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8:01 AM
@vzn Who said what? If you can't find it on the internet, I probably made it up myself, but don't want to claim that it's from me, jus in case it isn't. :-)
 
@JohnRennie I believe VLQ flags can only be cast on posts less than a week old.
@JohnRennie this was cleared with the mods in advance - we don't really have any comments to make.
@DanielSank here's the clear explanation: for two weeks, starting tomorrow, the mods will not decline any VLQ flags. That means people are free to flag things which ordinarily wouldn't be considered VLQ, and they will sit in the low-quality review queue for as long as it takes for the community to deal with them.
 
8:19 AM
@JohnDuffield My guess would be to have a look at inkscape, but it's way beyond my abilities; sorry
 
@Sanya : thanks.
 
@JohnDuffield you're very welcome and if you decide to have a look at inkscape, I have always had the impression that there is quite a large community online with a lot of tutorials and the like being available
but again, there might be better suited programs I don't know
 
@CuriousOne : no, they're all there.
@Sanya : I'll definitely try it. Thanks again.
@0celo7 : we've had our little run-ins.
 
I second that; Inkscape is good for general figures. For specific types of drawings, there may be more specialized software (e.g. JaxoDraw for Feynman diagrams) but Inkscape will in theory do anything.
Personally I use TikZ for most of my figures; I like the results better. But the learning curve is higher than Inkscape.
 
8:38 AM
@DavidZ tikz is the Tex-programming-like figure creation tool?
 
Yep
381
Q: Nice scientific pictures show off

ThomasTask Show off your best scientific illustration ! The main purpose of this question is to share beautiful scientific pictures, preferably with an educational aspect. Content Your post must contain a nice picture and the associated code. One can post several pictures, but it must be done in...

 
ah ok ... in case I ever do a PhD, I might consider investing the time to learn that :D
that's a very nice thread - with lots of work in each picture I'd bet
 
@JohnRennie : we won't make the world a better place by parroting bad science either. For example this answer has over a hundred upvotes, and it's wrong. We will make the world a better place by encouraging people to do their own research and think for themself.
@JohnRennie : there is no consensus. But there are some high-rep users who want more power. And as we all know, power corrupts.
 
@Sanya This doesn't represent any official policy, it's just the way things have worked out. Having said that, I'm delighted that an educational resource like the PSE is available. I just wish it had existed in the 1970s when I was an enthusiastic young physics nerd!
 
@DavidZ : thanks for the input.
 
8:51 AM
@DavidZ Let the experiment commence! We can presumably discuss this in the chat next Tuesday.
 
Add to favourites. I find things like this particularly interesting:
 
@JohnRennie Well, this coming Tuesday we have DanielSank's AMA, but the next chat session after that, we can definitely discuss it.
 
@JohnRennie it's just refreshing to see someone describe the status quo instead of upholding the flag of "PSE is for research level questions only (which are less than 5% of our questions, but still!)". Apart, I can understand your sentiment, I'd have been happy myself to have discovered this website at the beginning of my studies too :)
 
@DavidZ I thought that was on the 12th. That's what the pinned message says.
 
Oh wait, I thought it was supposed to be during the chat session
@DanielSank to clarify, the 12th is Friday. Are we having your AMA on the 12th or during the next chat session, on Tuesday?
cc @vzn if you have anything to say about this
 
8:55 AM
It's a good job one of us is paying attention :-)
 
Yeah... good thing! 3 or 4 people were involved in changing that date and somehow none of us noticed the discrepancy.
 
@DavidZ : see this meta question where DanielSank says the 12th.
 
I know about the meta question. All these sources are copying from the same exchange we had in chat, where we agreed on the 12th, but at the time I thought the 12th was the date of our regular chat session.
 
@JohnRennie probably my tone was a bit off and I shouldn't have written that - but I just get tired of seeing questions where I feel that the OP can't even take the time to write down a decent question
this is a sign that I'm taking things too seriously and should stay calmer ...
 
10:05 AM
@JohnRennie Please remark that I linked an answer and not the question.
 
Noted
 
10:16 AM
Suppose I have an equation $A+Bx=y(C-Dx)$, where $A,B,C,D$ are nonzero constants . If I assume $C-Dx=0$, then I get $x=\frac{C}{D}$ but at the same time I get $x=-\frac{A}{B}$. Does it means that $\frac{C}{D}=-\frac{A}{B}$ or because this equation is a deg 2 polynomial in 2 variables, the two x I calculated are actually two different solutions of x?
 
I fail to see how x = \frac{C}{D} solves in that case ... anyway, shouldn't we first worry about y=0 and $y \neq 0$? Then we can proceed from there, no?
 
if $y=0$ then I also get $x=-\frac{A}{B}$
 
that is the only solution, isn't it?
in that case
 
if $C-Dx=0$ then since the y term disappears from the equation, y can be any real number
 
yes, but at the same time, we still have $A+Bx=0$
which cannot hold for $x= C/D$ unless A and B are chosen to make it work
 
10:25 AM
I see, so in that case $A,B,C,D$ will be constrained by $C-Dx=0$ and the original equation
 
yeah
I think for $y \neq 0$, the solution is just x = \frac{yC - A}{B+yD} unless B+yD is zero
in which case there is no solution
 
For $y\neq 0$ we have $A+Bx=Cy-Dxy$, which becomes $x(B+Dy)=Cy-A$ If $B+Dy=0$ then $x \in \mathbb{R}$ and $y=\frac{A}{C}=-\frac{B}{D}$

Solving polynomials of multiple variables is harder than the single variable cases. I guess I should read up some algebraic geometry...
 
well, in general \frac{A}{C}=-\frac{B}{D} won't be the same, but you're right, in that case, there is a solution
 
[For the theoretical guys to be coming in the h bar later of the day/night]
http://physics.stackexchange.com/questions/273032/what-exactly-is-a-photon

This question reminds of one thing:
How do we know experimentally a photon has no substructure in that it is not simply an overally neutral object made of massless positive and negative charges such that it has some nth order multipole moment where $n > 2$
 
All: I have asked a question: What exactly is a photon?
I will put a 400-point bounty on it.
 
10:41 AM
>In fact that's worse than useless, in that

it perpetuates the popscience myth that photons are forever popping into existence like magic, spontaneously, like worms from mud.

Sadly this myth is there in the photon Wikipedia article too. It isn't true. Hydrogen atoms don't twinkle, and magnets don't shine. Because as anna said virtual particles only exist in the mathematics of the model. So, who can tell me what a real photon is? Or refer me to some kind of authoritative informative definition that is accepted and trusted by particle physicists? I say all this because I think it's of param
As an edit suggestion, If I were you, I will try to remove any terms that is perceived as subjective or emotional by others , such as "myth".

When i aks questions, I often strip of any terms that might sound neutral to me, but probably sound emotionally or subjectively driven by others in order to get others to focus on the issue without the distractions by emotions
 
@Secret : we know the photon is not made of massless positive and negative charges because radio-wave photons can be 1000m long. And because of electron-positron annihilation to two (or three) gamma photons.
@Secret : noted. I've edited the question accordingly.
 
From what you said, you mean if a photon is made of masslss charges that it is impossible to get it to very long wavelengths?

Also how does the observation of electron positron annihilation ensure the photon to have no substructure?
 
10:57 AM
:Secret : annihilation means you start with two charged particles and you get two photons. When you perform gamma-gamma pair production it's the reverse process. And mass and charge always come as a package. There are no stable massive particles that have no charge. Even a neutron has charge. Its magnetic moment is the evidence that it has charge like a hydrogen atom has charge. It just doesn't have any net charge.
I have to go I'm afraid. By for now.
 
 
3 hours later…
1:51 PM
[Scifi ]Yes, I have finally derived the equations for back to the future style temporal mechanics!

Bonus point for getting a condition that can check for ontological loops

Now we are into business
(If only we can figure out a way to resolve that goddamn bottleneck... then it will be purefect)
 
2:16 PM
Can a Schwarzschild black hole appear in nature?
 
no
 
K cool
 
@SirCumference learn calculus yet?
 
@0celo7 Yes
Single variable
 
Oh, can you explain the proof that continuous functions have primitives
 
2:30 PM
 
what
that's needed to define integrals
 
I halfassed the integral part of my lectures. I'll need to read up on them again
 
then you don't know calculus, I'm afraid
 
C'mon man
I know single variable differentiation from head to toe
All the seemingly endless rules that come with it
 
There's like three
linearity, chain rule, product rule
Can you prove them?
 
2:33 PM
Quotient rule, er, lemme think of them
I used to be able to
 
Quotient rule is chain rule + product rule, try again
 
I bet I can again using the difference quotient
@0celo7 Why you testing me tho
 
I dunno
that Canadian kid died and I've supplanted you as my protege
 
Canadian kid?
ACM?
 
Yes, @ACuriousMind
She's gone
 
2:35 PM
ok
The difference between ACM and me is that I have a lot less experience in this
and physics as a whole
 
I don't care about physics
 
you should understand basic mathematics
 
You're on the wrong site
 
Let's do some homological algebra
 
2:37 PM
Oh god
 
I can teach you, dw
 
Last time you tried to teach me about black holes, you showed me a false proof
 
A differential complex is a series of the form $C^\bullet:\cdots\to C_{p-1}\stackrel{\mathrm d}{\to}C_p\stackrel{\mathrm d}{\to}C_{p+1}\to\cdots$ such that $\mathrm d^2=0$.
 
What are we doing
 
de Rham cohomology
 
2:41 PM
Why are we doing this
 
It's useful for e.g. string theory
 
What's string theory useful for though
 
not much, admittedly
$\odot$
ooooo
I might have to get CLN in the future
Although I want to learn symplectic topology too
And twistors
too much to learn!
 
Chow Lu Ni
I have Brendle but it's too informal
informal books are pretty aggravating
I don't want all the details but when I do want details, I can't get them
but CLN is massive
Also Mucho Expensivo
 
2:52 PM
@0celo7 *Muy expensivo
You just said "A lot of expensive"
 
I'm not a Mexican
Or Brazilian or whatever
 
Neither am I
 
@DanielSank you around?
 
3:09 PM
All right, I have a potentially stupid question about GR and black holes
Anyone willing to answer?
 
Well, you can at least post it here
 
All right
Say I'm approaching a black hole, and a lightyear away from me, an observer is using a magically-powerful telescope to see what's happening to me
Now, as I get closer and closer to the apparent horizon, time dilation between me and the observer increases, right?
 
yep
gravitational time dilation increases
 
Okay, so will time dilation reach infinity once I've reached the apparent horizon, or absolute horizon?
 
Ah, that needs more of a GR expert than me to answer
 
3:14 PM
okay, regardless of that question
Once I've reached the event horizon, then will time outside of the black hole have speed up so much, the black hole has already evaporated due to hawking radiation?
 
From an outside observer's perspective, yes, I would imagine so, because it takes an infinite amount of time for you to reach the event horizon
I guess there is some process of you falling toward the horizon and the horizon receding away from you
 
So from my perspective, as I approach the black hole, would everything have sped up so fast that — poof, the black hole is gone before I notice?
 
Again, that needs a better GR expert than me to answer. All the usual treatments use Schwarzschild black holes, which don't experience Hawking radiation and are unchanging in time.
 
I thought Schwarzschild black holes were only different from Kerr black holes in that they don't rotate?
All right, if we applied real physics though, I would be spaghettified before I reached the event horizon, right?
 
@SirCumference That sounds right. I suppose one could get the same conclusion with a Kerr black hole; the important part is that it's static in time. (Which means no Hawking radiation.)
@SirCumference Not necessarily, not if the hole is very large
 
3:46 PM
@DavidZ Why's that?
Wouldn't the difference in tidal forces still rip you apart?
 
4:12 PM
en.wikipedia.org/wiki/Help:Template This is incomprehensible alien language
 
@Secret What language?
Wiki markup?
 
I don't understand what it is saying, for example I want to grab this template en.wikipedia.org/wiki/Template:NumBlk for use in my wiki, but I cannot find any code of sorts
 
What kind of wiki is it?
Wikia?
 
yes
 
K, I'll help ya out
So you want to create a new page on your wiki called "Template:NumBlk"
 
4:17 PM
yup, I have done that already, the question is now where to find the code
 
All right, on that page
On wikipedia
There's code that doesn't show up unless you view it in the editor. That code is surrounded by <includeonly></includeonly> tags
If you go to edit that page, and you copy everything between those tags, you'll have your code
Here's it specifically
{{#if:{{{1}}}|{{Repeat|<dl><dd>|{{str len|{{{1}}}}}}}|}}
{|style="border-collapse:collapse; background:none; margin:0; border:none;"
|style="vertical-align:middle; border:{{#if:{{{Border|}}}|thin solid|none}}; padding:0.08em;" class="nowrap"|<p style="margin:0;">{{{2}}}</p>
|style="vertical-align:middle; width:99%; border:{{#if:{{{Border|}}}|thin solid|none}}; padding:0.08em;"|
<p style="margin:0;">
{|style="border-collapse:collapse; background:none; margin:0; border:none; width:99%;"
|style="border:{{#if:{{{Border|}}}|thin solid|none}}; padding:0.08em;" rowspan="2"|<p style="margin:0; font
 
ok this is weird, in my browser of the same page, I cannot see any of the above code
 
Uh, no
Just make sure you copy from here
You're copying from the documentation page, not the real template
Copy everything within the <includeonly></includeonly> tags
 
ok
 
@Secret Did it work?
 
4:23 PM
checking...
 
Basically, code within <includeonly></includeonly> will only show up when you edit the page
Anything within <noinclude></noinclude> will not affect your template's code; it will show up on the page of the template though
So you can write documentations, examples or instructions between <noinclude></noinclude>, and it won't affect the template
 
4:34 PM
@DavidZ What's up?
@DavidZ Good question. I don't know. I am now confused.
@DavidZ I don't suppose you'd be willing to edit the meta post so as to make that clear...?
 
@DanielSank the edit to the meta post is already done
 
That post is still twenty miles long. Whatever.
If we're going to change the date of the AMA again we should do it soon.
 
Yeah, I didn't want to remove stuff because I don't know what Emilio considers important - I just added a summary
As for the AMA, we can do it either during the chat session on Tuesday, the 9th, or we can do it on Friday the 12th. As the guest, it's up to you.
 
@DavidZ So, uh, could you elaborate?
 
I mean, if your availability only allows one day or the other, naturally we have to go with that.
 
4:38 PM
I'm available either way.
 
@Secret It work?
 
I'm mostly worried about people not coming if we change it again.
 
@DanielSank okay, well then... I think you're right, it's probably better to do it the 12th, as advertised. It won't be during a chat session, so the planning is entirely up to you and @vzn.
 
@SirCumference Not really, this thing is insane. It has a nest full of Xpd(blahblahblah) templates that follows before it
 
Then we will have our chat session on Tuesday the 9th as usual. I'll make sure somebody drops a note about the changed AMA scheduling during the chat session.
 
4:40 PM
@Secret Link me. I'll try to fix it
 
@Secret And what page are you getting the code from?
 
@SirCumference sorry about that, I had to go earlier. Basically, tidal force is related to how much gravity changes over a fixed distance. For a black hole, that's roughly proportional the size of the object divided by the radius of the black hole. For a large black hole, the tidal force is small.
 
@DavidZ Ah
Thanks
All right, new question. Time dilation is affected by speed, right?
 
4:42 PM
Say that I was at rest relative to the ground. Then suddenly, I sped up to $0.999c$ relative to the ground.
Theoretically, time dilation should make my time slower, and I would see the Universe age quickly before my eyes.
 
@SirCumference yes, in addition to gravity, speed is also a cause of time dilation
 
But from my perspective, I would see the Universe moving at $0.999c$ relative to me. So it should see me age quickly, right? The question is: which one of us ages quickly?
 
No, for speed it's symmetric. The universe sees you age slower and you see the universe age slower.
 
Would I see it aging fast, or would it see me aging fast?
So...overall, we age the same amount
No difference?
@Secret Try it now
 
You can't even compare the two. They're two different time lines.
 
4:45 PM
@DavidZ So wait. There would be any difference between my scenario and one where I was standing still, relative to the ground?
 
@DavidZ What things would you do in terms of "planning" if the AMA were during a chat session?
In other words, got any advice?
 
@DanielSank "Planning" basically involves pushing off anything I'd considered putting on the agenda during that chat session. (Or anything anyone else had suggested for the agenda.) For an AMA outside the chat session schedule, I don't think much planning is needed.
I guess it would be useful to have someone lined up to introduce you, unless you want to introduce yourself.
 
There's still a whole lot of stuff missing in the NumBlk. I am not sure exactly how many templates NumBlk needs and what those are doing
http://secretuniverse.wikia.com/wiki/Template:NumBlk
 
@DavidZ Meh, I'll introduce myself. This ain't a Harvard colloquium ;)
 
Sure, whatever works for you
 
4:48 PM
thanks
 
@Secret How about now?
 
The one thing that could be helpful is a sticky star in the chat room during the AMA explaining what's going on.
 
I would say we should make sure to have at least one moderator or room owner around, preferably more, just in case things get out of hand. (Not that they ever have, really) But I'll be here at the time.
 
Who has the power to do such things?
 
@Secret I just removed all the stuff between <noinclude> and </noinclude>. That stuff doesn't affect the template
 
4:49 PM
Any room owner can pin a message.
 
@DavidZ Well thanks for volunteering to be around :-)
 
Oh, I'm just saying I'd be here anyway
maybe not actively watching, but I'll at least be on my computer and pingable
 
@SirCumference The NumBlk template still return with Script Error when used, I am not sure what is missing
 
@DavidZ Ok then I'll ping you a bit in advance to set up the pinned message.
 
@Secret What pages are you using the NumBlk template on?
 
4:50 PM
@DanielSank sure, or any of the room owners - in case I'm not around early
 
Hmmmm, how to find out who is owner?
 
@SirCumference There's a page I am still writing. I need the NumBlk template to display equation numbers. Otherwise may need a rather code ugly workaround
 
@Secret Hold on
 
@DanielSank click on "info" at the top right
 
aha!
 
4:52 PM
David Z has added an event to this room's schedule.
 
I just noticed, we didn't have an event for the AMA
 
@Secret How about now?
 
It seemed to be working now. How do you fix it?
 
@Secret I put in two new pages: Template:Str len, and Module:String
I copied them from wikipedia
Tbh, I've never heard of a module before this
 
I see ,thanks
 
5:00 PM
@Secret Np, if you need more help just ping me
@Secret I also fixed a mistake on the color template
 
Can someone help me out with my 'rotating charged ring radiation' question? I simply dont seem to be able to smoothen things out using the poynting vector.
 
@DavidZ So, uh, will there be any difference?
 
5:31 PM
@SirCumference between what exactly?
 
Between my scenario and one where I was standing still, relative to the ground?
 
So, you're comparing a situation where you're moving at high speed relative to the ground, with one where you're standing still relative to the ground?
 
@DavidZ Yep
How would time dilation differ between me and the rest of the Universe
 
When you're moving (with respect to the universe), you see the rest of the universe going slow. When you're not moving, you don't.
 
@DavidZ But from the universe's perspective, you'll be the one aging slowly
 
5:40 PM
Yep
 
So if both me and the rest of the universe are aging slowly, we have nothing to compare it to...
So I'll be no older nor younger than the rest of the universe. How is that different from me standing still?
 
No, you can't even compare your age to the rest of the universe because you're not moving at the same velocity. You're in a different reference frame from the universe.
 
@DavidZ Oye...here's a new idea. I'm standing on a flat plane, and I travel at 0.99c relative to the ground. My clone is standing still relative to the ground. He counts for one second. I then instantly go back to 0mph relative to the ground. By that time, which of us will be older?
 
More time will have elapsed for the clone
 
But how? From my perspective, he's moving at $0.99c$. Shouldn't more time have elapsed for me?
 
5:48 PM
No, he's not moving anymore once you stop
That's the difference. You started and stopped. He didn't.
 
But relative to me, he was moving and then stopped, wasn't he?
 
Yes
That doesn't matter though
 
Why not?
 
You're in an inertial reference frame. He isn't.
Oh wait, other way around
His frame is inertial
Special relativity only says that all inertial frames are equivalent, it doesn't tell you about non-inertial frames
 
Hello everyone!
 
5:58 PM
@DavidZ This might sound stupid, but how do you know which frame is inertial and which isn't?
 
I want to find more informations about embedding distributions in functions of non-archimedean fields
But apparently it's all done by one bloody guy
Todorov
 
The one in which you don't feel any acceleration is inertial
 
@DavidZ Aaaaah
Now it makes sense
thanks
 
6:35 PM
@SirCumference The same way(s) you know in Newtonian mechanics. Test if an object subject to net zero real forces accelerates or not. Or by determining if the frame is in constant velocity motion relative some other frame already known to be inertial.
 
6:59 PM
Everyone is talking about Special relativity today.
 
¨What's so special about it!
 
It's not even good, really
 
7:16 PM
@0celo7 Relative to what?
@Slereah It's pretty special relative to Newtonian physics
 
@SirCumference What?
 
7:32 PM
I really REALLY HATE wikia Markup. Keep screwingly up for even the tiniest change
Just one tiny space, and then wikia adding <commaseparators> all over the article and resulting some text to become PNG blocks
 
7:57 PM
My friend thinks I'm a mechanic because I'm taking mechanical engineering classes
 
I'm a quantum mechanic
 
[Scifi] Grrr, I gave up fixing it, the formatting just kept on screwing up
http://secretuniverse.wikia.com/wiki/Ripple_dynamics
 
@Slereah Explain to her the difference between quantum mechanics and mechanics
She thinks mechanics are scientists
And that electricians are synonymous with electrical engineers
 
8:39 PM
Ok it's now readable, but good luck finding a way to fix it
 
user54412
9:04 PM
21
Q: Could a candle theorically melt iron?

user1869935The title question is rather ilustrative. I suppose the real question would be: Is heat cumulative? Put it back into an example: If I have a lit candle right beneath an iron bar, assuming the candle will lit indefinitely, and that the heat-losing rate is below the heat-getting rate. Will the ...

 
user54412
7 answers, and I think all of them assume we're allowed to insulate the iron but for some reason we can't insulate the system
 
9:28 PM
Does anyone want to start a study group for Landau and Lifshitz "Mechanics"?
 
vzn
9:44 PM
@DanielSank @#%& sigh bummed out over the date mixup. but dont think a new date is a big deal as long as we give ~1wk lead time to everyone & maybe state sorry date change. it seems to me like more ppl show up for the routine/ regular chat session & that better attendance in that slot is more likely. its not exacty a convenient time for me. think wknds might be better for multiple ppl who have jobs but then worry about building up attendance in a new slot. DS will go with whatever you choose
 
vzn
9:54 PM
@JohnRennie thx very much for catching this/ bringing it to attn, didnt check/ realize it wasnt on a reg tues mtg either :(
 
@WilliamOliver Yeah, maybe I, why not ?
I know Landau-Lifschitz equations in the context of magnetic domain walls.
and spin waves
 
@WilliamOliver I already know most of the subjects (but could use some repetition), but I have a copy, so feel free to ping me if you want to discuss anything.
 
10:09 PM
@caconyrn Yeah.
 
@PhysicsGuy Cool, also let me know if you know of anyone else who might be interested. Keep in mind I am a beginner. I've been reading it on my own, but I know it would be better if I had some other people to work with. I know the Newtonian formulation of classical mechanics, but never learned much more.
@caconyrn And thanks!
 
Landau and Lifshitz is pretty much a classic, you should find enough people having it around & happy to talk about it
 
@WilliamOliver I am just a student, too. No problem.
@WilliamOliver So you want to learn about dynamics of magnetic fields and ferromagnetic objects ?
 
@Sanya I'm looking for people who are looking to go through the whole thing and do problem sets. One reason is certainly to be able to discuss, but another is motivation to do the problem sets and to get through the whole thing.
@PhysicsGuy Well, I am looking to go through their books, starting with mechanics. Not really focusing on anything specific
Teaching myself cause I can't really afford college right now.
 
eh okay, I understand you ...
 
10:21 PM
@WilliamOliver Okay, but Landau and Lifshitz started at magnetic fields and moved on to Landau-Lifshitz-Gilbert mechanics, so on.
 
I think the second volume is on fields, but the first one is just on Lagrangian mechanics.
 
Lagrangian-mechanics are good. It can be derived directly from newtonian mechanics.
 
I didn't think d'Alembert's principle was directly a apart of Newtonian mechanics
 
It was.
the easiest formulation of it is in newtonian mechanics.
Well, it can be formulated in every mechanical formalism, it doesnt matter.
 
@0celo7 I don't know what d'Alembert's principle is yet exactly, but as far as I understand it, the theories are equivalent from a mathematical perspective. Its a just a different way of thinking about it. Using newton's laws some things are much harder to derive apparently, but everything can be derived eventually.
 
10:26 PM
We can also discuss Noehers theorem for Hamiltonian mechanics
 
I understand the proof for Noether's theorem for the Lagrangian, but I haven't gotten to Hamiltonian mechanics, thats at the end of the book.
 
Ah, okay.
 
@PhysicsGuy Hamiltonian vector fields of conserved quantities are symmetries and infinitesimal symmetries are Hamiltonian vector fields of conserved quantities if the first de Rham group vanishes?
 
Is this a question ?
 
Is that what you mean by it
 
10:30 PM
By what ?
 
The thing I linked to
 
I dont know what you are talking about, Im sorry.
 
@yuggib Let $F:\Omega\times [0,\tau]\to\Bbb R$ be smooth, where $\Omega\subset\Bbb R^n$ is open. Suppose $\partial_t F\le\Delta F+C\tau^{-2}$ on $(0,\tau]$, where $C>0$ is a constant. Why does the maximum principle imply $\sup_\Omega F(\cdot, t)\le C\tau^{-2}t$?
Oh, also $F(x,0)=0$ for all $x$.
Hmm, maybe I have to define $\tilde G=F-C\tau^{-2}t$, then plug that into the PDE, run maximum principle on that?
 
What is this ? This is fully out of context.
 
He will (probably) understand what is meant.
@PhysicsGuy well you said it 10 minutes ago, I can't help your terrible memory
 
10:37 PM
:/ ?
 
10:51 PM
Hello everybody :)
I encountered a very simple formula regarding potential energy, but don't know how to interpret it
$F = - \frac{dE_{p}}{dx}$
why does this equal F? I mean what does force have to do with potential energy? I know that Ep depends on the position
I am aware of intermolecular forces, but still...
I cannot understand how the derivative of Ep results in a force...
maybe that some example can help me understand
 
how are you defining potential energy
 
what do you mean? Ep = mgh
if I remember properly
 
in general I mean
what is $mgh$
 
m = mass, g = 9.81N/kg, h = height
 
not that
what is it
what does $mgh$ mean
 
10:57 PM
Ep is the energy a particle has due to its position
 
Where did it come from
that, what does that mean
 
idk from where it comes
 
what is the "energy a particle has due to its position"
 
that s the "deepest" I know
I think
 
@privetDruzia I think the confusion you may be having is that potential energy can change forms (Its not always mgh). I am pretty sure its more that the potential energy is defined using that equation, its not derived to be that way (or force is defined using that equation depending on how you look at it).
 

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