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9:33 AM
@Slereah Dunno, but Immanuel Kant claimed that it's a priori knowledge that space has 3 dimensions. Sorry, I can't give a good ref for that, I guess it's somewhere in Critique of Pure Reason (1781), but here's a recent (paywalled) article on the topic: jstor.org/stable/24704497
Around that time, there was some progress being made towards non-Euclidean geometry, but Kant published 5 decades before Lobachevsky & Bolyai published their works on hyperbolic geometry.
I wonder what impact it would've had on Kant's philosophy of space if he knew about hyperbolic geometry...
 
 
3 hours later…
1:00 PM
Is there a categorical idea for how one goes from a local description to a global description
Like going from a vector field to a diffeomorphism, or a PDE to a solution of that PDE, or a distribution to a foliation
How does one generally integrate objects
 
@Slereah isn't that just stating that the functor from the category of the global description (e.g. Lie groups) to the category of the local description (e.g. Lie algebras) has certain properties?
 
@ACuriousMind I mean it's category theory, so most probably it's about a functor
But that doesn't narrow it down much
 
An "obvious" functor
I'm trying to find of such a description to go from the connection form to the exponential map, basically
The exponential map is basically integrating the geodesic spray, I guess
Which depends on the connection form
 
from what connection form to what exponential map?
 
1:06 PM
Any affine connection, in this case
 
sure, but what has any exponential map to do with the connection
 
Aren't they related?
nlab claims that one of the property of the exponential map is basically $\nabla \exp_p = \exp_p$
 
if they are, it is not obvious to me :P
 
I mean the exponential map traces the geodesics of the connections
 
but I suspect you're talking about a different exponential map than the one I'm thinking about, that's why I'm asking you what exponential map you mean
 
1:08 PM
the one of the normal neighbourhood
I want to say "not the Lie group one" but then again they're mildly related
Also from what I can tell in a related sector, there is the Wario's equivalent evil twin of differential geometry called integral geometry
Which doesn't get discussed as much
 
@Slereah Okay, so what's wrong with saying that $\mathrm{exp}$ is defined by $\nabla \mathrm{exp} = \mathrm{exp}$ and $\mathrm{exp}(0) = 1$?
 
Well the whole going from local objects to global objects happens a lot in differential geometry
I am wondering if there are more general ideas on the topic
 
I'm not so sure this is a case of going from local to global - you get an $\mathrm{exp}_p$ at every point, you're just turning the global data of the connection form into equally global data in form of a collection of maps $\mathrm{exp}_p$
this isn't like the Lie group case where the exponential map at one point essentially determines the structure of the whole group
The only global-local passage here is that for any single $\mathrm{exp}_p$, it turns local data at $p$ into paths in its neighbourhood, where I'm not sure whether "paths in a neighbourhood" should be seen as local or global
 
it is a tricky one
The reason it bothered me is that the log map seems even less local, since for two separate points, you get a vector at one of them
 
The real local/global dichotomy for a connection is going between the description in trivializing patches and the description on the bundle as a whole
@Slereah but couldn't you equally well define that one to give you vectors at both points :P
 
1:19 PM
I vaguely recall that sometimes people use three levels of things
The level of the whole manifold, a neighbourhood or a single point
@ACuriousMind Well sure, you just get the parallelly propagated vector going the other way
but then using the parallel propagator is pretty non-local
It's literally integrated!
 
I'd really say that a connection is always global data, but a large part of the theory of connections can be done with the local data in a single coordinate patch
I'm not convinced trying to distinguish between "more local" or "more global" data for the various ways in which you can present the connection data in such a patch is a good way to approach this
 
1:39 PM
The only thing I can think of that's kind of what I'm trying to grasp at is the De Rham theorem I guess
the integration of forms over simplexes
 
1:53 PM
Maybe that can be related here, idk
Curves are just 1-simplex after all
From the general vibe I am getting maybe Cech cohomologies are involved
 
2:09 PM
Cech cohomology with values in $G$ is essentially counting how many different $G$ bundles you can build - the expression of a bundle in local patches essentially defines Cech cocycles, and two local expressions define isomorphic bundles when they are cohomologous in any refinement
 
sometimes science is more art than science, unfortunately
And I must rely on gut feeling for a while
 
3:02 PM
I just feel the need to drop in and say that I just had chocolate and pistachio gelato and that no other gelato I've ever had comes close
 
 
1 hour later…
4:05 PM
ew, pistachios :P
 
could be worse
 
glS
4:38 PM
@Mithrandir24601 pistachio is in my mind essentially the ideal ice cream flavour. Pity you can't easily (if at all) find a decent one is non-gelato countries =)
 
5:00 PM
@glS I tasted it in the shop (cafe) and was completely like 'ohhh wow, this is amazing!', asked for a scoop of that and chocolate, then when I got home and had them both together, was just totally and utterly blown away. Just sublime
I've had gelato in NI fairly recently and it's nice yes, but it's not the same as what the Italian cafe in Bristol does
 
glS
5:21 PM
definitely not the same, I can imagine
 
 
2 hours later…
glS
7:08 PM
@Mithrandir24601 if you want to try the real thing, there's going to be a quantum info conference in Palermo in September, is all I'm saying =)
 
 
3 hours later…
10:02 PM
0
Q: Some of the political lies and oppression in Physics Stack Exchange

Koorosh HeidarianStack Exchange is a great idea ruined by politics. Stack Exchange closes and deletes proofs, truth, and skepticism in questions, answers, even easily ban users from asking, vote down good things, etc. They have disappointed many individuals with truly scientific minds. Some of the political lies,...

 
 
1 hour later…
11:24 PM
well "levers are wrong" is certainly a new one for me
 
11:49 PM
^^ please mods close and delete this nonsense which has nothing to do with the meta site.
 

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