The h Bar

But you need to tell me who the poster is aimed at. The early version I saw looked as if it was aimed at mathematicians vaguely interested in physics.

0ßelö7

@JohnRennie the average STEM person

John Rennie

@0ßelö7 the average STEM person will have lost the will to live after the first equation.

2

0ßelö7

the first equation is merely the Einstein equation...

I figured everyone on SE does it, so normies must like it

Hm. Does Matlab indexing start at 1?

haha it does

John Rennie

Does anyone know if gravitational waves propagate in accordance with the Huygens-Fresnel principle?

0ßelö7

@JohnRennie the general consensus of the chat is that no one knows how the Huygens principle actually works

in particular since we all imagine it in 2 dimensions, but it's false in 2 dimensions

John Rennie

5:44 AM

Huh? It gives the right predictions for diffraction patterns.

0ßelö7

@JohnRennie The mathematical proof fails in 2 dimensions

It is demonstrably incorrect

So why it works is truly a mystery

@ACuriousMind @AccidentalFourierTransform and I were talking about this the other day

John Rennie

I guess that's tough for Flatlanders. Fortunately we don't live in a 2+1D spacetime.

0ßelö7

@JohnRennie Well, what about water waves?

We pretty much assume they work like other waves

@JohnRennie what's a good number for the cosmological constant in SI?

John Rennie

@0ßelö7 I did calculate this in an answer. Let me search ...

Hmm, 57 answers to look through

0ßelö7

~1E-52 1/m^2

John Rennie

5:49 AM

$ 2.036 \times 10^{-35}$ s$^{−2}$ according to this

0ßelö7

that seems so small

@JohnRennie oh fuck

I have to put the c's back in my equations

@JohnRennie shouldn't it be like J/m^3?

John Rennie

That is the same as $10^{-52}$ once you put the $c$s back in.

0ßelö7

@JohnRennie should it not have units of J/m^3...

it is an energy density, no?

or do you have to convert somehow

John Rennie

The dimensions are energy density multiplied by the dimensions of the gravitational constant.

Remember the equation is $G - \Lambda = 8\pi G T$

0ßelö7

@JohnRennie Yah, I think I have to fire up mathematica and compute the EFE with the constants

John Rennie

5:59 AM

This is trouble with getting used to setting G and c to one :-)

0ßelö7

not exactly my ideal 2AM activity

@JohnRennie I get the feeling that this realistic Lambda will be so tiny that it won't matter

I might need to crank it up

1J/m^3 seems reasonable

John Rennie

@0ßelö7 your universe will be closed and rather short lived :-)

0ßelö7

hmm, when do I have class

@JohnRennie Oh, I divide by G so there's 11 orders of magnitude for free

sadly it's still like 30 orders lower than everything else in the equations ;_;

John Rennie

I can't get the dimensions of $\Lambda$ to come out right ...

Aaaaah, it's $8\pi G/c^4 \mathbf T$. I forgot the $c^4$.

0ßelö7

6:14 AM

@JohnRennie So if I want $\Lambda$ to have the dimensions of $\rho$ I need $c^2\Lambda/G$

John Rennie

$c^4\Lambda/G$

0ßelö7

that's for $P$

according to wiki we have $\Lambda =8\pi (G/c^2)\rho_\mathrm{vac}$

John Rennie

That does work. I get the dimensions of $\Lambda$ to be $L^{-2}$.

0ßelö7

you said it was 1/s^2

John Rennie

Multiply by $c^2$ and you get $T^{-2}$

I guess it depends on whether you write it as a mass density or an energy density

0ßelö7

6:19 AM

so you want $c^6$ for the pressure term?

seems excessive

I guess the metric is dimensionless?

John Rennie

Yes, the metric is dimensionless.

0ßelö7

So the constant is indeed 1/m^2

christ

John Rennie

It just depends on what units you choose for the stress-energy tensor

i.e. is the EFE $G = \frac{8\pi G}{c^2} T$ or $G = \frac{8\pi G}{c^4} T$

0ßelö7

I guess I should assume everything can be diagonalized and just rederive the field equations

seems easier than whatever this is

so for a perfect fluid, should T have $\rho c^2$ ?

and just $P$

oh my god the Minkowski metric has a $c^2$

THEY'RE EVERYWHERE

John Rennie

:-) Just set them all to unity!!

0ßelö7

6:25 AM

@JohnRennie I'm doing numerics

I want to have some idea of the magnitudes of things

John Rennie

It's surprisingly painful when you start trying to calculate real physical quantities in GR

0ßelö7

I hate dimensional analysis so much

what the hell are the dimensions of $G$

@JohnRennie lmao this should be mass density not energy density

i give up

I'll give these people their money back

I can't do GR

John Rennie

The dimensions of G are $L^3M^{-1}T^{-2}$. If we take T as an energy density then the dimensions of T are $ML^{-1}T^{-2}$

Multiply these togeter and divide by $c^4$ and you get $L^{-2}$. Bing!

0ßelö7

right, I think I want the energy momentum to be diag (c^2 rho, P, P, P)

So I was right, modulo some pis

amazing

time to finish this thing

John Rennie

Yes if $\rho$ is the mass density.

0ßelö7

6:31 AM

@JohnRennie mhm

of course, I just realized I probably forgot some c's when I got that number from HDE

I can't really be assed to check that now

John Rennie

Christ this is hard! Or is Alzheimer's setting in early?

0ßelö7

@JohnRennie you and me both, bud

John Rennie

Fortunately I have to go and check my servers now. At least I can manage that ...

0ßelö7

again, my goal is to put people in comas by the second equation

they won't get to this part

0ßelö7

6:45 AM

@JohnRennie aaaaaand I've been using the wrong initial conditions all along

rip

I can't do calculations

@JohnRennie Ok what's like a truly ludicrous value for Lambda

I've got everything working but I can't see effects

Secret

The division between "natural" and "artificial" is pretty artificial. Obviously, humans are the dominant natural process that leads to the occurence of lasers. — ACuriousMind ♦ Aug 22 '15 at 16:44

Solution: Replace "natural" with "not directly caused by humanity"

John Rennie

@0ßelö7 Well $\Omega$ is about 5 protons per cubic metre. Multiply that by G/c^2 to get the corresponding $\Lambda$. Then vary by a few orders of magnitude either side.

0ßelö7

I think I made a black hole

the radius of this star is small

@JohnRennie I put 1/m^2

that might be too extreme

John Rennie

:-)

0ßelö7

@JohnRennie hahahaha

it's such a powerful constant the star is exploding

this is pretty cool

if you put a powerful enough constant the radius of the star is infinite

this is pretty interesting stuff

@JohnRennie you're gonna like this poster

DanielSank

7:44 AM

Hi, everybody.

Avan

7:54 AM

hello

Secret

8:23 AM

[Chemistry]

Annoying thing about deleting PBS pro jobs from cluster:

Often when the number is large, there's a 10-30s delay, and that count towards service units deducted from budget

Physics Meta

9:12 AM

0

Q: Classical mechanics

I have a question about my Physics Stack Exchange post: Classical Mechanics

Rumplestillskin

10:19 AM

Hello fellow struggling scholars!

Are there any Post newtonian approximation experts or enthusiasts currently perusing the chat room?

Sir Cumference

11:28 AM

Sigh, another rainy morning

Physics Meta

11:56 AM

0

Q: What does "temporarily deleted" mean?

I can't find in the help, or by googling, the meaning of "temporarily deleting" in I'm temporarily deleting this in accordance with our homework policy. Please don't post complete answers to the underlying problem in homework-like questions. To me it sounds like it's deleted for now, but wi...

support homework deleted-questions

Anonymous

1:14 PM

@SirCumference You don't like the rain ? I love cold rainy mornings as otherwise mornings are usually very warm here.

Sir Cumference

@Blue Er, rainy days are usually hot and humid

But worse, they make waking up a pain

Anonymous

Hot?

Sir Cumference

Entire sky is dark

Anonymous

That's strange

Sir Cumference

@Blue Yeah. I remember in high school earth science we learned the process of cloud formation

Warm air rises and collects water molecules, then cools and forms a cloud

So warmer days are more humid

Anonymous

1:21 PM

That would imply rainy days are cooler. The rain originates from a much cooler region compared to the land. Also the precipitation should reduce the temperature near the ground. Rain occurs below a certain temperature called dew point iirc.

Anonymous

Lemme find a source

Anonymous

I can agree that rainy days are more humid...so you might sweat more

Anonymous

Huh...lol...this seems more complicated than I expected. In places where it is usually cold there rainy days are warmer but in places which are usually hot the rainy days are cooler.

Anonymous

That's why we had different perceptions of rainy days :P

Anonymous

@SirCumference The reason for rainy days seeming warmer in cold places isn't the one you gave. The preceding nights are usually cloudy. So the long wave IR rays are not able to escape into space (sounds like greenhouse effect ?). Hence the following morning seems warm. However this reasoning is not correct for tropical regions.

Sir Cumference

1:33 PM

@Blue I've never heard that. To my knowledge, the fact that warm air rises means it should carry up the water molecules until it condenses.

Cooler air won't rise as much or pick up as many molecules along the way

Anonymous

If warmer air rises...then the surroundings near the land should cool down. :P You need to think about this.

Sir Cumference

@Blue Huh? Why? They're still getting loads of sunlight (until it rains)

Anonymous

Sunlight on rainy days ?

Anonymous

It's cloudy !!

Anonymous

By that logic non-rainy days should be even more warmer since sunlight enters directly with much lesser cloud density

Sir Cumference

1:36 PM

quora.com/Why-does-it-tend-to-be-warmer-on-rainy-days

All right, so I'm gathering that cloudiness keeps in heat

Which makes more clouds

Anonymous

google.co.in/…

Anonymous

See the first quora link

Anonymous

quora.com/Why-are-rainy-days-cold

Sir Cumference

@Blue That link starts with the sentence "I don't think the presumption is always true"...

Anonymous

See the second answer

Sir Cumference

1:39 PM

I could present this

quora.com/Why-does-it-tend-to-be-warmer-on-rainy-days

Anonymous

hko.gov.hk/m/…

Anonymous

I could present this ^

Sir Cumference

theweatherprediction.com/habyhints/226

Anonymous

But well...i don't need to. I stay in a tropical country and I can say from experience that rainy days are cooler over here.

Sir Cumference

> Cold air has a low capacity to contain moisture

@Blue That...doesn't seem possible

At least not from my Earth Science classes

Anonymous

1:41 PM

Okay. We can ask a question on physics stack exchange regarding this.

Sir Cumference

There's an Earth Science SE

Anonymous

Oh...then there

Sir Cumference

Actually, you should probably ask here. It's more active

Anonymous

1:55 PM

@SirCumference I'll surely ask a question on the main site once I'm done with my work today. Let me summarize my points: 1) In tropical places : On normal days the sunlight enters directly and heats up the land a lot. On rainy days due to the cloud cover much sunlight can't enter and it feels cooler. 2) In cold places: On normal days the sunlight entering is anyway low and the night are colder! But the cloud cover prevents heat loss in form of long wave IR and hence keeps the region warm.

Anonymous

That is why rainy days feel warmer in such places.

Anonymous

@SirCumference I suppose @JohnRennie can help us with this. :)

John Rennie

I don't think rainy days feel warmer in the UK.

Emilio Pisanty

↑ that

Anonymous

I guess it depends very much on the place, isn't it?

John Rennie

2:03 PM

More humid days feel warmer because sweating doesn't cool you as effectively ...

Anonymous

Maybe SirC lives in a very cold place

Anonymous

In Kolkata rainy days are usually much cooler

Anonymous

And we wait for it to rain during summer months

Anonymous

@JohnRennie Yeah, that might be another point! Does my reasoning ^ seem correct, though?

Sid

@Blue even if SirC is in US, they will surely be southern to JR

Anonymous

2:05 PM

9 mins ago, by Blue

@SirCumference I'll surely ask a question on the main site once I'm done with my work today. Let me summarize my points: 1) In tropical places : On normal days the sunlight enters directly and heats up the land a lot. On rainy days due to the cloud cover much sunlight can't enter and it feels cooler. 2) In cold places: On normal days the sunlight entering is anyway low and the night are colder! But the cloud cover prevents heat loss in form of long wave IR and hence keeps the region warm.

Sid

And by simple logic, in a place warmer than JR

John Rennie

@Sid Alaska ...

Anonymous

No. Check Alaska's average weather.

Anonymous

That logic is completely wrong.

Anonymous

Being south doesn't mean it will be warmer.

Sid

2:08 PM

Being South means it will be closer to equator.

Anonymous

@Sid Did you check the average weather?

John Rennie

I hadn't realised, but Chester is indeed farther north than the northernmost part of the USA (excluding Alaska). Well, well, you live and learn.

Anonymous

There are many more factors than closeness to the equator.

Sid

@Blue closeness to the equator is perhaps, the most important factor

Slereah

Alaska belongs to Russia you imperialist swines

Anonymous

2:11 PM

@Sid Altitude...

Sir Cumference

@Sid I'm only a bit south of him

Well actually now that I'm back in college, I'm quite a lot farther south

Anonymous

In which city do you live? @SirCumference

Anonymous

Maybe we can check out the weather reports

John Rennie

→ 1 message moved to trash

Anonymous

lol

Anonymous

2:13 PM

:P

Sir Cumference

@Blue Came from New York

John Rennie

@sid: be nice!

Sir Cumference

@Slereah Russia belongs to America tho

Anonymous

@SirCumference I'd say the opposite :P

Sid

But but... It's true. :P

Sir Cumference

2:14 PM

@Blue Yes, actually very true XD

Anonymous

Duh..I should be studying thermo now...:P

Anonymous

Too much procrastination

Anonymous

Cya! :)

Sid

@Blue yes, altitude matters but if you compare places at similar altitudes but at different distances from the equator, the average temperature will vary quite a bit

Sir Cumference

Can anyone recommend some good rock music?

Sid

2:16 PM

Actually, the max and min temperatures will vary a lot too

Sir Cumference

Preferably soft rock

John Rennie

@SirCumference there are many thousands of soft rock bands

Give us a clue. What bands do you know and like?

Sir Cumference

@JohnRennie I mostly listen to Billy Joel, Van Morrison, Cat Stevens, John Lennon, etc.

Not really that many bands

John Rennie

List of soft rock artists and songs

The following is a list of notable soft rock bands and artists and their most notable soft rock songs. This list should not include artists whose main style of music is anything other than soft rock, even if they have released one or more songs that fall under the "soft rock" genre. (Such songs can be added under Category:Soft rock songs.) Artists which have released music of various different genres throughout their career including soft rock as one of their main styles, may be included together with their notable soft rock songs. == 0–9 == == A == == B == == C == == D == == E ...

Sir Cumference

@JohnRennie Of course, not all of them are going to be good :P

That's why I'm asking for recommendations

John Rennie

2:23 PM

Good bands: The Eagles.

Sir Cumference

@JohnRennie Favorite song from them?

John Rennie

Hotel California of course :-)

Sir Cumference

@JohnRennie Oh, right XD

John Rennie

Bread - fave song Guitar Man

Fleetwood Mac - fave song Albatross

Genesis - everything up to And Then There Were Three is excellent.

Balarka Sen

Can I shoot an irrelevant recommendation which might or might not be up to your taste?

Sir Cumference

2:25 PM

@BalarkaSen Sure :)

@JohnRennie Already love it :)

John Rennie

The Beatles are far better than you think, especially the later albums.

Balarka Sen

This and this.

Sir Cumference

@JohnRennie I dunno. They're good, but I really haven't heard any of their songs that I love per se

0ßelö7

@BalarkaSen mooooooodules

Balarka Sen

@0ßelö7 I need to run right now though!

Sir Cumference

2:27 PM

It's kind of sad that there's no good soft rock nowadays

Every one I see is from the 60s-80s

John Rennie

That's because soft rock is mainly rubbish

And people finally realised it

Sir Cumference

@JohnRennie :(

John Rennie

It's true though. NB I didn't say it was all rubbish - just most of it.

Sir Cumference

The fact that it's been replaced with pop and (c)rap is disappointing...

Hell, heavy metal is just loud noise

Slereah

@SirCumference Tell that to Crimea

Balarka Sen

2:29 PM

@SirCumference not true

John Rennie

@SirCumference wrong

Sir Cumference

Oh?

Slereah

@0ßelö7 Rate my site m8

uridimma.hopto.org/site/articles/Mathematics/…

Sir Cumference

In my experience it's just been a lot of people screaming

John Rennie

@SirCumference and that was just the people forced to listen to it

Sir Cumference

2:30 PM

@JohnRennie But many of the most popular songs in history are soft rock

John Rennie

There's no accounting for taste

Sir Cumference

@JohnRennie ?

John Rennie

(says the man who likes Hawkwind :-)

Balarka Sen

I have been listening to black metal a lot recently and I really like what I hear

Sir Cumference

I dunno. Everything on the radio now is just repeated pop songs from a year ago

Balarka Sen

2:32 PM

Pick any earlier Ulver album

Sir Cumference

I swear I've heard despacito or however it's spelt a million times

0ßelö7

@BalarkaSen you don't look like you're leaving

John Rennie

@SirCumference Mainstream US radio is not noted for being innovative

0ßelö7

@Slereah no comments section?

Balarka Sen

the popular songs nowadays is mostly hip hop or trap for some reason

@0ßelö7 about to

Sir Cumference

2:34 PM

@JohnRennie Yet apparently some people like hearing the same annoying songs over and over...

John Rennie

2 mins ago, by John Rennie

There's no accounting for taste

Sir Cumference

@JohnRennie But why...?

Slereah

@0ßelö7 Not yet

I shall do it later

John Rennie

@SirCumference How on Earth would I know? I struggle to understand how many of the people I meet from day to day manage to tie their own shoelaces.

Sir Cumference

@JohnRennie Just started with Down and Out. This doesn't sound like rock :/

Hey @ACuriousMind

John Rennie

2:40 PM

@SirCumference ACM likes soft rock. Ask him for recommendations :-)

Sir Cumference

@JohnRennie K, thanks :)

ACuriousMind

@0ßelö7 It's a "don't count on replies on any sort of regular schedule" absence :P

Sir Cumference

@ACuriousMind So...have any recommendations for soft rock?

John Rennie

@SirCumference Down and Out? From Bugsy Malone?

Sir Cumference

@JohnRennie No, it's the first song in the Genesis album

John Rennie

2:42 PM

Oh, from And Then There Were Three?

ACuriousMind

@SirCumference Try this.

2

John Rennie

I would have said that was soft rock.

Sir Cumference

@ACuriousMind ...

John Rennie

@ACuriousMind :-)

Sir Cumference

I have that abomination in my Youtube history now. Thanks

ACuriousMind

2:43 PM

@SirCumference JR may have misled you about my musical preferences :P

John Rennie

Just a bit ...

Sir Cumference

XD

Anonymous

Aaahhh...I clicked on ACM's link with loudspeakers on. My mom thought I was studying in my room...but..lol

Anonymous

I should be careful while clicking links :P

John Rennie

@SirCumference: Dire Straits?

Sir Cumference

2:47 PM

@JohnRennie I'll check em out

Sid

@Blue lol

That's why you should always keep earphones connected. :P

John Rennie

Dire Straits - Money For Nothing

►

Sir Cumference

:(

John Rennie

Oh! Weird, why would it not be available in the US?

Anonymous

The uploader might have some grudge against ....

Anonymous

2:49 PM

Use a proxy

Anonymous

:P

QuamosM87

https://www.youtube.com/watch?v=4zLfCnGVeL4
Try this guys. Hotel California is also good in soft rock genre.

Sir Cumference

@Blue I didn't know you could cherry pick countries

Youtube has weird features

Anonymous

I think you can. I'm not very sure if that feature exists

Sid

I have a pretty easy question on Python that I am struggling with. Someone help?

Sir Cumference

2:51 PM

@QuamosM87 That doesn't really sound like soft rock

Anonymous

Go ahead

Sid

X-DSPAM-Confidence: 0.8475 - Basically we have to count the no. of lines that start with this, extract the values and find the average.

(Values as in the floating point values)

Sir Cumference

Though if I make a recommendation (assuming anyone likes my type of music), it would be this or this

Sid

Now, it's obvious I have to do a find() and then,basically slice the decimal part out. But, I am not sure what to find for, here.

We simply don't know what the values in floating points are..

Well... @blue any idea?

John Rennie

Are the lines always:

>X-DSPAM-Confidence: <number>

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