The h Bar

0celo7

16:00

@BalarkaSen Does one get the generalized tube lemma in the average analysis text?

Balarka Sen

It's certainly in Munkres.

ACuriousMind

And it appears it is chat session time!

John Rennie

But we have no DavidZ and no agenda :-)

So we can talk about anything EXCEPT THE F*****G REFERENDUM

ACuriousMind

lol

Jim

so about that referendum...

John Rennie

16:02

:: John was last seen running away ::

ACuriousMind

@Jim Careful, JohnRennie has been granted the power of room owner while you were away ;)

John Rennie

Ah, true, I'd forgotten about that.

I now have (demi) god like powers to ban users :-)

Jim

@ACuriousMind I'm hoping he gets an aneurysm from what I say quick enough to prevent him from kicking me

John Rennie

Actually I do have something to ask the panel, but it's about the homework policy.

So at this point you might all want to run away.

Jim

:: Jim was last seen running away ::

0celo7

16:05

@BalarkaSen Probably

user116211

@JohnRennie missing DavidZ.

0celo7

Chat why are you doing this

ACuriousMind

@JohnRennie Well...that's what these sessions partly are for, right? Meta discussion? ::sighs::

John Rennie

We all agree in banning basic homework questions - I feel fairly confident in saying that ...

Jim

@JohnRennie as established in the chat session of 1631

John Rennie

16:07

But we sometimes get questions that are kind of homework, but at a much higher level. Final year undergrad or postgrad.

user116211

Let's talk about England vs Iceland.

user116211

@JohnRennie, can we ;P

John Rennie

@MAFIA36790 :: John reaches for the kickmute button ::

user116211

damn.

John Rennie

Should we allow homework questions if they are high level enough?

Jim

16:08

example?

user116211

@JohnRennie That was one of the reasons for the contentions of the users prompted in making Physics Overflow.

John Rennie

I asked this a while back in:

19

Q: Graduate and above level questions aren't homework

I know, I know, it's yet another Meta post about our homework policy. Feel free to downvote and write abusive comments if you're heartily sick of posts about the homework policy. If you're still with me: the homework classification for closing questions has never literally meant that your teache...

I got a fairly negative response then, but I wonder if we've become a bit more relaxed since.

@Jim I don't have examples because this is just a nagging feeling and I haven't done any real research into the matter. It's just an impression I have.

The thing is that the zealous end of the spectrum wanted this site to be like math Overflow.

That was never going to happen so they went to Physics Overflow in a huff.

user116211

@JohnRennie yup.

user116211

I think homeworks are homeworks no matter what level they are of.

John Rennie

Given that we aren't going to get many, if any, research level questions, could we dilute the really crap questions by encouraging more grad level ones?

ACuriousMind

16:12

@JohnRennie No. I have a strong feeling that most of the homework-like questions on that level we get are from people who are in far over their head or who are just lazy.

user54412

@ACuriousMind I... understood it. What does that say about me?

user54412

(I've even cited it in an answer or two here)

user116211

@JohnRennie when did anyone block asking grad level questions?

John Rennie

Do you get many lazy students doing PhDs?

user116211

It's just that they are less populous.

user54412

16:14

@JohnRennie I don't understand it, but yes, the grad-level questions do come off as lazy to me. More so than the simple stuff, because there I imagine someone being forced to take physics against their will.

ACuriousMind

@JohnRennie I have no idea whether the people asking these questions are PhD students, but when I think of the questions of "high level" that I close as homework, they usually either have a "I have no idea what I'm doing" or "I didn't try anything before asking here" vibe to me.

user54412

And then there's also the in-over-their-head group ACM mentions -- there's a nonnegligible population of students who grab the hardest book they can find and try to make sense of it.

John Rennie

@ChrisWhite is that necessarily bad?

user54412

@JohnRennie Not necessarily.

vzn

@ChrisWhite that you have comprehension/ reverence for the history/ tradition/ lineage of physics. (surprised to say the least to hear a serious physicist refer to newton as "gibberish".)

John Rennie

16:17

@ACuriousMind I see comments from you criticising questions when I don't even understand the question. That suggests to me an answer might be interesting to site members even if the original question is homework.

Or interesting to me anyway ...

ACuriousMind

@JohnRennie Is that really the case in the homework cases? I'd not be surprised if that happened for some other close reasons, but now I want to see examples!

user54412

I'm usually skeptical that pole vaulting over an entire undergrad curriculum will work out for the person -- at best they gain a problem solving ability without any context or understanding. But hey, we answer low level questions by the curious who are clearly never going to become actual physicists, so perhaps this is the same sort of community service.

John Rennie

I haven't cared enough about the issue to look for specific examples. I can do, but I'm not going to bother if everyone is implacably opposed to the idea anyway.

ACuriousMind

I do note that unclear what you're asking votes have a hard time getting the necessary reviews on questions where the questioner (mis)used a lot of technical terms.

John Rennie

Well, no-one has spoken in support of my idea, so unless ACM and Chris are the only ones awake I guess it's a non-starter ...

ACuriousMind

16:21

@JohnRennie Well, I'm opposed to letting such questions be on-topic because in my perception the majority of grad-level-closed-as-HW questions really aren't good questions. You seem to be saying that there are sufficiently many good ones among them to rethink our policy so neither of us is going to move an inch here until someone gets evidence ;)

user116211

0

Q: Curvature of Hilbert space

That may appear as a dumb question, but: Does Hilbert space have a curvature, or is it a flat space? How and why?

quantum-mechanics hilbert-space metric-tensor

user116211

say again?

ACuriousMind

I would love to have more interesting higher level questions

John Rennie

Maybe if we can attract more graduate physicists (and fewer undergrads) the general quality would go up ...

At the moment I feel (again no proof) that we have a real issue with question quality.

It seems to me that these days I scan the overnight questions first thing in the morning and there are very few that look interesting enough to answer.

ACuriousMind

I share that feeling

But I don't think that there is much we can directly do to influcence quality

John Rennie

16:27

Well maybe if the site gets a reputation for helping (not necessarily answering) grad level questions we will attract more graduates.

ACuriousMind

Voting is the most efffective instrument we have for that

John Rennie

efffective = effective and only effective? :-)

@MAFIA36790 I was going to ask if Hilbert space is a metric space, but apparently it is

ACuriousMind

@JohnRennie But we currently do answer grad level questions, just not the homework-like ones.

And since my perception is that the questions we close are mostly bad ones, I don't think relaxing the policy would attract users that improve quality.

AnimatedPhysics

I do multi-particle gravity animations currently using Newtons law (ie. calculate force in 3d, add to velocity, add to position and re-display). I am interested in upgrading the calculations using the Kerr metric. An example of the calculations in matlab (I use Javascript) can be found here: yukterez.net/org/kerr.txt. Is this an appropriate place to ask if anyone knows anyone who would be interested in helping to work out the step by step x/y/z force on each particle?

John Rennie

Well, there's homework and there's worked execrcises, and unless things have changed since my day at grad level it's more about the worked exercises.

ACuriousMind

16:30

The quality issue to me is less the level of questions and more that many of them are malformed

user116211

@JohnRennie yep.

John Rennie

The problem may be superficially homeworky, but their point is to examine some concept.

user116211

@AnimatedPhysics Please save it for later; there is a discussion ongoing.

John Rennie

Maybe I'll go away and try and make my concerns more concrete, though the response today has left me with little enthusiasm for the task.

ACuriousMind

To be fair, the response was mainly me. I'm always the grumpy one in these discussions ;P

John Rennie

16:32

Apart from Chris no-one else has spoken, which I take to mean they either agree with you or don't care.

ACuriousMind

@AnimatedPhysics I guess this is an appropriate place, but whether you find someone is another question...

John Rennie

OK I have one other idea, which is unrelated, for a change in the way the site works.

0celo7

No we will not have tea time

John Rennie

So we'd need to ask the SE if they will do it, but I'd be interested to know if anyone else agrees with the idea.

Could we have the answer box look at the curent tags on the question and pop up tag related messages?

e.g. if the question is tagged homework could we have it pop up a warning not to give an explicit answer?

SE will probably say no anyway, but is the idea worth asking for?

ACuriousMind

That would require a change by SE

And would it be useful for tags that are not meta tags, which are anathema anyway?

John Rennie

16:37

No, I think it's only worth it for meta tags.

But then homework is a meta tag and we use it (a lot!!)

ACuriousMind

Yeah, I don't think we can convince SE to do that

Could probably deter new users to come and answer HW questions, though - that's the goal, right?

John Rennie

@ACuriousMind exactly :-)

Possibly also for resource-recommendation tags.

There's already a warning that appears on questions tagged as resource-recommendation, but I'm not sure how that gets there- it might be manually added by the mods.

ACuriousMind

Yes, it's a mod banner

user54412

But it's automatically added when the tag is, right?

vzn

SE has made almost no major chgs to individual sites on request/ afaik its nearly an urban legend around here. (think there are many good proposals however)

John Rennie

16:41

@ChrisWhite no I don't think so

user54412

Poor mods

John Rennie

I'm sure I've seen RR tagged questions that didn't have the banner

ACuriousMind

@ChrisWhite I think they usually add it when they convert the question to CW

John Rennie

@vzn to be fair I can see their point.

user54412

@vzn Though in some few cases they have. There are even a couple sites with different weights to up and down votes.

John Rennie

16:43

@ACuriousMind maybe what I'm thinking of is a way to automate the mod banner(s)

vzn

@JohnRennie what point do you mean? its more of a case of the software is mature & there are low resources to chg it, resources caught up just in day-to-day maintenance.

ACuriousMind

@ChrisWhite "A couple"? Any other than stackapps?

John Rennie

@vzn given that it's free, and still uncluttered by ads, it seems churlish to make a big fuss about it.

dmckee

@ChrisWhite That's something we have to stick on by hand. Thankfully there are only a few of those questions most weeks.

(BTW--I'm proctoring an exam right now, so I might be available. Normally I have class during the chat.)

user54412

@ACuriousMind Maybe note

Qmechanic

16:47

@ChrisWhite : Unfortunately, not.

John Rennie

Could we have a similar banner for homework, saying something like Don't give explicit answers?

vzn

@ChrisWhite would like to see a list somewhere of customizations, & suspect it would not be very remarkable, and maybe even kind of nearly-embarrassingly minor/ limited (& maybe there is no such list for such a reason) ... have seen many propose customizations over the years... many are in Meta Stack Exchange with high votes mostly languishing

user54412

@JohnRennie That would be quite a lot of manual labor, no?

John Rennie

@ChrisWhite yes ... unless there was some mechanism for automatically adding it when the question was tagged.

Qmechanic

@JohnRennie : Read this line in the news: Not been this embarrassed to be English since ... since ... since Friday! :D

John Rennie

16:51

@Qmechanic we've already banned any discussion of the referendum and the football :-)

user54412

Can John kick-mute a mod?

John Rennie

I'm willing to give it a try :-)

ACuriousMind

@ChrisWhite Why didn't you try that yourself? :P

vzn

getting late but DZ might ask, any physics news from anyone? (posted a few links myself lately)

John Rennie

But our ladies cricket team just whitewashed Pakistan, and the rugby team just whitewashed Australia. So it's not all bad news.

@vzn I thought the article about doing a numerical calculation of the cosmological metric was really interesting.

dmckee

16:53

@ChrisWhite Oh! Oh!. Pick me! Pick me!

AnimatedPhysics

@vnz an amazing number of black holes discovered in our galaxy lately.

John Rennie

There has been some interest in the idea that the accelerated expansion may be an artefact of ignoring back reaction.

vzn

@AnimatedPhysics really? milky way? at the center? can you post a link? yes black hole news is really popping these days

user54412

@dmckee Oh, it's not even an available action :(

dmckee

Well, now we know.

AnimatedPhysics

16:55

Black hole story chandra.harvard.edu/press/16_releases/press_062716.html

vzn

@JohnRennie heres a wild idea just want to throw out. have a funky physics book that proposes just as there is a "big bang" there is a "big crunch" sucking up the explosion "on the other end". in a perpetual cycle. anyone heard of such a thing? might write more on that sometime, apparently its taken halfway seriously by some...

AnimatedPhysics

From the article: Because this study only covered a very small patch of sky, the implication is that there should be many of these quiet black holes around the Milky Way. The estimates are that tens of thousands to millions of these black holes could exist within our Galaxy, about three to thousands of times as many as previous studies have suggested.

John Rennie

@vzn Eh?

A Big Crunch is going to require dark energy to have a very strange equation of state.

vzn

@AnimatedPhysics thx for the ref, it sounds remarkable wrt a "small" black hole not a lot larger than our own sun mass. huh!

@JohnRennie cant remember this exactly but isnt it known our universe is "exploding" in some particular "direction"? need to learn more

AnimatedPhysics

@vnz Can lots of little black holes all over our galaxy account for dark matter?

John Rennie

16:58

@vzn No.

Obliv

does spacetime curvature exist at the quantum level

Balarka Sen

@vzn Wasn't that Friedmann who proposed it first? I thought that has long been discarded.

ACuriousMind

@Obliv Nobody knows...

Slereah

@JohnRennie i recall some discussion about how adding back reaction of a phi4 field completelt fucks up cosmological models

Obliv

@acuriousmind so there's no geometrical interpretation of EM,strong & weak forces?

ACuriousMind

17:04

@Obliv That does not follow at all from what I said.

user54412

@Obliv Quantum mechanics and QFT take as given the spacetime of Newton/SR/GR. So yes. If you want an absolute truth beyond what some model says, well, good luck finding it.

ACuriousMind

What is unknown is what the proper theory of quantum gravity is.

John Rennie

@Slereah See this article:

newscientist.com/article/…

Though bear in mind that New Scientist's editorial policy is a bit wayward at times.

@Obliv Yang-Mills theories are geometrical, but it's a different kind of curvature. There's a question on the site somewhere about this.

user54412

The source article for John's story: arxiv.org/abs/1505.07800

John Rennie

15

Q: Can all fundamental forces be fictitious forces?

After reading many questions, like this and this, I wonder: is it possible to consider also the other fundamental forces, the electroweak interaction and the strong interaction or ultimately the unification of these, to be fictitious forces like gravity in the framework of general relativity? ...

general-relativity forces reference-frames interactions unified-theories

vzn

17:08

@JohnRennie (googling...) scientificamerican.com/article/universal-alignment

Obliv

@acuriousmind it's momentum that causes things to attract each other in GR right?

John Rennie

@Obliv No

Obliv

@johnR what is it then

ACuriousMind

@Obliv that's not even wrong because I have no idea what that's supposed to mean.

Things don't exactly "attract" in GR

Obliv

er

user54412

17:10

Well, one might say that since the stress-energy tensor is just the momentum flux tensor, then sure... Not that anyone would say such a thing without a lot more context.

vzn

@BalarkaSen "discarded" is a loaded phrase in science esp when there are so many/ huge unexplained phenomena begging for explanations... eg dark matter/ energy etc... "unresolved" is key concept/ theme...

John Rennie

@vzn that's just an observation that there may, repeat may, be unexpected large scale inhomogeneities in the CMB.

Obliv

I phrased that badly. It's momentum that causes an object to curve spacetime, attracting things to it, right? @acuriousmind

ACuriousMind

@Obliv It's the mass/energy/momentum of the object that curves spacetime. I'm not sure why you're singling out "momentum" here.

Obliv

well I'm under the assumption that massless particles curve spacetime

John Rennie

17:11

@Obliv momentum is one contribution to the stress-energy tensor, but only one. And in most cases it is not the dominany contribution.

@Obliv because they have a non-zero energy density.

Obliv

Well it follows that if something has momentum, it has energy, no? @johnR

vzn

hi all announcement/ reminder, there will be a special guest yuggib next/ 2wks from todays chat session, plz drop by, ask a question or two meta.physics.stackexchange.com/questions/7783/…

John Rennie

The stress-energy tensor treats matter density and energy density as the same, with just a factor of $c^2$.

Obliv

not necessarily requiring mass, though.

is this correct

John Rennie

@Obliv yes, massless particles gravitate. In fact very shortly after the Big Bang the photon density dominated the expansion dynamics.

ACuriousMind

17:14

@vzn Discarded is not a "loaded phrase". We discard models that are in conflict with observation, or that have been superseded by more predictive models.

John Rennie

The way we usually write the SE tensor the energy/mass density goes into $T_{00}$ and the momentum flux goes into $T_{0j}$ and $T_{i0}$. However $T_{00}$ is usually dominant.

Obliv

what makes certain particles harder to detect than others?

John Rennie

@Obliv coupling constants

Obliv

@johnR what does a coupling constant depend on

what is the 'strength' of the interaction in the field

John Rennie

@Obliv We don't know

ACuriousMind

17:18

@Obliv The strength of interactions of particles with other particles is an experimental input to the quantum field theory. It is not a predicted or derived quantity.

John Rennie

One day some unified theory may give us an explanation for all the coupling constants, but so far no such explanation exists.

vzn

@ACuriousMind ok (fine) so then who thoroughly analyzed and discarded a steady state universe?

Mike Miller

@ACuriousMind if you're looking for interesting questions I can keep asking you about $\delta^2$

Obliv

@acuriousmind isn't it true that larger/particles with more energy are harder to detect?

John Rennie

@vzn Is that a serious question?

ACuriousMind

17:19

@vzn Famously Hubble, among others.

vzn

@JohnRennie (sigh) look ("holier than thou") guys even the big bang is a hypothesis

ACuriousMind

Oh, that's it. "Only a hypothesis/theory" is the worst science trolling in existence. I'll not waste any single further word on you.

John Rennie

@vzn no it's not, it's a well defined mathematical model. To what extent the model describes the real world, well that remains unclear.

If we're showing signs of frustration it's because you're asking questions that an undergrad should be able to answer, and you're asking them as if they're somehow portentous.

vzn

@ACuriousMind yikes youre in your stern/ angry schoolteacher mode, dont know what you ate for breakfast :(

0celo7

God damn

ACuriousMind

17:22

@MikeMiller I swear the universe conspires against me to be able to ask that lecturer what he meant. I have not seen him in three weeks although I should have class with him weekly.

0celo7

@JohnRennie What questions?

vzn

@JohnRennie whatever! am not hearing any refs/ citations

0celo7

I'm not sure what you guys are upset about

I agree with @vzn

Obliv

@JohnRennie Isn't it also a hypothesis?

vzn

seems quite plausible to me that dark matter/ energy, long unresolved, may be related to universe expansion aspects

Mike Miller

17:24

I can also ask you questions that boil down to "Can you read this Witten paper and dumb it down for me?" :)

2

vzn

@0celo7 thx for the support you @#%& undergrad :P

John Rennie

@Obliv No, it's a mathematical model. It's the zero time limit of the FLRW metric. The hypothesis is that it's a good description of the real universe, but in any case we all think it's probably just a reasonable approximation to the real universe.

ACuriousMind

@MikeMiller Well, we already did that once ;) That's actually not bad because I usually learn a lot when I actually have to decipher a Witten paper

John Rennie

@vzn that's the point of the papers mentioned above. The approximations used in the FLRW metric may break down at long times and as a result the observed expansion may differ from the FLRW metric.

ACuriousMind

Alas, for today, I have to leave.

John Rennie

17:27

Which is a really interesting thing to work on, but has until recently been out of reach for computational reasons.

We're just beginning to get the first calculations that might shed light on this.

Though few cosmologists think that any big surprises are likely.

David Z

OK, I'm back. Sorry to have missed the chat session.

I've been very sick the past week and I figured dealing with that takes priority over chatting :-/

Obliv

@JohnR any idea how experimental particle physicists 'discover' particles? Don't they just collide beams of particles

what is the theory behind it

0celo7

QFT

Obliv

@davidZ hope you feel better :)

David Z

so do I

(thanks)

Obliv

17:32

@0celo7 was that to me? it helps to be specific man

Shadock

Hello

John Rennie

@Obliv if you measure the intensity of the scattered particles as a function of energy you expect a smooth plot. If you get a bump in that plot at a certain energy that indicates something happened at that energy. Usually that something is a particle being created and the energy is the particle mass.

There you go. This collider physics stuff is dead simple ;-)

Shadock

I don't understand why this reasoning from Feynman Lecture is true

"Because it is reflected"

It is really not obvious for me

Can someone help me please

John Rennie

@Shadock remember that force is the rate of change of momentum.

Emilio Pisanty

-2

A: Is it possible to "see" atoms?

Im not allowed to delete this answer. Only edit it ?

↑ needs some attention

I'm kinda done with it

Obliv

17:42

@johnR what determines whether a new particle is created? say $A$ collides with $B$ so $A + B = C + D$ or $A + B = E + F$ what produces one or the other?

John Rennie

Suppose the particle is moving to the right with momentum mv, then it hits the piston and bounces off, so now it's moving to the left with momentum -mv.

David Z

@EmilioPisanty I rolled it back

Emilio Pisanty

@DavidZ cool

John Rennie

The momentum change is initial momentum - final momentum = mv - (-mv) = 2mv.

Emilio Pisanty

I guess it's interesting to have a microscopist on board

but that answer definitely needs a lot of polishing

I did my best but I'm out.

Shadock

17:44

I understand the calculus but I have trouble with it

John Rennie

@Obliv that's described by quantum field theory. To create a new particle means adding a quantum of energy to the quantum field that describes the new particle. The probabilities for this happening are calculated using those pesky Feynmann diagrams.

Obliv

so it is probabilistic?

John Rennie

@Obliv yes. Typically there are many possible ways for the initial kinetic energy to end up in different quantum fields, and each has a probability.

Shadock

Trouble in thinking we give two time the momentum to the piston with only one hit

times*

Obliv

I suppose it's nearly impossible to isolate a single particle collision with controlled conditions to see what causes it anyway.

John Rennie

17:47

@Shadock well suppose the particle hit the piston and stopped, what would the momentum change be?

@Obliv well that's not true, because that's exactly what particle colliders do.

Shadock

If the particule stops its momentum will be 0 and the piston will have mv?

David Z

@Shadock Yeah, but that's not what happens.

Obliv

well @JohnR I mean a single particle colliding with another particle. I imagine a 'beam' of particles doesn't give precise information about the particles themselves.

John Rennie

@Shadock Yes, the momentum change is mv. Now suppose the piston fires the stopped particle away again at velocity -v. What is the momentum change?

Shadock

Why the piston would fires the particule at -v if the particule stopped and the piston moved a bit in the right because of the momentum the particule gave ?

John Rennie

17:52

@Obliv I got the impression from looking at your posts here on maths that you're already pretty accomplished in physics/maths. So maybe you should be looking for a more complete explanation that the cartoon one I've just given ...

@Shadock the point is that the particle can't just hit the piston and stop. If you calculate what happens when a light particle like a gas molecule hits a heavy object like a piston you have to do it by equation total energy and total momentum before and after the collision.

What you find is that if the incoming particle has velocity v, then after the collision its velocity is slightly less than -v. So the recoil velocity isn't exactly -v, but it's very close to it.

Shadock

Hum

John Rennie

So the momentum change isn't exactly 2mv, it's 1.9999(lots of 9s)mv.

But for most purposes we can just approximate it as 2mv.

Actually the slight decrease in velocity is why an expanding gas cools.

Shadock

Well I understand what you mean. And I also understood what I missed reading again the conditions of the problem. Than you for your explanations.

Obliv

@johnR i'm not at all accomplished but yeah I think I'm just hoping that I can understand it now. It'll be years before I can understand modern physics probably

Bass

@ACuriousMind: The partial integration I came up with is:

$$
\int_{t_i}^{t_f}dt\left[\frac 12m\dot x(t)^2-\frac 12m\omega^2x(t)^2\right] = \int_{t_i}^{t_f}dt\left[-\frac 12mx(t)\ddot x(t)-\frac 12m\omega^2x(t)^2\right]+\left[\frac 12mx(t)\dot x(t)\right]_{t_i}^{t_f}
$$

Is that what you meant?

Jim

18:13

so I'm looking for ideas for devices that the university physics club could build over the next year and wondered if anyone had any ideas. The type I'm looking for is something that looks cool or is a cool idea and having a practical usage is optional. For example, the first year, they built a tesla coil hooked up to a keyboard that plays music. Looks cool, shows physics, no practical usage.

last year they built a 3d printer. Cool idea, practical usage

Secret

18:27

I am going to bookmark chatroom log 28/6/2016 later as there are some really interesting stuff going on here that might provide some insights to my self study quantum project

kevin Tah N.

19:04

@0celo7 I noticed @ACuriousMind totally ignored my humbe request to correspond with him about some ads/cft stuff I am considering doing lol

I am sure people think I am still as stupid as I was back in the day . . . ignoring the fact that I have been studying daily for a few years now. . . . . sad. . . first and eternal impressions :^/

How do I bother Lumo*. . . he seems like he might have a lot of spare time

*Lubos Motls. . . . I want to share my secret crackpot(kidding) ideas with him

0celo7

19:51

@Jim A device that polishes 6x6 mm silver and copper, various alloys including gallium and uranium squares

kevin Tah N.

I created a room. "Drop by for left over pizza" not physics

Jim

20:15

@0celo7 so, how is that a cool physics idea?

0celo7

20:31

@Jim because it will help me with my job as a solid state physics undergrad?

user54412

@Jim Perpetual cloud chamber.

0celo7

@ChrisWhite Am I the only one completely unimpressed by cloud chambers?

Cloud chambers and g waves

I'm the only one who doesn't go bonkers over them

user54412

20:47

You have only ever shown disdain for physics in this room, which is a tad contrarian to say the least.

0celo7

@ChrisWhite theoretical physics

But also physics that has no use

And cloud chambers

kevin Tah N.

21:23

hey guys, consider coming to my room for some hay and water

ADS/CFT for Sophisticated donkeys

I am trying to produce a paper. You can come by and listen to ...

@ACuriousMind soooooo? I suppose me offer is laughable from the lack of enthusiasm.

HDE 226868

@0celo7 Hey! That was my E+M final project!

Not that it worked, of course. . .

Emilio Pisanty

@DavidZ Just saw your lock on that one.

Maybe give that user a place in chat where they can still ask?

0celo7

21:38

@HDE226868 the polisher????

Emilio Pisanty

As is they can no longer comment under that answer, so they can't comment anywhere on the site.

HDE 226868

@0celo7 A cloud chamber. The exclamation mark was indignant.

Slereah

21:52

Cloud chambers are neat

My university had one in the hall

0celo7

@HDE226868 Ah, well, we all have to do lame projects sometimes.

dmckee

@EmilioPisanty At my last job I supervised REU students most summers, and that REU program made them start giving presentations about their work (to the rest of the REU crowd) by week three of the program. This post reminds me very strongly of some of those first presentations.

Like this guy is still thrashing about in a poorly understood and nearly unorganized sea of fresh information about which he is very excited but over which he has no mastery to speak of.

Painful to watch/read, but a necessary part of getting them up to speed in a short time.

0celo7

22:08

Hmm, I wonder if I can use Lemma 2 in the appendix...

@dmckee Do you have kids?

dmckee

Nope.

0celo7

I wonder what it's like for STEM people who have children and have to watch their children being ruined by public education

I once had a teacher take off points because I did not put parenthesis around a fraction

I'm still mad about that.

dmckee

Then tend to be very active in their kid's education, and a lot of them send their kids to private schools (or home schooling) for a least part of their primary/secondary education.

The tendency around here is to accept the elementary schools and high schools, but keep them out of the middle school which are pools of stagnation in a lot of classrooms.

My department head has one in public high school and one in private middle school right now.

0celo7

@dmckee How expensive is private school?

dmckee

22:23

@0celo7 Some of the religious one are subsidized, but mostly they cost quite a bit. Several thousand dollars a year and up.

The big private one in town is right across the street from my uni and it looks more like a real college than this place. The dual credit students we get from them are mostly serious achievers who bring up the average level of student in intro classrooms.

0celo7

@ACuriousMind Is there a vector bundle $\xi\to M$ such that $TM=\xi\oplus \xi$?

Hirsch seems to think so but I'm stumped.

@dmckee Wow!

@dmckee I believe it

I took some dual credit classes in high school and I was the most motivated one there

dmckee

@0celo7 Do you know how much the state give a public school per students? It's more than the average tuition at a private school.

0celo7

@dmckee Yeah, probably.

dmckee

Part of the excess goes into dealing with difficult/disadvantaged students, and part goes into some bureaucracy and the rest goes ... well, I've never been quite sure.

0celo7

@ACuriousMind That doesn't even really make sense if $M$ is odd-dimensional either.

@ACuriousMind Do you know how to interpret this? I'm not sure how to show it but I'm still wondering how one applies that result to $TM$.

Emilio Pisanty

22:37

@dmckee Yeah, agree on the painful to watch bit.

I tried to get him/her to read the existing answer more carefully, but to no avail.

0celo7

@ACuriousMind Ahh, maybe one has to consider $TTM=\xi\oplus\xi, \xi\to TM$...which I know nothing about.

@dmckee government inefficiency?

Slereah

One day I want to ask a question on Stack Exchange about Fadeev Popov terms and call it "The mystery of the haunted Lagrangian"

dmckee

@0celo7 Blaming a budget on "government inefficiency" is like my students who want to blame every bobble in their lab results on "human error": there might be some truth to it, but it is completely unenlightening.

I think I read that public school teachers have systematically better compensation packages than those in private school which would be part of it. And I wouldn't be surprised in the administration had a similar compensation edge, but what else?

0celo7

@dmckee I blame my results on shitty equipment.

$50,000 polishing machine and I can't polish a piece of copper

@Slereah Might want to delete that.

Slereah

Which rule does it violate

0celo7

22:44

I got a week for a holocaust joke once

@Slereah It's offensive

Slereah

So I'm trying to work out self intersecting geodesics in thin shell wormholes

I am using some math I did not expect to use in GR

(Raytracing)

Also I suspect that alice handle wormholes do not have self intersecting geodesics either

At least not the basic one

0celo7

@Slereah what

Slereah

Raytracing is what you use in computer graphics for generating 3D shapes

dmckee

@Slereah Rays are geodesics, no?

Slereah

Yeah

dmckee

22:50

Sure for graphics purposes we work in a very simple space-time, but it is still the same stuff.

We do ray tracing for particle physics Monte Carlo, too. Both photons and massive particles.

Slereah

Hm

dmckee

I've actually gotten quite a bit of use out of the upper division graphics programming course I took.

Slereah

I suspect that a rotation of pi/2 isn't enough to avoid self intersecting geodesics

dmckee

And I took it for a lark. It just seemed like a good idea at the time.

Slereah

mb we need pi

0celo7

22:55

@dmckee Can profs take classes for free?

Slereah

In France you can take classes from a university for free

Universities are more or less open to the public

0celo7

@Slereah Communism is offensive to me.

dmckee

@0celo7 That was when I was in grad school, and I was taking a full load already, so no change in cost.

0celo7

@dmckee Ok, but that doesn't answer my question.

dmckee

The rule for profs depends on the school. Here I pay half tuition and no fees. Or most other profs will let you sit in unofficially.

Other places I've been had them free, or 25% cost but you did pay fees.

And I've heard of other arrangements.

Slereah

23:01

other "arrangements"

dmckee

You should see the spelling mistakes I make at the board.

But so far I've never misspelled "hadron", which one of my professors did.

Slereah

oh my

0celo7

@dmckee Hmm?

dmckee

Yep. Even grad students will giggle like twelve-year-olds with the right stimulus.

0celo7

> Let us think, say, of a surface or a solid made of rubber, with figures marked upon it. What is preserved in these figures if the rubber is arbitrarily distorted without being torn?

I have no idea :<

Slereah

23:04

I think the Correct equation for self intersection of geodesics is like

Pick the class of geodesics that intersect the disk 1 of the wormhole but not the disk 2

Write down their image after going through the wormhole

Find out if those lines intersect

Oh wait

I also have to check if the geodesic intersects the second disk in the opposite direction, maybe

I dunno

0celo7

@Slereah Can you answer my vector bundle question?

Slereah

I cannot

I am no wizard

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