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2:04 PM
@Jim on a string?
 
Jim
@0celo7 tied to it
 
Oki
 
Formatting for the book has now been determined
Next conundrum
Should I attempt the group the questions by category
or just list the questions 1 to 100
 
Jim
yes
 
2
Q: What is the review counter showing?

SteevenHere comes an odd little low-practical question out of curiosity: What does the review counter show? The little counter in the upper bar is always showing a high number (fluctuating a bit) even when review queues are empty. I might have missed an explanation somewhere when being sworn in with th...

0
Q: Why do defunct links to the old astronomy site still format correctly into a linked title?

Emilio PisantyThis question pointed out that there are still a number of links to posts on the old Astronomy SE site kicking around, and that they mostly redirect to new, unrelated questions and answers in the new Astronomy site. I feel those links should have been automatically edited at the time of the migra...

 
@ACuriousMind I think even email addresses can be viewed using data explorer (data.stackexchange.com/math/query/63580/…). I haven't used it though. Maybe it is a bug. Or perhaps SE allowed it intentionally.
 
I think I need to either find or investigate a solution that is basically the van stockum dust truncated (by somehow fiddling with f(r) so that the density of the dust will die exponentially as we move radially from the rotation z axis (and all axes parallel to it) and also away from the origin along the z axis.)
 
@anonymous Doesn't work (anymore), just tried it with mine
 
@ACuriousMind Oh that's good. :) Maybe we ought we write on Meta about that age thingy.
 
Jim
@Secret And I think you just had an aside within an aside
 
2:17 PM
@anonymous I have poked the Powers That Be about it
 
this Feynman diagram is looking at me weird
the book is staring into my soul :-/
 
@ACuriousMind the email is hashed so you can't actually obtain a user's email
but you can check if an email belongs to a particular user
 
@AccidentalFourierTransform The eye of Sauron Feynman
 
Jim
that feynman diagram scares me. Like I'm talking nightmares
I don't want to think about the equation that would represent
 
if it still works, it looks the email hash field is blank
 
2:22 PM
@Kenshin Never claimed you could - but that query claims it can find a user belonging to a given email, which it can't, I just tested it with my own.
 
"five minutes into netflix and renormalisation and he gives you this look"
 
@ACuriousMind yeah it appears the email hash field has been set to blank for every user in the users table
 
BTW did I tell y'all already that Zinn-Justin's book is great?
 
@AccidentalFourierTransform Yes
 
2:25 PM
Well, the goal is, if we can find a 4-density that can induce a metric allowing CTC (don't care if they are geodesics or not, as long you don't need to move at the speed of light to travel along them, i.e. not null CTCs) and said 4-density is bounded in the spatial dimensions (thus unlike the tipler cylinder and the van stockum dust in that they are infinite in at least on spatial dimension),
then it might be possible to scale this 4-density so that it gets small enough to be within the laser power that are currently accessible, and possibly small enough to get around that massive need of e
and that will mean, I need to take out my large notepad to do the calculations cause I cannot do any GR calculations in my head...
 
@ACuriousMind quick question: I have this skew-symmetric tensor $A_{\mu\nu}$ from which I can construct the scalar $A_{\mu\nu}A^{\mu\nu}$
in $d=4$ I also have the second scalar $\tilde A_{\mu\nu}A^{\mu\nu}$
how can I generalise the second scalar for arbitrary $d$?
 
as for those singularities that result from logarithmic and reciprocal maths expressions, I am not sure if tweaking f(r) can nuke them
 
@AccidentalFourierTransform I don't think you can. For even dimensions, the generalization is $A\wedge\dots\wedge A$ d/2-times, but I don't see one for odd dimensions
 
hmmm... ok, thanks!
 
Jim
2:29 PM
@Secret strictly speaking, you can't have null CTCs. The curves are (as per their name) "timelike", which means not spacelike nor lightlike. i.e. not null
 
0
Q: Which body has high moment of inertia (rigid body or soft body)

FawadI was reading explanation of why by spinning eggs on a table top,how will you distinguish a hard boiled eggs from a raw egg? In explanation it is given that Moment of inertia of raw egg is greater than boiled egg. So do really moment of inertia of soft body is more than rigid body? How can I ex...

that is off-topic?
 
oops sorry (yeah, they are called null closed curves, my bad)
 
Hello,

I have constructed a feynman diagram for a particular reaction, but I don't know if it's the lowest order. Are there any useful checks I can perform, or is it a case of 'knowing' what can occur?
 
@Jacobadtr you can use the Dyson expansion to first order in $\mathcal H_\mathrm{int}$ and check whether it agrees with your diagrams or not
 
@AccidentalFourierTransform I don't know what that is - this is for my first particle physics course. Can you link me to a resource?
 
2:33 PM
@YashasSamaga <https://www.quora.com/What-rolls-faster-a-boiled-egg-or-a-raw-egg> I think that is more of a physics question than biology. BTW did you see my solution to your question?
 
@Jacobadtr oh well, it is a rather broad subject
how were Feynman diagrams introduced to you?
any sort of analytical derivation from first principles?
 
@anonymous I know that you have commented a solution, I haven't tried it yet.
 
@Jacobadtr or were they just given as a set of rules with no justification?
 
@AccidentalFourierTransform Not really, we got a brief discussion of transition matrices, but they are mostly a visual tool we use in conjunction with conservation rules
 
"Since it has an action of the global symmetry U(1) acting by rotations near infinity, the moduli space must have genus zero" Sigh...Witten, I'm not as smart as you, would it kill you to spell out your arguments a bit more?
 
2:36 PM
@Jacobadtr well, the more "formal" derivation of the Feynman rules is more or less as follows: you want to calculate the transition element $\langle \text{initial}|\mathrm e^{-iHt}|\text{final}\rangle$
and if you expand the exponential in powers of $H$, you get the different Feynman diagramns
because each terms corresponds to a certain integral that can be represented diagrammatically
 
@anonymous the answer on quora is wrong
 
what I was suggesting is that the first non-trivial order is the one that is linear in $H$ in the Taylor expansion
 
@YashasSamaga Why?
 
so you could compare the analytical expansion in terms of operators to the expression you get from the diagrams, and check if they agree
 
because moment of inertia is a function of the distribution of the mass.
 
2:37 PM
that way, you can be sure that the diagrams are correct
 
@ACuriousMind oh yes it would
 
even if you lose some mass, the moment of inertia can increase.
 
It would also kill Mr. Wolf, whose book on algebra I am trying to read
 
The yolk in the boiled egg can get placed at the walls of the egg. It can also get fixed to the center of the egg. The moment of inertia will be higher for the case where the yolk is near the wall.
 
He uses the wonderful phrase "a glance at the proof also shows"
 
2:38 PM
I am assuming that the yolk is denser than the albumin part.
I am not sure about that. I am a vegetarian.
 
@0celo7 are there any perfect fluid in a box solutions, that is, I want the metric tensor to be given by a perfect fluid that is bounded in some finite region, say the interior of a unit ball?
 
@Jacobadtr but you don't know what any of that means, so nevermind
 
@AccidentalFourierTransform hmm, that makes some sense in principle. Is there a process you would recommend I look at to practice this? electron/positron annihilation for example
I understood some of it :)
 
@0celo7 At least it's a glance at the proof and not out the window!
 
@Jacobadtr well, as I said before, it is a very broad subject. The only way to understand what's going on is to read a book on quantum field theory, which in your case seems to be an overkill
if you really want to understand the details, you'll have to read a book on QFT
 
2:44 PM
@YashasSamaga The internal degrees of freedom of the liquid in the raw egg will affect its rotational motion and increase its friction and absorb part of the energy into internal motion. Two sliding eggs will retain the same velocity if there is no friction and initial rotation, they will not roll so it is a tie. If an initial rotation is given the boiled will go faster because the raw will be turning energy into heat due to the internal degrees of freedom.
 
@Jacobadtr on the other hand, if you just need help with your homework, you can post it here and I can try to help you with that
 
@Secret I don't know off the top of my head, have you checked Stefani?
 
So, in short they say that moment of inertia of boiled egg is smaller.
 
I see - well I think that might have to wait. I can't take QFT at university until next year because I need to finish QM2.

Thank you. I am looking at the interaction

$e^+ + e^- \rightarrow \mu^+ + \nu_{\mu} + e^- + \bar{\nu_e}$

I will try to find a way to share the diagram I have come up with for the interaction
 
@anonymous How can you claim without know how the mass distributes in the boiled egg?
The yolk might be at the edge of the egg.
 
2:47 PM
@YashasSamaga In boiled egg the fluid hardens. Its bio man.
 
The yolk is a solid thing inside the egg right?
 
@YashasSamaga No.
 
@ACuriousMind that's how I felt reading BBS and BLT. A bunch of random statements with no explanation and dubious mathematical rigor.
 
@anonymous Yes, you can have the yolk at the wall when it is hardening.
As far as I know, the yolk can be anywhere inside a boiled egg. It just won't move.
 
@YashasSamaga All the yolk can't stick to the wall
It can't be anywhere!
 
2:48 PM
I checked images at google.
 
If you shake the egg hard enough before boiling, it can :P
 
The yolk has moved towards one edge.
 
@ACuriousMind That would damage the egg first :P
 
It depends on the location of the yolk before it is boiled.
 
@Jacobadtr that is a somewhat complicated process
 
2:50 PM
@YashasSamaga Even then the liquid yolk hardens and that will decrease moment of inertia and degrees of freedom.
 
to be honest, I don't know off the top of my head which diagrams contribute
but Im sure that there are several diagrams
 
@anonymous What does degrees of freedom got to do with moment of inertia?
 
Jim
There seems to be more theories on the initial conditions of boiling an egg than there are about inflation
 
@Jacobadtr presumably, a lot of diagrams contribute to first order
 
@AccidentalFourierTransform This is what I came up with http://draw.to/Dq5bFF

sorry for the crappy drawing
I think everything is conserved which is a start
 
2:51 PM
@anonymous Have you considered the case where the liquid part inside the egg does not rotate at all whereas the shell rotates?
 
@Jacobadtr yes, that is a valid diagram, but there are many more diagrams that are valid too
 
That would reduces the moment of inertia to a very small number.
 
and the correct result is given by the sum of all valid diagrams
 
@YashasSamaga That is not possible in reality. There is friction.
 
because only the shell is rotating
@anonymous The shell can rotate faster than the liquid.
 
2:52 PM
@AccidentalFourierTransform I am mostly interested in whether there exists any lower order diagrams
 
@YashasSamaga And it does. The yolk will slow it down due to friction.
 
I still don't understand your freedom of degrees/internal movement argument.
It does not matter at all, imo.
 
Okay. Go and ask it on PSE if you don't believe me.
 
If you assume that only the shell rotates and the liquid slowly starts and tries to damp the shell's rotation. You shouldn't include the liquid in the moment of inertia.
 
@YashasSamaga The liquid MI matters a lot.
 
2:55 PM
You have got the concept of moment of inertia wrong :/
 
I got to go. Sorry.
 
The rotating part is the shell
the liquid is trying to damp the motion.
so while calculating moment of inertia, you count the shell only.
 
1
Q: Will a boiled egg or a raw egg stop rolling first?

JackIf we roll a normal egg and a boiled egg at the same time on a floor 1) with friction 2) without friction which one will come to stop first (if they will stop at all) and why? Can anyone tell me reason for this?

 
@Jacobadtr I don't think so. Your diagram is $\mathcal O(\alpha\ G_F)$. This seems to be the lowest order for your process. I am 90% sure :-P
 
@anonymous that is not moment of inertia
@anonymous that question is different
This is more of a semantics problem.
 
2:56 PM
Oh he Yashas
Hey
 
If the liquid does not move in sync with the shell. It shouldn't be included in the moment of inertia.
 
@YashasSamaga That is definitely moment of inertia. Just ask a question on PSE - Which has higher moment of inertia -boiled or raw?
 
because the calculations will go wrong.
 
@AccidentalFourierTransform haha, well thank you for your help. I will work through some more exercises and see if I come up with anything else
 
@anonymous Suppose the liquid never moved at all, only the shell moved. What would moment of inertia be?
You apply a torque $\tau$.
To calculate the angular acceleration, you'll have to use the moment of inertia of the shell only.
@Isomorphism hi
 
2:58 PM
@YashasSamaga Moment of inertia doesn't depend on whether body is rotating or not. It is a constant.
 
...you are not understanding what I amt rying to tell
Try to answer this
There is a hollow sphere.
You fill it with a liquid which has zero friction with the walls of the sphere.
Now when you try to rotate the sphere, you use moment of inertia of the shell only (hollow sphere)
Liquid is not counted
 
It's hollow. Hallow(ed) would mean it's been blessed by a priest or something ;)
 
@Jacobadtr I mean, you also have the diagram draw.to/D1R85CB right?
this is a lower order diagram
 
@ACuriousMind I fixed it already. idk why I used the wrong spelling even though I knew the correct one.
 
but it is trivial
so we dont take that one into account
 
3:00 PM
Human brain is too weird to understand.
@anonymous $I\alpha = \tau$, if you had considered the liquid to be included in I, you'd get the wrong answer.
The shell rotates independently of the liquid inside.
If the liquid behaves like a solid and rotates with the shell, then it'll be counted in I.
 
@Jacobadtr if you want to make the $e^-$ to interact with the rest of particles, you have to introduce two more vertices, which brings us to your diagram from before
 
@AccidentalFourierTransform Hmm, that is a good point. I don't know if that is considered 'cheating'
 
and therefore, I am 95% sure that your diagram is the lowest possible order one
 
Raw egg yolk is something like this ^
And you know how a boiled egg yolk looks
Now it is a maths problem
 
3:02 PM
Read my previous messages
 
Find out which has higher moment of inertia
 
You are having a serious conceptual error. Listen.
 
@Jacobadtr my diagram is valid but trivial. The $e^-$ doesnt interact with anything, so such a diagram is never considered. So I dont think your professors want you to consider it either
 
You have a shell.
 
"The shell rotates independently of the liquid inside"
 
3:03 PM
@anonymous It has a liquid inside it which has no friction.
 
This is a false argument.
 
ok you are reading it
 
@AccidentalFourierTransform That would be my guess as well - i'll mention it anyway to remove any doubt in my mind
 
@anonymous I am not talking about the egg.
 
@Jacobadtr ok :-)
 
3:04 PM
@anonymous If the liquid does not interact with the shell. The liquid has no role to play.
 
@AccidentalFourierTransform Thanks again for your help
 
youre welcome
 
@YashasSamaga Who has a greater moment of inertia: A shell or a sphere (of same mass)?
 
@anonymous Rote learning from textbook will harm you.
A solid "RIGID" sphere has more moment of inertia than a solid RIGID shell.
I am not talking about something rigid.
I am talking about a rigid shell which has a liquid inside it.
The liquid and the walls of the shell have no friction.
The moment of inertia for that body will include the shell only.
Just think. Does the liquid being inside the shell damp the rotational motion?
 
3:07 PM
@YashasSamaga Yeah that moment of inertia will be greater than the boiled case. So it will spin slower.
Conserve angular momentum.
I1w1=I2w2
 
@anonymous did you understand what I was trying to tell btw?
The argument started when you claimed that you should include inside thing in $I$ even when it does not move in sync with the shell.
 
@YashasSamaga yes
Did you understand mine?
 
@YashasSamaga So you agree now that boiled has lower I ?
 
nope
Can the yolk in the unboiled state change its shape?
 
3:10 PM
@YashasSamaga Then I am sorry to say you are violating angular momentum conservation.
 
@ACuriousMind There's some connection between quotients and affine diffeomorphisms that I'm not groking. It's not clear why e.g. two cylinders should be affinely diffeomorphic when they are quotients of $\Bbb R^2$ by different groups.
 
Wrong.
The mass of the shell is more distant in average from the center than the mass of the sphere, therefore the shell has a larger moment of inertia.
 
@Mostafa It was a typo.
 
@0celo7 I don't know what an affine diffeomorphism is
 
3:12 PM
@ACuriousMind A diffeo $f:M\to N$ that carries the connection on $FM$ to the connection on $FN$.
 
I am bored now. Let's continue tomorrow. :/
 
Also, what's your definition of a cylinder that you can get two of them by quotienting by different groups
 
@ACuriousMind Quotient them by the translational group generated by different vectors
topologically the same, sure
but the radius will be different
 
Ah. Technically, that's the same group, but a different action :P
 
Different subgroups of the isometry group of the plane if you want.
I can't tell if Wolf is an algebraist who is terrible at explaining geometry or a geometer who thinks geometry is obvious...
 
@0celo7 But a diffeomorphism need not preserve the radius? I mean, that the homeomorphism between the cylinders is also a diffeomorphism is clear, right?
 
@ACuriousMind Yeah, sure
But why does it carry the frame bundle of one onto the other?
 
Ronald Lawrence "Ron" Mallett (born March 3, 1945) is an American theoretical physicist, academic, and author. He has been a faculty member of the University of Connecticut since 1975 and is best known for his scientific position on the possibility of time travel. == Early life and education == Mallett was born in Roaring Spring, Pennsylvania, on March 3, 1945, and grew up in The Bronx in New York City, New York. When he was 10 years old, his father died at age 33 of a heart attack. About one year later, at age 11, Mallett found a Classics Illustrated comic book version of H.G. Wells' The Time...
 
So the issue is the connection, I'm assuming it's the LC connection you're talking about?
 
Ugh, I don't have enough GR knowledge to analyse his paper, let along trying to modify the intitial conditiosn to get a finite cylinder
 
3:16 PM
@ACuriousMind Yeah.
 
Just received my first (academic) spam E-mail ever.
2 months after it was added to the research group's website.
 
[To be checked] Ok if cylinders don't work, what about light travelling in a solenoidal configuration
 
@ACuriousMind I think the cylinder is misleading because the diffemorphism is so nice.
You can just expand/contract it.
How about two different tori?
 
@0celo7 Right, the metrics only differ by a factor, and metrics differing by a factor have the same LC connection, right?
@0celo7 The tori are not even all conformally equivalent, so this should fail
 
Crap, here I thought cylinders were simply connected :P
@ACuriousMind Apparently it's true for cylinders, tori, Möbius bands, and Klein bottles.
 
3:22 PM
Huh
Well, I really have no feeling for how strong the restriction for a diffeomorphism to be affine is
 
I'm not even sure what the diffemorphism is supposed to be.
@ACuriousMind Me neither.
Ok, these are all complete connected manifolds.
Some shenanigans with geodesics might work.
 
@HDE226868: thanks :-) I didn't realise you could get a bounty on a community wiki answer.
 
@YashasSamaga hmm, I think your answer is the best one can say without a careful investigation of the problem.
 
I can't even try it because I am a vegetarian. :(
 
Why?
 
3:26 PM
I think the content of an egg is too viscous to be able to rotate independently from the shell. (at least noticeably)
 
But it definitely does not rotate with the shell initially.
or does it?
I haven't seen the contents of an egg so I have no clue how viscous it is.
 
@YashasSamaga Really?? :o
 
...how?
 
The point I am trying to make is that including the contents of the egg in moment of inertia calculation is not fair.
 
Do they not have eggs in India?
 
3:29 PM
I don't eat them so I don't get them.
 
Do your parents not eat them?
Have you not seem them at a friend's house?
 
Nope.
I have seen the initial product and the final products only :P
proper egg and the final thing
 
By "final product" you mean a chicken?
 
Hehe
 
The omlet thing
 
3:30 PM
@YashasSamaga On TV?
YouTube?
 
@Mostafa street shops
 
@ACuriousMind Are you a vegetarian?
 
@0celo7 No
 
@0celo7 someone with that profile picture...
@YashasSamaga You can buy a few and experiment with them. (and it's kinda satisfying to crush them with your hands too)
 
I would probably puke.
 
3:35 PM
@Mostafa Not sure "it's satisfying to crush the eggs with your hand" is a convincing argument for a vegetarian :P
Also, what's wrong with my profile picture? :P
 
Crushing eggs with your hands is psychopathic.
 
@ACuriousMind or for a feminist
 
@YashasSamaga Don't be a baby.
 
Do I need to bring Ciri back so users can comment on how beautiful I am again? :P
 
@AccidentalFourierTransform Replace eggs with nuts (open balls) and you're golden.
@ACuriousMind I thought you didn't like Apple.
 
3:37 PM
@0celo7 wat
Ciri, not Siri
 
@0celo7 isnt eggs a synonym for nuts? in Spanish it is
 
:P
@AccidentalFourierTransform It is in German as well, but not in English.
 
oh well, it works then
 
I said not in English.
 
I know. ACM is from Germany. It works for him
 
3:38 PM
I refer to mine as "open sets."
@AccidentalFourierTransform I'm from Germany too but when you say "eggs" in the English context I think of ovaries.
@ACuriousMind Potato potato.
 
@0celo7 wair youre from Germany? but you live in the US, right?
 
@AccidentalFourierTransform I was born and raised in Germany, but I am American
 
He's a 'Murican with delusions of being German :P
 
@0celo7 huh, interesting. American parents, right? and do you speak G.?
 
One parent is American
I speak the Deutsch, yes
Very rusty though. I only use it when talking to German professors occasionally.
 
3:42 PM
2 hours ago, by Mostafa
Failure of The TheoreticalPhysics - Suppression of the RonMaimon insurgency - Demise of Lubos Motl and rise of John Rennie - Advent of ACuriousMind - .....
Advent of ACuriousMind - the singularity is upon us!
 
Demise of Lubos?
@Mostafa I am offended that I did not make it onto your list.
 
Hi @JohnRennie
what does "I've done the math enough to know the dangers of our second guessing" even mean?
 
@AccidentalFourierTransform what's the context?
 
@0celo7 I noticed he pushed a little harder those days that the competition was heated...but became more and more inactive after he found out he's lost :)
 
3:48 PM
@AccidentalFourierTransform Did you know Maynard is in the studio now recording the vocals for the new album?
 
@JohnRennie yeah :D
I was going to tell you that actually
 
he didn't lose
he's still the greatest mind of our generation
 
@0celo7 With 131k posts in The h-Bar, accounting for 17% of all its posts, you certainly have a prominent place in its timeline.
 
@0celo7 fortunately I'm a lot older than him, so I still have a chance at being the greatest mind of my generation :-)
 
Why isn't John Rennie a mod?
 
3:53 PM
@YashasSamaga Because it's a mug's job.
 
Aw lol.
 
Ask any of the mods - it's a lot of hassle and whatever you do someone complains!
 
yeah, tell me about it
 
Mods are horrible people
@JohnRennie is not
Hence he cannot be a mod
(@ACuriousMind I still <3 you)
 
::ahem::
 
3:57 PM
He's too human.
 
He's too dancer
 
@0celo7 I have been practicing boiling kittens and eating babies but I haven't made the grade yet
 
@ACuriousMind did you not read the fourth message there?
 
@0celo7 So I'm a horrible person and you love me. What does that say about you?
 
That I'm a basic b*tch
 
3:59 PM
@JohnRennie Kittens are far better when barbequed
 

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