The h Bar

Sir Cumference

20:00

@Blue Ikr, but I'm considering a math double major and analysis is a requirement

Anonymous

If the mangoes you ate were awful then those were not "real" mangoes

0celo7

@Blue No True Mango?

Mithrandir24601

@0celo7 Is it worth buying? I've an interest in stuff like that, but it's not relevant to my work/PhD

@0celo7 Complex analysis on the other hand, apparently is :P

0celo7

@Mithrandir24601 I use it all the time but I'm a GR guy. Also I got it for Christmas some years ago.

pZombie

i wish mangos came in banana form, easy to peel

0celo7

20:01

It is the best GR $\cap$ physics book, IMO.

Anonymous

@0celo7 wikihow.com/Pick-a-Good-Mango

Sir Cumference

Though tbh, the math requirements are supposedly as hard as the physics classes, which are by far my most demanding classes

Mithrandir24601

@0celo7 Hmm... I'll have a serious think about it... I've spent far too much money this week already, but £16...

0celo7

@Mithrandir24601 Oh yeah, get the cheap one

I got the expensive one

Mithrandir24601

Yeah, there's no way I'm spending >£200 on a book that's not even relevant

0celo7

20:03

@Mithrandir24601 Not relevant?

It's the second most cited GR book after MTW.

It's still a good reference.

Sir Cumference

@Blue I'll be honest, I'd have no time to study all that course information on my own. Not to mention it wouldn't grant me the same understanding as a uni class would.

Anonymous

@SirCumference I think it'd help to talk with the analysis lecturer and ask him whether he can guide you through analysis separately (beyond class hours). He/she should probably help you out. (That's exactly what I do when I have clashing timings)

Mithrandir24601

@0celo7 For what I'm doing (PT-symmetric Hamiltonians)

Sir Cumference

@Blue I thought about doing that, but when I checked who the prof was, it just says "Staff"

No other class does that...

0celo7

@Mithrandir24601 It includes some great disses

He basically calls everyone out on their bullshit that spacetime is actually curved, saying that doesn't even make sense

pZombie

20:05

Ocelo7 can you elaborate on that?

why doesn't it make sense?

Mithrandir24601

It's getting very tempting...

Anonymous

@SirCumference Perhaps wait for the classes to start. You'll come to know. Register for the one which you find more useful for the time being

0celo7

@pZombie "Spacetime" is a model. You can't point to it. It's a made-up thing

It represents reality, it is not actually reality.

We do not "live in a curved manifold"

Sir Cumference

@0celo7 Oh, pedantry

bolbteppa

Any time I've looked at astronomy books, the hand-waiving put me off

0celo7

20:07

@SirCumference In the 70s when people thought they could put GR in a QFT framework it was an important distinction. Quite important.

pZombie

Ocelo7 - everything is model. That's true. It's how we imagine things to be, based on the local experiences we have, some models seem to predict better than others

bolbteppa

Maybe after I read Weinberg's Cosmology I'll be able for the hand-waiving

0celo7

Because once you decouple the math from the physics you can try to make a better model.

And that is the moral of the story.

Don't star that you fools

Anonymous

Lol

Semiclassical

if you don't want people to star s*** you say, maybe say less s*** :P

4

Anonymous

20:09

Lemme flag the "you fools" message now :'D

0celo7

@Semiclassical It's actually not stupid shit. I was more concerned with the "insanity" part. Is that what you were referring to?

I do believe that John Duffield had some good intentions. Most of what he said was nonsense, but every now and then he made a good point.

Semiclassical

s*** is probably over the top

but if you say something here, you assume the risk that someone else will star it

Slereah

what good point has he made

0celo7

@Slereah Spacetime isn't in the night sky

Sir Cumference

Hmm, maybe I should just take the non-honors version

Slereah

20:11

Well, that's true I guess

0celo7

That is code for "don't forget that GR is just a model"

Slereah

I don't think he understands that himself

0celo7

GR is actually an approximation. There's no reason why there aren't higher order terms in the equation, but we just say they're small for reasons.

Slereah

The reason is that we don't measure them to be within experimental error

Sir Cumference

@0celo7 It's somewhat 100% accurate

0celo7

20:12

@Slereah we don't really have that many tests of GR

Semiclassical

uhh

we've got a few

0celo7

@SirCumference that's very ignorant

@Semiclassical modulo LIGO they're all in the very weak regime

Sir Cumference

as in, it's close enough to reality that we might as well call it "reality"

Semiclassical

Tests of general relativity

Tests of general relativity serve to establish observational evidence for the theory of general relativity. The first three tests, proposed by Einstein in 1915, concerned the "anomalous" precession of the perihelion of Mercury, the bending of light in gravitational fields, and the gravitational redshift. The precession of Mercury was already known; experiments showing light bending in line with the predictions of general relativity was found in 1919, with increasing precision measurements done in subsequent tests, and astrophysical measurement of the gravitational redshift was claimed to be measured...

for reference

0celo7

oh and there was the binary pular experiment

pZombie

20:13

you wouldn't be a good physicists if you ever called some theory to be real

Semiclassical

I have the modern ones more in mind

pZombie

or ever believed in direct observation. Never happened

Semiclassical

The Shapiro delay effect is pretty cool

0celo7

@Semiclassical I think I'm familiar with all of them. The pulsar one is strong gravity, but we still don't have direct observations of black holes.

pZombie

define direct observation

Semiclassical

20:15

if you mean that one is only testing a leading effect of GR, though, I see your point

0celo7

@Semiclassical That is exactly what I am saying

We don't have enough strong regime experiments to rule out any kind of f(R) gravity, AFAIK.

Semiclassical

I imagine there's at least some constraints on the size of the possible corrections, but some constraints != strong constraints

0celo7

And GR isn't quantum, so it's certainly not "reality".

pZombie

and quantum is not GR so it is not reality either

0celo7

Correct. Although QM is more of a foundation for reality than GR is.

Semiclassical

20:17

I'd put my money on quantum being more basically correct than GR, yeah

0celo7

An LQG-ist might disagree

Semiclassical

i wouldn't put much money, mind

Slereah

@0celo7 We have plenty

Have u not read that great book

By Samuel Lereah

pZombie

both are good theories in the sense that they allow for useful predictions, but they are both incomplete

0celo7

fug

Anonymous

20:19

@pZombie That's probably true for every theory in physics :P

0celo7

well, screw physics

Semiclassical

heh, look at the title/author of this paper: adsabs.harvard.edu/full/1970CoASP...2...35T

and then look at the publication date

Sir Cumference

How do I become a good cosmologist like @Slereah

Slereah

Am I a good cosmologist

I basically don't remember anything about cosmological models

Sir Cumference

I think?

Semiclassical

20:23

@0celo7 I guess the real question is how small of corrections we'd reasonably expect to exist

Sir Cumference

ok nvm

Slereah

I mean I could look it up I guess

but on the spot

I don't think I remember much

Sir Cumference

aren't you a cosmology student?

0celo7

@Semiclassical well, the classic argument is that if you want a term like $Riem\cdot Ric$ in the field equations, you need a coupling constant with some strange dimensions

and then you make out out of the planck units and it's tiny compared to $G$

so you just declare it zero

Semiclassical

hmm

so the scale you'd need to look in order to see corrections is huuuuge

0celo7

20:25

yeah or small scale with huge energy

Semiclassical

which I'm guessing is what you had in mind with that R in f(R)

0celo7

f(R) gravity just means the Einstein-Hilbert action is $\int f(R)$

Semiclassical

oh

0celo7

regular GR is f(R)=R

Semiclassical

I find it hard to get excited by beyond standard model physics, whether that be the standard model of particle physics or GR as the standard model of gravity

0celo7

20:27

same

Semiclassical

because it ends up being so hard to test

the one exception to that is neutrino physics. that's genuinely cool

0celo7

hard to test and anything harder than the Einstein equations makes me want to cry

lılostafa

@Semiclassical protip: If you want to star something like that, wait until it's not removable/editable and then do it ;)

3

Semiclassical

loool

Sir Cumference

@0celo7 Wait, do you like cosmology?

Rumplestillskin

20:28

Guys can someone help me out with a sanity check on index notation : $(\partial_a T_b) M^aM^b + (\partial_a T_c) M^aM^c$. Can I swap the $a$ index in the first expression to a $c$ and the $a$ in the next expression to a $b$?? I am having a massive brain fart as I just woke up and am wondering does it have to be the same change for the entire expression!!

0celo7

yes

Semiclassical

yeah, you're good

0celo7

@SirCumference idk, why

Sir Cumference

@0celo7 Curious

pZombie

is inertial mass an emerging property?

or is it something inherent elemental particles are supposed to posses?

Rumplestillskin

20:30

Aw good!! I answered a question like this last night and then when I woke up I doubted whether I was right and didn't want lead someone down the wrong path! Cheers!

Anonymous

@pZombie arxiv.org/abs/1206.7114

Semiclassical

the tricky thing is that, absent a theory which puts QFT and GR on the same footing, it's hard to say that we've fully explained the origin of mass

pZombie

i think a lot would reveal itself if anyone actually tried to write an accurate relativistic particle simulation even if it would be able to simulate just a fraction of a nanosecond. By accurate, i mean one that could be set to arbitrary accuracy, sacrificing simulation speed.

0celo7

@Semiclassical time for fortran ;_;

Semiclassical

god help you

pZombie

20:40

no simplifications should be contained within the simulation

the most general case

0celo7

@Semiclassical it's just asking me to implement RK4

seems like it shouldn't be the end of the world

Semiclassical

no

but i imagine it's not without its headaches

0celo7

I'm sure I'll learn some new things about fortran in the process however

like the god damn formatting

@Semiclassical without looking, what is the golden ratio

Semiclassical

1.618...?

it's something like $\frac12(1+\sqrt{5})$

0celo7

@Semiclassical Yeah I got a 95 on one of my fortran homeworks because I said that. The prof swears it's 1/ that

Semiclassical

20:48

well, to be fair, 1/1.618 = 0.618

0celo7

yes, it's a bit strange that way

Semiclassical

but phi >0 is pretty solid

0celo7

pretty solid?

Semiclassical

i mean that phi being greater than 1 and not less than one is something I'm pretty confident in

0celo7

me too

wonder how he will respond

my code is right if you use his wrong definitions

@Semiclassical are you a linux guy?

Semiclassical

20:52

lolnope

0celo7

@Semiclassical erm, how would I solve $y'= 2e^{4x/5}+3y$ analytically

Semiclassical

well, $y'-3y=\frac{dy}{dx}-3y=e^{3x}\frac{d}{dx}(ye^{-3x})$ I think?

0celo7

ah yes, integrating factor

thx

Semiclassical

so therefore that's equivalent to $\frac{d}{dx}(ye^{-3x})=2 e^{4x/5}e^{-3x}=2e^{-11x/5}$

in which case yeah

I had the integrating factor idea in mind, i just was able to do it by inspection here

0celo7

for some reason I thought you had to have homogenous for that to work but apparently not

@Semiclassical yay, got my grade bumped up

Semiclassical

21:09

woo

0celo7

@Semiclassical This should make you happy: if $V\in L^2_\text{loc}$ is positive, then $-\Delta +V$ has a nondegenerate strictly positive ground state.

Semiclassical

nice

0celo7

@Semiclassical er, if $V\to \infty$ at infinity

Semiclassical

kk

yeah, that makes sense

0celo7

@Semiclassical If you change signs I think you see degeneracy in general.

@Semiclassical Reed and Simon are so wonderful because I can go to them for something completely unrelated to QM and come away with some QM magic

in this case, positivity conditions for semigroups and I learn about ground states

Danu

21:42

@0celo7 near the end of Gromov's paper

he has the following

"Since $\pi_1(SO(n\geq 3))=\Bbb Z_2$, there are just two orientable vector bundles of rank $n\geq 3$ over orientable surfaces."

I don't understand this statement. I know how to do this over $S^2$, using the clutching construction. But to reduce say a torus to contractible pieces I need 2 cuts, not just one.

0celo7

@Danu Agreed, so you'd have to worry about the gluing conditions at the two cuts.

I don't know, there is probably some consistency condition then. @BalarkaSen would know.

@Danu I'm trying to think of how the bundle looks on the polygonal representation of the surface.

Danu

21:59

Trivial on the interior

you're just gluing along the 1-skeleton

0celo7

@Danu Yeah it's trivial but it should be "pre-twisted" so that when you glue you don't have to do any more twisting

like if you cut a Mobius strip

Danu

It's not going to be so easy to visualize

0celo7

Indeed.

It's an $n\ge 3$ thing.

Danu

even e.g. the non-trivial $S^2$-bundle over $S^2$ is already $\Bbb C\mathrm P^2\#\overline{\Bbb C\mathrm P^2}$

I once found a way to sorta visualize it but it didn't last long :P

0celo7

lol yeah I was trying to imagine the clutching construction on $S^2$ as a warm-up and was reminded why I'm not an algebraic topologist

Danu

22:05

I have it written out

but even with the notes it's very hard I remember

I managed to derive some intuition from the real projective case, which is much easier to visualize

0celo7

@Danu This seems like something that would be in Milnor-Stasheff but I haven't read it.

Danu

The kind of topology that all these 4-manifold guys are fucking good at

@0celo7 Don't think it's in there.

0celo7

Unrelated but do you use HBO Go?

Danu

no

Why do you ask?

Also, does that even exist in EU?

0celo7

@Danu I keep having this problem where it thinks I'm using it on multiple devices but I'm not

was wondering if there's a trick to resolve the issue

google is unhelpful

Bernardo Meurer

23:51

@JohnRennie Are you around?

ACuriousMind

@BernardoMeurer If he were at this time that would be highly unusual :P

Bernardo Meurer

@ACuriousMind Yeah, I just always try anyway so when he wakes up he pings me back

@ACuriousMind Do you happen to be familiar with audio equipment?

ACuriousMind

@BernardoMeurer Would it not be more efficient if you just pinged him with whatever you actually want from him?

@BernardoMeurer Not at all, I'm afraid

Bernardo Meurer

@ACuriousMind No, I want to be wanted

ACuriousMind

dont we all

Bernardo Meurer

23:59

@ACuriousMind True that

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