The h Bar - 2019-11-15

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Physics Meta

01:50

-1

Q: Homework Question Closure Disagreement

I disagree with my physics homework question being put on hold. I'm new here, so I'm open to being wrong and would appreciate guidance if that's the case. I've read the relevant meta-posts. Firstly, I'm from Math Stack Exchange, where homework questions are openly asked and relatively well recei...

Aaron Stevens

02:48

quantamagazine.org/…

7

JMac

03:32

@AaronStevens "Update on November 14, 2019: Following the publication of this article, the neurostatistician Manjari Narayan noted on Twitter that the identity found by Parke, Zhang and Denton also appeared in an unpublished 2014 manuscript by Piet Van Mieghem. In a comment on his blog, Tao confirmed that it does seem to be the same identity." What a rollercoaster

arxiv.org/abs/1401.4580

yuvraj singh

04:14

How is PM 2 RING.

Aaron Stevens

04:24

@JMac yeah really. I was sent this article because Denton got his PhD at Vanderbilt where I am currently at.

Still crazy to have an interaction with Tao

-1

A: Centripetal and Centrifugal acceleration/force

Centripetal force is required to keep the object moving in a circle by constantly pulling it towards the center. The centrifugal force just arises due to the object's tendency to continue motion in a straight line. It isn't a force applied to any body. Its direction is opposite to that of centr...

Keeps pinging me to remove the downvote that isn't mine

user8718165

@AaronStevens sorry for the pestering...I thought you were the one. And moreover...I'm happy to correct errors. We're here for learning.

@AaronStevens I think if there's a downvote, there's room for improvement and I'd be very glad to know where I could improve. Anyways, not being arrogant ;)

So I misinterpreted it as your vote hence I was asking if I could improve and get rid of that down-vote. I'm happy at 0.

yuvraj singh

04:40

@AaronStevens pm 2 RING changed his name

user8718165

@AaronStevens We had been discussing about improving the answer so I was having the false impression.

Sir Cumference

05:16

argh why does flash need to be retired

there are plenty of nostalgic flash games

5 hours later…

Emilio Pisanty

10:17

4

Q: On the method described by Purcell for finding the magnetic field by measuring the force on a test particle

The following text is a method for finding the magnetic field as described in Purcell's Electricity and Magnetism (page $151$, the top part). Measure the force on the particle when its velocity is $\bf{v}$; repeat with $\bf{v}$ in some other direction. Now find a $\bf{B}$ that will make $\tex...

magnetic-fields vectors vector-fields

Seriously?

that got on HNQ?

🤷🏻‍♂️

I really don't get this shit sometimes

2 hours later…

Kyle Kanos

12:10

@EmilioPisanty yeah, the algo seems a lot messed up for us on physics

1 hour later…

PolarBear

13:23

Can someone please help me point out the things I should learn to be able to solve this?

I'm unable to solve this even after doing my lecture slides and I've an exam tomorrow and I'm utterly confused

user image

PolarBear

13:34

Could someone please help or refer to some website/video please? I don't know what to search for.

I would be really really grateful.

Ryan Unger

13:49

@Slereah @bolbteppa check the instructor registrar.princeton.edu/course-offerings/…

Slereah

14:11

@RyanUnger It's the guy!

Slereah

14:29

Recommending his own book for his class

How self centered

skillpatrol

it would be if there were no other recommendations

I once read Dirac only used his book and when asked questions he said go read the book

Slereah

heh

why even go to the class

skillpatrol

14:49

> Harish-Chandra (1987) remembers dropping out of his lectures when he found that they were the same as his book. When asked to clarify something during a lecture, he repeated it exactly since it was already expressed the best way.

Slereah

Dirac was a robot all along

bolbteppa

@RyanUnger that's incredible, totally wasted on you :p

Slereah

Does @RyanUnger still hate string theory

bolbteppa

How about AdS/CFT which is GR'ey

Slereah

It's still a quantum field theory

skillpatrol

15:01

he's a pure mathematician now

Slereah

Is he

Why is he doing GR lectures

skillpatrol

GPA boosting :P

Slereah

Aren't the mamericans supposed to studies the humanities too

What are his humanities courses

skillpatrol

ask'em

Slereah

I just did I suppose

bolbteppa

15:04

He just doesn't know what AdS/CFT is all about and is prematurely stifling himself

Slereah

is it about love

bolbteppa

It would tap into that GR thing of his and not be a waste

skillpatrol

@Slereah they say he was more of a perfectionist

Abhas Kumar Sinha

15:18

@bolbteppa $$ \psi = a e^{i \frac{S}{\hbar}}$$ true na?

where S is the action...

bolbteppa

$\approx$

Abhas Kumar Sinha

@bolbteppa ah, but doesn't that mean the probability which is defined as $\psi \psi^{*}$ is independent of both S and $\hbar$...?

bolbteppa

The equality is just approximate

Abhas Kumar Sinha

@bolbteppa okay, but that approximate equality implies probability is independent of S and hbar?

bolbteppa

It gets more and more accurate in the classical limit in which $\hbar$ goes to zero forcing $S$ to be extremised giving classical mechanics back

Abhas Kumar Sinha

15:26

@bolbteppa weird...! That's what I was asking, for any function, $$f(x) \stackrel{def}{=} a e^{i y}$$, has it's square of modulus independent of $y$ and is equal to $$ |f(x)|^2 = a $$... So, whatever limits we put on $y$ is meaningless... Isn't it?

because $$f(x) = a (\cos y + i \sin y)$$ whose square of modulus is 1 as $\sin^2 y + \cos^2 y = 1$

bolbteppa

It's explained in Landau's QM section 6

Abhas Kumar Sinha

@bolbteppa I'm asking because I was reading that part.

page 20

Ryan Unger

@Slereah humanities in grad school?

Slereah

I dunno

I don't know how the universities work there

Ryan Unger

Princeton math has no course requirements

Slereah

15:29

Practical

Not gonna study poetry then?

bolbteppa

@Slereah @RyanUnger we should post proper full derivations of every equation from arxiv.org/pdf/0712.0689.pdf here

Ryan Unger

@bolbteppa I’ll be sure to heckle

Slereah

Or underwater basket weaving

Ryan Unger

I don’t care about AdS-CFT

Slereah

Should we

bolbteppa

15:31

$\psi \neq a e^{iS/\hbar}$ though, it's equal to that + ...

Abhas Kumar Sinha

@bolbteppa hm......

Are @skillpatrol @skullpetrol and @skullpatrol same person?

@skullpatrol ah okay...

bolbteppa

The only point of analysing $\psi \approx a e^{iS/\hbar}$ is that $\psi$ should reduce to that more and more in the classical limit, and so we can use this to derive things like the Schrodinger equation or the momentum operator, this is made more formal in the quasi-classical chapter

Abhas Kumar Sinha

@bolbteppa How? I don't see if $\hbar \rightarrow 0$, this reduces to classical equations. Because in case of classical equations, the probablity function should be equal to 1 or $a$ in this case, which doesn't happens to be.

Slereah

@AbhasKumarSinha it's the whatchamacall it

Stationary wave method

Abhas Kumar Sinha

ofcourse I assume $\psi$ is normalized to understand the intitution easily...

@Slereah wut?

bolbteppa

15:37

If $\hbar \to 0$ then $e^{iS/0}$ is nonsense, only way it can make sense is if $S = 0$ also (or $\infty$)

Abhas Kumar Sinha

@bolbteppa So that $e^{i S/0}$ is a classical mechanics equation?

bolbteppa

No it is nonsense

Abhas Kumar Sinha

@bolbteppa so you mean that nonsense is equation in classical forms?

bolbteppa

$1/0$ is nonsense, it's the same idea

Abhas Kumar Sinha

@bolbteppa so how can we get classical mechanics equations from it which makes sense...

bolbteppa

15:40

The only way it can make sense is if e.g. $S = 0$ as well so that we get $0/0$

But this forces $S$ to be minimised

Which is classical mechanics

Abhas Kumar Sinha

@bolbteppa ah okay... That makes sense...

Slereah

S isn't 0 when minimized >:|

Abhas Kumar Sinha

@Slereah who said that?

Slereah

Me

24 secs ago, by Slereah

S isn't 0 when minimized >:|

cf here

skillpatrol

lol

Abhas Kumar Sinha

15:42

@Slereah "* I don't have a measurement problem, I can stop whenever I want *" - @Slereah

@bolbteppa Thanks sir :)

bolbteppa

@RyanUnger you will be laughed at when they find out, there, that you don't care about AdS/CFT

Slereah

Also I'm not sure 0/0 is something to aim for to "make more sense" :p

skullpetrol

0/0 = 0*(1/0)

Slereah

not wrong

skullpetrol

0*a = 0

Slereah

15:50

Does it mean that $$\lim_{x \to 0} \frac{x}{x} = 0$$

PM 2Ring

@yuvrajsingh No, I did not change my name!

Abhas Kumar Sinha

@PM2Ring hello :)

PM 2Ring

@AbhasKumarSinha Hello.

Abhas Kumar Sinha

@PM2Ring How's everything there, heard a lot of wildfires and stuff.

yuvraj singh

Hi. So I saw the fire is almost taken the whole Sydney.

@PM2Ring

Are you fine.

Abhas Kumar Sinha

16:02

that's why I asked. I think that there is still official emergency there.

for a week

yuvraj singh

Why there are tree account of the same name, are you same. Or different users.

Abhas Kumar Sinha

@yuvrajsingh different names, look carefully

yuvraj singh

Sorry my mistake.

@PM2Ring Can you suggest me a good on thermal statistics.

@AbhasKumarSinha do you have any idea.

Abhas Kumar Sinha

@yuvrajsingh sorry no idea for thermal statistics and thermodynamics...

yuvraj singh

OK.

PM 2Ring

16:08

@AbhasKumarSinha I'm fine. The area I live in is too urbanised for wildfires (BTW, in Australia, we call them bushfires). But I did smell the smoke for a few days: eucalyptus tree smoke has a fairly distinctive odour.

3

Abhas Kumar Sinha

@PM2Ring ah, good to hear that you are safe :)

yuvraj singh

@PM2Ring hi.

Abhas Kumar Sinha

@PM2Ring I was active on twitter hashtags asking random people how was the condition there. Most of them were unaffected...

PM 2Ring

@yuvrajsingh Sydney does have a lot of trees, and large nature reserves that can be affected by fire, but it also has a lot of very urban areas. Those areas do have trees, but they generally aren't in dense clusters.

yuvraj singh

That, s good.

@PM2Ring

PM 2Ring

16:16

Here's a photo from Wikipedia of one of the main roads in my area. The road is split, with a corridor of trees in the middle.

Abhas Kumar Sinha

@PM2Ring People don't die from bushfires due to actual fire. They actually die because of smokes. See Brazil fire, where 90% of the people were affected due to affected air. Only small tribes have direct effect from fires, otherwise, it's not a big problem.

yuvraj singh

@PM2Ring do you have any suggestions.

PM 2Ring

@yuvrajsingh No, I don't.

Abhas Kumar Sinha

Smoke inhalation

Smoke inhalation is the primary cause of death for victims of fires. The inhalation or exposure to hot gaseous products of combustion can cause serious respiratory complications.Some 50–80% of fire deaths are the result of smoke inhalation injuries, including burns to the respiratory system. The hot smoke injures or kills by a combination of thermal damage, poisoning and pulmonary irritation and swelling, caused by carbon monoxide, cyanide and other combustion products. == Signs and symptoms == Symptoms range from coughing and vomiting to nausea, sleepiness and confusion. Burns to the nose, mouth...

yuvraj singh

Can we see you house in the image.

Abhas Kumar Sinha

16:22

@yuvrajsingh hehehehe... Cool idea..

PM 2Ring

@AbhasKumarSinha True, but Sydney is very large, and I live quite near the coast, so the smoke from bushfires gets dispersed by the time it gets to my area.

Abhas Kumar Sinha

@PM2Ring that's good :) (A house near coast <3)

PM 2Ring

@yuvrajsingh No, but that area isn't far from my place, and I often walk (or catch a bus) along that section of road

yuvraj singh

Do you have sea near your house, if yes, do you have a pic.

PM 2Ring

Few people die from bushfires in Australia, but plenty of people lose their houses and valuable items.

Abhas Kumar Sinha

16:28

@PM2Ring that's sad...

PM 2Ring

@yuvrajsingh Well, the beach is a couple of kilometres away. Before I moved back to Sydney 2 years ago, I lived near the coast. I could see the ocean, and a couple of islands, from my balcony.

Abhas Kumar Sinha

@PM2Ring wooooooooooooooow

PM 2Ring

@AbhasKumarSinha It's a complicated situation. People like to live in "leafy" areas surrounded by trees, so the expensive areas have more trees. But the trees get overgrown, and a bit too close to the houses...

Abhas Kumar Sinha

@PM2Ring btw, it's summer there?

@PM2Ring trees are not a problem, they don't catch fire when green...

PM 2Ring

For tens of thousands of years, the Australian Aboriginal people managed the forests by regular burning to prevent a build-up of fuel. That put a certain kind of evolutionary pressure on the forests. Many Australian plants actually need fire for the seeds to germinate.

Abhas Kumar Sinha

16:36

@PM2Ring true, it should be done...

PM 2Ring

@AbhasKumarSinha Summer is starting. We've had a few warm days. But 2 weeks ago it was still getting chilly enough some nights for me to turn on the heater.

Abhas Kumar Sinha

@PM2Ring same here :)

weather here - satellite.imd.gov.in/imgr/animation3d/sanew_3d.htm

yuvraj singh

What precaution you people are taking. @PM2Ring

PM 2Ring

@AbhasKumarSinha It does get done, but sometimes it doesn't get done at the proper frequency, so fuel can build up to dangerous levels.

Abhas Kumar Sinha

@PM2Ring hmm....

PM 2Ring

16:46

Some people living next to areas that need regular burning don't like it when there's a fire nearby, and complain if there are too many "burn-offs". But they complain even more if the burn-offs don't get done and their nice neighborhood bushland turns into a raging bushfire that destroys their house.

Ryan Unger

@bolbteppa lmao why would I care about physicists laughing at me

Abhas Kumar Sinha

@PM2Ring well, in India Aboriginal people live in isolation, away from human settlements and the government has some areas reserved for them so that they can enjoy their own rights to live like that. So, on one complains here

PM 2Ring

@AbhasKumarSinha Australia is a rather dry country. Australian forests are full of eucalypts, which regularly shed bark. So it doesn't take long for fuel to accumulate. And once a bushfire is blazing, even green trees will burn, especially the smaller ones. They're full of flammable eucalyptus oil, and it doesn't take a lot of heat to dry the smaller branches enough to make them burn.

Abhas Kumar Sinha

@PM2Ring oh okay

user image

@PM2Ring this is the block where I live

yuvraj singh

@PM2Ring have seen Delhi pollution level AQI EVEN reach to 937

Abhas Kumar Sinha

16:56

@yuvrajsingh AQI 340 in UK - 1000 people died same month, AQI 1300 in Delhi - no deaths

bolbteppa

Nobody cares about the value of $0/0$ in this case

1 hour later…

Typo

18:21

Hello, is there anyone here with an understanding of electric circuits?

JMac

@Typo What level of understanding are we talking here?

Typo

high school and above

since I'm in high school

JMac

Sure, I understand them at that level, what are you looking to ask about?

Typo

I want to understand how neutral wires and grounding works in mains electricity

I don't understand how/why the neutral wire doesn't have current through flowing it (or if it does, why it isn't shocking you)

I've looked at some answers on the site about it but most of them talk about phases and such which are way beyond my scope. I still went out of my way to learn about it but I can't seem to connect the dots here

normally with an AC source, the two ends of the wire should act identically but in mains, the live and and neutral wires are different because of neutral wire being grounded

what does the grounding change?

JMac

@Typo I'm not super well versed in electrical wiring specifics, but the neutral wire should be at the ground potential. You should also be at the ground potential. Current flows from high potential to low potential, with greater current the greater the difference. If you and the wire are approximately the same potential, minimal current flows between you and the wire; which is what would harm you.

If you touched the hot wire, part of you would be ground voltage, and another part would be the mains voltage. This would cause a large current due to the large difference; which can harm you. The same would happen if you shorted a neutral to a hot wire.

Typo

18:34

I see but what is happening to flow of electricity in the neutral end?

why exactly is the neutral wire at low potential?

JMac

@Typo It has current; it just would rather keep traveling through the wire than through your body, unless your body was at a lower potential. There's some resistance involved with going between a copper wire to your body, I'm pretty sure its much greater than the resistance of just going through the wire, so the current mostly chooses the wire.

@Typo Because it's grounded at the panel, and electrical codes are designed so that it should remain at ground potential. Sometimes that actually fails, and you can get shocks from the cases of objects; for example.

Typo

what does it mean to be grounded? I assumed it just meant part of the wire goes through the ground, how does that change the original potential of the AC source?

JMac

@Typo Grounded means "at the same potential as the ground". So if you have the neutral potential at the same potential as your ground, it should be the same potential as the people standing on the ground, because we are essentially at the same potential as the ground since we're directly connected to it most of the time.

Novalium Company

In inflation, since income and prices go up, that shouldn't mean that people are "getting poorer" (since they get more money and therefore can buy the same amount of stuff), but money devalues?

JMac

(if not, current would also be flowing between us and the ground)

Typo

18:44

That would be funny

I think something may have clicked. I just need a bit more clarification

JMac

@NovaliumCompany Money devalues, but the value of all their stuff goes up approximately the same amount as inflation. They wouldn't be getting wealthier, even though they make more money; but they wouldn't be any poorer, because as you mention they can buy the same amount of stuff.

Novalium Company

Cool, so why is too much inflation bad? People think they have more money and before prices have gone up buy a lot of stuff, and later they can buy less stuff cuz they've spent their money while thinking they were rich and not have small amount of money and the prices are high? But why are prices high if there is less demand at that point? Is this were deflation happens?

Typo

The grounding is accomplished by splitting the neutral side of the circuit into and connecting 1 part of it to the ground and another becomes the neutral wire?

into two*

JMac

@Typo I don't know the specifics, but I don't think you really split the neutral in two. In a circuit, the neutral is just connected to the ground at the panel, so that it should be at the same voltage as ground. There can also be ground circuits though as far as I know; which are typically used to make sure that the casings of high voltage equipment doesn't go above ground potential and shock you when you touch it. I'm not an electrician or electrical engineer though, so I may be a bit

off

Typo

Even still, would the principle still be the same?

JMac

18:55

@Typo Possibly, but again that's getting too far out of my knowledge. I'd basically be guessing

Typo

Alright, thank you so much

Most my confusions are cleared

I still have a final question

All this explains why the neutral wire itself is neutral.
Say a live wire is connected to an appliance, as in the connection comes in from one end of the appliance and exits out the other and is connected to the neutral wire. Would touching the connection where it exits the appliance be harmful?

JMac

@Typo There's current running through it, but for the current to travel through you instead of the wire is a path of greater resistance typically, if the neutral has no breaks. electronics.stackexchange.com/questions/76726/… Really good explanation here that words it much better than I can from a real electrical engineer.

It shouldn't be harmful, but it definitely can be harmful.

Typo

alright thank you for all the help

JMac

19:16

physics.stackexchange.com/a/514016/127931 Does anyone understand what this answer is saying... It was at +6 but to me it doesn't seem to answer the question remotely.

Rishi

Hi , is anyone there

Aaron Stevens

20:03

@JMac Yeah seems like one of those answers that is talking physics but not really getting to the point of the question.

Loong

20:29

-7

Q: theory of everything is bullshit

The missing link that's giving us all depression by: KY-LA-20 I am so frustrated by what I have realized today that I have to say something about this before I start to go insane wondering if the catholic church was literally controlling Stephen Hawking all along or if he just didn't care about ...

Novalium Company

hello things.

JMac

@Loong Classic "I think I've figured it out" word-salad post.

Novalium Company

@JMac Can I ask u some questions about inflation?

JMac

@NovaliumCompany You can, but I would advise against it if you want accurate answers (assuming you're talking about cosmic inflation, and not like balloons or something).

Novalium Company

Economic inflation.

JMac

20:39

@NovaliumCompany Oh right, we talked about that. You can ask but, similar to cosmic inflation, I don't know a whole lot about it.

Novalium Company

So government prints too much money --> income rises --> demand rises --> prices rise --> you can buy less stuff --> money devalued. Or simply, income rises --> scarcity is reduced --> value lowers. That's inflation in a nutshell right?

JMac

@NovaliumCompany I'm really not sure of the order of cause and effect or anything.

Aside rant, again on this question, how does this get +3 in 10 minutes? physics.stackexchange.com/a/514086/127931

Novalium Company

Alright, no problems. I'mma talk with my father, he should know.

Good night atoms.

ACuriousMind

@JMac Suspicious voting behaviour is a fine occasion for a custom mod flag (but in this case I'll look into it without one anyway).

JMac

@ACuriousMind I literally just starting digging a bit more and was about to flag it. Thanks though.

Definitely some weird patterns there as far as I see.

ACuriousMind

20:51

@JMac No worries. Thanks for being attentive!

JMac

@ACuriousMind Haha no problem. Not like it was subtle with the really quick votes, unclear answers, and random dog pictures.

user199721

21:03

i have a query about generating functions of canonical transformations: does the function i choose need to have half of the variables chosen from old co-ordinates and half from the new ones?

Oh wait no, i think i got it. we need it to be half old and half new...

1 hour later…

peterh says reinstate Monica

22:22

I have an idea. Maybe in the singularity in a black hole, actually matter does not compress to infinite. Instead, a new state of matter appears, where the mean energy per particle is around the Planck-energy.

The density would be the planck-density, $\approx 10^{90} kg/m^3$.

Scientist Smith YT

22:49

@Slereah Ah. I got it!!! Thank You! Sorry, that helped me figure out the answer to a problem I'm having with stabilizing a fusion device I'm working on.

@Slereah Sorry I'm just to excited. Anyways, Thank You for the help.

Sir Cumference

23:14

wait question upvotes are now +10?

who thought that was a good idea

ACuriousMind

2 days ago, by DanielSank

I came here to complain, but I can see that @ACuriousMind is already handling it.

;P

Sir Cumference

keep doing what you're doing then

there was once a time when SE consulted their users about big changes

wonder whatever happened to that

Scientist Smith YT

23:46

True. SE seems to be going down a not so great path by destroying the only thing keeping them going. The users.

A company with a building isn't anything but a building without it's employees.

peterh says reinstate Monica

@SirCumference I've won 900. Before that, I won 500 for a trivial answer to a HNQ. I went from 5500 to 6900. Soon I will be 10k+. Maybe 2-3 years. I could do the rest also in a year, but then I should remain silent on the meta and I should target HNQs, I dislike both.

@ScientistSmithYT Wait, giving more rep for questions, why would it be destroying users? I think, rep plays only a very little part in the motive of most question posts. The most typical motivation of a question post is that the OP wants to know the answer.

@ScientistSmithYT Furthermore, the rep gained by questions is little not only because its ups were only 5. Another cause was that the communities don't like to vote questions up. The expected value of the ups is much smaller, maybe a half or third of the expected up-count of the answers. While taking care a question, defending it from "nice" commenters and VtC threats, understanding and reacting the answers, makes it actually bigger work than posting an answer and smiling on the ups.

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