The h Bar

4:38 AM

Hi. I was thinking about the time evolution of a free particle and one of the peculiar things that I hadn't noticed before is that the time evolution of a single free particle depends on the mass of the particle.

See, the explicit formula for the time evolution of a free gaussian wave-function here: en.wikipedia.org/wiki/…

This is very weird and in stark distinction with classical mechanics. I mean the inertial of a particle is not supposed to enter the equations of motion of a particle until a non-gravitational force is involved.

I was wondering if anyone has come across some nice discussions on this aspect. In particular, I can't seem to put a finger on what about QM is responsible for this aspect arising.

user434058

5:51 AM

@schn You know it's Griffiths when you see that boldface r in Kauffman font :)

user434058

@JMac Isn't it called talking past each other?

Yes, it is usually called "past each other"

"Talking over" is usually used in "talking over your head"

usually, but not always :-)

pal

John Rennie

6:15 AM

Wow, it's Good Friday. With all the fuss and bother about covid Easter kind of slipped out of sight.

@FakeMod Talking past each other

Hopefully the Easter bunny will help people get their minds off all this fuss @JohnRennie

6:30 AM

Chocolate helps

:-)

6:44 AM

2

@Loong :D

I prefer white chocolate over dark.

John Rennie

@Loong it must be ten years ago I first saw that joke but it still makes me laugh :-)

8:13 AM

I knew it was risky to use that question as a dupe target for this BH question: physics.stackexchange.com/q/542760/123208 Oh well.

Q: Confused about a dielectric charged sphere in a space with no external Electric field

Anyone here? I need quick math help. I'm doing my homework and I can't find the formulas for sinA/2 = ? cosA/2 = ? and tgA/2 = ?

nvm lol

I had to google and click the first link

it was a pretty complicated and tiring process

user434058

9:48 AM

@skullpatrol Thanks!

user434058

I find it funny that people write a 100 letter/word question however are too lazy to write "please" and instaed write "pls" or "plz", or even worse, "plzzzzzzz" :D

user434058

10:21 AM

0

Consider a dielectric sphere with a non-zero density charge on the surface, outside the sphere there is vacuum. Given the radius and the density, the problem asks to find potential and Electric field inside the sphere, but I got confused: if there is no Electric field outside of the sphere, does ...

user434058

Am I wrong in thinking that the field inside the dielectric sphere would simply be zero?

user434058

If there's any polarization, it would be spherically symmetric, and while applying Gauss' law inside the sphere, we would get zero flux due to zero charge enclosed... Is this correct?

B. Brekke

@FakeMod To me it seems like Gauss law would work just fine and I would conclude the same as you

user434058

@B.Brekke Thanks! I haven't dealt with dielectrics much, so I was a bit confused :)

Q: Why does a running fan causes some soundproofing in the room?

@DvijMankad You need to compare the evolution of the wavefunction not to classical mechanics of a point particle, but to classical mechanics of a probability distribution. I believe you get a similar mass-dependent spread for a classical phase space density from the Liouville equation for free particles

(this is intuitive because the phase space distribution is in terms of position and momentum - the same phase space distribution therefore represents different velocities for particles with different masses and hence spreads faster or slower depending on the mass)

@FakeMod Says the person who writes '+ve' instead of 'positive' :P

user434058

10:37 AM

@ACuriousMind hehe... I mean that was chat, not a question ;P

user434058

11:57 AM

0

I notice that when I switch on my fan inside my room, the room becomes soundproof to a great extent to the outside noises. The greater is the speed of the fan, the greater is the soundproofing. When the fan is switched off, the soundproofing is almost gone. Even little sounds from outside can c...

fluid-dynamics acoustics flow turbulence

user434058

Now the fan has taken the place of the outside noise, right? Isn't it as simple as, "just because someone is shouting, you will stop hearing the whispers"?

user434058

Or am I underestimating some advanced acoustic phenomenon which isn't trivial?

@FakeMod The fan sound is aporoximately white noise, that is, it has roughly equal energy over a broad frequency range, and the brain tends to ignore such signals pretty quickly. So it's good at masking transient noises.

White noise

In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines, including physics, acoustical engineering, telecommunications, and statistical forecasting. White noise refers to a statistical model for signals and signal sources, rather than to any specific signal. White noise draws its name from white light, although light that appears white generally does not have a flat power spectral density over the visible band....

user434058

@PM2Ring Thanks! BTW I produce electronic music so I am familiar with this white noisey stuff :) FWIW, white noise is mostly used for risers/uplifters and impacts (swooshes). Pink noise is used for mixing a track. White noise can also be used. And it is used as reference for the mix, because you want almost equal energies in all the frequencies, so you compare your sound with white/pink noise to see how close are you to getting an equal distribution.

That article has a white noise sound sample you can play. A fan usually has a distinct "hum" sound as well as the white noise, due to the resonant frequency of the motor + blades.

user434058

12:09 PM

@PM2Ring I think the loudness has a major role to play here. A silent fan which produces white noise, might not work effectively...

@FakeMod Sure. It has to be loud enough to drown out those transient noises.

user434058

@PM2Ring For me, my focus mostly depends on what I am doing :P

user434058

The noise gets noticed only when I am doing the stuff which I don't like.

Jan 11 at 15:04, by ACuriousMind

> Don't advertise your recent questions. If you just posted something on the main site, give it some time, and don't tell people to go there and look at it. Particularly by pinging people. That's rude. Those who can answer are already watching the queue on the main site!

user434058

@Mathgeek This might help: physics.stackexchange.com/a/189773/258881

user434058

12:21 PM

@Slereah Who is the guy in your display picture?

Sir Cumference

12:46 PM

Was stack exchange named after the stock exchange?

Or is it an eerie coincidence

Q: Why does a book leaned up against a wall sometimes fall over after being stable for many hours?

Jeff Atwood had a chimney replacement company.

Aaron Stevens

1:43 PM

7

This problem has been vexing me for years. Sometimes, you lean a book up against a wall and it stays there (the static frictional force balancing gravity, etc), but then after a long time (hours or even days) it suddenly falls down without any obvious external perturbation (e.g. bumping the she...

newtonian-mechanics friction vibrations stability

Another speculation question that is going to get very popular

user434058

2:26 PM

@AaronStevens It got tweeted, and it can be understood by a layman $\Rightarrow$ raining upvotes and answers

Does expectation of momentum in the stationary ground state vanish identically?

Aaron Stevens

@ManasDogra Stationary ground state of what?

energy eigenfunction of a non relativistic quantum mechanical system with arbitrary potential

John Rennie

3:54 PM

@ManasDogra $\langle \hat p \rangle$ is basically the average momentum. A particle in a bound state doesn't go anywhere so the average moment is always just zero.

Oh,yes...Thank you,Sir,but is there a mathematical way to prove it?(Maybe this is not the right place to do here,but still just asking).

@ManasDogra See physics.stackexchange.com/q/100277/50583

How I Taught Myself an Entire College Level Math Textbook

►

I wanna try that

Gonna be good training for my brain. I believe it'll teach me different ways of looking at things

Q: A suggestion box for downvotes (and upvotes, if necessary)?

@ACuriousMind Thank you

Physics Meta

4:15 PM

0

I think that the main reason for a downvote is to convey that a post is not up to the mark, or does not show proper effort on behalf of the person who posted it. In other words, a downvoted post has to be improved. [Note that I'm not saying this out of any dislike for downvotes, but as a means ...

feature-request comments down-votes

@ACuriousMind I see, thank you! Although, I still find this a bit weird--just the physical fact that one can measure the mass of a free particle.

I mean the fact that you can distinguish between two free particles and say which one has a higher mass without subjecting them to any force (potential).

@DvijMankad How are you "measuring" here?

you can't just look at a particle and determine its wavefunction experimentally

If I produce an array of gaussian initial states with a known sigma and then measure their positions at a later time and plot them to retrieve the probability distribution among positions at a later time then based on how fast the sigma has changed, I can calculate the mnass

mass*

No, I am preparing them in a known state, let's say

@ACuriousMind Also, you kind of can, if you have multiple "copies" of the same state, right? Like if you can figure out a tomographically complete set of operators which you, in principle, can.

@DvijMankad Yes, in the limit of infinite measurements you can recover the wavefunction in that way. But your position measurements interact with the particle, how is that different from subjecting a classical particle to an interaction with a non-gravitational force in order to figure out its mass?

Haha, that's not as clean as I'd like it to be, but sounds right :P

5:40 PM

Is there a way to view only my messages on this chatroom?

@NovaliumCompany You can limit the transcript search (upper right corner) to specific users, including yourself

I only see a search bar

type something into it

On the results page you can then enter users

Oh got it

I will be deleting all the embarrassing and too personal messages I've posted here. I contacted SE but they said they don't offer an option to delete all messages. I have to flag the ones I want to be deleted to a moderator...

So now I'll be reading all of my messages from the past 2 years great

srsly?

5:47 PM

yes...

:(

I've shared really personal stuff here, I don't want the wrong people finding them

@NovaliumCompany Before you waste your time doing that: No moderator will delete more than a few of these messages. Generally we only delete messages on request when there's a good reason (e.g. unintentional disclosure of PII) beyond just the author wanting them deleted.

Can you imagine if the world finds out that Steve Jobs was participating in some chatroom and everyone can read all of his embarrassing thoughts, shared personal feelings and info... I don't want that happening to me

I guess I'll have to do it the hard way - become super rich, buy SE, delete my messages

But seriously, I don't know what to do

Live with it (and stay more anonymous in the future if you don't want such records to exist)

5:55 PM

Isn't there anything I can do about the past messages?

Does changing my name change the name on the past messages?

Yup,

Sort-of - it doesn't change it in any pings toward you, so it's usually pretty easy to figure out what name someone was using around the time a message was written if there are any replies to it

pff

@ACuriousMind help me, please

I.e. if you changed your name now, the @NovaliumCompany a few messages above wouldn't

On the other hand, "Novalium Company" is a pretty pseudonymous name. Maybe you should just choose a different pseudonym for your future endeavors if you don't want them to be connected to anything you said here

@ACuriousMind I'll do that

6:02 PM

Don't forget to change your avatar :D

doing that :D

coolio

my email is not publicly visible right?

@NovaliumCompany No, it is not visible

Name and pic changed, hopefully they update soon

6:10 PM

@JingleBells?

lol I have an idea

I'll delete every message I post from now on xD

(in the time span I have)

Sir Cumference

luckily, I haven't posted my real name on this chatroom.... .... right?

Go back and search.

I'm too scared to put my real name in any search box

SE's gonna steal it and then be like "nope, we're not taking it down"

6:17 PM

...weren't you the one that didn't understand why I didn't want companies to see all my data?

fak

@ACuriousMind is it safe to put my real name in the search box of the SE transcript search?

@ACuriousMind we'll soon be transcended to computers so who cares

Put in a bunch of fake names too

@JingleBells Then why would you be scared to search your name...?

@skullpatrol genius

@JMac I knew someone would point this out.

I would hope so. It's obviously conflicting.

6:21 PM

done, luckily I haven't been dumb enough to post my name

@JingleBells You'll just have to hope they don't have a witch on payroll that can curse you by knowing your True Name

I searched for like 10 other fake names so yay

plus, I only typed my first name

I hate when people associated with the magical arts figure out my True Name. It's such a hassle to get out of their complete control they have over you. Waste of a week.

@JMac I know a Nigerian mage that can do that for you in a day. You just need to send me a few dollars so I can establish a secure connection between you and him.

@ACuriousMind I appreciate it, but I've already been in contact with three or four. I'm paying quite a bit in Nigerian mage protection contracts right now; but it takes a very powerful mage to control me now, so I'd say it's worth it.

6:26 PM

I have no idea what you're guys talking about lol

:54047750 DAAMn, I'm just now seeing what high of a rep u have

@skullpatrol why r u deleting ur messages lol

@JingleBells We we basically joking about magical fantasy things, and also mixing in some Nigerian scams in there for good measure.

rpg.SE actually gets a fair bit of magic spam (mostly offering death spells or love spells)

The more I revisited this picture and the actual experimental procedure that generates it and the more I reflect about my understanding about quantum mechanics and quantum field theory

6:28 PM

Lose Yourself to Dance

►

the more I think quantum interactions is really a super elaborate fluid like thing, except unlike fluids it does not have any notion of viscosity

I forgot about that album.

@JMac it always appears on my youtube mix playlist

@vzn

I listened to it quite a bit when it came out, but haven't in a few years.

6:32 PM

is it weird that I have been under quarantine for like a month, I haven't seen a single human being (except my family) and I feel super good. The absence of people hasn't yet driven me crazy. Ahh the power of being an introvert

I can only imagine how extroverts must feel

I mean, if you look at quantum states in some way, its really some "stuff" that get disturbed by a measurement, and that disturbance get registered by the measuring device as an observable

I think quantum field theory got the right idea on how infinite degrees of freedom took place at the quantum state

But again, I am pretty ontic in the interpretation camp, so yeah

I am a strict 1:1 ambivert. I felt lonely and relaxed at the same time

For the relax bit, no need to deal with tons of people I don't trust

@FakeMod soundcloud.com/user-599486398/cool_beat/s-66NI9kCtcKP for u

For the lonely bit, not enough stimulating and deep conversations and interactions on the meaning of life

Hopefully one day I'll find people with whom I can discuss cool stuff, not drugs, smoking, partying and social status bullpoop

So this is what I woke up to this morning:

6:38 PM

@JMac omg snow! I haven't seen that in a while

Oh no! Someone appears to have vandalized your car's license plate!

@ACuriousMind I think it's the paint mage

@ACuriousMind They used that crazy kind of spraypaint that just hangs overtop of everything like a black mass too. It's awful.

@JMac Cool car btw

did you programmed?

Programmed?

6:39 PM

coded

@JMac Vantablack vandalism is a serious concern

C++, Python, Javascript..

Like do I code?

yes

I learned a bit of coding but I don't do it for a living or anything.

6:40 PM

I mean, what's ur job

Wait, are you an adult? Is it cool being grown up

I have so many questions

"Project coordinator". I help manage construction projects (on the heating/plumbing/ventilation side usually).

@JMac cool!

Welp, that explains the higher-than-average interest in passive houses ;P

Yeah I took an energy management technical elective too, and ended up getting pretty into it.

Ahh how I want to meet most of you in real life. It's gonna be so cool. You'll see some 17-year-old kid and I'll see some adult guys, a bit weird tho. It'd be so cool to meet someone I know that I have many interests in common.

Which is your favorite car brand? (question to all)

6:49 PM

I don't like cars :P

oh yep, I forgot :D

I still like my BMW.

I still haven't gotten my license. I should be taking it now but coronavirus.

I like Chevrolet

(Hawaii 5-0 implanted that in my brain)

"If you wanna be like McGarrett, buy Chevy"

I watched the last Hawaii 5-0 episode a few days back... man.. what an ending

I had to watch it in chunks, so I don't break down crying

@JingleBells The original series used a Mercury Marquis, not a Chevy.

@JingleBells Yeah I'm not much of a car person either. I bought a Civic from my dad when he upgraded. Then I hydroplaned it off the road and totaled it. Since I walked away fine I decided I was happy with buying another Civic.

7:04 PM

@Loong in the original series, McGarrett looks like he would crack his hair if he smiled.

1968

I haven't watched the old Hawaii 5-0

But I've watched all of the episodes of all seasons of the new one

@skullpatrol and parked like a Russian Porsche Cayenne