The h Bar

One of my profs once said there's more physics in a cubic centimeter of a neutron star than in entire subfields of physics, and I'm pretty sure he's right.

0celo7

like I've heard dark energy violates energy conservation...but how is that energy even defined

FenderLesPaul

6:13 PM

@ChrisWhite I've heard that said before yeah

a grad student here described the different physics that goes into his neutron star research

and it was a craaazy variety of physics

user54412

@0celo7 $T^{00}$ is a pretty good start

Slereah

I heard a stress energy tensor so I ran here

Dark energy is proportional to the volume

When the universe expands, you get more of it

Or less of it, more accurately

Since it violates the NEC

0celo7

less of it?

Rigor

< it

0celo7

ok but I thought there is no universal definition of energy in GR, so how and why is dark energy defined that way

Slereah

6:18 PM

what way

Dark energy is just a term of the form $\Lambda g_{\mu\nu}$

It's not too hard to see that it is $\propto V$

0celo7

what is the definition of energy in general

Danu

@0celo7 It's just local.

We're talking about energy densities here

Slereah

yes

Danu

Just defined through $T_{\mu\nu}$

Slereah

Energy would be $\int T_{00} dV$, but that varies with coordinates

Dark matter itself isn't the reason why energy isn't conserved, though

It's just a consequence of the dependance of the FRW metric on time

FenderLesPaul

6:23 PM

@0celo7 it's conserved if you include exchange between the matter fields and gravity

0celo7

can you show that?

FenderLesPaul

well you'd have to first settle on a definition of gravitational energy

which has to be quasi-local

but once you do that yes the rest is easy; the total energy of the quasi-local region is always conserved

by construction

0celo7

so globally there is still no conservation?

FenderLesPaul

in any space-time

well you can't define the gravitational energy globally in FRW, at least not in any usual way

since it's not asymptotically flat

but presumably you could take a quasi-local region

and just keep increasing its size

user54412

Even in a stationary, asymptotically flat spactime with a nice Cauchy surface and everything, if someone says "energy [density]" I can't tell if they mean $T^{00}$, $T^0_0$, or $T_{00}$, nor can I tell if $x^0$ is the coordinate that is constant along the surface, or orthogonal to the surface.

0celo7

6:27 PM

what does "quasi-local" mean then

if you can just make it large

Danu

@ChrisWhite I think the correct definition for the energy density measured by an observer is $u_\mu u_\nu T^{\mu\nu}$.

where $u^\mu=\partial_\tau x^\mu$

This, at least, is the "obvious" scalar to form out of $T_{\mu\nu}$

0celo7

how do we know that the cosmological constant is a constant in time

Danu

@0celo7 We don't

FenderLesPaul

@0celo7 just means a finite region

Danu

not exactly, at least

Rigor

6:31 PM

Where is ACM?

0celo7

he seems to be gone a lot lately

Danu

He got a girlfriend (or boyfriend)!

A bar isn't complete without rumors about the regulars

6

0celo7

why was I not apprised of this

oh my goooood I have LA homework due tonight

Rigor

Get to it.

user54412

@Danu But who is the observer? :p

0celo7

6:35 PM

@ChrisWhite God

user54412

@0celo7 And what exactly is HIS 4-velocity?

0celo7

@ChrisWhite he is the hypersphere, listen to HIM

that is HIS frequency

user54412

Aug 20 at 1:24, by 0celo7

Feb 21 at 18:08, by ACuriousMind

I want to listen to the hypersphere. That is my goal. We can hear sums. So if I add up every single point on the sphere I can hear it. Those graphs are the 3d one. The equation to number 5 is shown above. — user1698948 21 mins ago

Danu

@ChrisWhite Whoever you want. That's why it's a good definition, I guess.

36 secs ago, by Chris White

Aug 20 at 1:24, by 0celo7

Feb 21 at 18:08, by ACuriousMind

I want to listen to the hypersphere. That is my goal. We can hear sums. So if I add up every single point on the sphere I can hear it. Those graphs are the 3d one. The equation to number 5 is shown above. — user1698948 21 mins ago

0celo7

we can hear sums

:'D

user54412

6:38 PM

chat: prepare to find your recursion limit

0celo7

every time

Rigor

Sorry wrong sound.

3 mins ago, by Danu

36 secs ago, by Chris White

Aug 20 at 1:24, by 0celo7

Feb 21 at 18:08, by ACuriousMind

I want to listen to the hypersphere. That is my goal. We can hear sums. So if I add up every single point on the sphere I can hear it. Those graphs are the 3d one. The equation to number 5 is shown above. — user1698948 21 mins ago

0celo7

39 secs ago, by Rigor

3 mins ago, by Danu

36 secs ago, by Chris White

Aug 20 at 1:24, by 0celo7

Feb 21 at 18:08, by ACuriousMind

I want to listen to the hypersphere. That is my goal. We can hear sums. So if I add up every single point on the sphere I can hear it. Those graphs are the 3d one. The equation to number 5 is shown above. — user1698948 21 mins ago

Danu

sigh

Rigor

Have we found the limit?

0celo7

6:46 PM

try iterating!

Jul 24 at 20:40, by Danu

^took it too far... sigh

@Danu not yet

FenderLesPaul

@0celo7 let's work through the problems in Padmanabhan

I have nothing to do this weekend

apart from usual singing lesson

:p

0celo7

@FenderLesPaul ok we'll skype...tomorrow?

FenderLesPaul

@0celo7 mmk

0celo7

I'm busy tonight unless you wanna either A. come to the pizza social with me B. go out with me if I find a group to go out with or C. join teamspeak if I play BF4

doing LAHW now

Rigor

Pizza social?

0celo7

6:54 PM

yesh the nuke dept. gives us lots of free shit

mainly food

Rigor

First football practice, now this?

0celo7

I have things to go to every day of the week for two weeks...

trying out new things

Rigor

Cool.

"Get out there."

2 hours ago, by John Duffield

@0celo7 Good thanks, though seeing as it's Friday night, I have to go. PS: don't stay in your room too much. Get out there.

0celo7

I am!

but I need to work

FenderLesPaul

I wanna get out there

but people are scary

0celo7

6:58 PM

@FenderLesPaul just tell them you can do their calc homework

FenderLesPaul

I already get used enough for HW I'd rather not do it willingly :p

2

Rigor

Start charging

:O

FenderLesPaul

what was removed

I must know

Rigor

Obscene

-_-

Go to the GR room please.

Yeesh!

FenderLesPaul

yeeeeesh

I meet people when doing gigs

but outside of that people are like

super complicated and stuff

Rigor

7:05 PM

Yep, some are more complicated than others.

It also depends on what context they're in...

NeuroFuzzy

new question on "ToA"s. Theory of aaaaaaverything!

Rigor

ToA?

NeuroFuzzy

typo

sic

Rigor

:D

0celo7

sic?

is that the sound you make when you slit a throat?

FenderLesPaul

7:12 PM

eww

0celo7

dude Skyrim is extremely violent

especially with my blood mods :)

FenderLesPaul

-6

Q: Natural Philosophy

My question is an extension of the celebrated question on the moon’s existence if unobserved. “do we still have tides on earth if the moon is unobserved?”

deep

Rigor

Looks like one of those "if a tree falls in the forest..." questions

0celo7

it is

the tree does not fall, unless Einstein saw it

but Einstein saw the moon, therefore there are tides

QED female dogs

damn my book is in but I'm busy

FenderLesPaul

8:01 PM

typing notes is sooo boring

T_T

0B3

Do you type them to learn or to post on the internet?

FenderLesPaul

oh this is for research

so it's mostly for my professor to go over to make sure I didn't fuck up calculations

and to organize things for future publication

0celo7

you probably did

FenderLesPaul

probably

0B3

I had a weird dream again.

0celo7

8:07 PM

maybe you're doing too much QM

0B3

I was reading about these objects called "dirac crystals." too bad they don't exist.

@0celo7 too little, the dreams are probably wake up calls.

FenderLesPaul

8:25 PM

I dreamt I was a carrot

going to battle against an onion

0celo7

dooooone

FenderLesPaul

but we were both killed by an army of pickles

twas very dramatic

0celo7

Pizza social in a bit

Rigor

Any chicks?

0celo7

lol it's a bunch of engineers

probably not

Rigor

8:30 PM

-_-

0celo7

-_-

Danu

9:35 PM

lolwat

Here, the internal variables $y \in \mathbb R^4$ represent an infinite-dimensional vector space; this vector space is a Hilbert space. — kryomaxim 22 mins ago

FenderLesPaul

11:12 PM

o.o

FenderLesPaul

11:39 PM

yay Kiritsis arrived

dat new book smell

0celo7

got Carothers

FenderLesPaul

I was told I should start thinking about my statement of purpose

I hate applications