The h Bar

GPhys

3:06 AM

Are MTW wizards?

koolman

3:40 AM

Good morning

thought for food

4:00 AM

Good morning.

DHMO

7:59 AM

@JohnRennie for the SE of hydrogen atom, have we taken gravity into account?

John Rennie

No. The gravitational force is so low compared to the electrostatic force that we can ignore it. The error introduced by ignoring gravity is far smaller than other approximations made in the SE e.g. ignoring relativistic corrections.

DHMO

@JohnRennie I see, thanks

@JohnRennie is it that the helium SE is impossible to solve or very difficult to solve?

John Rennie

@DHMO there is no analytic solution. That's because it is a three body problem with all three bodies interacting.Even in classical mechanics the three body problem has no closed form solution.

DHMO

@JohnRennie why is there no analytic solution?

John Rennie

There's a theorem somewhere that explains why the three body problem has no closed form solution. I can't remember what it's called but I imagine a Google would find it.

DHMO

8:08 AM

ok, thanks

thought for food

Does this sentence sound ok @JohnRennie "You don't trust me but at all?"

"Don't you trust me but at all?"

John Rennie

That sounds odd. What sentiment are you trying to convey?

thought for food

Not sure, that's what we were discussing in the English language and usage room.

Complete lack of trust, I believe.

John Rennie

8:24 AM

Then just say: "You don't trust me at all" or as a question "Don't you trust me at all?". The word but is out of place.

thought for food

That is exactly what we said.

"but" does not belong

John Rennie

I've just had a look and I think that Faerd was just having fun.

No-one would use the phrase in the way he suggested.

thought for food

I see, thanks for looking :-)

Kenshin

Hello

thought for food

hi

DanielSank

8:38 AM

I need to vent this:

reddit.com/r/AskReddit/comments/5qdccw/…

thought for food

lol

definitely leave the beans at home

GPhys

reads homework carefully

it decelerates too D;

John Rennie

Your homework decelerates? That's after you've thrown it out of the window? :-)

GPhys

8:53 AM

between Landau and Lifshitz, Sean Caroll, and a pdf of MTW I found online, I managed to get a handle of tensors well enough to calculate the travel time for a spaceship leaving earth with a constant acceleration of 10 m/s and traveling 250 lightyears - the time measured on board and on earth

and now it decelerates at the midpoint at 10 m/s :<

John Rennie

Hint: The two halves of the journey are symmetrical ...

Why do you need tensors for that? Just use the relativistic rocket equations.

GPhys

because if I can't figure out how tensors work then I can't finish the rest of the homework :D

John Rennie

I assume you referred to MTW chapter 6 where they deal with constant proper acceleration in SR. However that chapter is not beginner friendly!

GPhys

you don't say

I got $x=c^2g^{-1}(\cosh(g\tau/c))-1)$ and $t=cg^{-1}\sinh(g\tau/c)$

John Rennie

If you work through their calculation step by step it does make sense, but they go through it so briefly that it's utterly incomprehensible unless you already know what they are talking about.

GPhys

8:57 AM

for the first half, which gave me reasonable numbers

where g is the acceleration

John Rennie

40

Q: How long would it take me to travel to a distant star?

Suppose I wanted to travel to one of the recently discovered potentially Earth-like planets such as Kepler 186f that is 490 light years away. Assuming I had a powerful rocket and enough fuel, how long would it take me?

GPhys

I spent most of my time justifying each of the steps

which I think I can do now, at least

John Rennie

Susskind also does the derivation - in one of his Theoretical Minimum books I think. And his version is somewhat clearer.

GPhys

compares your equations to my equations

to be fair (to how much I actually understand) MTW did the hard part

John Rennie

Susskind's derivation is here

GPhys

9:04 AM

oh hey

you did Kepler 186f which is what my question is asking about

John Rennie

When I saw your commebnt just now I nipped back in a closed timelike curve and wrote that post :-)

GPhys

the only difference is my question quotes 500 lightyears as the distance and asks for the round trip time instead of the one-way time

GPhys

9:18 AM

I was gonna just redo the initial conditions at 250 lightyears for -10 m/s until it was back at earth

and I feel like I'm just bumbling it because I've been at this too long

or perhaps I need to make coffee

Secret

9:41 AM

Metallic hydrogen: It's shiny, its round. It's soooooooooo rrooooooooouuuunnnndddd

GPhys

hmm

user228700

@JohnR: Morning :-)

John Rennie

Hi :-)

How is life in sunny Chennai today? It's flipping freezing in the UK :-)

user228700

Oh, it is? What temperature is it?

John Rennie

user228700

9:51 AM

Holy crap.

user228700

Sweet mother of God, I might really die, were I in the UK at the moment.

John Rennie

Die?

user228700

Yep.

John Rennie

I suspect not. You might swear and curse though - like I'm doing :-)

GPhys

huh

user228700

9:53 AM

@JohnRennie I was exaggerating, of course, but I might pass out. Aren't you used to the cold by now?

John Rennie

Yes, -1 to -3 isn't really that bad. In my part of the UK that's about as cold as it gets. In any case the temperature is forecast to rise in the next day or so.

user228700

That's good. Not to make you jealous or anything, but it's a very warm 26 degree Celsius over here :-)

John Rennie

The UK is smack bang in the middle of the collision zone between cold dry European continental airflows and warm wet Atlantic air flows. So the temperature jumps around all over the place depeding on which airflow is winning on any particular day.

I would find 26C a bit too hot.

My preferred temperature range is 15 - 20C.

user228700

@JohnRennie I would find 40C a bit too hot.

John Rennie

And to be fair the UK is in that range quite a lot of the year.

40C?

user228700

9:57 AM

Hmm, OK :-)

user228700

@JohnRennie Yessir!

user228700

37-40C might prove to be much hotter than I like. My preferred range is 27-33.

John Rennie

The highest temperature ever recorded in the UK is about 37C. Above 30C and everyone is complaining about the heat!

user228700

April 14, 2016:

user228700

> "Yesterday saw the hottest day in Tamil Nadu for the year 2016. Karur, Vellore, Trichy & Tiruttani recorded temperatures in excess of 41 degrees while Salem, Dharmapuri, Tirupattur, Palayamkottai & Madurai recorded above 40 degrees. "

Secret

10:01 AM

It is expected next tuesday sydney will reach 38 C

user228700

Haha, I just imagined @JohnR trying to pronounce the names of those places!

user123733

In this why mv$^2$/r is acting radially outwards

John Rennie

@Kaumudi.H The pronunciation of Indian names written in English is fairly straightforward isn't it?

user228700

@JohnRennie Yes, but I hardly suspect that you'd be able to get it right, with ur accent and all.

GPhys

@JohnRennie so I originally solved $\frac{\mathrm{d}u^0}{\mathrm{d}\tau}=gu^1, \frac{\mathrm{d}u^1}{\mathrm{d}\tau}=gu^0$

user228700

10:04 AM

After all, you did think it was cow-moo-dee, not -thee.

GPhys

for g my acceleration

John Rennie

@Kaumudi.H Well, to be fair that's a slightly exceptional case.

user228700

You'll find many such exceptions when Tamil is written in English :-)

John Rennie

You should try pronouncing Welsh or Irish words

How do you think Dún Laoghaire is pronounced?

GPhys

for this the boundary conditions were obvious; I used $t=0\implies\tau=0$ and $x=0\implies \tau=0$ and made use of the fact that $u_\mu u^\mu =1$

user228700

10:07 AM

@JohnRennie Erm, I'd pronounce it like done-la-og (like orgre) -hare

John Rennie

How to Pronounce Dun Laoghaire

►

user228700

Erm... :-)

GPhys

but I seem to be screwing something when I try to set it up with negative acceleration

oh

flips signs

Secret

0

Q: Travel time classical mechanics VS special relativity

Suppose a traveler is going from point A to point B at close to light speed. From the point of view of the traveler, what is the difference of travel time using classical mechanics or special relativity? Can we have a formula giving the difference of travel time experienced by the traveler depe...

special-relativity spacetime time-dilation

John Rennie

Or Coedpoeth, which is a town near where I live.

Secret

10:10 AM

How t get newtonian mechanics from relativity: Make your light cones infinitely wide

user228700

@JohnRennie I'm not sure how oe would be pronounced here.

GPhys

note to self: keep track of negative signs next time

John Rennie

It's like the oi sound in void or the oy sound in boy

user228700

...that is not what I thought.

John Rennie

Welsh and Irish are Celtic languages and they are radically different from English.

The way the written form represents sounds is completely different and massively confusing for non-native speakers.

user123733

10:12 AM

@JohnRennie can you help me in that

John Rennie

The Irish form of my name is Sean. Care to guess how that is pronounced?

user228700

@JohnRennie I'd normally pronounce Sean in the same way that I'd pronounce Shawn but wtf, Irish form?!

John Rennie

Spot on, Shawn is correct :-)

user228700

I've heard in movies, that's why I was correct.

user228700

Normally, Indians might pronounce Sean as seen or Siyan.

John Rennie

10:16 AM

@user123733 The acceleration of the car is the rate of change of its velocity.

In this case the speed, the magnitude of the velocity, is constant so it is the direction of the velocity that is changing.

So you can work out the direction of the acceleration by looking to see how the direction of the velocity changes.

And the force on the car is then just $\mathbf f = m \mathbf a$, so the force is in the same direction as the acceleration.

@Kaumudi.H aha yes, e.g. Sean Connery

user123733

@JohnRennie but centripetal force always act towards centre

user228700

Lol, no, I didn't (until a few seconds ago) even know who that is. I've heard the name in random movies.

John Rennie

I have a friend called Eilidh. Guess how that is pronounced.

user228700

Aye-lid?

user228700

Oh crap, aye as in not i, like the pirate slang.

user228700

10:21 AM

Let me rephrase that.

user228700

eyelid? :-P

John Rennie

@user123733 The centripetal force is the force on the car. This is obviously always inwards otherwise the car would travel outwards. The centrigugal force is the force the car exerts on the ground, and that is outwards.

@Kaumudi.H Aylee - ay as in hay, lee

user123733

@JohnRennie but the equation was written for car

user228700

@JohnRennie WTF, why the "dh", then?!

John Rennie

@Kaumudi.H shrug - that's the Irish language for you :-)

user228700

10:24 AM

.__. Many many people mispronounce my name.

user228700

So much that I give a fake name at restaurants and such, simply for convenience.

DHMO

Irish orthography

Irish orthography has evolved over many centuries, since Old Irish was first written down in the Latin alphabet in about the 8th century AD. Prior to that, Primitive Irish was written in Ogham. Irish orthography is mainly based on etymological considerations, although a spelling reform in the mid-20th century simplified the relationship between spelling and pronunciation somewhat. There are three dialects of spoken Irish: Ulster (now predominantly in County Donegal), Connacht (Counties Mayo and Galway), and Munster (Counties Kerry, Cork, and Waterford). Some spelling conventions are common to all...

user228700

@DHMO Thanks :-)

John Rennie

@user123733 Suppose the car is stationary. Then the force the ground exerts on the car is upwards. Yes?

user123733

Yes

John Rennie

10:31 AM

In the valley the force on the car due to the circular motion is inwards, i.e. also upwards, so the two forces add. Yes?

user123733

You mean centripetal force is inwards

And normal force is outwards

John Rennie

I mean the force the ground exerts on the car to change its velocity. That force acts inwards.

user123733

@JohnRennie the ground only exerts normal force

Or frictional force if possible

John Rennie

The force the ground exerts on the car can be divided into two bits. There is the constant force due to gravity (which is always straight up) and the force due to the circular motion (which is always pointing towards the centre of the circle)

At the bottom of the valley the centre of the circle is straight up, so both forces point straight up.

user123733

Ok

John Rennie

10:39 AM

So at the bottom of the valley the total force is $mg + mv^2/r$. Yes?

user123733

Yes

John Rennie

OK. At the top of the hill the gravitational force still points straight up, but now the centre of the circle is below the car i.e. straight down.

user123733

Yes

John Rennie

So the two forces act in opposite directions and the total force is $mg - mv^2/r$.

user123733

But if we write it like this F$_N$ + mv$^2$/r = mg

Then we can say both normal and centripetal are in same direction

John Rennie

10:44 AM

Well that expression is just $F_N = mg - mv^2/r$ rearranged.

user123733

Ohk

Got it thanks

GPhys

11:05 AM

@JohnRennie The return trip is fully symmetric to the leaving trip, right?

John Rennie

@GPhys Yes

Why wouldn't it be?

GPhys

Well, I assumed it was, but I was setting up the initial conditions for the return trip anyway to appease myself and had fudged a number somewhere for the third time

although that may be because I'm trying to do it on my phone :P

GPhys

11:29 AM

@JohnRennie I'm convinced I'm genuinely doing it wrong, but I'm not sure where - I'm trying to set up the equations of motion at the midpoint for an acceleration opposite its velocity so that I can solve for the time when it reaches the midpoint again. Previously I had the general solutions $gt=A\cosh(g\tau)+B\sinh(g\tau)+C$, $gx=B\cosh(g\tau)+A\sinh(g\tau)+D$ from the differential equations

Secret

11:53 AM

old news: sciencedaily.com/releases/2015/01/150126170634.htm

My processing style is both analytic and holistic. In particular, I am bad at multitasking while somehow my brain kept records of an insane amount of peripheral sensory information mostly unconsciously as evidenced by the rich content of my dreams logged for 6 years

It is still unknown why the subconscious think there is such need to keep so much peripheral data

Secret

12:17 PM

-2

Q: How to explain to a child how magnetism works?

If one had to explain the concept of magnetism to a child of roughly 7 years of age, what would be the simplest approach in conveying the fundamentals (magnetic fields & lines of force) thereof? Assume that the child's IQ is of average measure.

electromagnetism magnetic-fields

Explaining to a kid about <insert concept> type question but look at the votes are highly different

Probably because the dimensions of electric current is a lot more nontrivial to answer

Secret

12:46 PM

House of mirrors: Sydney
It is possible to effectively "walk" down this hallway: The fun thing about these mirror configurations is while the image look straight, they are in reality twisted paths bouncing around the mirrors

Emilio Pisanty

1:14 PM

@Secret floor plan?

or even better, video of someone actually walking it

Secret

I have not videoed the scene, but I do took a sequence of photos that demostrates effectively walking down the hallway (will post shortly) The official site also does not have a map, though google maps might have an aerial view of it owing that it is open to the sky

For a video, this guy's give a rather complete impression on what it feels like to walk inside the maze

Even if you know how the maze works, the mirrors are so clean that it is still disorienting enough that you sometimes run into mirrors

Ok, turns out I actually videoed a short segment. There isn't much people at that time but should be enough to see people walking around as seen in the view of the hallway image

thought for food

1:45 PM

Fluoride sodium?

how...inorganic ;)

Bernardo Meurer

2:52 PM

@ACuriousMind Are you around?

anonymous

Hi @BernardoMeurer :)

More maths ?

Bernardo Meurer

@anonymous Howdy :)

I just solved it

I had forgotten that $x>0$ always

I was showing that a sequence was decreasing

anonymous

Cool. So studies over for today ? :D

Bernardo Meurer

Hahaha, nope

I have my analysis exam on the 30th!

And after that I have my digital systems on the 2nd

and Chemistry on the 4th

anonymous

Oh I see :P All the best. Tough schedule

Bernardo Meurer

2:57 PM

but I already passed DS and Chem, so it's okay

I might not even show up for Chem because fuck that class really

@SirCumference Do you run Linux?

FrancescoS

Hi guys. Here, arxiv.org/pdf/1601.05000v2.pdf (pag. 8 and 9) the author states the $\phi^4$ massive theory in $d= 4- \epsilon $ dimension has a fixed point which is unstable. He says that the instability comes from the fact that $\phi^2$ is a relevant operator even at the fixed point whereas the interaction $\phi^4$ becomes irrelevant. Is there a quick way to see this, without going through all the calculations once the beta-function of the couplings is given?

Then, he states also that the are two regimes in which the $Z_2$ symmetry is preserved or broken. I don't understand why it should be broken

ACuriousMind

3:29 PM

@BernardoMeurer Only briefly

Also, just post your analysis questions instead of asking me if I'm around, there are enough other able people here to help with integrals ;)

Bernardo Meurer

@ACuriousMind It wasn't integrals dood

I wanted to tell you I love you

but okay then

ACuriousMind

@BernardoMeurer No need to tell me, I already know.

Bernardo Meurer

Seriously though I just wanted to ask if when I want to show that a sequence is decreasing it suffices to show that $a_{n+1}-a_n < 0$

@ACuriousMind :P

ACuriousMind

@BernardoMeurer :P

Bernardo Meurer

Also, to show that a point is a local min/max do I only have to show that the first derivative is 0, or do I also have to show that the second derivative is not zero?

ACuriousMind

3:35 PM

@BernardoMeurer The latter, otherwise it could also be a saddle point, like at zero with $f(x) = x^3$.

Bernardo Meurer

@ACuriousMind saddle point? Is that an inflection point?

ACuriousMind

I don't know what an "inflection point" is, but a saddle point is a point where the both the first and the second derivatives are zero.

It looks like $f(x) = x^3$ at the origin, and not like a parabola like maxima/minima do.

Bernardo Meurer

@ACuriousMind We learned an inflection point as being a point where the function changes from increasing to decreasing or vice-versa. Like the origin at $f(x) =x^3$

I guess we're talking about the same thing

also the third derivative must not be 0 for it to be an inflection point IIRC

ACuriousMind

...$x^3$ is always increasing...

Bernardo Meurer

Inflection point

In differential calculus, an inflection point, point of inflection, flex, or inflection (inflexion (British English)) is a point on a curve at which the curve changes from being concave (concave downward) to convex (concave upward), or vice versa. A point where the curvature vanishes but does not change sign is sometimes called a point of undulation or undulation point. In algebraic geometry an inflection point is defined slightly more generally, as a point where the tangent meets the curve to order at least 3, and an undulation point or hyperflex is defined as a point where the tangent meets the...

I meant concave to convex

oops :P

ACuriousMind

3:41 PM

Yeah, so the derivative has an extremum at such a point.

Bernardo Meurer

@ACuriousMind Yessir

Emilio Pisanty

4:59 PM

@ACuriousMind man, HNQ is terrible. +29-12 should give you an indication that maybe, just maybe, it's not a great question.

DanielSank

6:03 PM

Hi, everybody.

Emilio Pisanty

@DanielSank Hi Daniel

DanielSank

@EmilioPisanty 'sup, bruh?

Emilio Pisanty

@DanielSank revelling in yet another step towards an Epic badge

bless start-of-term, man

five rep-caps in seven days

DanielSank

@EmilioPisanty whatzat?

Jeez, 200 rep fifty times!

I think I've done that maybe three or four times.

Emilio Pisanty

@DanielSank nine, apparently

the gold badge is 150, which is utterly unrealistic

but maybe I have a shot at the silver

within some kind of reasonable time frame

Bernardo Meurer

6:14 PM

@EmilioPisanty JR did it twice already :P

Emilio Pisanty

@BernardoMeurer yeah, well

Bernardo Meurer

He's not human though

So I guess it doesn't count

Emilio Pisanty

he's also got four times as much rep as I do, and I've got waaaay too much to begin with

DanielSank

Goodness, you do have a lot.

Emilio Pisanty

@DanielSank I should give more away

DanielSank

6:16 PM

What's the highest amount that actually affects priveleges?

Emilio Pisanty

@DanielSank I could give 23k away and keep all privileges

DanielSank

ah

Emilio Pisanty

@DanielSank but frankly the one that matters is 10k

DanielSank

indeed

Thanks again for pushing me over that ;-)

Emilio Pisanty

well, maybe I do vote to delete on occasion, but I'm not sure how effective it is

I don't think we have enough of a critical mass of active, reviewing 20k+ers for the mechanism to really work

and the analytics turn out to be pretty boring

most of the time, anyway

this is curious, though

DanielSank

6:20 PM

@EmilioPisanty not sure what you mean

Emilio Pisanty

DanielSank

Referring to what, PSE?

Emilio Pisanty

@DanielSank yeah

DanielSank

That's really weird.

Emilio Pisanty

I don't know what that news360.com is or why it's there, that's pretty curious

@DanielSank with 20k you can vote to delete bad questions and bad answers, but you need three 20k+ votes to delete, and this very rarely happens so far as I can see.

DanielSank

6:22 PM

ah

Emilio Pisanty

@DanielSank The problem with the analytics is that it essentially stops there.

^ that's all the info you get

So you can't tell which questions got the most news360 referrals, or which news360 pages did the most referring

which is what would be useful for actually using those stats to promote the site or something

It doesn't even tell you which avenues of referral come from stackoverflow

I imagine almost all of it is through HNQ, but there's no way to tell

DanielSank

Right, referral from the hot network questions would lead to many arrivals from Stack Overflow simply because it has a lot of traffic.

lol, great minds...

Emilio Pisanty

though on the other hand the referral traffic is a relatively small fraction of the total traffic

DanielSank

yep

Search engines ftw.

Emilio Pisanty

really, what the stats say is "87.5% of PSE's traffic comes from Google"

DanielSank

6:30 PM

:-D

Emilio Pisanty

but that's probably only for new visitors

or maybe where it says 'direct' is where it captures all the times I click on my PSE bookmark?

I don't even know if that's in there or not

DanielSank

@EmilioPisanty probably

Emilio Pisanty

@DanielSank in which case, the stats also say "casual (first-time??) visitors outnumber regular contributors by 10 to 1"

and they also say "there's this news360 web recommendations app that provides 35% of referrals, i.e. 5% comparatively more referrals than facebook does", which is curious enough

Emilio Pisanty

7:08 PM

0

Q: The site that refers the most visits to Physics Stack Exchange is…

I was recently talking on chat about the analytics information available to 25k+ users. This is normally pretty boring, but I went to have a look and I was somewhat surprised. These are the top ten sites that have sent us traffic over the past month: (full statistics here.) A lot of this comes...

discussion site-promotion

Physics Meta

0

Q: The site that refers the most visits to Physics Stack Exchange is…

I was recently talking on chat about the analytics information available to 25k+ users. This is normally pretty boring, but I went to have a look and I was somewhat surprised. These are the top ten sites that have sent us traffic over the past month: (full statistics here.) A lot of this comes...

discussion site-promotion

CR Drost

That we have 10 times as many casual visitors as regular contributors is no surprise. The news360 thing is maybe interesting but beating out Facebook is not very hard as it's not exactly something you share with your family, "look, ma, I understand relativity now!"

John Rennie

7:28 PM

OK, I suspect this is wasted effort because the OP will run away screaming in terror. But it was fun to do :-)

0

A: How do you find the actual acceleration of a comet approaching the Sun at near c?

The trajectory of a freely falling body, like a comet is given by the geodesic equation: $$ \frac{d^2x^\alpha}{d\tau^2} = - \Gamma^\alpha{}_{\mu\nu}\frac{dx^\mu}{d\tau} \frac{dx^\nu}{d\tau} \tag{1} $$ Unless you're a GR head this probably looks appalling, but it isn't as complicated as it seems...

khaverim

If anyone would be so kind as to confirm... physics.stackexchange.com/questions/308006/…

CR Drost

7:46 PM

@JohnRennie Upvoted for effort. :D but yeah I think probably the better thing would have been to just say "what's actually additive in special relativity is the rapidity..." and just give them that partial answer rather than the full monty.

Emilio Pisanty

8:54 PM

@CRDrost sure, facebook is not the kind of site that would send traffic our way

but it is a thousand times bigger

Mostly I'm curious as to what that use case is. I understand vaguely how you'd end up here through facebook, and what kind of threads that will mostly send you to, and what kind of questions get the most stackoverlowers. I would like the same kind of intuition for this new source.

Sir Cumference

10:12 PM

@BernardoMeurer Mac. Why?

ACuriousMind

10:26 PM

@EmilioPisanty Yeah, such an amount of downvotes should be taken into account by the HNQ algorithm. Due to various social and systemic factors, a +31/-14 split is a signal for a significantly worse question than a +17/0 one (most excellent questions never get to +17 to begin with)

5

GPhys

People are getting their ass kicked at the start of this grad Statistical Physics course

I'm thanking the stars I had an excellent difficult undergraduate thermodynamics course

He was the best professor I've had for anything, but he retired the semester after he taught the course I took :\

GPhys

11:44 PM

@JohnRennie I figured out my issue after some sleep. Are there any books in particular that you feel give good discussions of special relativity - specially with wrt four-vector use - and perhaps more problems to work?

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