The h Bar

12:06 AM

If I have a planet rotating a star, when we consider the total energy it is the linear kinetic energy plus the rotational kinetic energy

is the ‘rotation’ the spinning of the planet about its own axis or about the stars axis?

enumaris

@alarge I don't get anything other than the hourly rate

no bonus, no benefits, no nothing

I think only equipment (and space obviously) is provided

@alarge I am an employee of the company that is contracting out my services

alarge

Right, so what does it then mean if they match the offer, as in what did you factor in etc? (I seem to remember you waiting to see whether your current employer is going to match, but maybe I'm just confused)

enumaris

12:24 AM

The offer was for full time employment

but by match I meant match the base salary basically

but obviously I have to consider all the factors

enumaris

12:41 AM

@alarge by "current employer" I meant the company that "practically" employs me - the one that I work for - not the company that I "technically" work for (by law). The company that I do the work for (or the client if you will) wanted to convert me to full time employee.

alarge

Ah so you contract through an agency? And your contract doesn't prohibit you from moving from the agency to the client?

enumaris

there's some stipulations in the contract

I can't just convert immediately since then the agency won't make any money

those are details

Jake Rose

12:58 AM

Can anybody see how that last line is true?

alarge

1:20 AM

@JakeRose What do you mean by the last line?

dmckee

2:40 AM

@SwapnilDas A beautiful subject, to be sure, but also one that features "straightforward" problems that resulting in turgid equations requiring the most damnedly clever mathematical trickery so solve. Which makes it as frustrating as it is beautiful.

GodotMisogi

3:12 AM

I like having colds because the medicine the campus provides gets me nicely stoned

@ShaVuklia It's ironic shitpost. Unfortunately post-ironic shitpost hadn't been invented at the time

GodotMisogi

3:30 AM

@danielunderwood Something like that. The "operator algebra" method takes a linear ordinary differential equation and treats it like an algebraic equation in $D$, where $D$ is the derivative operator. So indeed in general you're factoring the derivative operator into its "roots" $\prod_i (D - a_i)y(x) = f(x)$ and then using first order linear ODE theory to draw conclusions about the solutions

Mostly, the key lies in $(D-a)^{-1}R(x) = \exp(ax) \int \exp(-ax)R(x)\,\mathrm{d}x$ for constant $a$, and can be more general with $\exp(ax) \mapsto \exp\left(\int a(x)\,\mathrm{d}x\right)$

Leaky Nun

7:58 AM

@JohnRennie you can travel to the past in the event horizon because of the schwarzschild metric right? what does that even mean? how do you even define time?

also if space inside the event horizon unidirectionally flows inward, then where is the new space coming from?

John Rennie

@LeakyNun as far as I'm aware the Schwarzschild metric does not allow travel to the past

Leaky Nun

I don't even know what the normal spacetime interval implies

everything is squared, so the directionality is lost?

how does $\Delta s^2 = \Delta x^2 - c^2 \Delta t^2$ imply that time flows unidirectionally?

John Rennie

@LeakyNun space does not flow inwards to the black hole. In fact space isn't a physical thing and doesn't flow at all. It is possible to choose coordinates that are flowing inwrds. These are the Gullstrand-Painleve coordinates. But you have to be clear that it's just the coordinates that are moving not anything physical.

@LeakyNun time doesn't flow at all in GR. There is no concept of the flow of time in physics generally. In fact there isn't a directionality of time apart from the one offered by the second law of thermodynamics.

Leaky Nun

well I mean something like (x,t) = (0,0) can cause (1,1) but not the other way round

spacetime interval is about causality right

John Rennie

Yes, but causality is bidirectional. Two points are causally related if they are linked by a timelike curve. But whether A causes B or B causes A is a human distinction.

Humans certainly experience time flowing, and flowing in only one direction. But this idea of time flowing does not exist in physics.

For all we know it is an artefact of the way human consciousness works.

Leaky Nun

8:08 AM

hmm

interesting

John Rennie

I should add that this whole idea of the flow of time is controversial. Basically no-one really knows what is going on.

The idea that the flow of time is an illusion of human consciousness is called eternalism:

Eternalism (philosophy of time)

Eternalism is a philosophical approach to the ontological nature of time, which takes the view that all existence in time is equally real, as opposed to presentism or the growing block universe theory of time, in which at least the future is not the same as any other time. Some forms of eternalism give time a similar ontology to that of space, as a dimension, with different times being as real as different places, and future events are "already there" in the same sense other places are already there, and that there is no objective flow of time. It is sometimes referred to as the "block time" or...

Leaky Nun

what does it really mean for an object to travel

sure, I see there's an electron in (x=0,t=0)

and also one electron in (x=1,t=1)

are they the same electron?

John Rennie

In GR an object like an electron is described by a world line. This is a curve in 4D spacetime so it exists in time and space. This curve is a static object i.e. it does not evolve with time - it occupies both space and time.

Your two positions are two points on the world line.

Leaky Nun

I don't think that answers the question

John Rennie

We generally use a parameter called the proper time to describe where points are along the line i.e. the distance measured along the world line between two points is the proper time.

But the electron does not travel along the world line.

Leaky Nun

8:21 AM

let's say I have two electrons travelling towards each other with the same velocity

at t=0, x1=-1 and x2=1

John Rennie

To write down the equation of the world line we have to choose some coordinates, and when we do this we have space coordinates and a time coordinate and we can write a velocity as dx/dt.

Leaky Nun

and then at t=2 you detect two electrons at x=-1 and x=1 with velocities -1 and 1 respectively

Physics Meta

0

Q: On Bra & ket notation

I was asking a QM question recently, and wanted to use the bra & ket notation; however, the mathjax does not support the necessary packages, as far as I can tell. Is it possible to add the necessary requirement to the Mathjax system so that we can use the bra & ket notation easily while asking q...

Leaky Nun

let's say I have two electrons travelling towards each other with the same velocity, so at t=0 we measure x1=-1 and x2=1. then at t=2 you detect two electrons at x=-1 and x=1 with velocities -1 and 1 respectively. did the two electrons switch positions?

John Rennie

But we could choose other coordinates and we'd get a different "velocity".

The velocity $dx/dt$ is a choice of coordinates not something fundamental.

It turns out there is a fundamental velocity called the four-velocity, but this is always equal to the speed of light!

@LeakyNun that's a meaningless question because it depends on your choice of coordinates

Leaky Nun

8:24 AM

what do you mean?

John Rennie

This is one of the hardest things to understand in GR. We are used to thinking of space and time coordinates as physically real because we experience them every day. But what GR tells us is that coordinates are just a choice of how to label points in spacetime and aren't physically meaningful.

This is why different observers can observe the same thing in radically different ways. Because the coordinates they have chosen don't match up.

ACuriousMind

@GodotMisogi Unfortunately, stoned while sick is only moderately enjoyable :P

John Rennie

@LeakyNun I need to work now for a bit but I'll be around later.

Leaky Nun

ok thanks

Slereah

hello

ACuriousMind

8:29 AM

@LeakyNun It is consistent with this observation that a) the electrons started moving with that velocity right after t=0 and indeed switched positions but also that b) the electrons didn't move at all between t=0 and t=2 and just gained that velocity. You simply haven't given enough information to say anything for sure.

Leaky Nun

t=0:x1=-1,v1=1,x2=1,v2=-1
t=2:x?=-1,v?=-1,x¿=1,v¿=1

ACuriousMind

If you're trying to ascertain what "it means for an object to travel", then electrons - or any other fundamental particles - are a particularly bad choice since they're all indistinguishable from each other. They have no "identity" in the naive sense. That is, you first need a firm idea of what uniquely identifies "an object" before you can ask how it "travels" :P

Leaky Nun

ok let's do this. when 0<=t<=1, I observe two electrons in positions -t and t and having velocities 1 and -1 respectively; when 1<=t<=2 I observe two electrons in positions -t and t having velocities -1 and 1 respectively

Slereah

@ACuriousMind just consider a universe with only one electron

ACuriousMind

@Slereah Poor lonely electron

@LeakyNun Are you using electrons for any particular reason or can we substitute classical billard balls? :P

Slereah

8:34 AM

you can consider the trajectory of an electron

but you're gonna have a nasty surprise

Leaky Nun

@ACuriousMind ok, let's use billard balls

ACuriousMind

Well, then it's easy - if we work out the classical elastic collision we know that they hit each other in the middle, and each is now returning in the direction it came from.

We then also know that the two balls must have exactly the same mass.

Leaky Nun

how do you know they didn't just pass through each other :P

ACuriousMind

Because billard balls don't do that

Slereah

what if they quantum tunnel

ACuriousMind

8:36 AM

And if you really want to know you can just paint one of them differently

Existental ontological problems solved by application of paint - that's classical mechanics for you :P

Leaky Nun

ok can we go back to electrons? :P

Slereah

you can also use a sticker

ACuriousMind

@LeakyNun Sure. Electrons have no definite trajectory and your setup doesn't work for them to begin with.

@Slereah or googly eyes

Slereah

I think googly eyes would interfere with the geodesic motion

GodotMisogi

9:38 AM

@ACuriousMind Most of my symptoms are gone fortunately, so I'm just stoned

And there's nothing like studying GR when you're high as a kite

Slereah

9:56 AM

I wouldn't recommend it

Physics Meta

10:59 AM

0

Q: Why don't we have the physics MathJax extension enabled? Should we?

As a recent question points out, it would be nice if the site's MathJax environment included something similar to the physics LaTeX package. Fortunately, the list of third-party extensions for MathJax does include a physics extension. I'm not sure if this has been considered before and discarded...

discussion feature-request mathjax

Secret

12:23 PM

0

Q: Why does less Knowledge imply more Correlations?

So I've been studying this issue for some time now and I think I get the basics. Correlations quantify (if we are talking about 2 systems) how much information we can gain about system 1 by measuring system 2. Ok now entanglement is a type of correlations which only occurs in quantum mechanics...

quantum-mechanics

That's actually an important concept when understanding entanglement. The amount of mutual information is maximised in a bell state for example

Secret

12:43 PM

0

Q: Is is possible to have a commuting observables only in a single direction?

In quantum mechanics, for two observables to be compatible, successive measurements of the observables, say A and B, should yield the same result as earlier, i.e if we do the measurements with the order $A \to B \to A$, the result from the first A and the last A should be the same, and similarly,...

quantum-mechanics observables

That would break associativity, and thus the operator algebra is no longer quantum mechanics. what do you guys think?

Anonymous

3:05 PM

@EmilioPisanty It would be nice if you could summarize the advantages of having the physics MathJax extension for our site, in your meta post. The SE devs usually have to be convinced that such an extension is really needed, and ask us whether we can justify the slower loading time, etc.

Emilio Pisanty

@Blue I'm not terribly invested in the change, either

Anonymous

FWIW, Robert Cartaino and Jon Ericsson once told us that these changes are essentially irreversible, so they're generally very cautious of adding new extensions.

Anonymous

@EmilioPisanty Makes sense. :)

Emilio Pisanty

heck, I'm not invested in the change at all, tbh

I bring it up because it's something we should consider

and some people might quite want it

if they do, they can argue for it

Abcd

@JohnRennie Are you there for 30 s technical issue?

John Rennie

3:14 PM

Is typing \rangle really that much of a pain?

@Abcd yes

Emilio Pisanty

honestly, though, the stuff in the test page isn't enough for me to think "ooooh, that would really be useful, we should totally install it!!!"

Abcd

@JohnRennie How to remove "3D print" from the picture?

Emilio Pisanty

heck, it still looks buggy to me

John Rennie

Abcd

@JohnRennie how so fast?? I have to collect all these pics for my project...

Emilio Pisanty

3:17 PM

Abcd

v. old joke

John Rennie

@Abcd in most paint apps just use the select tool to draw a rectangle around the stuff you want to remove then press delete. That deletes whatever is selected and replaces it by the current background colour.

Abcd

chemtube3d.com/Rearrangements-Pinacol.html

@JohnRennie Would it be OK to include the 3D picture in the printed project??

Or will it look odd?

John Rennie

I think it would look OK

Abcd

Ah then I'll have to click so many screen shots and connect them through arrows.

student

3:22 PM

Last time I looked at MathJax, I was under the impression that the physics extension is essentially dead

Abcd

3:48 PM

@JohnRennie Do you know the fastest way to click those pics? At present I use snipping tool but its a problem because first I have to select, then I have to click save, then I have to change name.

John Rennie

@Abcd that's basically the way I do it, though I use the "print screen" key then paste it into my Paint app

Anay Karnik

5:02 PM

hey @JohnRennie

John Rennie

@AnayKarnik hi

Anay Karnik

could you provide some citation for that effective distance thing you mentioned in your answer about coulombs law in different media?

John Rennie

@AnayKarnik I don't have a citation. It's just something I've always known. I guess I must have learned it when I was a student, and was four decades ago :-)

Anay Karnik

I am actually doubting the result. Has it occured to you that the result may not really be true?

John Rennie

@AnayKarnik the method has always worked when I've used it. Have you have found a calculation where you're sure it gives the wrong answers?

Anay Karnik

5:14 PM

How have you checked the result?

John Rennie

@AnayKarnik yes, many times over the last 40 years

Anay Karnik

How?

John Rennie

By getting the correct answer

kartik c.p

Relation and functions

enumaris

welp

Anay Karnik

5:19 PM

Yeah, I mean what other method have you used to compare with this result? @JohnRennie

enumaris

decided to go to the other company...had to reject the offer from my current company...stressful

John Rennie

@AnayKarnik I don't remember now. After I found the method worked every time I stopped checking, and that was decades go.

@AnayKarnik your basic equation for the electric field of a charge is Q/(4 pi e r^2). Yes?

Sid

@JohnRennie That would be the best citation ever. :P

John Rennie

@Sid :-)

Sid

@enumaris Who's paying more effectively?

enumaris

5:23 PM

at the end of the day it wasn't really about money since the current company roughly matched the other offer

it was about the opportunity vs the team dynamic

Anay Karnik

@JohnRennie Yes.

kartik c.p

Relation and functions,inverse and trigonometric function, matrices,determinants,continuity and differentiability, application of deriventives,integrals, application of integrals, differential equations , vector algebra. Which of them I need for physics (GR,SR,ME )

John Rennie

@AnayKarnik And for a dielectric e = ke0. If we make this substitution we get Q/(4 pi e0 (\sqrt{k}r)^2).

And we can write the effective distance R = \sqrt{k}r to get Q/(4 pi e0 R^2)

So for this simple example of a point charge you can see how the effective distance works.

Sid

@kartikc.p You would need pretty much everything

Anay Karnik

Yeah I have no trouble understanding it for single medium. @JohnRennie

enumaris

5:29 PM

@JohnRennie thanks for your help by the way in this whole process. :)

Anay Karnik

@JohnRennie Tho, that doesn't prove how it works for multiple media.

John Rennie

@enumaris thanks, though I doubt I helped much! But it's always good to have people to bounce ideas off even if it's only to clarify your own thoughts.

@AnayKarnik Any more complicated system is ultimately built up from point charges. I guess there must be a proper proof of the substitution somewhere or I wouldn't have learned it, but where you'd find it I don't know.

@AnayKarnik next time you have a test try my method and see if it matches the answer key.

enumaris

@JohnRennie I think your advice was quite helpful :D

John Rennie

@enumaris thanks :-)

enumaris

now I gotta work on a lot of logistics heh

Anay Karnik

5:33 PM

@JohnRennie If one of the charges is zero, your method will give answer = 0. But, won't there be force due to the existence of induced charges on boundary of the mediums?

kartik c.p

@Sid From which one I need to start

John Rennie

@enumaris Would you consider moving closer to the job? Obviously not right away, but maybe if after a year or so it's looking good?

@AnayKarnik if you're now asking about the interaction between a single charge and a neutral dielectric slab that's a completely different problem.

enumaris

I was thinking of moving to an intermediary location...like maybe 20 miles away from the job rather than 30 miles away...but due to the way traffic works that doesn't actually take all that much off my commute time lol, maybe 15 minutes...

I'm also considering changing cars to something that self drives XD

Sid

@kartikc.p Functions

kartik c.p

@Sid you mean relation functions?

Anay Karnik

5:37 PM

@JohnRennie Umm, isn't that just a reduction of the same problem by making one charge infitisimally small, if not zero?

John Rennie

@AnayKarnik I'll have to think about it.

vzn

@enumaris congratulations! am interested to hear details on the new job :)

enumaris

it's at a newly built SAP innovation center heh

Emilio Pisanty

anyone up to investigating a mystery?

3

Q: Definition of a field line?

Ok so I finished by A-levels last year (english exams 18 year olds take) and we defined in my physics course we defined field lines (for an electric field) as: The path a free positive test charge would follow if acted upon no other force but the force due to the field itself. Reading more in t...

electric-fields definition

the Linked page says there's a link between that and this

but for the life of me I cannot find it

vzn

@enumaris so what kind of data do they work with? what are they trying to optimize? are you working with other data analysts/ scientists? any phds? etc

Sid

5:41 PM

@AnayKarnik you dare doubt JR! Blasphemy. :P

Emilio Pisanty

there's one in this comment under a deleted answer, but that shouldn't count, right?

Anay Karnik

@JohnRennie Could you check the result again? I am really confused since I am not able to find it anywhere else on the internet.

One of my professors mentioned it might not be correct as he hasn't ever seen a proof of it.

John Rennie

@AnayKarnik no, basically. I am not paying you to use the method and you should feel free to use a more complicated method if you prefer. That's up to you. If/when you have a problem of this type you can easily compare the results using the method to the answer key. Over to you.

On the other hand, if you manage to disprove it I would certainly be interested to see the details.

Anay Karnik

Umm, I did provide a counterexample.

Emilio Pisanty

@AnayKarnik which answer is this?

John Rennie

5:50 PM

@EmilioPisanty it must be that comment to the deleted answer. A "feature" I guess.

@EmilioPisanty this

Avnish Kabaj

Symthe's Static and Dynamic Electricity, Chapter 5, Section 5.303 Point Charge and Dialectric Plate

Reference https://www.physicsforums.com/threads/calculating-the-coulomb-force-between-2-charges-in-space.66008/

Haven't checked but looks like the proof is above ug

Or maybe it's ug dunno

John Rennie

ug?

Avnish Kabaj

Undergraduate

Anay Karnik

Also, you are being a bit offensive here. It was merely a request. It's not about you paying me or not. You have posted the answer on a forum. I was just expressing that I doubt the result and asking for proof, politely. Also, I feel you should have mentioned the fact that result is unproven as it might misguide a lot of people if it turns out that its not true.

@JohnRennie

Avnish Kabaj

Whoa whoa buddy

Sid

5:59 PM

Should we expect another YouTube video saying JR and Physics.SE is full of crap? :P

John Rennie

No offence was meant and I'm sorry if I gave offense. But at the end of the day this is not an issue that is going to keep me awake at night. If you were asking me about general relativity or quantum mechanics I would feel a greater investment in the issue. As it is, this is just something I learned decades ago.

Emilio Pisanty

@AnayKarnik frankly, this is a bit out of line, as is this. You're making a request and JR is declining rather more politely than I'd expect him to.

John Rennie

I would add that I've used the method to go through problems with various JEE students in the problem solving room and it has always matched the answer key, so the evidence I have is that my memory is not letting me down, at least in this respect.

Anay Karnik

I am sorry guys I just took offence on that "I'm not paying you part...".

Emilio Pisanty

@AnayKarnik the answer provides the method but it's very clear that it's provided as-is without further guarantees

no one is forcing you to use it

John Rennie

6:02 PM

@Sid not another naked physicist! :-)

Anonymous

6

Q: How did this question get a 'linked' question that isn't mentioned in comments or answers?

Looking at How can I best use Stack Exchange as a Q&A site for my own project? which has been closed as a duplicate (quite right), we see three 'linked' questions in the sidebar; however, only two of those questions are given as duplicate-close-targets. Use Stack Overflow as the official support ...

support linked-questions deleted-comments

Avnish Kabaj

Avnish Kabaj the pirate is still on it

Anonymous

Could be a deleted comment. Maybe the mods can check?

John Rennie

@AvnishKabaj getting a copy of Smythe's book?

Anonymous

If not, time for a bug report!

user2723984

6:04 PM

@Sid wow, that video is golden

Avnish Kabaj

@JohnRennie yeah djvu

Though

Emilio Pisanty

10

A: How did this question get a 'linked' question that isn't mentioned in comments or answers?

It was linked in a now-deleted comment. There can be a delay between the link being removed and the "linked questions" list being updated. You'll noticed that this has happened in the meantime; the link isn't there anymore.

"there can be a delay"

five years?

Anonymous

@EmilioPisanty Yeah, something's fishy.

John Rennie

@AvnishKabaj it's a free download

enumaris

@vzn they are in the stages of building up the center. The building isn't even built yet. Mostly there's software engineers there, but there should be 1 other data scientist at this time (that I'm aware of). Their current use case is they want to do something in the healthcare field - smart hospitals or something like that.

John Rennie

6:06 PM

@AvnishKabaj this is a better link

Emilio Pisanty

Can you comment on how long that delay can be? I'm curious about why this question is marked as Linked to this one; the only thing I can find is this comment, which has been deleted for 4+ years. — E.P. 16 secs ago

if he answers, great, if not, then meh

I'm not that invested

Anay Karnik

I am sorry guys, I realise I have been a bit out of line and disrespectful here. I really don't to be. Also, I appreciate @JohnRennie helping lot of us out, don't get me wrong. I hope we are at peace.

don't mean to be*

John Rennie

That's OK. I must admit I'm a bit surprised Googling can't find it. All these decades I've just assumed it was a routine technique.

Sir Cumference

So I'm trying to get into some languages that could be useful for astro research. Should I get started with SQL or NoSQL?

Emilio Pisanty

@SirCumference astro research uses SQL or noSQL?

what on Earth for?

@JohnRennie to be fair, though, I don't understand how your method would work in something like this

Sir Cumference

6:18 PM

@EmilioPisanty I'm helping out with a telescope, I remember when I got started I was asked if I had experience with SQL

enumaris

@SirCumference SQL and NoSQL are different ways to store and then access data. They aren't really "languages" in the sense like C++ or Python.

Emilio Pisanty

ostensibly your method would predict a reduction in the force felt by the plates

but what charges would produce it?

enumaris

they do a very specific task

not general purpose at all

Emilio Pisanty

I mean, you just get two thin layers of polarization charge on the boundary layers of the dielectric

but their effect should cancel out

the force should be the same, I think

John Rennie

@EmilioPisanty treat that as two capacitors in parallel

enumaris

6:20 PM

Which one you need depends on how your data is stored...if it's stored in a SQL database, you will need SQL, if it's stored in a NoSQL database you will need NoSQL.

Sir Cumference

@enumaris Well they seem rather useful. I'm trying to decide what to spend my winter break learning.

enumaris

Depends on what your goal is.

Emilio Pisanty

@JohnRennie wait, which two capacitors?

user2723984

@SirCumference I don't know if this is obvious but I worked with some solar physicists for a while and apparently they all use Python

John Rennie

Emilio Pisanty

6:21 PM

183

A: Is SQL or even TSQL Turing Complete?

It turns out that SQL can be Turing Complete even without a true 'scripting' extension such as PL/SQL or PSM (which are designed to be true programming languages, so that's kinda cheating). In this set of slides Andrew Gierth proves that with CTE and Windowing SQL is Turing Complete, by construc...

John Rennie

Oh wait, did I misinterpret the diagram?

Sir Cumference

@user2723984 IIRC there was a mention of SQLAlchemy at the time, which I think is Python-based

Emilio Pisanty

@JohnRennie it feels like that to me

enumaris

If you want to learn how to store and access data for traditionally structured data, then SQL is the way to go. If you want to learn how to access and store other kinds of data, then NoSQL is the way to go. If you want to learn how to "code" or something like that, then choose neither. Choose C++, Python, Java, or something like that

John Rennie

@EmilioPisanty I thought it was showing a capacitor with a dielectric slab half inserted

enumaris

6:21 PM

@EmilioPisanty yeah, theoretical turing completeness is not the criterion I use when choosing which "language" to learn XD

Emilio Pisanty

@JohnRennie no, it covers all of space transversally

Sir Cumference

@enumaris People use Java for research?

John Rennie

Is it a capacitor with a dielectric slab hovering in the middle?

Emilio Pisanty

but there's a distinct air gap between either plate and the dielectric slab

@JohnRennie yes

enumaris

@SirCumference not so much, if you are looking into coding for research, then probably C++ or Python...or Fortran if you are going to be using legacy systems...

Emilio Pisanty

6:23 PM

@enumaris oh, no indeed. But SQL is still a "language" in the same sense as C++ or python

enumaris

In theory yes, in practice, no not really

Emilio Pisanty

it's just not a useful language in the same sense as C++ or python

Sir Cumference

@enumaris I mean at this point I'm just trying to learn something useful in the 10 days before college resumes. Would SQL or NoSQL be quicker to get the hang of?

Emilio Pisanty

as the great Jony Hudson once said, just because it's Turing complete doesn't mean it's useful

enumaris

If all you want is to do some simple queries and access some data, then yes it's "quicker". If you want to be building databases from scratch, then hell to the no.

John Rennie

6:24 PM

Sir Cumference

@enumaris Which, SQL?

enumaris

I don't have much experience in NoSQL

John Rennie

@EmilioPisanty Effective spacing is $a + b\sqrt{k} + c$

enumaris

so I can only really speak for SQL in terms of how hard/easy it is to get a hang of

it's pretty easy to get to the level of "ok, here's a database and a bunch of tables, I need to access this data this way, how do I do that?"

Emilio Pisanty

@JohnRennie for the potential? or for the force too?

enumaris

6:25 PM

It's quite a different level to get to the level of "Ok, I want to store this (huge and diverse) kind of data, how should I go about building a database to accomplish that?"

Emilio Pisanty

how does the introduction of the slab reduce the electric field at the upper plate?

enumaris

I think probably Python is the quickest path to the most useful "language" - just my opinion of course.

Emilio Pisanty

it took me a couple of years to realize that the problem was basically asking us to write a brainfuck interpreter in Mathematica

@JohnRennie no, as in: what physical charges are responsible for the (presumed) reduction in the electric field?

all you're doing is introducing two layers of charge with opposite sign

their combined electric field is zero outside of the slab

John Rennie

@EmilioPisanty don't know, don't care, it works.

enumaris

@EmilioPisanty lol

John Rennie

6:28 PM

Electrodynamics is the Devil's work anyway

Emilio Pisanty

in short, though

@SirCumference brainfuck

CaptainQuestion

I have the pdf

Emilio Pisanty

@enumaris it's even funnier - in the end, we didn't solve it with Mathematica. One guy just sat down and solved it by hand.

enumaris

Does that make that guy turing complete?

Emilio Pisanty

@enumaris yes

Sir Cumference

6:30 PM

@EmilioPisanty Ya know I think I'll just stick to HQ9+

CaptainQuestion

darn this account cant upload

Sir Cumference

> HQ9+ is a tutorial-complete programming language, but I often receive complaints that it is not Turing-complete — typically from people who are not sense-of-humor-complete.

Ha.

CaptainQuestion

No wonders @JohnRennie Doesn't remember

@AnayKarnik

enumaris

hmmm

John Rennie

rubbish

Abcd

6:37 PM

chemtube3d.com/Rearrangements-tiffeneau_demjanov.html

John Rennie

@EmilioPisanty sorry, no, you do that sort of capacitor problem as three capacitors in series.

Abcd

@JohnRennie can you tell which background will look better on white A-4 size paper - black or white?

(click black above 3D picture)

Emilio Pisanty

@JohnRennie your result looks right to me for the capacitance

just not for the force

John Rennie

@Abcd As a general rule I'd say use white if you're printing it on white paper.

Unless you have a really good printer solid black tends not to print that well.

Abcd

@JohnRennie Oh okay, I felt that white wasn't able to highlight the charges and push-arrows properly,

John Rennie

6:40 PM

@EmilioPisanty no it's wrong for the capacitance, as you can easily see by setting $a=b=0$. That should give $\epsilon A/b$, which it doesn't.

@Abcd try it and see I guess ...

Emilio Pisanty

@JohnRennie eh

@JohnRennie I have no idea what you mean by this, though

Abcd

@JohnRennie i dont have coloured printer at home, I'll have to go to the shop and get it printed tomorrow.

Emilio Pisanty

putting in fake conducting plates at the dielectric boundaries?

John Rennie

Emilio Pisanty

@JohnRennie then the fake plates experience different forces and they'd be "forced" apart, wouldn't they?

John Rennie

6:43 PM

@EmilioPisanty yes. The boundaries are equipotential surfaces so you can put a notional conducting plate there without changing anything. Then it just becomes three capacitors in series

Emilio Pisanty

that method is OK for the capacitance

John Rennie

Trust me, it works. I wouldn't lie to you (often)

Emilio Pisanty

@JohnRennie so what force do you get?

same as without the slab?

John Rennie

The force is half the field times the charge on the top and bottom plates, and the field is the potential difference divided by the effective distance $a + \sqrt{k}c + c$.

I have to go - actually I had to go 5 minutes ago but dashed back to correct my equation for the capacitance. We can pick this up tomorrow if you really care :-)

Emilio Pisanty

@JohnRennie if the force on the plates really does change with the introduction of the slab, then I really do want to understand why.

in other news, though

State of the Stack in 2019: we totally didn't have a complete meltdown late in the year regarding whether the mechanisms for interaction between the company and the community actually work.

Loong

7:14 PM

According to my car, the coldest spot on my way home from work was −20 °C.

made me a bit worried

Anonymous

7:25 PM

As for the force on capacitor plates, I always found the energy method more intuitive.

Anonymous

5

A: Attractive force between capacitor plates

The energy of the capacitor is $U= \frac{\epsilon_0}{2} S\,\mathrm d E^2$ where $S$ is the area of a plate. If we increase of $\Delta d$ the distance of, say, the right plate from the left one, keeping fixed the charge $Q$ on each plate, $E$ does not change and we find a variation of energy $$\De...

t t t t

7:40 PM

@SirCumference an admin broke it that way. I had broken it another way.

that admin believes my profile is now "fixed"

so ive no idea what he's done but I can only thank him.

@Blue are you here? I was wondering where quantum computers are turing machines

or are they able to solve problems a normal computer cannot do even with infinite amount of tempus?

Anonymous

@tttt I don't understand the question. Do you mean whether a quantum computer can be simulated by a Turing machine?

Emilio Pisanty

@tttt Quantum computers are Turing-complete

there's a complete embedding of classical computers within quantum computers

It's conceivable that there are problems which can be solved in a quantum computer (given arbitrarily large amounts of memory and time) that aren't solvable on a classical Turing machine

the current thinking is that that isn't the case

This is known as the Extended Church-Turing Thesis

Church–Turing thesis

In computability theory, the Church–Turing thesis (also known as computability thesis, the Turing–Church thesis, the Church–Turing conjecture, Church's thesis, Church's conjecture, and Turing's thesis) is a hypothesis about the nature of computable functions. It states that a function on the natural numbers is computable by a human being following an algorithm, ignoring resource limitations, if and only if it is computable by a Turing machine. The thesis is named after American mathematician Alonzo Church and the British mathematician Alan Turing. Before the precise definition of computable function...

However:

there are two vastly different questions: the set of problems which can be solved, and the set of problems which can be solved efficiently

the main separation that's thought to exist between classical and quantum computers is on regarding the efficiency with which they can solve certain problems

Quantum computers are able to solve in polynomial time certain problems which are thought to have a worse scaling on classical computers

Note the "are thought to", though - all of this is conjecture at this point, and hard proofs are hard to come by.

Indeed, a rigorous proof that quantum computers are better at any given problem than a classical computer would constitute a solution to the P-vs-NP problem, of Millennium Prize fame

If you want to read more on the subject, I'd recommend perusing Scott Aaronson's blog.

Anonymous

7:56 PM

Interestingly, one of Scott Aaronson's students (Ewin Tang) proved that several of the quantum algorithms which were earlier thought to provide exponential speedups over existing algorithms, have classical equivalents (in 2018).

Anonymous

As Emilio says, quantum computers can be completely simulated by Turing machines and it can itself simulate a Turing machine. The main question is about efficiency.

Novalium Company

watup fellas

just coming to check out what the gang is doing :D

Anonymous

Probabilistic Turing machine

In computability theory, a probabilistic Turing machine is a non-deterministic Turing machine which chooses between the available transitions at each point according to some probability distribution. In the case of equal probabilities for the transitions, it can be defined as a deterministic Turing machine having an additional "write" instruction where the value of the write is uniformly distributed in the Turing Machine's alphabet (generally, an equal likelihood of writing a '1' or a '0' on to the tape.) Another common reformulation is simply a deterministic Turing machine with an added tape...

Novalium Company

Sup @Blue

how's life?

Anonymous

Quantum Turing machine

A quantum Turing machine (QTM), also a universal quantum computer, is an abstract machine used to model the effect of a quantum computer. It provides a very simple model which captures all of the power of quantum computation. Any quantum algorithm can be expressed formally as a particular quantum Turing machine. Such Turing machines were first proposed in a 1985 article written by Oxford University physicist David Deutsch suggesting quantum gates could function in a similar fashion to traditional digital computing binary logic gates.Quantum Turing machines are not always used for analyzing quantum...

Anonymous

8:07 PM

@NovaliumCompany Hey!

Anonymous

@tttt You might also want to look at quantum and probabilitic Turing machines.

Anonymous

> One of the central questions of complexity theory is whether randomness adds power; that is, is there a problem which can be solved in polynomial time by a probabilistic Turing machine but not a deterministic Turing machine?

Anonymous

Note to future self: I personally have a strong feeling that P=BPP=BQP and that quantum computing is moot. (Blue - 2019) :P

Novalium Company

@Blue How's life?

Anonymous

@NovaliumCompany Not bad. How's school treating you? :)

Novalium Company

8:19 PM

@Blue Great actually! I read a book on emotional intelligence and stuff began to change... I'm much more open and talkative :D

Anonymous

Oh, that's great!

Novalium Company

I analyze situations and change my perceptions :d

Anonymous

Which book?

Novalium Company

I forgot :D it was on my kindle

Anonymous

Heh.

Novalium Company

8:20 PM

Now I'm watching turkish massage online xD

Anonymous

Turkish massage? Oo

Novalium Company

... :D

How's uni?

Anonymous

@NovaliumCompany Medium. Some classes are interesting. :)

Novalium Company

Do you wanna become an electrical engineer?

Anonymous

@NovaliumCompany Nope.

Novalium Company

8:23 PM

Then why do you waste your time studying EE?

Anonymous

It's too late to change now. I guess some of the things I'm learning now will help in the future, regardless of my future career choices. :P

t t t t

Thanks you @Blue and @EmilioPisanty

Anonymous

@CaptainQuestion For the case of (homogenous) dielectrics inserted between parallel capacitor plates just the 1D Laplace equation should suffice.

Anonymous

9:00 PM

@JohnRennie I think Emilio has a point here. We need to consider only the field outside the dielectric for the force on one plate, isn't it? The dielectric doesn't affect that external electric field. That field still remains ${\bf E}=\frac{\sigma}{2\epsilon_0}\hat{n}$ (i.e. the field due to one of the plates of the capacitor).

Anonymous

3

Q: Force between plates of a capacitor with dielectric a battery

My teacher told me that the force will increase $k^2$ times, where $k$ is the dielectric constant, but I don't see how. To start with, with no dielectric, the force between the plates is given by $\frac{q^2}{2A\epsilon_{0}}$. If I do insert a dielectric, and The plates are connected to a battery,...

electrostatics capacitance dielectric

Anonymous

$\mathbf{E} = -\nabla V = -\frac{\partial V}{\partial x} \hat{x}$ (in 1D). If you're writing $\mathbf{E}=-\frac{V}{d_{\text{eff}}} \hat{x}$, you're already pre-supposing a linear relationship between $V$ and $x$, which may not be the case (I think).

Leaky Nun

Adam and Steve are immortal beings in an otherwise empty universe with a spacetime that is the manifold S^1 (a circle). Steve is moving with a constant velocity w.r.t. Adam, so Adam perceives Steve as having aged less than Adam in every encounter. However in Steve’s perspective it is Adam who is travelling in a constant velocity, hence having aged less every encounter. Who is right?

Anonymous

9:28 PM

To be clear, I'm not convinced that "the field is the potential difference divided by the effective distance $a + \sqrt{k}c + c$". The primary reason being that we already know that the electric field is not a constant within the span between the plates (considering an inserted dielectric).

ACuriousMind

@LeakyNun You're incorrect about what they would see. Half-way between each encounter, the direction of the shortest path of light changes, and they see each other age so that their ages match when they encounter each other again. See physics.stackexchange.com/a/43803/50583

enumaris

horton hears a who

ACuriousMind

...huh?

enumaris

hwat

Anonymous

@enumaris Watching animated movies these days? :P

enumaris

9:42 PM

I've always watched my share of animated movies

Leaky Nun