1:45 AM
@EmilioPisanty Wow!
@EmilioPisanty Looks like you took care of it.
Regarding the octobot: awesome! Thanks for sharing that.

2:40 AM
@0celo7 You there?

yes.
doing Riemannian geometry problems

Do you have Larson?

@peterh Surely a user who has been around and knows the ropes should be held to a high standard. Otherwise you're in the position of favoring the very people best prepared to live up to the standard we try to set.

No
Bob has it.

@0celo7 Ah, okay
I'm solving exercises

2:43 AM
but if you post a screenshot I can help.

I just found the online answers :)
Didn't know that was a thing
I'm just calculating intercepts
I know this stuff, but I figured I might as well do the whole book
These asshats only give answers to odd-numbered questions and only make even ones hard

correct

@Jim In all seriousness I think the historical reason is that when the network had that as a reason there developed—on a site that shall remains nameless to protect the guilty; I'm looking at you, Stack Overflow—a pattern that looked like people selecting that reason as a way to avoid thinking too hard about what was or was not wrong with questions. Coincident with that change Shog9 suggested that poorly thought-out questions are unclear ...

3:15 AM
@0celo7 There's no symmetry in $xy^2 = -10$ right?

what symmetry
it's invariant under $y\mapsto -y$, so there might be.

in regards to x axis, y axis or the origin

x axis probs

I figured it could have x axis symmetry
but when I plotted it didn't look like it

you won't see x axis symmetry unless you plot the whole thing

3:20 AM
Aha! My plot was too close up
@0celo7 What about $xy-\sqrt{4-x^2}=0$
Origin!

origin, maybe

Called it

I don't know what these look like

I wish I could down-star starred comments that I think are incorrect.
6

@DanielSank Anything particular in mind?
@BernardMeurer how do you know what symmetry these things have

3:32 AM
@0celo7 If you change $x$ for $-x$ and the resulting equation is equivalent then it has y axis symmetry
if you do that for $y$ it has x-axis symmetry
if you do that for $x$ and $y$ it has the origin as it's symmetry axis

Proof?

It's trivial

My topology prof said if he sees "trivial" or "clearly" in a homework proof, he stops reading

If a signal inversion on $x$ produces an equivalent equation it means that for any given $y$ determined by an $x$ that same $y$ can be obtained by $-x$
Which will clearly provoke symmetry
the proof extends to $y$ with ease

provoke symmetry...interesting choice of words.

3:35 AM
and the origin one makes sense in my head but I can't prove it
Or just interesting?
Ah, because the $(+x,+y)$ quadrant and the $(-x,-y)$ quadrant are diagonally opposed
I see, I see
@0celo7 Makes sense?

I'm proving something, hold on.

So, the more velocity dispersion a cluster has, the more the velocities of its stars will differ from the mean?

AHA
If the closed and bounded sets are compact, the metric is complete.
Now how to prove...

Crap, guess no astronomers are in
@ChrisWhite Sorry to bug you, but could you just tell me if what I said above about velocity dispersion is true?

I'm going to need more paper

3:50 AM
@BernardMeurer I'm deep into a problem
how many books on my desk

10

@ChrisWhite Or, more specifically, is the velocity dispersion simply the standard deviation of velocities from the mean?

dang

to be fair 5 are not related to this problem
three Riem geo books, one topology book, one diff topology book

3:53 AM
@0celo7 Wait, you like statistical physics, right?

huh?

That a yes?

no, and I don't know where you got that crazy idea from.

Dammit D:
I'm just kind of hoping at this point
Thinking that, the more I believe it is true, the more likely it will actually be true
Actually I would make a horrible statistical physicist...

Ah yes.
After an hour, I understand the hint.
Now to show I can actually prove the hint.

4:12 AM
That's enough exercises for tonight
G'night!

4:23 AM
@SirCumference Stat mech is awesome.

4:41 AM
@DanielSank I know it's cool and all
But that assumption I made kind of proves how horrible a statistical physicist I'd make

esp fluid dynamics =D

5:23 AM
@SirCumference Oh come on.
What level are you at?

Well...I'm just starting college soon

@SirCumference Oh for heaven's sake, you cannot possibly know what topics you're good at yet.

True

When I was in college I thought I hated statistics and noise, etc.
Now it's one of the areas where I really know more than the average physicist!
It just really became interesting to me in junior year of college.

You've got a point, I suppose I'll keep looking into it

5:25 AM
I thought quantum mechanics sounded stupid too.
Now I mostly do noise in the context of quantum mechanics :|
Oh, also I wanted to be a theorist.
...and now I'm an experimentalist.

5:54 AM
@DavidZ Here's one of those comments I was talking about (a new one):
Welcome to Physics Stack Exchange. This is an awesome site for physics Q&A, but it has some guidelines that we follow to keep the quality up. Posts asking for a specific problem to be worked out are generally considered "homework-like" and have special rules which you can read about in the help center. As written, this post will probably be closed, but if you could please edit it to show your work and identify a specific aspect of the problem on which you need help, we'll be happy to answer :D — DanielSank 1 min ago

@JohnRennie ...no? Except in the sense that all (most) comments are meant to improve the post and thus provide a public service

I thought everyone knew that $H\rightarrow\infty$ as $t\rightarrow 0$ for the FLRW metric ...

@DavidZ Spoken like a true mod.

@DanielSank yeah, I've seen them, but the thing that I think will be useful is the distribution of different reasons. That's why someone needs to collect a lot of them.

5:58 AM
@DavidZ Oh, I couldn't remember why I was supposed to collect these comments.
>.<

@JohnRennie actually, I didn't know that offhand
It does make sense though, with $H = \dot{a}/a$
P.S. seems off topic but maybe it's just me:
1

How much damage would a space probe cause if it can get about 4 lightyears away in 10 years and doesn't have any brakes when it arrives? Could it cause a global extinction event on the planet it was sent to? Do actual concept probes think about this?

@DavidZ I almost always go for anything other than off-topic.
This one could be closed for asking too many separate questions.

@DavidZ It would only require $\dot{a}$ to remain finite at $t=0$ for $H\rightarrow\infty$ but in fact $\dot{a}\rightarrow\infty$ as well.
@DavidZ it's not without interest. The calculation is straightforward if you assume a mass for the probe, and NASA does care what happens to its probes, which is why they deliberately crash them when the mission is over.

@JohnRennie When is it ever straightforward to calculate damage? ;-) But I suppose I see your point.
At the very least, it does strike me as insufficiently low effort.
I would want to see at least some attempt to turn the idea "how much damage" into some physical concept, like how much energy is released.

Hey, can anyone tell me if my answer to this question was correct?
0

The Moon phases can be defined by the phase angle between the Earth, Moon and Sun; for example, at 0°, the Moon is defined as full, while at 180° it is defined as new. If you want to know how bright the Moon is at a given angle, we would use the phase angle to find the apparent and absolute magni...

I just need someone to check it over

6:24 AM
@DavidZ This is why I ask people to focus on one specific question.

I suppose this could be an informative case study: how should we best formulate the reason it should be closed?
(just throwing that out there to think about)

@DavidZ It's a good question
1. Title is terrible.
3. "How much damage" is sort of unclear what you're asking.
4. Asking about extinction events involves parameters way outside the realm of what we normally call "physics". Not sure if there's a close reason for that.
5. I have no idea what a "concept probe" is, so again it's unclear what OP is asking.
...aaaaaaand it got an accepted answer :|

Times like this are when I second-guess my restraint in using the mod close vote :-/
Anyway, we can still fix it up by editing

I'm putting my money that the question gets on the Hot Network

6:40 AM
@DanielSank That one I could excuse because presumably someone could look up the order of magnitude energy required to cause an extinction-level event. That part of the question isn't really physics, but I think it's acceptable that questions sometimes ask how to relate the physics part of it to something else. It's debatable though.

@DavidZ Indeed we see that several good answers were generated.
However, the question is still crummy. It got an ok answer because the energy involved in the crash is so obviously below what's needed for an extinction event, but this is something OP could have figured out in two seconds with basic kinematics and Wikipedia.

7:04 AM
Yeah, and I downvoted accordingly.

@DavidZ Hmmm, downvote but no close...
This notion still confuses me.
I'm hard pressed to think of a question which deserves a downvote but isn't either too broad, unclear what you're asking, or some other close reason.
Maybe I'musing close votes where down votes belong because I don't like down-voting...

Could be. Though it is true that there is some correlation between downvote-worthy questions and close-worthy questions.

@DavidZ "Some". Lol.

lol
But in this case I do mean to make a point, that downvoting and closing serve somewhat different purposes and I really don't think every downvote-worthy question should be closed, nor vice-versa.

@DavidZ Why exactly would I downvote a question which isn't in violation of the rules covered by closing?
Or is the idea that slight violations should receive the down vote?
(To me, a down vote is more severe)

7:11 AM
That's part of it - downvoting is meant to be less severe than close-voting.

@DavidZ See there's our problem right there.

How is that a problem?

Close voting gives the user a very well defined (ideally) path toward re-opening. As stated previously by me, the close reason itself is a short lesson in how to ask better. A closed question does not affect one's rep.
Down votes offer no information and do lower rep.

I would take that as a sign you care too much about reputation ;-)

@DavidZ It's not about what I care about. What do you think a user with 4 rep and who is wanting to use the chat room cares about?

7:13 AM

Also, let's not ignore what I said about down-votes offering the OP no constructive information, while the close reasons do.

@DanielSank One example might be, if the asker makes significant wrong physical assumptions while explaining his question. Or maybe not even wrong, but you strongly dislike his assumptions.
2

From Harvey Friedman's manuscript on "Order Invariant Relations and Incompleteness": DEFINITION 4.4. A $\Pi_1^0$ sentence is a sentence asserting that some given Turing machine never halts at the empty input tape. A $\Pi_2^0$ sentence is a sentence asserting that some given Turing machine hal...

@ThomasKlimpel So, why is that down-vote but other types of crappiness are worth of closign?

That question for example has 9 downvotes (you can't see, because you only see the sum of the vote counts), but never received a single close vote.

7:15 AM
And I was going to say, even though it is desirable that closing a question gives the asker instructions on how to improve it, fundamentally the purpose of closing is to establish that the question is inappropriate for the site, and so severely inappropriate that (in its current form) it doesn't deserve to get an answer at all.

@ThomasKlimpel Why would you assume I can't see up and down votes? ;)
@DavidZ It's interesting to read this. Even as a n00b I considered down votes much worse than closure.

Because it is a question on mathoverflow, and you don't have sufficient reputation on mathoverflow to see downvotes?

Closure comes with an explicit route to re-opening. On the other hand, when people down-vote, that's it. Your question is screwed forever.

@DanielSank No, not necessarily. People edit downvoted questions and improve them, and they get upvotes afterwards.
And closure doesn't always come with an explicit route to reopening.

@DavidZ I wonder how often the down votes go away...
@DavidZ No?

7:18 AM
Some questions get closed because they're fundamentally inappropriate for the site, and there's no way to fix them up to change that.

@DavidZ indeed true
I guess that's a minority. I could be wrong, of course.

No, I think you're right that the majority of closed questions could be fixed up and made reopenable.

For example, every "homework" closure has a path to success, even if at this present time that path is rather obscure.

I'm a little wary of saying every homework-like closure, but yeah, maybe so.

@0celo7 about to finish my holidays

7:22 AM
@DavidZ Well, rather a lot of them.

Oh, of course most of them. I agree with that.
I'd just be wary of coming to believe so strongly that every homework-like closed question is salvageable that it becomes axiomatic. AFAIK there may be such a thing as an unfixable homework-like question.

I suppose that while I may use language indicating that I think such and such always indicates so and so, I wouldn't assume any of us to ever have a mental construct so immune to revision.
But that's me being lazy.
And good communication can't be lazy :)

Well, I'd say good communication can be lazy as long as it doesn't interfere with clarity ;-)

@DavidZ You live dangerously, sir.

lol

7:29 AM
With respect to the above question, some downvotes were actually for editing the question (removing a part of the question which made it less clear and self contained). Some people strongly dislike this sort of editing, because it indicates that the initial question was bad...

@DavidZ Hey, could you mail me an embarrassingly large bucket of szechuan peanuts?

Depends on how much you pay me ;-)

@DavidZ I'm willing to pay for the product and a reasonable handling charge.
Not kidding Szechuan peanuts are my favorite snack and hard to find here.

@ThomasKlimpel wait, so people downvoted because the question was edited to make it better? Weird.

::facepalm::

7:32 AM
@DanielSank Honestly I haven't got the first clue where to get them in large quantities. Assuming we're talking about the same thing, I've only had them at restaurants.

@DavidZ Dang.
Hello @peterh, and what is wrong with the site today? :P

@DanielSank I long did the copy & paste, thus I solved a link-onlyness problem.

@peterh very good
At least I think so. I'm not 100% sure what you mean.

The grocery stores don't sell much of anything in large packages except rice and oil. It's a little weird (for me) and/or inconvenient. I have to either go to the store and buy the same things every 3 days, or pick up like 12 tiny containers of noodle sauce at a time.

@DavidZ Do most people make their own?

7:35 AM
@dmckee It is not so trivial, simple copy-paste didn't work between the arxiv and the mathjax, they use different libraries. I had to download the latex source, copy-paste it and then substitute the non-existent commands with in mathjax existing ones.

Also, how do I get baozi to come out nice and fluffy? They're always a bit rubbery when I do it.
@peterh Ah, well your work is appreciated.

@DanielSank Now I don't know, but after the CET worktime I will have time for a little bit of moderation and then I will explain which question shouldn't have been closed :-)

@peterh Very good.
The Earth still revolves around the Sun.

@DanielSank ask Seasoned Advice. I love eating Chinese food but I'm sadly lacking in expertise on how to actually make it.

Hot diggity!

7:38 AM
My apartment doesn't have a stove or oven so I don't really get to cook

How to survive?

Eat out a lot

wow
I guess in China this is possible.
$0.15/baozi last time I was there. It's super-cheap. I pay like$1 for dinner at the campus cafeteria.
Yep

amazing

7:41 AM
When I got the apartment I hadn't been here for long, and I figured I would continue eating out regularly so it didn't seem like a problem that there was no stove. In retrospect, that was not such a good decision.

::shrugs::
Chinese food is delicious.
ma po tofu
::drool::

Yeah, that's true. They make it a lot better at restaurants than I could do myself anyway.
hehe

There's a little joint opened near my apartment about a year ago...
They make this dish... it's a miniature wok served with a little fire underneath to keep it warm... full of funny mushrooms I'd never seen before, bacon, onions, ginger, chili peppers, szechuan peppers, and a few other goodies.
Ho boy that's good.

Does this mean anything to you?
-1

Now, as we know according to Einstein's general theory of relativity, no particle with non zero rest mass has attained the speed of light in vacuum. But is it possible for a similar particle to attain the momentum as that of the speed of light associated with some wavelength in vacuum?. Will it h...

Yes.
But OP is rather confused.

7:59 AM
@DanielSank mmmm, yeah, sounds like some things I've had here. It is super-delicious.

8:23 AM
[Super random thought]
What if the 750 GeV bump is actually a real, but an event so rare that it is like once in a lifetime as seen in that time scale thus it get buried and averaged out into noise by the later volume of data of non detection in 2016...?

Well, (practically) non reproducible results are beyond the realm of enquiry of science anyway thus let's continue to analyse the other data sets

That would be a classic case of an untestable (in practice) hypothesis.

I think one reason why of the three branches of culture: Science, Magic and Religion, science being so successful might have something to do with how most events in nature are highly reproducible

Even if some time far in the future someone found out our unvierse is not necessary mathematical, the love of predictability of it is still quite a significant observation
True, our reality is fundamentally probabilistic as revealed by quantum mechanics, but even that has a great deal of reproducibility. It seems if our universe is really part of a multiverse not only we have the correct range of constants for life to develop,
we also have a nice subset of rules that are at least statistical in nature, hence highly reproducible
Looking more on this and reflect in the worldbuilding perspective, it does wonder me how people actually think in a world ruled by magic, a place governed by nonstatistical and nondeterministic rules
(For religion it is simple to think about, because the only rules are controlled by one or a group of intelligent entities)

8:49 AM
http://www.sciencedirect.com/science/article/pii/S026240791631510X

This will mean cyber warfare can potentially be reaching a whole new level
Also that will mean Skynet is coming

9:22 AM
Hi guys! I need to check this formula $e^{-A}\partial_\mu e^{A} = \int_0^1 ds e^{(1-s)A} \partial_\mu A e^{-(1-s)A}$ where $e$ is the exponential, $A$ is a unitary matrix. is this formula correct?

9:44 AM
@FrancescoS Yes, it is correct. The proof rigorously involves Lie-theoretic arguments probably amounting to a proof of BCH, but the "direct formal approach" on the Wiki page of the exponential map works well enough, provided you assume the existence and interchangability of the limits.
@DanielSank I downvote questions e.g. because I think they are lazy (i.e. I think anyone able to formulate that question should be able to find the answer on their own), focus on irrelevant issues in an overly pedantic manner, seem designed to start arguments rather than actually learn something (those can be phrased in a way that they don't actually are closeable as such), make assumptions I think are completely silly thereby making the question useless...
None of these reasons is something that as such would warrant a close vote, with the possible exception of laziness in more egregious cases
I'm sure I can come up with other downvote-but-not-close reasons, those are off the top of my head

@ACuriousMind thanks

10:01 AM
@FrancescoS Wait
There might be some sign issues
Ah, no, it's correct
It's just written a bit strangely for my taste, setting $\alpha = 1-s$ seems a better integration variable.

10:16 AM
https://en.wikipedia.org/wiki/Negative_mass
someone has updated this article since my last visit 2 years ago, before that there is no section that said "Arrow of time and space inversion"

@Secret So? Wikipedia articles get edited, that's pretty much one of their defining properties.

Well, you don't expect highly speculative physical topics to update that frequent given their general non detection

Uh, Wikipedia articles don't just get edited when new information comes in. They get edited when someone finds an error, thinks something should be phrased differently, thinks of another thing the article could mention and writes a new section,...

10:55 AM

(NB not so recent)
Well then, crossing fingers when you guys finally get that exhaust detected, then
Until then I am going to just continue to read my quantum

@Secret Afaik it is done by probability calculations. Not a single 750GeV thing was detected, but a lot of measurements seemed as if it had existed.
@Secret A particle is considered existing if it was detected with 5 sigma, which means the probability of misinterpreted random data is smaller as 1:100000. This 750GeV thing has reached 3 sigma, after that a non-detection came out.

Unless I am mistaken, I think the sigma measurement is in some sense a measure of how reproducible a bump is
if you have small bumps that keep popping up at the same region, then as we average the data the bump is going to persist

@peterh That...doesn't expain anything. For most short-lived particles, we detect the decay products and add their invariant masses together to get the mass of the resonance/unstable particle that produced them. That's not a "probability calculation". There is, however, random noise as a background thast could conceivably, from time to time, create such bump without there actually being a particle.

and thus we are then confident that the probability of a misinterpretation is much less than (insert number)

The $\sigma$s denote the confidence with which we say that any deviation from the mean background is an actual process and not a fluke of the noise.
It's not that any data was misinterpreted if such a deviation disappears again (where "disappearing" means the $\sigma$ gets smaller, or it doesn't appear at all in a repeated measurement).

11:04 AM
@ACuriousMind Exactly this is what also I tried to say :-)
@ACuriousMind Btw, calculating the probability of the false detection given the measurement data, how would you name it?

@peterh I'm not exactly sure what you mean. The proper name for what the $\sigma$s denote is confidence, or the probability that a given event is due to random effect rather than a systematic reason.
But there's no "false detection" - the bump is really there. It's just that such bumps may be random flukes, and they may be particles. Once $5\sigma$ is surpassed, we say we are "certain" that it's not random, and that it's a particle.
The exact calculation of the $\sigma$ depends on the models used and probably employs statistics beyond my rudimentary knowledge

11:37 AM
So if we have particle events so rare that they cannot achieve 5$\sigma$ say within 2 years of observation, then we are going to miss those and unable to distinguish them from the noise?

@Secret What have I said that gave you that impression?

> the bump is really there. It's just that such bumps may be random flukes, and they may be particles. Once 5σ is surpassed, we say we are "certain" that it's not random, and that it's a particle.
plus
My own experience in analytical chemsitry on how some signals often get swamped by noise despite they are there

@Secret So, how does that imply that we can't detect faint signals by measuring for longer than 2 years?
The point of a systematic signal is that it's not going to disappear - it will be there in all data. Since we constantly accumulate data, any real signal will eventually become statistically significant.

Well, that's kinda what I am thinking about for the 750 GeV bump (or other bumps that might came up). It might be possible the 750 GeV bump has a frequency of say 2 bumps per 4 years, and thus it might looked like it faded away from 2015-2016, but might be systematic (except it does not look like one because of the time window the reports came out). Well it just whenever I think about statistics and signals I tend to

@Secret What? Such a bump represents an incredible number of events (I can't say how many zeros you need for it), it can't have a frequency of "2 bumps per 4 years"

11:46 AM
(cont.) think about the relationship between the time window of a data collection and the strength and frequency of a signal, and knowing that there are cases where at short enough time windows a systematic signal can lok like noise. Just some random reflection on this subject matter, nothing much really add to the discussion.

You're suggesting that you see a certain particle slightly significantly in the first ten million events and not at all in the second ten million events. That...can't really happen.

well it's all probability, nothing can tell you a series of coin toss will nto all turn up heads for some throws despite heads and tail are 1/2 for each throw
and every machine has a detection limit

@Secret Sure. But if I show you a coin that shows head 1000 times in a row and then only gave tails, I sure hope you're not going to just shrug "Well, that's probability for you"!
Sure, that can happen by chance, but there are much more likely explanations.
If you're going to this length to doubt results, you might as well doubt if gravity really always makes things fall down to earth or if we've just had a lucky streak so far :P

It's highly unlikely, but not possible. But as you said, if you have so many coincidences, then there bound to be some underlying explanations. That's the fun thing about the interplay between probability and signals. We scientists knew how to strife the balance between these two and thus can find out which events are worth further investigation and which of them were noise

Now I have no idea anymore what your point is.
I'm saying that we will detect particles if they are there and in the accessible energy range sooner or later. The longer we don't detect them the more likely there aren't any

11:55 AM
The nature of that means there is going to be something we missed out and we hopefully will catch them some time later when we are either more lucky or with better equipment to decrease the detection limit. (i will mentioned about my point on the next post before my next block of paragrpah cause right now I am one post behind in the responding)

"The nature of that means there is going to be something we missed out"...no, it doesn't. This is not valid logic: Just because it is theroetically possible that we miss something doesn't mean that there actually is something we missed. In fact, it is very unlikely.

I came in today to see several different and equally good answers to my question about why we don't have insufficient effort as a close reason. Thank you, Physics.SE community, for being so overwhelmingly prompt and persuasive

A friend of mine asked me on the Internet if divergent theorm has any use on Evolution (life science). It makes me wonder, how exactly do we apply a math theorem to phonomena/modeling?

My point is that there are events that might look like noise because we have not let them long enough, and the longer we don't find them means the least likely they actually exists (because results like this means we are pushing the bounds of how frequent such event theoretically can be lower and lower, until it is so low that it effectively is nonexistent). We might have miss something, but as you mentioned, with that huge volumes of data (and hence replicates) it is highly unlikely.
(cont.) Perhaps the conclusion of this thought is that should the 750 GeV bump pop up again, I won't find that surprising

@Shing I'm not sure I understand the question.
You model something mathematically by choosing something that represents the "essentials" of that which you want to model: Vectors to model location, numbers to model temperature, etc.
Once you have those mathematical objects, you can apply whatever theorem about them you know to them.

12:10 PM
@ACuriousMind take my friend's question as a example: does divergent theorem have any use on Evolution?
// my thought was: since there is really nothing in Evolution that either 1.) has large amount of number, 2.) behave like field. So there is probably no any use in Evolution. (some google would be easy, I want to practice my reasoning skill though)
however, since I have rare experience on modeling, so I am not confident on my line of reasoning.

@Shing "Evolution" is a very broad field, and I wouldn't be that surprised if some of the more mathematical approaches to population genetics somewhere used it. What's the point of this question? Why does it matter if a particular theorem can be used in a particular field?
The question strikes me as the equivalent of: "I've got this hammer - is evolution a nail?"

Perhaps phrasing it in another way will flesh out what my main point is:
Imagine I am a researcher sifting through the collider data
2015: O wow, a bump at 750 GeV. Looks like the theorist have some explaining to do
2016: Meh, it's just noise and statistical fluke, too bad
2017: The bump is back, now that's interesting...
Having said that, I don't recall of any historical event that has the extreme analogue of getting 1000 heads, followed by 100 tails and then followed by 1000 heads, thus I must be too emotionally devastated or something and too much wishful thinking
anyway, unlike most other discussion, this one does not really have a goal in mind at all, more like sharing about my thoughts in that issue
@Shing for divergence theorem to work, you need something smooth and quite symmetric in the context of evolution. But as far I knew, evolution is more like tree diagrams plus some randomness, I kinda doubt there is something smooth enough to make use of divergence theorem
Most population genetic models, as well population dynamics of various species are statistical, as far I am aware
Having said that, it seems they do use vector fields in some models, thus there might be cases where divergence theorem and other vector calculus is being used
The most common thing I saw when reading and attending some talks in mathematical biology, is that they use a lot of coupled differential equations

12:28 PM
@Secret ...why would one need something "quite symmetric" for the divergence theorem?

I have not really processed that piece of information I learnt back when I first heard about it in h bar, but one possible guess is that while the theorem applies to all shapes, in terms of computation it is most convenient for objects with high symmetry
e.g. we use gauss theorem for symmetric systems mostly, or to simplify some integrals

That the theorem might not be convenient doesn't mean it doesn't apply.

noted

@ACuriousMind Would you happen to know about smooth exhaustion functions

@0celo7 I don't know what an exhaustion function is to begin with

12:36 PM
It is a function that allows you to smoothly exhaust a manifold with compact sets.
Preimage of half infinite closed intervals are compact

Yeah, I don't know anything about those

@ACuriousMind I'm starting to think you don't like geometry that much

Hi all

12:51 PM
@ACuriousMind I was wondering if they're always globally Lipschitz
Probably not tho
I think the existence of proper Lipschitz functions is equivalent to completeness, which doesn't help here
Else any manifold would be complete

Short electrostatics question. Given that a dipole moment of a charge distribution is defined as $$P = \int r' \rho(r')d \tau '$$ where $r'$ is the position vector of the source charge, and $\rho$ is the charge density. Why does it follow that if we displace the origin by an amount $\vec{a}$ then the new dipole moment $\bar{P}$ is $$\bar{P} = \int \bar{r}' \rho(r')d \tau ' = \int (r'-\vec{a})\rho(r')d \tau'$$ rather than $$\bar{P} = \int \bar{r}' \rho(\bar{r}')d \tau '?$$
I might be missing something simple? Give me a hint if possible, thanks.

1:37 PM
@ACuriousMind

ha, that's pretty much it "I've got this hammer, I wonder if next time, I see a nail, I can tell it is a nail".

I think there is some merit in it. last time I was studying Monte Carlo, I did not realize it was actually just a matrix until I was told.... and I hope next time I investigate anything -- I will know it is a nail, when the instant I see it :)

1:47 PM
Finally! I just finished building my own HeNe laser
Got it lasing this morning
Today is a good day

2:00 PM
@0celo7 I don't see any reason why planetmath.org/exhaustionfunction has to be Lipschitz? Where have you come across these apart from SCV?

2:23 PM
Reading this newscientist and my very common association pattern of evolution and cosmology caused a weird thought to be generated:
What if the universe begins like this scenario of life origins, where there are many bubbles of spacetime or vacua, each with its own set of physical laws and constant. They then interact with each other and then slowly, only one victor is left behind and expand into the unvierse we knew of today
Does out current data of the quite even CMB have ruled out the possibility of such scenario? What other potential phenomenon will be experimentally observed if the above is a model of the universe origins?

@Secret Apart from the "victor" (what does that even mean?), this sounds like Smolin's cosmological natural selection.
@Jim Firin' da lazors is a good way to start the day, I guess
@Secret Let's talk about that sentence. You say "Reading this [...] caused a weird thought to be generated:". What's wrong with "After reading this, I thought:"? It's highly unusual to speak of your thought processes as if you were perceiving them as an outside observer like you did there.

I can explain that one, it has to do with my personality that has the tendency to immitate scifi lab anouncement system, which are often quite impersonal (and explains why some teachers note I am better at academic writing tone than most of my peers)
I will explain the previous question later, as I am reading the smolin theory you referred

Well, I guess at least you are consistent.

@bolbteppa SCV?

2:40 PM
@ACuriousMind Well I am not really sure (given my limited high energy physcis knowledge), and I suspect nobody really have any idea what happens if completely different set of physical laws and parameters clashes together. But suppose they do interact in some compromised fashion, then similar to some nonequilibrium systems, eventually the dynamics will result in one factor to dominate others as it slowly settle into some equlibrium. So yes it has an evolutionary aspect similar to Smolin's except its
more like packing the whole multiverse model into just one universe and explain how the only bubble that dominates is the universe we are in
(P.S. No black holes involved)
But like all models, it has to be falsifiable, and I am suspecting the above "competing universes as the origin of our own universe" might have left behind relics of the interactions with the other bubbles in the CMB or something similar, but given the CMB is quite smooth, I kinda doubt this model is valid
Otherwise, I am not sure if there are other testable predictions

The Delirium over Beryllium / Tanedo, quantum diaries
@Jim congrats what are you gonna do with it?

3:01 PM
laser something, clearly.

@vzn Crossing my fingers when it gets real. But it does seems these experiments will help the nuclear physics sector to talk more with the high energy physics sector

@Secret the "fuelless thruster" controversy is interesting, thx for sharing, started to hear about it awhile back. am starting to think it might be (new) evidence for "spacetime fabric" theory... aka (cue full alert!) antigravity o_O

I won't go all the way to spacetime based on that result... But I always suspect there has to be some kind of photon thrust somewhere (As I cannot imagine photons like to be trapped in that cavity for ages)

@Secret think it suggests we dont understand a basic property of "spacetime physics" if the thruster works and it cant be explained by conventional theory

no the latest paper said they have figured where they theoretically think the exhaust came from, and its a pair of photons that destructively interfere with each other all the way throgh

3:06 PM
@vzn Try to take over the world!

@Secret yes, but why did it take years to come up with that explanation? and anyway am sure it will be under contention. think its the birth of a new form of physics
@Jim did some photon-counting experiments with one of those handheld laser pointers years ago, it was really cool, alas it was pre-blog so not much writeup, always wanted to return to it

We'll see, echoing my discussion with dmckee and DavidZ, it seems most of the trailers for BSM seemed to be happening in low energy sectors

@Secret was just musing on that myself. so whither goes "Big Physics™"? it does seem like its not returning "bang for the $" lately... or B$ that is... have some blogs suggesting investments in massive particle colliders should be "hedged" with other stuff ie simulations etc

Well if BSM pop up in low energy sectors, it can help steering researches in the low energy sectors. Having said that, accelerators are still needed after all table top vs accelerators are complementary to each other

@Secret agreed its all complementary to some degree but one wants more return on B$prjs otherwise the means is not justifying the ends so to speak etc... actually one wonders if theres too much emphasis on B$ prjs... its cool to see some glimmers of the community thinking along these lines aka "desert" musings etc

3:12 PM
I am not that worried, the particle physics community have been doing great in keeping itself in pace
it will only be a worry if it is desert all the way down

@Secret actually it reminds me of US space program/ NASA reaching "end of the line" with the space shuttle... some similarities etc

Btw for the EMdrive, that latest paper I am actually more interested in something unrelated. Suppose the exhaust is really in the form of coherently destructively interfering photons,then I am actually more interested in using that drive as a source of these photons
I have always dreamt of building a laser that is completely invisble in all parts of the EM spectrum
for unknown reasons
Probably have something to do with my affinity to nothingness personality

@Secret lol sounds like a contradiction

Well, looking at oxymoronic phenomenon as if they are ordinary is my nature
In a worldbuilding perspective, I can get two meanings from the oxymoron "static motion"
One is an object that is moving while it seems standing still
Another is a more exotic object which moves when time stopped in its tracks

@Secret reminds me of solitons =D

3:20 PM
Really? solitons are a hundred times less mind screwy than these things. If I recall it's just a wave that does not decay as it travels

@Secret think particles ≈ 3d solitons.

that's probably horribly wrong

@0celo7 what?

nothing

@0celo7 maybe in a few decades or centuries current modern physics will probably look "horribly wrong" :P o_O

3:46 PM
@vzn : I don't think there's any similarities myself. I'm not a fan of colliders, and to be honest I think they've been something of a distraction that has painted particle physics into a "stamp collecting" corner. I am a fan of the space program however. So much so that my next work is going to be dedicated to NASA. And public domain!
@Secret : another is a standing wave.

@JohnDuffield am all for space exploration myself & am a huge fan of the mars rovers & other robotic exploration... but "devil is in details"... which reminds me, whats your take on Musk?

@vzn : when it comes to space, he isn't thinking big enough.