of distance, and my view is that spatial distance (separation) is ultimately just a way of talking about the extent to which events are "unrelated." It begins to look like a traditional distance only at large enough scales.

Regarding photon mass, it was thought for a long time that neutrinos had no mass, but it was eventually discovered that they do have mass after all. Hence, I would be inclined to keep an open mind even about something as "sacred" as that. However, mass itself is again an emergent concept in my view, so the questions of what a "photon" really is and what "mass" really is are things that cannot be taken for granted.

You'll have to remember that my background is mostly mathematical, and therefore I'm inclined to consider the possibility of things that most physicists "know" are wrong. This might be useful in some cases; in others it only reflects my own ignorance. Take care,

Ben

Dear Yuri,

I did look through the comments on your thread, but I am afraid I don't quite understand. It seems you are suggesting there is a simple dimensional relationship that explains the observed value of the cosmological constant. This would be great, but it's not obvious to me. Do you mind explaining a little more? Take care,

Ben

Dear Yuri,

I got two out of the three articles, and I'm sure I can find the other one. I'm not sure why the first didn't come through. I don't know why you got that error message... that is the correct address. In any case, thanks for the articles; fortunately, they were easy to read, but included some information I did not know. I think I understand what you are suggesting about the relationship between the cosmological constant and Planck's constant, but don't you think that perhaps the cosmological constant is a little too small? Take care,

Ben

Dear Vladimir,

I appreciate the feedback! And I was quite amused by your metaphor of the three little pigs... although I think the physicists have given mathematicians at least as much of a headache over the years with ideas like path integrals and delta functions!

As a matter of fact, as I wrote on your thread, the math I use here is simply whatever seems necessary to get the job done... the basic physical idea of cause and effect is the motivation. The length limitation for the contest makes it a bit difficult to explain things adequately and still fit in everything you want to say.

I was going to wait to rate the essays until I had read them all, but I will rate yours now just so I don't forget. Take care,

Ben

Dear Sreenath,

I appreciate the feedback! It's true that I doubt a lot of the modern assumptions, but this arises mostly from my doubt about the ultimate physical relevance of manifolds. In my mathematical work, I have come to appreciate how very idealized and mathematically convenient objects like continuum manifolds and algebraic varieties are, and it seems to me that many of the properties that make them mathematically convenient do not arise in any natural or necessary way in physics. Many people think that convenient properties such as the least upper bound property in the order theory of the continuum can be assumed without worrying about their ultimate physical reality, based on the belief that any sufficiently fine approximation will suffice for measurement purposes. However, these properties determine the symmetry groups whose representation theory governs the properties of particle states, so the difference is an important qualitative one, not simply a small quantitative one that vanishes in the limit. My approach is to begin with the concept I view as most central to scientific process, namely cause and effect, and explain as much as possible in these terms. Ultimately, it may not be enough, but it is an approach with obvious motivations and clear and simple principles, and one that has not been adequately explored.

Regarding your essay, I view it positively even though your approach is much different than mine. I don't know if your equations will turn out to be correct, but the advantage of your approach is that you go into very specific details, and it should be possible to evaluate it one way or the other in a reasonable time frame. Like mine, I think your approach is worth trying, which is really all one can ask for. Take care,

Ben