So when people say there are 6 independent d.o.f. in GR they are either ignoring initial conditions in the Bianchi identities or ignoring diffeomorphism invariance which, on fixing coordinates, reduces the d.o.f.?

...gauge fixing is not fixing coordinates (see e.g. physics.stackexchange.com/q/346793/50583 and its linked questions/answers), but yes.

hmm

3 m's in hmmm plz

oh no it's the hmm police

hide your m's

hm hm hm

arrests Semiclassical

hmmmph

hmmmmmm.......

Feels like magic but also kind of makes sense

arrests bolbteppa you've been warned

#Free.D.O.F.

@Blue Australian hmmm?

@Blue no, that's acceptable

mmmh

@danielunderwood don't push it

3 m's right there. My layer says it's perfectly legal

@Blue It's a (feeble) Southern hemisphere (->upside down) joke.

these topics found by topic modeling is...vaguely interpretable...

hmmm...not super easy to interpret though...

o.o

have you done topic modeling in the past?

lol, sounds like some guys thought "what are the buzzwords right now?"

and then came up with that paper

yeah, I'm not trying to suggest it's bs

only its title is funny

Quantum computing is going to bring some interesting job titles...I've already seen a posting for a Quantum Architect, but I could imagine Quantum Engineer, Quantum Salesman, who knows what else

meep, just got a flu shot

if the anti-vaxxers are right, I should be developing autism in a few days maybe

my arm is already a little sore

must be the side-effects kicking in

D:

I dunno about those flu shots dude

I haven't gotten one, but no flu for me!

on a more serious note, supposedly getting flu shots is not really about you if you're a healthy younger person

it's more for those with weaker immunities who you will be infecting with the flu if you don't get a flu shot

I actually heard something pretty similar to that on a podcast recently

@Blue The flu may just manifest in minor symptoms for you (e.g. a few sneezes) but you're still carrying and spreading it.

But people with weaker immune systems will suffer much more badly under it and it's also dangerous for pregnant women.

yep

Physics question

What happens to the Time-like Killing field in Schwarzschild geometry inside the event horizon?

that field vanishes for all r<2M or just for r=2M?

It turns spacelike?

chirp chirp

sorry talk of physics isn't allowed here

Well that's definitely the mathiest intro to GR I've seen

I'll have to read through it a bit. Everything that I've seen on GR felt to be lacking on the math side except perhaps MTW. And I have yet to figure out what they're talking about half the time

I saw this while scrolling through and I find it much funnier than it probably should be

Notation that isn't easy to write by hand always makes me roll my eyes lol

Not as bad as the Fraktur symbols for Lie algebras I suppose

Or $\xi$...such an annoying thing

Wald is pretty mathematically sophisticated

Wald is the main common one that I haven't seen

Actually there's another, but I don't remember the name. The one that isn't Carroll or MTW

Carroll was decent compared to other ones I've seen though

Schutz?

Hartle?

That's the one

That's the two

these lectures start from topology...

what is this, a mathematician's approach?

everyone knows you just "use the math" where it suits you

The first time I took a GR course, I think I about went insane trying to figure out what tensors were. It was a good 2 years before I saw a mathematical definition and they made a lot more sense

so

By first time I mean the only one. And by GR course I mean a course that briefly covered GR along with SR and cosmology

what's a tensor?

The understanding I have now is that it's $T: TM \times \cdots \times TM \times T^*M \times \cdots T^*M \to \mathbb{R}$ where $TM$ is the tangent space and $T^*M$ is the cotangent space of a manifold $M$...then you get to define a manifold and those spaces :D

uhoh

Actually it may be the (co)tangent spaces at a single point

Looks suspiciously like a tensor field to me...

But it makes me feel a lot better than "some geometrically invariant sort of thing with indices"

Is that a tensor field in disguise as a tensor?

jk jk I won't make you append all the |_P's lol

ahh good that's what it was

you gotta specify the multi-linearity though

Non-linear mappings of vectors and co-vectors to real numbers are not Tensors

this is what you get for trying to be the mathematician

Ahh right. I always seem to ignore think of linearity as obvious and leave it out for some reason

Are non-linear maps like that their own thing? Or do they just get called maps?

"determinants of matrices are multilinear functions of the columns" gets forgotten surprisingly often