The h Bar

1:00 PM

@Kenshin I don't know (not being a poli-sci person) of reliable metrics for ideology between countries through time.

@Secret A small-c conservative thinker would point out (a) 1940ish is an unusually broad middle, so regression to a mean's naturally going to take hold; (b) looking carefully at the note on the "5th party system" you'll see that we're probably measuring the "wrong" axis for the past 20-30 years in that ^^ chart--who knows how mixed/centrist it might seem along a more-informative axis?

Kenshin

Can someone please help me with a quantum mechanics question

0celo7

@Semiclassical too bad I have class right now. I think this epsilon trick might work.

nitsua60

@Kenshin Just go ahead and ask.

Kenshin

Ok so I worked through the SE in an infinite potential well between 0 and a

nitsua60

okay

Kenshin

1:09 PM

the solution was psi = A sin (kx)

the problem is, doesn't this imply that the wavefunction persists even beyond the infinite potential at 0 and a?

Shouldn't we expect a solution that looks like a gaussian distribution (bell curve) that contains the wavefunction within the potential well?

nitsua60

@Kenshin That's the solution for 0<x<a, no?

Kenshin

nope

nitsua60

You're telling me that psi satisfies the TISE outside [0,a]?

(It's been twenty years, but I don't think so...)

Kenshin

I dunnno

I mean I'd hope it doesn't, but it appears to me that it does

I've probably made an error somewhere tho

Qmechanic

Speaking of

1

Q: Has the definition of "statistical mechanics" changed from its original meaning?

I guess this never registered with me when I read the Feynman Lectures on Physics in the past. But I have wondered, from time to time, what distinguishes statistical mechanics from, say, kinetic theory. Like most people, I assumed the distinction was that statistical mechanics is a probabilisti...

Due to often/daily mistagging of kinematics posts as kinetic-theory by newbies, it may be a good idea to make kinetic-theory a synonym of the statistical-mechanics tag. Keep in mind that there is no point in having a refined tag system if it is applied wrongly. Thoughts?

2

Kenshin

1:14 PM

kinetic-theory and statistical mechanics are seaparate in my opinion

nitsua60

@Kenshin Just try it. Take the second derivative, multiply by whatever, add V (infinite!), and see if you get a multiple of psi. If not, that psi is not the solution in that region of the potential.

Kenshin

@nitsua60 well V isn't infinite

I have V infinite only at the points 0 and a

V is like a dirac-delta distribution at those points in a sense

nitsua60

@Kenshin Oh! You've got two delta-potentials?

Kenshin

yea

nitsua60

I thought you were talking about infinite square well.... Sorry.

Kenshin

1:16 PM

np, I probably should have been doing a square well

so do you think in the delta-potential well, the state just tunnels straight through?

nitsua60

@Kenshin Well, think about the region outside of [0,a]. If there's behavior out there, what would you expect?

Kenshin

yeah id expect to equally likely find the particle inside or outside the box

because i haven't explicitiy specified whether the particle is inside or outside in my set up

I think that makes sense, thanks bro

nitsua60

@Kenshin That's not what I'm saying. (And be careful with "equally" there.)

Kenshin

yeah

I mean, it wouldn't be a normalisable solution

nitsua60

@Kenshin Yeah, but we won't let that stop us =)

Kenshin

1:19 PM

but the fact I've only specified two potentials in the SE, the particle will form a wavefunction that isn't necessarily in the box

and I think that is why the wavefunction appears to just pass straight through the potentials

nitsua60

@Kenshin Right. So you've got to think about your free-space solution, then about transmission/reflection, &c.

Kenshin

yea

too hard for now

I think I'll just change the problem to square potential

but at least I understand the one I was working on a bit more

nitsua60

@Kenshin Mind if I ask what you're working on? ("Change the problem" raises an eyebrow.) Some sort of presentation or something?

@Kenshin glad to hear it =)

Kenshin

just working through a textbooks that's all

for independent learning

it actually was a square potential in the textbook i just didn't read it properly ;)

but then I still wanted to understand how my interpretation would work

Semiclassical

@0celo7 lol, it’ll still be there when you finish

nitsua60

1:23 PM

@Kenshin Okay, that makes sense. That's typically tackled pretty early, then at some point a single delta potential, and by then you should be able to tackle a collection of deltas (like having two at x=0 and x=a) without too much trouble.

Semiclassical

One way to look at it is that a delta-potential is what people use to describe a “hard-core” interaction between, say, an electron and the nucleus

So the two only interact if there’s a possibility of the two coming into “direct contact” so to say

nitsua60

@Semiclassical Hey, let's keep it family-friendly. (he he he)

Semiclassical

As such, the interaction does nothing if the wavefunction happens to vanish at the delta-potential since the probability density of finding the particle there is zero

Lol

0celo7

@Semiclassical I thought we decided it was insane to expect that

Semiclassical

Hah

Secret

1:29 PM

0

Q: Would it be possible to travel from A to B on spacetime with a gravity generator?

Say you have two points A and B on the plane of space, represented in 2D by the black line. Let's also say you have a gravity generator that can generate infinite amounts of gravity without destroying itself and can be turned on and off at will. When you place that in the middle it will caus...

spacetime spacetime-dimensions

@Slereah you happen to know some wormhole solutions that behave like this?

Semiclassical

It’ll still be there for you to obsess over when class finishes:P

It’ll still be there for you to obsess over when class finishes :P

nitsua60

@Secret Yeah, I was just looking at that. In fact, at all three that just came across the ticker. You-all get quite the variety of questions around here.

Secret

Our not perfect, but still managed to do the job homework policy helps retain weird and interesting questions

meanwhile MSE is flooded with homework vampire questions

Kenshin

great question

@Semiclassical thanks that helps

Secret

Based on what Slereah taught so far, we cannot simply change the topology of spacetime, thus I will imagine there will be issue at that fold

nitsua60

1:33 PM

@Secret Do you-all tend to handle homework-help in here, or is that forwned upon?

Kenshin

If I have a hardcore interaction, like you mention, will it always be positive roots

for coefficient A

Secret

@nitsua60 Well, I am lazy, the other guys here does msot of the question management, thus I am a bad person to ask

nitsua60

(I ask because I was around last night when someone posted a question from their HW, got it closed, came in here, and just got crickets. Rather, got everyone else continuing their conversation while they were all "any help? anyone?")

(And I noticed that this room isn't listed as a resource in the meta on "where can I ask for HW help," so I figured I'd ask before I chimed in and helped that kid.)

Secret

well as far I recalled, we do sometimes help answering people homework questions, but onyl if they show enough work. That said, there are other JEE and whatever exam comrades that helps. In addition, there'sa problem solving room for that purpose

nitsua60

@Secret Well, your activity graph says you're here a bunch, so I figured you'd have seen.

@Secret Okay, cool. Thanks.

Secret

1:37 PM

above all, Johnrennie is usually very nice and answer most people queries which is how he goes stardom

Problem Solving Strategies

General chat for high school physics. For MathJax see [here](m...

but for main, we usually will close homework like questions in order to keep the main relevant for researchers

Kenshin

@nitsua60 there is a site called physics.qandaexchange.com that they also use

nitsua60

@Secret Sure--that makes sense. I just wasn't sure if I'd have been "polluting" this room if I'd helped them out in here. In Math's main chat, for instance, they're always willing to walk a student through where they're having trouble with something.

Secret

yeah we are usually free to help them as long they show enough effort and really stuck in chat, though sometimes when conversations flow fast they get swept away by the chat flow

Semiclassical

if there's a few conversations competing I do like to go to a separate room

and if I'm not interested in answering a question I usually just don't engage with it

(probably could deal with it more constructively but tbh being seen as someone who is willing to answer questions gets draining)

Secret

I am usually strict when it comes to question. The person has to show enough effort for me to notice it (and myself free enough of course)

and I usually don't like to answer newtonian questions because those contraptions are just tedious

Semiclassical

1:46 PM

yeah, there's a level of due diligence I usually look for

Secret

Interestingly though, sometimes there are homework questions that are somehow interesting and it inspires me a completely new idea to test at some higher level

Sir Cumference

'Guys? The Town is supposed to be good, and I thou--' 'PHYSICIST STEPHEN HAWKING DECLARES NEW FILM BEST IN ALL SPACE AND TIME' 'No, I just heard that--' 'SHOULD SCIENCE PLAY A ROLE IN JUDGING BEN AFFLECK?' 'I don't think--' 'WHAT ABOUT MATT DAMON?'

Semiclassical

oh, I think I managed to find an error in Griffiths E&M (or at least something that's misleading as written)

Constantine Black

Hi. Anyone having an idea on this: Let's say we have a trace over four vectors, lets call them k,l,m,n, so we have Tr[klmn]. If the vectors have extra indices on another space, let's say a and b, so that we have in fact Tr[k_a l_b m_a n_b], can I somehow simplify this quantity more? Thanks.

Semiclassical

use mathjax please

also, I have no idea what the trace over four vectors means.

you can't multiply vectors together.

(not in without saying more about the product at any rate)

Secret

1:55 PM

For example, that ball hitting spring question 2 days ago will make you think about the case when the ball hits the spring too hard such that even if it fully contract cannto store all the excess kinetic energy. To find out where it will go turns out to be not so trivial as the original question since the spring will be pushing as the ball bounce away from it

Secret

I knew what a trace of a diagonal matrix mean, but not a vector

John Duffield

@vzn : reddit shadowbanned me. I have a special place in physics hell reserved for them.

0celo7

loooooooooooool

Constantine Black

Ok, sorry, on my pc mathjax doesn't show up on screen for chat.
The product comes from the calculation of Feynman amplitude, so in fact we have 4 spinors contracted. That's what I meant. Srednicki, chapter 46 has an example.

Let's say we have a trace over four vectors, lets call them k,l,m,n, so we have $Tr[klmn]$. If the vectors have extra indices on another space, let's say a and b, so that we have in fact $Tr[k_a l_b m_a n_b]$, can I somehow simplify this quantity more? Thanks.

Does it show up ok now?

Semiclassical

for mathjax in chat use the link in the room desc

I have vague memories of those kinds of calculations, but it's been literal years so I'll pass

Secret

1:58 PM

ok, spinors are waaaayyy over my legue, I ma pass this question

Problem Solving Strategies

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