The h Bar

ACuriousMind

00:00

@Sofia Done

user54412

@ACuriousMind What's wrong is that he's intuiting parallel transport wrong

user54412

If what looks curved is a "straight" geodesic, then one should expect that parallel transporting that spin arrow won't do what the diagram makes it do

ACuriousMind

That makes sense.

user54412

That is, he's keeping the projection onto the spatial tangent the same, rather than the spacetime tangent

ACuriousMind

@0celo7 That sounds less like "GR for mathematicians" and more like "GR for people who want to hate GR"

Sofia

00:03

@ACuriousMind Many thanks. Now, just for rescuing me from my ignorance, can you tell me why was it expectable that the object spin?

0celo7

@ACuriousMind "Pseudo-Riemannian Geometry with Applications to Physics"

ACuriousMind

@Sofia The user is using the intuitive statement that everything follows "straight lines" in GR. If something is following a straight line, it will always point in the same direction compared to its velocity. If the object didn't "turn", it would look "backwards" after making the curve around earth, and hence not follow a straight line

As ChrisWhite says, this is wrong because the "velocity" you have to think about in GR is the four-velocity, not the spatial velocity.

0celo7

@Sofia The straight lines in GR are straight lines in 4 dimensions, not 3.

Sofia

@ACuriousMind and @0celo7 thank you both!

0celo7

I have two lab reports to do.

Sofia

00:11

@ACuriousMind and @0celo7 Aaaaaa! So, what you both say it that in the vicinity of the Earth the space-time is not so terribly curved! It's that what you say?

0celo7

But they don't have to be typed!! :: dances ::

Time to procrastinate! (Procrastinate = learn QFT/CFT)

ACuriousMind

@Sofia It's not so terribly curved that you can't mostly get away with pretending it is not curved at all

But for precision calculations like GPS or something you need to take the curvature into account

0celo7

@Sofia @ACuriousMind in the lowest order approximation we say that time is curved around the Earth.

The next highest is a spatial curvature.

Sofia

@0celo7 I lost you both.

0celo7

I'll explain. Have to type.

ACuriousMind

00:15

@Sofia Let's say "not so terribly curved" is a good description. The earth is far from being a black hole, fortunately

0celo7

The action of Newtonian gravity is $$S=\int dt(\tfrac{1}{2}mv^2-m-m\phi)$$

For small $v$ and $\phi$ we have $$S=-m\int dt(1-\tfrac{1}{2}v^2+\phi)\sim -m\int dt\sqrt{1-v^2+2\phi}=-m\int \sqrt{(1+2\phi)dt^2-dx^2}$$

Sofia

@ACuriousMind GPS? It's an apparatus that my family uses for finding the best way on the roads. What does that have to do with gravity?

0celo7

The last term is just the length of a curve in a curved time metric

Sofia

@0celo7 slowly please!

0celo7

@ACuriousMind Slick, eh?

Sofia

00:18

@0celo7 yes! And?

0celo7

@Sofia And what? That's the fastest motivation I know of that shows curved space-time is reasonable.

ACuriousMind

@Sofia The GPS satellites you use for the navigation need to take into account that time is slowed when you are deeper in a gravitational well. That is one of the best things to tell people when they ask you why they should believe in GR - they find their way with it every day!

Sofia

@ACuriousMind Whaaat? Do you want to tell me that the GPS uses General Relativity?

ACuriousMind

@Sofia Yes. Yes, it does!

user54412

What's interesting is that while that question is wrong in spins flipping 180 degrees in parabolic orbits, there is probably some precession

0celo7

00:21

@ChrisWhite I forget...is spin Fermi-transported?

user54412

@0celo7 is Fermi-transport another word for parallel transport?

0celo7

Naw. I'll look up the definition.

Sofia

@ACuriousMind Ooooo! The people that program the GPSs have to know GR?

0celo7

@ChrisWhite While looking up the definition I rediscovered that it is Fermi-transported.

ACuriousMind

@Sofia I think they just need one or two formulae, so they don't need training in GR, but they use its formulae, yes.

0celo7

00:24

@Sofia GPS uses very carefully calibrated timers to locate things. GR screws with those timers.

Sofia

@ACuriousMind many, many thanks. I am very impressed. I didn't know all these things.

0celo7

@ACuriousMind I wonder if they have to account for the slight rotation and oblateness of the Earth. @ChrisWhite do you know?

Sofia

@0celo7 and many thanks to you too.

ACuriousMind

@Sofia: From Wikipedia: "In 1956, the German-American physicist Friedwardt Winterberg proposed a test of GR - detecting time slowing in a strong gravitational field using accurate atomic clocks placed in orbit inside satellites. Calculations using GR determined that the clocks on the GPS satellites would be seen by Earth's observers to run 38 microseconds faster per day, and this was corrected for in the design of GPS"

GPS is really my favourite everyday example for why classical physics is not enough

0celo7

@ACuriousMind I never told you that I like cats too.

ACuriousMind

00:26

Along with the fact that the sun burns, that is :)

0celo7

@ACuriousMind Can you come up with another reason for why GR is useful?

user54412

@0celo7 The Newtonian potential is certainly measurably affected by oblateness, so yes. Frame dragging, on the other hand, has proven very difficult to detect even with dedicated instruments.

0celo7

But the sun burns whether or not we spent money on nonclassical research.

ACuriousMind

@0celo7 Useful? Not so much. The classic perihelion shifts or whatever they're called are nice examples, but not very useful.

user54412

@ACuriousMind And Sputnik wasn't launched until 1957!

Sofia

00:27

@ACuriousMind Hmmm! What you say! Very impressing. But, is it indeed GR what is used, or SR (special relativity)? Isn't the latter enough?

0celo7

@Sofia there is both gravitational and velocity time dilation.

ACuriousMind

@Sofia No, SR is only for electromagnetism+moving things. As soon as you have gravity in any sense, you have GR

user54412

The GR effect might be larger?

0celo7

@ChrisWhite Do modern high-precision GPS systems use high-order post-Newtonian calculations then?

Sofia

@ACuriousMind and @0celo7 A lot of thanks to you both. I'll have a look at that page in Wikipedia.

0celo7

00:29

@Sofia You can always learn GR you know :D

ACuriousMind

@0celo7 Yeah, I wasn't saying that's an example for the practicality of non-classical physics, I meant that's one glaring example of non-classical physics literally staring you in the eye everyday

0celo7

@ACuriousMind Dude, nucleosynthesis.

Life is not classically possible.

ACuriousMind

If you only spend money on useful things, I don't wanna talk to you anyway, I think ;)

user54412

@0celo7 I have no idea. Honestly, I sometimes wonder if they don't just have some heuristic correction algorithm (something that minimizes squares of residuals, say).

0celo7

@ChrisWhite They just adjust parameters until they know they have the correct position of a fixed, known, target?

user54412

00:35

@0celo7 Something vaguely like that. I'm not saying that's how it's done, but I could believe it.

user54412

If you were already doing something like that anyway, say to fix all sorts of poorly-quantified other uncertainties the real world throws at you, you might be able to get away with correcting for GR at the same time.

0celo7

@ACuriousMind What in the world does the special conformal transformation do?

Inversion, translation and inversion...

What?

ACuriousMind

@0celo7 It, uh, does something like translation and then doing an inversion on a circle. It's difficult to visualize

The best "explanation" I've found is that it is a transformation that does nothing to spherical waves.

But I don't think anybody has a good intuition for it, I guess that's why it's special ;)

0celo7

@ACuriousMind So the conformal group on the plane is the Lorentz group of spacetime?

COOL

ACuriousMind

@0celo7 What?

Where did that come from?

0celo7

00:44

The conformal group in $d$ dimensions is isomorphic to $SO(d+1,1)$

So in two dimensions it's isomorphic to $SO(3,1)$

Am I wrong?

ACuriousMind

@0celo7 Yes :D

0celo7

Wth

Pls explain

ACuriousMind

The conformal group in 2D is not $\mathrm{SO}$ at all

Two dimensions are special, and they are where CFT really shines

In two dimensions, the conformal group is infinite-dimensional

0celo7

Ah, this is the $d\ge 3$ chapter!

Oh, I see that sneaky (d-2) factor!

Sofia

01:06

@0celo7 I have no reason to learn GR. I would be happy if I could learn what is the wave-function, and how works the collapse.

0celo7

@Sofia Magic. (And projection operators.)

ACuriousMind

::Channeling @Jimdalf:: Magnets

Sofia

@0celo7 What you say!!!

@0celo7 I'll make a communication to Stockholm about your discovery.

0celo7

Do I get an acceptance speech? I have to thank @ACuriousMind for teaching me clopen sets.

@ACuriousMind This book (Di Francesco) is superb. I was a fool for looking at string theory before reading this.

ACuriousMind

@0celo7 See? Jamal and I told you CFT is awesome :)

0celo7

01:18

Not saying that yet, just this is better than trying to reconstruct CFT from nothing

@ACuriousMind So does the Ward identity relate classical field relations to symmetry operators acting on fields?

ACuriousMind

@0celo7 It basically tells you what the handwavy "Classical laws are implemented as operator laws" really means in the case of symmetries, yes

0celo7

@ACuriousMind Why does the energy-momentum tensor have scaling dimension 2?

Something about it being a density? What's up with that?

Kyle Kanos

I'm voting to close this question as off-topic because the question is of a too low level in physics. — Sofia 22 mins ago

@Sofia: You really need to stop using that as an excuse

It is not a valid close reason

ACuriousMind

@0celo7 Just look at how $T^{\mu\nu}$ transforms under general diffeomorphisms, and observe that the conformal transformations, especially scaling, are just special cases of them

Kyle Kanos

The Help Page says (emphasis mine) Physics Stack Exchange is for active researchers, academics and students of physics and astronomy. We welcome questions of all levels, but please stick to the following topics:

0celo7

01:31

@ACuriousMind Oh yeah. So does $T_{\mu\nu}$ have -2 scaling dimension?

@Sofia Didn't you chew some of us out today (possibly yesterday) for not answering a novice question?

Sofia

@KyleKanos How can it be *not a valid reason"? Questions showing that a poster has no idea of physics shouldn't be closed?

ACuriousMind

@0celo7 Mh, probably. I've never seen CFT with indices, you don't need them in 2D, really.

@Sofia No, that is not a valid close reason. That's a reason for downvoting (if it shows no effort)

Kyle Kanos

@Sofia It's not valid because all questions are welcome (so long as it conforms to the policies set forth). And it is totally okay for the poster to not know physics

We're here to try answering their queries

0celo7

@KyleKanos all questions within set rules

Kyle Kanos

^ That too

Sofia

01:36

@KyleKanos It seems to me contradictory with other things that you or the moderators told me.

Kyle Kanos

There is no such thing as "too low level"

ACuriousMind

It is a bit inconsistent though that "insufficent effort" has been used as a close reason.

Kyle Kanos

I disagree with insufficient effort closures

ACuriousMind

@KyleKanos Me, too, but I've seen John Rennie and others successfully close a question with that

Kyle Kanos

I know.

Sofia

01:38

@KyleKanos how a question that asks how to transform Joules in ergs (or something similar) may remain? I was always told that questions have to be useful for the public of this site.

@KyleKanos the public of this site is not of such a deplorable low level.

ACuriousMind

@Sofia The questions and answer here shall be useful for a generic public - not necessarily for the people who frequent this site

0celo7

If one of the goals of the site is to establish a database of questions and answers, it doesn't make sense to me to close for "insufficient effort".

ACuriousMind

I seem to recall some SE blog post stating that, eventually, most of a site's traffic should come from search engine rather than SE members

Kyle Kanos

Oddly, the bare poles on my "Insufficient Effort" Meta.Physics post show that not closing is more of a consensus.

ACuriousMind

@0celo7 Yeah, to me neither. That's what downvotes are for.

Kyle Kanos

01:40

+13/-6 for leave open, +7/-3 for VTC

Sofia

@KyleKanos you please tell me how can I not close a question that revolts me

Kyle Kanos

Easy, click out of the page.

ACuriousMind

@Sofia By downvoting it and then ignoring it

Sofia

@KyleKanos why do you mock me?

0celo7

@ACuriousMind What's your personal view on "homework-like" questions?

Kyle Kanos

01:42

I'm not trying to mock you. You asked a serious question and I gave a serious answer

That a question revolts you should be irrelevant to its closure

Sofia

@KyleKanos the answer wasn't serious. I stupid question is much more severe fault than a hard question/home exercise that the user didn't know how to handle. There has to be a reason and consistency in the rules here.

ACuriousMind

@0celo7 I close them. Solving/answering a homework problem is not really useful for anyone but someone stuck at precisely the same point. Sometimes I feel a little bad about it when the asker has really put in a lot of effort typing up all equations in LaTeX and such, but mostly they're lazily typed or even copypasted/photographed, and I don't feel sorry for those at all.

0celo7

@ACuriousMind That's your feeling on the matter, without regarding SE's rules?

That's what you would do if you were king?

Sofia

@KyleKanos I saw homeworks that needed indeed some trick that I couldn't expect from novices. But the questions were closed. And how to transform from units to units is acceptable? Where is the logic?

ACuriousMind

@0celo7 Yes. Answering homework questions would turn this into a very different place. One look at math.SE tells me that I would keep it the way it is here.

0celo7

01:47

OMFG 2-hour delay tomorrow

I haven't been to school in a week!

ACuriousMind

Really, I felt worse about closing homework questions before I tried to look at math.SE

0celo7

@ACuriousMind I want it so I can ask homework questions.

Kyle Kanos

@Sofia You can just click out of the page. There is nothing forcing you to stay there and close it.

It's a serious answer

0celo7

You can't answer my GR/ST questions :(

I see your point regarding Math.SE though. I've seen the same analysis problems from Rudin 100 times.

Sofia

@KyleKanos where is the logic? Don't you see that these two rules clash? You didn't answer me.

Kyle Kanos

01:49

Provide links, I cannot say with that

ACuriousMind

@0celo7 The main point is that for every HW question I'd like to answer there'd be a hundred I'd like to kill with fire, I think

Sofia

@KyleKanos leaving something unbearable like how to transform units is O.K. and a difficult and tricky home-work should be closed. How so?

Kyle Kanos

I imagine one asked it in the right way and the other didn't

Someone who's stuck on a HW problem (regardless of level and/or "tricks" needed) should not be asking here

0celo7

@ACuriousMind Perhaps HW questions are allowed with 1500+ Rep? For instance, I don't think my HW questions (chat questions are not HW questions) are worthy of fire killing, but they have been in the past.

Sofia

@KyleKanos I don't solve home-works, I give some indications, but to close an interesting home-work and leave stupid and infantilic questions?

ACuriousMind

01:51

@Sofia You could close a simple unit-conversion question with the homework close reason, I think. Just not with the reason you gave of "too low a level"

Kyle Kanos

Someone who doesn't understand the difference between Unit A and Unit B can ask here

@Sofia I disagree with the notion that homework problems are interesting.

And I also disagree with the notion that the remainder are stupid or infantilic

People have difficulties with lots of things

Units is one of them. Vectors another.

0celo7

@KyleKanos In Straumann I tried to solve a problem dealing with nongeodesic orbits in Schwarzschild spacetime. Is that uninteresting? (It was closed for homework reasons.)

ACuriousMind

And again, downvote questions you think are stupid (after considering if you couldn't have asked the same question some time in the past)

Kyle Kanos

@0celo7 If it's phrased as, "How do I solve this?" then yes

Sofia

@KyleKanos If you disagree that home-works may be interesting, I will have the pleasure to show you, when I meet something that indeed I like.

ACuriousMind

01:54

@0celo7 That's...not how it works, unfortunately. The janitorial priviledges are one thing, but to grant the right to ask certain questions by rep absolutely smashes the idea of "anyone can ask and answer here", which I think is one of the most important promises here

Sofia

@0celo7 Bravo!!!!

0celo7

@ACuriousMind @KyleKanos You both closed this. Can you please tell me exactly why?

1

Q: Non-geodesic circular orbit?

From N. Straumann, General Relativity Exercise 4.9: Calculate the radial acceleration for a non-geodesic circular orbit in the Schwarzschild spacetime. Show that this becomes positive for $r>3GM$. This counter-intuitive result has led to lots of discussions. This is one of those problems wh...

I never did figure it out, BTW.

Sofia

@KyleKanos there are many elegant home exercises, really pleasant and interesting.

infinitesimal

@ACuriousMind I agree.

0celo7

Random other question: is it kosher to use editing privileges to fix a single typo?

Sofia

01:56

@0celo7 you speak by my heart!

Kyle Kanos

@0celo7 It says in the close reason: it's a homework-like question in which you are asking us to help solve a problem. That's not the kind of site we are

0celo7

But I'm asking about a specific concept!

ACuriousMind

@0celo7 You're just asking how to solve a problem. There's no "conceptual question". I realize that the nature of advanced calculations is such that it can be that there is no conceptual issue, but that's just how it is. And, again, answering that would help almost no one but you and people working on the exact same exercise.

Sofia

@KyleKanos and @ACuriousMind you have to leave it to our judgement, we that try to show the users smart solutions, new ways to approach the things.

ACuriousMind

@0celo7 What's the concept? (really, I don't see it)

0celo7

01:58

@ACuriousMind It had nothing to do with the calculation. I didn't know how to approach the problem. That's a conceptual issue in my mind.

Kyle Kanos

@Sofia Interesting to you. I don't find them interesting. I find conceptual questions interesting.

0celo7

@ACuriousMind How to calculate a nongeodesic orbit.

I'm not even sure what that means. (nongeodesic orbit)

Sofia

@KyleKanos Who told you that a home-work cannot be conceptual? Who told you that it cannot teach other people new things?

ACuriousMind

@0celo7 Aha! So, the conceptual question would not be "How do I solve this problem?", but "What is a non-geodesic orbit, why is it interesting, and what is its acceleration?"

The latter question, without a reference to the specific problem, would likely not have been closed

Kyle Kanos

@Sofia I am using homework as defined for this site.

0celo7

02:01

@ACuriousMind I really want this question answered. Can I delete this one, write a new one and have you look over it?

Kyle Kanos

It is slightly different than "A problem out of a textbook." (textbooks often have conceptual problems)

ACuriousMind

@0celo7 Sure. You could even just edit it, the reopen queue works, although people like to complain it does not. (Posting in chat about it increases the speed of potential reopening, though :P)

Sofia

@KyleKanos can you indicate where I can find the definition?

0celo7

@ACuriousMind Nah, I'll scrap it.

Kyle Kanos

50

Q: How do I ask homework questions on Physics Stack Exchange?

What is the policy on asking homework questions on Physics Stack Exchange? What kinds of questions are considered homework questions? Are homework questions allowed? What should I include in a homework question? Why don't you provide a complete answer to homework questions?

discussion faq homework

0celo7

02:02

So I can't even mention that it was a HW problem that motivated this?

@KyleKanos That summary reads in my mind: "Self-studiers: sorry, but you're screwed."

Kyle Kanos

@0celo7 You can, but solving it can't be the main point. It might be okay to ask, "I got this as my answer, but I don't know what it means."

ACuriousMind

@0celo7 Sure you can mention that, no problem at all, if you think it adds to the post. Some people have an itchy finger on the close button when they see a pasted excercise, though.

Kyle Kanos

@0celo7 The self-study aspect has been brought up a few times.

ACuriousMind

But, generally, a good way to distinguish a HW from a non-HW question is - if I remove the specific excercise, is there a question left?

Kyle Kanos

That lead Qmechanic editing this post (at least I think it did):

9

Q: My question was closed on Phys.SE. Can you recommend me another internet site where my question might be on-topic?

My question was closed1 on Phys.SE. Can you recommend me another internet site where my question might be on-topic? Here we keep a list of other internet sites that might help students2 of physics. One site per answer. To keep the list at a reasonable size, please only include sites which fulfil...

discussion closed-questions homework

0celo7

02:05

/r/Physics is just as bad as here.

And their TEX is horrible, but that's beside the point.

Kyle Kanos

I've had nice interactions on Physics Forum

But that was nearly a decade ago

0celo7

I've seen blatantly false answers.

And without the up/downvote system, there's no way to tell (unless you know the correct answer ofc)

PhysHW.SE

pls?

ACuriousMind

@0celo7 I think someone tried something like that once

0celo7

@ACuriousMind Title: What is a nongeodesic orbit?

ACuriousMind

@0celo7 But do you really want that site just to get your questions answered? Think of how horrible the frontpage would look. Parabolas, electric circuits, finding the time something takes to fall. Over, and over, and over

0celo7

02:08

What I really want are freaking solution manuals.

Kyle Kanos

@ACuriousMind At least twice, actually

Dilaton (of all people) started one. And recently Peter Horvath

0celo7

GradPhysHW.SE then.

Kyle Kanos

Not sure if there's been other attempts by others

ACuriousMind

@KyleKanos Dialtion sounds like an elf :)

Kyle Kanos

@0celo7 PhysicsOverflow? They don't seem to mind doing HW (if it's grad+)

@ACuriousMind Fixed

0celo7

02:09

How fuzzy is the grad line?

ACuriousMind

@KyleKanos Awwwww. I liked it better the way it was

0celo7

Is "Graduate texts in physics" a good indicator?

Kyle Kanos

@ACuriousMind Well it's forever entrenched in your post

@0celo7 I honestly don't know. I don't normally venture over there

ACuriousMind

Meh. It's not the same. sniff

0celo7

@ACuriousMind Should I tag with HW or just let the tag master sort it out?

Sofia

02:11

@KyleKanos O.K. I saw the definition and the instructions. But may I ask not to hurry to close a home exercise that I'd recommend as useful, i.e. worth of the effort to rephrase and retain it in our site?

Kyle Kanos

@Sofia I'll have to dig it up, but I'm pretty sure StackExchange policy is to put the close votes in and not wait

ACuriousMind

@0celo7 Mhhh. Well, perhaps read the tag description and decide for yourself. I'm not sure what I would tag that with

29

Q: How soon should I "vote to close"?

Since it's currently impossible to delete a "vote to close",* when I see a poorly asked question, should I immediately vote to close, or should I comment, and give the OP a chance to improve his question? Waiting increases the chance that I'll forget, and never vote to close, potentially leading...

discussion vote-to-close

Kyle Kanos

29

Q: How soon should I "vote to close"?

Since it's currently impossible to delete a "vote to close",* when I see a poorly asked question, should I immediately vote to close, or should I comment, and give the OP a chance to improve his question? Waiting increases the chance that I'll forget, and never vote to close, potentially leading...

discussion vote-to-close

0celo7

My cat is going to get himself killed. Sleeping on stairs is a horrible idea.

ACuriousMind

:D

Kyle Kanos

02:13

Curse you @ACuriousMind!

ACuriousMind

@0celo7 Cats are surprisingly robust. Stepping on them is rather going to get the human wounded than anything else ;)

Sofia

@KyleKanos No! No! No! When I recommend a home-work to be retained, I take it under my responsibility to try to rephrase it, I don't leave it and come back after a week.

ACuriousMind

At least, the only one who got bleeding scratches from stepping on our cat was me :P

@Sofia Since there's no guarantee the question will not be closeworthy after your edit, voting to close regardless of that is still the thing to do. Reopening can happen.

Kyle Kanos

@Sofia I'm kinda opposed to the idea of modifying questions to make it on topic. I think making suggestions in the comments and let the OP decide if they want it reopened

Sofia

@ACuriousMind and @KyleKanos cursing a skeleton has any physical effect?

ACuriousMind

02:18

@Sofia Yes, it makes it stronger

I bathe in the unholiness!

Sofia

@KyleKanos Oh, they want, yes.

@ACuriousMind What about going to sleep? It has to be 3 o'clock in your country.

0celo7

@ACuriousMind Little Einstein reading about his field equations.

2

Sofia

@0celo7 Ahhh! What a pleasant photo!!! I like it!!!

ACuriousMind

@0celo7 Cute :)

Meh, I wish I could have a cat, but the place where I live is not really suited for pets.

0celo7

@ACuriousMind He's too squishy!

Sofia

02:22

@0celo7 Ahhhh! How pleasant!

0celo7

I can't have a cat as a freshman, sadly. He has to stay here for my first year of college.

ACuriousMind

@Sofia It's 3am. And I'm wide awake. The first day I have to rise early will be horrible indeed, but it is not in sight

Sofia

@ACuriousMind where do you live that you say that you can't keep a cat?

ACuriousMind

@Sofia It's too small, and on the third floor, right under the roof. I couldn't let the cat out, and my flatmate probably would also object.

Really the only thing I dislike about living here, the flat is nice otherwise

Sofia

@ACuriousMind Aaaaa, don't you live with the family? You took a room in Heidelberg with another student? Ohhh! And does your family care to bring you food?

ACuriousMind

02:26

@Sofia Hah, my family lives about three hours away, I have to make my own food

They do send cake sometimes, though, and of course I visit in the holidays

I'm not sure if my parents or I are more surprised that I've survived this long without major problems :D

Sofia

@ACuriousMind But, in that flat, you do have your own room, don't you?

ACuriousMind

@Sofia Yes, kitchen and bathroom are shared, but we each got our own room

Sofia

@ACuriousMind which major problems? You don't have time to prepare smth. to eat and stay hungry?

@ACuriousMind it is expensive to take a flat for students in Heidelberg?

0celo7

@KyleKanos before I work on a new question, do you know how to answer my old one?

What the heck is a nongeodesic circular orbit?

ACuriousMind

@Sofia Major problems rather like "I forget the oven and burn down the house", but no worries, no one seriously expected me to fail at living

infinitesimal

02:33

There's a joke about that...

ACuriousMind

@Sofia Yes, it's quite expensive to find a place in the city itself. Most students either live in student housing (which actually also are nice flats, mostly!) or in one of the smaller "suburbs" scattered around the city.

I think only Munich is more expensive for students than Heidelberg in Germany

Sofia

@ACuriousMind Well, lovely skeleton and vampire, I wish you good night and pleasant dreams. Don't stay so late, you would damage your biological clock. When the semester begins you may have problems. I go to sleep, it is terribly late for me.

0celo7

@ACuriousMind Please tell me what a nongeodesic orbit is :/

ACuriousMind

@Sofia Sleep well, then.

@0celo7 I have no idea except that it's probably not a geodesic ;P

0celo7

@ACuriousMind The "hint" says to use the tools developed in sect. 4.6.

But that section is about gyroscopes in geodesic orbits

So idk about that

ACuriousMind

02:39

That sounds not very helpful indeed

Kyle Kanos

@0celo7 Nope, GR isn't my thing.

0celo7

@ACuriousMind Does an orthonormal tetrad obey the geodesic equation?

ACuriousMind

@0celo7 I know nothing about tetrads, really.

0celo7

Argh.

@KyleKanos Cosmologists/Astrophysicists don't know GR?

@ACuriousMind I'm curious. Can you write the Schwarzschild metric from memory?

Kyle Kanos

I am not a cosmologist

I am an astrophysicist, but GR isn't my field

Supernova remnants are

I've taken a GR course, but my instructor was not very good at instructing

ACuriousMind

02:46

@0celo7 Uhh...not reliably. Something with $\frac{1}{\sqrt{1-(r/r_s)^2}}$, right? :D

0celo7

@ACuriousMind $$-\left(1-\frac{r_S}{r}\right)dt^2+\left(1-\frac{r_S}{r}\right)^{-1}dr^2+r^2d{\‌Omega}^2$$

ACuriousMind

I'm not good at memorizing any formulas, though. I tend to look them up when I need them

Argh, close :P

0celo7

Now I get what you mean by "absolutely no GR" :D

Did that omega come out right on your end?

ACuriousMind

No

0celo7

any idea why?

ACuriousMind

02:48

It shows a red \O followed by a mega^2

@0celo7 No, looks alright to me in the code

Kyle Kanos

Looks like you have an extra space or character there. Try deleting \O and do it again

0celo7

$$-\left(1-\frac{r_S}{r}\right)\mathrm{d} t^2+\left(1-\frac{r_S}{r}\right)^{-1}\mathrm{d} r^2+r^2\mathrm{d} {\Omega}^2$$

ACuriousMind

Testing: $\Omega^2$ ${\Omega}^2$ $\mathrm{d}{\Omega}^2$ $\mathrm{d}\Omega^2$

0celo7

Weird.

what the hell

ACuriousMind

lol, the Jim thread is gaining traction. 29 answers so far.

Kyle Kanos

02:53

2

A: A Jim for all seasons

Sweep all before you JimJimjeree and next month it could be... JimJimjeroo

ACuriousMind

@KyleKanos I have no idea what that one references

Kyle Kanos

Chim Chim Cher-ee (from "Mary Poppins")

►

0celo7

Can I post this?

If I further ask for a hint on how to calculate this, will it be a closeable HW question?

0

A: Laws of physics in curved space-time

The laws of physics are constant across the whole universe. The equations will be untouched. The speed of light will be the same. Time will not be the same. As you approach the speed of light time slows down. It will continue However, outside observer will see that time has elapsed in its lifetim...

that's wrong, right?

Kyle Kanos

Some of it is right, it just does not seem relevant

@0celo7 I would not vote to close it. It looks fairly conceptual to me

ACuriousMind

@KyleKanos Okay, now I know what it is, but I can't hear it because I'm in Germany :D

@0celo7 It's fine, I think

0celo7

03:01

@KyleKanos @ACuriousMind I would like to write: 3. If the equation in 1. is correct, then why does this behave differently than the geodesic case? I don't see how a term on the other side could change the behavior of the orbit this much.

Does that sound like I'm asking for a calculation?

Kyle Kanos

@ACuriousMind Is everyone in Germany deaf?

:D

ACuriousMind

@KyleKanos Heh. (I don't know if you know, but we have an agency called GEMA that's rather irritating when it comes to copyrighted music)

Kyle Kanos

Even on Youtube?

0celo7

@KyleKanos Especially on YouTube.

ACuriousMind

@0celo7 A bit, but just because you want to see what's the difference, so that's also fine. Go ahead, post it

Kyle Kanos

03:05

that sucks

0celo7

When I lived in Germany, YouTube sucked.

ACuriousMind

@KyleKanos Yeah, even there

Kyle Kanos

Can Tor block it?

ACuriousMind

@KyleKanos Yes, proxies/anonymizers help

Kyle Kanos

Sucks. But at least you now know the reference

If not the actual song

ACuriousMind

03:07

There was an addon for some browsers that automatically used a proxy for the few seconds it takes to overcome the GEMA check, but they found a way to recognize it and actually forced Youtube to block it

Many musicians don't even want the GEMA to do this, but some weird laws require them to grant GEMA that authority if they want to copyright their music at all.

0celo7

03:46

@ACuriousMind My lab...the momentum INCREASED

:: crais ::

@ACuriousMind Is momentum conserved during inelastic collisions?

NeuroFuzzy

Yes

0celo7

Exactly

So how the hell did it increase

NeuroFuzzy

Hah! "Your lab"?

0celo7

@NeuroFuzzy What?

@ACuriousMind What the fuck. I magic'd momentum.

NeuroFuzzy

@0celo7 "My lab...the momentum INCREASED". Whatevs.

0celo7

03:58

@NeuroFuzzy In my lab on linear momentum, the momentum somehow increased after an inelastic collision.

NeuroFuzzy

@0celo7 Well that's a problem! and your lab is in this universe, right?

0celo7

@NeuroFuzzy Yes. My classmates are simply saying it's error because the collision is not perfectly inelastic. That doesn't explain an increase in momentum.

NeuroFuzzy

@0celo7 well a few things come to mind if you want any ideas.

0celo7

@NeuroFuzzy I'm open, yeah

NeuroFuzzy

@0celo7 So I'm guessing you're using those standard sonic distance sensors, and two carts, maybe w/o velcro or magnets on them to make them stick together when they collide? If they bounce off of each other at all and you measure the velocity of the leading one, the leading one can have a higher velocity. The assumption that both carts have the same velocity would then cause the momentum to be miscalculated (ie, you'd calculate $2 v (m_1+m_2)$ when you really want $v_1 m_1+v_2 m_2$)

0celo7

04:10

The sensor was placed a bit back though

they weren't bouncing when the velocity after impact was measured, @NeuroFuzzy

NeuroFuzzy

they were firmly stuck together? @0celo7

0celo7

I'd imagine so. 20% increase in momentum, man.

That's just crazy. @NeuroFuzzy

@NeuroFuzzy My elastic collision lab has 0.14% error.

The inelastic one 20% in the wrong direction

Joe Stavitsky

what is the meaning of s^2 in the newton definition?

0celo7

@JoeStavitsky What Newton definition?

Joe Stavitsky

1 N = 1 kg⋅m/s2

NeuroFuzzy

04:22

@0celo7 I dunno! Sounds like bouncing to me!

0celo7

@JoeStavitsky Seconds squared.

Joe Stavitsky

right, but what does that mean for static objects? Or is it there just to cancel something later?

0celo7

@NeuroFuzzy So for "account for your error" I'm gonna put 1. micro black hole 2. bouncing

@KyleKanos Do you have any insight?

Kyle Kanos

@0celo7 Into what?

0celo7

@KyleKanos I did a lab where we crashed a moving cart into a stationary cart to model an inelastic collision.

Somehow the momentum after collision was greater than that of the impacting cart.

Kyle Kanos

04:26

I'd say some math was probably done wrong

NeuroFuzzy

@JoeStavitsky units have more generality than that. Think of it as "the acceleration that would be observed in a 1kg object, should the force be applied to that object."

Kyle Kanos

@JoeStavitsky What is a static object?

0celo7

@KyleKanos Holy crap. I'm going to kill Sara. I think she switched the carts.

Joe Stavitsky

@NeuroFuzzy o ok that makes sense

NeuroFuzzy

@0celo7 you can't now. Everyone in the chat room will know who did it.

Kyle Kanos

04:29

@0celo7 My students always managed to mess up the cart order, despite my telling them which order it should be

0celo7

I get <2% error if I switch the widths of the tabs on the carts.

Kyle Kanos

Bam

0celo7

@KyleKanos The problem is that my classmates who weren't in my group also report increased momenta

Kyle Kanos

Well tell them that they were wrong?

0celo7

@NeuroFuzzy So? No one here would testify against me.

And the logs can be wiped.

It's a justified crime.

@KyleKanos Your students sound like typical teenagers.

Kyle Kanos

04:35

Most of them were around 20, so close enough

0celo7

@KyleKanos 20-year olds doing cart labs?

Kyle Kanos

It's college physics

0celo7

I know, but isn't that freshman stuff?

Kyle Kanos

For physics majors, yes. For not physics majors, no

0celo7

@KyleKanos Teaching non-STEM physics must be rough.

Kyle Kanos

04:38

Well I only run the labs. But they're not that bad

dmckee

@0celo7 It is a different experience. They are harder to motivate with "we're laying the foundation" arguments, because they don't plan on erecting the building.

0celo7

@dmckee That's the way I feel about most electives besides Economics.

I gave my cat a glue stick to play with. He loses it in the most creative of places. I just retrieved it from a pant leg of a pair laying on the floor in another room.

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