Dear Ben,
Thank you for looking at my especially and thank you especially for your serious questions about my framework. I will attempt to answer them the best I can, and I hope that if I failed to be clear or if you disagree with something I say, you will let me know.
You asked: "What do you mean geometrically/topologically when you say that "spacetime reduces to a 2+1 dimensional analogue?"
A more mathematical statement would be this: consider a four-volume x^0x^1x^2x^3 in some frame (x^0 denotes time in this fourvolume). We can write very generally the following statement:
[math]
\lim_{x^1x^2x^3\rightarrow0}x^0x^1x^3x^4=k
[/math]
where k is taken to be a constant because of homogeneity and isotropy of space. Now, I believe that, although I have not seen this stated explicitly, it is generally tacitly assumed that
[math]k=0
[/math]
This would seem to be implied by the fact that we model elementary particles as point (i.e. zero dimensional particles) which means that we are not considering in our current mainstream theories any spaces or objects of intermediate dimensionality between zero and three.
I believe, however, that already special relativity gives us a hint that this conception cannot be quite correct. Consider that objects described by v=c undergo complete length contraction along the direction of motion. In other words *they are reduced by one spatial dimension* (as opposed to three).
So, in my view, it is not true that k=0. In fact, the first axiom of my theory specifically postulates that
[math]k=|U_{3max}|
[/math]
where the term on the right is defined as a constant quantity with dimensional units of areaxtime of variable shape (the variability in shape is analogous to the variability of a four-volume in different inertial frames even though its magnitude is constant as the Lotrentz factors for one direction in space and for time cancel).
Now, I agree that this is statement is still imprecise in the sense that it does not specify exactly how this limit is approached. This is an area I am still trying to figure out, but my suspicion is that due to the close association of mass with the emergence of spacetime in my theory (recall, acutal mass--> finite spacetime proper time--> actual worldline --> spacetime) this geometric limit manifests itself to us as a dynamical limit with dimensional units of action.
There are some additional ideas I have on this, but in order to make this response manageable, I will defer mentioning them to a future point in our discussion.
You said:"2. You refer to "actualizable worldlines," but Feynman's sum over histories doesn't give a probability amplitude for worldlines, it gives a probability amplitude for terminal events. It seems that you gain a spatial dimension when you consider all maps from "areatime" to "spacetime," but you lose the time "dimension" (causal direction or extent) when you measure, since all you know is the terminal state. Are you really changing dimensions, or just converting 2+1 to 3?"
Well, you are of course correct in your statement about probability amplitudes in Feynman's formulation and if you thought that this is what I claimed, then I failed to clearly express my ideas. In my defense, it was not so easy given the small amount of space available to summarize my idea.
I was implicitly already referring to the canonical formulation in my paper where I said something about the "vanishing probability amplitude"as the situation it referred to involved a two-state system.
In my formulation, as in standard theory, each path contributes equally to the overall path integral, so I think we completely agree.
You said:"You reject the dichotomy of existence and nonexistence, but I am not quite sure what you mean by "existence" in the first place. Evidently more than mathematical existence, since Feynman's world lines exist in that sense. The actualizations seem to exist and are distinguished by interacting dynamically with the 3 (+1) space. Is that the point then, that things "exist" if they interact dynamically with the space in which they live? If so, is areatime dynamical too?"
Excellent observation! Yes, with one qualification: Massless objects evidently interact with spacetime objects, too, but they always cease to exist during those events.
As for "mathematical existence" I believe (and please forgive if this sounds pretentious) that this involves an area of mathematics where there exists a distinction that has so far been overlooked. I recently gave a talk at a conference in which I presented my ideas, and the first 7 minutes of that talk may be relevant to your question. The talk is here:
http://youtu.be/GurBISsM308
I am sure that you will have additional questions, so I'll let you take look before discussing this further.
You said" If spacetime is not special, presumably areatime is not special either. Taking this to its logical conclusion, you could imagine maps from time to line-time, line-time to area-time, and so on. The constant feature here is a partial order which may be identified with causality. But then, we should realize that manifolds themselves are quite "anthropocentric" as well. If you take a step back and think of morphisms of partial orders, you get something like my "causal metric hypothesis" described here: On the Foundational Assumptions of Modern Physics."
Yes indeed, the first part of your response makes me think that you might not have seen the appendix to my paper, for it takes this idea exactly in that direction. Did you see the appendix?
As for the second part, yes, I will take a look at your paper and comment in your thread.
You wrote: "Finally, on a lighthearted note, string/superstring/M-theory involves "branes" of different dimensions too, but no one can agree on what the "M" stands for. Perhaps you should suggest "M" for "meta.""
Ha! I did not even think of that, that's funny. It reminds me of this quote by Nathan Seiberg at the 80th anniversary IAS conference:
"Most string theorists are very arrogant," says Seiberg with a smile. "If there is something [beyond string theory], we will call it string theory."
or, rather, M-theory :)
Finally let me mention that I present a more mathematical version of my theory in a paper called "A Dimensional Theory of Quantum Mechanics" which, if you like, you can access here:
http://hdl.handle.net/2027.42/83865
Basically, in my theory I start with a set of axioms and attempt to derive the usual Feynman formulation, i.e. sum over histories, each of which is associated with a phase factor e^iS/hbar(and its complex conjugate)which then allows one to go from there to canonical quantum mechanics.
I feel that there are several shortcomings in that paper, including some concepts that could be stated more precisely, some misconceptions that reflect my lesser understanding at the time, and some awkward wording which might potentially turn off readers, so I am working on an updated version, which, if you are interested, I can let you know about when it is completed.
Again, thank you for your questions and I hope I was able to at least address them somewhat.
Armin