The h Bar - 2020-01-17

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Nobody recognizeable

02:32

user image

If helmholtz free energy is discontinuous then why gibbs isn't. Ie g=h-Ts. I can't see why internal energy is discontinuous too?

vzn

03:30

@undefined fyi there are a whole set of new cutting edge experiments that show "macroscopic QM behavior"... highly recommend looking into them, and few around here comment or seem to find them remarkable, but its clearly a substantial paradigm shift in play. but yes the idea of waves being mostly a small scale phenomenon has been around for decades... it was once called "the infamous boundary" in one of my favorite popsci physics books... (by Wick)...

2 hours later…

Johan Liebert

05:29

Does anyone here knows a case in which energy is not conserved but momentum is?

Semiclassical

05:53

@JohanLiebert As far as kinetic energy conservation goes, the go-to example is an inelastic collision of two bodies

Of course, the total energy is conserved once one accounts for changes in internal energy

Johan Liebert

@Semiclassical i am considering total energy conservation.

0

Q: Does momentum conservation imply energy conservation?

I was trying to figure out situation in which energy is conserved and momentum is not and it was quite easy to find out one case which is of a stone tied to a string moving in uniform circular motion. Then I thought to consider the reverse situation in which momentum is conserved but energy is n...

newtonian-mechanics energy momentum conservation-laws symmetry

I have elaborated over here.

Semiclassical

Gotcha

enumaris

06:53

hmmm

undefined

07:06

@vzn thank you, vzn. I'll look into it

2 hours later…

Naveen Balaji

09:23

Hey all! I had this doubt- Einstein tensor vanishes identically in (1+1)-D, which means that the stress tensor, S will also vanish (following the expressions as given in Poisson). But, shouldn't S be non-zero to indicate the discontinuity in the first derivative of the metric at the shell? If it vanishes, how does it account for the matter placed on the shell?

John Rennie

09:43

@NaveenBalaji I think there can still be a non-zero cosmological constant appearing on the left side of the equation.

i.e. the equation reduces to:

$$ \Lambda g_{\alpha\beta} = 8\pi T_{\alpha\beta} $$

Naveen Balaji

09:59

@JohnRennie Given the metric, $dr^{2} + \gamma_{tt}(r,t) dt^{2}$, the stress-energy of the source on the screen happen to be null. This metric config. is reflected in the Ricci tensor and scalar since both of these are non-zero. Since the stress tensor, $S_{ab}$, is zero, $T^{\Sigma}_{\alpha\beta} = S_{ab}e^{a}_{\alpha}e^{b}_{\beta}=0$ and this indicates that there is no source right?

John Rennie

No. In 1+1D $R_{\alpha\beta} - \tfrac12 Rg_{\alpha\beta} = 0$ but $R_{\alpha\beta} - \tfrac12 Rg_{\alpha\beta} + \Lambda g_{\alpha\beta}$ can be non-zero.

4 hours later…

Johan Liebert

13:53

1

Q: Newtonian-Mechanics .Elastic reflection

Question from the olympiad. Two planes move perpendicular to each other with speeds $v = 1m/s$. A body flies at them with a velocity of $u = 1 m/s$ at an angle $a=45$ to the surfaces of the planes and is elastically reflected successively from each of the planes.No friction Question: Fin...

homework-and-exercises newtonian-mechanics kinematics collision

That user has posted a new question though this time it is a bit different. Should I answer it?

Yuvraj Singh...

14:49

@NaveenBalaji hello.

Jake Rose

@JohnRennie I was at girton last week, they have a bloody swimming pool and I can’t contain my jealousy

John Rennie

15:14

@JakeRose :-)

My niece still has to get the two As and an A* of course.

Johan Liebert

1

Q: I'm stepping down

You are given 4 positive integers: volume of the first container (v1), volume of the second container (v2), volume of the liquid in the first container (l1), and volume of the liquid in the second container (l2). Your task is to move (or "step down", if appropriate) some of the liquid from contai...

code-golf arithmetic integer

Lol (Some serious use of title!)

Novalium Company

15:33

I'm planning to get single super thin wires from one of those (ref to pic) and somehow insulate them individually. How would I do that and is there a better way?

user image

A single wire is as thin as a human hair. Is there a way to insulate it?

What if I dip it in some substance?

What if I dip it in melted hot glue or something?

John Rennie

@NovaliumCompany Lacquer

Novalium Company

How a spray gonna do it?

John Rennie

The spray dries and leaves a solid insulating layer on the wire.

Novalium Company

Alright, thanks.

tpg2114

You'll have to be pretty careful not to burn up single strands of wire

Novalium Company

15:41

I know, that's why I'll go for a bit thicker ones.

tpg2114

I am not an electrical engineer, but a single strand isn't going to carry more than milli-amps

Novalium Company

Can I just buy thick insulated ones? lol

@tpg2114 true, I'm reconsidering making it thicker.

tpg2114

Yeah, of course you can. Thin, thick, braided, not braided

All pre-insulated or not. With thick insulation like your picture, or very thin (like speaker wire)

I guess I should say the "of course you can" is from the perspective of the USA -- maybe it isn't as readily accessible elsewhere, depending on where you might be

Novalium Company

I found what I needed. I have a sample to show the store owner.

tpg2114

You can calculate how thick it needs to be based on your power needs plus a safety margin

Or use a lookup table -- I'm sure there's tables out there since everybody has to size the wire accordingly (thinnest you can get away with is better for weight and cost)

Novalium Company

15:45

I will calculate it.

I need to figure out the other electronics first :P

tpg2114

I was a very bad circuits person. Being colorblind makes reading resistors nearly impossible, and causes lots of burned up hardware.

Knight

0

Q: How can action and reaction forces be equal?

Suppose I push an object having a very small mass of 10 grams. The object accelerates, so does our hand. Since the mass of the object is less than us and so is the acceleration then how can it exert an equal force on our hand since $F=ma$?

newtonian-mechanics forces free-body-diagram

Can someone please tell me why @Qmechanic has added the free -body diagram tag, he must have done for a purpose but i’m unable to understand it.

Loong

16:06

@ACuriousMind twitter.com/gortok/status/1217909998023577602

> Moderators: since we can’t feature moderator resignations, we’ll just feature everything that talks about this craziness.

Qmechanic

:53262337: Usually this kind of problems arise for not properly analyzing the various forces. A FBD often helps.

Knight

@Qmechanic Okay. Sometimes I think how do you earn your reputations, beacuse @JMac told me that people having reputations above 2000 don’t receive +2 for edits. How is it that your reputation is increasing?

Is it because of your earlier posts getting voted up?

tpg2114

@Knight physics.stackexchange.com/users/2451/qmechanic?tab=reputation

You can look at anybody's reputation history

MKC

@tpg2114 would you please help me out in a physics question(sort of)?

JMac

@Knight Probably has more to do with posting answers almost daily and having a lot of them accepted physics.stackexchange.com/users/2451/…

Knight

16:17

Thanks @tpg2114

tpg2114

@MKC Don't ask about asking a question -- just post the question and people can answer it if they are able

And also don't ping random people to ask for help -- only ping those you know will be interested, otherwise just post the question and the folks around here can address it

Knight

@JMac But I never saw him posting answers, I know because he answers only high level questions which I never open.

tpg2114

@Knight You can also see answers posted: physics.stackexchange.com/users/2451/qmechanic?tab=answers

He's still actively answering things

JMac

@Knight I don't see many of his answers when I browse either for the same reason. That's why I checked his profile and linked to his recent answers. You can see he's still posting them daily.

tpg2114

Heh -- that's what I get for not looking at your link @JMac

Sorry for just repeating you

Knight

16:21

@JMac He is like a Spector

John Rennie

@Knight like a wall of sound?

Knight

@JohnRennie Wall of sound? This humor is very complicated.

John Rennie

Phillip Harvey Spector (born Harvey Phillip Spector, December 26, 1939) is an American record producer, musician, and songwriter who developed the Wall of Sound, a music production formula he described as a Wagnerian approach to rock and roll. Spector was dubbed the "First Tycoon of Teen" by writer Tom Wolfe and is acknowledged as one of the most influential figures in pop music history. After the 1970s, Spector mostly retired from public life. In 2009, he was convicted of second-degree murder and has remained incarcerated since.Born in the Bronx, Spector began his career in 1958 as co-founder...

MKC

@tpg2114 First of all, mostly all are random here and I tried to ask you as I you have interest in newtonian mechanics etc...

but if you want me to post here...well here it is physics.stackexchange.com/q/525519/218252

John Rennie

@Knight I suspect you meant he is like a spectre not Spector :-)

Knight

16:24

@JohnRennie aha yes. It’s because of lack of sleep I think (you can say lack of mind also)

tpg2114

@MKC Heh -- well, only 5% of my answers are in newtonian-mechanics so I can't say it's my forte. But I can look at the question later when I'm not working

MKC

Thanks for that . I hope others may help too.

John Rennie

@Knight anyone my age will immediately associate Spector with Phil Spector

Knight

@JohnRennie Did you like Roger waters ?

John Rennie

Yes, he is one of the greatest lyricists of all time.

Knight

16:27

@tpg2114 Sir can you please tell us what your stage name means, I mean your online name that is tpg2114

John Rennie

Not the greatest composer I have to say. Lyrics yes, tunes not so much.

tpg2114

On a totally unrelated note, I find Mystery Science Theater 3000 to be the ultimate background TV while working. The movies are bad enough that I don't really want to watch them, but every once in awhile there's something worth laughing about

Knight

@JohnRennie How he made his voice so heavy? His voice can break delicate show pieces of royal families (English royal families.

tpg2114

@Knight tpg2114 is my birth name. The name I use in person is fake.

Okay, that's not true. But this was already discussed yesterday in chat actually so I won't rehash it now

Knight

Wow!

John Rennie

16:28

@Knight Where did you hear that? Roger Waters has a rather ordinary baritone voice.

JMac

@tpg2114 I was gonna say. As of just yesterday I know what your name means.

John Rennie

We are talking about the same Roger Waters? The bass player from Pink Floyd?

Knight

Yes.

“We don’t need no education, we don’t need no thought control”

tpg2114

@JMac Somewhat ironic that I had never been asked that in the past 8 years I've had an account here, and now it's been brought up twice in two days

John Rennie

@Knight As far as I know Roger Waters's voice has never broken any crockery :-)

JMac

16:30

@tpg2114 I'll make sure to ask you tomorrow for good measure.

tpg2114

@JohnRennie Her Majesty was spotted with a hearing aid the other day -- perhaps she is a Pink Floyd fan and your statement needs revision :P

Knight

@JohnRennie m.youtube.com/watch?v=MAe_w9a_IN8

tpg2114

@JMac If I had the patience, I would increment the number every day from now on.

But... I'm pretty lazy.

John Rennie

@Knight yes, I know the song, but I don't think Waters's voice is particularly special in it.

Knight

@JohnRennie which one is your favourite?

John Rennie

16:34

My no. one favourite Pink Floyd album is Animals. That's the album before The Wall.

But it's a pretty dark album, and not to everyone's taste.

Knight

Okay

JMac

@Knight ... "Pink Floyd - We Don't Need No Education " lol not even the song name.

Knight

Have you ever heard Poets of the the fall?

tpg2114

@JMac Gotta trick the copyright scanning folks at YT!

Knight

@JMac actually I did that only to show how heavy his voice seems to me.

Anyone here knows Poets of the Fall?

tpg2114

16:36

@Knight Seek out Tom Waits if you want a deep voice. Although it also sounds like his throat is full of pebbles

JMac

@JohnRennie Animals is something else. I have to be in the right mood to listen to it; but when it connects, it really connects.

tpg2114

Not everybody's cup of tea. Johnny Cash is the other stereotypical deep-voiced singer I can think of, especially later in life

John Rennie

@JMac I just love it. It's so brooding and intense.

Knight

@tpg2114 He sounds very well, I thought that he was performing just before me in person

John Rennie

@tpg2114 my favourite Johnny Cash song is his cover of a song by Trent Reznor of all people.

tpg2114

16:39

@JohnRennie Which song is that? I may know the Johnny Cash version but am too uneducated in music to know it was a cover

John Rennie

Johnny Cash - Hurt

tpg2114

Oh yeah, that's fantastic

JMac

@JohnRennie Even Trent Reznor likes that version better IIRC

John Rennie

Just the opening chords send a chill down my spine.

Knight

Can anyone tell why most of songs are written for love? Is it because youth is infected by it and hence will listen to it?

tpg2114

16:42

I wouldn't say most are

John Rennie

@Knight you'll find out in due course :-)

Knight

Hahahahahaa

tpg2114

But, in general, art is a vehicle to express and connect with emotions and the essence of being human

Knight

I think if some statistics were to be done then we will find that most of the songs will have this phrase “I cannot live without you” or something similar

John Rennie

@tpg2114 if you consider chart songs, i.e. not the weird racket I listen to, then I bet over 50% are to do with one human's lusts for another human.

JMac

16:46

@Knight Most is definitely too strong a word. Some songs express those things, but there are also plenty of songs that deal with living without someone. Let alone all the songs not about that at all.

Knight

@JMac All those singers who say “I cannot live without you” in their songs, are generally seen giving interviews after that song.

tpg2114

And what is more emotional or essentially human than love

JMac

@Knight But a lot of those songs are possibly written about significant others. But yeah, obviously that is an exaggeration for most people.

John Rennie

@tpg2114 lust, avarice, sloth and the other four deadly sins I can't remember. Love comes way down the list :-)

JMac

@JohnRennie Man, I always have to watch that video when I'm reminded of it.

Knight

16:51

Which video?

JMac

Such a good music video for the song.

@Knight The Hurt music video.

Knight

@JohnRennie Do you believe that love is the cruelest thing a human can face?

2

Well I do

John Rennie

I know people who have been hurt by it, but I also know people who have rejoiced in it.

JMac

Why would you consider love cruel? To me, love is one of the parts worth sticking around for. It's the loss that I find cruel.

John Rennie

I don't think there is a simple answer. If you refuse to love for fear of the lows you will never enjoy the highs.

Knight

16:54

Let’s change the topic, I’m getting a little personal.

It feels very nice when a small child wants to be with you otherwise his mother would send him to school. For a moment it feels so elevated.

ACuriousMind

17:14

@JohnRennie All the deadly sins are just love of something that's gotten a bit too pathological, when you take them apart ;)

John Rennie

@ACuriousMind ah, now you mention it I've just remembered my favourite deadly sin: gluttony! :-)

ACuriousMind

Your love of food has never been a secret!

John Rennie

I suspect many would regard my cooking as pathological.

Sha Vuklia

17:42

heya guys, anyone specialised here in quantum information?

ACuriousMind

Some are, but why not just ask your question? Maybe some clever generalist can answer it, too ;)

Sha Vuklia

(haha yea I'm typing)

it might be that I asked my question before, but what is a good book on quantum information theory? I read parts from Preskill for a course in quantum physics at the beginning of this year, and I've read an introductory book before that was linked to computation too, but as of now, I don't have like a good book that should serve as a basis?

ACuriousMind

@ShaVuklia Maybe we've already got you covered?

Knight

18:00

ACM why you picked such an ugly picture for your profile. You are now bound to explain it.

The person in your profile looks like a hangman

ACuriousMind

@Knight It's the protagonist of Disco Elysium

Yuvraj Singh...

What happen to you race @ACuriousMind

ACuriousMind

Assuming you mean face, it was time for a change again. I switch it every once in a while.

Yuvraj Singh...

Change pic it doesn, t suit you.

ACuriousMind

Thank you for your assessment.

Yuvraj Singh...

18:09

Can you describe who is that in pic?

@ACuriousMind

ACuriousMind

I just said who it is

Yuvraj Singh...

Ah, yes.

Sha Vuklia

@ACuriousMind hmm.. I mostly see recommendations for quantum information + some other topic/field. have you read a good book by any chance?

ACuriousMind

I have read many good books. Unfortunately for you, very few of them were textbooks, and none on quantum information :P

Sha Vuklia

oh haha, alrights

Johan Liebert

18:14

-6

A: How does one measure Earth's speed of revolution around the sun?

Brain washed programmed space cadets (all imagination in the empty space in mind) europeans/the Neaderthals are sickos cant measure nothing all rhetoric bs the earth is a horizontal plane we all stand up..@90L degree Attention stance tilt off center and you topple over beyond 2.35 inch fake ti...

Knight

18:46

ACM plays video games, wow! I shall play them too.

tpg2114

You don't need to emulate everything random strangers on the internet do

Although, video games are pretty fun

Naveen Balaji

19:46

@YuvrajSingh...what up!

Novalium Company

When I say Backpack I, do you read it as Backpack One or Backpack "I"?

Knight

20:01

@tpg2114 what’s the difference between video game and computer game?

tpg2114

@Knight I would say all computer games are video games. But not all video games are computer games, because they happen on things other than "computers" (meaning PC/Mac/Linux). Maybe there's another definition, but that's how I would define them.

Same goes for other subsets -- all mobile games are video games, but not the other way around. Console games are video games, but not the other way around.

Novalium Company

As we know in a conventional NPN transistor, small current flowing through the collector to the base allows for a much higher current to flow from the collector to the emitter. Sure, but what happens to voltage? P = IV should tell us that the output voltage between collector and emitter should lower?

Knight

@tpg2114 Thank you. I think ACM is old that’s why he called PC game as video games. Video games were something I used to play on TV 📺 by using some very bad consoles. The year was somewhere around 2006

Loong

20:53

jlericson.com/2020/01/17/leaving_stack.html

4

JMac

21:03

@Knight "I think ACM is old that’s why he called PC game as video games." That's a weird way to look at it. Most people consider games on PC to be video games. I don't think age has much to do with it. A lot of video games on consoles also come out on PC

vzn

21:23

in theory salon, 26 secs ago, by vzn

Study finds billions of quantum entangled electrons in 'strange metal' / physorg

2 hours later…

enumaris

23:00

@ACuriousMind Trip to Germany incoming, you free to meet up in Walldorf on the 29th?

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