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Avantgarde
Avantgarde
12:04 AM
@ACuriousMind Yeah it's an interesting interview.
Well, luck does play a role, for sure. But the pragmatic won't count on it when hiring others. Past records are the most tangible testimonials when gauging how useful a person will be for your company/organization.
 
 
2 hours later…
bolbteppa
bolbteppa
2:20 AM
Original Faddeev-Popov article is interesting too sciencedirect.com/science/article/pii/0370269367900676
 
vzn
vzn
3:16 AM
re simons theres an old expr wrt the elite/ "1%" referring to economic inequality/ mobility... pulling up the ladder behind them ... just read lengthy/ remarkable articulate/ erudite article on that... the 9.9% is the new american aristocracy theatlantic.com/magazine/archive/2018/06/… but at least simons is involved in a lot of philanthropic activity much of it science/ grant related. also driving force behind great quanta magazine...
other remarkable (astronomy) news this reminds me of benard convection one of the remarkable aspects/ phenomena of emergence... Weird hexagon on Saturn is way bigger than scientists thought nbcnews.com/mach/amp/ncna906541 en.wikipedia.org/wiki/Rayleigh%E2%80%93B%C3%A9nard_convection
 
 
2 hours later…
Abcd
Abcd
5:48 AM
@JohnRennie I had updated from Windows 8 to Windows 10. Then I got my laptop formatted so idiotic Windows 8 is back. How do I get windows 10 back?
 
John Rennie
John Rennie
You can download the Win10 install files from Microsoft. Let me dig out the web address ...
 
Jasper Loy
Jasper Loy
6:15 AM
Hey @JohnRennie I am thinking of buying a laptop today. I just wanted to ask you if 725 USD is a good deal for an i7-8550U processor, 256GB SATA SSD, 8GB RAM, non-backlit keyboard, 15.6 inch FHD non-IPS display. Actually it is this model laptopmedia.com/review/…
 
John Rennie
John Rennie
@JasperLoy I'm not up to date with the current laptop market, so I'm not the best person to advise you.
 
Jasper Loy
Jasper Loy
@JohnRennie I see. Thank you!
 
John Rennie
John Rennie
I invariably buy laptops on eBay that are two to three years old because they are dramatically cheaper. But you do need to know what you're doing to go down that route.
 
Jasper Loy
Jasper Loy
Yeah, I don't trust ebay.
 
John Rennie
John Rennie
But Dell Inspirons are well respected. It certainly won't be a bad laptop.
 
Avnish Kabaj
Avnish Kabaj
6:25 AM
Why is entropy for the system $$\frac{Q_{rev}}{T}$$ but for the surroundings $$\frac{Q_{actual}}{T}$$
 
John Rennie
John Rennie
@JasperLoy getting a new laptop gives you security because it comes from a trusted supplier and you get a warranty. That's the safe course. I always buy from eBay but then I'm a long time computer nerd and I'm happy to repair the laptop myself if anything goes wrong with it.
 
Abcd
Abcd
@AvnishKabaj who said
 
Avnish Kabaj
Avnish Kabaj
@Abcd Bruce h mahan
 
Abcd
Abcd
@AvnishKabaj please send pic (to provide more context)
 
Avnish Kabaj
Avnish Kabaj
Not at my house right now
Later
 
user157860
user157860
6:34 AM
@JohnRennie, hi John, can we continue the discussion on the door (handle vs. hinge) problem?I think I cannot pull (nor push ) the door, only an impulse (like a kick) can work
 
John Rennie
John Rennie
@user157860 I'm working for about half an hour. Back later.
 
Abcd
Abcd
How is programming applied physics?
Both are completely unrelated
 
Abcd
Abcd
6:45 AM
@AvnishKabaj Which edition do you have? Is it coloured or black and white? Also, is it a good book for preparation?
 
John Rennie
John Rennie
@user157860 any experimental physicist (I was an experimental physicist for 15 years) will tell you to immediately try the simplest experiment you can. Even if it ends up failing it will tell you how to improve the experiment. So get a luggage balance, or similar, and try the experiment. See what happens.
15 mins spent trying the experiment is worth any number of hours of discussion about how hypothetically it might be done.
 
user157860
user157860
@JohnRennie, I meant we cannot consider a force pulling/pushing the door but only an impulse, because a force pushing in one direction and progressing with the door will soon not be perpendicular any more, shall I draw a sketch if it is not clear?
 
John Rennie
John Rennie
If you're holding the end of the balance move your hand to keep the balance normal to the door.
 
user157860
user157860
7:29 AM
@JohnRennie, one should advance and rotate the force at the same time without losing contact or changing the applied force, that can't be scientific, can it?, isn't it better an impulse?
 
John Rennie
John Rennie
7:54 AM
@user157860 you're overthinking this. Try with a balance first and see what happens. Keeping the balance normal and at exactly the same constant force will be hard and there will be some experimental error as a result. But you can estimate the experimental errors and see how much they affect your final result.
 
user157860
user157860
@JohnRennie, I posted my first answer: physics.stackexchange.com/questions/173987/…, can you tell me if it is correct?
what do you make of the accepted answer, it's wrong, but do you get what it's trying to say=
 
Secret
Secret
8:23 AM
[Random]
magic burning a piece of wood at a distance
maybe able to be made via some kind of counterfactual communication setup with a high power laser such that the destructive interference ends only at the surface of the wood
This has the advantage of a lasting action that cannot be blocked because nothing passes the middle
2. Quantum computing provide a computational memory impossible with classical computers as the whole atom of universe is used, at a finite region
this means two finite numbers can be infinite relative to each other if extra constraint such as the size of the universe is imposed
 
 
1 hour later…
djsmiley2k
djsmiley2k
9:53 AM
Soooooo something itneresting i noticed with these two cubes on my desk
their 1cmx1cmx1cm cubes of machined titanium
and I've been rubbing them together (you can see the maching marks showing they were cut in a perticular way
however now I've managed to get them to 'stick' together
and I'm wondering if it's just friction o_O
Someone just linked me to this
which seems like the same thing is happening
 
 
1 hour later…
ACuriousMind
ACuriousMind
10:55 AM
@djsmiley2k If the surfaces of your cubes are very flat, then it's possible that it's wringing.
the more important question is why have you got two cubes with such a flat surface? :P
 
djsmiley2k
djsmiley2k
11:24 AM
because they were cool :D
 
Anonymous
@djsmiley2k Jo blocks are available in most machine workshops. And they're designed to have completely flat (and by extension nearly frictionless) surfaces. The sticking of Jo blocks, I guess, is a pretty well known phenomenon
 
Anonymous
So the reason certainly isn't friction :P
 
djsmiley2k
djsmiley2k
a distinct lack of friction?
 
Anonymous
@djsmiley2k Sure
 
djsmiley2k
djsmiley2k
I just found it cool
 
Anonymous
11:34 AM
@djsmiley2k It's not only cool but also useful! Jo blocks are normally stacked to create desired reference lengths
 
Anonymous
Also, metals are easier to flatten than other materials
 
Anonymous
Over and above, titanium has a very low value of $\alpha$. So its length doesn't change with temperature (practically). So those blocks form good references
 
user 2646
user 2646
what happened to your avatar? @Blue
 
Anonymous
@user2646 Changed!
 
Anonymous
Well, sorta inspired from the Watchmen
 
Anonymous
11:41 AM
Watched it last week. It was a pretty cool movie. Don't know how I managed to miss watching it earlier. :)
 
user 2646
user 2646
the other one was more sinister loooking
 
Anonymous
That one was a dark Deadpool logo :P
 
Anonymous
This current avatar stands for apathy
 
Anonymous
16
Q: Why did Dr. Manhattan choose the hydrogen atom for the symbol on his forehead?

Ankit SharmaIn the alternate versions of Watchmen, when Dr. Manhattan's is forced to choose a symbol, he made one on his forehead representing a hydrogen atom. Why did he choose this symbol? How does it relate to him?

character watchmen
 
user 2646
user 2646
interesting...
 
Anonymous
11:48 AM
I highly recommend watching the movie btw
 
user 2646
user 2646
thanks, i'll put it on my list
 
Novalium Company
Novalium Company
12:16 PM
Sup everybud. We know that $a = \frac{dv}{dt}$. If $v(t) = t^2$ for example, then at $t = 3$ the velocity will be $v = 9m/s$ and the instantageous acceleration at that point in time will be $a = 6m/s^2$ since the derivative of $\frac{dv}{dt}$ will be equal to $2t$. Is my basic understanding of derivatives correct?
 
user 2646
user 2646
yup
 
ACuriousMind
ACuriousMind
@NovaliumCompany Yes, but your usage of units could be a bit more consistent ;)
 
Novalium Company
Novalium Company
@ACuriousMind Sorry :D?
 
user 2646
user 2646
how would you correct that @NovaliumCompany?
(usage of units)
 
Lozansky
Lozansky
@NovaliumCompany What unit is $t$ measured in?
 
Novalium Company
Novalium Company
12:21 PM
Oh, I have forgotten the seconds in $t = 3s$?
 
user 2646
user 2646
yup
 
Novalium Company
Novalium Company
Oh xD My mistake sorry
 
ACuriousMind
ACuriousMind
Well, that's one issue. The other is that if you plug $t=3\mathrm{s}$ into your equation for $v(t)$, you get $v = 9\mathrm{s}^2$, not $v=9\mathrm{m}/\mathrm{s}$.
 
Lozansky
Lozansky
Well then the velocity has units of $s^2$
 
Novalium Company
Novalium Company
Well, that's weird. :D
 
user 2646
user 2646
12:23 PM
2 mins ago, by user 2646
how would you correct that @NovaliumCompany?
 
Novalium Company
Novalium Company
@user2646 You are asking how I would make the units of the velocity not $s^2$ but $m/s$?
 
user 2646
user 2646
yup
 
Novalium Company
Novalium Company
Pff, I have no idea :D
 
Semiclassical
Semiclassical
This week’s colloquium talk: history of density functional theory. Not bad
 
ACuriousMind
ACuriousMind
@NovaliumCompany Well, since this is just an example pulled from thin air, the simplest way is to introduce a constant in front of the $t^2$ such that their product has the correct units.
 
Semiclassical
Semiclassical
12:26 PM
Ugghhhh
 
user 2646
user 2646
whatever happened to the Langlands project lectures?
 
ACuriousMind
ACuriousMind
@Semiclassical It's Sundrum! I've even asked a question about his and Randall's model. Why are you ugghhhhing? :P
 
Novalium Company
Novalium Company
So $v(t) = ct^2$ where $c = \frac{d}{t^3}$. Pretty sure this doesn't make sense but.
 
Semiclassical
Semiclassical
Because “fifth dimension” just sounds silly and speculative to me
 
Lozansky
Lozansky
@NovaliumCompany How about $c = 1 [m/s^3]$?
 
Semiclassical
Semiclassical
12:31 PM
Maybe that’s too much of a knee jerk reaction
 
Novalium Company
Novalium Company
@Lozansky Isn't this the same as what I wrote?
 
Lozansky
Lozansky
12:46 PM
@NovaliumCompany Not really. I am saying $c$ is a constant equal to $1$, measured in units of $m/s^3$. You are saying $c$ is a function inversely proportional to the cube of $t$ (the independent time-variable). Also, you have introduced a mysterious $d$ (which I know is meant to be a constant with value $1$ measured in $[m]$, but this is not clear from the expression).
 
Emilio Pisanty
Emilio Pisanty
@NovaliumCompany $v(t) = t^2$ is terrible form, and it should never be used if at any point downstream from that formula you're going to put things in with units.
$v(t) = ct^2$ is slightly better, though I would strongly discourage you from a variable name (like $c$) that is associated with a velocity.
$c=d/t^3$ is wrong, though.
(a) you haven't defined $d$, but more importantly (b) you've already used $t$ for a variable, which would mean that $c=d/t^3$ is not constant.
if you just want to indicate dimensionality, you use square bracket notation
i.e. $[c] = [d/t^3]$
but preferably $[c] = [L/T^3]$.
 
Novalium Company
Novalium Company
Got it, so the best thing would be to do what @Lozansky said? $c = 1 [m/s^3]$
 
Emilio Pisanty
Emilio Pisanty
@NovaliumCompany that's an incorrect usage of the square bracket notation.
You just say $c=1\: \rm m/s^3$.
 
Lozansky
Lozansky
@EmilioPisanty It's how engineers use it
 
Emilio Pisanty
Emilio Pisanty
@Lozansky If you're writing for an engineering publication and there is a solid engineering-specific style guide to back it up, then it's ugly notation but acceptable in that context.
 
Novalium Company
Novalium Company
12:54 PM
Ok what is the right notation and overall what's the right thing to do with the case of $v(t) = t^2$ to get the right units? Maybe just not use it at all?
 
Emilio Pisanty
Emilio Pisanty
Absent either of those conditions, the notation falls under SI Brochure or NIST Guide To The SI style-guide territory, and both make it very clear that that usage is incorrect.
@NovaliumCompany personally, I would use $$v(t) = (1\:\mathrm{m/s}) \times \left(\frac{t}{1\:\mathrm{s}} \right)^2$$
or just $v(t) = kt^2$ with $k=1\:\rm m/s^3$.
 
John Rennie
John Rennie
@EmilioPisanty two rep limits in a row. Don't you just love the HNQ? :-)
 
Novalium Company
Novalium Company
I prefer the last one :D
 
Emilio Pisanty
Emilio Pisanty
depending on the context and which aspects require more clarity and emphasis.
As I said above, though, I would above using $c$ for anything that isn't a velocity.
i.e. if $[c]\neq[L/T]$ then $c$ is probably the wrong symbol to use.
 
Novalium Company
Novalium Company
@EmilioPisanty I'll just use $b$ for example?
 
Emilio Pisanty
Emilio Pisanty
12:59 PM
@NovaliumCompany that works.
@JohnRennie I do.
 
John Rennie
John Rennie
@EmilioPisanty :-)
 
Lozansky
Lozansky
@EmilioPisanty We also write stuff like $\dim(k) = LT^{-3}$, is that correct at least? :P
 
Emilio Pisanty
Emilio Pisanty
30
Q: Square bracket notation for dimensions and units: usage and conventions

Emilio PisantyOne of the most useful tools in dimensional analysis is the use of square brackets around some physical quantity $q$ to denote its dimension as $$[q].$$ However, the precise meaning of this symbol varies from source to source; there are a few possible interpretations and few strict guidelines. ...

conventions units notation dimensional-analysis
you're probably referring to usage 4 in that answer's list
 
Lozansky
Lozansky
Yup, cool
 
Emilio Pisanty
Emilio Pisanty
it's one possible correct usage
the only thing that matters is that you use your notation consistently, and that you provide appropriate definitions for any non-standard notation.
everything else is just style
 
Novalium Company
Novalium Company
1:04 PM
So in $v(t) = ct^2$ where $c = 1m/s^3$. Where $c$ is just a constant equal to 1 with units of ($m/s^3$). Is that correct?
 
Emilio Pisanty
Emilio Pisanty
... which can also be 'wrong' if you're writing in a venue that enforces a strict style guide, of course.
but, as documented in that answer, the official style guides differ on that issue.
@NovaliumCompany no. $c$ is a constant equal to $1 \: \rm m/s^3$.
"equal to 1 with units of X" is language that's just going to cause you more confusion down the line.
 
Novalium Company
Novalium Company
@EmilioPisanty Ok, for now I'll just accept it although I don't really understand the $1 m/s^3$ part?
 
Emilio Pisanty
Emilio Pisanty
@NovaliumCompany what's there to not understand?
you're already OK with writing $c=1 \: \rm m/s^3$, right?
just replace the $=$ with "equals".
 
Novalium Company
Novalium Company
Yes, but what is this number 1, having units of m/s^3?
 
Emilio Pisanty
Emilio Pisanty
@NovaliumCompany No. It is a physical quantity, period.
It happens to have the numerical value $1$ when expressed in meters per second cubed.
 
Novalium Company
Novalium Company
1:10 PM
Hmm, so I shouldn't try to make sense of the $1 m/s^3$ part? It's just a physical quantity with those units?
 
Emilio Pisanty
Emilio Pisanty
You can say "the numerical value of $c$ in m/s$^3$ is 1", though
 
Semiclassical
Semiclassical
it's also smart to choose notation which isn't liable to be misinterpreted
 
Novalium Company
Novalium Company
$b=1 \: \rm m/s^3$ is a physical quantity of what?
 
Semiclassical
Semiclassical
and $v(t)=ct^2$ definitely lends itself to misinterpretation given how frequent $c$ is used to label speed
"$c=1$ in units of m/s^3"
 
Novalium Company
Novalium Company
So the constant $b$ is just a physical quantity of nothing really, created just to get the units of $v(t) = bt^2$ right, is that it?
 
Semiclassical
Semiclassical
1:15 PM
note that $v'(t)=a(t)=2bt$ and $a'(t)=2b$
so one way to understand $b$ is that it's (half) the rate at which the acceleration is changing
 
Novalium Company
Novalium Company
Ok, I'm gonna stop with $b$ here, becuase it's getting even more consfusing.
 
Semiclassical
Semiclassical
so if we choose to express our positions/times in units of meters and seconds respectively, then acceleration is expressed in units of m/s^2 and so b would be in units of m/s^3
 
Novalium Company
Novalium Company
Do I have to use such constant every time I use derivatives in physics?
 
Semiclassical
Semiclassical
It really has nothing to do with derivatives per se.
The point is that physical expressions need consistent units.
 
Novalium Company
Novalium Company
And we create these constant to get the right units?
 
Semiclassical
Semiclassical
1:20 PM
Well, suppose I have an object in free fall. I'll have v(t)=-gt. Would you say that I "create" g=9.8 m/s^2?
 
Novalium Company
Novalium Company
Nope...
 
Semiclassical
Semiclassical
Nor would I
I mean, g=9.8 m/s^2 is a human creation insofar as the definitions of meters and seconds are human creations
but g itself exists regardless of those definitions
 
Emilio Pisanty
Emilio Pisanty
@NovaliumCompany you have to use such a constant every time you use expressions that would otherwise have different units on either side of an equation.
 
Semiclassical
Semiclassical
By contrast, $v\propto t$ or $v\propto t^2$ is fine
 
Emilio Pisanty
Emilio Pisanty
@NovaliumCompany well, it looks like it's "of nothing" in this context because that's where your $v(t) \propto t^2$ came from in the first place
 
Novalium Company
Novalium Company
1:23 PM
Can you give me one more simple example like $v(t) = -gt$ so I can get my head around it.
 
Semiclassical
Semiclassical
In the interest of getting this out of the mechanics context, consider the case of some system in thermodynamics e.g. some volume of gas
 
Emilio Pisanty
Emilio Pisanty
if you were doing this in a real-world physical situation then you would have specific details of the forces and kinematics that produced your $v(t) \propto t^2$ motion, and the constant of proportionality would then have a clear physical meaning in terms of the details of those forces and kinematics.
 
Semiclassical
Semiclassical
If I want to increase the internal energy of said gas by $\Delta U$, I'll need to increase the temperature by $\Delta T$
 
Emilio Pisanty
Emilio Pisanty
it looks "unanchored" in this situation because your entire calculation is unanchored.
 
Novalium Company
Novalium Company
or reduce the volume?
 
Semiclassical
Semiclassical
1:26 PM
@NovaliumCompany Sure, I should say "at fixed volume" (and fixed particle number for that matter)
to the extent that the change in internal energy is small, I expect the change in internal energy to be proportional to the change in temperature
i.e. $\Delta U\propto \Delta T$
However, temperature is measured in units of Kelvin and energy is measured in units of Joules
 
Novalium Company
Novalium Company
A very very stupid question, does proportional mean equal? Or it means as the change in internal energy goes up or down, then change in temp. goes up or down respectivly?
 
Semiclassical
Semiclassical
$A\propto B$ (or $A\sim B$---same idea) says that if I change $B$ by a certain factor, then $A$ changes by the same factor
 
Emilio Pisanty
Emilio Pisanty
In a real-world application, you might have e.g. the requirement that the particle's acceleration increases linearly from zero to $a_0$ between $t=0$ and $t=t_0$. That would then mean that the acceleration reads $a(t) = a_0 \ t/t_0$, and you integrate that to get $v(t) = \frac 12 \frac{a_0}{t_0} t^2$. Then has the form $v(t) = bt^2$ and now the constant of proportionality $b=a_0/2t_0$ has a clear physical interpretation in terms of your initial description of the situation.
 
Novalium Company
Novalium Company
A factor is a number basically?
 
Semiclassical
Semiclassical
a pure number, yes. no units
 
Novalium Company
Novalium Company
1:29 PM
Right.
 
Semiclassical
Semiclassical
e.g. if I have an object moving at a fixed speed, then waiting twice as long means that the object moves twice as far
 
Novalium Company
Novalium Company
So time and distance in this case are proportional?
 
Emilio Pisanty
Emilio Pisanty
If you don't provide that context, then $b$ won't have an 'explanation' because there's no context to explain it in.
 
Semiclassical
Semiclassical
No. Distance and time are proportional, when the velocity is fixed
 
Novalium Company
Novalium Company
Oh, yep, sorry, my mistake.
 
Semiclassical
Semiclassical
1:30 PM
also, distance relative to the position at time t=0
more generally, fixed velocity means that the change in position is proportional to the change in time
and the (constant) velocity expressed the relation there: how many units of position change per unit of time
 
Novalium Company
Novalium Company
Got it. Meaning, if the distance increases with +2 for example, the time would also increase by 2?
 
Semiclassical
Semiclassical
No.
Suppose I'm moving at 60 miles per hour. Then in one hour I"ll move 60 miles and in two hours I"ll move 120 miles
 
Emilio Pisanty
Emilio Pisanty
@Semiclassical btw I suspect you'll get a chuckle from this charge distribution.
 
Novalium Company
Novalium Company
Yep, got the idea. I've also heard of inversely proportional which I suppose is if one increases twice for example, the other will decrease twice?
 
Semiclassical
Semiclassical
If by "increase twice" you mean "doubles", then yes
Proportionality is not a story about additive change. It's about multiplicative change
 
Novalium Company
Novalium Company
1:36 PM
Yep, got it now :D Thanks
 
Semiclassical
Semiclassical
so an inverse proportionality relation would mean that doubling one quantity will halve the other
in the case of interest, you have $v(t)\sim t^2$
which would mean that doubling $t^2$ would double $v(t)$
or, in terms of $t$: doubling $t$ would increase $v(t)$ by a factor of 4
 
Novalium Company
Novalium Company
Pff, I don't know but I feel so stupid, my mind isn't working properly, I can't concentrate ;\
 
Semiclassical
Semiclassical
However, that's not enough to actually tell you what $v(t)$ is. You could have, at t=1 second, that $v(t)=1$ meter/second. In that case, $v(2t)=4 m/s---the velocity has increasd by a factor of 4
but you could just as well have that v(t)=343 m/s when t=1 second i.e. the speed of sound (if I'm remembering that right) in which case v(2t) = 1372 m/s
 
Novalium Company
Novalium Company
Ok, I'm gonna take a small break because my mind is overheating and about to explode, I don't know why. I'll think about these things and come back later, thanks so much for the help!
 
Semiclassical
Semiclassical
Exercise for myself: Suppose $|\psi\rangle = |\uparrow\rangle^{m}|\downarrow \rangle^{ n}$ (in the sense of m- and n-fold tensor product). Compute $S^2|\psi\rangle$ where $\vec{S}=\sum_{k=1}^{m+n} \vec{S}_k$
 
Slereah
Slereah
1:45 PM
sounds like it's gonna be some Clebsch Gordan bullshit
 
Semiclassical
Semiclassical
to be clear, $|\uparrow\rangle = |\frac12\,\frac12\rangle$
so there's not a lot of clebsch-gordan BS possible there given how simple the actions of $S_z,S_\pm$ on $|\frac12\,{\pm\frac12}\rangle$ are
Still gotta write it out tho
 
Emilio Pisanty
Emilio Pisanty
1:59 PM
@Semiclassical that's why they invented the notation $|{↑}⟩^{\otimes m}$.
 
Semiclassical
Semiclassical
yeah, i'm just being lazy
 
Koolman
Koolman
2:28 PM
What would be the best place for discussing about programming ?
@JohnRennie
 
Semiclassical
Semiclassical
depending on the programming, here might be a good place if only b/c of how many of the physicists here have transitioned to programming (quite a few machine learning people)
 
Abcd
Abcd
To improve your code^ (code review)
Everyone knows that ^
 
Semiclassical
Semiclassical
so I finally figured out what I was remembering from Penrose. he was referring to a construction of Majorana, one which was later picked up by Schwinger and used to formulate his boson representation for spin-s states
which I have seen before but forgot about. So that wraps it up nicely
 
user157860
user157860
2:53 PM
@Semiclassical, hi guys, anyone willing to answer a simple query on math?
 
Anonymous
@user157860 Hi. Just ask your question, without asking if anyone is willing to answer you. And then, wait patiently. It is absolutely useless to ask whether anyone is willing to answer your simple query on math, because they don't know what your "simple query" is, and whether it is "simple" in the first place (by their standards).
 
Novalium Company
Novalium Company
$a = \frac{dv}{dt} = \lim_{\Delta t\to 0}\frac{v(t + \Delta t) - v(t)}{\Delta t} = \frac{d^2x}{dt}$. Is this right?
 
user157860
user157860
@Blue,thanks Here:en.wikipedia.org/wiki/Harmonic_series_(mathematics) ...wiki says that the harmonic series is divergent, but haven't we all studied that the sum (or integral) tends to $e=2.4828...$
(I wasn't sure math was allowed here)
 
Semiclassical
Semiclassical
@NovaliumCompany yes.
 
Anonymous
@user157860 "haven't we all studied that..." No, I haven't studied anything like that. The proof of divergence is clearly given on that page
 
Semiclassical
Semiclassical
3:04 PM
also, $e=2.71828...$
 
Anonymous
@user157860 It is allowed, but you need to install the bookmarklets from here: math.ucla.edu/~robjohn/math/mathjax.html
 
Semiclassical
Semiclassical
one does have $e=1+\frac{1}{2!}+\frac{1}{3!}+\frac{1}{4!}+\cdots$
but not $e=1+1/2+1/3+1/4+...$
 
Anonymous
Yup, I think @user157860 is confusing the second for the first ^
 
Semiclassical
Semiclassical
it's funny how many series amount to "take reciprocals of some infinite subset of the positive integers and try to sum them"
 
Anonymous
BTW, we need something like this for our room: sopython.com/salad!
 
Semiclassical
Semiclassical
3:08 PM
if that subset is the factorials, then you get e. if it's the subset of squares, you get pi^2/6
 
Anonymous
That's the Stack Overflow Python chat's website ^. I wonder who is paying for hosting the site though
 
Semiclassical
Semiclassical
if it's the subset of cubes, no one knows :)
 
ACuriousMind
ACuriousMind
@Semiclassical I think it's to be expected, isn't it? Taking reciprocals of a growing series of integers is the easiest way to produce a sequence that goes to zero, which is necessary for the series to converge
 
Fawad
Fawad
 
Semiclassical
Semiclassical
oh, sure.
 
Anonymous
3:10 PM
@Fawad Eh?
 
user157860
user157860
@Semiclassical @Blue, please check this proof 1 + sum of inverse even numbers=1 + sum of uneven numbers 1/3+1/5.... which is surely less than previous , then total less than 3. What is wrong?
 
Semiclassical
Semiclassical
it's a natural enough. but when one starts out with series one just sees a bunch of seemingly arbitrary sums
 
Fawad
Fawad
How to proceeed?
 
Semiclassical
Semiclassical
@user157860 you're trying to manipulate a divergent series as though it's a convergent one. that won't work
1+1/3+1/5+1/7+... is divergent, just like 1+1/2+1/3+... is
if you want to do something like 1+1/3!+1/5!+... < infty, then such manipulations will make sense
 
user157860
user157860
@Semiclassical, 1/2 +1/4 tends to 1, right?
 
Anonymous
3:11 PM
@Fawad Not feeling like attempting to prove combinatorial relations now. Ask on the main site maybe :P
 
Fawad
Fawad
Np
 
Semiclassical
Semiclassical
@user157860 1/2+1/4+1/8+... +1/2^n+... does. 1/2+1/4+1/6 +...+1/2n+... does not
the latter is again a divergent series.
 
Fawad
Fawad
@Semiclassical
 
user157860
user157860
@Semiclassical, I see, thanks
 
Fawad
Fawad
in Mathematics, 53 mins ago, by Manish Kundu
How do I find : S=∑k^2 * (n-k)C(n-r) from k=1 to k=r
 
Semiclassical
Semiclassical
3:14 PM
tediously.
 
ACuriousMind
ACuriousMind
@Fawad Please do not ping random users in an attempt to get a question answered. If someone wants to answer your question, they will do so on their own.
5
 
Semiclassical
Semiclassical
No.
 
Anonymous
Gosh.
 
ACuriousMind
ACuriousMind
@Semiclassical Isn't "a bunch of seemingly arbitrary symbols" a good description of any math field one is only starting out with? :P
 
Semiclassical
Semiclassical
lol
true
except geometry. then it's a bunch of seemingly arbitrary pictures
 
Abcd
Abcd
3:19 PM
What did fawad post that got him banned?
 
ACuriousMind
ACuriousMind
@Abcd What does it matter to you?
@Semiclassical Depends on the geometry - I know plenty of geometers who only rarely draw pictures ;)
 
Semiclassical
Semiclassical
point
(plus a picture is itself a symbol, just visual rather than textual)
 
ACuriousMind
ACuriousMind
Or only draw..."pictures". I will never forget the algebraic geometry lessons with a fuzzy bunch of lines on the board and the TA going "This is the generic point" :P
 
Anonymous
Speaking of which.....does anybody still do research in Euclidean geometry? Or is it just "competitive math" for high schoolers these days
 
Abcd
Abcd
@ACuriousMind Curiosity
 
ACuriousMind
ACuriousMind
3:23 PM
@Blue I don't think there are many unanswered questions in pure Euclidean geometry
 
Semiclassical
Semiclassical
I think the main things you see nowadays in Euclidean geometry are finding other ways to prove what's known.
 
ACuriousMind
ACuriousMind
Or games of "how weak can I make these axioms and still show stuff?"...
 
Semiclassical
Semiclassical
actually, one realm in Euclidean geometry where you probably still do have active work is polyhedra in higher dimensions
 
ACuriousMind
ACuriousMind
But technically that's not Euclidean geometry anymore then, more like sub-Euclidean geometry :P
 
Semiclassical
Semiclassical
which does connect to group theory inasmuch as one wants to describe the symmetry groups of said polyhedra
2D or 3D Euclidean geometry seems pretty well exhausted, tho
 
Tanner Swett
Tanner Swett
3:43 PM
So there's this interesting question over at World Building: worldbuilding.stackexchange.com/questions/3474/…
I could make the question a little more formal by asking something like: how would gravitational potential behave in a non-Euclidean space?
 
ACuriousMind
ACuriousMind
@TannerSwett That's just General Relativity :P
 
Tanner Swett
Tanner Swett
You could easily define a metric space that's "Euclidean space with a portal added".
@ACuriousMind Oh, perfect. :D Sadly, I don't know general relativity.
So, it's pretty easy to define gravitational potential in a non-local way. Something like: gravitational potential is the sum, over all massive particles in the universe, of the mass of the particle divided by the square of the distance to that particle.
Can I characterize gravitational potential locally, though?
 
Anonymous
@TannerSwett Damn, the theorists would make good money if they went into the movie-making/story-writing industry :P
 
Tanner Swett
Tanner Swett
If I simply look at some region of space near a massive object, what does the gravitational potential look like?
If I knew the gravitational potential at the boundary of that region, are there some differential equations I could solve to find the gravitational potential everywhere within the region?
 
John Rennie
John Rennie
@TannerSwett the problem is that gravitational potential defined in that way is observer dependent.
 
ACuriousMind
ACuriousMind
3:49 PM
@TannerSwett Unfortunately, there's no such thing as a well-defined "gravitational potential" in general relativity.
 
Tanner Swett
Tanner Swett
Let's set aside general relativity, then, and assume I'm just talking about Newton's gravity.
Perhaps the Laplacian of gravitational potential is zero in regions which don't contain any matter?
Aha, this is Poisson's equation for gravity, right? en.wikipedia.org/wiki/Poisson%27s_equation#Newtonian_gravity
The Laplacian of gravitational potential at a point is proportional to the density of matter at that point.
 
enumaris
enumaris
yep
that's true
 
Fawad
Fawad
@Abcd your curiosity is your curiosity none of his curious mind
 
Abcd
Abcd
4:29 PM
@Fawad What did you say?
 
ACuriousMind
ACuriousMind
Apparently we're discussing this on meta now, so let's not additionally discuss it here.
 
Abcd
Abcd
Woah I dont believe this.
 
Emilio Pisanty
Emilio Pisanty
Apparently they're blue now and the ask question button is still red... There you go! — Catija ♦ 6 mins ago
whaddayaknow
 
Semiclassical
Semiclassical
@ACuriousMind btw, let me know if you update that one question re: the trivial representation
 
Physics Meta
Physics Meta
0
Q: Bad behaviour from mod

FawadRecently I was struggling with a problem so I asked for help from two people. 1st from Blue . I talked to him before and he is nice dude ready to help. Then I saw semiclassical who is also cool and very active on math chat. He helped me so many times (just some proof) before also in math chat so ...

feature-request moderators
 
enumaris
enumaris
4:46 PM
pain
 
Secret
Secret
Just when I thought only AntiFa vs far right is a clusterf8888
No, a clusterf888888 is too mild, we are dealing with Condensationf888 here
 
John Rennie
John Rennie
(deleted)
 
Emilio Pisanty
Emilio Pisanty
5:18 PM
@PhysicsMeta oooohhhh, thanks for digging this up
I'd forgotten about it
@ACuriousMind (to be clear: what's not acceptable is telling other users to shut up, not the mere usage of the acronym)
2
 
Anonymous
lol
 
Emilio Pisanty
Emilio Pisanty
I would suggest making the assertion more precise. Telling other users to shut up is rude but, depending on context, may be acceptable if done politely. Using profanity while doing so is what takes it over the line.
please do not star any of that.
 
Anonymous
Anyhow, the situation looks pretty irreparable now, especially since the user in context has some sort of personal vendetta which has lasted for more than a year. (The last comment by the OP makes it clear)
 
enumaris
enumaris
why john rennie delete his answer
 
Fawad
Fawad
 
ACuriousMind
ACuriousMind
5:24 PM
@EmilioPisanty I guess you're right. I added "in a rude manner" as a qualifier. Which should not be taken to mean it's always acceptable to tell others to shut up if you're doing it politely.
@EmilioPisanty Maybe we should add a "Don't post anything you don't want to see on the starboard" to the room description :P
2
 
Anonymous
That ^
 
Anonymous
<3
 
Anonymous
Hmm, yeah. There might be someone flagging these
 
ACuriousMind
ACuriousMind
→ 20 messages moved to Trashcan
 
Anonymous
Phew
 
enumaris
enumaris
5:30 PM
awww
2
 
ACuriousMind
ACuriousMind
@Semiclassical I'm still thinking about whether it's salvagable at all.
 
Emilio Pisanty
Emilio Pisanty
@ACuriousMind it isn't.
I don't know what you're talking about, but it's almost certainly not salvageable.
 
ACuriousMind
ACuriousMind
@EmilioPisanty I was about two sentences into a rebuttal before I read that second message :P
 
user54826
user54826
holy snail I just discovered I have the rights to check the upvotes and downvotes to questions.... WOW. but it's been months I could do that
that's freaking amazing. unfortunately I am interested to get the statistics of the website, so I need like 25 k points lol
 
ACuriousMind
ACuriousMind
@user54826 I think the feature is intentionally a bit hidden because it's somewhat expensive on the database
 
Mithrandir
Mithrandir
5:35 PM
@user54826 You can get almost all the same data and more using Stack Exchange Data Explorer.
 
Anonymous
There were some user scripts to view upvotes/downvotes before reaching the rep threshold
 
Anonymous
Maybe some exist on the Stack Apps
 
Anonymous
Haven't checked in a while tho
 
Mithrandir
Mithrandir
@Blue I've got one installed as a Chrome extension.
 
user54826
user54826
man so that question physics.stackexchange.com/questions/258837/… has 2 upvotes and 2 downvotes. WOW geezus im so freaking happy i feel like a super powered man
 
Anonymous
5:37 PM
137
Q: "View Vote totals" without 1000 rep

Rob W Screenshot About The vote counts are a great tool to determine whether an answer is disputed or not. Unfortunately, not many of us have enough time to join all Stack Exchange websites and get 1000 reputation. This script unlocks the "View Vote counts" feature for those who are not logge...

script chrome-extensions greasemonkey bookmarklet
 
Emilio Pisanty
Emilio Pisanty
@user54826 they're... less useful than you think
> LaTeX Warning: A float is stuck (cannot be placed); try class option [floatfix]
@ACuriousMind is this a situation where it's OK to tell RevTeX to shut up?
 
Abcd
Abcd
@Fawad you use an i-phone?
 
Anonymous
@EmilioPisanty Umm, is it compiling fine otherwise? That issue seems to be very common while using revtex
 
user54826
user54826
how do you like my new username
uh
 
enumaris
enumaris
@EmilioPisanty rise up!
 
user54826
user54826
5:42 PM
not changed?
 
Fawad
Fawad
@Abcd no ipad
 
enumaris
enumaris
I'm occupying this chat, you can't make me leave except by extraordinary measures!
 
Abcd
Abcd
@Fawad Okay.
 
enumaris
enumaris
don't tase me bro
 
coniferous_smellerULPBG-W8ZgjR
coniferous_smellerULPBG-W8ZgjR
strange my username is changed on my profile page but not here
 
ACuriousMind
ACuriousMind
5:43 PM
@EmilioPisanty The float is...stuck? Does that mean it didn't find any position where it fit?
@coniferous_smellerULPBG-W8ZgjR Chat profiles update with a delay.
 
coniferous_smellerULPBG-W8ZgjR
coniferous_smellerULPBG-W8ZgjR
I see @ACuriousMind thanks
 
Emilio Pisanty
Emilio Pisanty
@Blue oh, it compiles fine. The figures are right where they need to be. Toggling floatfix doesn't do anything to the float placement or make the warning go away.
 
Mithrandir
Mithrandir
...unless a mod is feeling nice and presses the "refresh profile" button, which I did.
 
Anonymous
Oh, floatfix is pretty useless :P There are other ways to explicitly shut off those warnings tho
 
Emilio Pisanty
Emilio Pisanty
@Mithrandir huh
 
Anonymous
5:44 PM
I don't remember the exact command, but lemme check
 
Emilio Pisanty
Emilio Pisanty
@Blue oooohhh, are there? if you have links handy, I'm interested.
0
Q: How much electrons can absorb a single photon?

ArturIn the one hand, energy can be emitted\absorbed only by a discrete portions. In the other hand - according to Coulomb law the higher distance - the lower force, because count of force lines per area decreases with distance. This fact always confused me: the problem is that it's difficult(or imp...

quantum-mechanics photons quantum-electrodynamics virtual-particles
anyone feeling brave?
 
Anonymous
16
A: RevTex4-1 warning: "Repair the float package"

egregThe revtex4-1 class defines several similar messages, for instance 351 \@ifx{\eqnarray\eqnarray@LaTeX}{% 352 \class@info{Repairing broken LaTeX eqnarray}% 353 \let\eqnarray\eqnarray@fleqn@fixed 354 \newlength\eqncolsep 355 \setlength\eqncolsep\z@ 356 \let\eqnarray@LaTeX\relax 357 \let\eqna...

 
Anonymous
Iirc the silence package works well
 
Anonymous
If you have to block specific warnings
 
Anonymous
@EmilioPisanty I mean the silence won't help you fix the root of the issue i.e. the "float" placement issue, but it will just suppress those warnings. I don't know if that's what you're looking for
 
enumaris
enumaris
5:55 PM
Gandalf? Is that you?
 
Emilio Pisanty
Emilio Pisanty
@Blue yeah, I'll take it.
Like I said, I just want revtex to shut up and leave that space for useful warnings instead.
 
enumaris
enumaris
not nice to tell off revtex bro, what did revtex ever do to you? It's trying it's best...
 
Anonymous
@EmilioPisanty I can understand the trauma with revtex :P The worst part: It moves footnotes to bibliography unless you force it not to!
 
Anonymous
The journal guys who maintain revtex sure have some .... issues
 
Mithrandir
Mithrandir
> "Gandalf," the old man repeated, as if recalling from old memory a long disused word. "Yes, that was the name. I was Gandalf."
 
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