The h Bar

12:08 AM

@BalarkaSen give up this crazy math thing and go make friends with en.wikipedia.org/wiki/Shiraz_Minwalla and help the world :p

Balarka Sen

I have enough trouble making friends with, and therefore helping, myself

vzn

1:33 AM

> "We hope the laws that we find will be beautiful," says Franck Wilczek, an MIT physicist who won the Nobel Prize for his work on the strong nuclear force in 2004. "We hope that they'll exhibit symmetry, and that they will explain a lot of things in terms of a few hypotheses — so that you get more out than you put in."

↑ interesting BT thx for sharing. the dirty secret of standard model is that its extremely complex, with many moving parts! almost nobody points this out. how many physical constants are there, has anyone counted? incl all particle masses? 21st century epicycles anyone? have long conjectured a soliton theory will eventually actually predict/ derive/ determine many particle masses and explain a lot of things in terms of a few hypotheses, getting far more out than put in... o_O

bolbteppa

1:55 AM

@vzn "It is interesting to note that 15 out of the 18 (the 9 Yukawa fermion mass terms, the Higgs mass, the Higgs potential v.e.v., and the four CKM values) are related to the Higgs boson. In other words, most of our ignorance in the Standard Model is related to the Higgs." spinor.info/weblog/?p=6355

You know well that epicycles were on the right track motls.blogspot.com/2008/07/myths-about-epicycles.html :p

Semiclassical

2:55 AM

@bolbteppa I think epicycles get a bad rap. For one, the whole notion of 'epicycles upon epicycles' is simply a myth; people did attempt to modify Ptolemy's scheme, but for the most part people didn't because (1) the calculations would be too involved for that period in time and (2) Ptolemy's model worked quite well for the level of data they actually had.

Another fact that people forget: With elliptic orbits, you have the sun at one focus. The other 'empty' focus is physically not meaningful but it still exists. It turns out that, if the orbit is not so different from circular---as is indeed the case for a number of planets---then the empty focus approximates the equant in the Ptolemaic model.

gory details here: math.nus.edu.sg/aslaksen/projects/heilbron/…

the claim re: the empty focus appears here but there's not a lot of detail: adsabs.harvard.edu/full/2005JRASC..99..120B

bottom line being that Ptolemy's model, while certainly wrong on principle, was very good at describing the quality of astronomical data people had access to for a very long time.

and that's where a clear dis-analogy with the modern standard model emerges: We've got a huuuuuge amount of good data for the standard model.

Anonymous

5:08 AM

Is "computation basis state" some standard terminology in QM? I haven't ever come across that before

vzn

5:20 AM

@Semiclassical the purpose of a "model" is not merely to "describe data"...

Semiclassical

depends what you're doing.

if you're trying to do science, definitely.

vzn

@Semiclassical definitely what?

Semiclassical

the purpose of a model in science is indeed not just to describe/predict data

what I'm questioning is to what extent medieval astronomers sought to 'understand' the cosmos versus simply attempting to catalogue and predict it

vzn

think of saying it another way. "the idea of the geocentric universe gets a bad rap..."

Semiclassical

yes.

there's a reason it took so long for us to realize it was wrong.

vzn

5:24 AM

@Semiclassical it was pre-scientific revolution. but notice how the copernican revolution almost exactly coincided with the scientific revolution. think thats not a mere coincidence.

Semiclassical

for the level of astronomical data we actually had access to back in the medieval period, the Ptolemaic model was entirely adequate and reasonable.

vzn

@Semiclassical it was also... "wrong"...

Semiclassical

yes. but we only know that now, after the fact.

knowing that wasn't simply saying "oh hurr durr why should we put earth at the center"

vzn

May 11 at 1:27, by vzn

> Whether you can observe a thing or not depends on the theory which you use. It is the theory which decides what can be observed. ---Einstein https://en.wikiquote.org/wiki/Albert_Einstein

One in four Americans 'don't know the Earth orbits the Sun' and only half believe in evolution / independent

2

Semiclassical

And of those who know the earth orbits the sun, I imagine a substantial number would suppose that the reason we have seasons is because the earth is closer to the sun in the summer than in the summer.

But my reason for being disturbed by so many Americans not knowing the earth revolves around the sun is not because "it's so obvious" but because it wasn't---the discovery of such was such an achievement in human understanding and took a lot of work.

vzn

5:36 AM

you remind me a little of a philosophy teacher in college (philosophy of AI class) who told me that the medieval debate about "how many angels can dance on the head of a pin"... (paraphrasing) "gets a bad rap"...

Semiclassical

Not knowing the history of theology well enough, I'll decline to consider such.

But I consider that a silly analogy, frankly.

vzn

some might think defense of epicycles (~½ millenia old discarded theory) is silly.

Semiclassical

Depends on what we're defending.

vzn

← really does try to sympathize/ empathize with ignorant humans, doesnt always succeed, ie ½ millenia old ancestors struggling to find the truth/ light in the darkness... :P

Semiclassical

Given the wealth of astronomical data one had access to a few centuries later---plus, y'know, the advent of Newtonian mechanics and calculus---it would indeed be silly to suppose the Ptolemaic system were literally true.

But that is not the position that the medieval astronomers were in.

From where they stood, the Ptolemaic model was perfectly satisfactory in order to predict where Mars was going to be seen in the sky on a given night.

vzn

5:45 AM

even copernicus basically criticized the ptolemaic astronomers, maybe you dont share his view on that.

Semiclassical

That's so vague a statement as to be worthless.

Criticized them on what? It can't be on observational grounds.

vzn

you quoted it yourself. in here.

Semiclassical

Yes, because I actually bothered to read up on what he objected to.

vzn

Apr 30 at 19:56, by Semiclassical

"Moreover, they have not been able to discover or to infer the chief point of all, i.e.
the form of the world and the certain commensurability of its parts. But they
are in exactly the same fix as someone taking from different places hands, feet,
head, and the other limbs - shaped very beautifully but not with reference to one
body and without correspondence to one another - so that such parts made up
a monster rather than a man"

↑ sounds like a criticism to me...

Semiclassical

Just saying "oh he criticized the ptolemaic model" is so vague as to be useless

Yes, and he was (I think?) the first to realize that the ptolemaic model had that problem.

but you'll notice there: it is not a problem of describing the motion of one planet. it is the problem of placing all of the planets into a single system of motion

That was, indeed, an achievement.

But it was an insight gained from having a very deep understanding of how the Ptolemaic model worked.

Plus, it's an insight into theoretical astronomy. The main practical use of astronomy that I know of in those days was in order to cast accurate horoscopes.

vzn

5:51 AM

lol ok yeah

if you want to defend the ptolemaics, maybe you might also like to look into newtons alchemy... am sure he had "good intentions" too... o_O :P

Semiclassical

I'm fully prepared to defend any person who is using the tools, data, and practices they have access to in order to make what progress they can.

It is easy to mock the Ptolemaic model; it was far harder to come up with evidence in order to overturn it.

vzn

this is such a strange conversation, am struggling with words/ thoughts/ responses. latest idea: maybe 1 or both of us (or neither) have an ancestor who was a ptolemaic astronomer.

Avnish Kabaj

My apologies

This was not a piracy talk

Semiclassical

Mostly I just think that talk of geocentrism being 'obviously' wrong and heliocentrism being 'obviously' right is anachronistic.

Once you've got Newtonian physics, the former is of course quite implausible and untenable.

But before that? Best you had was Aristotle and the notion of uniform circular orbit as the most 'perfect' motion

vzn

in Computer Science, Jan 26 '14 at 18:23, by vzn

"a great many ppl think they are thinking when they are merely rearranging their prejudices" –wm james psychologist

Semiclassical

6:02 AM

Ok?

vzn

not sure if "anachronistic" is the right word, but you seem to be referring to "conventional wisdom" and how it can shift over time... and just because someone "knows" the conventional wisdom doesnt mean they really understand it...

Semiclassical

I guess I'd say it's like saying a push sign on a door is 'obvious.' If you haven't learned to read in the first place, then that's not going to be obvious.

vzn

sig other says (lying on couch reading this on our new wall prj): "to be able to have wisdom you need to have a lot of empathy". maybe your point is something like to have empathy with ptolemaic astronomers...

Semiclassical

I'm not sure 'empathy' is the right word, given how tied to emotional state that is.

But it requires one to be able to think from the standpoint of those astronomers, to consider what tools and data they actually had access to.

vzn

hey, do admit they were humans, struggling in their moment, deja vu, know the feeling, can certainly relate to that

@Semiclassical it does also remind me of that expr "moving the goalposts"...

Semiclassical

6:06 AM

Right.

And to assess medieval astronomers by the standards of Newtonian physics, let alone modern science, is to shift their goalposts substantially.

vzn

lol sig other getting very emotional because you didnt immed agree with her reframing :P

Semiclassical

Well, I think empathy is just not quite the right word. Close but not quite.

vzn

lol missed by that )( much

vzn

6:21 AM

> So this is potentially as many as 26 parameters in the Standard Model that need to be determined by experiment. This is quite a long way away from the “holy grail” of theoretical physics, a theory that combines all four interactions, all the particle content, and which preferably has no free parameters whatsoever.

Semiclassical

that makes the epicycles example look simple by comparison :P

but the breadth of data for the Standard Model dwarfs that of planetary astronomy, I think

So the constraints which the Standard Model is subject to are a good deal more stringent. In that regard I'm hesitant to take the Standard Model to task.

vzn

@Semiclassical agreed, but scale is relative. the philosophers of science havent exactly noticed this but new theories are far more complex than previous ones. standard model is very large, but so is human grasp of the data it covers. dont think its "too big to fail" so to speak... (to use an infamous 2008 expr wrt US banks...)

Semiclassical

I don't know the state of Standard Model tests well enough to say either way.

vzn

@Semiclassical afaik its nearly ~½ century old.

Semiclassical

sounds right

the tricky thing is also that there's stuff which is 'beyond' the Standard Model in the sense that there's a few ways to incorporate the phenomena and it's not clear which one is right

that's beyond the Standard model but not out of reach of it.

And then there's stuff which is not within the domain of the standard model and so can't be predicted by it.

vzn

6:34 AM

admittedly/ concede its unthinkable to question the standard model at this point, but on other hand, there is mass swathes of advanced theoretical physics research that does not really fit into it. think someone should try to do some kind of inventory of that, and the results might surprise some ("conventional thinkers")...

Semiclassical

The tricky thing is to find unambiguous examples which are both beyond the Standard Model but can't be easily incorporated into it (and still fall within its domain)

vzn

eg string theory is not exactly "standard model" right? or significant parts of string theory? etc!

Semiclassical

Could not tell you.

Time will tell. I'm curious if this observation of sterile neutrinos will hold up.

But I won't know it tonight, so time to sleep

later

vzn

one thing about the neutrino stuff thats clear to me is that it seems the data size is too small, it takes too long to collect very few samples, it would really shock me if thats the "falling hair" that causes a major crack in the foundations...

same here nice chatting bye

Physics Meta

7:32 AM

0

Q: Do we need a new moderator?

One of our moderators was last seen on the 22nd of May; it's now the 3rd of June (the same year, luckily). He's also a mod on Chem SE; he was last seen there 1 month ago. I don't want to name him, even though it's obvious, but he was a really nice and helpful guy when he was active 3 years ago (h...

Secret

I really hope there are no experimental error screwing up the sterile neutrino results this time. I want to play with 1eV ghostly bags of stuff!

topologically_astounded

8:02 AM

@JohnRennie: I posted a question which I wanted to bring to your attention. Read many of your responses elsewhere, they were wonderful. Thanks

John Rennie

8:20 AM

@topologically_astounded that's really too broad a question. We tend to like posts that ask a single question and be answered with a single answer.

Emilio Pisanty

8:43 AM

@DanielSank quantum computery is probably rather too much of a stretch

The closest I can do is singular optics

I can share more details but I need a private channel

Anonymous

9:25 AM

@Mithrandir24601 Need some halp again: What does this mean - "Therefore, in a sense, the classical FFNN only considers the situation in which all
the hidden layer neurons are activated, while QPNN traces all possible situations
simultaneously and computes their expectation value." on page 8

Secret

tardis.wikia.com/wiki/Weeping_Angel

I think it is safe to say that "time energy" is really heat when an open system slowly dissipates as entropy is produced

Anonymous

I had the impression that even a feedforward NN traces all possible situations

Mithrandir24601

@Blue Sounds very much like: Classical FFNN is classical and so, the neurons can only have a specific number, while QPNN allows for superpositions of neuron/quron values

Secret

which from the perspective of someone in the present, is an instant source of energy since the total amount of heat dissipated from $t_0$ to $t_1$ as the system age will be concentrated at wherever the system will be in the present

Anonymous

@Mithrandir24601 Quron's activation values are still numbers only even in a QNN

Anonymous

9:29 AM

Maybe they're trying to say something from (11)

Mithrandir24601

@Blue but you can have a superposition of $\left|0\right>$ and $\left|1\right>$ for the qurons in the hidden layers, right? Otherwise, there's not going to be any advantage to using a quantum computer in the first place

(I haven't read it, but it sounds very much like the standard pop-sci classical computers can trace one path at a time, while a quantum computer can do all paths simultaneously)

Anonymous

I think they're hinting at the fact that the conditional probability of getting a certain output state given a certain input state is a summation over all the possible states of the hidden layer (the index i). While in classical FNN the activation of the hidden layer gets fixed if the input layer activations and the weights are known. My point is: Even in a QNN we would know the state of the hidden layer given all the weights, isn't it ?

Anonymous

@Mithrandir24601 Sure but my point is that that superposition is determinable if we have knowledge of the weights from the input layer to the hidden layer

Anonymous

So we aren't tracing through more possibilities than what a classical network already does

Mithrandir24601

9:45 AM

@Blue In principle, if you know all the weights (which you might not), sure, you'd know the state, but that's exactly like saying if you know the circuit, you know the final output state - this is true, except that actually calculating the state is exponentially hard.

I really think you're reading too much into this one sentence - I've lost track of the times I've read such things and all it's ever meant is that you've got a quantum computer doing the operation and it uses superposition, entanglement etc.

@Blue Sure, but this is how you get the speed-up. Remember, a QTM isn't any more poweful than a classical TM, but a physical QC can solve (certain) problems much faster than a classical computer

Anonymous

Okay that makes sense then. My only remaining issue is this part: "the classical FFNN **only** considers the situation in which all
the hidden layer neurons are activated"

Anonymous

That doesn't sound right

Anonymous

Even in a classical FFNN, you can have some inactive neurons in the hidden layer

Anonymous

Yet you can get an output

Anonymous

So that part doesn't make sense

Mithrandir24601

9:51 AM

@Blue Yeah, this is something that doesn't really make sense to me either

Do you have that link to the article again?

Anonymous

link.springer.com/article/10.1007%2Fs11128-017-1692-x

Mithrandir24601

@Blue Oh good, thanks :)

Anonymous

I'm now starting to have some doubts about the quality of the paper and whether I should proceed with it at all :/

Anonymous

Found two mistakes till now

Anonymous

One of them is rather more about terrible notation

Anonymous

9:55 AM

On page 7 where they are calling $(\beta_2)_1^i$ a transition matrix

Mithrandir24601

@Blue I've heard this before about QNN stuff - most of the people who work on NNs don't have any physics experience, while most of the people who know the physics don't have any experience with CS, so there are apparently a fair number of 'birthing issues', for want of a better word

Anonymous

Guess I'll only stick to the famous guy's papers after this one :P Lloyd also has a similar paper about Hopfield networks which he uses for genetic sequence recognition. At least I can trust those :P

Anonymous

@Mithrandir24601 And also very little literature exists in this area!

Mithrandir24601

@Blue Yeah, Lloyd is apparently a genius or something

@Blue Yep, that's... confusing... I also really didn't like their (ab)use of Einstein notation

Anonymous

I still couldn't understand what Lloyd's profession is. A mechanical engineering professor who does machine learning and quantum mechanics. I wonder why he doesn't join the CS or electrical department instead. He hardly had any work in mechanical engineering other than the control theory stuff which hardly anyone would call ME. :P

Mithrandir24601

10:00 AM

@Blue Maybe they view it as being able to set all the weights for that neuron to 0 or something. Not clear, but the only thing I can think of that makes sense

Anonymous

We can set all weights to a neuron in the hidden layer to 0 in both classical and quantum case I think

Mithrandir24601

@Blue don't see/know of any reason you couldn't...

(OK, that's really not saying anything :P )

Novalium Company

@Blue Sup.

Anonymous

Hmm, well I guess I'll proceed with the next part and fry my brains a bit more. Thanks for the help

Anonymous

@NovaliumCompany olla

Novalium Company

10:09 AM

I learned about permutations and combinantions but it's really hard logic to grasp. But I revised derivatives good enough.

Anonymous

Gòod. I'm coming in a few minutes

Anonymous

Bathing now lol

Novalium Company

Oks :D

CooperCape

For real tho, how does one pronounce Lebesgue.

Novalium Company

@Blue Would you tell me what you would like me to learn on my own, because today I won't have much time to chat?

CooperCape

10:17 AM

I feel like I butcher in my mind every time.

ACuriousMind

@CooperCape Approximately like "lebek", with stress on the second syllable

CooperCape

Ah, nice one. Thanks :p

At least now I have a way to read things.

Anonymous

@NovaliumCompany Watch this fully and ask me if stuck

Novalium Company

10:33 AM

@Blue Isn't there other video or something. This is such a bad quality, and looks hard.

Actually @Blue what are you even teaching me :D?

Anonymous

@NovaliumCompany Classical mechanics

Novalium Company

What exactly in classical mechanics?

Anonymous

Lagrangian and Hamiltonian

Anonymous

Necessary to understand quantum mechanics properly

Anonymous

And is also helpful for some parts of relativity

Novalium Company

10:36 AM

So I shall google Lagrangian and Hamiltonian Mechanics and learn them?

Anonymous

Good luck doing that :P

Anonymous

If only it were that easy :)

Novalium Company

Then why not learn something else?

Anonymous

That lecture series I linked is "gold", and by one of the best theoretical physicists I've ever known

Anonymous

It will take some patience to get through but it is worth it

Anonymous

10:38 AM

@NovaliumCompany Like?

John Rennie

@NovaliumCompany I think Lagrangian and Hamiltonian mechanics might be a bit advanced for you right now

Novalium Company

And In the end, what will I know ?

Anonymous

I mean I could teach you the watered down versions of special relativity and QM too, but I don't really like that pedagogy

Anonymous

You could refer Shankar's lectures for that

Anonymous

Meant for pre med students

Novalium Company

10:40 AM

The problem is that I don't know what I want to know :D

Physics Meta

0

Q: Voting button captions on meta

In meta, when you run your mouse over the upvote button for questions, it displays the same hint as it does for the normal SE: "This question is well-researched and clear..." or something of the sorts, and the upvote button for answers says "This answer is useful". I think that this needs to be ...

feature-request voting

Anonymous

@NovaliumCompany Then perhaps go for something easier like Shankar's physics lectures for medical students at Yale

Novalium Company

wut?

Anonymous

As JR suggests, maybe that'll be better for you

Anonymous

youtube.com/playlist?list=PLW7IIVtPOpdZF1362eJn5sJlaaqFJWRe4

Anonymous

10:43 AM

For quantum mechanics I don't know any such (good) lecture series at an easy level though

Novalium Company

Ok, let me first learn everything else except qm and relativity.

John Rennie

@NovaliumCompany I'd start with whatever you are most interested in

Novalium Company

^

John Rennie

When I was your age I was fascinated by general relativity. It only took me 40 years to learn it :-)

(learn bits of it, that is)

Anonymous

The problem is that you could be interested in quantum field theory, but if you don't have the necessary background (which you need to work hard for), it will turn you into a 15 year old crackpot rather than a 15 year old genius :P

CooperCape

10:46 AM

@Blue I was gonna say that could become an issue if you're interested in things like that.

John Rennie

@Blue I'm not sure that's true. The basic ideas in QFT are pretty simple.

ACuriousMind

@JohnRennie The problem is that you can't do anything with the basic ideas

Any specific prediction requires loads of technical calculations

John Rennie

At age 15 your aim should be to get the basic idea of things, then start down the rabbit holes as you try to learn the detail :-)

@ACuriousMind a 15 year old isn't going to be calculating scattering cross sections :-)

Anonymous

@JohnRennie After seeing people like Balarka I think my views about that changed :P

ACuriousMind

@JohnRennie Yes, but again, you can't do anything else with it, so I'm not sure I see the value in learning QFT if you're not going to compute cross sections.

John Rennie

10:48 AM

@Blue OK, OK, most 15 years olds ... :-)

Mithrandir24601

@Blue If you want to be able to do stuff with QFT at age 15, then you'd need to start learning physics before you're 15 :P

ACuriousMind

There are basically no neat phenomena that are explained by QFT but not QM - all the stuff you need QFT for is hidden away in colliders and superconductors and other "exotic" circiumstances

John Rennie

@ACuriousMind I can't calculate scattering cross sections, but I found it enormously gratifying to at least grasp the basic idea of QFT.

ACuriousMind

@JohnRennie I imagine a large part of that gratification came from finally understanding what all the misleading pop-sci presentations really wanted to say. But if you had not been exposed to such bad accounts of QFT, I imagine your desire to learn it properly would have been not as strong

Anonymous

@Mithrandir24601 I agree with that. For that reason, now I wish someone had introduced me to stuff like linear algebra and calculus when I was 10, rather than making me mug pages of history and geography (99% of which I've forgotten now)

Anonymous

10:53 AM

And I wish I had spent the time reading pop science books in doing something actually interesting (like real science)

ACuriousMind

QFT's selling point is not its ontology, not that the basic ideas suddenly explain something not explained before - it's that its specific, quantitative predictions are the best we know.

John Rennie

> finally understanding what all the misleading pop-sci presentations really wanted to say

YES

That is exactly why is was so gratifying!

Mithrandir24601

@Blue Ahh, no - I wouldn't say that! If taught well (which may not be the case, to be fair), arts and humanities are so important, so ignoring them in order to learn physics isn't a great idea either

John Rennie

I'd never thought of it in those terms before, but you've put your finger bang on it!

Anonymous

@Mithrandir24601 The point is that the way I was taught history and geography was more of a torture

Anonymous

10:54 AM

I'd prefer doing something more interesting like calculus in that place

John Rennie

One lifetime soon I will write a series of books XXX for non-non-nerds where I'll explain what pop-sci books really mean.

Mithrandir24601

@Blue I enjoyed reading the murderous maths books - they made the subject interesting, which is really important for children

Anonymous

Of course history and geography are important, but I'd rather pick them up out of personal interest rather than them being forced into my throat

Mithrandir24601

@Blue This is reasonable, yeah

John Rennie

@Blue My instinctive response to that would get me immediately banned :-)

Mithrandir24601

10:57 AM

I didn't realise that history was interesting until about Uni :P

John Rennie

@Mithrandir24601 the History SE is fascinating reading for when you want to idle away a few hours.

Mithrandir24601

@JohnRennie My interest is quite niche...

ACuriousMind

Man, I really liked history in school. The teacher had one of the sharpest tongues I've ever heard and was incredibly disdainful of the "memorize these three dozen dates" approach to teaching history.

Novalium Company

Guys, for now I will learn what's interesting to me and what I would actually want to know. Thanks.

John Rennie

@Mithrandir24601 not uncommon. For a long time I was fascinated by the Mesopotamian civilisations before eventually learning that humans tend to behave in similar ways throughout history. Sam is fascinated by the collapse in societies around 1000 BC.

Mithrandir24601

11:02 AM

@ACuriousMind I liked my teachers quite a lot, but just didn't find the subject interesting, until I discovered historical martial arts

Novalium Company

Guys, what's the point of L... and Himel... mechanics?

CooperCape

lagrangian/hamiltonian?

Novalium Company

yep...

ACuriousMind

@NovaliumCompany You're not the first to wonder that ;P physics.stackexchange.com/q/89035/50583

Novalium Company

Actually guys, what is the point of QM, why so many people learn it?

ACuriousMind

11:07 AM

Because there are phenomena in the real world that are explained by QM but not by classical mechanics?

Novalium Company

I mean, what can we do with QM?

ACuriousMind

I don't think everyone who learns physics is concerned with what they can "do" with it; I certainly wasn't.

Novalium Company

So in the end, you've learned QM and you don't know why?

CooperCape

practical application of qm

Just for an example

ACuriousMind

But as an example of practical application, see e.g. quantum Hall effect, transistors and semi-conductors, etc.

Novalium Company

11:09 AM

Ok, now it makes sense.

ACuriousMind

@NovaliumCompany I know exactly why - because I wanted to learn how the world works.

Anonymous

@NovaliumCompany People learn QM to explain natural phenomena...the things observe around you

Anonymous

A scientist's job is to question why the world (or rather the universe) is the way it is

Novalium Company

Ok sorry, makes sense.

ACuriousMind

To me, curiosity - the desire to understand that which you do not yet understand - is entirely disjoint from any concern of what one can use such understanding for.

Anonymous

11:11 AM

While an engineer looks for applications of quantum mechanics, say for making better computers

Novalium Company

Engineering is more my side then :D

Anonymous

And a mathematician's job is to mock both of them for being non-rigorous :)

CooperCape

Dec 15 '17 at 20:53, by Blue

$\pi=e=1$

Yeet

Mithrandir24601

@Blue I do know mathematicians who are technically quantum engineers, just to confuse things a bit :P

Anonymous

@Mithrandir24601 They mock themselves. No paradox here

Mithrandir24601

11:15 AM

@Blue hehehe. Almost. They mock the non-mathematicians, the non-mathematicians mock them ;)

Novalium Company

For now guys, I'll just learn what interests me. I'll improve my 'before 12th grade' physics and when I am comfortable with it, I'll dig deep in to the small holes.

John Rennie

> I'll dig deep in to the small holes

Careful, they go down a long way! :-)

Novalium Company

...

See you for now people :)

Also guys, if I go to a university with physics, I'll still learn these stuff so...

Anonymous

11:34 AM

Okay, I'm sort of aware of a classical mux

Partha Sarker

Hello, people.

If I want to start learning Diff Geo from Spivak's 5 volume series, what prerequisites should I learn?

Anonymous

11:53 AM

Anything "differential" normally needs multivariable calc for a start

Anonymous

Actually some linear algebra would be useful too

topologically_astounded

@JohnRennie @JohnRennie

@JohnRennie Yes I understand that it is a broad question. More frustatingly, I couldn't even come across that question whose duplicate I ended up asking.

heather

12:26 PM

morning

ACuriousMind

afternoon

Sha Vuklia

hey guys, a changing magnetic field induces an electric field, but that doesn't mean that each electric field is caused by a changing magnetic field, or does it?

ACuriousMind

@ShaVuklia No, it doesn't. E.g. the field of an electric charge is certainly not caused by a changing magnetic field

Sha Vuklia

but the electric field of the free charge does induce some changing magnetic field, doesn't it? since $\nabla\times E=-\partial B/\partial t$

or does that equation only hold for induced electric fields?

ACuriousMind

The electric field of a free charge is irrotational, i.e. $\nabla \times E = 0$.

Sha Vuklia

12:38 PM

oh right

ofc

alright thanks!

lılostafa

1:36 PM

@vzn Only 53% of adults know how long it takes for the Earth to revolve around the Sun.

Only 59% of adults know that the earliest humans and dinosaurs did not live at the same time.

Only 47% of adults can roughly approximate the percent of the Earth's surface that is covered with water.*

source: survey by the California Academy of Sciences sciencedaily.com/releases/2009/03/090312115133.htm

@DanielSank You once said you work with COMSOL in your research. Do you have any experience with its Electromagnetic Waves, frequency domain solver? I've tried everything to reproduce the band diagram of the structure presented here, without success

Slereah

@lılostafa One solar year

obviously

None of that sideral year!

also I think those are specifically for Americans

which are a special people

Secret

1:51 PM

[][][]:

The Scope - The Triglavian Collective Revealed

►

Harsh flux of spacetime is soooo lame

DanielSank

2:09 PM

@lılostafa I haven't used it myself. Others in the group do.

ACuriousMind

2:32 PM

→ 7 messages moved to SecretLabs (SE Branch)

Secret

that triglavian stuff is technically part of the above mess that is being moved

It is interesting that it stays

ACuriousMind

@Secret Take it as an indicator of the comprehensibility of your posts that I was unable to see that.

Jake Rose

Can somebody point out where im going wrong here

$w=\int_{r}^{\infinity}f.dr$

CooperCape

\infty*

Jake Rose

Thats the work done by me moving an object from infinity to r away from some body in a gravitational field

(wont let me edit)

CooperCape

2:43 PM

:c

Jake Rose

So that is -ve of the work done b gravity which is $\int_{\infty}^{r}GMm/r^2$

The limits should be swapped on my first expression

So potential energy = $-\int_{\infty}^{r} GMm/r^2dr$?

Anonymous

That negative sign shouldn't be there

Jake Rose

Why not?

Anonymous

$r$ is decreasing

Anonymous

$dr$ is negative

Anonymous

2:49 PM

First of all, let's clear this up: What is the definition of GPE?

Jake Rose

Work done by an observer moving a mass from infinity to some distance r reversibly

Anonymous

The direction of the force the observer has to exert is opposite to the pull of gravitation and the direction in which the object is being pulled by the gravitational field, right?

Jake Rose

Yes

Anonymous

So the observer (in your words) is doing negative work

Anonymous

Now write the equation for that (for the work done by observer)

Jake Rose

2:51 PM

I thought thats what I had written :')

Anonymous

Always use the vector notation in such cases

Anonymous

I don't see you using the vector notation

Anonymous

Say $\vec{F}_g$ is the gravitational pull on the object

Anonymous

And $\vec{r}$ is the position vector of that object w.r.t to the body which is pulling it gravitationally

Anonymous

Now write the gravitational potential energy in those terms

Anonymous

2:53 PM

Vectorially

Jake Rose

$-\int_{\infty}^{r} \vec{F}_g \vec{dr}$

?

Anonymous

Okay, cool. Can you explain how you wrote that?

Jake Rose

The total work done by the force of gravity from infinity to the point, then as work done by me=-work done by gravity you have to include a -ve

So is the problem that with my integral if I dont do it vecotrally Im making an implicit assumption that dr is increasing

So I need a -ve to correct for that?

Anonymous

Let's break it into more parts. We are trying to find the work done by you, in moving the object from infinity to $r$. What's the work done by "you" in moving it from a location $r$ away from the body exerting the gravitational pull to a location $r-dr$ away from that body exerting a gravitational pull, on the object which you're supposed to move? Remember that the force you're exerting is $-\vec{F_g}$.

Transcript for