The h Bar - 2020-03-30 (page 1 of 2)

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knzhou

00:15

PSA: everybody who can vote on tag synonyms should!

2

I've been on this site for 5 years and I only just now realized this was possible.

Many of the proposals by Qmechanic have been floundering, without positive or negative votes, since before I even joined.

vzn

00:43

@Secret remarkable pov! or maybe unremarkable considering the author...

6 hours later…

ACuriousMind

06:59

@knzhou Yeah, it's so hidden that I regularly forget this is possible even though I've known it for a while

Slereah

07:10

Yabadabadoo

Johan Liebert

07:26

Ikidivakidi dim?

Loong

07:52

Hmpf, our canteen is closed now. :-(

And I have got isopropanol from our storage; so we can make our own disinfectant now.

user image

Student404Mus

Does Theoretical Physics got criticism regardless String and SUSY?

Are there books for it?

Slereah

That is not a sentence.

@Loong and cocktails

Loong

08:09

mmmh

Maybe I will just build a flamethrower.

Johan Liebert

@Loong yes, you should be prepared for zombies.

Current death toll: 34,000 (exact)

One third of which is from Italy.

Slereah

not counting all the hidden deaths from various countries underreporting it

Johan Liebert

Yes, hadn't Iran underreported it's death toll, it might have been at the first place.

Same goes for China.

Loong

Hm, zombies. Good that we have our fence and armed guards.

Johan Liebert

08:28

Well zombies can get into the room via digging. ( I am it in Resident Evil, don't know which part that was)

Slereah

Also Russia, I hear

It could reasonably be at 100k right now

Guru Vishnu

@Loong Just curious, is that the "nuclear-grade" disinfectant you mentioned a few days ago?

Loong

08:45

No, that's just ordinary isopropanol. We will dilute it and mix it with glycerol and some H2O2.

Slereah

@Loong No fruit juice?

Loong

and I need cocktail umbrellas

Slereah

Nuclear-grade cocktail umbrellas

Loong

designed against airplane crash

Johan Liebert

09:10

user image

Well these are before and after images of a painting. Even though I have seen it being made with my own eyes, I still can't believe these are real. THE PAINTING IS A PAID ACTOR.

Novalium Company

I just found out there's some virus called coronavirus that's apparently a big thing. don't know what's all the fuss about

I feel left out in this chatroom and I don't know what to say so I behave like an idiot so I can get a drop of attention. Please ignore me.

Student404Mus

We use the spin operators as angular momenta with their commutations relations, on the other hand when we deal with the spin matrices we use the anti-commutation relations. Does anyone figure this out?

What's the consistency here?

Johan Liebert

@NovaliumCompany no it's a really really small thing!

ACuriousMind

@Student404Mus I have no idea what you mean. Are the "spin matrices" the gamma matrices?

Student404Mus

pauli matrices

since we know, $s=\sigma /2$

ACuriousMind

09:23

you're being very unclear again, sorry

Student404Mus

Why? Aren't Pauli matrices the ones which describe the spin in quantum mechanics?

ACuriousMind

Sort-of, but let's say yes. You can take their commutator or you can take their anti-commutator, both are valid operations to do with any matrix. What's your question, exactly?

Student404Mus

Maybe you have answered me indirectly. Let's say the conventional usage is the commutators, where in rare occasions we use anti-commutators. So, I answered myself. it depends on the case we deal with.

Gamma matrices are sort of extension to Pauli matrices in space that took into account the time as dimension.

and the spinor components are spinors themselves.

four components in total.

Novalium Company

09:40

Is $(-\frac{12}{13})^2$ equal to $\frac{144}{169}$ or -$\frac{144}{169}$?

(I'm doing my retarded online school homework)

I have $sin^2a + cos^2a = 1$ where $sina = -\frac{12}{13}$

it's $\frac{144}{169}$ right?

Johan Liebert

@NovaliumCompany yes it is.

Novalium Company

@JohanLiebert thank

ACuriousMind

@NovaliumCompany The fact that you weren't sure of the answer would suggest the homework is, in fact, useful.

Novalium Company

@ACuriousMind I knew you'd say something like this. I'm just in a rush and my mind is a mess now.

And useful is pretty relative.

I don't want to become a mathematician nor a physicist.

ACuriousMind

Knowing how to square a negative number is pretty far from specialized knowledge only professional mathematicians might need!

Novalium Company

09:55

True. I'm not saying it's useless. I'm saying that I've long accepted that according to my school, blindly applying formulas without asking why is considered mathematics.

Sir Cumference

well according to high schools yep

in university it's the opposite, you're expected to be able to reason why these things are true

so by grad school you can come up with them by yourself

Student404Mus

10:44

Let's define a tensor of rank $k$ ( angular momentum in spherical harmonics) parametrized by $\kappa$ (projection of angular momentum). Let's assume we know the law of transformation of such tensor by,
$F_{\kappa}^{(k)}=\sum F_{\kappa^{\prime}} D_{\kappa^{\prime} \kappa}^{(k)}(R)$
We want to illustrate a fact as follows. We take for e.g., a rotation around the z-axis by an angle $\phi$, $F_{\kappa}^{(k)}=e^{-i \kappa \varphi} F_{\kappa}^{(k)}$. Thus, for $\kappa \neq 0; F_{\kappa}^{(k)} =0$. Now the point is, since this relation holds in every frame why do we deduce it vanishes for $\kappa …

Something is hidden to complete the proof, or I'm missing the point?

Slereah

@ACuriousMind See here tho

timecube.2enp.com

Educators are lying bastards.
-1 x -1= +1 is WRONG, it is
academic brilliantity and is boring.
The educated brilliant should acknowledge
the natural antipodes of+1 x +1 = +1and
-1 x -1 = -1 exist as plus and minus values
of opposite creation - depicted by opposite
burritoes and opposite hemispheres. Entity is
death worship - for it cancels opposites.

ACuriousMind

11:04

Uh.

Sir Cumference

@Slereah Is this some kind of bot-generated text?

Novalium Company

@JohnRennie You should make a bread burger. Two buns with a slice of bread in between. Desperate times... or you could also replace the bread in the middle with toilet paper.

John Rennie

@NovaliumCompany I have made and enjoyed pie sandwiches i.e. two slices of bread with a meat pie in between them.

ACuriousMind

@SirCumference If it's a bot, it just passed the crackpot Turing test because that's almost indistinguishable from some of the weirder screeds by people with strange beliefs about math I've seen :P

Sir Cumference

I do have to wonder where @Slereah finds these things :P

Loong

11:21

Time Cube was a personal web page, founded in 1997 by the self-proclaimed "wisest man on earth," Otis Eugene "Gene" Ray. It was a self-published outlet for Ray's theory of everything, called "Time Cube," which polemically claims that all modern sciences are participating in a worldwide conspiracy to teach lies, by omitting his theory's alleged truth that each day actually consists of four days occurring simultaneously. Alongside these statements, Ray described himself as a "godlike being with superior intelligence who has absolute evidence and proof" for his views. Ray asserted repeatedly a...

ACuriousMind

Having the "See also" section of the Wiki article about your work consist solely of "Crank (person)" must be a special kind of achievement.

Slereah

@SirCumference This is old fashioned schizophrenia

like our ancestors did

@JohnRennie doesn't need to be told about bread burgers because he comes from England, where this is considered cuisine

A toast sandwich is a sandwich made with two slices of bread in which the filling is a thin slice of toasted bread, which can be heavily buttered. An 1861 recipe says to add salt and pepper to taste. == Victorian recipe == A recipe for toast sandwiches is included in the invalid cookery section of the 1861 Book of Household Management by Isabella Beeton, who adds, "This sandwich may be varied by adding a little pulled meat, or very fine slices of cold meat, to the toast, and in any of these forms will be found very tempting to the appetite of an invalid." == 2011 publicity == In November 2011 the...

John Rennie

I can see that it would have an interesting texture - the crisp toast against the soft bread.

Though I feel no immediate urge to make a toast sandwich.

skullpatrol

@Slereah ancestors that believed in the aether wind in outer space?

Slereah

There's nothing fundamentally wrong or unscientific about aether winds

it just happens to not be a theory that works

John Rennie

11:35

0

Q: Why Einstein's GR considered perfect theory of gravity even though it has geometrical base?

In my view, GR is imaginary geometrical explanation of how gravity works and fundamental science behind why gravity works is still unknown.

general-relativity gravity

There, that's told you Albert!!

skullpatrol

:-/

Slereah

let him battle with John Duffield

skullpatrol

July 31 at 11:30

skullpatrol

12:14

The Good The Bad and the Ugly Finale

Auberron

Was Einstein wrong that his theory of relativity suggests universe expands with decelerating speed?

Slereah

Basically anything that people in Einstein's time believed about cosmology is not to be trusted

because that was before a lot of modern findings about cosmology

Basically anything before 2000 is probably not to be trusted regarded cosmology

Auberron

If that's the case how the theory of relativity suggests universe is expanding with declerating rate?

Slereah

12:29

It doesn't specifically

There are cosmological models where it does

and models where it doesn't

depending on the matter content

insomniac

Hello world, Opinions requested (be gentle, Aummm) : physics.stackexchange.com/questions/540348/…

Yuvraj Singh...

13:09

Hi guys!

@ACuriousMind he has posted question link?

user434058

Should I ever give hints to an OP instead of answering the whole question? If yes, then should I give the hints in the comments or write an answer which contains the hint(s)?

ACuriousMind

13:25

@YuvrajSingh... Who?

Yuvraj Singh...

@ACuriousMind insomniac!

ACuriousMind

@FakeMod I'm struggling to come up with a scenario in which you would prefer to give "hints" rather than a full answer in which the question is not simply off-topic for being HW-like.

@YuvrajSingh... Indeed, they would appear to have done so. Why're you pinging me about it?

Yuvraj Singh...

As you told me the posting question link is prohibited, am I right?

So what I would say is to delete the link!

user434058

@ACuriousMind Well... For me giving hints is the best option when the question is simple enough to be solved by the OP (this is generally when OP is talking about a specific example and does not have any conceptual doubt) but if you were to write an answer explaining every step then it might take good lot of time. (By good lot, I mean at least half an hour).

Yuvraj Singh...

It is like promoting question, did n, t it? @ACuriousMind

ACuriousMind

13:31

Mar 4 at 16:16, by ACuriousMind

@YuvrajSingh... it's not forbidden to post question links, we just ask people don't advertise their own questions shortly after they're posted

Yuvraj Singh...

Yup.

ACuriousMind

Will you stop already claiming that I said anything about posting question links being forbidden?

JMac

Also this was someone asking for opinions on an answer. Not completely different; but somewhat different.

Yuvraj Singh...

I agree. And sorry to ping you again and again, but he has posted the link of the question, without giving any reason for doing!

ACuriousMind

And yes, I'd also ask people to not advertise their recent answers. But I'm not here 24/7 and if you don't like people doing that just tell them yourself. Especially if a user does that only once, there's no need to do anything else in my opinions.

JMac

13:36

@YuvrajSingh... Presumably he had intended to link to his answer on the question; which is why he said "opinions requested".

Yuvraj Singh...

OK. Will I be authorized to say that?

user434058

Is it better to draw diagrams using TikZ (assuming that you know how to use it, which would require you to dedicate about 100 hours of "practice") and to draw your diagram you would need to write the code yourself (which again might take significant time for non trivial drawings) OR should you just use a WYSIWYG software for drawing and insert that drawing(in SVG format) to your LaTeX document?

Yuvraj Singh...

@FakeMod use Google draw, much better!

Simple and fast

user434058

@YuvrajSingh... yes but I thought I would rather ask someone.

Yuvraj Singh...

In my opinion it is the written theory and calculation which make the answer more useful in physics world, so we need diagram which help in depicting the situation so instead of using time In writing diagram codes we should prefer more time In content.

ACuriousMind

13:44

@FakeMod If you can produce a diagram that satisfies your expectations without TikZ, there's no reason to use it

tpg2114

@FakeMod I've used TikZ quite a bit and... I'm rarely happy that I did. The perk is that I can reuse the code in lots of documents and I try to make it general, so I can change colors or scales or whatever via macros. The reality though

Once I published the figure somewhere, I can't reuse it directly anyway

So I rarely have gotten multi-use out of it, except for perhaps one paper, my thesis, and related slides. Which means I used all that customization for like... 3 instances only.

There are times that I do it anyway, but that's because I find it easier than trying to find, then learn, a wysiwyg program to do it

user434058

@tpg2114 I refer pgf manual a lot of times, experiment a lot of times, tweak the co-ordinates to get em right and after all that hardship, I am finally able to get something. The effort seems too much.

ACuriousMind

There isn't really any inherent advantage of using TikZ over embedded graphics. It's good if you care about version control (meaningful diffs for TikZ code vs. meaningless diffs for the binary blobs that most graphics files are), it's good if you often draw diagrams with the same "building blocks" since you can then reuse the code, but if you don't care about these things and you can get the same result with something else there's no reason to switch

user434058

@ACuriousMind It's just the feeling of LaTeX being superior which makes my ego stop me from using anything other than TikZ

user434058

When I firdt saw TikZ and its power, I was enraptured, but the hardwork makes me gloomy.

user434058

14:01

Sh*t man!!!! You are writing a beautiful answer to a question, you've completed 75% of it and suddenly a notice pops up which says that the question was deleted....... Damn!! All that gone wastel!?! It's ok, I am not angry. This was just a momentary rant-burst.

user434058

@ACuriousMind Not forcing you but please answer my question on hints when you're free!

user434058

Why do people use "BEAMER" in LaTeX? It looks ugly when compared to the "ARTICLE" class/layout?

ACuriousMind

@FakeMod I'm still not sure in what case the question would not simply be off-topic. Anyways, never post anything in comments that is even part of an answer - partial answers are answers, too. See also the classic meta.stackexchange.com/q/225370/263383, "hints" seem to me to be the mostly-eaten apples

@FakeMod Uh, because beamer is for presentations and article is for, well, articles?

user434058

@ACuriousMind Got it, thanks!

user434058

@ACuriousMind But still, if I were ever to make a presentation, I would make it in an article layout!

ACuriousMind

14:06

I'm not sure how that would work. Not even the dimensions of the page match the usual dimension of a projected screen

article is made to be read through continuously, not be projected one page at a time

user434058

@ACuriousMind Yup and they also say that the beamer fonts look much better when projected, but there is an inherent love in me for the style of the article document class. Anyways LaTeX is awesome!!!

user434058

In LaTeX, I'm in that starting phase of finding something new and being super excited bout it, but it will soon fade away.

tpg2114

Using article instead of beamer is the equivalent of using ms word to make presentations instead of PowerPoint... I mean, maybe it's possible but there's no way it's optimal

What I'd like to figure out is a way to hack eps files to have latex macros or something, so I can make sure fonts and sizes match exactly, instead of incessant iteration between matplotlib and latex

insomniac

14:23

I see that there was a conversation about the link I posted, so let me add to it: I was taking a bit of a beating (very metaphorically. I mean, on one hand, likes and dislikes don't matter, but on the other hand, they do hurt) for holding a view, which, as I have mentioned several times on the post, is the mainstream view of understanding the system OP has posed a Q about.

All I wanted was scholarly views. I have been using this website for years as an integral component of my own education. I know that there are people capable of them. That is all I wanted

user434058

@tpg2114 That's a nice analogy! I'll try making something in beamer one or the other day.

insomniac

I find it a bit unfortunate that someone in the chat above described it as "advertising". Modernity, I guess. Teaches us all to boil a person down to some fixed set of motives (which, in the modern world, always are ulterior, and always look like they ultimately have to do with money)

user434058

user image

insomniac

Anyway. I wanted to do no such thing. I would still, as I said earlier, solicit scholarly debate. tty all later. I need to pour some ice water on my steaming ears.

ACuriousMind

@insomniac I wasn't really talking about you, specifically. I understand the desire to solicit feedback, and the reason that we generally ask people to not do that for their fresh own posts in here is simply that, if everyone did that, the chat would be flooded with such requests

insomniac

14:33

I respect that

but (and pardon any arrogance; its just my righteous rage) : I do feel that some debates are more worthy of posting here than others

The average around here is not very high

You are not included in that ; I read and learn from your answers

but the majority of questions around here are from either (A) highschoolers or (B) undergraduates who are suffering the effects of privatization of education.

I have no problem with that ; I know what it is like to not have a good professor AND not have a TA to ask questions

Such questions, I agree maybe should be moderated here (although, really, why ? This is a chat afterall, not the main site)

Most of the time here people are just chitchatting here (and NOT about Physics)

ACuriousMind

@insomniac I also sympathize with that view, but we're a site for all levels of education and this is the general chat for users of the site, so we can't really in good conscience enforce a level restriction here that's not present on the site itself.

insomniac

So really, how does posting questions impoverish the quality of this chat?

ACuriousMind

It's not that any single such post is bad, it's that we don't want many of them. If we allowed it in principle, we'd have to debate it anew with every impatient new user that arrives here and just wants an answer to their question by spamming it here. And so "don't advertise your recent posts here" was born in order to have something to point everyone towards. (Have you heard of the categorical imperative? :P)

JMac

@insomniac It's the tragedy of the commons basically. One person doing it isn't really a big deal in terms of chat impact; but if everyone did it, the chat would just be flooded with questions and answers.

insomniac

@ACuriousMind : Anyway, sorry, my mind is de-phasing a bit: thanks for at least not ignoring me. I was feeling a little lynched before you addressed me, so thanks for that. I really need to cool down a bit, so I'll take your leave. Thanks.

Also : there is no tragedy of the commons. A very well argued view is that the modern world we are a part of, that is, Private Property, circulation of capital before all else (I mean, just look at how worried everyone is about the economy, when it is ventilators that is going to really decide how this shit will all turn out. But Try telling that to Wall Street Journal) etc, rests on the systematic taking away of the Commons.

What happened out of intentional construction cannot be termed a tragedy as such.

JMac

14:48

@insomniac There's no tragedy of the commons if you regulate things like we've done here; but if you just let everyone to promote their own questions, you would run into that problem due to the shared nature of the chat.

skullpatrol

15:06

whelp, that was tense...all I can say to insomniac is "try to get some sleep?"

joke^

:-)

insomniac

Yes. stand by and pull out your popcorn bags (but don't intervene). That is exactly what modernity teaches us.

Not that I am judging

Feeling upset due to exchanges on social media is also a very modern phenomenon. I stand guilty.

skullpatrol

I've tried to intervene many times and always end up suspended

insomniac

What situation cannot be rectified by suggesting that someone who is agitated should take a nap, take a chill-pill

skullpatrol

true

insomniac

Its what men have done to their wives for a long time. We all learn to play along.

Anyway. I really have a life outside of this as well (as I am sure you all do). So, I feel I must release you all. Show is over.

skullpatrol

15:12

cya pal

ACuriousMind

@insomniac Honestly, feeling upset because of an online exchange is exactly as valid as feeling that way because of an IRL exchange. If anything I've said upsets you I'm sorry for that.

@skullpatrol You haven't been suspended in four years and none of the instances before that looks like "trying to intervene" and more like "stoking the flames".

Just as you did here - there was no need for you to comment on the conversation, and especially none to tell insomniac to "get some sleep" (subtly suggesting they were being unreasonable when they were doing quite well at staying civil).

John Rennie

@FakeMod when that has happened to me in the past I have posted my own version of the question then posted my answer to it. This for example.

Guru Vishnu

16:08

Is it possible to merge the answers of this question with the one QMechanic mentioned in the comments. Both have received answers and are exactly the same and asked by the same user.

And I don't know which one to VTC as a duplicate.

ACuriousMind

@GuruVishnu merges are messy but possible. Please use a custom mod flag to request merges (no need to do that here now, but chat's not generally the right venue for this)

Novalium Company

16:28

Boom boom pow

Johan Liebert

Can someone help me understand a political statement? (though I think it's kind of hate speech but I don't understand what that statement means)

Secret

Negative dimensions:
to actually get -2+-2=-4, we need a notion of "antidimension". If dimension is the freedom to set parameters, and constants are parameters already set, then negative dimension is a "coercion on parameters"
which interestingly... special relativity does somewhat that with the negative metric signature on time
That negative sign leads to the degrees of freedom to be partitioned out into future present and past
it means there are correlations along that degrees of freedom that makes it less than it is

tpg2114

@JohanLiebert I don't know that this room is the right venue for discussing potential hate speech from politicians. There might be better places (politics.SE?) to have an educated discussion about it. But I don't know how their site views potentially controversial statements

Generally speaking, those kinds of controversial/sensitive topics should be discussed with people who are experts in the area and aware of the nuances of what's going on -- if you want a real analysis of the meaning.

bolbteppa

Periodic reminder qft is really hard

Johan Liebert

@tpg2114 OK thanks!

Slereah

16:57

@bolbteppa you should do string theory instead

Strings are simple

I have a drawer full of them

Aaron Stevens

17:11

@knzhou physics.stackexchange.com/questions/540329/…

I don't really use other SE sites. Where is there a "bad name" of PSE?

Slereah

proforhobo.com

knzhou

Everywhere I've seen a reference to Physics.SE anywhere else on the internet, in the past 4 years, it's been negative.

Here's an example of the kind of thing I don't like:

Aaron Stevens

What is usually said?

knzhou

Here I wrote a completely trivial answer to a trivial calculation and got +7 for it.

And the same day I wrote a slightly less trivial answer and got -2 for it.

As far as I can tell, the second answer is superior to the first in every conceivable way.

But the first was asked in the context of field theory, and the second was in the context of Newtonian mechanics.

Slereah

That's a common problem of physics communities

the invasion of the homework people

Aaron Stevens

17:24

I doubt the down votes on the second one are because of quality though

But I agree, I am not sure why the first one got so many votes

knzhou

Okay, if my second answer deserved a -2, then why doesn't my first answer deserve the same?

It's far more trivial!

Aaron Stevens

IMO the first question is off topic anyway

knzhou

I'm convinced that the real criterion for votes is whether the question uses words found in a high school physics class. Regardless of quality that means the question is facing an uphill battle from the start.

I teach a lot of high school students and I tell them about Physics.SE.

Aaron Stevens

I disagree. There are many simple Newtonian mechanics questions that are well-received here. The problem is that there are also many Newtonian mechanics questions that are off topic as well

knzhou

They uniformly have bad experiences. They're extremely bright, too, (often ranking in the top 20 or top 5 in the country) but their questions are mostly about Newtonian mechanics and the like.

ACuriousMind

17:27

@knzhou Voting is broken, the only alternative that has no problem with allocation of votes is to not have votes at all, which has its own problems.

Aaron Stevens

@knzhou But that wasn't what you were referring to in the comment I linked to. What is said across the internet about PSE in regards to that?

I guess I just want to know what others are saying about PSE

knzhou

Sometimes these folks come by r/physics and complain about PSE while asking their question.

Also, the software engineers hanging out on Hacker News are bitter about it, but they kind of dislike all SE sites.

And again, the high schoolers I meet in (closed) discussion groups don't have positive experiences with it.

Johan Liebert

@AaronStevens well I have seen many negative comments about PSE on the internet. One of them is the first thing you get while searching PSE on YouTube. See(👇)

Aaron Stevens

Complain about what? Are bitter about what?

Johan Liebert

. youtu.be/rTfnTbu_6-g

knzhou

17:33

Anyway, what set me off about that baby question is that it's fine to close for homework, but there was absolutely no need to talk down to the OP, treating them like an absolute idiot, like this.

Rishi

@AaronStevens My classmates in high school and college all state that they think it's for pretty advanced students (typically upper div or grad-level), after looking at the front page.

Johan Liebert

Though he (the one in the video) is insane.

But it's the first thing that you get on YouTube.

Aaron Stevens

@knzhou I didn't think that comment was talking down to anyone. Even though I don't quite agree with it, I think it is a legitimate point.

The comment doesn't say anything about the OP though. It only talks about the question

Johan Liebert

I have reported it as hate speech but there has been no action from YouTube.

ACuriousMind

@JohanLiebert It's been posted here before, John is well aware of it :P

Aaron Stevens

17:36

@Rishi Does that give PSE a bad name?

Rishi

@AaronStevens No. It just discourages them from participating (through answering or asking).

Aaron Stevens

@Rishi But does the site discourage them, or is that their own feelings?

I am not taking a side currently. Just exploring the ideas

knzhou

@ACuriousMind This video reminds me of a couple users that are now (thankfully) gone...

I've always felt that the ideal question policy would be to kick out malicious wackos. Not to kick out people asking questions phrased without tensor indices.

Rishi

@AaronStevens I'm afraid I don't exactly understand the question.

knzhou

You can tell somebody malicious coming from a mile away, and they can do so much more damage to the site than a curious high schooler can.

But malice doesn't break any concrete rules...

Aaron Stevens

17:40

But the site isn't against curious high schoolers in particular

Curious high schoolers probably tend to ask Newtonian mechanics questions that then tend to be closed, but the closure isn't due to the question being by a curious high schooler asking about Newtonian Mechanics

@Rishi I just didn't know if you thought the site actively tries to make users feel like they can't participate if they are not advanced enough

Rishi

17:55

@AaronStevens I don't think so. It doesn't actively try.

Loong

18:09

user image

Aaron Stevens

@Loong Well.... there it is

Charlie

18:35

does light have a worldline or do only timelike particles have "worldlines"?

M.N.Raia

Goodmorning/ afternoon/night for everyone!

Please, if you could, take a look on a question of mine in Computational Physics; they have not a proper physics tag

0

Q: Difficulties on Mathematica code to solve Christoffel Symbols of a particular metric

DISCLAIMER: PLEASE DO NOT PUNISH ME FOR THE .JPGs I) The Problem There's a particular metric $[1],[2]$ in general relativity which is written as: $$ds^{2} = -[c^2-v_{s}^2f(r_{s})^2]dt^2+v_{s}f(r_{s})dtdx+v_{s}f(r_{s})dxdt+ dy^2+dz^2 \tag{1}$$ So my question is: How can I calculate the Christ...

computational-physics mathematica

ACuriousMind

@M.N.Raia Hi, please don't post questions you've just posted in here, if everyone did that we'd drown in that kind of messages.

Slereah

@Charlie you can do worldlines for null particles

but things get harder

because there's no natural parametrization

ACuriousMind

(see also our guidelines linked in the room description)

Slereah

You don't technically need to have natural parametrizations, but it does help for a lot of calculations

Charlie

18:49

@Slereah ty

bolbteppa

19:37

@Slereah You also have a shelf of string books iirc

Slereah

I don't have a lot of string books

Maybe 4

bolbteppa

I have at least 8

Slereah

what is best string book

bolbteppa

GSW

Charlie

Is there a reason some GR notes are reluctant to use the term "tensor product"? Is that frown on mathematically or something?

fornwed*

frowned**

Slereah

19:46

Do you have an example?

Charlie

Well my lecture notes for my course, and there have been instanced i've been reading online and have gotten a few lines in before i realised that's what was being discussed bc they didn't use that term

Slereah

I mean more instances of where they should use tensor products but do not

Charlie

i see "multiply", "direct product", even "combine" sometimes

Slereah

Well a lot of GR uses components instead of vectors

Charlie

i was just wondering if there was some good reason not too

Slereah

19:48

Technically you can't do tensor products of those

Charlie

ah i suppose

ok rt

ty*

Slereah

$u \otimes v = u^\mu v^\nu (e_\mu \otimes e_\nu)$

Multiplying two components is an implicit tensor product

bolbteppa

Who cares to specify that a real vector lives in a tensor product of copies of $\mathbb{R}$, more important things to direct one's energy to

Slereah

I live in a copy of $\mathbb{R}$

bolbteppa

Time is a flat circle of infinite radius

Slereah

19:51

the long circle

17

Q: Why is the Long Line not a covering space for the Circle

I know of several reasons why the long line can't be a covering space for the circle, but I'm more curious in what exactly goes wrong with the following covering map. Let $L$ be the long line and define $p: L \rightarrow \mathbb S^1$ by wrapping each segment of the line around the unit circle o...

algebraic-topology covering-spaces

heh

The real reason people don't use the tensor products is that writing everything in actual tensor notation is a huge pain

$$\mathrm{Riem}(X, Y) - \frac{1}{2}g(X, Y) \mathrm{Ric} = T(X,Y)$$

Einstein notation is amazing and you should use it

bolbteppa

I can't imagine ever using that notation

Slereah

Besse uses something like that

it is pretty bad

Oh wait, this should be the Ricci tensor, not Riemann tensor

bolbteppa

20:07

What's the Ricci scalar in this brave new world's notation

Slereah

Of course, the Ricci tensor is $$\mathrm{Ric}(X,Y) = \mathrm{Tr}(Z \mapsto \mathrm{Riem}(Z,X)[Y])$$

bolbteppa

My god...

Slereah

And the Ricci scalar is then $$R = \mathrm{Tr}(\mathrm{Ric})$$

So the EFE are $$\mathrm{Tr}(Z \mapsto (\nabla_X \nabla_Z - \nabla_Z \nabla_X - \nabla_{[X, Z]}) Y) - \frac{1}{2}g(X,Y) \mathrm{Tr}(\mathrm{Tr}(Z \mapsto (\nabla_W \nabla_Z - \nabla_Z \nabla_W - \nabla_{[W, Z]}) W)) = T(X,Y)$$

Or something like that

now solve the Schwarzschild metric

bolbteppa

Written in God's own language

Now write it in axiomatic set-theory language

Slereah

The Schwarzschild metric is the unique maximally extended solution of topology $\mathbb{R}^2 \times S^2$ with $T = 0$ and a set of Killing vectors that form a continuous group of transformations with orbits homeomorphic to $S^2$

@bolbteppa There is some axiomatic-based GR stuff

I have a book on it

it is quite ugly

Apparently you can describe the Riemann tensor in terms of parallel transport

$$\frac{d}{ds}\frac{d}{dt}\tau_{sX}^{-1}\tau_{tY}^{-1}\tau_{sX}\tau_{tY}Z|_{s=t=0} = \left(\nabla_X\nabla_Y - \nabla_Y\nabla_X - \nabla_{[X,Y]}\right)Z = R(X, Y)Z$$

I wonder if you can define the EFEs in terms of holonomies

sciencedirect.com/science/article/abs/pii/000349169190225W

Apparently so!

bolbteppa

20:29

In terms of parallel transport it arises from $\nabla_{\mu} A_{\nu} = \partial_{\mu} A_{\nu} - \Gamma_{\mu \nu}^{\rho} A_{\rho}$ being zero $\nabla_{\mu} A_{\nu} = 0$ so that integrating $ \partial_{\mu} A_{\nu} = \Gamma_{\mu \nu}^{\rho} A_{\rho}$ over a closed curve gives $\Delta A_{\nu} = \oint \Gamma_{\mu \nu}^{\rho} A_{\rho} dx^{\mu}$ and applying Stokes gives Riemann, in God's language

Slereah

Question is

What is the Ricci tensor and scalar in terms of holonomies!

I also remember that the photon bounce can be expressed in terms of holonomies, since at its core it's just a closed loop of specific curves

maybe you can express the EFE entirely as measurements

I should probably learn EM in terms of holonomies first

Slereah

20:45

journals.aps.org/prd/abstract/10.1103/PhysRevD.24.2160

that seems like a thing

ACuriousMind

@Slereah Curvature is infinitesimal holonomy (sometimes known as the "Ambrose-Singer theorem")

Slereah

I am aware, yes

I'm wondering if you could express GR entirely out of it

and more specifically, observable holonomies

which I guess are holonomies made of timelike and null lines

ACuriousMind

In general, two curvatures are gauge equivalent if they agree on their holonomies along all loops

So sure, in principle you can express everything involving curvatures with holonomies, but essentially you're just replacing the curvature everywhere by the formula that expresses it in terms of infinitesimal holonomy :P

bolbteppa

or it could be like the transition from $F=ma$ to Lagrangians unifying the laws of physics :p

Slereah

very tiny experiments

Slereah

21:14

@ACuriousMind but the point is more that perhaps there is a formulation where non-infinitesimal holonomies of the connection give us some integral of the stress energy tensor

ACuriousMind

@Slereah Sure, they give you the path-ordered integral $\mathrm{e}^{\oint R }$ along some path :P

Slereah

Hm

Would $\exp(\oint T)$ correspond to some measurable quantity

I guess it's kind of the averaged energy condition?

Since the averaged energy is given by $$\int_\gamma T(\dot{\gamma}, \dot{\gamma})$$

I have The Loop in my paper

which is just like

Lines made by observers and test particles

I must throw every possible math things on that loop to find out what I can squeeze out of it

The Synge world-function, Gauss-Bonnet theorem, Wilson loops, everything!

Hopefully some delicious science will come out of it

Balarka Sen

21:40

My dudes

What is a quick, catchy, clickbaity explanation for direction of light to be self-orthogonal?

I don't want math, just clickbait

@ACuriousMind @Slereah

Slereah

what do you mean by "self-orthogonal"?

EM field orthogonal to direction?

Balarka Sen

Lightlike curves on a Lorentzian manifold are $\gamma$ such that $g(\gamma', \gamma') = 0$, no?

Tangential to the light cone at every tangent space

Slereah

Errr I guess the basic reason is that it has to be the same in all frames?

Balarka Sen

That makes sense

Then it can't be anything other than the kernel of the Minkowski metric

Fine, good

Good clickbait

Slereah

"kernel of Minkowski metric" isn't super clickbait but sure

Ten kernels of the Minkowski metric you won't believe

Balarka Sen

21:45

Lmao

I'm writing a blogpost on hyperbolic geometry so taking time to "physically" explain the hyperboloid model lol

Slereah

The hyperboloid is also a shape

Balarka Sen

That's true

Slereah

which is easier to visualize than Minkowski space

Balarka Sen

The metric is weird tho

the metric is induced from the Minkowski metric so

Slereah

americanhistory.si.edu/collections/search/object/nmah_213480

get one for the house

Balarka Sen

21:48

what about hyperboloid of two sheets

thats the real deal

cant get one model, have to get two

Slereah

you can craft a hyperboloid of two sheets out of an egg

Cut the shelf in half

Balarka Sen

I don't believe you that that's a hyperboloid

Seems fishy

Slereah

probably not

I'm not sure what common object is a hyperboloid

bolbteppa

The speed of light is constant, this means $c^2 dt^2 = dx^2 + dy^2 + dz^2$ which means it's self-orthogonal $c^2 dt^2 - dx^2 - dy^2 - dz^2 = g(v,v) = 0$

Can't do any better than that

Balarka Sen

the hyperboloid of two-sheets is easy to "encounter" if you flip your pen along a slanted axis

which many do

suddenly it was a cone and oops you didnt pivot it correctly so now its a hyperboloid

@bolbteppa yeah ill write something to the frame-invariant effect

Slereah

21:57

You can draw the standard diagram

user image

Balarka Sen

i mean i guess it makes sense to say it's the cone if you say all events in the future of the cone point (the event in the space) and all events in the past must satisfy g(v, v) < 0

those are the points from which you send something at at most the speed of light to the cone point and in time past or time future that thing reaches there

Slereah

I have an idea

Hyperboloid of 3 sheets

It will be amazing

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