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Sir Cumference
Sir Cumference
12:10 AM
Howdy
 
FenderLesPaul
FenderLesPaul
12:36 AM
@BernardMeurer white boards suck
 
Bernard Meurer
Bernard Meurer
@FenderLesPaul You suck :(
White boards save my poor eyes
against the oppression of chalk
@ACuriousMind You there?
 
 
1 hour later…
Downgoat
Downgoat
1:49 AM
Hello, is anyone here? I have a physics question...
 
Helka Homba
Helka Homba
How can we help?
 
Downgoat
Downgoat
Hello @HelkaHomba do you know physics?
 
user116211
Go ahead @Downgoat...... any willing one would answer...
 
user116211
when they are available...
 
Downgoat
Downgoat
So I was trying to make a gravity simulator and I got this code...
 
Helka Homba
Helka Homba
1:50 AM
@Downgoat I know of physics
 
Downgoat
Downgoat
planet.velocityX += (Sun.mass / Math.pow(Sun.x - planet.x, 2)) / 1e10;
planet.velocityY += (Sun.mass / Math.pow(Sun.y - planet.y, 2)) / 1e10;

planet.x += planet.velocityX;
planet.y += planet.velocityY;
I'd appreciate any help :3
 
user116211
@skillpatrol: o/
 
skill patrol
skill patrol
\o @MAFIA36790
 
Bernard Meurer
Bernard Meurer
@Downgoat You here?
What a pleasure
 
Downgoat
Downgoat
@BernardMeurer yup
 
Bernard Meurer
Bernard Meurer
2:01 AM
Interesting bug
 
dmckee
dmckee
@Downgoat Your calculations here are obviously incorrect.
The thing in the denominator that must be squared is the radius, not a component thereof. Only after you have the magnitude of the force do you find it's components.
And yes, there are ways to shortcut it, but what you've done here is not one of them.
Secondly, you need to make sure that the resulting force is attractive which you have not done here.
Finally, this simply quadrature is not symplectic and will have a tendency to slowly add or subtract energy to the system even after you fix the surface bugs (which might be OK if you are doing it for Programming Puzzles & Code Golf, but probably isn't for any other purpose).
The game you tried to play there will only work is the vector (in this case the vector between the positions of the two bodies) appear to the power 1 (i.e. linearly and in the numerator).
 
Downgoat
Downgoat
@dmckee when you so it's not symplectic, I went to the wikipedia page and it does say "celestial mechanics", so am I misunderstanding what you're saying?
 
dmckee
dmckee
2:17 AM
@Downgoat You've misunderstood. Using a algorithm for celestial mechanics doesn't make it symplectic, being symplectic makes it suitable or proper for doing celestial mechanics.
The identifying characteristic of symplectic integrators is that they conserve energy properly. The one you're tying to use does not (as almost everyone who has tried to write this kind of simulation without recurse to a textbook has found out after they got the bugs worked out; my experience doing so was about 25 years ago).
But don't worry about that yet. Start by solving the surface bugs so that it will at least almost work. It's not worth trying to puzzle out symplectic methods until you've got the basics under control.
 
Downgoat
Downgoat
hm, okay... I guess I'll have to read more on what I'm doing before I continue... One more question I have is, when you say "that must be squared is the radius", do you mean the distance between the two bodies?
 
dmckee
dmckee
@Downgoat Yep. Sorry about the unclear language. I was teaching the central force chapter of classical mechanics a couple of weeks ago and slipped into the habit of assuming we were working in the CoM/reduced-mass framework.
 
Sir Cumference
Sir Cumference
@dmckee Hey Smithy, can you do me a favor?
Check over this code
 
 
4 hours later…
user54412
6:34 AM
@Downgoat I feel the urge to plug this.
2
 
lucas
lucas
6:52 AM
Hello. I have a question. May someone answer me please?
Is there anyone?
 
Physics Meta
Physics Meta
7:17 AM
0
Q: How can I type math?

lucasI don't know how I can type math in physics.stackexchange when I ask a question or answer. I type math in the Office word and use print screen. May anyone help me?

support
 
 
2 hours later…
user116211
9:16 AM
 
user116211
Damn, made a mistake.... please reject this edit.
 
ACuriousMind
ACuriousMind
10:26 AM
Refer a psychiatrist or keep an iron knife next to you. It stops bad dreams. — Anubhav Goel 24 mins ago
...what?
Is this a joke I'm not getting, or merely a non-sensical comment (the iron knife part)?
 
David Z
David Z
I don't get it either
In any case, joke or not, I don't see how it's constructive, so I deleted it.
 
ACuriousMind
ACuriousMind
Googling shows me that it is a folklore belief that putting a knife under your pillow (or sometimes specifically an iron knife) wards against bad dreams
 
David Z
David Z
Hm, interesting. Still not a constructive comment, but interesting.
 
ACuriousMind
ACuriousMind
Yeah, I agree it wasn't constructive
The Google hits are rare though, it seems not to be very widespread, or at least not very well-documented
 
David Z
David Z
@ChrisWhite I've been thinking. Of the few computational questions we get, most of them seem to be about this issue with non-symplectic integrators. Should we have a canonical question+answer on that? Or should we just point people to that question on Computational Science? The latter would be effectively declaring such questions off topic for us.
 
Slereah
Slereah
11:01 AM
I would not suggest sleeping with a sharp object in your bed
One time I had some thumbtacks in my bed, it made for an uncomfortable awakening
 
Xylius
Xylius
11:35 AM
Well, it doesn't have to be sharp..
A knife can technically be absent mindedly defined as being a post sharpened object that could once cut..
And you could always put a sheath on it..
 
skill patrol
skill patrol
12:04 PM
the comment was "obviously" made tongue in cheek :P
without really meaning what one is saying or writing.
 
Slereah
Slereah
I bought some chocolate
My diet :O
 
skill patrol
skill patrol
dark or white?
 
Slereah
Slereah
 
skill patrol
skill patrol
12:47 PM
bon appetit :-)
 
yuggib
yuggib
1:11 PM
Equicontinuous functions are nasty
or better, equicontinuous functions are nice but proving equicontinuity is nasty
 
Jiminion
Jiminion
So this is a poor representation of vacuum energy?

https://i.ytimg.com/vi/J3xLuZNKhlY/hqdefault.jpg
 
ACuriousMind
ACuriousMind
@Jiminion I have no idea what that picture is supposed to depict.
 
Slereah
Slereah
I guess it's a measurement?
Energy measurement of the vacuum
mb
I dunno
 
Jiminion
Jiminion
@ACuriousMind It is part of a video showing stuff winking in and out of existence. It is supposed to be what's happening inside a proton (and responsible for most of its mass).
 
Slereah
Slereah
Well maybe try to find how they actually got this picture
 
Jiminion
Jiminion
1:21 PM
I think it was by Mather and Smoot?
 
Secret
Secret
2:07 PM
Originally I planned to ask this question inspired from today's fuel cell lecture, but a bit of googling found the answer

If resisance is defined to be dV/dI, then strange result will be obtained when you tried to take the slope near the peak power...

The fact that resistance is V/I and V/I is linear means we are good
 
ACuriousMind
ACuriousMind
@JohnRennie @AccidentalFourierTransform Why did you vote to leave open here? The question is obviously too broad.
 
Slereah
Slereah
You're too broad
 
skill patrol
skill patrol
2:26 PM
 
AccidentalFourierTransform
AccidentalFourierTransform
@ACuriousMind for several reasons, but mainly because the question blew up, and IMHO it's the best question in the "hottests questions" list this month.
also, the question is supposed to be turned into "community post", which are always too broad, so I thought to let it be
 
ACuriousMind
ACuriousMind
@AccidentalFourierTransform Um...it's not supposed to, we don't use community wiki like that here. The only current use case for community wiki are resource recommendation questions.
 
AccidentalFourierTransform
AccidentalFourierTransform
well, the question reads "I suggest, at some point, we collate all of the answers into a single community post."
I'm new here
 
John Rennie
John Rennie
@ACuriousMind yes I agree it's not really a good fit for the Physics SE, but I was curious to see what the answers would be. So I suppose I voted for selfish reasons really.
And I think it has attracted some fascinating answers.
 
AccidentalFourierTransform
AccidentalFourierTransform
I thought it was the standard procedure to turn list-questions into community posts, but I guess I was wrong
 
ACuriousMind
ACuriousMind
2:39 PM
@AccidentalFourierTransform MathOverflow and math.SE do that, I think, but we have never established that here as far as I know
 
AccidentalFourierTransform
AccidentalFourierTransform
@ACuriousMind and, well, I like the question and I enjoyed reading the answers :-)
anyway, why did you ask me and JR, but not JD haha
 
ACuriousMind
ACuriousMind
@JohnRennie Yes, it has. It's a good question - for another site. I don't like the idea of leaving open a question that is formally clearly a big list question without clear answer because it is interesting since it suggests we only stick to policy to get rid of questions we don't like.
@AccidentalFourierTransform Well...I would have to lift the ignore to see his reply :P
 
AccidentalFourierTransform
AccidentalFourierTransform
@ACuriousMind and I must admit my decision was partially influenced by that gluon question from last week.
 
John Rennie
John Rennie
@ACuriousMind I can't argue with you since I'm clearly in the wrong on this one, but I'm unconvinced it's doing the site any harm. Given that I routinely run out of close votes by midday I would much sooner use them for the floods of homework questions.
 
AccidentalFourierTransform
AccidentalFourierTransform
both questions were off-topic, and both of them got several thousands of views
but this time the question is about physics
I dont like the idea of closing questions that arouse that much interest
 
John Rennie
John Rennie
2:49 PM
@AccidentalFourierTransform what was the gluon question from last week?
 
ACuriousMind
ACuriousMind
@JohnRennie This one
 
AccidentalFourierTransform
AccidentalFourierTransform
@JohnRennie yes, from last Thursday
 
ACuriousMind
ACuriousMind
@AccidentalFourierTransform But that interest is mainly off-site interest from users who do not participate here.
I think how many views or votes a question has should have no influence one whether it is on- or off-topic
 
John Rennie
John Rennie
Hmm, I thought that question was just crap. I voted to close it, though it seems I'm in the minority on this one.
 
ACuriousMind
ACuriousMind
@JohnRennie Well, the question is crap. It's just not off-topic. It also took far longer for reopening than it took for closing, so I'm not sure you are in a minority.
 
John Rennie
John Rennie
2:53 PM
Ah, I didn't realise it had been closed then reopened. I'm tempted to vote to close again :-)
 
Obliv
Obliv
@BernardMeurer What do you mean? I didn't say I needed anything :o
 
AccidentalFourierTransform
AccidentalFourierTransform
@JohnRennie lets do it ;-)
 
ACuriousMind
ACuriousMind
@JohnRennie I don't think you can vote again, but try it.
 
Obliv
Obliv
did you get confused by the guy above your message having a similar profile picture lol?
 
AccidentalFourierTransform
AccidentalFourierTransform
@ACuriousMind Im not sure the interest is from users that don't participate here. After all, the closing of the question provoked quite a stir from high rep users...
 
John Rennie
John Rennie
2:55 PM
@ACuriousMind just VTC'd :-)
 
Secret
Secret
Personally, I think it is a trend that questions are getting more and more cross discipline
while I don't have much comment on this particular one (other than it is an interesting angle to look at this particular topic, thus very narrow), similar cross discipline questions are going to be widespread as problems become harder and harder to solve by just one discipline
 
John Rennie
John Rennie
There's nothing disciplined about that question. The answer is that anything that used gluons to see couldn't farther than a femtometre and would quickly be eaten by other organisms whose eyes used boring old photons.
 
Physics Meta
Physics Meta
0
Q: Canonical question for two masses falling in Newtonian gravity

David ZSome variation of this question gets asked a lot: We have 2 point masses, $m$ and $M$ in a perfect world separated by radius $r$. Starting from rest, they both begin to accelerate towards each other. So we have the gravitational force between them as: $$F_g = \frac{GMm}{r^2}$$ How d...

discussion questions duplicates canonical-questions
 
Obliv
Obliv
if something is increasing eventually but maybe not immediately (there are deviations) do we say it is generally increasing? like say $p(x)$ increases when $x$ increases by some order of magnitude but it may be immediately increasing/decreasing for increments below that order of magnitude of $x$?
@JohnRennie Well the discipline they are referring to are fields/areas of studies. You would have to know physics to know how seeing gluons would impact evolution, correct? You also know some bio because you knew the organism would be at a disadvantage. So, it is cross-disciplined.
 
John Rennie
John Rennie
3:11 PM
@Obliv I'm afraid I think it's a stupid question. If you known what gluons are you know it's ridiculous to suggest them as a mechanism for sight. If you don't know what gluons are you shouldn't be mentioning them in a question like that.
5
 
Obliv
Obliv
Tbh I don't even know how one could use gluons to see.
gluons don't travel very far i'm guessing?
maybe a better question would be: could there exist very small organisms that use gluons to 'see'?
veerrry small
 
Secret
Secret
IMO, if I am a strict person I will agree that question is really stupid because the answer is so trivial

However what makes it interesting (at least to me) is because not many people think about angles like this

There are a lot of other questions that are MUCH less stupid than this, but still suffer from the same problem of being asked, because people are generally not aware that such question can exist in the first place

That is, (at least to me), I found a question in general can be hard not because its answer is hard (often times it is very trivial) but because of our tendency to ove…
 
Slereah
Slereah
Let's see if Arxiv has some cool new papers
 
Secret
Secret
The above then caused me to wonder and generated a meta question (NB it is not very physics but more psychology or neuroscience):

What are the known bias and other things that can cause people to overlook the possibility to ask some question, such as very good questions that have complicated answers but remain undiscovered until muhc sooner. What governs the discovery of those interesting (and sometimes important) questions in general?
The more I learn the more I realise how hard is to be aware or to discover a good question
 
Slereah
Slereah
"Wormholes minimally violating the null energy condition"
Oh Lobo
Why is Lattice Field Theory so focused on QCD
Wouldn't QED be a better introduction
 
Secret
Secret
3:22 PM
In conlusion there are a couple things that makes good questions hard:
1. The answer itself, obviously
2. How to ask the question so that others can understand it
3. What question to ask
*4. How to avoid overlooking really nice questions
 
Slereah
Slereah
Much better
 
ACuriousMind
ACuriousMind
@Slereah Um...because QED is perturbative, so there's no need for lattice methods,, and you don't see the main ideas of putting a non-Abelian gauge theory on a lattice in QED?
 
Obliv
Obliv
@Secret that's like asking how do the minds of geniuses work? I was always under the assumption that normal people had a narrower outlook on a question whereas geniuses were able to see the question with a larger field of view (more creativity, seeing connections, etc)
 
Slereah
Slereah
"As a result we obtain a discrete version of an algebraic Quantum Field Theory"
noooo
Not again
@ACuriousMind Sure but for an introduction
Would it not be gentler
 
ACuriousMind
ACuriousMind
For an introduction you usually put scalar fields alone on a lattice
 
Slereah
Slereah
3:25 PM
Yeah, or that
 
ACuriousMind
ACuriousMind
At least, that's what I got as an intro
 
Slereah
Slereah
But they went straight for QCD in that intro
Skadoosh
A bit harsh if you ask me
That's like if a GR book uses the Mixmaster metric as the first case
what's a good intro to scalar fields in LFT that isn't just a powerpoint presentation
 
Secret
Secret
@obliv
To elaborate point 4, consider the following:
(NB below is not intended to be mathematic, so please don't say I use maths wrongly because there is no maths in what I am going to say below, but that's really the best I can do to present it in English because my mind is like a computer more than a person)

Suppose in a given topic we have a bunch of questions Q1, Q2, Q3 etc.
Some questions such as Q1 have easy assumptions and easy answers which anyone with enough common sense can ask (and does not provide any insights). These are the stupid questions to most
IMO discovering the good questions is not unlike hunting for rare particles in colliders. Even worse than particles, questions are vague entities/concepts and generally don't obey conservation laws, thus we don't have a way in general to discover them (at least based on what I know...)
 
Slereah
Slereah
Let's see
 
Obliv
Obliv
@Secret well yes, you don't need to have a huge memory to recite $\pi$ if you're reading it off a piece of paper. If you somehow managed to have a control group of people of varying genetics to live the life of Einstein or any other significant scientists, you would get varying results on whether they asked Q3. I think it's how the brains develop through genetics & environment that lead people to be able to ask questions like Q3.
you need to think like someone who is capable of asking Q3 AND be proficient in some field of study
 
Slereah
Slereah
3:37 PM
I know 10 decimals of pi and it's basically useless
 
Obliv
Obliv
@Secret i think you would enjoy talks by Nick Bostrom if you haven't heard of him already. He talks about increasing the discovery rate of Q3 sometimes.
 
Slereah
Slereah
$\pi = 3$ is pretty much all you need
$3 + \frac{1}{8}$, tops
 
Obliv
Obliv
I hope you're not studying to be an engineer @slereah lol
you would cause a lot of damage :p
 
Slereah
Slereah
Man I've seen some astrophysicists not even caring if they had a margin of error of two orders of magnitude
 
Secret
Secret
1. be proficient in some field of study: This is hard but straightforward
2. capable of asking Q3: I sometimes felt like some luck is needed even if you are very proficient in the field. It's really like if you encounter a neutrino, whether it causes a recoil in your atoms, no matter how hard you try to look for them
 
Slereah
Slereah
3:40 PM
Ancient Egypt used $3 + \frac{1}{8}$ and they built the pyramids!
 
Secret
Secret
@Obliv Hmm, will check that out
 
David Z
David Z
@AccidentalFourierTransform we don't use CW mode for anything except resource recommendations (which I was opposed to)
 
Obliv
Obliv
I agree I think sometimes you just have to be in the right place at the right time doing something to see something profound. This applies mostly to physics though @secret
@Slereah try building something modern and let me know if it works out. Anything that moves in circles is preferred :p
wait would building an accelerator be impossible if you have the wrong number for $\pi$ :O
 
AccidentalFourierTransform
AccidentalFourierTransform
@DavidZ well yes, now I know it :-P
Im a newbie here!
 
user54412
@DavidZ A canonical answer might be worth having here, especially since the motivation (conservation of energy) is so physically motivated. That question is the one I personally most often link to, since every physically-inclined coder at some point writes such a code and then wonders why it just doesn't work.
 
AccidentalFourierTransform
AccidentalFourierTransform
3:44 PM
@DavidZ Ive seen many big-list questions on Math.SE turned into CW
 
John Rennie
John Rennie
@DavidZ I use community wiki for my canonical Q/As ...
 
David Z
David Z
@JohnRennie ah, well that's your choice
 
AccidentalFourierTransform
AccidentalFourierTransform
I guess I extrapolated
 
Secret
Secret
(Another interesting question spawn from the above question (because my mind is zipping all over the place in a mess like what Acuriousmind and my postdoc said) is a philosophical but not very useful question)

Suppose we do found a way to massively accelerate the discovery of Q3, is it meaningful to let that grow forever?

(The above question is inspired when the concept of "endless growth economics" crosses with "Finding Q3". You will see many instance of me like this, randomly cross pollinating cocnepts form all fields at random while most of the time perhaps as Acuriousmind, Slereah, my…
 
user54412
@ACuriousMind I didn't see it in review, but I was so impressed by it being not as opinion-based as it could have been that I probably would have forgiven its breadth.
 
user54412
3:45 PM
Actually, broken up into smaller questions might have made for a very nice collection -- particle colliders, dark matter detectors, earth-based telescopes, space-based telescopes, etc.
 
John Rennie
John Rennie
@DavidZ Isn't the whole point of CW to encourage multiple authors to contribute to a question or answer?
 
Obliv
Obliv
@Secret You should study some philosophy because that borders a concept of finite technology. It's basically the idea that there exists a finite amount of potential technology. I was skeptical at first but realized it seems very plausible.
Nick Bostrom talks about that also :p
 
David Z
David Z
@JohnRennie Yeah, but that was more relevant in the early days of the site when there were no suggested edits. You needed some large amount of reputation (3k?) to be able to edit posts that were not your own.
Community wiki mode lowered the editing threshold to 500
 
John Rennie
John Rennie
OK, though I thought the point was with a CW question it wasn't a single author getting all the rep.
 
David Z
David Z
That was never supposed to be the point - just a side effect.
 
user54412
3:52 PM
@DavidZ While your objections to overusing CW are well taken, I think part of the reason many of us are okay with it is that there's a certain... uncomfortable... feeling one gets seeing individual answers earn massive reputation simply because of breadth (as canonical answers might). We accept that ceteris paribus broader questions will get more attention, but it sometimes just feels wrong to benefit too much from that.
 
Secret
Secret
Sometimes I felt like I think like a (possibly inefficient computer)

If Acuriousmind still remember my sharing about the idea of a chunky concept (which is actually more rigorously defined in Cognitive theory https://en.wikipedia.org/wiki/Chunking_(psychology)) and given that he have observed me long enough to know my quirks, he would have noted how when I learn a new concept that can be chunked, it is like finding a new parameter to be added to the growing database of parameters

In the next few minutes, I will often act like a computer going overdrive and randomly placing random chunks t…
 
John Rennie
John Rennie
@DavidZ I think it's quite a big deal. I would be hesitant to write a canonical Q/A if I thought it looked as if I might just be rep whoring.
 
David Z
David Z
@ChrisWhite Yeah, I get that. It's a natural attitude to have, but it's not healthy for the site IMO. Anything that is good enough to be on the site should be good enough to be rewarded with reputation.
 
Secret
Secret
While I do take a look at the reputation of the answers, I often end up reading all the answers available fearing that I will left a stone unturned and thus missing out important information
 
John Rennie
John Rennie
@DavidZ CW doesn't stop a question/answer being upvoted, and there's still the inner glow that comes with the approval of your peers. It just removes the more mercenary aspects of it.
 
David Z
David Z
3:57 PM
What do you mean by "mercenary aspects"?
 
user54412
@DavidZ I (and I'm guessing John too) am more concerned with my own practices than those of others'. As in, I'm not pushing for a rule that no one can write canonical answers unless CW.
 
David Z
David Z
@ChrisWhite Yeah, to be clear, that's what I'm arguing against: rules requiring CW mode to be used for certain kinds of questions/answers.
If someone wants to voluntarily give up any reputation they might gain from a post, I don't mind that so much.
 
user54412
Anyone up for SQL? I'd be interested to see, as a function of total rep (or rate of earning rep), typical users' gini coefficient for their contributions (lots of okay ones vs. a few very large sources).
2
 
Obliv
Obliv
@Secret Actually, you're not that different from many people in a way. What you're describing seems to be analogous to finding a puzzle piece and trying to find where it fits in your brain. I do that a lot too when I learn something profound in physics. I have no idea if it's healthy but I imagine it's a waste of time if you do it with very little knowledge of something (ie me with physics).
 
John Rennie
John Rennie
@DavidZ OK I see your point and I agree.
 
Obliv
Obliv
4:03 PM
Since most things you learn (when you're a novice) are thoroughly investigated and most of what you think does not apply.
 
Physics Meta
Physics Meta
0
Q: How is this question still open?

tpg2114Usually people get mad because we close things too quickly without considering whether they are really on topic or not. But What are the next generation physics experiments? has been open for 2 days and only has 2 close votes, despite being the very definition of too broad. Am I missing somethi...

discussion scope vote-to-close
 
Slereah
Slereah
4:43 PM
So anyway
How do you deal with operator ordering outside of the action in path integrals
What's the difference between $\langle \phi(x) \pi(y) \rangle$ and $\langle \pi(x) \phi(y) \rangle$
 
AccidentalFourierTransform
AccidentalFourierTransform
@Slereah $\delta(x-y)$?
 
Slereah
Slereah
Well obviously, but how do you know that
 
AccidentalFourierTransform
AccidentalFourierTransform
@Slereah well, $\langle \cdot \rangle\equiv \langle 0|\mathcal T\ \cdot |0\rangle$.
 
Slereah
Slereah
In path integrals both would be $\int \mathcal{D} \varphi(x) \ \varphi(x) \pi(y) e^{iS}$
 
AccidentalFourierTransform
AccidentalFourierTransform
and $[\phi(\boldsymbol x),\pi(\boldsymbol y)]=\delta(\boldsymbol x-\boldsymbol y)$.
 
Slereah
Slereah
4:49 PM
If the ordering shenanigan is in the action, you can use Ito integrals to deal with that
But what happens outside
 
AccidentalFourierTransform
AccidentalFourierTransform
well, path integrals are bullshit so who cares :-P
 
ACuriousMind
ACuriousMind
@Slereah: The path integral has no idea what $\pi$ is.
 
Slereah
Slereah
Well $\partial_t \varphi$
Whatever
 
ACuriousMind
ACuriousMind
You can't compute that expression, it has no meaning in the path interral formalism.
@Slereah No
The reason you have $\phi,\pi$ in the canonical formalism is because it is Hamiltonian
 
Slereah
Slereah
Why not tho
 
ACuriousMind
ACuriousMind
4:51 PM
But the path integral formalism is not Hamiltonian, you are not allowed to use the Hamilton canonical momentum in a Lagrangian expression
 
AccidentalFourierTransform
AccidentalFourierTransform
@ACuriousMind in general, $Z=\int\mathcal D\phi\ \exp(S[\phi,\pi])$, where $S=\int \pi \dot\phi-\mathcal H$...
 
ACuriousMind
ACuriousMind
@AccidentalFourierTransform No, "in general", we assume that thing has been integrated out ;)
 
AccidentalFourierTransform
AccidentalFourierTransform
only if $\mathcal H$ is quadratic in $\pi$ you get $S=\int\mathcal L$
@ACuriousMind not always it can be integrated out
the hamiltonian expression is more general
 
ACuriousMind
ACuriousMind
@AccidentalFourierTransform Exactly - that is a basic assumption of QFT
@AccidentalFourierTransform Yes, but if you write the Hamiltonian expression, you also have to write $\mathcal{D}\pi$
 
Slereah
Slereah
Why can't you use $\partial_t \varphi$ either, tho
 
ACuriousMind
ACuriousMind
4:53 PM
@Slereah How is that supposed to be a Lorentz invariant observable?
 
AccidentalFourierTransform
AccidentalFourierTransform
@ACuriousMind yeah, I forgot about $\mathcal D\pi$...
 
Slereah
Slereah
Not all observables are Lorentz invariant!
Most are not, really
 
AccidentalFourierTransform
AccidentalFourierTransform
14
Q: Time ordering and time derivative in path integral formalism and operator formalism

Jia YiyangIn operator formalism, for example a 2-point time-ordered Green's function is defined as $\langle\mathcal{T}\phi(x_1)\phi(x_2)\rangle_{op}=\theta(x_1-x_2)\phi(x_1)\phi(x_2)+\theta(x_2-x_1)\phi(x_2)\phi(x_1),$ where the subscript "op" refers to operator formalism. Now if one is to take a time de...

quantum-field-theory path-integral greens-functions
 
ACuriousMind
ACuriousMind
@Slereah Please write down again what you want to compute, and why you think you can't
 
AccidentalFourierTransform
AccidentalFourierTransform
relevant ↑
 
Slereah
Slereah
4:57 PM
$\langle \varphi(x) \pi(y) \rangle = \int \mathcal{D}\varphi(x) \ \varphi(x) \partial_t \varphi(y) e^{iS}$
$\langle \pi(y) \varphi(x) \rangle = \int \mathcal{D}\varphi(x) \ \varphi(x) \partial_t \varphi(y) e^{iS}$
What's the difference between those two terms
 
ACuriousMind
ACuriousMind
None, because you omitted the time ordering
 
Slereah
Slereah
Oh right
Hm
Does the time ordering of two operators imply that they have no ambiguity in ordering?
 
ACuriousMind
ACuriousMind
Well, if you look at the question @AccidentalFourierTransform linked, there is a bit more to it
 
Slereah
Slereah
I'll look into it
Tho maybe later
I ate a whole bunch of candy so I have to work out :V
My precious gains
They are in peril
 
user54412
Is it time for the weekly "what is Academia.SE thinking?" rant?
 
Slereah
Slereah
5:09 PM
Probably about tenure
 
user54412
Do tenured people think about tenure?
 
Slereah
Slereah
No they just stop thinking
No need to anymore
 
user54412
lol
 
Hari Prasad
Hari Prasad
5:34 PM
Does dark matter(If it exists) has anything to do with weak interactions? and WIMPs?
 
Slereah
Slereah
Dark matter is just a generic term for matter you can't see
It depends on what dark matter is
Some models of dark matter interact weakly yes
Also cardio is done
Now for some lifting
 
dmckee
dmckee
@HariPrasad WIMPs are in some sense the "leading" candidate theory right now. Not because there is a consensus that they are the right answer, but because we can test that hypothesis now. I think the on-deck theory if that fails in axions.
The story of missing mass and dark matter has been a long one filled with "We know that X, Y, and Z are ruled out so we're trying to detect W", followed by knowing that W, X, Y, and Z are ruled out and starting to search for V...
 
Slereah
Slereah
5:51 PM
i do love the standard photo of the discoverer of dark matter, btw
 
Hari Prasad
Hari Prasad
@Slereah Who is he?
 
Slereah
Slereah
Zwicky
He theorized dark matter in the 30's
 
Hari Prasad
Hari Prasad
@Slereah But he didn't discover it.
 
Slereah
Slereah
Well he found the discrepency between the galaxy's rotation and matter density
 
Hari Prasad
Hari Prasad
@Slereah Could it be that we lack technical skills in doing those measurements, rather than theorizing a new kind of matter? Is there any other reason why Dark Matter should exist?
 
Slereah
Slereah
5:56 PM
Well that's the thing
There's matter we can't detect currently
What that matter is is another problem
There has been a lot of theories
 
Hari Prasad
Hari Prasad
You are a PHD student right?
 
Slereah
Slereah
Nope
 
Hari Prasad
Hari Prasad
@Slereah Graduate or postDoc or just a researcher?
 
Slereah
Slereah
I have a degree
Not doing physics now tho
 
dmckee
dmckee
@HariPrasad Dark matter solves several problems. The rotation curves of the galaxies. The velocity profiles of galaxy clusters, the aggregation problem in cosmology.
Though it increasingly looks like you need at least two temperatures (meaning at least two masses).
 
Hari Prasad
Hari Prasad
6:04 PM
@dmckee Hm... But what intrigues me is, what would happen if we fail to find Dark Matter?
 
John Rennie
John Rennie
@HariPrasad we can calculate the maximum density of ordinary matter that the Big Bang could have produced. We can do this because the matter density affects the ratio of the light elements. When we do the calculation we find there just isn't enough ordinary matter.
The conclusion is that there must be another form of matter out there producing the observed gravitational fields.
Since we can't see it we assume it doesn't interact strongly with light, and that means it doesn't interact via the electromagnetic force.
 
Hari Prasad
Hari Prasad
@JohnRennie what if it doesn't interact with light at all? Then what could be the other ways to find them?
 
John Rennie
John Rennie
@HariPrasad Current searches are based on finding interactions via the weak force. This is what the Weakly Interacting bit of WIMP refers to.
 
Hari Prasad
Hari Prasad
@JohnRennie @dmckee @Slereah Thanks this helped me :D
 
Slereah
Slereah
I wonder by the way
 
John Rennie
John Rennie
6:10 PM
I should note that neutrinos are exactly this sort of weakly interacting particle, and we detect them by their weak force interaction with other particles. However dark matter can't be neutrinos because neutrinos have relativistic speeds.
2
 
Slereah
Slereah
Is there a realistic cosmological model that isn't homogeneous
Like a metric that takes into account the large scale structures
 
dmckee
dmckee
@JohnRennie And we know how many light to medium mass flavors there are from the decay parameters of the Z boson. But there could, in principle be massive sterile flavors.
 
John Rennie
John Rennie
True, and a massive right handed neutrino would conveniently provide the masses for the three neutrinos we can see. As I understand it the various dark matter searches are getting to the point where thy can start ruling out some possibilities.
 
dmckee
dmckee
@Obliv The range of gluons is on order of the size of a proton. It's like John said, if you know what they are the answer is trivial.
 
user507974
user507974
hey guys, im having a bit of a derpy moment
im doing a spherical diffusion equation, fixed BC's at finite intervals, with radial symmetry
that means I basically should be writing down Legendre polynomials as the ansewer right
 
John Rennie
John Rennie
6:16 PM
@dmckee that flipping question is still open, and you're moaning at me for failing to close the one on large scale experiments in physics!
Why does the notification box that appears in the chat occasionally notify me about questions posted in other sites? I've just seen a notification for:
0
Q: In a fantasy setting, what would races of varying intelligence need to learn in order to complete quests?

Bishop MinterFor humans, elves, and gnomes who have average to high intelligence and wisdom I see them excelling in mathematics, physics, sciences, etc. These skills could be used to complete quests in any number of ways. For example, determine the volume of liquid needed to fill a certain container so that...

physics weapons intelligence fantasy-races
Which seems a fun question, but I'm puzzled as to why it appeared in a notification here.
 
NeuroFuzzy
NeuroFuzzy
Minotaur school!
 
John Rennie
John Rennie
Hmm, I note it has been tagged as Physics. I wonder if that prompts the SE to notify here.
 
Slereah
Slereah
yes
The bot gives posts on meta and on worldbuilding if tagged with physics
 
user507974
user507974
@Slereah interesting, i guess if i ever decide to push through with my physics driven magic system i know what tag to use to draw the literate in
 
ACuriousMind
ACuriousMind
@JohnRennie A bit ago, we added some physics tags from other sites to appear there. It's worldbuilding.physics, music.acoustics and mo.math-phys, possibly some more
 
Slereah
Slereah
6:27 PM
What about science questions from the Islam SE and Christian SE?
0
Q: Are angels visible at the speed of light?

Muhammad I was wondering if the angels are made of light, could we see them if we traveled at the speed of light?

islamic-philosophy science malaikah theology
 
user507974
user507974
@Slereah lolnope i dont see that going well
@Slereah case and point
 
Slereah
Slereah
How do you Lorentz transform an angel
Are they vectors or scalars
 
ACuriousMind
ACuriousMind
Depends on what the needle whose tip they're dancing on is
 
Slereah
Slereah
0
Q: Black Holes, is it okay to believe in them?

FarhanSalam, I am studying for a physics degree and we are beginning to study Black Holes. But I am not sure whether or not these comply with the Quran, any justification would be appreciated.

science scientific-proof
 
John Rennie
John Rennie
@Slereah that question has a good answer though!
 
Slereah
Slereah
6:33 PM
I say black holes are a sin!
Repent your geodesically incomplete solutions!
 
John Rennie
John Rennie
There are of course physicists who would agree with you :-)
 
Slereah
Slereah
You know I wonder why there aren't more physics research into non-linear distribution theory
It would help out for both QFT and GR
40
Q: How do young earth creationists reconcile the age of the universe with the speed of light, and visible distant objects?

aceintheholeI am not trying to be argumentative, this is an earnest question, as this question got me in huge trouble when I was growing up attending a southern baptist Christian middle school. This question (and people's reactions to it) is actually one of the things that lead my away from the church as a t...

creation science young-earth-creation
 
John Rennie
John Rennie
6:51 PM
Just when you all hoped I'd stopped ranting on about vacuum fluctuations ...
This looks an interesting book on the subject. Has anyone read it?
 
user507974
user507974
@Slereah what if they are tensors
 
Slereah
Slereah
I have not
 
user507974
user507974
God, why cant creationists just retcon the age of the universe into their holy writings by saying something like Gods Words were so vast and unfathomable for a common man of that era that 13 billion (which god said was "a really long time") was interpreted as 10000 years
 
Slereah
Slereah
Because literalism
 
user507974
user507974
@Slereah why should man be able to understand the literal word of God, thats pretty arrogant on man's part to assume
and a thousand years ago I'd be put on fire for saying this
@JohnRennie you said earlier neutrinos are "weakly interacting" by this did you mean they interact with the weak force or that their interaction mechanism was low in magnitude
 
Slereah
Slereah
6:59 PM
both!
 
John Rennie
John Rennie
@user507974 both!
 
user507974
user507974
 
John Rennie
John Rennie
I'm not sure I see the relevance of that ...
 
user507974
user507974
@JohnRennie isnt QED basically the theory of electron self interactions
 
John Rennie
John Rennie
No, I don't think so
 
user507974
user507974
7:06 PM
isnt that what self energy is?
 
John Rennie
John Rennie
The self energy exists in classical electrodynamics, but it's infinite. QED gives us a description of the field around an electron that doesn't require infinite energy, but this is one application of QED rather than the heart and soul of it.
 
user507974
user507974
@JohnRennie whats the heart and soul then?
 
Slereah
Slereah
QED is just the quantized version of EM
 
Jiminion
Jiminion
<snarky>John is good at telling what things are not, rather than what they are!</snarky> :)
 
John Duffield
John Duffield
@HariPrasad : no.
 
Jiminion
Jiminion
7:20 PM
Virtual Particle Representation (valid?):

https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwirseHT953MAhXonIMKHVLGCukQjRwIBw&url=http%3A%2F%2Fwww.physics.adelaide.edu.au%2Ftheory%2Fstaff%2Fleinweber%2Fteaching%2FActionXs24t36gradient.html&psig=AFQjCNETKqTDfoxnp6kU0RO6tDRbvcYULw&ust=1461266384830797
 
John Duffield
John Duffield
@Slereah : yes.
@Jiminion : no.
 
Slereah
Slereah
That's not a virtual particle representation
Those are just field values
 
John Duffield
John Duffield
@Jiminion : see this by anna v: "virtual particles exist only in the mathematics of the model".
 
Jiminion
Jiminion
@JohnDuffield So fields REALLY exist?
 
user507974
user507974
7:36 PM
I feel like this is a task for @ACuriousMind
user image
8
 
Bernard Meurer
Bernard Meurer
Omg
what a glorious image
 
user507974
user507974
thanks
i felt like we needed something like that here
 
Bernard Meurer
Bernard Meurer
Truly amazing hahaha
 
dmckee
dmckee
7:54 PM
@JohnRennie I think that room owners can control the room feeds. I'm not sure who hooked us up with the science-based tags for worldbuilding, but that's what is going on.
 
ACuriousMind
ACuriousMind
@user507974 I have been called?
 
Bernard Meurer
Bernard Meurer
@ACuriousMind You're basically a celebrity
 
user507974
user507974
so there is a bit of an uncertainty going on basically about QED, the physical meaning of virtual particles, and if self-interaction is a thing in QED
 
ACuriousMind
ACuriousMind
@user507974 "Self-interaction" is a bit of a nebulous term. The self-energy is the contribution of Feynman diagrams in which the electron emits and then again absorbs photons to the propagator. This "dresses" the mass of the electron, as it shifts the pole compared to the free theory. It's not far fetched to call these contributions "self-interactions".
However, the Feynman diagrams depict just a systematic way to organize the deviation of QED from a free fermion theory. It is not justified by anything in the formalism to literally conceive of an electron state emitting or absorbing actual, real photons. The photon lines in these diagrams do not belong to actually occupied states.
 
user507974
user507974
@ACuriousMind I recall in my QM class doing a sort of integration of shells of a sphere that represented an electron in the middle with its probability density across the surface of the sphere. Does this have any physical significance and relevance to the discussion?
 
ACuriousMind
ACuriousMind
8:07 PM
@user507974 No. The particle states one uses in QFT are usually sharp momentum states, they are rather delocalized. Localizing particles in a relativistic theory is a bit tricky
 
Slereah
Slereah
Eh
You can use coherent states
Those are well localized
 
ACuriousMind
ACuriousMind
Yes, but that has nothing to do with what one calls self-energy, or what one draws in the Feynman diagrams.
 
Slereah
Slereah
Self energy even appears in classical theory, really
It's not that weird a concept
 
user507974
user507974
@ACuriousMind so as oppose to a self-interaction for an electron its better to think of these deviations from a free fermion theory as basically some kind of optimization that naturally occurs in the electrons that results in a distribution that differs from the free fermion
it just so happens that the best pseudo-particle theory is this Feynman diagram picture
 
ACuriousMind
ACuriousMind
Well, I don't know about "better". It is a correction term to the propagator, i.e. a correction to the behaviour of a lone electron in the interacting theory. It's fine to call it "self-interaction", just don't think about emitting photons
 
Slereah
Slereah
8:15 PM
Overall it's usually not a very productive use of your time to ask yourself is something is "real" in physics
 
user507974
user507974
@ACuriousMind virtual or not, at least if we are looking for a physical interpretation
 
Slereah
Slereah
Just ask yourself what element of the model is observable in the end
 
user507974
user507974
@Slereah I think there is something to learn from asking yourself what "is" real. It's like understanding phonons. Even if they dont exist as a particle proper there is a real mechanism underlying the collective excitations.
 
Slereah
Slereah
It can be instructive
But don't spend too much time on it because in the end, nothing is "real" in those models
Those are just mathematical abstractions
Although there are plenty of reasons in particular why virtual particles aren't real
 
Obliv
Obliv
wow **** me I just tried to prove something for an hour and a half missing a definition that would have made this infinitely more helpful Q_Q
 
skill patrol
skill patrol
8:25 PM
missing a definition always hurts
T_T
 
Jiminion
Jiminion
It seems to be all about the math. If the math works and is predictive, then the underlying reality (whatever that is) doesn't matter.
 
AccidentalFourierTransform
AccidentalFourierTransform
missing your dog hurts more T_T
 
Jiminion
Jiminion
Outside of a dog, solving Physics problems gives me great comfort. Inside of a dog, it's too dark to solve Physics problems.
2
 
skill patrol
skill patrol
:D
 
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