- #1
Mwyn
- 26
- 0
ok on a scale from one to ten exactly how likely is it for M-theorie to be true?
Mwyn said:ok on a scale from one to ten exactly how likely is it for M-theorie to be true?
notevenwrong said:... The last remaining hope, that somehow one could get predictions by a statistical analysis of the landscape, has just collapsed...
Mwyn said:ok on a scale from one to ten exactly how likely is it for M-theorie to be true?
kneemo said:...Noncommutative geometry, however, has surfaced implicitly in other approaches to quantum gravity, such as ... dynamical triangulations...
marcus said:One can speculate that once QG researchers have arrived at a useful quantum theory of spacetime and its geometry, (if they do, and to me right now CDT looks like the most promising approach), then a new theory of the particle fields and forces of matter can be constructed over it. the idea is to get the underpinnings right and then do the overlay.
marcus said:On what page, in what CDT paper?
kneemo said:...This is tantamount to saying that dynamical triangulations are built from D0-branes (vertices) and fundamental strings (edges)...
marcus said:I believe you may have confused two things which are different mathematically, because on a naive level they "look" the same to you.
Strings live in a differentiable manifold. Mathematically they are different objects from edges in a piecewise flat continuum.
The space of CDT does not live in a smooth manifold---it is not embedded in anything with a differentiable structure.
Indeed the space of CDT IS NOT PIECEWISE FLAT AND IT IS NOT MADE OF SIMPLEXES! This is very important to understand. the space of CDT is the limit of (an ensemble of) piecewise flat continua.
kneemo said:...Noncommutative geometry, however, has surfaced implicitly in other approaches to quantum gravity, such as ... dynamical triangulations...
kneemo said:My point is that a CDT is a derived concept. I've read through the CDT papers and have nowhere seen how to acquire a triangulation from more basic principles. When the authors eventually figure out how to do this, instead of presupposing the existence of a triangulation, they will realize they are doing noncommutative geometry.
Kea said:..and what I have been trying to tell you for a long time.
...
kneemo said:...Noncommutative geometry, however, has surfaced implicitly in other approaches to quantum gravity, such as ... dynamical triangulations...
...
me said:...this is what you said that interests me and I would like you to substantiate with some online article and page references.
It is fine with me if you reference a page from an article by Alain Connes on non-commutative geometry. I just want to see some connection established.
marcus said:So find me a page in some CDT article and a page of NCG that I can study and compare and see if they are talking about the same stuff. then I can evaluate for myself whether I think the connection is just vague handwaving or whether there is some substance to it.
Kea said:Hello Marcus
As far as I am aware, the words dynamical triangulations are not synonomous with CDT. In particular, in the paper
Construction of Non-critical String Field Theory by Transfer Matrix Formalism in Dynamical Triangulation
Yoshiyuki Watabiki
http://arxiv.org/abs/hep-th/9401096
which is referenced by
On the relation between Euclidean and Lorentzian 2D quantum gravity
J. Ambjorn, J. Correia, C. Kristjansen, R. Loll
http://arxiv.org/abs/hep-th/9912267
there is a background connection with the old Matrix theory.
The point is that there is a long and complicated history to the CDT papers.
Do you really want to ignore the evolution on the more mathematical side of things?
...I admire the CDT papers, but they are not fundamental. At least, I don't see anything in them that is.
...
marcus said:Let's follow up on this interesting difference of opinion. You see nothing fundamentally new in CDT, and I do. Let us talk it over. It might help clarify the ideas!
Kea said:There seems to be a reluctance to accept that more abstract modern mathematics might have a simplicity sublime enough to do physics.
Of course the mathematics looks complicated. Goodness knows I find it complicated. But who is the judge of what is simple? Posterity more than you or I. I've always felt I was much too stupid to understand anything that wasn't simple, and yet I find the combinatorics of Descent Theory to be essential to QG. Maybe I'm wrong.
Kea said:I will go away and look at the Reconstructing the Universe paper.
Kea said:I'm sorry, Marcus. I have a problem with the first sentence (but I will keep going). They say:
...at the shortest scales.
What does that mean?
Kea
marcus said:I was in love with elegant abstract modern mathematics presumably before you were born (you are a postdoc now right?) which is why i specialized in math.
Kea said:I'm very confused by the second sentence: Because of the enormous quantum fluctuations predicted by the uncertainty relations... Are they assuming that the UP applies 'as is' to QG?
marcus said:Do you have any questions about paragraph 3 of the introduction, at the bottom of page 1, which begins
"In the method of Causal Dynamical Triangulations..."?
Kea said:Still reading...
marcus said:but I have standards of concreteness which I apply in your case, and in the case of anyone claiming to be a mathematician. We don't want people waving their hands and just spouting words, we want to know exactly what the words mean.
I am telling you FAR from being reluctant I would simply LOVE it if you could give me a reliable set of page references that would show me that CDT (causal dynamical triangulation approach to quantum gravity, not something else applied to something else, but THAT) can be derived from Alain Connes NonCom Geom.
show me how CDT comes from something presumably more fundamental in NCG
kneemo said:Hi Marcus
Let us return to the path integral in eq. (1) of hep-th/0105267. The path integral is re-written as a discrete sum over inequivalent triangulations T. A basic question is: how does one acquire just one of the many inequivalent triangulations T? And given a specific triangulation T_0, what action is performed to acquire a new triangulation T_1?
.
The geometry of dynamical triangulations pg. 12 said:Dynamical triangulations are a variant of Regge calculus in the sense that in this formulation the summation over the length of the links is replaced by a direct summation over abstract triangulations where the length of the links is fixed to a given value a. In this way the elementary simplices of the triangulation provide a Diff-invariant cut-off and each triangulation is a representative of a whole equivalence class of metrics
Aaron Bergman said:In article
<Pine.LNX.4.31.0503091443440.19481-...man.harvard.edu>,
Urs Schreiber <Urs.Schreiber@uni-essen.de> wrote:
> Lubos' blog entry on deconstruction made me have a closer look at this
> stuff, which I should have had long before.
>
> If I understand correctly a quiver can equivalently be defined as a
> functor from some graph category to Vect. Given some graph, it associates
> finite vector spaces to vertices and linear operators between these to
> directed edges.
That's not the definition of a quiver. A quiver is just a directed
graph. A quiver representation is a vector space at each node and a map
for each arrow. The graph should define a category and you could look at
the functors from this category to k-Vect.
Equivalently, you can form the quiver algebra. Let the arrows be denoted [tex]a_j[/tex] and, to each node j, have an idempotent
[tex]e^2_j=e_j[/tex]
You have two functions source and target which take these to nodes. For
the idempotents, both the source and the target maps take the idempotent
to its respective node. The source and target map for the arrows is
obvious. Then, take the free algebra on this set subject to the relation
that a product ab is nonzero iff [tex] t(b)=s(a) [/tex] .