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12:39 AM
@ACuriousMind ah dang
new font looks weeird
at least on mac
 
I'm not a fan but...eh, might just be not being used to it. Changing the font on anything you're used to in a different font looks weird
 
 
7 hours later…
7:21 AM
Hey all! is it possible to measure the Riemann Curvature using this setup?
Where the grey diagonal line is a one waymirror
(not sure how its done in LIGO)
 
8:02 AM
Does anyone know a good book on binomial series
 
123
8:14 AM
Hi All..
 
 
3 hours later…
123
11:36 AM
What is exact and inexact differential in terms of thermodynamic coordinates? Pls explain in easy to understand example.
 
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@ACuriousMind Thanks, In book it is clearly explained how work is path function. But how how heat is path function? Any exmaple
 
Since $\mathrm{d}U = \delta Q - \delta W$, if heat were a path function and the differential $\mathrm{d}Q$ of a function $Q$, then $\delta W = \mathrm{d}Q - \mathrm{d}U$ would also be exact - it would be the differential of $Q-U$ since $\mathrm{d}$ is linear.
 
12:23 PM
@ACuriousMind Is a co-chain just a linear combination of differential forms multiplied with a prefactor
?
 
is this graph correct for Electric field vs R
 
And does this prefactor change at every point?
 
@MoreAnonymous a linear combination of differential forms is just a differential form :P
 
@ACuriousMind True!
:P
 
12:25 PM
a co-chain is just a functional on chains
 
trying to wrap my head on this
 
the answer by joshphysics shows how every differential form is equivalent to a co-chain on the smooth chains (i.e. "paths" over which you can integrate)
this equivalence is also at the heart of why the deRham cohomology is the same as the ordinary cohomology of a smooth manifold
 
I'm not really good at cohomology
 
then just ignore my remark :P
 
why is he mapping $S_k \to Z$ ??
where $S_k$ the set of all singular k-cubes in $R^n$ ?
 
12:35 PM
it's just a definition; another way to phrase this is to say that chains are formal linear combinations of his "cubes" with integral coefficients (which he shows later is equivalent to the definition in terms of functions)
 
thanks this helps^
 
I think his answer is pretty good at explaining what the intuition behind this is - "gluing" cubes together and assigning a "weight" to each cube
note that most usual math texts will talk about simplices, not cubes, but that's a non-essential difference
 
Yea ... I think I get it now. Just one quibble. How does he know that that heat should be mapping: cubes to a number and not say a cube infinitesimal?
 
I don't know what a "cube infinitesimal" is.
A 1-cube is just a path. Thermodynamics tells us there's a way to assign a quantity "heat" to any process, and processes are paths in thermodynamic state space.
you shouldn't try to mix rigorous approaches (like joshphysics' approach here) with non-rigorous ideas like "infinitesimals", it rarely goes well
(there are rigorous approaches that use something like infinitesimals but they require different machinery)
 
So here's one way of thinking of it (for one dimension):

$ \int_0^\infty f(x) dx = c $ ... Taking $x \to 2x$:

$ \int_0^infty f(2x) d x = c/2$

This is the same as saying in Riemann integration as count only the even strips. Then the question becomes how does one find the integral for an arbitrary sequence?
13
Q: The Definite Integral Problem (with a twist)?

More AnonymousThe Definite Integral Problem (with a twist) In the Riemann integral one essentially calculates the area by splitting the area into $N$ rectangular strips and then taking $N \to \infty$. Here's something I asked myself related to the Riemann integral. Let's say I split the area into say $3$ st...

This is more precisely what I'm talking about
^
Also please use @MoreAnonymous? (it helps when Im multitasking)
 
1:14 PM
@MoreAnonymous I don't see what that question has to do with what we are discussing
if you want to talk about infinitesimal paths, then the rigorous notion of an infinitesimal path is just a tangent vector (equivalence classes of paths is one of the definitions for tangent vectors). A functional on tangent vectors at every point is an assignment of a cotangent vector to every point, which is another possible definition of what a differential form is (given the assignment is smooth in the appropriate sense)
 
@ACuriousMind He's putting a weight (a number) on a finite cube. My question was not do it for the limiting case
an infinitesimal cube
 
the infinitesimal notion of a $k$-cube is just a collection of $k$ tangent vectors
a functional on these is a $k$-form
 
Agreed
 
@MoreAnonymous Morally your idea is the idea of differential forms, yes. You put weights on small cubes.
 
But my point is I can always multiply a coefficient $a$ to the say a one form at point $P$ $dx|_P $ and get $a dx|_P$ then at point $dx |_{P +\delta}$ I can multiply $b$ and get $b dx|_{P + \delta}$. I can continue doing this game for $N$ points and take the limit $N \delta = c$ where $N \to \infty$ and $\delta \to 0$
 
1:22 PM
I am not sure if there is a deeper connection
 
@BalarkaSen Neither am I
:P
 
@MoreAnonymous "at a point" you don't have a 1-form, you just have cotangent vectors
 
@ACuriousMind Can't I go to a one form using the musical isomorphism ?
 
no, I mean, a 1-form is an assignment of a cotangent vector to each point
"a 1-form at point $p$" = "the cotangent vector the 1-form assigns to $p$"
on a contractible domain - such as $\mathbb{R}^n$, you can write every differential form as linear combination of basis forms $\mathrm{d}x^i$ as $\omega = \sum_i f_i\mathrm{d}x^i$ where the $f_i$ are functions
there's no need to do this on some discretization and then take a limit or anything, it just works out of the box in the smooth case
 
@ACuriousMind Yes but how do we know heat is smooth then?
 
1:26 PM
that's the usual phyiscs assumption - everything is smooth until it isn'T :P
note that "smooth" does not mean you can't integrate it over things like a zig-zag path - there's a natural way to integrate a smooth form over piecewise smooth paths
 
@ACuriousMind yes thy physicists everything is nice convergent smooth etc etc XD
@ACuriousMind You can integrate my thingy too :P
Depending on how open minded you are to extend the definition of integration
 
@MoreAnonymous I haven't fully understood what you're proposing since you have to really define what your limits are and why they should converge, but I don't understand why you're even trying to do this.
 
@ACuriousMind I believe the answerer does provide a proof of convergence?
6
A: The Definite Integral Problem (with a twist)?

mathworker21Let $b_r = \sum_{d \mid r} a_d\mu(\frac{r}{d})$. We prove that if the $b_r$'s are small enough, the result is true. Claim: If $\lim_{n \to \infty} \frac{\log^2(n)}{n}\sum_{r=1}^n |b_r| = 0$ and $f$ is smooth, then $$\lim_{k \to \infty} \lim_{n \to \infty} \sum_{r=1}^n a_rf\left(\frac{kr}{n}\righ...

@ACuriousMind On why?
 
I also don't understand what that question has to do with differential forms or heat
rigorous axiomatizations of physics assume sufficient differentiability for everything to work out - there's no benefit to not assuming it
if you want to ask "how do we know heat is smooth?", you have to first explain what your axiomatization of heat is that does not already include that
and then you have to explain why anyone should use that axiomatization
 
True
I guess to get something like I'm proposing one would need a fundamentally different form of process. I don't think you would get it from something nice like $F = ma$
 
 
3 hours later…
4:27 PM
@ACuriousMind Heat shockwaves?
Ever walked out of an air conditionned store in the summer?
I'm pretty sure it's discontinuous there
 
 
3 hours later…
7:43 PM
hmm
feels like every time something potentially world changing shows up there's a giant asterisk to it
not doge coin, just cryptocurrency in general
 

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