The h Bar

Sir Cumference

00:10

Howdy

FenderLesPaul

00:36

@BernardMeurer white boards suck

Bernard Meurer

@FenderLesPaul You suck :(

White boards save my poor eyes

against the oppression of chalk

@ACuriousMind You there?

Downgoat

01:49

Hello, is anyone here? I have a physics question...

Helka Homba

How can we help?

Downgoat

Hello @HelkaHomba do you know physics?

user116211

Go ahead @Downgoat...... any willing one would answer...

user116211

when they are available...

Downgoat

So I was trying to make a gravity simulator and I got this code...

Helka Homba

01:50

@Downgoat I know of physics

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;

but the planet sings off into interstellar space

I'd appreciate any help :3

user116211

@skillpatrol: o/

skill patrol

\o @MAFIA36790

Bernard Meurer

@Downgoat You here?

What a pleasure

Downgoat

@BernardMeurer yup

Bernard Meurer

02:01

Interesting bug

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

@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

02:17

@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

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

@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

@dmckee Hey Smithy, can you do me a favor?

Check over this code

jsfiddle.net/qXP7H/…

user54412

06:34

@Downgoat I feel the urge to plug this.

2

lucas

06:52

Hello. I have a question. May someone answer me please?

Is there anyone?

Physics Meta

07:17

0

Q: How can I type math?

I 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

user116211

09:16

physics.stackexchange.com/review/suggested-edits/127356

user116211

Damn, made a mistake.... please reject this edit.

ACuriousMind

10:26

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

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

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

Hm, interesting. Still not a constructive comment, but interesting.

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

@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

11:01

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

11:35

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

12:04

the comment was "obviously" made tongue in cheek :P

without really meaning what one is saying or writing.

Slereah

I bought some chocolate

My diet :O

skill patrol

dark or white?

Slereah

skill patrol

12:47

bon appetit :-)

yuggib

13:11

Equicontinuous functions are nasty

or better, equicontinuous functions are nice but proving equicontinuity is nasty

Jiminion

So this is a poor representation of vacuum energy?

https://i.ytimg.com/vi/J3xLuZNKhlY/hqdefault.jpg

ACuriousMind

@Jiminion I have no idea what that picture is supposed to depict.

Slereah

I guess it's a measurement?

Energy measurement of the vacuum

mb

I dunno

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

Well maybe try to find how they actually got this picture

Jiminion

13:21

Empty Space is not Empty! (Dr. Lawrence Krauss)

►

I think it was by Mather and Smoot?

Secret

14:07

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

@JohnRennie @AccidentalFourierTransform Why did you vote to leave open here? The question is obviously too broad.

Slereah

You're too broad

skill patrol

14:26

abstrusegoose.com/395

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

@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

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

@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

I thought it was the standard procedure to turn list-questions into community posts, but I guess I was wrong

ACuriousMind

14:39

@AccidentalFourierTransform MathOverflow and math.SE do that, I think, but we have never established that here as far as I know

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

@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

@ACuriousMind and I must admit my decision was partially influenced by that gluon question from last week.

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

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

14:49

@AccidentalFourierTransform what was the gluon question from last week?

ACuriousMind

@JohnRennie This one

AccidentalFourierTransform

@JohnRennie yes, from last Thursday

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

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

@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

14:53

Ah, I didn't realise it had been closed then reopened. I'm tempted to vote to close again :-)

Obliv

@BernardMeurer What do you mean? I didn't say I needed anything :o

AccidentalFourierTransform

@JohnRennie lets do it ;-)

ACuriousMind

@JohnRennie I don't think you can vote again, but try it.

Obliv

did you get confused by the guy above your message having a similar profile picture lol?

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

14:55

@ACuriousMind just VTC'd :-)

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

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

0

Q: Canonical question for two masses falling in Newtonian gravity

Some 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

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

15:11

@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

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

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

Let's see if Arxiv has some cool new papers

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

"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

15:22

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

cft.edu.pl/~kijowski/Odbitki-prac/SCAL-LAT.pdf

Much better

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

@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

"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

For an introduction you usually put scalar fields alone on a lattice

Slereah

15:25

Yeah, or that

ACuriousMind

At least, that's what I got as an intro

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

@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

catdir.loc.gov/catdir/samples/cam033/2002016584.pdf

Let's see

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

15:37

I know 10 decimals of pi and it's basically useless

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

$\pi = 3$ is pretty much all you need

$3 + \frac{1}{8}$, tops

Obliv

I hope you're not studying to be an engineer @slereah lol

you would cause a lot of damage :p

Slereah

Man I've seen some astrophysicists not even caring if they had a margin of error of two orders of magnitude

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

15:40

Ancient Egypt used $3 + \frac{1}{8}$ and they built the pyramids!

Secret

@Obliv Hmm, will check that out

David Z

@AccidentalFourierTransform we don't use CW mode for anything except resource recommendations (which I was opposed to)

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

@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

15:44

@DavidZ Ive seen many big-list questions on Math.SE turned into CW

John Rennie

@DavidZ I use community wiki for my canonical Q/As ...

David Z

@JohnRennie ah, well that's your choice

AccidentalFourierTransform

I guess I extrapolated

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

15:45

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

@DavidZ Isn't the whole point of CW to encourage multiple authors to contribute to a question or answer?

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

@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

OK, though I thought the point was with a CW question it wasn't a single author getting all the rep.

David Z

That was never supposed to be the point - just a side effect.

user54412

15:52

@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

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

@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

@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

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

@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

15:57

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

@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

@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

@DavidZ OK I see your point and I agree.

Obliv

16:03

Since most things you learn (when you're a novice) are thoroughly investigated and most of what you think does not apply.

Physics Meta

0

Q: How is this question still open?

Usually 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

16:43

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

@Slereah $\delta(x-y)$?

Slereah

Well obviously, but how do you know that

AccidentalFourierTransform

@Slereah well, $\langle \cdot \rangle\equiv \langle 0|\mathcal T\ \cdot |0\rangle$.

Slereah

In path integrals both would be $\int \mathcal{D} \varphi(x) \ \varphi(x) \pi(y) e^{iS}$

AccidentalFourierTransform

and $[\phi(\boldsymbol x),\pi(\boldsymbol y)]=\delta(\boldsymbol x-\boldsymbol y)$.

Slereah

16:49

If the ordering shenanigan is in the action, you can use Ito integrals to deal with that

But what happens outside

AccidentalFourierTransform

well, path integrals are bullshit so who cares :-P

ACuriousMind

@Slereah: The path integral has no idea what $\pi$ is.

Slereah

Well $\partial_t \varphi$

Whatever

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

Why not tho

ACuriousMind

16:51

But the path integral formalism is not Hamiltonian, you are not allowed to use the Hamilton canonical momentum in a Lagrangian expression

AccidentalFourierTransform

@ACuriousMind in general, $Z=\int\mathcal D\phi\ \exp(S[\phi,\pi])$, where $S=\int \pi \dot\phi-\mathcal H$...

ACuriousMind

@AccidentalFourierTransform No, "in general", we assume that thing has been integrated out ;)

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

@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

Why can't you use $\partial_t \varphi$ either, tho

ACuriousMind

16:53

@Slereah How is that supposed to be a Lorentz invariant observable?

AccidentalFourierTransform

@ACuriousMind yeah, I forgot about $\mathcal D\pi$...

Slereah

Not all observables are Lorentz invariant!

Most are not, really

AccidentalFourierTransform

14

Q: Time ordering and time derivative in path integral formalism and operator formalism

In 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

@Slereah Please write down again what you want to compute, and why you think you can't

AccidentalFourierTransform

relevant ↑

Slereah

16:57

$\langle \varphi(x) \pi(y) \rangle = \int \mathcal{D}\varphi(x) \ \varphi(x) \partial_t \varphi(y) e^{iS}$

$\langle \pi(y) \varphi(x) \rangle = \int \mathcal{D}\varphi(x) \ \varphi(x) \partial_t \varphi(y) e^{iS}$

What's the difference between those two terms

ACuriousMind

None, because you omitted the time ordering

Slereah

Oh right

Hm

Does the time ordering of two operators imply that they have no ambiguity in ordering?

ACuriousMind

Well, if you look at the question @AccidentalFourierTransform linked, there is a bit more to it

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

17:09

Probably about tenure

user54412

Do tenured people think about tenure?

Slereah

No they just stop thinking

No need to anymore

user54412

lol

Hari Prasad

17:34

Does dark matter(If it exists) has anything to do with weak interactions? and WIMPs?

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

@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

17:51

i do love the standard photo of the discoverer of dark matter, btw

Hari Prasad

@Slereah Who is he?

Slereah

Zwicky

He theorized dark matter in the 30's

Hari Prasad

@Slereah But he didn't discover it.

Slereah

Well he found the discrepency between the galaxy's rotation and matter density

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

17:56

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

You are a PHD student right?

Slereah

Nope

Hari Prasad

@Slereah Graduate or postDoc or just a researcher?

Slereah

I have a degree

Not doing physics now tho

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

18:04

@dmckee Hm... But what intrigues me is, what would happen if we fail to find Dark Matter?

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

@JohnRennie what if it doesn't interact with light at all? Then what could be the other ways to find them?

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

@JohnRennie @dmckee @Slereah Thanks this helped me :D

Slereah

I wonder by the way

John Rennie

18:10

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

Is there a realistic cosmological model that isn't homogeneous

Like a metric that takes into account the large scale structures

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

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

@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

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

18:16

@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?

For 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

Minotaur school!

John Rennie

Hmm, I note it has been tagged as Physics. I wonder if that prompts the SE to notify here.

Slereah

yes

The bot gives posts on meta and on worldbuilding if tagged with physics

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

@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

18:27

What about science questions from the Islam SE and Christian SE?

0

Q: Are angels visible at the speed of light?

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

@Slereah lolnope i dont see that going well

@Slereah case and point

Slereah

How do you Lorentz transform an angel

Are they vectors or scalars

ACuriousMind

Depends on what the needle whose tip they're dancing on is

Slereah

0

Q: Black Holes, is it okay to believe in them?

Salam, 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

@Slereah that question has a good answer though!

Slereah

18:33

I say black holes are a sin!

Repent your geodesically incomplete solutions!

John Rennie

There are of course physicists who would agree with you :-)

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?

I 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

18:51

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?

amazon.co.uk/Quantum-Vacuum-Introduction-Electrodynamics/dp/…

user507974

@Slereah what if they are tensors

Slereah

I have not

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

Because literalism

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

18:59

both!

John Rennie

@user507974 both!

user507974

@JohnRennie imgflip.com/i/12trx2

John Rennie

I'm not sure I see the relevance of that ...

user507974

@JohnRennie isnt QED basically the theory of electron self interactions

John Rennie

No, I don't think so

user507974

19:06

isnt that what self energy is?

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

@JohnRennie whats the heart and soul then?

Slereah

QED is just the quantized version of EM

Jiminion

<snarky>John is good at telling what things are not, rather than what they are!</snarky> :)

John Duffield

@HariPrasad : no.

Jiminion

19:20

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

@Slereah : yes.

@Jiminion : no.

Slereah

That's not a virtual particle representation

Those are just field values

John Duffield

@Jiminion : see this by anna v: "virtual particles exist only in the mathematics of the model".

Jiminion

@JohnDuffield So fields REALLY exist?

user507974

19:36

I feel like this is a task for @ACuriousMind

8

Bernard Meurer

Omg

what a glorious image

user507974

thanks

i felt like we needed something like that here

Bernard Meurer

Truly amazing hahaha

dmckee

19:54

@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

@user507974 I have been called?

Bernard Meurer

@ACuriousMind You're basically a celebrity

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

@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

@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

20:07

@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

Eh

You can use coherent states

Those are well localized

ACuriousMind

Yes, but that has nothing to do with what one calls self-energy, or what one draws in the Feynman diagrams.

Slereah

Self energy even appears in classical theory, really

It's not that weird a concept

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

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

20:15

Overall it's usually not a very productive use of your time to ask yourself is something is "real" in physics

user507974

@ACuriousMind virtual or not, at least if we are looking for a physical interpretation

Slereah

Just ask yourself what element of the model is observable in the end

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

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

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

20:25

missing a definition always hurts

T_T

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

missing your dog hurts more T_T

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

:D

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