The h Bar - 2013-06-18

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Ϛѓăʑɏ βµԂԃϔ

8:35 AM

@ManishEarth What's that Boo o.O

Monsters, Inc... :D

-4

Q: I want dmckee♦'s closing rights to be removed

I asked Which beam splitting ratio has uncoated glass for 420 nanometers coherent laserlight?. Question was closed as not a real question by dmckee♦ yesterday It's difficult to tell what is being asked here. ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reaso...

support closed-questions

David Zaslavsky

9:03 AM

Chat session today!

In case I'm not here when it starts (7 hours), just go on without me. As always. But I can probably be around for it this week.

1 hour later…

10:15 AM

user image

7

10:35 AM

0

Q: Can a reminder-type concept be introduced?

I've noticed that - while browsing through questions and answers - I regularly come across an interesting question/answer but reading and understanding the answer or answering the question would take quite some time. That's fine, but we all have other things to do as well, don't we? So it happens...

feature-request

4 hours later…

2:06 PM

A homework problem I'm struggling with, but I don't think it's worth a question, hence I thought I'll ask it here. How does one call a second order PDE with a boundary condition that periodically changes with time (like u(L,t) = cos(t))? I have googled for 2nd order PDE with periodic boundary conditions but can't find any example of this...

2:41 PM

@BartArondson Are the derivatives with respect to time?

Could you give the exact pde?

3:15 PM

u_t = u_yy with u(0,t) = 0 and u(L,t) = U cos wt

But I found that this is called Stokes' second problem, so I'm researching that now.

3:51 PM

what are strategies for identifying primitive periods for periodics of the form asin(bx+c)+dsin(ex+f)+....nsin(ox+p)

aside from fft of cousrse :)

David Zaslavsky

4:02 PM

All right, so if I'm not mistaken, I believe we have a chat session ;-)

user54412

@JoeStavitsky 99% of the time, Fourier transforms are the best way to go - I can't really think of any other way

@DavidZaslavsky Huh, 9:30 already :)

The solution to ALL THE PROBLEMS: Fourier transform

Want to get rid of the tight-binding schrodinger equation? Fourier transform

Want to analyse music? Fourier transform

user54412

Want to find exoplanets? Fourier transform

@ChrisWhite how?

Look for spikes in the FT of the incoming EM waves?

Spikes corresponding to the revolution frequency or some such?

user54412

FT the time series of radial velocities of the host star

user54412

4:14 PM

exoplanet people call it a periodogram

ah

So I presume the exoplanets rotation induces some kind of regularity to the radial velocity which shows up as a spike?

David Zaslavsky

I do say all of physics is Taylor series and Fourier transforms ;-)

yep lol

user54412

@ManishEarth radial velocity = motion of the star along our line of sight

@ChrisWhite kk

But what exactly does the exoplanet do to the radial velocity?

4:17 PM

@DavidZaslavsky Not just physics, Engineering is too...

Also, someone shared this on FB: physics.aps.org/articles/v6/69

David Zaslavsky

Exoplanets pull a star around a little bit with their gravity, so the star's radial velocity has a component with the same period as the planet

What's the difference between a 4-quark and a superposition state of two pairs of mesons?

@DavidZaslavsky ah. basically what I said :P

user54412

@ManishEarth well, the earth is going 30 km/s around the Sun in a circle; since the Sun is 3*10^5 times as massive as the Earth, that means the Sun is moving in a circle with a period of 1 yr and velocity amplitude of 10 cm/s

user54412

So a sine wave

user54412

4:19 PM

elliptical orbits are trickier

My goal is to predict the shape of the output of a sum of simple sines. Easy enough to do first derivative test once you know the period, but I cannot see how to predict the period yet

@ChrisWhite iirc elliptical components are still pretty easy to locate on an FT, right?

For example can we show that the period has one, and only one local maximum?

David Zaslavsky

@ManishEarth a pair of mesons wouldn't be bound together by the strong force

I might get a blog post out about that later this week

@JoeStavitsky enwp.org/Beat_(acoustics)

@DavidZaslavsky ah right

user54412

4:20 PM

@ManishEarth yeah - it will be a broader peak rather than a perfect delta function

user54412

but it's not like we're oversampling the data anyway, so everything is a broad peak

oh right

Talking of 4-quarks and pentaquarks: in general, when is something declared a particle rather than just a resonance? Is it just an arbitrary "the lifetime looks long enough to call it a particle..."?

@twistor59 Yeah, that's what confused me

user54412

@JoeStavitsky If you want the period before the pattern exactly repeats, then it is the least common multiple of the periods going into the sum

4:23 PM

@DavidZaslavsky but then wouldn't there be pentaquarks in the nucleus? meson-proton/meson-neutron interactions do occur, right?

David Zaslavsky

@twistor59 they're kind of the same

And those ought to be governed by the strong force

David Zaslavsky

@ManishEarth yeah, the interactions do occur, but that doesn't mean the mesons and nucleons are bound together

@ChrisWhite, but doesnt this say in the comment that thats incorrect? math.stackexchange.com/questions/164221/…

@DavidZaslavsky So what defines "bound"?

David Zaslavsky

4:25 PM

You don't say that a photon and electron are a bound state just because they interact

@ChrisWhite checked answer, 6th comment

ah, I see

David Zaslavsky

@ManishEarth bound states have lower energy than the same particles with infinite separation, basically

user54412

@JoeStavitsky (1) I did just wake up, so anything I say may very well be completely wrong :P

@DavidZaslavsky ah

4:27 PM

@ChrisWhite, exciting :)

user54412

(2) my definition of LCM gives infinity for 1 and 2 pi

@ChrisWhite, sorry, that I will need help with

user54412

It's basically what that answer is saying - you need integer multiples of the periods

@ChrisWhite, well that follows from the definition of a harmonic

user54412

any interesting papers or news out there, anyone?

user54412

4:34 PM

astro-ph is all technical (read: boring) today ;)

David Zaslavsky

There was the tetraquark thing

Other than that, hep-ph is kind of boring as well

(and that paper was from PRL anyway)

There was also some news recently about a discovery of a new supernova remnant somewhere nearby, IIRC

Quiet today

The one time I get time to participate in the chat and everyone's missing :S

@ChrisWhite I must admit, I don't trawl the arxiv for interesting stuff, I trawl the blogs....

user54412

I was reading an answer written by @Pulsar

user54412

@Pulsar your answers are usually quite long, and also quite good :)

4:47 PM

Like long answers? Stalk Terry Bollinger :P

user54412

@twistor59 trawling the arxiv is my penance for not going to my department's daily coffee-and-arxiv event today :P

Coffee and arxiv - like it...

@ManishEarth ...and long questions with lots of diagrams. But nice to see all the effort going in.

@Chris Thanks, I do my best :-)

My longest non-MSO question is this

Dunno if anyone else thinks this, but the questions have taken a turn for the better over the last week or so. There have been some really good ones.

4:50 PM

@twistor59 Last week I've been too busy to browse as much :(

user54412

@ManishEarth I remember being very impressed with those diagrams

user54412

I'm far too lazy to generate diagrams for my posts

David Zaslavsky

@twistor59 yeah, I suppose so. I think it's part of the natural up-and-down cycle in question quality

@ChrisWhite Bollinger's?

user54412

If it's not on wiki, I use words

user54412

4:50 PM

@ManishEarth yours

David Zaslavsky

i.e. statistial fluctuation

Ah thanks :)

@ChrisWhite those can actually be made in 5 mins. In Paint. :P

@DavidZaslavsky yeah probably. We live in hope.

Still nothing like the quality pre-Maimongate

@ManishEarth just looked at your long question. Do you play the flute?

@twistor59 Yep.

@ManishEarth Ah. figures. Wife used to play a bit, but I could hardly get a note out of it.

4:55 PM

the trick is to first practice with just the head of the flute. A lot. Once you can control that sound, you're set :)

user54412

I had a music instructor in school who made new flute players keep a piece of paper up against a wall with just their breath - they had to hold it there for a long time to practice breath control

user54412

only after they mastered that were they allowed to use the flute itself ;)

@ManishEarth yep, that's what she said too.

@ChrisWhite Sort of like wax on, wax off?

:P

I've done other breath control practices, but the first thing taught was making noise from the flute

@ManishEarth too late Manish. I saw your lol! "Flutes" and "blowing" could lead us into bannable territory

4:59 PM

@twistor59 Well, at first glance I thought you were saying "that's what she said".You were saying that, but you didn't mean it the way I thought you did :P

A second later I realized my mistake

Sometimes euphemisms hit you in the back of the head when you're not looking :S

@ManishEarth Just as I thought!

@twistor59 FYI: such discussions actually are allowed on chat. The DMZ (IT Security) and The Comms room (Server Fault) chat rooms are major culprits

David Zaslavsky

OK, well our hour is up, so I have to run. see everyone later!

Cya!

Whaaat? you mean I've been nice and proper on here .... all for nothing?

Cheers David

5:04 PM

@twistor59 Well, chatroom regulars get to dictate how they want the room to be like

Offensive language is usually a no-no network wide, but you see the f word occasionally and it's not always dealt with -- doesn't need to be if the room doesn't mind

@twistor59 So if chatroom regulars have an issue with such discussions, it's best to stop. If they don't mind, it's perfectly OK

@ManishEarth OK, all is clear. Well, I have to f-off home now. Nice talking, bye everyone....

lol

cya :)

user54412

As a regular here, I therefore propose this chat room be dedicated to collaborative writing of the first Physics.SE erotic novel, entitled Quantum Entanglement

user54412

:P

user54412

(with my luck, something with that name already exists...)

5:13 PM

@ChrisWhite LOL

@ChrisWhite . xkcd.com/305

5:25 PM

@ChrisWhite There is an old Sid Harris cartoon along those lines. Shows a professor working at his typewriter (yes, that old). The text reads:

Chapter 7. THE STRUCTURE OF THE NUCLEUS OF THE ATOM "What?" exclaimed Roger, as Karen rolled over on the bed and rested her warm body against his. "I know some nuclei are spherical and some are ellipsoidal, but where did you find out that some fluctuate in between?" Karen pursed her lips. "They've been observed with a short-wavelength probe..."

// I have his book

And project selected cartoons at the start of many classes.

user54412

that's excellent

6:23 PM

From the recent questions regarding planetary collisions, I've learned how hard it is to dislodge a planet from its orbit and send it flying out of the solar system. Okay here is my silly question: Imagine a rogue exoplanet hurtling through the solar system in the plane of the ecliptic, at a velocity of 10 meters per second relative to the sun (barycenter). Imagine further that its trajectory just happens to be maximally distant from all eight planets plus Pluto during its transit.

Would it still perturb one or more planetary orbits significantly and could the effects be felt in our lifetime. (Let's assume its mass is about equal to Jupiter's.)

(Make that ten KILOmeters per second, 10 m/s is kinda slow...)

... and could it be expected to pull a large number of comets out of the Oort cloud (and/or Kuiper belt objects) into the inner solar system.

6:40 PM

@EugeneSeidel I think that the perturbations would be there, but not really significant

We'd get a bunch more leap seconds in a year (/etc)

If it comes close to us though, we have a problem

Yes, but I am asking maximally distant

Yep

After all, the other planets do affect our orbit

In a well-known/well-documented way

This would be no different afaict

Note that if you're assuming "maximally distant", it won't pass through the inner solar system

Or are you suggesting that it goes more or less straight through?

In which case, a Jupiter-like entity would wreak havoc

O.K., I have this doubtless erroneous notion that the ensemble of planets is a finely tuned "mobile" balanced precariously on a knife's edge

(What sort of havoc would fd fun to calculate)

@EugeneSeidel Well, newtonian orbits don't destabilize

thinking it would cross every orbit including Mercury's

6:45 PM

oh ok

and then narrowly miss the sun?

right, missing the sun

(or just pass through it)

Yeah, that could have major effects

I'll see if I can simulate that on Mathematica without crashing my computer (not now though)

you kidding about "passing through the sun"? would it not burn up

@EugeneSeidel For hypothesis' sake. Of course it can't really pass through, but if it was assimilated into the sun there could be lasting effects

i see

6:47 PM

Since we want to test the parts about orbits, we can just assume it disappears after crossing all the orbits

(also, yeah, I was partially kidding)

(Also, technically it wouldn't "burn". No oxygen or other oxidising agent. It would just melt and integrate into the fusion process.)

I suspect @ChrisWhite has software that can simulate these things :)

yes, sorry for writing "burn"

No biggie :)

thanks, i'll check back in later to see if an astronomer also weighs in. but your comments are already useful :)

yw :)

Actually, another thing I'd like to simulate is if the Earth-Moon system could be put in a stable elliptic orbit around Jupiter

user54412

@ManishEarth well, I have methods:

user54412

6:51 PM

12

A: What is the correct way of integrating in astronomy simulations?

You've essentially got the answer - no need for the factor of 0.5. Essentially you have a two-dimensional system of first-order ODEs: \begin{align} \dot{x} & = v \\ \dot{v} & = \frac{F}{m}, \end{align} where everything is a function of time except presumably $m$, and dots denote time-derivatives...

user54412

you should be warned, unless you are extremely careful, most n-body gravity sims will destabilize numerically long before they would physically destabilize

@ChrisWhite Oh, I know that. I was doing some collision simulation as part of a CS extra credit assignment.

user54412

and there are a lot of misconceptions out there - many people think Runge-Kutta integrators work for this, but they don't

It was hard to get the graininess from taking over

user54412

@EugeneSeidel most destabilizing effects of planet-planet interactions are resonances - they build up over many orbits

user54412

6:54 PM

one example is Jupiter's moon Io

user54412

it is tidally heated because, being in an eccentric orbit, it experiences a time-varying gravitational field from Jupiter

user54412

as a result, it is very volcanic

user54412

but where does that energy come from? you would think the orbit would circularize with all the dissipation going into heating the planet

@ChrisWhite So even if Jupiter passed through the inner solar system, keeping as far away from the planets as possible, there wouldn't be much of an effect because it would be there for only a short time?

user54412

the answer is it is in a resonance with some of the other moons, and they keep tugging at it in the same part of its orbit

user54412

6:55 PM

@ManishEarth in general, yeah

user54412

there is only so much momentum you can transfer in a single pass

@ChrisWhite tidally heated? My physics mind accepts that as tides are quite powerful but it still seems rather exotic to me

user54412

the rock is distorted - the whole moon basically is an ellipsoid whose axes are constantly changing length

wow

the rock?

O.o

user54412

Io is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus. With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean satellites—Europa, Ganymede and Callisto. Several volcanoes produce plumes of sulfur...

user54412

7:00 PM

about 10^{14} W of power

user54412

and a stress of about 1 part in 3 * 10^4 over the course of a 2-day orbit

@ChrisWhite wow

This answer physics.stackexchange.com/a/33177/8143 says, "If a sufficiently massive object passed very close to Earth, the gravitational interaction (no collision) might eject Earth out of the solar system." I guess that's in line with what Manish said, but I was curious about the least perturbation a large planet crossing all planetary orbits in plane of ecliptic would have

@EugeneSeidel Yeah, that would basically be a reverse slingshot effect

We usually throw stuff at planets (and the Sun) so that they'll slingshot around (I think it gives a speed boost among other things)

Here, if the planet comes fast towards you, you get slingshotted

(that's what it looks like to me)

But yeah, that doesn't say what happens if you're quite far away

Yes I've read about that every planetary probe visiting the outer planets uses slingshot technque

7:07 PM

yep :)

and what about would the rogue planet pull in comets or kuiper belt rocks into the inner solar system

probably

that could be a problem

But those may just get left behind in the asteroid belt

user54412

"The outer Oort cloud is believed to contain several trillion individual objects larger than approximately 1 km... with neighboring objects typically tens of millions of kilometres apart"

user54412

The Oort cloud (named after Jan Oort), or Öpik–Oort cloud, is a hypothesized spherical cloud of predominantly icy planetesimals that may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. The Kuiper belt and the scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth of the Oort cloud's distance. The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational domi...

huh

user54412

7:14 PM

you could perturb things if you were close enough

so maybe one tagalong comet

@ChrisWhite How do you do that trick of quoting from Wiki

user54412

but chances are you won't be close to many things

@EugeneSeidel paste the link

that's all there is to it? thanks

user54412

7:15 PM

@EugeneSeidel the link has to be its own line in chat - then a preview is automatically generated

user54412

works for any stackexchange link too

post links, user profile links, comment links

chat message links

SE software has all the bells and whistles :)

yep :)

May 22 at 18:27, by ManishEarth

Cool new feature

^^ like that

ok thanks @ChrisWhite and @ManishEarth you've both been very helpful

7:20 PM

you're welcome :)

I learned some interesting things too

'specially the Io thing

0

Q: why aren't the points added up

I have 33 points in physics SE. Not much but enough to make upvotes. I recently had a question on chemistry SE. What I found that Points don't add up between the two sites so I can't upvote. But as I know the two websites are linked. In what way are they linked and how.

discussion support status-bydesign

Ϛѓăʑɏ βµԂԃϔ

8:07 PM

Oops... Missed the chat today... :/

@ManishEarth: Regarding that -13'd meta post, why's this community mod messing things up..?

He didn't

That wasn't messed up, I agree with the close

I just would have closed it as TL

Ϛѓăʑɏ βµԂԃϔ

What about the revision history...? :O

locked, unlocked, notice added, removed, locked, unlocked..! o.O

@ϚѓăʑɏβµԂԃϔ Because I forgot to change the lock period to "permanent"

Ϛѓăʑɏ βµԂԃϔ

(along with a lot of funny rollbacks in the midst of these..)

By default locks are 1 hour

The lock was to prevent the rollbacks

In the case of a rollback war, we lock

Ϛѓăʑɏ βµԂԃϔ

8:09 PM

Oh... so, by default - community unlocks it...

@ManishEarth Ah.. I know that... I've seen a post (edit war between Ron & David)

Found a new way to access the internet... My dialup reaches speeds upto 25 KB/s these times..! (12'o'...)

Whilst during daytime, speeds can't even cross 8 KB/s -_-

@ManishEarth BTW, What is the importance of adding a notice (precisely..?)

I saw notice addup only for bounties...

What's the use of content dispute...?

user image

@ϚѓăʑɏβµԂԃϔ standard lock reason for rollback wars

@ϚѓăʑɏβµԂԃϔ There are actually many types of post banners

Ϛѓăʑɏ βµԂԃϔ

@ManishEarth Ah... similar to VTC reasons...!

Protection, Lock, Close, Delete, Bounty, Post Notice

Ϛѓăʑɏ βµԂԃϔ

Cool :D

user image

IIRC a post will only display a max of 3 of these though

Oh, and migrate notices

Ϛѓăʑɏ βµԂԃϔ

8:16 PM

@ManishEarth Migration is only for OT... Isn't that so..?

Notices for that too..?

No, incoming migration

Ϛѓăʑɏ βµԂԃϔ

Got it... :)

8:27 PM

0

Q: How much force is required to lift this car (all details/info provided)?

I have the details and info, but not the knowledge on how to determine the weight in any sensical manner. Approximately, on or off, or exact, about how much force is need to lift this car from the back bumper. What I tried: I have tried to estimate the center of mass or rotation point, but fai...

homework forces

reopen?

I doubt it -- still not enough work provided

also rather broad

but i gtg for now

Ϛѓăʑɏ βµԂԃϔ

@ManishEarth Yeah... Not necessary to reopen...

C'ya later :)

Could you leave a comment explaining what still needs to be done (tell him to read the HW policy fully)

Ϛѓăʑɏ βµԂԃϔ

My dialup cloneifies everything...! Crappy thing -_-

@ManishEarth Within a few mins... I'll do :)

Explain that it is a bit broad too

gtg

Ϛѓăʑɏ βµԂԃϔ

bye...

Ϛѓăʑɏ βµԂԃϔ

9:02 PM

@ManishEarth Last year (when I visited English room for the first time), I saw Cerberus saying, a***ole, etc. and when I flagged it as offensive, everyone in the room exploded like - "WHO DID THAT?"- Then, I showed my nice blue (smiley) balls in front of everyone :)

Cerberus asked me, "I'll call Bin Laden, an a***ole, What would you say?"

Nothing to speak :X :D

9:16 PM

0

Q: Previous next buttons on the main page

As the activity on Physics Stack exchange is increasing and steadily, It is getting difficult to browse through questions. Roughly ~ 9 hours or so, the questions disappear from the main page. One is forced to search either using tags, which is somewhat inconvenient and one is likely to miss inter...

discussion feature-request

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