The h Bar

0celo7

00:22

@FenderLesPaul wow

Come up with a solution in your sleep?

Btw have you done exercise 10.1 in Wald?

That's another one in the "hopelessly stuck" column

Kyle Kanos

There seems to be an abundance of questions involving $\delta T/\delta g_{\mu\nu}$ the last day or two :/

0celo7

@KyleKanos I think my comment on the latest one sums it up

unless is there another one?

shieeeet

Kyle Kanos

He deleted the question in which you commented about it being 0

Then posted this one:

0

Q: What is the value of the stress energy tensor?

If we are living in a portion of space-time where the metric is very close to flat space and we know that the stress energy tensor is negligible at this portion of space-time is it ok to assume that $\frac{\delta T_{\mu\nu}}{\delta g^{\mu\nu}}$ is also negligable?

0celo7

@KyleKanos cf. chat.stackexchange.com/transcript/message/22904417#22904417 for my realization there's a new one

Kyle Kanos

Yes

0celo7

00:28

I still don't completely understand what "X tensor components are small" even means, because they can be made "large" by a suitable coordinate trafo.

Kyle Kanos

Well, you can comment that to him

0celo7

It's a problem I have with all GR treatments

Maybe you can define a topology on the space of tensor components or something

then define a notion of "close"

then define the notion of "close to 0"

Kyle Kanos

It's also funny that he spelled it negligible and negligable in that post.

0celo7

404'd

@KyleKanos I know Straumann, which is ultra pedantic, mentions the problem of what "small tensor" even means, but is pretty much like ¯_(ツ)_/¯ "can't do GR without it"

Kyle Kanos

He deleted that new post to edit the old post

0celo7

00:36

@KyleKanos I obtained that paper from Rindler et. other guy.

The 1971 one.

Kyle Kanos

Karlov

0celo7

I'm pretty sure it has what I need.

I need to review the Fermi derivative.

@KyleKanos yeah

Kyle Kanos

Still had it open in my PDF viewer

0celo7

You could have just sent it to me

Saved me 10 minutes in 95 degree DC weather

Kyle Kanos

It was paywalled

0celo7

00:39

I know

I got it for free at the LoC

They let me make a copy

Kyle Kanos

That was nice of them

0celo7

@KyleKanos Did the paper make sense to you? You've probably never dealt with Fermi transport.

I'm gonna play Skyrim for a bit. Is anyone gonna bother watching? No use streaming if not

FenderLesPaul

@0celo7 sadly that's the only exercise in the chapter I haven't attempted but I can try if need be; any of the other ones I can readily comment on though

and no I haven't solved it yet sorry! busy eating my horribly unhealthy dinner

0celo7

4

Q: Application of Cauchy-Schwarz with Sobolev norms

I'm working through the problems in the initial value formulation chapter in Wald's General Relativity. A short summary of the problem. I have to show that $$\sup_{x\in A}|f(x)|\le C||f||_{A,k}$$ where $A\subset \mathbb{R}^n$ satisfies the uniform interior cone condition, $C$ is a constant, $k>...

real-analysis multivariable-calculus inequality pde inner-product-space

^^As far as I've gotten.

Kyle Kanos

@0celo7 Bits of it made sense, but not really the whole picture

0celo7

00:46

@KyleKanos The big thing is defining what "constant acceleration" even means.

Kyle Kanos

As in is it constant w.r.t. time/proper time/space?

0celo7

it's constant wrt. a fermi transported inertial frame

stream is live btw

@KyleKanos I take that back. Constant direction is what is needed to be defined.

FenderLesPaul

oh I'd be down to watch

where's the link again?

0celo7

twitch.tv/revolver_0celo7

Kyle Kanos

twitch.tv/revolver_0celo7

0celo7

00:53

shit my laptop is really slow

second monitor

FenderLesPaul

dope thanks

0celo7

all mods tonight

until ~9:45

THE PUPPY

George Smyridis

Hey guys. The following question might seem a little idiotic or even pointless.

I thought about posting it... but its really short so it thought to ask you guys

Kyle Kanos

01:08

Feel free

George Smyridis

suppose we have a point moving in space. So then can we say that

v= (1/Δs)(xv_x +yv_y +zv_z) ?

Kyle Kanos

delta s is?

George Smyridis

s is sqrt{x^2 +y^2 +z^2)

I am reading the Feynman Lecture and it has some pretty basic stuff. I know the velocity is the derivative of position, so when i differentiate this pops up (i think).

Kyle Kanos

v = d **x**/dt

George Smyridis

So i wonder if its right and it's indeed that same to sqrt{v_x^2 + v_y^2 +v_z^2}

So i can do it only to its axis?

Kyle Kanos

01:16

It's easier with latex: $\mathbf{v}=d\mathbf{x}/dt$

So if $\mathbf{x}=x\hat{x}+y\hat{y}+z\hat{z}$, then the time-derivative of this is just the time-derivative of the components times the unit vectors: $d\mathbf{x}/dt=dx/dt\hat{x}+dy/dt\hat{y}+dz/dt\hat{z}$

George Smyridis

Ohh thank you. I got it

:D

Kyle Kanos

Which would be $\mathbf{v}=v_x\hat{x}+v_y\hat{y}+v_z\hat{z}$.

Dotting this with itself (to give you magnitude) would be $v=\sqrt{v_x^2+v_y^2+v_z^2}$, as you said

0celo7

01:52

I fixed it.

Well that was a god run except for the end there.

Night guys!

FenderLesPaul

night!

0celo7

@FenderLesPaul Did you see my MSE post above? I'd appreciate you looking at it

FenderLesPaul

@0celo7 sure thang dude

and I'll try to figure out whatever Straumann is trying to do

0celo7

cool beans

obe

02:05

@0celo7 I'm not, I didn't move from the quantum chapter yet.

I told you, starcraft II.

FenderLesPaul

is that the blizzard game about the aliens?

obe

Idc about the story.

DanielSank

03:27

Dat title

(see version before I edited it)

Kyle Kanos

made title good

DanielSank

@KyleKanos You might be amused by some of my edit messages.

What should I have written?

Kyle Kanos

Oh poop, forgot the "lol" bit :(

That was funny

I do things like that alllll the time

DanielSank

@KyleKanos It's all I know how to do to keep myself from depression when editing titles on this site.

Question on energy?

Kyle Kanos

And you'd be proud: my son has taken an interest in cephalopods.

DanielSank

03:32

Electricity: how does it work

@KyleKanos YES!

Kyle Kanos

I showed him the camouflaging one and he was amazed

DanielSank

@KyleKanos I found this book quite interesting.

@KyleKanos Heh, there are so many camouflaging ones! Show him some videos of cuttlefish too. Those things are amazing.

Kyle Kanos

@DanielSank 200 pages is a bit long for a 5 year old

DanielSank

@KyleKanos Of course. I just figured that as parents tend to learn a lot about their childrens' interests that this might be up your alley :-)

Kyle Kanos

Hmm. Does it say anything about how to cook them in the book?

DanielSank

03:37

jerk

Kyle Kanos

:D

DanielSank

I'm sad now.

Imagine someone eating your family dog.

Kyle Kanos

Hmm

Well I don't have a dog...

they're like kids except they never grow up, always depend on you, never get a job and never leave you alone

DanielSank

^ Truth

They do not borrow your car, ask for money, and hate your for ~ten years, however.

Kyle Kanos

I don't forsee any problems with them borrowing the car or asking for money. I'm working to avoid them disliking me

I've also been giving my son an interest in magic

We watch a lot of Penn & Teller Fool Us

DanielSank

03:42

That's a really great show.

Do you have a deck of cards on hand?

Kyle Kanos

Slightly out of arms reach

DanielSank

Want to learn a trick?

Kyle Kanos

sure

DanielSank

Ok, grab the deck and separate out the jacks.

Kyle Kanos

Done

DanielSank

03:45

Ok, take the four jacks in a little four card pile facing you, and put a fifth card at the bottom.

In practice you have to do this real sly so nobody knows there's a fifth card.

Kyle Kanos

Face up or down?

DanielSank

All cards facing you.

You just have a little five card pile, which everyone else thinks is a four card pile of jacks.

Kyle Kanos

Okay...so what's the next step?

DanielSank

Ok, so you have this little pile. Hold it with your fingers on the long edges, palm toward you, and with your other hand peel off one card at a time. In your head, you are counting the cards for the audience.

So, you hold the pile and show the top card, that's 1.

Peel off a jack and show the next, that's 2.

Peel another showing the next jack, that's 3.

Peel the third revealing the fourth (with the fifth secret card behind it), that's four.

At this point you have three jacks in your left hand and two cards in your right - a jack with a secret card behind it.

Ok?

Kyle Kanos

Got it

I guess the stack should be J J J 3 J (assuming I picked a 3)?

DanielSank

03:57

Exactly!

You put those peeled off jacks behind the fourth jack and the 3.

Now place that little stack on top of the deck, face down with the rest of the cards.

Now pick up the deck.

These are circus jacks which do amazing tricks.

Deal out the top four cards face down in a row.

The jacks are lining up for their performance.

You now have, face down in a row, J J J 3 and the deck has a J on top.

ok?

Kyle Kanos

Got it

DanielSank

Ok.

"Jack #1 is the one who can climb instantly from the bottom to the top of the deck"

Put the deck down on J #1, flourish, and reveal that he pops off the top of the deck!

Pick up the deck making sure not to show the bottom card.

"Jack #2 starts on the top and dives down into the little pool of water on the bottom"

Put Jack #2 on top, flourish and riff the cards or whatever, and with a snap flip the deck over. The jack appears on the bottom!

Kyle Kanos

I see where this is going. The 3 will go in the middle?

DanielSank

Yes. Stick #3 face down in the middle while the rest of the deck is still face up.

(This one is the trapeze artist who flips from the middle to the top coming off the top in the other direction)

Then the bogus card is shoved into the middle and you spread the deck showing that he (the contortionist) has inverted himself!

This always surprises people, it takes 2 minutes, and almost no setup. It's the most effective trick I know for a party.

You can precede this with whatever trick you want which gets someone else to "find" the jacks for you.

or whatever

Kyle Kanos

That's clever

I'll have to practice that some

DanielSank

04:03

Yeah, it's awesome.

Don't repeat it if people ask you to. They know what to look for the second time :-)

Your 5 year old on the other hand...

Kyle Kanos

He's sharp though. He caught on with the 2 cards & 1 coin trick

On the 3rd try too

DanielSank

@KyleKanos Don't know that one.

Kyle Kanos

It's simple: take any two cards & a larger coin (quarter, half-dollar, sakagewa dollar, etc)

Layer as ||o (|=card, o=coin)

pinch with the fingers, reveal top card & place back on top of 2nd card

Reveal 2nd card, place back beneath the coin: |o|

Then just slide the coin out between the cards

Works better if you show the cards with different hands (less obvious that way)

DanielSank

In ||o what's on the bottom?

Kyle Kanos

top -> bottom, so coin's hidden at the bottom

So no one sees the coin & it just appears between the two cards

DanielSank

04:08

So I pick up top card. Audience sees back of bottom card.

Kyle Kanos

Right-o

DanielSank

Then I replace top card.

Then with other hand slide second card out, and replace.

But replace under coin.

How to reveal coin?

Kyle Kanos

Yes..You've gotta transfer the pinched coin from one hand to the other

DanielSank

Lift up top card again?

Oh, wait a sec...

Kyle Kanos

I do the reveal as letting it slide between the two

DanielSank

04:10

I thought this stack was sitting on a table.

Kyle Kanos

Do a quick flick on the two or wiggle them or something, then have a helper hold their hand out

DanielSank

It's in my hands?

Kyle Kanos

In your hands

DanielSank

I'm trying to figure out how to make this surprising.

How to hold stack?

Kyle Kanos

Pinched between a thumb & index (maybe middle too) fingers, face down

Longways is along your line-of-sight

DanielSank

04:12

Ah.

Now I see.

Have to set up subtly!

Kyle Kanos

Yes you do

DanielSank

In high school I met a guy who was good.

He would do tricks in the hall, etc.

Kyle Kanos

Holy crap...it's after midnight!

No wonder I'm tired...

DanielSank

One time he did a trick surrounded on all sides by spectators, at the end, he asked the victim for the time.

Kyle Kanos

I gotta get to bed

DanielSank

04:14

Victim looks at watch, find giant coin under wrist band.

Good night.

skill patrol

Later pal.

vzn

05:05

10

Q: Statistical analysis of data in Physics

Analysis of data is integral in bridging the gap between theory and experiment. How much do the results of the analysis depend upon the choice between Bayesian and frequentist methods? For instance, consider experiments in particle physics. This paper by Louis Lyons states that particle physicis...

experimental-physics statistics data-analysis data

Danu