The h Bar - 2015-05-22

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user54412

2:29 AM

@JulianMoore If you are clarifying something, then you can just edit what's there. If some new question has come up, even if closely related, I suggest starting a new post.

user54412

As much as possible, the question to be answered shouldn't be a moving target, lest people grow wary of being sucked into a longer thread than they thought they were entering.

user54412

So apparently Biology has graduated too

5

4:24 AM

0

Q: Can I undo a flag? Has this feature been added?

I accidentally clicked the flag button while on mobile. I have done this twice now and it would be nice to be able to undo the flag within a 5 minute window or something if I accidentally clicked it. Just like I can edit comments within 5 minutes before it's locked in. There has been discussion...

feature-request

2 hours later…

6:32 AM

@DavidZ shall I delete this question then? It sounds like it isn't going to be very useful?

@DavidZ and if you can do anything about my accidental flag that would be great. I accidentally marked as spam.

6:51 AM

@StanShunpike either way is fine; it doesn't hurt to have it there.

and I declined the flag. No harm done.

7:12 AM

@DavidZ super! Thanks. Okay, I will leave it for now.

@DavidZ so both chem and bio graduated?

Was the division to graduate them made at the same time?

They are sort of the other main "hard sciences" so to speak.

7:35 AM

I don't know anything about the timing of the decision, but you can check on their site metas and see how closely the graduation announcements were timed

Maybe @ManishEarth knows something

8:15 AM

@ChrisWhite Thanks - it's the same question, same fundamental point so it should stay where it is.

2 hours later…

10:03 AM

@StanShunpike no idea cc @DavidZ

We were told a while back that we would be graduating soon, and then the designs turned up

@StanShunpike you mean "stamp collecting worth talking about" :P

10:50 AM

@alarge Thanks a lot for the algo. I'm not sure why we don't get the same value at r=0 as in the article though, since we chose the same parameters ...

@ManishEarth Relevant xkcd

11:47 AM

Yay, 'nother shiny gold badge aka "certification that I spend too much time here".

12:06 PM

@ACuriousMind yeah, that was what I was referring to :p

@ChrisWhite It's ready

12:33 PM

@ACuriousMind You and I should get gold badges for spending way too much time in chat.

oh hey! I just logged in to procrastinate from homework.

Holy crap, you're on a decent EST time!!

It seems like I only talk to you at 10PM+

huh, I guess! I pulled another all-nighter after finishing my quantum monte carlo stuff

you will probably never see me on at this time again :D

hopefully...

5AM?

What time is it for you?

12:36 PM

yep!

It's 8 on the east coast.

Sitting in class, trying to read HE. People are too annoying though.

The pacific looks nice this morning!

Which UC are you at again?

san diego

Ah, I'll be in LA for Labor Day and Thanksgiving, visiting my sister.

12:41 PM

so, my girlfriend lives in LA. Guess which famous physicist her old highschool carpool's brother (or uncle or something like that) is?

"ed" :p

six degrees of kevin bacon and what not

@ACuriousMind 102 pages into HE. It's enjoyable, yet vague at times. No eureka moments yet.

@NeuroFuzzy I don't know any famous physicists in that age range. Maybe I do. My answer is: no clue

@0celo7 By "carpool" i mean the driver not a schoolmate, so there's no age range restriction. ED. Ed! THAT ed.

Witten!

Oh, the man who sounds like a woman?

lol, that's direct

His voice is pretty feminine. Good physicist though.

12:56 PM

Well. Back to doing physics! I wish class was over already so I could read useful books.

3 hours later…

4:15 PM

@ACuriousMind Do you have access to HE?

@0celo7 In electronic form, yes.

@ACuriousMind I need help on the proof of Prop. 4.5.1, found on p. 104.

I don't understand the first part of the second to last sentence.

I don't understand why that curve is timelike.

@0celo7 Have you written down the norm of the tangent vector?

Timelike means negative norm, right?

Yes, of course. I'm confused by the construction of Y. I don't see why I need the fact that the exponential map is timelike.

Wait, does the exponential map preserve timelike, null, etc.?

@0celo7 I believe it does.

It's the identity in a small neighbourhood, after all :P

4:26 PM

Ok, so we know that Y is timelike. So all I have to do is show beta has a timelike tangent?

Because if beta is timelike, then the exponential map is a timelike curve.

@0celo7 I believe so, and that follows from the assumption on $g(Y,\partial_t)_\gamma$

(together with timelikeness of $Y$ and $\gamma$)

What I don't get is that identity is defined at the point q. Is the point q related to r at all?

When I do the norm of beta, I get a term with the norm of gamma's tangent, but at r.

@0celo7 $r$ is not a point, it is the parameter of the curve $\beta$.

The notation is confusingly chosen

I know it's not a point, but since it's the curve parameter, doesn't q correspond to a specific r?

@0celo7 Yes, $r=0$, since $\beta$ is a curve in $T_q$.

And the origin of $T_q$ is mapped to $q$ under the exponential map

4:31 PM

Ok, so why may I assume the inner product of Y and d/dt is negative for all r?

Is it possible I only have to show beta's tangent is timelike at r=0?

It is possible, but I do not immediately see why that would be

Do you agree that with the given info, we can't do any better?

I would agree, yes

This may be a typo, I don't see any particular reason why the definition of Y has to be restricted to q.

@0celo7 I'd be inclined to say one can extend the assumption to all $t$, but...well, you'd have to show such a vector field actually exists.

4:44 PM

Any ideas?

I wouldn't even know where to start :P

Perhaps I'll have a eureka moment while I make dinner, but I doubt it.

5:21 PM

@pZombie: Not just Einstein, but anyone proposing a new theory would get their post closed as non-mainstream on this site (which probably happens a lot more often than you think). We are not a replacement for peer-reviewed journals, period. We do not advance science here, we correct misunderstandings of known & accepted physics. — Kyle Kanos 19 secs ago

0

Q: Splitting EmDrive questions into reference-request and reference analysis

We just got another EmDrive question, Emdrive in relation to newest NASA article - Does it extract energy out of the QM vacuum?, and much as you would expect it swiftly got closed and an OP probably went away feeling fairly upset. I feel we get enough EmDrive questions that this is something we o...

discussion non-mainstream-physics

5:41 PM

@ACuriousMind Holy crap, the next proposition is crazy. He doesn't explain the notation or half of the steps.

5:55 PM

@ACuriousMind 1. I don't know why rho is the square root of sigma, it's not like you can move a square root inside of an integral. 2. Why is the definition of lambda in terms of the exponential map admissible? Isn't any curve constructed from the exponential map a geodesic? 3. The heck is f(t)? 4. How is sigma here the same as in lemma 4.5.2? There's no integral there.

5. I don't understand that decomposition of the tangent vector to lambda. 6. Why greater than in the first equation that uses an inequality? 7. I obviously don't understand the last relation because I don't know where f(t) came from.

:/ I don't follow this one at all.

6:15 PM

@0celo7 Uh, well, it isn't obvious to me either from just reading it.

Lol, none are immediately obvious?

Yeah, I just wanted to say that I can't help you here :P

6:40 PM

@ACuriousMind Does that mean you won't give it any more thought?

It's one thing to not understand a little detail in a proof, but in this one I do only understand a single step.

@0celo7 Right now, at least, I've other things to read and think about, sorry

@ACuriousMind Is the metric constant in a normal neighborhood, i.e. can I move $g$ outside of the integral?

@0celo7 I don't know what a normal neighbourhood is

@ACuriousMind That's not good.

7:17 PM

@ACuriousMind See Straumann Sect. 15.3 if you're interested.

Geodesics in normal neighborhoods have interesting properties, and might just be the key to this!

I have a long weekend, so I will solve this problem.

I think the key is that the coordinates of the geodesic in a normal neighborhood are $v^it$, where $v^i$ are the components of the initial velocity.

This is proven in Straumann, I don't think HE ever states it.

7:41 PM

@0celo7 You don't have a searchable HE?

I have a scan.

I have a searchable, and a print, will that be of any assistance to you?

I'd like the searchable. I'm not sure if it will help in this case, though. This is just a really tough proof that they should have taken at least a full page for.

Out of time.

8:36 PM

@Icosahedron Out of time?

9:15 PM

@0celo7 You missed it?

1 hour later…

10:31 PM

From what I hear & understand, OP has the same problem every other CS major has: if it's not being done "this" way it's wrong & I need to fix it.

Fortunately the highest two voted answers tell him he's wrong and just to (more or less) ignore it

91

Q: Where should my team start with becoming "modern"?

I'm a relatively new developer, fresh from college. While in college and during subsequent job-seeking, I realized that there were a lot of "modern" software development methodologies that my education was lacking: unit testing, logging, database normalization, agile development (vs. generic agil...

agile software-developer professional-development

10:50 PM

@Icosahedron I was running errands.

11:36 PM

@Icosahedron You on /sci/?

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