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12:36 AM
@Slereah to be fair it's a pretty garbage book
but just about the only place where one can find these results
 
1:01 AM
hey
 
1:20 AM
900 page sobolev space book
 
1:37 AM
Anyone here has ever had to implement a numerical algorithm where angular velocities were involved?
 
2:26 AM
@dmckee writing a DoE report
it's terrible
 
 
3 hours later…
5:39 AM
@JohnRennie
 
@AlbertEinstein Morning :-)
 
@JohnRennie, A very good morning. How are you?
 
Pretty good thanks, and yourself?
 
I am fine, too
@JohnRennie, when does a man in the lift weigh more, when the lift accelerate upward or downward?
 
@AlbertEinstein This should obvious from your own experience. Do you feel heavier or lighter when the lift starts moving upwards?
 
5:45 AM
Lighter when it goes upward, is it?
 
Look at it this way. Suppose the lift starts accelerating downwards at 1g i.e. 9.81 m/s². What would you feel then?
 
@
@JohnRennie, heavier
 
If you jumped off a cliff you'd start accelerating downwards at 9.81 m/s². So if the lift is also accelerating downwards at 9.81 m/s² it means the lift isn't supporting you at all. You'd be falling as if you had jumped off a cliff.
 
user228700
6:06 AM
@JohnR: Morning :-)
 
Morning :-)
You must be about to jump on the train by now.
 
user228700
No, no. 11 hours to go...
 
Wow, you board the train around midnight?
 
user228700
10 PM!
 
Then travel overnight. That's a killer journey! Is it a sleeper train or do you have to try and sleep in the seat?
 
user228700
6:12 AM
Yes. Is "killer" a good adjective?
 
user228700
No, the train has berths to sleep in but they're not very comfortable.
 
@Kaumudi.H well, hopefully not. Though if it's literally true you won't be worrying about the exam results :-)
 
user228700
Ah, haha, yes :-) The train is too shaky, a bit too hot and the berth is terribly thin to have a good night's sleep.
 
user228700
Remember the two times I said "I'mma go to the town today"?
 
Yes?
 
user228700
6:23 AM
I wasn't able to go both times because I'm not allowed to roam around here on foot by myself -_-
 
Presumably due to concerns about attacks on women?
 
user228700
No, not even attacks. The people in this village... gawk. A lot.
 
That seems harmless enough, if a bit unsettling. Is the village society that different to Chennai? Do they have rules about unaccompanied young women?
 
user228700
No, they don't have rules but if I walk around wearing sweatpants and a T-shirt, they'll gawk like crazy. Gawk, whisper among themselves, etc.
 
user228700
It's a bit strange and new to them, that's all; women here wear drastically different sorts of clothes.
 
user228700
6:28 AM
In any case, yes, my parents don't want people gawking so yeah, wasn't allowed.
 
user228700
But my dad's taking me out in a bit! (What a lengthy segway!)
 
To be fair it probably wasn't that different here in the UK 50 years ago when teenagers first started wearing radically different clothes.
 
user228700
Yes, I suppose...
 
Though these days you could wear pretty much anything and people would just shrug.
 
user228700
Hmm, yes, OK :-)
 
6:31 AM
So this week you've endured rubbish train and bus journeys, done an exam, and spent the rest of the time effectively in prison!
 
user228700
That's one way to put it, yes :-P
 
... being starved :-)
 
user228700
Haha, no, that would be a bit harsh; I've been well-fed and all but I haven't been liking the food is all :-)
 
I used to be very fussy about food when I was young, but these days I'll eat anything. Yes, there are my favourite foods and foods that I'm not keen on, but at the end of the day food is food and is always better than no food.
 
user228700
> anything
 
user228700
6:35 AM
Oh, I know :-P
 
My sister in law used to work in the Ukraine, and the cooking there is, erm, different.
At one meal she was served cubes of pork fat stewed with cabbage, and she had to eat it to be polite.
 
user228700
That's, erm, wow :-|
 
Yes, that's what she said. Though she expressed herself rather more forcefully :-)
 
user228700
:-/ I...see.
 
If it's too traumatic to discuss I understand, but I'm curious about how th food is different from what you'd eat in Chennai.
The pictures of food you've posted in the past are pretty much what I understood vegetarian Indian food to be. So how does it differ in Kerala?
 
user228700
6:40 AM
Haha, you're being overdramatic! :-) It's certainly not traumatic at all.
 
user228700
Oh, it's not too different from what my mum makes.
 
user228700
Not different at all, actually. It's just the my grandma makes it all in some other way and the end-product doesn't end up tasting just as good.
 
Ah OK, so it's more that the flavours and or textures are different enough to be unappetising?
 
user228700
Sort of, yes.
 
user228700
I'm trying my very best not to be bitter about the food, of all things.
 
6:43 AM
My brother's mother in law (my aunt in law?) is a terrible cook and going for meals at her house is always a bit of an ordeal :-)
Especially as my mother and brother are both excellent cooks.
 
user228700
Ah :-) That's a bummer.
 
:: shrug ::
 
user228700
Right, I gots to go to the town (at last!)
 
We don't eat there often, and as I've said food is food.
@Kaumudi.H Have fun! :-)
 
user228700
Food is foot, yes :-)
 
user228700
6:49 AM
@JohnRennie Bye! :-)
 
9:47 AM
Now to prove that the musical isomorphisms have components $f_{\mu} g^{\mu\nu}$
 
10:14 AM
@ACuriousMind: well the photon is a massless boson - not a scalar but still a boson - so that seems a reasonable question, if a little ill informed.
 
@JohnRennie I consider the question utterly lacking in research effort. It's a "reasonable" question, but finding out that the photon is described by quantum electrodynamics where it's associated to a vector field should not be hard for someone reading a book on conformal field theory.
 
@ACuriousMind I must admit I agree with you on this. But in that case I would say so in comment to make it clear what you mean.
 
On the other hand, if the asker is aware of QED and just wondering whether a conformal massless scalar field might somehow be an alternative description, the question is lacking severely in elaboration.
If it's the former case of no research, then the question is unsalvagable. If it's the latter case, it can be salvaged by answering the comment I posted.
 
10:31 AM
I just used the term "currying" in a physics thing
Is that widely known enough to not have to explain it
 
As in currying favour?
 
I could not do a single question in my Physics exam yesterday
 
@Slereah Pretty sure it would be better to explain it unless your target audience is computer scientists :P
 
As in mapping a function of two arguments $f$ to a function of one argument $f(x, -)$
since that is the process by which one obtainds a mapping from vectors to one-forms
 
That's partial application, not currying
 
10:36 AM
isn't that what currying is
 
Currying is making a function $X\times Y \to Z$ into a function $X\to\mathrm{Functions}(Y,Z)$. What you're doing is currying the function and applying it to a specific $x$.
 
ah alright
 
Outside of contexts where the "nature" of functions plays a role, you shouldn't feel the need to use the word currying, since humans are pretty good at "automatically" doing that
 
I used to be big into lambda calculus so I got a lot of curry on my mind
 
10:38 AM
That is, just writing $f(x,-)$ is easier than explaining the abstract notion of currying.
 
user228700
@BernardoMeurer What? You're kidding, right?
 
@Kaumudi.H Nope
I got a nice fat 0
 
user228700
Dude. Geez, that sucks, man :-/
 
sorry, that grade is too low, we'll have to ban you from the chat
you'll have to go to the humanities chat now
 
user228700
@Slereah Quit bashing the humanities branch!
 
10:41 AM
@BernardoMeurer: for a computer engineering course you seem to have to do a lot of maths and physics. Not easy maths and physics either.
 
What if I do it in song
 
8
Q: Is there any exception to the law of "Cause and Effect"?

AminI've heard that 'God' is the only exception to the law of Cause and Effect which means that He is not dependent in His existence on anything. Can it be philosophically true? If we accepted this exception how could we make sure that the law would not be broken once again? Can we even call it a phi...

A world without causality is a strange place
 
@JohnRennie My course is really bonkers
 
Does the physics course matter? Can you just ignore it and proceed anyway?
 
It is a place where only correlations exists, but no way of explaining things (because you don't have the notion of A lead to the occurence of B
 
10:42 AM
It's driving me crazy, I'm transferring out of here, this place makes me loose my hair
 
I wonder what Tom Lehrer's thesis was about
let's see
 
All my courses matter, I must pass all of them always
Alas there's still 2 more exams where I can replace my grade
 
R. E. Fagen and T. A. Lehrer, "Random walks with restraining barrier as applied to the biased binary counter," Journal of the Society for Industrial and Applied Mathematics, vol. 6, pp. 1–14 (March 1958) MR0094856
T. Austin, R. Fagen, T. Lehrer, and W. Penney, "The distribution of the number of locally maximal elements in a random sample," Annals of Mathematical Statistics, vol. 28, pp. 786–790 (1957) MR0091251
Stochastics
 
@Kaumudi.H how was town?
 
Annals? (glad they have not mispelt it losing one more n)
 
user228700
10:44 AM
@JohnRennie Oh, it wasn't much :-) I had an ice-cream and a lemon soda. The scenery is beautiful, as always.
 
I am so bad at doing actual computations.
 
lol
That's pretty actually accurate for me.
 
Feynman was the guy who did mental calculation as a hobby
A great party trick
 
Yeah. Also cool integral computations etc
 
10:49 AM
I guess that's what people did before television was invented
 
Trick calculations are actually an important skill I simply just lack.
 
Well, it's a fine skill to have, I don't know about important
nowadays everyone carries a calculator
good old Tom Lehrer
 
having a bag of tricks is not a bad thing to have. i guess i just envy people with that skill
on a less frustrating note, watched The Thing yesterday. so awesome
 
which version
 
1982
not the prequel shit
 
10:51 AM
Well, The Thing was itself a remake
 
Ah yeah
 
I think it was The Thing from Outer Space.
 
itself from a book
 
from another world, got it
well John Carpenter is just great. even though I like left-wing horror movies more
 
10:53 AM
what
Also John Carpenter made Dark Star, which is great
It's the only country song I know about special relativity
 
left wing = horror from inside human body instead of outer space
 
Then just go eat at a kebab
 
kebab is tasty, not horrifying
 
The horror only comes later
From inside your human body
 
only if you're a weakling
 
10:56 AM
@Slereah I've been there :-)
 
I had a week where my diet was mostly McDonalds, kebab and alcohol
The horror from my human body was very compelling
 
11:33 AM
How does one prove the existence of the inverse metric
I can prove that there is indeed the components for an inverse metric, but is that enough to prove it's a tensor field
 
It is a tensor if it transforms like a tensor.
 
11:48 AM
You don't actually believe that, do you?
@Slereah you just prove that the inverse of a matrix is a matrix
 
Hm
If I have $a \in M_i$ and $b \in M_j$, with $M_i = \{i\} \times M$
 
@Slereah by definition??
 
How do I write that $a$ and $b$ correspond to the same point
@0celouvskyopoulo7 The definition is just $g(\omega^\sharp, Y) = \omega(Y)$!
It's not terribly hard to prove, but still
 
Plug in basis vectors you fool
Why are people demanding Trump release his tax returns? I thought he did it already.
 
Do I just write like $\pi(a) = \pi(b)$???
Where $\pi$ is the projection to the second component of the Cartesian product
It seems a bit much
 
11:55 AM
Wot
 
^I second this wot
 
I'm trying to write an equivalence relation between points of copies of the same manifold
 
aka wot wot
 
I could just write $a = b$, but that is terribly mundane
 
@ACuriousMind skyrim is a buggy piece of crap
I'm not hard saving enough
 
11:58 AM
I've got a bunch of copies $\bigsqcup_{i \in A} M_i$ and I want to write that two points $a \in M_i, b \in M_j$ correspond to the same point
 
Lydia just vanished and when I player.moveto her, I can't initiate dialogue.
So I have to load my previous save and lose an hour
 
@Slereah Everyone will understand if you just write that $a$ and $b$ are identified if they correspond to the same point in $M$.
 
Yeah
It's just so very unrigorous :p
Now to prove that for any closed subset $S \subset M$, $S \neq M$, an identification of $M_i$ is not Hausdorff
 
@BalarkaSen I have a manifold $N$ with a compact embedded hypersurface $M$, and $N$ has positive curvature. By Bochner's theorem $H^1_\mathrm{dR}(N)=0$. Supposedly $M$ now divides $N$ into two pieces. Is this the same argument as for Jordan-Brouwer?
 
Wait, does the subset even have to be closed
Line with two origins is the identification along $\Bbb R \setminus 0$
 
12:04 PM
I have no idea what you're even talking about.
 
Actually I think I may have it backward
It needs to be open
 
Hello there
 
@0celouvskyopoulo7 Nullhomologous hypersurfaces bound manifold on one side. This is not very trivial to prove but is essentially the consequence of being null.
 
just trying to find a generic method to have non-Hausdorff manifolds
 
@BalarkaSen what?
 
12:07 PM
That is not a question so I am going to pretend that message does not exist.
 
Why does it have to be hullhomologous
 
OH. I don't know why I thought that...
Well, it is not true. $S^2 \times p$ in $S^2\times S^1$ is a hypersurface which has $H^1 =0$. But it does not divide it into two pieces...
 
No, the manifold has $H^1=0$.
Not the hypersurface.
 
Ah, man I am really misreading stuff
Then the hypersurface is nullhomologous :)
 
Is "what" an acceptable question now?
 
12:11 PM
No, because the hypersurface is a homology class of codimension 1, and it's Poincare dual is a cohomology class of dimension 1. Since dimension 1 has NO cohomology, that is zero
Poincare dual = 0 => homology class = 0
=> nullhomologous
 
Dimension 1 has no cohomology?
 
@Slereah There is a difference between formality and rigor. It's perfectly rigorous, just not formal :P
 
Oh, by assumption.
 
:P
Using $N$ for the ambient manifold is really confuzzling man
 
Oh, so it's nullhomologous.
@BalarkaSen This book is nothing but one big confusion.
 
12:14 PM
what are you even reading
 
@0celouvskyopoulo7 I see you still do not accept that cohomology does not need a subscript indicating how it was computed :P
 
@ACuriousMind You know, I learned sheaf theory and de Rham's theorem and whatnot, and I decided to reject it all.
The only cohomology is de Rham baby
@BalarkaSen P. Li, "Geometric Analysis."
 
singular is love, singular is life
 
@BalarkaSen So is this in Hatcher somewhere?
 
That null hypersurfaces bound an embedded manifold on one side? No, it's nontrivial to prove.
If you want I can try to remember the proof
 
12:19 PM
He says "by an exact sequence argument"
@BalarkaSen Nah, I could probably find it if I wanted
@BalarkaSen In any case, it's used to prove a lower bound for the first eigenvalue for $M$
So nothing very interesting for you
You just need to know that there's a component of $N-M$ with boundary $=M$
@Slereah I'm going to give chap 7 of HE a read
I think I'm ready
 
Is that the Cauchy one
 
indeed
@ACuriousMind Even HE confuses diffeo and isometry :/
 
 
1 hour later…
1:48 PM
hey guys
 
hey
 
~~~~~~~~~~~~Beginning Transmission~~~~~~~~~~~~~
0
Q: Had any experiment being proposed/done that show that time is linear?

SecretIn classical mechanics and nonrelativistic quantum mechanics, time $t$ is a parameter to describe the evolution of a system. In discrete dynamical systems, we typically take $t$ to be linear and increases in uniform steps. In relativity and quantum field theory, time and space combine to form sp...

Probably the craziest question I ever wrote in PSE
(an no, I have searched the whole PSE and found no similar questions, afaik)
 
i'm sure it's my fault but i don't understand ur question!
 
@Secret: your question is meaningless. Time is just a coordinate like $x$, $y$ and $z$.
It makes no sense to ask if the $t$ coordinate is linear any more that it does to ask if the $x$ coordinate is linear.
 
"time" is just defined by events in physics
In the rest frame of a clock, time is just defined by that clock
it's the coordinative definition of time
 
2:06 PM
hmm... so whether using proper time or coordinate time, it is a number assigned for each event thus it is meaningless to talk about whether it behaves linearly or nonlinearly?
 
just ask yourself what it means for time to be nonlinear
 
Proper time is a coordinate time. It is the time coordinate for the observer at rest.
 
In a physical setting, time is just measured by the repeating of an event that we assume to be periodic
 
The mapping between the time different coordinates of different observers can be, and is, non-linear. But in any one coordinate system time is just the distance along an axis.
 
If the repeating of an event can't be assumed to be periodic, then you basically have no hope to measure time
 
2:12 PM
Slereah: Hmm... then nonlinear time will imply the event is aperiodic. However, if it is aperiodic, then there isn't any well defined standard on how it can be used as some kind of benchmark. Since it does not evolve in some "regular" fashion, I am not sure if one can use such event as some kind of "ruler" to measure time

(I typed too slow, you beat me to it)
I assume a quasiperiodic event will also suffer a similar problem...?
 
its more or less philosophy
not physics
i guess
 
Quasiperiodicity has a regular pattern but no fixed period, thus I suppose it can still be used I guess....
 
i think GR says there is no sacred time, and we're all alone
so i think we dunno really what is time, we just measure sth to be social
 
You can make up some random theory where you have time being nonlinear in rest frame according to specified conditions, and then convert back those times measured to "real" time, but that would be a really dumb theory
It would run afoul of Occam's razor
you are just adding features to the theory that are not necessary
 
All of physics runs afoul of that
An old man in the sky is far simpler than superstrings
 
2:18 PM
Well, you can make up a physical theory that is 100% without extra features
It's called the Craig axiomatization of a theory
It makes very ugly theories, though
And you can't extend those theories at all
 
@Slereah what's that?
 
Well for physical theories, you have what are called theoretical constructs and then you have measurable things
You can make a theory with only measurable entities and no theoretical constructs
but then it would be terribly unwieldy
For instance, you won't have a notion of "temperature"
Only measurable events such as what instruments show
you will get 100% the same predictions but those are all disjointed, conceptually
There's no obvious link between what a mercury thermometer shows and what an electronic thermometer shows
 
Millimeters of mercury baby
@Slereah why do you know random things about axiomatization?
 
@Slereah so doesn't it only separate rules?
 
Craig axiomatizations is discussed in "science without numbers" and "The philosophy of quantum mechanics"
 
2:25 PM
how is science without number?
 
@mathvc_ It gives you a mathematical formula where, for a list of measurable quantities, you get another measurable quantity
"Science without number" is a book on the axiomatization of classical physics with Hilbert geometry
so there are no numbers and no functions involved
Only points and order relations
and congruence relations
 
oh is it related to geometry interpretation of theories?
 
Not really
 
nominalism
 
yes
 
2:32 PM
$$\iint_\Omega h|\nabla f|\, dV=\int_0^\infty dt\int_{\Gamma(t)}h\, dA_t$$
why a double integral on the left?
$$\iint_\Omega d\omega=\int_{\partial\Omega}\omega$$
wot
@ACuriousMind ???
 
To specify that it's on at least two dimensions?
 
it's a reference text on spectral geometry, I think the reader is supposed to know how integrals work
I found that Stokes theorem from his riem. geo. book
jesus, who knows what he means
 
does it really matter
 
probably not
 
2:59 PM
what is the current state of experimental GR, anyway
what's the next big experiment they're planning
More G wave astronomy?
 

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