The h Bar

DanielSank

00:13

Which of the following do you guys think is more representative of what theorists do when they want natural constants to disappear:
1) "I work in units in which $\hbar=1$."
2) "In the following, we redefine the symbol $\hat{H}$ to mean $\hat{H}/\hbar$."
?

Kyle Kanos

I'd say 1

ACuriousMind

@DanielSank What they say: 1. What they actually (should/implicitly) do: 2.

DanielSank

@ACuriousMind You just made my month.

o/\o

user54412

@ACuriousMind But we can't make it too easy for readers to restore the constants. Exercise for the reader and all that.

DanielSank

@KyleKanos Because of habit or because it makes more sense?

Kyle Kanos

00:17

I'd say probably more habit than anything. It's mostly just a convenience thing for writing really

0celo7

@ACuriousMind Why is 2 better?

ACuriousMind

@0celo7 For one, so that $\hbar\to 0$, or semiclassical expansions in $\hbar$ don't become nonsensical. If it were literally a value we could set, it would not be a parameter to control the scale of quantum effects, and not a parameter one could meaningfully expand in.

DanielSank

@0celo7 Also, if I write $i \hbar \partial_t |\Psi\rangle = H |\Psi\rangle$, that equation is written with absolutely no reference to any "units".

Units are things like "meter", "second", etc. Physics equations don't involve units.

Kyle Kanos

@tpg2114: grep -rni "jameson" only presented AMRVAC which has Jameson time stepping as an alternative to other schemes

So no artificial dissipation :/

DanielSank

We can choose to define $\Omega \equiv H/\hbar$ in which case we have
$i \partial_t |\Psi\rangle = \Omega |\Psi\rangle$.

Note that this is a redefinition of the Hamiltonian, not a choice of units.

@ACuriousMind Heh, I would argue that expanding in a dimensionful constant is not particularly meaningful either...

0celo7

00:28

But we can pick units so it is 1.

Is doing so mathematically inconsistent ?

DanielSank

@0celo7 Well, yes, you can do that, but then your equations don't make any sense.

ACuriousMind

@DanielSank Hm...yeah, you're right, we're actually expanding in things like$\frac{S}{\hbar}$, I think.

DanielSank

@ACuriousMind Yep.

Nobody ever says that though, nor do they actually write the expansions in a way that respects that fact.

Usually it's the reciprocal though.

Kyle Kanos

@DanielSank Does writing a dimensionless form of the equations (e.g., these) count as #1 or #2?

DanielSank

Definitely #2.

Kyle Kanos

00:34

Okay, then I do #2 way more than #1

DanielSank

@KyleKanos Yeah, that's not the same thing as declaring e.g. $c=1$.

if you say "$v$ means $v/c$" then your equation is still meaningful.

Dang it. I'm listening to some Indian style music in which the percussive instrument sounds exactly like the @ ping on the chat room.

Kyle Kanos

I'm listening to Caleb Hart talk to like 4 different people

DanielSank

@KyleKanos Is he that speed runner?

Why do my neurons care about that?

Kyle Kanos

He is a speedruner

He's doing Pentafecta Thursday: Rockman&Forte (?) and MM 1,2,3,4

ACuriousMind

MM?

Kyle Kanos

00:41

Megaman

And it's actually Megaman X series he's playing

So it really should be MMX 1,2,3,4

DanielSank

@KyleKanos Oh snap!

Those are some of my favs!

link?

Kyle Kanos

twitch.tv/calebhart42

DanielSank

Has he done MM3 yet?

Kyle Kanos

He's finishing up X2

DanielSank

Oh.

Dang.

Kyle Kanos

00:43

Like literally about ot be swapping cartridges

DanielSank

I thought MM not MMX.

Dang.

Kyle Kanos

Oh, sorry to disappoint

DanielSank

@KyleKanos Dat walljump.

gonenc

@Qmechanic do you happen to know why my "unclear what you are asking" flag on this version of this question declined? After I flagged the question, it was edited by Kyle Oman and only after then it became clear what OP was trying to ask.

btw I'm just following the advice of Manishearth and "ping one of the moderators in chat before going off to meta" :)

ACuriousMind

@gonenc Automatically declined because of three leave open votes in the close review.

DanielSank

00:50

@gonenc It's possible that it was seen in the review queue, and the reviewer decided to edit and then decline the close vote.

gonenc

@ACuriousMind I see...

Kyle Kanos

I only saw Kyle Oman's edit to the query & thought it was sensible enough.

gonenc

@KyleKanos oman's edit is indeed sensible enough. but the original one that I flagged was utterly impossible to understand :)

well at least it was for me

apparently oman achieved to understand it :)

ACuriousMind

Don't worry, declined flags have no detrimental consequences - and in this case, declined simply means that the close review did not decide to close the question.

Kyle Kanos

@gonenc Eh, looking at it I think I would have the same thoughts the other Kyle did. Perhaps that's hindsight speaking, though.

gonenc

00:56

@ACuriousMind are you sure? cause AFAIK disputed flags are the neutral ones, whereas declined flags are the negative ones

@KyleKanos well having read oman's edit I too am able to understand the previous version now.

Kyle Kanos

40

Q: What is the difference between disputed and declined flags?

I'm looking at my flag count, and I see a couple 'declined' flags and a few 'disputed' flags, and I'm wondering: what is the difference between the two? I've had both disputed and declined flags where the action I expected from the flag was taken, so I can't quite nail down what they mean (other...

> Disputed flags are considered neutral; they do not count toward flag ban.

Probably, though, the bigger thing is the ratio

I've got 512 helpful flags and 23 disputed & 5 declined; I'm not particularly worried about the 5% decline+disputed rate (or even the 0.9% declined rate)

gonenc

@KyleKanos that probably is the case

anyhow it is good to know why it was declined

user54412

Hmm, there seem to be fireworks going off nearby. This may be the start of 3 consecutive nights of explosions in the air.

DanielSank

@ChrisWhite 'Murica!

How else to celebrate one's independence?

Kyle Kanos

@ChrisWhite Ah, but do you know why there's going to be 'splosions in the sky? Because these people sure don't...

gonenc

01:06

@DanielSank is it 4th of july already?

DanielSank

@gonenc No but some folks start early.

gonenc

@DanielSank like two days early? that is IMHO too early to celebrate :D

DanielSank

@gonenc Apparently you don't have fireworks which need exploding at hand.

user54412

@KyleKanos Clearly they need more explosions in the air to help teach them.

2

DanielSank

^ LOL

gonenc

01:10

@KyleKanos the declaration of independence signed in 1964?! that is awful

Kyle Kanos

@gonenc 19-84!

DanielSank

@gonenc 'murica?

Kyle Kanos

That guy is pretty funny. he goes around that pier and finds people to sign petitions for ridiculous things (e.g., banning the American flag, supporting illiteracy)

gonenc

@KyleKanos that video is hilarious and sad in a way

skill patrol

05:58

@ChrisWhite question #1: what kind of textbook would you recommend to somebody who wants to learn physics?

Perhaps, the Feynman lectures :-)

That certainly would put an end to "sitting in on a physics class belittling it all as poorly defined." In my opinion.

Or perhaps something from the PSSC?

Your point is valid is what I'm trying to say, Sir.

NeuroFuzzy

06:27

@skillpatrol Is it just me or is (question to the audience) the most important thing just reading consistently and working through hard points. That seems to be my sticking point.

The consistency I think is the difference between independent study and an actual university course.

Gaurav

@KyleKanos Of course, they must be cherry picking ignorant blokes. How else could a nation of almost 100 pc literacy possess such specimens in sizeable proportion as in the videos ?

David Z

0

Q: Importance sampling for Coulomb potential

The integral I have to solve is: $$I=\int\int d \mathbf{r}d \mathbf{r}' \frac{\Phi(\mathbf{r})\Phi(\mathbf{r}')}{|\mathbf{r}-\mathbf{r}'|}$$ It is a six-dimensional integral which I am going to solve using the Monte-Carlo integration method with importance sampling. My question is how do peopl...

computational-physics integration coulombs-law stochastic-models

(1) off topic (2) if so, where to send it (if anywhere)?

opinions (and corresponding votes) wanted

JiK

@DavidZ (2) I'd say it's an exact fit for stats.stackexchange.com/questions/tagged/importance-sampling but others disagreed yesterday.

David Z

06:43

Yeah, I'm hoping to get some more eyes on it

user54412

07:15

Not seeing anything too wrong with it. I mean, if someone were to ask about tweaking MC to work for the Ising model or radiative shock simulation or cosmological BAO measurements, I feel it would have better reception here. The fact the 1/r distributions are easy doesn't make this any less physics.

2

user54412

Not that I'm particularly attached to the question.

Gaurav

@NeuroFuzzy Which is to say according to you that people who do independent study are less consistent ? You're not clear there.

user54412

@skillpatrol Something written by a particularly clear-minded and insightful physicist, like Purcell or Carroll. Nothing by Spivak or Arnold, since they just care about symbols. And nothing by Griffiths (not insightful), Jackson (plain wrong), or Weinberg (confusedly complicated exposition).

user54412

Of course, many subjects in physics have no textbook that meets my requirements...

user54412

Honestly, though, I think @NeuroFuzzy is onto an important point -- you learn by working out problems. Also by talking to your peers who are also working out problems (this is what a lot of MOOCs are missing, to their detriment).

David Z

07:24

@ChrisWhite I have to strongly disagree about Griffiths

user54412

I figured someone would.

David Z

Sure, it might not be insightful (for some definition of the word) but in that case I think "insightful" is not what a learner needs

Gaurav

Apart from that, I do think independent study does help you to study at your own pace, you get to work your way into the concepts on your own terms, which gives a fuller understanding of the given topic at hand.

Consistency helps you keep focus, but it's not impossible for someone doing self-study to be consistent as well.

user54412

@DavidZ And I think many would agree with you. I've always felt, though, that undergrad physics curricula are all wrong. They focus on teaching physics for future-generic-people ("this is what society has come to learn") rather than physics for physicists ("this is how people who make discoveries actually think").

David Z

Hm, that's not really the impression I've gotten. At least not in the courses that use Griffiths' books.

user54412

07:38

You might also have had good profs. I don't have much positive to say about the two courses I had that used Griffiths.

Gaurav

That's why I think, even if rare, physicists who've done considerable self study like Feynman come out as being strong at concepts at every level. Rare because so few enthusiastic about self-study can be consistent at it too.

JiK

I remember having a strong opinion about Griffiths but I can't remember what it was.

Gaurav

Self-study is also the reason (i think) why such people are strong at pedagogy. Perhaps this also translates as having enough confidence to teach, and in turn an eagerness to communicate concepts to learners.

JiK

I think insightful is precisely what I would have needed in my studies. I learned all stuff required in the courses but in the end I didn't feel I was any wiser in anything outside the topic of the course. And I don't have any physical intuition, i.e., ability to know which model to choose where.

If I could travel back 9 years, I'd tell the freshman-JiK to choose learning resources wisely, to maintain enthusiasm for self-study (and tell him to join Physics.SE the minute it is created). University courses didn't teach much about understanding, and learning to understand things without good books or other resources didn't give me much confidence on my skills. And based on what I've read in Physics.SE in past year, my lack of confidence was indeed jutsified in many cases...

Gaurav

07:58

@JiK Of course, formal courses are meant to only guide a learner into gaining expertise at something of his/her interest.

JiK

But of course, that is just my opinion, and probably it's not wise to trust "If I'd done this and that I'd be a nobelist instead of not wanting to get a physics PhD".

I also saw a huge difference between BS level and MS level courses: in the first, the teachers acted like they were teaching students; in the latter, teachers acted like they were teaching future coworkers. The latter resulted in a much more insightful course; too bad I had lost a nontrivial part of my motivation in the way.

Gaurav

Should we set a limit to the number of questions asked within a question ?

-1

Q: Can you correct the following thinking process ?

Each element in universe is forming at an instance. Now when i observe the existence of two visible points to me , they appear to be present at all the intervals of my observation because my observation is part of formation interval of universe. Question : Space has dual nature. Existence and...

universe big-bang space matter

JiK

08:13

@Gaurav I'm thinking whether that question would make as nice poem as physics.stackexchange.com/questions/192299/…

Gaurav

Ha !

@JiK The poor guy got downvoted to death. Clearly shows how unappreciative physicists at PSE are to quality poetry.

JiK

@Gaurav Personally I don't appreciate the question marks at the end of each line; that might be the reason for the downvotes.

skill patrol

08:30

If math is the poetry of logical ideas, then physics is the poetry of physically possible logical ideas.

The results can satisfy the equations used but not the physical conditions of the problem situation.

David Z

@Gaurav the limit is technically 1

Interesting question on reddit: reddit.com/r/AskScienceDiscussion/comments/3bxoyl/…

2

Gaurav

08:46

@skillpatrol nice insight there

Mew

09:11

hello

Mats Granvik

10:43

What would happen if I created really cold solid hydrogene or helium like a bose einstein condensate and took it and put it in a coconut and and sealed the hole. Everybody know that coconuts are really hard to crack, so then I would heat up the coconut with the near absolute zero bose einstein condensate and put it in a tokamak at a really high temperature and pressure. Would it explode violently?

JiK

@MatsGranvik Seals are not as hard to crack (even with a club) as coconuts.

Gaurav

11:20

@MatsGranvik I assume you're more interested in how awesome the explosion would be, as I am too. F*ck physics.

2

gonenc

@JiK god i didn't notice that he actually wrote a poem. it actually rhymes, nice catch :)

Bosoneando

@gonenc Well, all the questions end in 'time' or in 'observed'. Not a great display of poetry... ;)

David Z

12:34

For the record: I don't know if it's written explicitly anywhere, but offensive flags are used to seed SE's automatic anti-trolling scripts, so it's better not to edit out offensive content from posts. cc @innisfree and everyone else who was involved in that post.

Jims Bond

what post?

David Z

this one (deleted)

Jims Bond

yikes

0celo7

13:02

@ACuriousMind Is German tech support available today?

I'm reformatting my partitions to give me 500GB for Windows; I have no clue what I'm doing.

Gaurav

Should I flag this as Engineering-related ?

ACuriousMind

@0celo7 Bad timing, I have to go in about ~5mins. I haven't used multiple OS on the same computer in quite a time, though, so not sure if I could help you anyway

0celo7

@ACuriousMind Crap. I'm totally incapable of downloading the Windows ISO file it seems.

Last time I tried it on a Win 7 PC, now I'm doing it on my own Win 8. Maybe that will work.

Jims Bond

If I were to use multiple OS, I'd get a nice copy of VMware Fusion and dual boot windows on top of Mac. That way you get the best of both OS simultaneously

0celo7

@JimsBond Too complicated (for me) and probably drops performance.

Bootcamp is supposed to take the Win ISO and just install Win normally on a partition.

My biggest challenge is getting the right file from Microsoft.

Last time I just messed around in Disk Utility and somehow got it to work.

Jims Bond

13:11

I try not to have 2 OS on 1 machine. If you have to shutdown the computer and restart it to access the other OS, it's not really different from having a separate computer, eh? Except now you can't run both simultaneously

0celo7

@JimsBond Some of us can't afford two computers.

Gaurav

And the combined size of two computers.

Jims Bond

@0celo7 Neither can I :( I make do with Mac at work and Windows at home. (The work computer isn't mine, btw)

JiK

At home I use Windows normally and Linux in a virtual machine (VirtualBox) for programming. The only problem I've encountered is that GPU computing won't work inside the virtual machine.

0celo7

@JiK I've heard this as well from my brother and sister (programmers).

David Z

13:21

Yeah, but if you're using GPU programming you're probably in a position to care about the reduced performance of a VM.

0celo7

@DavidZ Will a VM also decrease gaming performance?

David Z

Yes, as far as I know

JiK

@DavidZ Not if I'm using my home computer only for practicing and testing and not any serious computationally intensive tasks.

0celo7

@DavidZ That rules out me using a VM then.

David Z

The decrease may or may not be significant, depending on your needs

@JiK true

0celo7

13:23

I'm already maxing out my poor laptop GPU.

David Z

It depends on what kind of game and how powerful the host computer is. But for something resource-intensive, I wouldn't expect a VM to meet your needs.

I've tried playing Starcraft in a VirtualBox guest, and it worked, but I could tell it was stressing the computer even using the lowest settings.

Kyle Kanos

@Gaurav Of course he (and obviously others like him) are cherry picking the "idiots." I'd also suggest that being able to read has little to do with one knowing what the hell is going on around you.

Physics Meta

1

Q: Reset username for user

So I found this user. His needs nuking. Can a mod do that?

discussion

Kyle Kanos

13:47

@ChrisWhite I think that Griffiths is alright for an introductory course in E&M. One of the problems I have with it as a course book, though, is that his solutions manual is all over the internet & many/most students won't actually learn it b/c they'll find the manual and just copy (even the wrong answers)

Jims Bond

@KyleKanos As an undergrad, I loved it when the solution manual was accessible. It meant I could teach myself the proper methods to answer questions. Now, I hate it because it means students don't need to hire a tutor as often

Why yes, I am sometimes a bit selfish. How did you know?

Kyle Kanos

I was aware of PhysicsForums before the idea of searching for solution manuals

JiK

As a teacher I'd love to have solution manuals accessible. Then I could spend more time answering real questions regarding to understanding the concepts than answering simple questions about some mathematical details in the solutions.

Kyle Kanos

Another chatroom fight...WTH man

Jims Bond

can't we all just get along?

0celo7

13:57

@JimsBond Reformatting my partitions. See you on the other side.

Jims Bond

@0celo7 yes, come to the dark side

0celo7

@JimsBond (I'm just expanding my Windows partition size, should be painless.)

Jims Bond

okay then, embrace the light side

JiK

@0celo7 "should be painless" are a good choice for last words

0celo7

@JiK Last time I tried installing Windows on a Mac, it took 5 hours.

It should take less than that this time.

Kyle Kanos

14:03

I have come to the conclusion that Data Scientists are really Psychohistorians.

Kyle Kanos

14:15

Also: there's a lot to data that I had no idea could be there

Jims Bond

@KyleKanos does that mean you made it into that course/program thing?

Kyle Kanos

No, I think the registration period is going on still

yesterday, by Kyle Kanos

So a company I applied to (data mining) is testing me to see if I really know how to do the job. Kinda smart on them b/c it weeds out all the people who know nothing.

^That's what I'm talking about now

Jims Bond

Ah I see.

Kyle Kanos

I was given 15 days to review data & write a report on it. So I'm totally learning R and data mining on the fly for this

And it's actually pretty cool

Jims Bond

I believe you

I'm doing the same thing right now with risk management for banking

Kyle Kanos

14:19

Sadly, though, the log-in on the test site said test-takers had until 15 Aug to register and then a pair of test (one 15 days, the other 30), so I'd probably not hear back until October if I were to get the job (which there's obviously no guarantee that I'll even finish part 1)

Jims Bond

wow

Kyle Kanos

Yep. But it's still a prospect

I'm currently trying to see if there are "users" on this fake data that have moved countries (and then cities and then IP addresses)

Jims Bond

::2 months from now we see Kyle Kanos on the street holding a sign: "Will do physics for food"::

5

Kyle Kanos

Two months from now, if I don't have a job, I'll probably be moving (wife & kids as well) back in with my parents to save some money

Jims Bond

Again, same with me (minus the wife and kids part)

On the other hand, without the wife and kids, I could just get an old refrigerator box.....

David Z

14:49

-2

Q: Where will a sparkling (CO$_2$) water bottle have more bubbles when opened

If you take an unopened sparkling water bottle to higher altitude (7000 feet) or under sea level (-8 feet), and then open it, in which location will it bubble more?

homework-and-exercises pressure hydrostatics

Reopenable?

Jims Bond

I'd say yes

But that's why we have a voting system

because I can't be trusted on my own

skill patrol

15:03

I concur.

Jims Bond

^ see, told you I can't be trusted

skill patrol

We gain trust the old fashioned way, we earn it :P

DanielSank

15:45

@DavidZ Why... was that ever closed?

Not enough input from the asker?

Kyle Kanos

@DanielSank The original was (probably) closed as homework

DanielSank

So it needs more "concept"?

Kyle Kanos

Rick's edit (which I disapproved of b/c it invalidates Jerry's answer) changed the question

Jims Bond

Yeah, I'd call that a radical change

The original was about pressure, not bubbles

Kyle Kanos

@DanielSank Actually, it seems it was a custom reason, but since it was accepted and closed, I'm not sure that we can determine it

@JimsBond I concur, but OP did mention the bubbles in a now-deleted comment to Jerry's answer

DanielSank

15:49

That edit is pretty bogus.

Jims Bond

I rolled it back because it invalidates an already present answer

Kyle Kanos

I considered rolling it back when I saw it went through

DanielSank

@JimsBond Good move.

0celo7

Hola from 0celo7's PC 2.0

@JiK Worked beautifully.

DanielSank

I wonder if it's as simple as that answer says...

The pressure in the bottle depends on things like temperature because the solubility of gas in the liquid is temperature dependent.

I wonder if there's also some effect from external pressure in a nontrivial way.

Kyle Kanos

15:54

WTH....

-2

Q: What happens to sound at speeds greater than mach 1?

So what we know about the Doppler effect is that if the origin of a sound is moving, the sound waves will be closer together in-front of the origin, and farther apart behind it. This creates a difference in pitch. Knowing that I am going to try an work through a problem and would love to know if...

acoustics speed

-1

Q: Would you hear a sound in reverse when its source travels with twice the speed of sound?

As I already said in the title, I wondered, whether a stationary listener would hear the sound of an airplane traveling with twice the speed of sound in reverse due to the doppler effect. (of course, he wouldn't hear anything at all before the airplane passed by and the sound reaches him) ~Mystery

acoustics speed doppler-effect

Is that really two different users asking the same question about Mach 2+ sounds?

skill patrol

16:06

That's a lot of users with only 1 rep >8-/

Did high school just get out for summer?

0celo7

Wow, this isn't 4chan.

skill patrol

It could be someday!

0celo7

Are you Sofia?

(For those who don't know, she said this place is doomed to become a porn site.)

Jims Bond

I think high school let out last week

JiK

@KyleKanos I don't quite understand your comment "The animation at the bottom of the page you linked and the link I provided should tell you what the M>2 sound would be." Doesn't the animation show that the circle that's emitted first is the last one to reach an observer at the right end?

skill patrol

16:16

So summer school has started?

0celo7

@JimsBond I graduated 3 weeks ago.

@KyleKanos Steam is downloading at 21 Mbps now.

skill patrol

Sofia is a well intentioned enough pal @0celo7

Jims Bond

@0celo7 Yeah, but around here the graduating students get out of school a bit earlier than the rest. Or maybe it's just the different systems. All I know is our schools let out on June 25

I thought for sure our site would have a go-to "Are we sure the universe is actually expanding?" question. But I can't find a decent one that discusses if there are possible alternatives OR why we are sure it is expanding. It's got to be here somewhere. Anyone want to help me find it?

Bosoneando

@JimsBond For example, physics.stackexchange.com/questions/34164/… physics.stackexchange.com/questions/68512/…

Jims Bond

yeah, those are really good. But they just deal with redshift.

I'm pretty sure we should already have a question I can mark this as a duplicate of

0

Q: Why is universe expanding?

Okay, this question may sound silly: base on the observation Besides an expanding universe, would there be other possibilities? Would it be possible, say, there exists a fundamental repelling interaction among all partices, which is proportional to distance (e.g. $Ae^{ar}$, where $A$ extremely s...

cosmology universe space-expansion

And if we don't, I'd be kind of ashamed. I really should have noticed that earlier

David Z

16:33

Some people might appreciate this: reddit.com/r/WritingPrompts/comments/3bzs0u/…

2

@DanielSank It was closed as homework-like, though I'm not sure offhand why the reason doesn't show up anymore

Jims Bond

@DavidZ It was good, well written, and creative at first

but it got lazy and sloppy and the end was the worst

Gaurav

16:52

@KyleKanos Indeed, the comment intended instead to question the relation between literacy and general awareness, since it is still the indicator of societal awareness is many countries.

@0celo7 Did Sofia really say that ? She had nothing more than mild disagreements about homework questions at the time I had to take break a few months ago.

Kyle Kanos

@JiK Not exactly. An observer would have all the waves reach him first then shifted sounds reach him. Is this "reversed" as the way OP thinks it to be? Definitely not.

@Gaurav Eh, I'm still not sure that I'd correlate literacy and general awareness. Literacy, I think if anything, is a general "rule of thumb" that your education system is functioning.

DanielSank

@0celo7 Sofia said a lot of things, dude.

@DavidZ That post :D. The rest of reddit today >:(

New rule: starting a post with the words "Okay" or "OK" is against intergalactic law.
What say you?

Kyle Kanos

17:11

Would "Alright" be a caveat or acceptable?

Gaurav

@DanielSank Please tell me how the hell you manage to be up to date on affairs of multiple physics sites.

skill patrol

It's going to be pretty hard to make a law against the most used word in the English language.

innisfree

@DavidZ ah I see now why editing is a bad idea. Why isn't the spam detector also applied to usernames? Can't it detect and flag particular words (and variations e.g. w0rdz) that suggest someone is here to cause trouble?

Gaurav

@DanielSank I'd probably be exhausted by the time I get over the politics of the day on PSE. I'd happily give reddit a skip.

JiK

@KyleKanos In any case neither the animation nor the Mach wave Wikipedia page that you link to give any hint that the wave of the sound emitted at the beginning of the animation would reach the observer at the same time as the emitter.

Kyle Kanos

17:19

Er, the text right next to the image contains the following statement:

> The shock wave advances at the speed of sound v, and since it is built up from all of the combined wave fronts, the sound heard by an observer will be quite intense

JiK

"The shock wave advances at the speed of sound $v$"

Kyle Kanos

Kinda hard to interpret that to mean "it comes at different times!"

JiK

The plane advances at the speed $2v$

Kyle Kanos

And?

JiK

So how does the sound from the beginning reach the observer at the same time as the plane?

Kyle Kanos

17:21

Uh, what?

DanielSank

@Gaurav Uh, I went to reddit/askscience to see if anything interesting was going on and the top post is an explanation of why half the site is closed down. Didn't exactly go out of my way to find out about it.

@Gaurav I've mostly given up on posting to r/askscience. Stack Exchange's system is so much better that there's just not much point.

JiK

@KyleKanos (Let $v=0$) At $t=0$ the plane is at $x=0$ and emits a sound A. At $t=1$ the plane is at $x=2$ and emits a sound B. The observer is at $x=3$. When does he hear the sound waves of A and B?

Jims Bond

@skillpatrol We're making a law against "Sorry"?

No wait, that's only most used in Canadian English. Sorry

Kyle Kanos

@JiK Is v the plane's speed or speed of sound? Because either way, your question makes little sense as written.

JiK

@KyleKanos $v$ is the speed of sound

Kyle Kanos

17:25

Okay, a zero speed of sound makes little sense

Gaurav

@DanielSank Unless, you're looking for a site to ask 'homework-related questions' of course. :D

JiK

@KyleKanos What... I meant $v=1$...

Kyle Kanos

Well you wrote 0

But 1 makes more sense

skill patrol

I always thought it was "eh" @JimsBond :P

David Z

@innisfree you'd have to ask the SE team, but you can't tell for sure whether an account is legitimate or not based on its username

@Gaurav well, /r/askscience isn't for homework help, but yeah, the threaded discussion model is generally better for that sort of thing

Jims Bond

17:28

@skillpatrol I think they're tied. Especially because they occur so frequently together with "sorry, eh?"

skill patrol

:D

Kyle Kanos

Since the plane's supersonic, the observer at 3 would hear both sounds simultaneously. That's the whole idea of the Mach cone

JiK

@KyleKanos Yet you quoted that the shock wave travels at the speed of sound, not at the speed of the plane.

Kyle Kanos

@JiK Uh, because it does travel at the speed of sound

I'm not sure what you're not getting here

JiK

Then isn't the plane faster than the shock wave?

Kyle Kanos

17:30

Sound waves travel at the speed of sound.

@JiK Yes...that is possible, you know

JiK

So... At what point does the observer hear the shock wave?

Kyle Kanos

Sometime after the object at M>1 passes by

Did you look at the videos I linked at the end there?

JiK

What is "some time"

Kyle Kanos

If $t=1.5$, the plane passes $x=3$. Observer at $x=3$ would hear the sonic boom at $t>1.5$

JiK

So would that sonic boom contain components that were emitted at $t=-100,x=-200$?

Kyle Kanos

17:33

Depends on the positions of the two objects moving and such

@JiK That is what a sonic boom is.

I mean, obviously the sounds would be damped by normal inverse-square laws

JiK

And because the shockwave travels at the speed of sound, that would mean that the sonic boom is heard after $t>203$

Kyle Kanos

So the bulk of what you hear is things closer in time/space to you

Not sure where 203 is coming from, but the sound from the shockwave would come after the object passes you, yes

I think the video I mention shows this way more effectively than the words are describing here

JiK

err, I mean 103

0celo7

Fallout NV decided to break itself.

Kyle Kanos

Many jets breaking the sound barrier.

►

0celo7

17:35

It won't load saves anymore.

JiK

@KyleKanos How should that video show anything?

Kyle Kanos

Because it shows pretty clearly real-world examples of supersonic jets producing sounds as I'm trying to describe

JiK

@KyleKanos You're saying that the components emitted at $(t,x=2t)$, no matter what $t$ is, reach the observer at $x=3$ at the same time.

Kyle Kanos

For an object that is (a) moving towards you and (b) sufficiently close, yes. That's what the Mach cone depicts (because it's what happens)

Gaurav

Due to the irrational diversity of proper nouns in English vocabulary, I propose we purge all of them except Jim, which henceforth will serve as a synonym of the rest, which removes the need to produce new proper nouns at all. What say ?

JiK

17:41

@KyleKanos If the time of the shockwave is $T$, then I can't see how the component emitted at $(-T,x=-2T)$ would reach the observer at time $T$ as it has distance $2T+3$ to travel in time $2T$.

Jims Bond

@Gaurav Jim Jim Jim Jim Jim

Gaurav

Jim, Jim ?

Jims Bond

Jim..... Jim Jim

0celo7

JIM JIM JIM JIIIIIIIIIM

Jims Bond

@0celo7 Why are you shouting my name? I'm right here. I can hear you

0celo7

17:44

@JimsBond Jim'm Jim...Jim.

Jims Bond

Ah, okay. That makes sense

Kyle Kanos

@0celo7 I'm pretty sure it's Snake? Snake? SNAAAAAAAAAAAAAAAAAAAAAAAAKKKKE

Jims Bond

!

Kyle Kanos

@JimsBond Is somebody there?

Jims Bond

-2

A: Correlation between speed, distance, mass, etc between a planet and a star (or moon and planet)?

The time diffrence caused by gravity according to me is due to its rotation. since motion affects the rate at which time flows according to theory of relativity. as we know the time near a black hole flows really slow also its rotational speed is close to half of the speed of light. since time fl...

Kyle Kanos

17:47

@JiK That's why the Mach cone pictures can be helpful, as can the video I linked above

Jims Bond

I have no mercy

Kyle Kanos

True

JiK

@KyleKanos But it isn't for me. I can only understand that there is a shock wave, not that the shockwave contains every sound the moving object has ever emitted and there is never absolutely nothing else heard and can't be.

Kyle Kanos

What is a shock wave then?

0celo7

@JimsBond #rekt

Gaurav

17:49

What the J*m, Jim ?

Jims Bond

I probably made him cry. But he'll learn and become a more humble physicist for it eventually

Kyle Kanos

@DanielSank Hmm, might be a bit arrogant of us to declare ourselves Makers of Rules of intergalactic space here. There could be dragons out there in the Galaxy.

JiK

@KyleKanos Apparently something that travels at the speed of sound, and yet contains signals from events that occured at locations and times from where a signal traveling at the speed of sound couldn't yet have arrived. I'm having a hard time grasping the idea just by looking at Youtube videos that say BANG.

skill patrol

The email address should be removed

Jims Bond

The whole thing will be deleted anyway

0celo7

17:51

@JimsBond He's an actual physicist?

Kyle Kanos

@JiK Sound is a compression wave (p-wave or pressure wave) that has a characteristic speed. When you push a bunch of these compressions together (due to supersonic speeds), the amplitude increases

But it still travels at that characteristic speed

Gaurav

Alright guys, it's morning and I need to go to the Jim.

Jims Bond

@0celo7 I doubt it. But I imagine he's a prospective one. Wants to be, probably

Probably younger than you right now

@Gaurav +5

JiK

@KyleKanos Yes, that's how I understood shockwaves until todeay.

Kyle Kanos

Well that's all that's happening here. The sound waves are being compressed (amplitude increases) and travelling together at the same speed; as a stationary observer you'd hear the compressed sound (the shock wave) at once, and then the Doppler shifted sounds afterwards

JiK

17:58

I still don't understand what happens to a sound that's emitted at a time and place from where it can't have reached the observer by the time the observer hears the sonic boom.

Kyle Kanos

18:14

@JiK I'm currently editing an animated gif of a supersonic wave right now. Hopefully it will work & help

JiK

@KyleKanos Thank you!

JiK

18:35

Hmm, also a comment to this answer of the duplicate question and this paper (Sec. 2.2) linked in the what-if seem to agree with me.

...that there is a time-reversed signal after the sonic boom (obviously dominated by the signal the plane emits after it has passed) but they don't give any reasoning

Kyle Kanos

18:49

Red it obviously first sound

Black is supersonic sound-producer

If you put your finger somewhere below the moving object you'll see all the sounds hit you at once (shock wave)

Gaurav

@JimsBond Sorry to play spoiler, who's the physicist guy you guys were talking about ?

Kyle Kanos

Then you'll notice that all the subsequent sound waves are actually in sequence.

JiK

I see plenty of waves between the shock wave and the red wave.

Kyle Kanos

Sure, the object is still producing sound

But put your finger somewhere and you'll see that all the waves hit your finger at the same time

Unless you're right next to it

JiK

No, the red wave doesn't hit my finger at the same time

If I put my finger close to the right end of the picture and some distance down from the object's path, I first see the shock wave, then some other waves, then the red wave, and then some waves that are emitted after the object has passed.

Kyle Kanos

18:55

Then you're putting your finger too close

:D

JiK

The question didn't specify that the observer's distance from the object isn't small compared to the distance the object has travelled.

Kyle Kanos

Well it didn't specify anything except M>2 and reversed sounds

JiK

Yes, and if you paint the second circle blue, there'd be a significant portion of area where the blue wave hits before the red wave.

Kyle Kanos

Took me 45 minutes to do that one! Not sure I want to spend another 45 min getting the second wave

JiK

:D

Kyle Kanos

18:58

I probably should figure out how they made the gif and just color it from the start

I mean, it's just expanding waves at some speed, shouldn't be too hard to program :/

And I think this image is kinda misleading/incorrect:

If the boombox is supersonic, then those sound waves should be making a Mach cone and it doesn't really look like one

JiK

The intersection points of the circles are sort of making a Mach cone, but the discretization interval is too long for it to look like a cone.

Jims Bond

@Gaurav Oh, just some new user that posted a "my theory is that time dilation is not due to gravity but is actually......" answer

Kyle Kanos

Wow, someone calling David Z out in the form of an edit

innisfree

19:25

haha, just saw that. David Z, your work is appreciated, and fortunately that kind of behaviour is rare here and quickly removed.

Physics Meta

19:54

0

Q: Identifying Metals at Home

I'm posting this on meta because I'm not entirely sure if this is the site for it. The sidebar suggested I that this was the place to put it, so I'm doing here. Is this question appropriate for this stack exchange? If not, where ought this question go? Mouse over to see the questionable question ...

discussion

DanielSank

@KyleKanos I like dragons.

We're buddies. Nothing to worry about. They'll understand.

JiK

@KyleKanos Remember, you made me do this:
http://i.imgur.com/izad0kE.gif

Kyle Kanos

Needs a new line:

Also: source code?

Or was that gif editing?

JiK

Source: pastebin.com/D4MsBftY Disclaimer: This is just trial&error, I don't have any idea what some of the commands do.

Next idea would be continuous colormap for all circles, so one can see that the old circles are time-reversed but the new circles are in original order and that the new circles are smaller = larger intensity = you maybe can't hear the time-reversed part even if they are real.

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