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12:00 AM
it's just tough to quickly recall a word's meaning when i have to think of it's exact definition. much easier to just think about how it's defined in terms of other things i'm talking about at the moment
e.g. i'd immediately recognize how open and closed sets are defined when i'm talking about boundaries. but if i'm talking about limit points it becomes a lot more of a headache
then trying to define limit points/isolated points nicely in terms of the easy interior/boundary/exterior points becomes really messy
hell i even found an asterisk when trying to define isolated points in terms of limit points (i.e. it must not be a limit point, and it must also be in the set)
I think the problem is that you're expecting topology to make intuitive sense
But our intuition operates mostly on "nice" topological spaces like metric spaces, where much of the finer points of topology are completely irrelevant
i mean the definitions themselves are somewhat intuitive, but there's a new term for every subtle change in definition
In mathematics, an adherent point (also closure point or point of closure or contact point) of a subset A of a topological space X, is a point x in X such that every neighbourhood of x (i.e. every open set containing x) contains at least one point of A. A point x is an adherent point for A if and only if x is in the closure of A, thus x ∈ Cl ⁡ ( A ) ⟺ ∀ U ⊂ X open : x ∈ U...
Like why do we need this, we already have limit and isolated point
Is it too hard to say "in the closure"
"Adherent" isn't really a useful way to think about it, not more than "limit". these names are terrible
@SirCumference The "also point of closure" remark seems to indicate some share your sentiment
then there's this crap
In mathematics, a subset A {\displaystyle A} of a topological space is said to be dense-in-itself if A {\displaystyle A} contains no isolated points. Every dense-in-itself closed set is perfect. Conversely, every perfect set is dense-in-itself. A simple example of a set which is dense-in-itself but not closed (and hence not a perfect set) is the subset of irrational numbers (considered as a subset of the real numbers). This set is dense-in-itself because every neighborhood of an irrational number...
Which is distinct from the definition of "dense" (i.e. everything is an element or limit point of X)
Since everything would be literally dense in itself (everything's an element of it's own set)
Come on at least choose a better name, I spent 30 mins confused before realizing it was a new term
I think much of this terminology stems from the fact that topologists often try to look at definitions that are equivalent in "nice" settings and then try to figure out how much you need to weaken the axioms in order to make the two notions diverge
That is, the terminology is not intended to be pedagogical in any way - many of these notions are irrelevant to any applications of topology to other fields, it's terminology purely invented for some papers exploring the distinction between these notions in exotic settings most non-topologists won't ever encounter
12:12 AM
i guess so, but even the more common terminology just has a bunch of asterisks
even algebra is more neatly structured and named than this
it's like a bunch of people came up with useful terms, but they never communicated so the terms don't mix so nicely with each other
all right i ought to chill about this
@SirCumference Or you could make a really big deal about it; make a blog with like a million posts on the topic; and also post it in the comment sections everywhere else even slightly related. Just become "that guy" who always brings it up. There's nothing to lose but your sanity!
12:27 AM
okay point taken lol
i'm just waiting for the moment where i forget an asterisk and inevitably mess up on a test
@SirCumference If you despair over the meaning of all of it, take solace in the fact that topology can also be just pointless
that's going in my book of math puns :P
3 hours later…
4:22 AM
@ZeroTheHero coolio
3 hours later…
7:17 AM
Q: Why this is opinion-based?

Aleksey DruggistThis question is closed as being opinion-based. Perhaps I do not understand English well, could someone explain in more detail what opinion-based is on this question?

7:32 AM
I'm confused about P/E ratios. For example, a P/E ratio of 30 means that investors are willing to pay 30$ for 1$ they earn? Like... wtf? Aren't they loosing 29$ each year then. I'm confused.
Who would trade 30$ for 1$...?
What's even the point of that ratio?
8:29 AM
nvm I watched the right video
"Apple really is the most futuristic company out there
They have already adjusted their prices for the next 50 years of inflation!"
a comment i found on youtube xDD
1 hour later…
9:59 AM
I wonder if there's a polite way to tell a prof their 9am mandatory attendance is extremely destructive for sleeping with insomnia and studying during the day
A gun
will do the trick
with a silver bullet
Doesn't seem very polite...
"To get the attendance credit you must be asking questions, not sitting in the back quietly"
Like what the hell, this is a physics class taught to 4th year adults
10:08 AM
Do you want me to give you extremely annoying questions to ask him
Lol sure
What class is it
Ask him how the contact manifold stems from the statistical manifold
why did Boltzmann et al commit suicide?
10:11 AM
plz mister, what is the Fisher information metric of your system
it is very important
@skullpatrol he almost certainly suffered from clinical depression.
Was it suicide
or was it
I'm not sure if that condition was understood in Boltzmann's day. The Victorians in the UK used to call it melancholia.
Stat mech has a bad rep for that though
Suicide is no new condition
10:13 AM
@skullpatrol No kidding, it's first week and I'm already in despair
goes back to time immemorial!
@skullpatrol I'm not sure that the incidence of clinical depression is higher amongst physicists than amongst the general population.
Oh wait by bad rep I assumed you meant attendance not suicide
At any rate it's 5:10 and I'm not realistically gonna get sleep with this stress
Ask the prof the reason he wants maditory participation @SirCumference
maybe he's lonely
10:15 AM
They always insist it's the correct way to learn
@skullpatrol Because no-one would attend if it was optional? :-)
Which is a joke since I can't even understand their accent in lecture
@JohnRennie Bingo
Maybe the power he feels forcing you to do it thrills him
I've heard unis need to keep their rep by making sure students attend classes
10:16 AM
A minuscule amount of power corrupts absolutely
How about earn a rep by not having miserable students
One day you will start learning fluid mechanics, and you will look back upon your stat mech lectures with wistful longing.
It's not the material tho, it's the inane attendance policy. Not sure how I'll be able to sleep and study normally like this
fluid mechanics isn't that bad, at least at the uni level
@SirCumference drugs
Just give me a textbook and test me please
10:18 AM
What % is the participation worth @SirCumference
Which is the difference between a B+ and A
maybe for your attendance questions, only ask him really mean questions
Does he follow a textbook?
Perhaps he will stop asking after a while
"Sir, how could your mother love such a face?"
"Are you sure that this is the best you could achieve in life?"
@skullpatrol Officially he does, though when I asked if this week's material is in the book he had no idea
10:21 AM
"Do you think anyone will mourn you when you're gone"
Maybe he just wants me to attend lecture
@Slereah You seem more aggressive toward him than me :P
I am a bit surprised. How can community user make such a decision?
@SirCumference I'm offering solutions
10:23 AM
@JohanLiebert I think Community means the author of the question accepted the suggested edit.
@JohnRennie and I think the suggestion might have been anonymous?
@JohnRennie yes the suggestion was anonymous.
Ah, no, it was an edit from a non-site member (i.e. anonymous) and that's why it shows as Community. It was approved by stafusa and DavidZ.
"A Wikipedia factoid also compares the Sun's heat-per-unit-volume to the heat produced by an active compost pile, although the energy production from compost varies with temperature—since a hot compost pile kills off the organisms that do the composting."
I wonder where Ryan is
One of his friends is also giving me mandatory attendance
Maybe he can talk him out of it
1 hour later…
11:41 AM
Is it legal if I put Steve Job's picture and name on my website?
The website will make money.
Is there anything illegal if I put Steve Jobs or Bill Gates' picture, name and description on my website which I plan to commercialize and make money from?
I'm planning to build a website where I'm going to make money from their faces and names. Is that legally fine?
I'm pretty certain that portrayals of celebrities' faces are regarded as their property and you'll need a licence to use them in a commercial venture.
12:09 PM
What if I modify/filter the picture in such a way so that the face is still recognizable but legal?
@JohnRennie Can I private message you somewhere? I really want to share my idea with you and get your opinion.
12:35 PM
@NovaliumCompany you can message me on Facebook, but I have no special skills in commerce nor any interest in it so I doubt I can say anything useful.
@JohnRennie I have q question can I ask here.
Actually I have a planet let spherical planet.
And it is surrounded by some gravitational field.
And I draw the arrow all around that planet.
Now if I place my self between the arrows just like my head perpendicular to direction of arrow do I feel any tidal force?
@YuvrajSingh... Are you hovering at a fixed distance from the planet or falling freely towards it? This makes a difference because if you are falling freely you will experinece a tidal force, though it's a very small one.
No I am not free falling.
12:52 PM
Let me draw a diagram ...
user image
I've deliberately exaggerated this to make it clearer.
Th gravitational force always points towards the centre of the planet, so the forces at your head and at your feet are at slightly different angles. They converge slightly.
And that means they have a non-zero component along your length. So the gravitational forces are slight compressing you.
So, yes, you do feel a tidal force in this situation, though unlike the usual tidal forces it is compressing you not stretching you.
@JohnRennie force due to my gravitation field.
@YuvrajSingh... this is just due to the gravitational field of the planet
@JohnRennie cool :) what about magnitude of this force
1:03 PM
Just consider an infinitesimal mass $dm$ at your head and another at your feet, and you can calculate the forces using simple trigonometry.
@JohnRennie one more question?
I love how tidal forces compress the smaller object and yet, once it crosses the Roche limit, the object is ripped apart. It's a satisfying dichotomy
@YuvrajSingh... I need to go, but I'll be back this afternoon.
1:35 PM
Q: Why do Chemistry and Physics SE use text numbers instead of lining numbers?

theorist[N.B.: I use a Mac, but I'm assuming the following is not Mac-specific.] I've noticed that, font-wise, SE sites can be divided into two categories: 1) Those that employ a sans-serif font (e.g., TeX SE, superuser SE, and this one). These display numerals as "lining figures" (~full-height number...

1 hour later…
2:39 PM
some1 recommend mi buk to rit pliz
2:50 PM
@NovaliumCompany You seem in dire need of this book :
3:08 PM
@ACuriousMind thanks for the link to Black Sabbath. Interesting but really * no * connection with my avatar... :)
3:24 PM
I’m sitting in on the intro physics lecture for today on osmosis
@Slereah xDDDD
I just started Dune, hope it's good.
@Semiclassical Now focus on something else so that you can learn osmosis by osmosis.
And the prof gave this explanation for osmosis: higher solute concentration dictates lower water concentration, and diffusion across a semipermeable membrane produces a flow of water from the higher water concentration to the lowe
Or, more simply: water dilution drives osmosis
@NovaliumCompany I really enjoyed Dune. It's a very well known sci-fi book. There's also like a bunch of sequels; but the advice for those is just keep reading until you stop enjoying it, apparently they just keep getting slightly worse.
Bottom line being that that view of osmosis is out of date by about half a century
So this is frustrating
(The lecture doesn’t linger on that, but if he tests on the “correct” explanation of osmosis I’m going to have a real problem)
And the thermodynamic explanation goes back to Gibbs in 1897
3:33 PM
@Semiclassical I think I've always only heard the simplified explanation as well. I was personally never tested on it or had to apply it since HS chemistry though; but I don't remember anyone clearing up that it wasn't diffusion. I don't think we ever covered it in physics though, so I'm not overly surprised chemistry had a misleading interpretation.
If I read the blog post (and some of the other sources) correctly, then one should have the following simple counter example: sodium fluoride, dissolved in water, should increase the water concentration rather than increase it
@JMac that said, this is supposed to be a physics course
If all he asks them to do is compute the osmotic pressure or predict the direction of flow, then fine—they just need to know that osmosis directs solvent flow from higher solute concentration to lower
But if there’s any concept multiple-choice questions...
3:55 PM
Yeah, I'm fine with what I learned because in the context, we weren't necessarily good at physics and able to describe it in an accurate way; since it was a chemistry class. If the professor asked about it an expected that wrong answer; that's definitely quite bad.
That said, I'm going to break out my usual rant; that I had a "Advanced Math 11" teacher try to convince our class that 'double rounding' was reasonable. I.e. 0.45 would round up to 1 in the nearest integer because 0.45 rounds to 0.5 which rounds to 1. So teaching a common misconception about osmosis barely phases me compared to that.
4:10 PM
note: faze = phase is another misconception :>
Q: Why is law of conservation of angular momentum (seemingly) being violated over here?

Johan Liebert Description of the system Assume two point masses one at the point $C$ and the other on the circumference of the circle with radius $R$. They are attracting one another gravitationally and no external forces are acting on them. The point mass at C has very large mass such that it is at the COM...

Does anyone think that my derivation of the equation for angular momentum of the external particle is wrong (I ask this because I have got two answer which are implying that $\ell =rp$ and not the one I have derived).
4:36 PM
@JMac I think I'll try to read only the first book. I'm really excited because there's a movie and I want to see how close my imagination is to the movie.
4:53 PM
@AaronStevens do you think the central mass has an angular momentum of $\ell _{central} = -Rp \sin {\omega t}$? It's quite hard to derive it for central mass!
Do you guys think the world is headed for another global recession (or maybe even depression)?
5:25 PM
Is the global economy currently in an expansion, boom, recession or depression?
I feel like just how after the first world war people called just "world war", never assuming there would be a second one, the same way we call just "great depression". xDD
Let's just hope the people are not greedy.
@JohanLiebert I am still thinking about it, but I think JEB's answer is correct. Although some unsaid assumptions in his analogy might be bypassing your issues. I am not entirely sure yet, and I don't know if I will have the time in the near future to think more carefully about it.
@ZeroTheHero My pleasure. Giving Eloy a listen right now ;)
@ACuriousMind Eloy? That's a bit proggy for a die hard metal fan! :-)
I can tolerate other genres in small doses ;P
@AaronStevens I have derived it and am going to post the answer in some time. But I would need your help to check if I am doing anything wrong somewhere.
5:34 PM
During an economic recession, whom are the people selling their stocks to? After all, everyone panics and everyone wants to cash out. Who the hell buys their stocks at that time?
I have a couple of Eloy albums somewhere, if they haven't got lost over the years, but I would have put them in the prog mainstream as in pleasant to listen to but not terribly innovative.
@AaronStevens This is the diagram. (Not good because I use my photo editor to make these.
@NovaliumCompany Presumably people willing to gamble that they'll make a lot of profit from the cheap stocks once the recession is over.
@ACuriousMind So this is where the Warren Buffet quote comes from: "be afraid when others are greedy and greedy when others are afraid".
@Semiclassical Whoa. First time I hear of that!
5:46 PM
i did talk with the prof after the lecture, though not at length due to time
@JohnRennie I find the strong German accent of the singer both hilarious and fitting for some reason
didn't exactly move it
there was a particular assertion made ("The semipermeable membrane exerts the force that drives solvent flow") which the prof thought was clearly wrong
but which seems rather reasonable to me
The good prog bands tend to be the well known ones - they became well known for a reason. Though I would have to concede this isn't universally true as the inexplicable popularity of Emerson, Lake and Palmer testifies.
@AaronStevens @JohnRennie can the universe have a net angular momentum? (I mean does this make any sense for the universe to have a non-zero angular momentum?)
It could do. The Gödel metric describes a universe with a non-zero angular momentum.
However observation suggests the net angular momentum of the universe is small.
@JohnRennie while making an argument in my answer to my own question I assume that the total angular momentum cannot be a non-zero (and this simplifys the situation). So what should I do?
You'll have to catch me tomorrow. By this stage of the day my brain has largely ceased to function.
@JohnRennie OK then I would put this as an unstated assumption and then modify it tomorrow after discussing with you.
6:28 PM
A: Why is law of conservation of angular momentum (seemingly) being violated over here?

Johan LiebertThanks for the answers they gave me motivation to try deriving angular momentum for the central mass. The following picture says most of it. (A description is coming ahead): Description The outer mass is at a distance $R$ from the com and has a linear momentum as $P$. The inner mass has is ...

@AaronStevens you can have a look at my answer. Is it correct?
7:06 PM
Does anyone know a relatively short and simple sci-fi book I could read that also has a movie that I can watch afterwards?
@NovaliumCompany BTW from what I've heard, the Dune movie is very different from the books. I personally haven't heard much good about it.
@JMac I gave up Dune. Read a summary, not my taste.
Tried watching the movie, my opinion is that it's trash.
What about it led you to give up?
Well, before trying to read the book I decided to read a quick summary. The annoyingly complicated names and political style of the book are not my type.
It uses unnecessarily twisted linguistics, which pushed me away as well. I'm a fan of simplicity.
(or at least not overcomplicating things unnecessarily)
Fair enough I guess, though I tend to avoid reading summaries for books before I read them and usually just stick to the blurbs to see if that interests me. Personally, I also wouldn't give up on books just because of naming conventions; but I guess I kinda grew up reading fantasy so I'm just really used to that.
7:20 PM
Jagarok of planet Niximasex lives under the rule of Petrubelium....... no thanks
It's just too much. Anyways, I think a 2020 movie of Dune is coming so... I guess I'm kinda excited for that. (now knowing the story)
Recursion was amazing tho.
The way I see it, it's not much different than describing anything, besides that the people and place names are just not ones you're used to; but that can happen without fictional place names.
It's not the main point I'm not attracted to Dune
@JMac Do you know a relatively simple sci-fi book I can read that has a movie?
Or just... anything. I want to read something
We have a 10 day "flu vacation" here in Bulgaria soo I want to invest my time reading books.
I think you might like "harder" science fiction. Dune does actually have some pretty heavy fantasy elements, and it sounds like those are a bit off-putting for you.
Have you read The Martian? I haven't read the book; but it's supposed to be really good, and I thought the movie was really good. It's pretty much all real science, so quite different than Dune.
@JMac I have watched the movie a few times. Not sure if the book will be exciting (now when I know what happens)
Wish I could have read Ready Player One before watching the movie.
7:39 PM
Another one where I've seen the movie but haven't read the book is Contact en.wikipedia.org/wiki/Contact_(novel) by Carl Sagan. The movie was pretty cool again; has to do with aliens and SETI stuff.
I'll dive into Contact. Seems interesting.
7:55 PM
I just borrowed a digital book.......
that's just genius.
They are lending it for 14 days because you know.... it's not like they have an infinite amount of it....
oh wait a minute
I have 14 days to read Contact.
Will do it in 3.
Thanks archive.org
4 hours later…
11:29 PM
@ZeroTheHero You might enjoy this Deep Purple tribute:
Ok, it's not quite as good as the original, but I think they did a good job. IMHO, Jon Lord was one of the greatest rock keyboard players.
11:46 PM
@PM2Ring Amen to that.

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