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Sir Cumference
Sir Cumference
12:08 AM
@EmilioPisanty holy hell i didn't think that was possible XD
 
Mo_
Mo_
12:40 AM
@ACuriousMind The ground state of a double well like this (with negative $V_0$) is always bound, right?
 
Secret
Secret
1:36 AM
 
vzn
vzn
1:50 AM
@bolbteppa nicest thing youve said in months :P yes am long aware solitons + fluid dynamics are nearly interchangeable in various ways. as for it being "hard nonlinear pde stuff" am aware too; maybe that explains some of the possible widespread aversion to it. but then isnt this just a variation on the drunk looking under the lamppost for his keys? :|
 
Secret
Secret
 
Mazura
Mazura
2:28 AM
Y'all ever watch The Mechanical Universe and Beyond series from 1986? What were we wrong about, if anything, in those episodes, now that it's 2019?
 
Sir Cumference
Sir Cumference
2:54 AM
Huh, TIL "college" has a very different meaning in the US compared to countries like the UK
 
Secret
Secret
> We will see that the notion of ‘jumping’ to a QRF becomes ill-defined: not all the variables can be cast in relational terms, and a choice has to be made as to which degrees of freedom are relevant to the situation studied
wut...
 
 
3 hours later…
Dicul Smerd
Dicul Smerd
5:34 AM
Can anybody help me to know how can one move only in the direction of time but not in space in curved space-time , when body is absolutely rest to all frame of reference
 
John Rennie
John Rennie
5:56 AM
@DiculSmerd there isn't a simple answer to that because it's not clear that we actually move in time in the way that people think.
We all have the perception that time passes, but it's entirely possible this is an artefact of the way the human brain works, and time doesn't really flow at all.
 
Dicul Smerd
Dicul Smerd
6:16 AM
@JohnRennie Proceed
 
John Rennie
John Rennie
@DiculSmerd I'm not sure I can say much more. No-one knows whether time really flows or not. Even the most experienced physicists cannot agree on the subject.
 
Dicul Smerd
Dicul Smerd
@JohnRennie it was first stated by enstien right?
 
John Rennie
John Rennie
As far as I know Einstein didn't make any specific comments about whether time flows or not. The first such comment I know of dates back to Saint Augustine about 2000 years ago!
 
Dicul Smerd
Dicul Smerd
@JohnRennie Then how gravity works . Space-time is curved . Even though I may at rest I'm pulled towards earth ? Then it does mean that I am moving time right. This what I know about GTR
 
John Rennie
John Rennie
6:33 AM
@DiculSmerd in GR you calculate a geodesic which is a posh name for the curve that a falling object traces out in spacetime.
But this is distinct from the idea that time flows.
 
Dicul Smerd
Dicul Smerd
 
John Rennie
John Rennie
I don't think that's a very good answer ...
To be fair this is a hard area to understand.
When we say there is a force between two bodies we ean that when we humans watch the two bodies we see them accelerate together.
But humans experience time moving forward. Whether it really does or not is irrelevant - that's what we experience.
So when you talk about the force between two bodies you have already implicitly assumed by asking the question that time flows.
 
Dicul Smerd
Dicul Smerd
7:06 AM
@JohnRennie whatever it is please answere that question in the best form you can (i think it’s whole GR) but still.. , I can understand what you trying to say ( it’s too difficult to grasp) but I want get rid off that question
 
John Rennie
John Rennie
@DiculSmerd I'll have a look at it later today.
 
Dicul Smerd
Dicul Smerd
@JohnRennie Ok thanks
 
Mo_
Mo_
@JohnRennie Hi!
question:
I have solved a problem quantum mechanically and also using classical statistical mechanics, and obtained a quantity which is essentially a probability times some length $a$. Using QM the answer has become $a$ and using classical statistical mechanics $ae^{E_0/kT}$.
I'm trying to interpret this result to make sure it's correct; currently, the two answers would agree if $kT \to \infty$. Do you think it makes sense?
 
John Rennie
John Rennie
8:00 AM
@Mo_ I guess $kT \to \infty$ is the continuum limit so it makes sense the answers would agree there.
 
Mo_
Mo_
@JohnRennie Yeah. But normally when $kT \to \infty$ it's the quantum answer that gets close to the classical answer...here it's the other way!
 
 
2 hours later…
ACuriousMind
ACuriousMind
10:35 AM
@Mo_ I think so, yes
 
starunique2016
starunique2016
@JohnRennie Sir.are efficiency in formation and percentage yield the same thing
 
John Rennie
John Rennie
@starunique2016 in a chemical reaction?
 
starunique2016
starunique2016
yes...
@JohnRennie
 
John Rennie
John Rennie
Percentage yield is normally the yield you actually get divided by the maximum yield possible.
I'm not sure I know what efficiency in formation means ...
 
starunique2016
starunique2016
but...is there any term known as efficiency,,
ok sir thank you..
can you name any sources where i might find the differnce
 
John Rennie
John Rennie
10:51 AM
@starunique2016 I think I've seen the term efficiency used to mean the percentage yield, but I'm not sure it has a precise meaning i.e. different people might use it in different ways.
 
starunique2016
starunique2016
thank you sir...i think you are correct
 
 
2 hours later…
ACuriousMind
ACuriousMind
1:04 PM
Huh, I didn't realize that there were already effectively analogues to quantum interpretations - i.e. very different stories about what the formalism means but that do not change any predictions - for string theory until I wrote this answer.
 
 
2 hours later…
Anonymous
2:41 PM
In case anyone's interested in this bug report...I couldn't figure out any reason so far.
 
ACuriousMind
ACuriousMind
@Blue The room owners for which the message numbers are shown are exactly the ones that also appear in the list "currently in room" to the right.
I.e. I suspect that the number of posts is only queried for users in the list on the right, but that the user cards are reused if the user appears more than once on the page.
 
Anonymous
@ACuriousMind Ah! But why would they do that? It doesn't make much sense to me.
 
ACuriousMind
ACuriousMind
@Blue I do not think that it is a deliberate design decision :P
 
Anonymous
Right, now that JR is in, it immediately updated!
 
Anonymous
Yeah, looks like a bug. :P
 
ACuriousMind
ACuriousMind
2:48 PM
A scenario for how this can happen would be the following: 1. There's a central service for building usercards that takes whether or not to put the number of messages on it as a parameter. 2. The code for the owners doesn't set this parameter, the code for the other users does, because they were written at different points in time in different places in the code.
3. There's a logic for buffering user cards somewhere that just reuses a card instead of querying the builder service if it sees it's the same user, and does not care for the "number of messages" parameter (e.g. because the parameter was introduced after the buffer was written).
In a large code base, it's not reasonable to expect any single developer who wrote parts of this to have forseen the consequences for the room info page :P
 
Anonymous
@ACuriousMind Interesting, that does make sense!
 
ACuriousMind
ACuriousMind
It's equally likely that it's an obscure bug with no reasonable explanation, but this is how this could happen even in well-designed code :P
 
Secret
Secret
3:32 PM
The concept is interesting enough, but I think it will be more interesting is if the laws of physics allow the laboratory to boost into these QRFs
imagine boosting the laboratory's frame so that the superposition collapsed into a sharp position state (because as seen from some third QRF, both the laboratory and the quantum state being investigated are now in an entangled state) that will be really cool
Moreover would it be cool to suddenly see your whole room turned into a superposition state because relative to the room, you have boosted yourself into a superposition of momentum states?
Finally that final suggestion there in the article, on the possibility to generalise that to general relativity, suggest in theory it may be possible to do a boost, so that a superposition of times will become a superposition of spatial positions
More wile ideas in the other room
 
Secret
Secret
3:52 PM
in SecretLabs (SE Branch), 46 secs ago, by Secret
Let a dynamical event X. Bring X into a superposition in time in the laboratory frame. Transform into X's frame so that the laboratory becomes a superpositon of positions. Project out the desired position of the laboratory (which is originally biased with high probability), transform back to the original frame, you now basically travelled back to an early stage of the evolution hence time
@vzn these QRF really capture the imagination
 
John Rennie
John Rennie
4:10 PM
Chicken and ham pie with a cream sauce. Mmm :-)
 
dmckee
dmckee
4:55 PM
@ACuriousMind In other words, when adding features to a large code-base just finding out what parts of the code need up-dating is a daunting task.
And then it takes longer than you expected or wanted.
And then—if you haven't been diligent about performing keep-up merges—the final version-control check-in involves a wide-ranging and messy merge conflict.
 
ACuriousMind
ACuriousMind
5:32 PM
@dmckee Well...if our predecessors had been diligent about separation of concerns and single responsibility, it would be much easier ;P
 
Mo_
Mo_
6:09 PM
@ACuriousMind Watch this for a higher job security: youtube.com/watch?v=MTCYhbfSAuA
 
ACuriousMind
ACuriousMind
@Mo_ The legacy code base is already full of anti-patterns, I don't need to add new ones :P
Well, anti-pattern at least suggests there's a pattern to it!
 
danielunderwood
danielunderwood
6:51 PM
@JohnRennie nice placemat!
 
Anonymous
@danielunderwood Huh, I thought that was a sheet of paper. :P
 
Anonymous
Wait, is it? That picture is too confusing. I can't judge its (the "placemat"'s) thickness!
 
Sir Cumference
Sir Cumference
Howdy folks
 
Anonymous
oWdY
 
danielunderwood
danielunderwood
lol I think it's a stack of paper
But it doubles as a placemat
 
Anonymous
6:57 PM
@JohnRennie Is the yellow thing rice? O_o
 
Anonymous
@danielunderwood Sounds reasonable. :P
 
PM 2Ring
PM 2Ring
TIL, John Rennie still has a tractor-feed printer. Or once had such a printer & has left-over paper. ;) I don't even a own a printer anymore, but I still have a boxfull of ancient tractor-feed paper.
 
ACuriousMind
ACuriousMind
Jan 22 at 17:10, by John Rennie
I still have a stack of several thousand sheets of line printer paper. I keep it thinking it might come in useful one day :-)
 
PM 2Ring
PM 2Ring
I can relate to that. I suppose I should just use it for notepaper, etc. But it seems wasteful... OTOH, not using it at all is even more wasteful.
 
Novalium Company
Novalium Company
7:28 PM
@JohnRennie I love how you constatly post what you're eating. Let's make this a cooking chat room.
 
dmckee
dmckee
Grading freshman lab reports.
Every time I do this my loathing for The Discovery Channel and other purveyors of half-assed, dramatically framed and voiced, populist "documentaries" grows a little more intense.
It's a ()&#$)(*&$#( report about a standing wave lab. You do not need to invoke Life, The Universe, And Everything in your introductory remarks.
 
Anonymous
@NovaliumCompany It already is! (And we love it.) :D
 
Anonymous
If you want to talk food, we're all always ready. ;)
 
PM 2Ring
PM 2Ring
7:46 PM
@NovaliumCompany I think you'd enjoy browsing the Less Wrong site. There's a lot of interesting information there, especially on the topic of artificial intelligence. Some of the info is questionable, or downright incorrect, but hey, they don't claim that they're right, just less wrong. ;)
 
ACuriousMind
ACuriousMind
Whatever you do, just don't read the "quantum" part of the "sequences" :P
 
PM 2Ring
PM 2Ring
Yeah, Eliezer Yudkowsky is a pretty smart guy, but he's mostly self-taught, and in some areas, like QM, he has some serious misconceptions. But he writes so authoritatively that it's easy to assume that he always knows what he's talking about.
 
John Rennie
John Rennie
@Blue Grated potato.
 
Anonymous
@JohnRennie Aha. :)
 
John Rennie
John Rennie
@Blue it goes crisp in the oven and gives the meal a lovely texture.
 
Anonymous
7:57 PM
I remember having something very similar a few months ago (mine had rice though). Tastes pretty good!
 
Anonymous
I can't eat too much potato though...the aftertaste is a bit disturbing. But otherwise its mostly nice.
 
Anonymous
This dish has a specific name...I forgot...
 
ACuriousMind
ACuriousMind
If your potatoes have a distinctive aftertaste, consider using fresher potatoes :P
 
PM 2Ring
PM 2Ring
Potato shouldn't have a disturbing aftertaste... unless the potatoes were green. :D And what ACuriousMind said.
 
Kyle Kanos
Kyle Kanos
Potatoes could have an aftertaste if you cook them with spices
But that's more the spices than the taters...
 
Anonymous
8:05 PM
Ah, I think it's called Chicken (and Ham) Shepherd's Pie. Looks like a British speciality.
 
Anonymous
That seems like it. And we were served with some bread and rice.
 
Kyle Kanos
Kyle Kanos
Mmm, shepherds pie...
 
Anonymous
@ACuriousMind Dunno, lol. I've never liked potatoes as such.
 
Anonymous
Shepherd's pie
Shepherd's pie (lamb) or cottage pie (ground/minced beef) is a meat pie with a crust or topping of mashed potato.The recipe has many variations, but the defining ingredients are minced red meat ("cottage pie" refers to beef filling and "shepherd's pie" refers to lamb), cooked in a gravy or sauce with onions and sometimes other vegetables, such as peas, celery or carrots, and topped with a layer of mashed potato before it is baked. The pie is sometimes also topped with grated cheese to create a layer of melted cheese on top. == Etymology == The term cottage pie was in use by 1791, when the potato...
 
Anonymous
Oh, wow. There are so many varieties!
 
danielunderwood
danielunderwood
8:14 PM
@Blue shepherd's/cottage pie is more of a tomato sauce with veggies inside and mashed potatoes on top
 
PM 2Ring
PM 2Ring
The best potatoes I've ever tasted were cooked by a Peruvian chef. Even though he was just using local Australian potatoes, he somehow managed to evoke some ideal potato magic when he cooked them. But he said our potatoes were pretty ordinary compared to what was available back home.
 
danielunderwood
danielunderwood
Or at least in my experience...I'm from a far away land though
 
ACuriousMind
ACuriousMind
I'll never understand how you can call anything with meat in it a "pie" :P
 
Anonymous
@danielunderwood According to the Wiki page:
 
Anonymous
> The recipe has many variations, but the defining ingredients are minced red meat ("cottage pie" refers to beef filling and "shepherd's pie" refers to lamb), cooked in a gravy or sauce with onions and sometimes other vegetables, such as peas, celery or carrots, and topped with a layer of mashed potato before it is baked.
 
Anonymous
8:16 PM
@ACuriousMind Must be a Brit thing!
 
Kyle Kanos
Kyle Kanos
The Irish do it too
Probably due to British influence... Though it could be the other way around
 
Anonymous
They're just a pond away. Not unexpected. ;)
 
Anonymous
@PM2Ring Some dishes with potatoes can be really nice, but it's mostly because of the spices (at least in the Indian cuisine perspective).
 
PM 2Ring
PM 2Ring
Speaking of cottage pie, see this excerpt from A Slurry Tale, which is how Pulp Fiction would have turned out if it had been written by Shakespeare.
2
 
Anonymous
@PM2Ring Loool
 
ACuriousMind
ACuriousMind
8:23 PM
@PM2Ring I'd pay to see the full version
 
PM 2Ring
PM 2Ring
@Blue Indeed! I'm quite fond of Indian cooking, although the South Indians tend to use a little too much chili. :) I've been vego for the last 3 or 4 decades, and Indian restaurants always have several vego dishes on the menu.
 
Anonymous
I think ~90% of the Indian restaurants abroad are run by the South Indian folks...and a majority of them are vegetarians...so... :P
 
Anonymous
Among South Indian dishes, I love dosas. They're light, filling and tasty.
 
PM 2Ring
PM 2Ring
@ACuriousMind The script was "translated" in an online collaboration, and it was great fun to watch it develop. After it was completed it got published as a proper play, and has been performed. But now the full script is no longer freely available on the net... I guess it's probably hiding in the Wayback Machine, though.
 
Anonymous
8:40 PM
Hmmm, I'm out of downvotes for today.
 
ACuriousMind
ACuriousMind
Then you're also out of upvotes, since they draw from a shared pool of 40 votes/day.
 
Anonymous
@ACuriousMind I only downvote on Physics SE these days. :P
 
Anonymous
40 per day seems too less. Too many terrible questions!
 
Anonymous
Gotta get some points on Physics so that I can cast close votes.
 
dmckee
dmckee
@Blue There are plenty of bad posts, of course. And if you are diligent you could use up your daily votes that way. But for my own sanity I prefer to seek out the deserving posts and vote them up.
 
PM 2Ring
PM 2Ring
8:58 PM
@Blue Bad questions are a problem, but that's mostly because they're off-topic, or because they tend to attract low quality answers. But at least they can be close-voted. Bad answers are much worse for the site, especially those containing incorrect information.
 
Anonymous
@dmckee I definitely would like to up-vote the good content more but that unfortunately eats up the number of available votes I get per day. I leave the upvoting duty to the site regulars. The current problem with Physics SE is that ~50% of the front page is occupied by homework-dump-type, no-effort-shown and terrible-formatting questions. :/
 
Anonymous
@PM2Ring True, but I can't really afford to downvote answers much until I manage to amass some considerable reputation.
 
Anonymous
Low quality and wrong answers definitely need more downvoting!
 
ACuriousMind
ACuriousMind
@Blue Have you tried ignoring homework-and-exercises and other frequent hw tags such as the kinematics family?
 
Anonymous
@ACuriousMind That would solve the problem for me, but not the community.
 
ACuriousMind
ACuriousMind
9:01 PM
I don't think the community has a problem with homework-type questions. Most of them get closed.
 
PM 2Ring
PM 2Ring
I'd probably have enough rep to close-vote, if I didn't spend so much rep on downvotes.
 
ACuriousMind
ACuriousMind
But closure takes a while, so the front page will always have them, no matter what
 
Anonymous
Yeah, but they still occupy the main page for most people. Downvoting bad content on the other hand eventually blocks the posters.
 
ACuriousMind
ACuriousMind
@Blue So does closure. But recurring homework posters are not the problem either, most of the really bad questions get asked by drive-by askers who just stumbled on the site, dumped their question, and will never return.
I really don't think you should specifically focus on downvoting questions that get downvoted and closed anyway.
I mean, I downvote a lot of these questions, too. But often when a question is already at -1 or -2 and closed - or obviously on the track to being closed, I will conserve my vote for a post where it actually adds signal instead of effectively wasting it on that one.
 
Anonymous
I wonder if we can get through with a "delete bad content" policy instead of simply closures. Probably SE's not gonna approve though.
 
PM 2Ring
PM 2Ring
9:06 PM
Physics SE has a very tough homework policy. Pity that we can't adopt it on SO. We generally CV pure no-effort assignment dumps as Too Broad, and others get closed for lacking a Minimal Complete Verifiable Example, but that still leaves an awful lot of garbage. And of course, rep farmers will often manage to get an answer in before the question is closed.
 
Anonymous
The ordinary roomba deletions take a month...
 
Anonymous
> Questions that are extremely off topic, or of very low quality, may be removed at the discretion of the community and moderators.
 
Anonymous
The help center does say this. ^
 
ACuriousMind
ACuriousMind
@Blue Doesn't scale. Do you really think >20k users want to sift through all the terrible stuff deciding what gets deleted? We're juuuuuust on the brink of having enough reviewers, we certainly wouldn't have enough deleters.
Also, deletion is basically inauditable by a large fraction of the community since they can't see deleted posts (which is arguably the intent), so one should be careful in making it a regular part of the process
 
Anonymous
@ACuriousMind I was speaking deletions by mods (at least for closed questions which are not salvageable according to mod discretion). Basically, I'm suggesting a more aggressive close/delete policy that involves mod powers more. The main problem with that of course is: "most of the moderation should be done by the community" philosophy.
 
Anonymous
9:11 PM
And then you'll have folks shouting MOD ABUSE! :)
 
ACuriousMind
ACuriousMind
Moderation by mods scales even less well than moderation by high rep users!
Moderators are exception handlers, not roombas.
 
Anonymous
@ACuriousMind Yeah, that's the problem! :P
 
PM 2Ring
PM 2Ring
In some SO chat rooms we use delv-pls to request high rep members to consider casting a delete vote. But we normally only use it on stuff that won't be roomba'd, unless it's really bad & needs immediate nuking.
 
ACuriousMind
ACuriousMind
Well, that only works on a site like SO where deletion privileges aren't rare. The entire population of physics.SE capable of casting an immediate delete vote on a closed question is on a single page.
I realize the current state is not optimal. But I think it is close to the best we can do without sacrificing core values like "everyone can ask a question"
 
PM 2Ring
PM 2Ring
@ACuriousMind Fair point. But with a bit of chatroom based coordination, those with del-voting powers can be more effective, including 10k+ people.
 
Anonymous
9:24 PM
Indeed. Things get a bit tough when we expect both Nobel laureates and laymen to ask questions on the same site. The "everyone can ask" philosophy is nice, but it's not something I can get myself to agree with.
 
ACuriousMind
ACuriousMind
@PM2Ring Effective? Certainly. I'm worried that it is not particularly transparent, though. Most users have no idea chat exists and having a bunch of delete votes descend on your question because someone posted it in a chat feels much more arbitrary than the standard review process.
Arguably, there should be a deletion review queue, but again, the number of people who could review is rather small so it probably wouldn't reach critical reviewer mass.
 
PM 2Ring
PM 2Ring
Everyone is permitted to ask a question, but clearly not everyone is capable of asking a good question... or even reading the full Help tour before attempting to ask a question
 
Anonymous
What advances a nation or community is not so much to prop up its weakest and most helpless members as to lift up the best and most gifted so as to make them of the greatest service to the country. I prefer this constructive philanthropy which seeks to educate and develop the faculties of the best of our young men.
 
ACuriousMind
ACuriousMind
@Blue For all the times that I've been called an elitist, I do believe that "low-level" vs. "high-level" is not the issue here. I've seen absolutely terrible technical questions about advanced QFT, and very well thought-out questions about elementary every-day mechanics. The bad questions don't come from "laymen".
They come from a) lazy students trying to use us for homework b) people bad at communication c) people with idiosyncratic understanding of how the world and/or the site works and an iron-clad belief they're right.
 
Anonymous
A lovely (controversial) quote I once read. Now that I think about it, I do at least partially agree with it.
 
Anonymous
9:30 PM
@ACuriousMind That's totally true. Yup.
 
PM 2Ring
PM 2Ring
@ACuriousMind Yet another fair point. OTOH, a big advantage of chat vs a review queue is that in chat we can discuss the merits (or lack thereof) of a post, and come to a consensus, whereas in a review queue you're flying solo.
 
Anonymous
There can absolutely be great questions about everyday physics too. But like all good questions, they take some time and effort to produce, which most new users aren't willing to put in.
 
ACuriousMind
ACuriousMind
@PM2Ring That (reviewing solo) is actually a benefit! It makes it easier to form your own opinion about a post, while in a chat, your view will have been tainted by what the others said about the post before you even read it.
 
Anonymous
I definitely don't expect that all QFT or string theory on our site are good. I totally loved the "cooling a cup of coffee with the help of spoon" thread on the other hand!
 
ACuriousMind
ACuriousMind
That way, we're actually getting five different opinions for a close vote, and not just the opinion of one eloquent user who talked the other four into voting
 
Anonymous
9:33 PM
706
A: Cooling a cup of coffee with help of a spoon

drhodesI We did the experiment. (early results indicate that dipping may win, though the final conclusion is uncertain.) H2O ice bath canning jar thermometer pot of boiling water stop watch There were four trials, each lasting 10 minutes. Boiling water was poured into the canning jar, and the ...

 
Anonymous
Just look at the effort this individual went to, to write an answer! :) Even the question shows considerable effort.
 
Anonymous
This is exactly the kind of content we want to encourage. It doesn't matter that it's about "everyday-physics"! All that matters is the feeling of "I really learnt something from there...something that I won't find in textbooks...something that shows the beauty of real physics!"
 
ACuriousMind
ACuriousMind
Of course, yes, we could be going for a Wikipedia-style consensus moderation via chat. But closure is supposed to happen quickly, and reaching an actual consensus can be very slow.
 
PM 2Ring
PM 2Ring
@ACuriousMind I suppose so. But when I see a delv-pls request, I try to visit it and see for myself what's wrong with it befoe I read other's opinions.
Closure often needs to be fast. Deletion can happen at a more leisurely pace.
 
ACuriousMind
ACuriousMind
@Blue You know, there's survivor's bias there. Of course it's mostly users who are overwhelmingly unwilling to put in effort, because those who aren't willing generally won't become regulars. And of course new users mostly ask about low-level physics because that's what the vast majority of humans know. So the association of low effort with low level is to be expected, but it's more of a statistical misfortune than an indictment of low-level physics.
 
Anonymous
9:39 PM
@ACuriousMind Yup, I agree with all the points there.
 
Anonymous
It's just that when I myself: "do we need such users?"...I hear a recurring no. I can totally understand that my philosophy completely differs from SE's philosophy though. They're a for-profit company after all. And with a stricter policy we'd be cutting down on a lot of traffic.
 
ACuriousMind
ACuriousMind
@PM2Ring It can't be leisurely if Blue wants a clean front page :P
But yes, you're right.
@Blue Depending on who exactly you mean by "such users" there, I either completely agree or am immensely glad you don't moderate this site :P
 
Anonymous
Lolol
 
Anonymous
I mostly meant "users who are overwhelmingly unwilling to put in effort".
 
Anonymous
If SE were a dictatorship and I were the dictator, I'd nuke those accounts on sight. So yeah, you should be glad. :P
 
PM 2Ring
PM 2Ring
9:47 PM
I wish more answerers made an effort to do a dupe search before answering. But even some high rep members tend to answer fairly obvious dupes...
 
Anonymous
Answering dupes is fine if you're adding some new perspective. But if it's just regurgitation of old points, then yeah, you're doing it wrong.
 
ACuriousMind
ACuriousMind
@Blue Again, the vast majority of low-effort askers just asks a single terrible question. You could nuke every single one of them and you still would have a steady influx of these questions. And sometimes, users are willing to improve and learn.
Most of the time when I leave one of my comments giving tips for improving the question, I get nothing. But sometimes, I get an "oh, sorry, sure" and suddenly a terrible one-liner morphs into a well-formed question. Sometimes, the carrot gets you more than the stick. And I would hate for a single one of these users to be turned away because we weren't willing to assume good faith at least once.
4
I believe it is not a coincidence that the best crowd-sourced repositories of information on the internet -- Wikipedia and SE - both strictly insist on community moderation and on assuming good faith.
 
PM 2Ring
PM 2Ring
With homework questions, I sometimes write a comment asking the OP if they have a conceptual question regarding their assignment. But most of them don't get it, even if I link them to the relevant FAQ on Physics meta.
 
ACuriousMind
ACuriousMind
@PM2Ring Did you know you can use comment templates?
 
Anonymous
@ACuriousMind Oh of course, those cases are there. I was just exaggerating it a bit. But yes, those users would certainly fall in the "willing to show effort upon being provided guidance" category and I wouldn't want to turn them away either. I'm all for guiding and helping; I'm just against being used and and doing someone else's work for them.
 
ACuriousMind
ACuriousMind
9:53 PM
I used to not comment on many of these posts because typing the comment every time was really annoying. Now it's like two clicks and I do it almost every time I interact with a new user.
(you have to tolerate the occasional insulting response, though :P)
 
Anonymous
Yes! That's why I installed the auto comment script and created around 10 custom comment templates for QCSE. It feels great when I see my guidance is helping someone improve their post.
 
PM 2Ring
PM 2Ring
@ACuriousMind Kind of. I use them on SO, when I'm on my computer. I haven't investigated installing them on my phone's browsers. But I guess I could copy & paste them manually.
 
Anonymous
@ACuriousMind Heh. We've developed a thick skin over time. :P
 
Anonymous
I already have 10 posts complaining about me on meta.
 
ACuriousMind
ACuriousMind
@PM2Ring I don't think there's a good way to use them on mobile
I don't use SE much on mobile
 
pZombie
pZombie
9:57 PM
help me understand why this would not work imgur.com/a/MHr3TB0 before i create another fail question on SE
Initially, the valve will be closed, and both chambers inside the pipe will be depressurized to the point where water starts boiling at ambient temps. When the max pressure at the bottom chamber is reached, the valve releases and the turbine is spinning (or any other method to extract part of the energy of the steam)
the thermal shield will be closed during this period. As i would assume, since the steam gives up energy, the whole system cools down, allowing the steam to go back into water and we can repeat the cycle by removing the thermal shield at the bottom and allowing the heatsink to heat up the water again into steam
 
PM 2Ring
PM 2Ring
@ACuriousMind The mobile view is handy, but a bit limited. But it's easy enough to switch to the full site view when you need it, and full site is now a lot more usable on mobile devices, with the new responsive design.
 
ACuriousMind
ACuriousMind
@pZombie ...why would the pressure in the bottom chamber increase at all?
 
Anonymous
@PM2Ring I think you can use those scripts with Tampermonkey on Firefox (in Android devices).
 
Anonymous
Monica Cellio mentioned she does that.
 
PM 2Ring
PM 2Ring
@Blue Interesting!
 
ACuriousMind
ACuriousMind
10:01 PM
@PM2Ring I know, but usually when I browse our site, I'm looking for things to answer, and typing answers on a little phone screen feels terrible, especially if you have to type MathJax.
 
pZombie
pZombie
@ACuriousMind Isn't that what happens when water turns into steam? The first time of course, when we depressurize both chambers, we already did the work but after that i would assume that it would go on by itself, extracting energy from the environment
 
ACuriousMind
ACuriousMind
@pZombie Why is the water in the bottom chamber turning into steam?
 
pZombie
pZombie
@ACuriousMind Because we set the pressure such that water turns into steam at ambient temperature
 
ACuriousMind
ACuriousMind
@pZombie How did you depressurize the bottom chamber if the valve is closed, and how did not the water start to boil immediately when pressure dropped low enough for it to do so, thereby maintaining that pressure?
 
pZombie
pZombie
@ACuriousMind Both chambers have interfaces to do the depressurizing. And yes, i expect in the first cycle that water would start boiling at the bottom chamber during the depressurizing. But we have to do this only 1 time
 
ACuriousMind
ACuriousMind
10:08 PM
@pZombie But you can't do it at all. Unless you posit a "magic instant depressurizer", I would expect the water to turn into steam at a rate that maintains the "boiling pressure", and you only reach a pressure below that once all the water has evaporated.
 
pZombie
pZombie
of course, if that is an issue, we can change the initial setup. depressurize the whole pipe first with the valve open. Get some water inside which would instantly boil and then use a piston to push into into the valve and close it. And then go from there
@ACuriousMind ok, forget about how we do the pressurizing. Just think about an initial state, where water at the bottom chamber is boiling and reached the max pressure just from being subject to ambient temperatures. We release the valve and it shoots up to spin the turbine. Will the water not cool down by losing energy to the turbine?
and if it does cool down, supposed we closed the thermal shielding such that the pipe is no more subject to ambient temps, will the water not collect back in the first chamber?
 
ACuriousMind
ACuriousMind
@pZombie Yes, it will cool down due to expansion and interacting with the turbine.
 
pZombie
pZombie
@ACuriousMind And if we chose the pressure inside the pipe such that this cool down would be enough to turn the steam back into water, wouldn't we then end up with all water flowing back into the first chamber? At which point, we close the valve, remove the thermal shielding at the bottom and heat up the water back to ambient temps where it starts boiling again?
 
ACuriousMind
ACuriousMind
@pZombie I don't understand the role of the heatsink yet, but if the cylinder is thermally isolated if you close the heat shield, then you just have a cylinder full of steam, with as much of it condensing (and yes, flowing back to the bottom) as is needed to reach equilibrium
@pZombie No, if all the steam turns back into water, that would suggest that it is a state closer to equilibrium for all the water to be fluid, which would mean it wouldn't have evaporated in the first place.
 
pZombie
pZombie
@ACuriousMind the thermal shield at the bottom can be opened or closed. When the water is back inside the chamber, the valve closes, the thermal shield opens at the bottom, such that the heatsink can heat up the water again, using ambient air
so the steam won't turn back into water completely i assume
because it did not give up enough energy to the turbine
what about gravity and steam losing momentum to the walls of the pipe? Does that matter at all?
 
ACuriousMind
ACuriousMind
10:22 PM
@pZombie The turbine extracts the kinetic energy of the steam jet pushing into the upper chamber. It cannot extract the phase transition energy, so it doesn't cool the steam below the condensation temperature,
i.e. it this isn't a cycle - the steam pushes into the upper chamber, pwoering the turbine, but then most of it just stays steam. Note that since the water reached "max pressure" in the bottom chamber, but the combined chamber has larger volume, you're now below max pressure, meaning even more water is evaporating instead of condensing.
 
PM 2Ring
PM 2Ring
When water changes phase from liquid to vapour it absorbs heat without changing temperature. So in the reverse process, condensing water vapour releases heat.
 
pZombie
pZombie
@ACuriousMind the combined pressure of both chambers, hence the whole pipe, would be chosen such that water just about boils (at ambient temps). We then somehow push the boiled water into the first chamber and close the valve. The initial setup.... Then when the water shoots out of the valve, it will lose energy to the turbine. So the whole system will have less energy now than before.
@ACuriousMind less energy i guess does not necessarily translate in cooler steam/water from what i get
 
ACuriousMind
ACuriousMind
@pZombie Sooo...you're creating a vacuum in the upper chamber?
 
pZombie
pZombie
@ACuriousMind while that was not my intention, i don't see why this would be an issue. Let's go with the vacuum
point is, that if we get the water to just about boil inside the pipe at ambient temps just before we compress it into the chamber, once we open the valve and it gets back into the whole pipe while having lost energy to the turbine, the whole system would end up with less energy
less energy i assumed would suffice to turn the water back into liquid
 
ACuriousMind
ACuriousMind
@pZombie Again, you need a "magic depressurizer" to even get this initial state. You can't depressurize the two chambers without water instantly starting to boil. If you need to continue until the pressure is low enough such that the pressure after shoving everything into the bottom chamber is low enough for it to still boil, you're going to just suck all the water out as steam :P
I'm not saying that the whole setup would work if you could get the initial state (because I don't really understand what you're trying to do here :P), but you can't even get to the initial state!
 
pZombie
pZombie
10:34 PM
ok, let me make it simpler then and get to the initial state i require using another method
we setup the experiment at 20°C ambient temps. We fill a pipe with water, both chambers connected with the valve open. Then we depressurize the pipe, but not to the point where water boils. The pressure is chosen such that water would boil at 40°C
we close the valve and move to another place, where ambient temps are 40°C.... water at the first chamber should now begin to boil and you have your initial state of the experiment
 
ACuriousMind
ACuriousMind
That actually sounds as if it would work. Okay, so you open the valve, the turbine spins, and now you have a single chamber with some steam and some water. What now?
 
pZombie
pZombie
well, that is the question. The steam lost energy to the turbine. Does this or does this not translate into the water getting colder?
then my turbine cannot substitute a condenser i guess
 
ACuriousMind
ACuriousMind
Hmmm, I think you would get some drop in termperature of the whole system, yes.
If you now connect it back to the enviroment, you basically have made an air conditioner!
 
pZombie
pZombie
something tells me that this won't work still
 
ACuriousMind
ACuriousMind
But it'll get less effective with every cycle unless you invest energy into pushing the steam in the upper chamber back into the bottom
If you don't do that, the energy you can extract in the limit of infinite cycles will just be the initial work you did in depressurizing and pushing.
 
pZombie
pZombie
10:47 PM
i guess some complex physics is required to make such a statement as you just did
 
ACuriousMind
ACuriousMind
It's just conservation of energy. But yes, if you want to explicitly compute the energy extracted in each cycle you'll have to do some annoying thermodynamics
 
pZombie
pZombie
thanks for clearing that up. That question has been just binned
@ACuriousMind also, conservation of energy is not the issue here. The energy comes from the environment used to heat the water back up when the heatsink is engaged.
@ACuriousMind the environment around the heatsink would get colder but since there is plenty of air around the heatsink, that would quickly be taken care of
 
ACuriousMind
ACuriousMind
@pZombie Ah, true. You will be able to also extract energy from the environment. But each cycle, the temperature will drop less - because it doesn't return to the initial 20C at the beginning, and not to the new initial temperature the second time, etc.
 
dmckee
dmckee
Of course if you fix the "it doesn't return to the initial [conditions]" bit you have produced a machine that can be analyzed with the second law of thermodynamics.
 
ACuriousMind
ACuriousMind
11:11 PM
@dmckee Still logging on from a coffee shop or did you find something better?
 
dmckee
dmckee
@ACuriousMind I bought a WiFi hotspot and a medium data plan for it, but the throughput varies and is rarely very impressive.
 
ACuriousMind
ACuriousMind
Ah :/
 
dmckee
dmckee
Today I'm at the coffee shop again. But because I wanted coffee not because I needed internet.
 
pZombie
pZombie
@dmckee the 2nd law of TM will have to explain why i won't be able to get my first chamber back with all water as in the initial state of the experiment. Because if i can get all water back into the first chamber after the first cycle, i can reproduce the initial state by heating it back up again with the heatsink
@dmckee So there has to be a limit dictated by the 2nd law to how much energy i can get out of the steam through the turbine, limiting the cooling effect on the whole system
 
ACuriousMind
ACuriousMind
@dmckee Heh. I don't drink coffee on the weekends because otherwise it doesn't work during the week ;)
 
dmckee
dmckee
11:17 PM
@pZombie The awe-inspiring/deeply-frustrating thing about the second law is that it doesn't care how you establish a cycle, it just tells you about limits on what a cyclic heat-to-work converter can do.
 
ACuriousMind
ACuriousMind
^that. The second law is the explanation. It's just a high-level one derived from the very same principles you'd use to compute each time step of the process at a low-level.
Reductionism isn't always preferable.
 
pZombie
pZombie
@dmckee the limits of how much heat can be transferred into useful work however is not the crux of the problem here since heat is getting added to the system by the heatsink/ambient air after each cycle
 
dmckee
dmckee
I didn't really begin to fathom just how deep the 2nd law goes until I had taught the subject (so, my ... fourth? time through the subject seriously).
And you can build it on foundations that seem absurdly parsimonious. I mean, conservation of energy and the ergodic hypothesis are enough if you are willing to detour through some statistical mechanics on the way.
How, the heck does that work?
 
pZombie
pZombie
@dmckee well, we have established that the whole system drops some temperature, due to the steam losing energy to the turbine after the valve is opened. The system is certainly below ambient temp at this point. We open the thermal shield at the bottom and re-attach the heatsink, subject to ambient air in the environment. Once the water boils again, we close the heat shield, detach the heatsink, open the valve, the cycle repeats
@dmckee The only way for this experiment to not work, is for the energy extracted by the turbine to not be high enough to turn all steam back into water, falling back into the chamber such that we can close the valve again and repeat the cycle
i am idealizing a bit here of course, assuming a perfect heat shield and no energy required to do all the attaching/detaching of the heat shield and heatsink at the bottom
 
dmckee
dmckee
I have no idea what the machine you are talking about looks like. Couldn't see the image (possibly because of my script blockers). Until you have both (a) dug deep into thermal physics and (b) seen enough example analyses it is very tempting to think that you can "fool" the universe on these things.
Give me a moment to find a link to a "hard" case on the site.
 
pZombie
pZombie
11:30 PM
your script blockers do not allow to visit imgur?
 
ACuriousMind
ACuriousMind
@pZombie It can't be high enough, because initially there was vacuum in the upper chamber, and then there's some steam left, setting a new base pressure/temperature for the whole chamber. Then you heat the bottom again, some more water evaporates, it expands into the upper chamber, and then there's a new, higher base pressure/temperature.
If it was possible that the system returned to the initial temperature, then you'd have some setup with 100% efficiency, violating the maximality of Carnot efficiency
 
dmckee
dmckee
91
Q: Where is the flaw in this machine that decreases the entropy of a closed system?

QuantumFoolI was thinking about a completely unrelated problem (Quantum Field Theory Peskin & Schroeder kind of unrelated!) when the diagram below sprang into my mind for no apparent reason. After some thinking, I can't figure out why it wouldn't work, other than the theoretical reason that it systematicall...

thermodynamics energy entropy
 
pZombie
pZombie
@ACuriousMind but we already established how we would generate the initial state without there being a vacuum and both chambers having the same pressure
 
dmckee
dmckee
@pZombie I can "visit" the site just fine. But the site relies on scripts to display the images. (And just think about that for a minute, they want to run code on your computer to show you a picture, for FSM's sake! How bonkers is that?!?)
 
pZombie
pZombie
@ACuriousMind "ok, let me make it simpler then and get to the initial state i require using another method
we setup the experiment at 20°C ambient temps. We fill a pipe with water, both chambers connected with the valve open. Then we depressurize the pipe, but not to the point where water boils. The pressure is chosen such that water would boil at 40°C
we close the valve and move to another place, where ambient temps are 40°C.... water at the first chamber should now begin to boil and you have your initial state of the experiment"
 
dmckee
dmckee
11:37 PM
Anyway, you have to dig in deep across several field to figure out how the machine in that link doesn't violate the 2nd law, but Carnot still ends up laughing.
 
ACuriousMind
ACuriousMind
@pZombie Yes. The point is that you can't return to the initial state with that setup. In the initial state, the upper chamber was in vacuum (or at a particular pressure), after the first cycle, the pressure is higher.
 
dmckee
dmckee
And that kind of thing happens over and over again when you try to fool thermal physics.
 
ACuriousMind
ACuriousMind
If it returned to the initial state, then, again, you would have an efficiency of 100%, which cannot happen.
The neat thing about conservation laws and no-go theorems is that they make reasoning easier. The whole reason Noether's theorem is so brilliant is that solving problems via conservation laws is often tractable when the exact dynamics of the problem are not
Insisting that we solve the exact dynamics instead of applying the very laws derived from the same principles the dynamics rely on is not useful
On the imgur thing: I think @enumaris's (now ex-)employer literally blocked access to the imgur domain :P
 

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