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0celo7
0celo7
12:04 AM
anyone here?
@ACuriousMind help pls with Yang-Mills
Obi-Bajoran, you're my only hope
 
ACuriousMind
ACuriousMind
12:21 AM
@0celo7 What seems to be the problem?
 
0celo7
0celo7
@ACuriousMind I was going to ask you why my E&M lab has $E\approx \delta V/\delta d$ instead of the minus, but they're taking the magnitude :)
 
ACuriousMind
ACuriousMind
"Yang-Mills"...you're getting crafty ;P
 
0celo7
0celo7
@ACuriousMind I need to invent the sheaf topos of geodesic balls...then you can never refuse to work on them ;)
@ACuriousMind My QM homework wants me to compute the mean and standard dev for a Gaussian. Doesn't the latter need complex analysis?
I remember once showing the calculation to my chem teacher in high school and I think it did
oh no that's for fourier transforming a Gaussian
 
vzn
vzn
1:06 AM
 
0celo7
0celo7
1:34 AM
@NeuroFuzzy omg please tell me you remember some E&M
 
NeuroFuzzy
NeuroFuzzy
I do!
 
0celo7
0celo7
Under what conditions will the electric field between the electrodes of a parallel plate configuration be uniform
I don't even know what this means
 
NeuroFuzzy
NeuroFuzzy
Hah! Ummm that's a bit of an awkward question. I would answer that the plates would have to be infinite planes, that's probably what the asker is going for.
 
0celo7
0celo7
Yes, that makes sense
Thanks
 
Bobbie D
Bobbie D
2:36 AM
Can someone help me figure out what my QM professor means by this question: s18.postimg.io/xvsbt6yqh/image.png ? Does he just want me to explicitly compute $\Psi(x,t)$? If so, doesn't that require a Hamiltonian?
 
NeuroFuzzy
NeuroFuzzy
3:12 AM
@BobbieD Equation 4 gives you the time evolution...
so sure, it's assumed there's some weird hamiltonian out there
but you don't need to know it
 
Bobbie D
Bobbie D
OK, so I need to find $f(k)$? Then I need to do a Fourier transform of $\Psi(x,0)$ to find $f(k)$ and then evaluate the integral in equation $4$ correct? I did that (using Mathematica) but I got a pretty nasty looking result.
So I assumed I must be doing it wrong.
 
0celo7
0celo7
Mathematica probably didn't simplify enough.
 
Bobbie D
Bobbie D
OK. Thanks. Now I know that I'm doing it correctly at least. :)
 
NeuroFuzzy
NeuroFuzzy
3:28 AM
@BobbieD I am bored and worked through it. It all works fine with Mathematica! You get a standard spreading wavepacket (With a terms along the lines of $\frac{1}{\sqrt{x+i t}}$)
Try using assumptions in your integration commands? I added an option to my integrate function, "Assumptions -> {a > 0, Element[k, Reals], m > 0, [HBar] > 0, Element[t, Reals]}"
("[HBar]" meaning I typed in "<escape>hb<escape>" to get the special character)
 
0celo7
0celo7
@NeuroFuzzy if you're really bored do you want to help me with some geometry
 
NeuroFuzzy
NeuroFuzzy
@0celo7 Lol, if I can plug it into mathematica in a few mins :P (JK, shoot!)
 
0celo7
0celo7
3
Q: Every Riemannian metric is conformally related to a complete metric

0celo7If $(M,g)$ is a Riemannian manifold, there is a metric $\tilde g=hg$ on $M$ which is complete, where $h$ is a positive smooth function. I've been given the hint to let $f:M\to \Bbb R$ be a smooth exhaustion function, which means that $f^{-1}(-\infty,c]$ is compact for all $c\in\Bbb R$. Then one s...

differential-geometry riemannian-geometry
 
Bobbie D
Bobbie D
I just worked it out by hand. Did you get $\dfrac{ae^{-k^2/(a^2+\hbar t/2m)}}{(2\pi a^2)^{1/4}\sqrt{a^2+\hbar t/2m}}$?
 
NeuroFuzzy
NeuroFuzzy
@0celo7 Can't help. I'll be taking a graduate differential geometry course next quarter, so I might be able to answer in two months or so!
 
0celo7
0celo7
3:33 AM
@NeuroFuzzy Maybe. Although it's doubtful you'll cover Riemannian geometry in one quarter.
Assuming they start from sctratch
 
NeuroFuzzy
NeuroFuzzy
@BobbieD By "k" do you mean "x"?
 
0celo7
0celo7
Although this problem should be elementary
 
Bobbie D
Bobbie D
Yes. Typo.
 
0celo7
0celo7
I'm using shit like geodesic balls but that seems really unnecessarily
 
NeuroFuzzy
NeuroFuzzy
@BobbieD I think that's almost correct, except I think you're missing an i in there. That it should be $\sqrt{a^2+i \hbar t/2m}$.
erm two i's. One in the exponent and one in the radical. (should be $\hbar t i / 2m$)
 
Bobbie D
Bobbie D
3:37 AM
Ah yes, I forgot the $i$'s. OK. Cool. Thanks!
 
0celo7
0celo7
Hmm
what if i use a really weird contradiction proof
like
I find a sequence so that $f$ blows up
now this sequence can't converge, since $f$ would blow up at a point, which violates continuity
similarly, $x_n$ can't have a convergent subsequence
because $f$ would blow up at the limit of the subsequence
of course it could converge on a limit point outside of the set
but that still means $f$ blows up somehow
I need the most general form of Bolzano...
Bolzano is equivalent to compactness, not good
 
 
1 hour later…
Secret
Secret
4:52 AM
sciencedaily.com/releases/2016/08/160829140420.htm Water is a major topic in friction researches
 
 
1 hour later…
Noah P
Noah P
6:14 AM
@DavidZ Is this on-topic?
0
Q: Physics of a Coffee Percolator

Noah PThis morning, as with every morning, I had my coffee. However, today it was burnt, because I slightly overfilled the water. I use an Italian Percolator on a gas top. Normally, you put it on to boil, and once you hear the water bubbling, the coffee is ready, and chamber C will be full of coff...

thermodynamics water convection
Or do I need to alter it?
 
John Rennie
John Rennie
@NoahP that strikes me as an engineering question rather than physics ...
 
Noah P
Noah P
@JohnRennie You think? :( I'm just wondering why the steam needs to be released for the thing to work
@DavidZ Could I reword it? I'm interested in the physics of why the pressure has to be released etc for the thing to function. Or is this just inherently an engineering problem?
 
David Z
David Z
6:31 AM
@NoahP Off the top of my head I can't think of a way to reword it to make it on topic here, but I'll let you know if I come up with anything.
 
Noah P
Noah P
@DavidZ Okay, thanks. The answer there at the moment is good, though confused as to how it was posted after you'd put it on hold! Hoping for another
 
David Z
David Z
@NoahP There's a grace period where if someone is writing up an answer while a question gets closed, they have some amount of time to post it after the closure.
 
Noah P
Noah P
@DavidZ Ahh okay, that makes sense
I'll end up accepting it if I'm not able to have it re-opened by the end of the day (UK, 7:30 am here)
 
David Z
David Z
Basically, understanding why the pressure has to be released for the coffee maker to work is an engineering thing (or, what we call engineering). It depends on the design of the device more than the physical principles of its operation.
 
Noah P
Noah P
I'm more interested in why the build up of pressure prevents the coffee from rising
I see how it comes across as engineering, but thats certainly not the intention
I'm more interested in the idea that there may be a pressure gradient or something @DavidZ
 
David Z
David Z
6:37 AM
That was an unnecessary ping ;-)
 
Noah P
Noah P
Apologies :P
 
David Z
David Z
No worries
If I were going about it, I'd try this: first do some research on the physical effect that allows the water/coffee to rise through the central pipe in the first place. At least to the point of knowing what it's called. Then you can rewrite your question to ask whether that physical effect is affected by pressure.
 
Noah P
Noah P
Okay, I'll have a go at that then
Would a ping be suitable if I manage to do that? :P
 
David Z
David Z
lol
Yeah, why not
 
Noah P
Noah P
Okay
 
David Z
David Z
6:39 AM
I mean, if I just posted a message within the last few minutes I'm probably still in the room and don't need to be pinged
but it's not a big deal, relatively
 
Noah P
Noah P
Sure, okay. Wasn't sure of 'ping etiquette' but that clears it up :P
 
David Z
David Z
With a question like yours, I'd suggest structuring it like this: (1) present the motivation for your question - that is, your observations of the coffee pot (2) discuss your efforts to learn about the physical principles underlying those observations (3) ask your question about the physical principles, in a way that isn't too specific to your motivation. You're basically missing step 2.
That's just off the top of my head, of course, so take it with a few grains of salt, but I think it should give you a sense of the direction to go in when you rewrite the question.
 
Noah P
Noah P
Okay, thanks, having a go now
 
Noah P
Noah P
6:55 AM
@DavidZ edited!
1
Q: Physics of a Coffee Moka Pot

Noah PThis morning, as with every morning, I had my coffee. However, today it was burnt, because I slightly overfilled the water. I use an Italian Percolator/Moka Pot on a gas top. Chamber A is filled with cold water, and B with ground coffee. As you heat the water, steam builds up, increasing the...

thermodynamics pressure water convection
 
David Z
David Z
I don't really see a meaningful difference. Sure, you added more words, but they're saying basically the same thing as the earlier version of the question.
 
Noah P
Noah P
Oh. I thought I'd explained how it works and made it more clear
 
David Z
David Z
I mean, to me it still looks like a question about a coffee pot.
 
Noah P
Noah P
Okay
@DavidZ Take 3, third time lucky?
 
David Z
David Z
Hm... well, I have to think about this. I read it over a couple more times, and maybe the physics is in there, but there are a bunch of irrelevant details about the coffee pot which are distracting from the main question IMO.
(above re: revision 2)
 
Noah P
Noah P
7:07 AM
If I take out the pointless stuff would that help?
 
David Z
David Z
Yeah, maybe. You could post a draft here in chat so we can review it without editing the post itself (until we're done).
 
Noah P
Noah P
Oops
Trimmed it down, there was one paragraph which seemed like an obvious candidate
I make that revision 3? :P
 
David Z
David Z
I guess you do have a 5-minute grace period to change your edit without making a new revision
So, yeah, removing that paragraph definitely helped IMO
 
Noah P
Noah P
Suitable for re opening now?
 
David Z
David Z
Maybe... it's getting closer, at least
 
Noah P
Noah P
7:12 AM
Arghh you're killing me
 
David Z
David Z
It takes time. I've got to read each new version enough times to push out the previous version from my memory :-P
 
Noah P
Noah P
Ahaha okay
 
David Z
David Z
eh, I guess it's fine now. Not a great question IMO but not worthy of unilateral closure.
The title sucks though
 
Noah P
Noah P
I'm open to suggestions
I couldn't think of what else to put
 
David Z
David Z
"Why wouldn't an increase in steam pressure force water out of a chamber faster?"
 
Noah P
Noah P
7:15 AM
Consider it done
 
David Z
David Z
Probably still not great, but way better than the current one
(if you edit that in, also strip out the "Thanks!")
and I would suggest putting the two pictures on one line
i.e. remove the line break between them in the Markdown
 
Noah P
Noah P
Done that now, its annoying how they dont quite line up
 
David Z
David Z
ahh, yeah it is. I think it's still better than before because it's easier to see the entire question. But you could download the images and crop them appropriately in an image editor.
 
Noah P
Noah P
I'm not quite that upset by it
 
user54412
@NoahP fwiw the "engineering" close reason has been misappropriated since it was developed 3 years ago
 
user54412
7:26 AM
It used to be that we got lots of questions like "I need to build a thing, can you give me the specs?" or "how can I construct some object in a cost-efficient manner in my factory?"
 
Noah P
Noah P
Ohh okay
 
user54412
So we delineated engineering as being the application of physical principles, with science being about the principles themselves.
 
user54412
Unfortunately, once those sorts of questions stopped coming in, the community found itself with a tool (the close reason) with no purpose, so the tool was tacitly but near-unanimously repurposed to close questions that give concrete mention to everyday objects.
 
Noah P
Noah P
Fair enough
 
user54412
We should probably revisit this at some point, because most of what is closed as "engineering" isn't such according to the meta post, so policy in practice has gone out of alignment with stated policy.
 
David Z
David Z
7:31 AM
Which meta post is that?
 
user54412
15
Q: Are engineering questions appropriate for this site?

Chris WhiteOften we get questions of the form "How to build this particular device?" or "Why was this design implemented in such-and-such product?" Are these questions appropriate for Physics Stackexchange?

discussion faq scope
 
David Z
David Z
Ah, right
 
Secret
Secret
https://www.youtube.com/watch?v=HnVtIgJ0SqY

Only the projection makes sense, otherwise all I see is a random jumble of reflective objects on a rotating stand.
I wonder if that's a feature of early days kinetic art, that it simply cannot brought out that impression when in 3 dimensional space
 
David Z
David Z
This is certainly something we can revisit, but I do believe that the first version of Noah's question qualified as engineering (or more importantly, not on topic) according to the meta post.
 
user54412
Actually, I think this is a datapoint that shows that disallowing things with policies really does work -- we stopped getting the sorts of questions that originally inspired the policy.
 
user54412
7:36 AM
The same thing happened with homework in a way. The policy emphasized showing work, and I'm pretty sure there was a precipitous drop in no-work homework questions, if nothing else.
 
David Z
David Z
@ChrisWhite We still get them from time to time.
It would be nice if there were a way to collect data on that without a whole lot of manual labor (reading and rating questions)
 
user54412
some, sure, but my intuition is that they're much less common
 
David Z
David Z
That could be
 
user54412
In other news, today I found out I was added to a (legitimate) email list... by getting a message about "online child sexual grooming." Can't wait to see what else comes through this list.
 
Secret
Secret
7:52 AM
@DavidZ a small question about particle phenomenology. When we are trying to find BSM particles, are we guided by the theorist models and search for collision patterns that matches those models, or do we also do some kind of systematic categorisation such as having the computer to filter away all common collision patterns such as gluon gluon productions, and group together similar patterns which cannot be matched to any known models for further analysis?
 
David Z
David Z
Both
 
Secret
Secret
So for the latter case, there is so far no interesting patterns that are recurrent enough that warrant a possibility of unknown particles?
 
David Z
David Z
Right, not currently. There was that 750 GeV bump that would have been such a pattern if it weren't just a statistical anomaly.
 
Secret
Secret
hmm I see
 
John Rennie
John Rennie
@0celo7 I actually took this seriously for a few minutes. I should know you better by now :-)
 
Secret
Secret
7:56 AM
It seems the next breakthrough is likely to be either related to the proton puzzle or the beryllium anormaly
 
David Z
David Z
Or dark matter
 
DanielSank
DanielSank
Wow, the real @ChrisWhite!
 
John Rennie
John Rennie
@DavidZ: since you're around and clearly have nothing better to do :-) Can I ask you to reconsider the close vote on:
1
Q: What would happen if a charge could travel at $c$?

user104372We know that an electron can move at $c-\epsilon$, but can we figure out what would happen if a charge moved at c, surfing the wave of the electric field it has produced? (If it sounds better, consider a Weyl fermion) It would be pushed forward, repelled by its own field, but there are 2 possibl...

speed-of-light
 
Secret
Secret
Speaking of that, some suggest to use the equally mysterious fast radio wave burst to probe for the possibility that dark matter are primodial black holes
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.091301
 
John Rennie
John Rennie
While we know of no massless particles with an electric charge I don't know of anything that forbids them.
 
DanielSank
DanielSank
7:59 AM
@JohnRennie Wouldn't that be very, very odd?
To me "charge" means "couples to electromagnetic field".
 
John Rennie
John Rennie
Presumably above the electroweak transition energy all particles are massless, though I'm not sure if we can talk about electric charge at these energies.
 
DanielSank
DanielSank
@JohnRennie o_O
 
John Rennie
John Rennie
Well gluons carry a (strong) charge and they are massless ...
 
Secret
Secret
We only knew that massless composite particles are not well defined in QFT as per discussion with ACM and (forgot name), but I too don't see what symmetry arguments will rule out massless elementary charged particles
electric charge by noethers theorem is tied to U(1) symmetry, right?
 
David Z
David Z
@JohnRennie If the question were edited to mention that and remove the last paragraph, I'd offer the 5th reopen vote
 
user54412
8:01 AM
@DanielSank o/
 
John Rennie
John Rennie
I would be quite interested to know if massless charged particles are prohibited by some law/symmetry, or if not how they would behave.
@DavidZ Done!
 
David Z
David Z
OK
 
user54412
Perhaps massless charged particles affect each other electromagnetically the same way they would gravitationally? As in, parallel paths do not affect one another, and antiparallel paths interact by a factor of 4 more than classically one would predict?
 
David Z
David Z
@JohnRennie Oh, wait, you didn't add the thing I wanted you to add, but that's okay, I made an edit of my own
 
user54412
hmmm, 4 is about the ratio of dark matter to normal matter...
 
user54412
8:05 AM
::goes off to write crackpot paper::
2
 
Secret
Secret
Well, for one , dark matter is not known to be charged, else we will have seen it electromagnetically a long time ago
also what if photons and massless charges coexists, then how will electromagnetism be modified?
 
DanielSank
DanielSank
@ChrisWhite ::Puts colander on head and follows::
 
Secret
Secret
we knew photons are the force carriers of EM. Will the existence of massless charges complicate this picture and somehow make EM shorter ranged than observed due to the extra possibility of self interactions, similar to what happens with gluons?
 
Noah P
Noah P
@DavidZ I posted it on engineering SE too, out of curiousity, and its going to be closed there...
 
DanielSank
DanielSank
Oh goody, are we debating what counts as "engineering" again?
Guess I'll link this as a point of reference:
30
Q: Experimental Physics & Engineering

Kyle KanosI know that most of us are "paper theorists," but I think we need to remember that experimental physics is physics too! We have several tags for experimental physics (with tag excerpts): experimental-technology Use this tag for questions pertaining to the limits, management, and operation ...

discussion scope close-reasons
 
Noah P
Noah P
8:15 AM
I fear I may have set off a small debate
 
DanielSank
DanielSank
@NoahP It's an old debate, and one with which I'm rather familiar.
Is there a post in danger of being closed?
 
Noah P
Noah P
To be fair, my initial question did fit the engineering close policy
It was closed, but opened again after editing
 
peterh
peterh
@DanielSank As I know, one of the mistakes the TheoPhys committed was that they made experimental physics offtopic
 
DanielSank
DanielSank
@NoahP Ah.
 
Noah P
Noah P
1
Q: Why wouldn't an increase in steam pressure force water out of a chamber faster?

Noah PThis morning, as with every morning, I had my coffee. However, today it was burnt, because I slightly overfilled the water. I use an Italian Percolator/Moka Pot on a gas top. Chamber A is filled with cold water, and B with ground coffee. As you heat the water, steam builds up, increasing the ...

thermodynamics pressure water convection
That one
If you look at the edit history you'll see what i mean
 
DanielSank
DanielSank
8:20 AM
At this point I'm more interested in understanding how your coffee maker works.
The cartoon diagram is good but more info would be nice.
 
Noah P
Noah P
The water in Chamber A is heated
 
user54412
^^ agreed -- this has got to be the single most complicated way to make coffee
 
Noah P
Noah P
Pressure increases because of the steam
And then the water is forced through B
Up the small pipe
and flows into C
And then you get to drink the liquid gold
 
DanielSank
DanielSank
@ChrisWhite Hahaha, no way dude.
This is more complex:
 
John Rennie
John Rennie
Moka pot
The moka pot, also known as a macchinetta (literally "small machine"), is a stove-top or electric coffee maker that produces coffee by passing boiling water pressurized by steam through ground coffee. It was patented for the first time in Italy by the inventor Luigi De Ponti for Alfonso Bialetti, in 1933. Bialetti Industrie continues to produce the same model under the name "Moka Express". The moka pot is most commonly used in Europe (especially Italy) and in Latin America. It has become an iconic design, displayed in modern industrial art and design museums such as the Wolfsonian-FIU, Museum of...
 
Noah P
Noah P
8:23 AM
Ooh what is that? @DanielSank
@JohnRennie Join the party
 
DanielSank
DanielSank
@NoahP Japanese coffee maker.
 
Noah P
Noah P
Is there an amazon link to that?
 
DanielSank
DanielSank
A dude who worked in the same department as me while I was in grad school used a device like that.
He took coffee making seriously. He parametrized the process and did systematic studies of how flavor and other qualities depend on each parameter.
He outfitted his espresso machine with pressure gauges to help collect the data.
 
John Rennie
John Rennie
@NoahP I mostly drink instant coffee and cheap instant coffee at that. For me coffee is sonething that provides a bit of distraction while I'm in super-concentration mode. I frequently reach for my mug and discover it's empty with no memory of having drunk any of it.
 
DanielSank
DanielSank
Best coffee I've ever had by a huge margin.
 
Noah P
Noah P
8:25 AM
Oh wow
@JohnRennie Ahh okay. I'm one of those strange people who enjoys it and enjoys drinking it
 
DanielSank
DanielSank
Yeah, his box of coffee tools looked a lot like what you use in the microfabrication cleanroom.
 
user54412
@DanielSank Is that an alembic? Are you distilling lead into gold?
 
DanielSank
DanielSank
Notebook, timer, various glassware, etc.
 
Noah P
Noah P
My my
 
DanielSank
DanielSank
@ChrisWhite Duh. All the cool kids are doing it.
@NoahP Ask and ye shall receive.
 
Noah P
Noah P
8:27 AM
Ah, that has a burner attached
 
DanielSank
DanielSank
wtf, we get worldbuilding.se notifications now?
Ok I think I understand the failure of the coffee maker if the water is filled too high.
 
Noah P
Noah P
Ooh go on
 
DanielSank
DanielSank
Floris mostly got it, but I think it could be stated more clearly.
 
Noah P
Noah P
Could you make an answer out of it?
 
DanielSank
DanielSank
Well just a minute.
Let's see if I'm being smart or dumb first :)
No sense in posting misinformation.
 
Noah P
Noah P
8:30 AM
Okay
 
DanielSank
DanielSank
If the water line is below the valve, then excess pressure is relieved by steam leaving the safety valve.
 
Noah P
Noah P
Yes
 
DanielSank
DanielSank
However, if the water line is above the valve, then excess pressure is relieved by water leaving the safety valve.
 
Noah P
Noah P
Yes, that makes sense
 
DanielSank
DanielSank
So here's what I'd like to know: during normal proper operation, can you feel steam coming from the safety valve? Even a little bit.
 
Noah P
Noah P
8:31 AM
Yep
 
DanielSank
DanielSank
Eurika!
Alright, so we've established that normal operation increases the chamber A pressure above the safety valve threshold.
Therefore, if you overfill, then during normal operation we will get water flowing through the safety valve.
 
Noah P
Noah P
Sure
 
DanielSank
DanielSank
Now, I would think that this shouldn't matter.
 
Noah P
Noah P
Thats what I thought
 
DanielSank
DanielSank
In either case the valve should simply keep the chamber A pressure at a certain maximum level.
However, it is possible that if water flows through the valve, then the valve ceases to function properly.
This is not a crazy thing to happen.
 
Noah P
Noah P
8:33 AM
Ahh
 
DanielSank
DanielSank
I'm not saying I'm sure. It's a possibility.
If that were the case, then we'd have solved the mystery of the crappy tasting coffee!
@ChrisWhite Dr. White, a consultation?
Oh also, @NoahP I don't think we've met. Hi, I'm Daniel.
 
Noah P
Noah P
Hi, I'm Noah
:P
 
DanielSank
DanielSank
Noah, your profile description is hilarious.
 
Noah P
Noah P
So the malfunction of the valve releases too much pressure, meaning the water has to boil to reach the pressure needed to force water through into chamber C, which in turn burns the coffee?
And I have no idea what it is
I don't get the hilarity of it? :P
 
DanielSank
DanielSank
@NoahP It's brief and to the point. I dunno, it's just funny when compared to others that include flowery quotations from famous physicists, etc.
 
Noah P
Noah P
8:39 AM
Ahh okay
Glad it amused you :)
 
DanielSank
DanielSank
@NoahP I'm not sure.
My hypothesis would explain why overfilling produces less liquid in chamber C.
I suppose it would also lead to longer extraction times, which yields sour coffee, I think.
 
Noah P
Noah P
Yeah, and the increased temperature would burn the coffee
 
user54412
seems reasonable
 
Noah P
Noah P
Worth an answer
 
DanielSank
DanielSank
@NoahP I dunno. Do we really want to post speculations and just-so stories?
 
user54412
8:42 AM
I wonder if a compressibility effect is at play too.
 
Noah P
Noah P
Seems like a valid postulation to me
 
DanielSank
DanielSank
@ChrisWhite Thought about that but I'm not seeing it.
@NoahP Yeah but generally we avoid that around here.
 
Noah P
Noah P
Ahh ok
 
user54412
I'm not quite seeing it either, but it just comes to mind whenever there is both water and vapor in a system.
 
DanielSank
DanielSank
If two people both posted reasonable but different explanations, how could you choose?
@ChrisWhite Indeed.
Wait a minute.
That valve probably introduces a non-negligible flow impedance.
I mean, even when open.
The water flow rate with the valve open is probably lowered in such a way that the pressure isn't limited to the design value.
Hmmmm, that doesn't explain the reduced filling of chamber C.
This would make a great practical qualifier exam problem.
Is there a thermometer on that thing, @NoahP?
 
Noah P
Noah P
8:48 AM
Nope, you just take it off when it makes the boiling noise (when it works normally)
 
DanielSank
DanielSank
darn
 
Noah P
Noah P
Would be no way to insert one either
 
DanielSank
DanielSank
...and I don't suppose you've got it outfitted with a pressure gauge?
 
Noah P
Noah P
No chance
 
DanielSank
DanielSank
Gotta go to bed.
I'll think about this tomorrow.
 
Noah P
Noah P
8:50 AM
Okay then
 
DanielSank
DanielSank
If you can take more data that might help ;)
 
Noah P
Noah P
I'm not really sure how I would :P
 
Secret
Secret
http://arxiv.org/abs/1608.01683

(Learnt from NewScientist)
Causality in superposition. Mathematically it is not very surprising because two measurements are in general noncommutative thus the order matters. While this experiment have demonstrate how to prove two causal processes are non separable, hence in a superposition, I wonder if we can go one step further by getting some kind of interference pattern out of it that is based on the superposition of the control bit?
 
Secret
Secret
(Warning, extreme speculation alert). If this effect is scalable all the way to the macroscopic level of histories, then this experiment suggest an implication that perhaps the probability of what event we experienced some infinitesimal time in the future is actually depends on the superposition of all other histories, and all points to the past and future that are within some coherence "range" (maybe spatial or temporal I am not sure)
In other words, maybe initially it is all random versions of histories distributed randomly in spacetime and then their superposition somehow give rise to some overwhemly high probability in one direction in time that lead to what is known as the future direction
that is, causality might be an emergent phenomenon rather than fundamental
 
Secret
Secret
9:12 AM
(Back to the experiment comments)
I think the next thing they should do is to push the limits further on how many measurements, how far and how long the causality superposition can sustain. If this is macroscopic it will surely help on the quantum communication researches in increasing the calculation of some problems
 
user116211
And indeed, in real life, I speak fast enough that you don't realise you are hearing 1000 something words in a few minutes — Secret 4 hours ago
 
user116211
@Secret: kudos ;P
 
Secret
Secret
(Even more speculation) If this causality superposition can manifest at the level of histories and histories can be described using hilbert spaces, then it should be possible to evolve it in such a way so that time flows backwards because you might just end up projecting to a base state that form this superposition where causality is reversed
Having said that, I don't think histories exhibit superposition as there are too many interactions to otherwise decohere it like crazy
another reason is that perhaps at the scale of histories, perhaps the notion of superposition is not even a well defined concept or does not exist
> Kid, baseless quantum speculation is not very healthy. Please go back to study
 
 
3 hours later…
Emilio Pisanty
Emilio Pisanty
12:14 PM
@NoahP @DanielSank @ChrisWhite @JohnRennie and others in the cofee-maker debate may find this one interesting
 
0celo7
0celo7
12:26 PM
@JohnRennie what did you take seriously?
 
Jim
Jim
10
Q: How many photons are there in the universe?

physnolimitsAssuming that the universe is a sphere with a radius of $10^{25}$ m and the medium temperature is 3K, how many photons there are in the universe?

homework-and-exercises thermodynamics cosmology thermal-radiation cmb
I'm vtc as "do this math problem for me"
 
John Duffield
John Duffield
@JohnRennie : you can't have charge without mass.
 
Jim
Jim
@JohnDuffield not with that attitude
 
KNP
KNP
Hey can someone help me with choosing university units?
 
John Duffield
John Duffield
@Jim : and when it comes to particles, you can't have mass without charge. Even a neutron has charge. It just doesn't have any net charge.
 
Jim
Jim
12:39 PM
@KNP potentially. What is the choice?
@JohnDuffield whether or not it is the correct attitude doesn't change the fact that you can't do it if you hold that attitude. Thus, I can keep saying "not with that attitude"
 
Secret
Secret
http://copaseticflow.blogspot.com.au/2012/09/an-intuitive-way-to-spherical-gradient.html

Spherical coordinates with no chain rule. YMMV
 
John Duffield
John Duffield
@Jim : you can't do it because of the wave nature of matter.
 
Jim
Jim
@JohnDuffield not with that attitude
 
KNP
KNP
@Jim Next semester is my third year and I'm studying physics and maths. I cannot decide between two choices: I can get graduate courses, cosmology and particle physics. It's the first option. Second I can select bachelor courses but in various topics like chemistry or solid state. I dunno go in details or just spread my superficial knowledge :/
 
Jim
Jim
are you planning to do a graduate degree?
 
KNP
KNP
12:45 PM
Yeah but I would finish my double bachelor in three years later.
 
Secret
Secret
it depends on whether you want to focus more on chemistry and solid state, or whether you prefer more cosmology and high energy physics. Which one sounds more like your cup of tea?
 
John Duffield
John Duffield
@Jim : photon momentum is a measure of resistance to change-in-motion for a wave propagating at c. But when you trap that photon in a mirror box, it increases the mass of the system. Because mass is resistance to change-in-motion for a wave going round and round at c.
 
Jim
Jim
@JohnDuffield What's your point?
 
KNP
KNP
Of course I prefer fundamental physics so I have to be comfortable with cosmology and particle physics but I alternatively I can choose more bachelor courses. What's your idea if you were me?!
 
Jim
Jim
@KNP I'm a cosmologist. So my opinion would be biased
 
Secret
Secret
12:48 PM
My opinion is to alway go with your passion unless you have a much stronger reason to not do so
 
Jim
Jim
@Secret Generally true: Do whatever you want unless you have a good reason not to
 
Secret
Secret
For me, to 1st order, my interest in chemistry, condensed matter and fundamanetla physics are equal, thus I am not baised at 1st order
 
Jim
Jim
what about 2nd order?
 
KNP
KNP
@Jim und do you offer a junior student who wants to be more comfortable with GR's maths to get Manifolds course or differential geometry from maths department?
 
Jim
Jim
@KNP that's a tough one. I'd lean towards manifolds. But diff geo is also quite useful
 
John Duffield
John Duffield
12:51 PM
@Jim : like I said. You can't have charge without mass. See what Wilczek said: "Today we trace Bohr's rules to the fact that the proper quantum mechanical description of electrons involves wave functions, whose oscillation patterns are standing waves."
It's like the photon in the box. Minus the box.
 
KNP
KNP
@Secret for me It's fundamental physics and maths. But I really like to know more about chemistry and molecules generally as well
 
John Rennie
John Rennie
@EmilioPisanty how could there be a better use of beam time? If only I'd thought of this when I was doing neutron scattering at Grenoble :-)
 
Jim
Jim
@JohnDuffield This is going to get awfully tedious. At some point, you need to realize the joke of my saying "not with that attitude"
 
peterh
peterh
@JohnDuffield Gluons have color charge without a mass.
 
KNP
KNP
@Jim got it thanks!
 
peterh
peterh
12:53 PM
@JohnDuffield Although my naive non-QFT intuition says, they should get energy in eachother's strong field, and thus get mass.
 
John Rennie
John Rennie
@0celo7 your claim that you were going to mix up cerium and hydrofluoric acid and heat the whole lot to 1700C (presumably in a pressure vessel). But if you ever decide you're really going to do this let me know and I'll video it so we can put on YouTube :-)
 
Secret
Secret
Speaking about HF, probably because of 0celo7's discussion, I had a quite horrible dreamt yesterday about that there is a high school analytical chemsitry session and they use boiling HF to distill the analyte. Got my hands spilled with the stuff and feared I will die soon. Luckily the technician took my sample and said I am fine as my body don't have HF in it
 
John Duffield
John Duffield
 
peterh
peterh
@JohnDuffield What about in QGP?
 
Secret
Secret
@KNP How deep in chemistry do you want to go? also what is your double bachelor on besides physics?
 
John Duffield
John Duffield
12:58 PM
@0celo7 : and maybe we could take some photographs afterwards.
@peterh : IMHO a QGP is rather like pea soup. There are no actual peas in pea soup.
 
peterh
peterh
@JohnDuffield For me seems this virtual-real distinction so phylosophic. You can't see also a "real" (i.e. non-virtual) photon, except if you interact with it, what means, you destroy it. Its energy will be used up to excite rodophsin in your retina.
 
Jim
Jim
@JohnDuffield THERE ARE NO PEAS IN PEA SOUP!? What else haven't you told me!? This changes everything. My whole life is turning upside down
 
John Duffield
John Duffield
@peterh : I can make a real electron (and a real positron) out of real photons.
 
Secret
Secret
@Jim It still amaze me how QGP is fluid like according to the LHC reports
 
peterh
peterh
@JohnDuffield It is quite interesting to compare your world view with acms. He seems for me to deny the existence of a reality behind the formulas. You seems to me denying the formulas behind the reality.
 
Jim
Jim
1:04 PM
@Secret I'm sorry, I can't bother thinking about that right now. I just learned there are no actual peas in pea soup. Why aren't more people concerned about this?
 
John Duffield
John Duffield
@peterh : I'm not denying the formulae. I'm just pointing out that some things in the formula are abstract things rather than real things. A proton is a real particle. It's modelled as being made up of quarks and gluons. But the gluons in a proton are virtual particles. A proton is not some mess of quarks and gluons ready to spill out like beans from a bag.
Or peas.
 
Secret
Secret
That reminds of a glue ball...
::Protip, I can become drunk when seeing exceptionally rrrroooooouuunnnnddd objects::
 
John Duffield
John Duffield
@Secret : glueballs remain hypothetical.
Right, I'm off to the dentists. Bye.
 
0celo7
0celo7
@JohnRennie no not in a pressure vessel
Just in our tube furnace in 100% oxygen
 
Jim
Jim
@JohnDuffield what are those lumpy floaty bits in the soup if not peas?
 
KNP
KNP
1:27 PM
@Secret i want to know how quantum mechanics is used to describe matters characteristics like triple point temperature and so on. And I've doubled in math
And of course statistical mechanica and thermodynamics
 
Secret
Secret
Then it sounds like condensed matter course will be a good choice (because a lot of stat mech and quantum is used to explain the phase transitions in solid states systems). As for chemistry, your interest seemed to suggest that computational and quantum chemistry (both are like 2-3rd year courses) will be most aligned with your chemistry side of your interest
 
0celo7
0celo7
@ACuriousMind I had a dream!
 
Secret
Secret
Describe dream
 
0celo7
0celo7
What if I take $h$ to be a Whitney-smoothed supremum of balls on which $f$ is bounded
@Secret I was talking to Riemann and he suggested this
 
Secret
Secret
well, does dream riemann's proof hold when you check it in your woken life?
 
0celo7
0celo7
1:42 PM
Not sure
I need $$r(x)=\sup\{r>0\mid \text{$f$ is bounded on $B_x(r)$}\}.$$
Then Whitney smooth so that $h(x)>r(x)$.
Maybe it should be $h^2>r$.
 
Noah P
Noah P
How many flipping dreams do you have? @0celo7
 
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