2:54 AM
Anyone know of some high-quality interviews between two physicists? Nothing dumbed down, but rather fully rigorous physics being talked about.

5 hours later…
7:50 AM
SHYSICS
velcom sir tu my paitn deveLOPment siris
QM says that the universe is quantized (not continuous). Why is that? Because electrons jump on specific discrete energy levels (and therefore the photons emitted have specific frequencies)?

> QM says that the universe is quantized (not continuous)
Actually it doesn't. That's a common misconception.
It says bound states have quantised energy levels.
Unbound states like free particles do not have quantised energy levels.

@JohnRennie Lol I got this sentence from an official QM summary - google.com/url?sa=t&source=web&rct=j&url=https://…

That first statement is misleading. It's true only for bound states.

Mmalright, I believe ya
what is an unbound particle? (i tried googling)

8:06 AM
A bound particle is one that is confined to some region of space. For example the electron in a hydrogen atom is bound because it is trapped in the hydrogen atom and can't escape from it.
But suppose I shine uv light on the H atom to ionise it so the electron escapes and goes flying off to infinity. Now the electron is a free particle.

A UV ray is just high energy photons flying to the H atom but does that manage to push off the electron so the H atom becomes ionized?

Yes, the atom absorbs the photon and the absorbed energy is converted to the energy of the electron.
The binding energy of the electron in an H atom is 13.6eV, so if the photon energy is greater than 13.6eV the electron is ejected from the atom.

8:23 AM
Got it. So a free flying particle is not quantized?
What if you shoot electrons in vacuum? Will they be continuous? I mean, if there's no energy states provided by the atom's nucleus, it makes sense there will be no quantization

Yes, the free states have continuous energies/momenta

If that's so, can we infer that the atom's nucleus (protons and neutrons) is what "creates" quantization?

y no?

hello..

8:31 AM
First, let's be clear that the free particle is still quantum (like "as opposed to classical" not like "as opposed to continuous", I know, terminology is terrible). Second, there is plenty of situations where you have a discrete spectrum for an observable without a nucleus - all you need is some sort of potential, in this case the Coulomb potential from the nucleus, but the source doesn't matter
And some observables, like spin, are always discrete independently of any potential

do you guys know about quantum magnet?

@ACuriousMind Got it, thx

@ACuriousMind did a user flag for converting an answer from community wiki?
I've forgotten the user name but they had accidentally converted their answer to community wiki and asked me to change it back.
I suggested they flag the question for mod attention.

@JohnRennie I don't think I'm supposed to tell you what flags other users are raising but let's say that if someone raised such a flag and it really looked likely an accident we would like remove the wiki status from the answer.

Sorry, I just meant had they understood what I meant and submitted the flag?
I'm not actually interested in their question :-)
Though I had a quick look and neither of their questions are currently community wiki so I guess they did.

8:43 AM
I got what you meant, we just generally consider flags (or rather who raised them) to be private information, even if in this specific case it wouldn't do any harm to disclose it

Agreed!

9:26 AM
In classical mechanics, is the choice of inertial frame in which to solve a problem considered a choice of gauge?

no, a choice of coordinates is not a gauge

damn

but much of the introductory literature is very poor at explaining what a gauge is, so don't worry :P
But that's not surprising because you need a bit more advanced concepts to properly state what a gauge theory is: It's clearest in the Hamiltonian formulation, where a gauge theory is one in which there are constraints so that the actual surface on which the motions lie is a subsurface of the total phase space. On this surface, the constraints generate transformations, and you can consider the orbits of points under these. A gauge is a (smooth) choice of one point from each gauge orbit.

when you say "orbit" are we talking about the group theory kind of orbit?

yes
"transformations" are groups (or algebras), so they have orbits. That one isn't really a difficult concept - the orbit of a point under the transformations is just the set of all possible points you can get by applying one of the transformations to the point
E.g. the unit sphere is the orbit of any point at distance 1 from the origin under the group of rotations

9:40 AM
I'm kind of at a point where I understand the phrases you're using to describe this but I have yet to see a "choice of gauge" in action so it's still a bit lost on me

0

When I read old posts, I come across some informative comments below the questions which are off-topic, i.e., not aimed at asking for clarification, but they do add some value. I have noticed such comments being deleted under new questions for attempting to answer instead of using the answer box....

it doesn't help that the first example of a gauge system we usually encounter is electromagnetism, where we have fields making everything a bit more complicated

I've been putting off learning electromagnetism for so long now lol
I haven't encountered a situation where I've desperately needed it yet so i've just been living without it :P

(also its nature as a gauge theory isn't usually explained there in the way I did above, either :P)

10:18 AM
Is it possible that rich people buy so many homes, land and cars just so they can safely store their wealth somewhere? It's more about having a place to store your money rather than actually living in all of the houses. Is this how it is?

What advantage does that have over using a bank?

It's safer? I'm not sure, that's why I'm asking

Land, homes and cars still have upkeep costs

Alright, let me ask this way - Where do the rich store their money?

I'm not sure what you mean by "is that how it is?", there's no limit to how much stuff you can buy with your money but it not always optimal to just by expensive things
Banks?

10:23 AM
Where do the rich store their money?

You can collect interest and invest your money when your assets are liquid
I'm not sure what you mean, do you expect the rich to have large Mr. Burns style vaults in their houses or something? Why don't you think they would just use a bank?

I don't know what they use, that's why I'm asking.

a bank lol

hmm cool, do they get interest on their money?

everyone gets interest on their money

10:25 AM
lol so their money get more and more by just having them in a bank
(they are taking the risk of bankrupcy tho)

Yes that is how banks provide incentive for people to store their money in banks

what about buying houses and land? how does that work? They hope that they can probably sell the house at a higher price in the future?

Yeah sure, if you have your money in a bank you are taking the risk that the financial market will collapse
You can invest in houses and land if they generate income, you can rent out housing, improve land etc.

hmm got it. What about cars?

Depends what kind of cars you're buying

10:28 AM
really expensive and a lot

Old cars appreciate in value, new cars decrease in value, they're a luxuary good

I mean, is there some reason to store money in cars?

Again depends on whether you're expecting their value to go up

hmm got it

lots of wealthy people just like owning expensive cars

10:29 AM
lol

as a status symbol or whatever

So mainly banks, real estate and land is where rich people store their money?

it's like buying an expensive watch, at a certain point your watch isn't reading the time more accurately, it just looks more expensive
I mean you'd have to ask a rich person what they do with their money, I'm sure there are good and bad ways to handle your finances and people do all kinds of things with their money

mm alright, one last question
When we have someone who has built a large public company and owns a really big percentage, we say that he is worth let's say 50 billion because the many shares he/she owns of his/her company is counted as assets. If that's so, in order for the person to get some of the money in cash and be able to buy stuff, he/she will have to sell part of the stocks (therefore give up ownership of his/her company)?

If what they want to buy costs more than the amount of cash they have then sure, they could sell their shares I guess
But to my knowledge it's not common for a person to have literally every single penny they own invested in their own company's shares
Then again I don't own a company so I'm not 100% sure

10:36 AM
Hmm got it, but the person can eventually buy out his/her stocks (ownership) back.

This is now getting a bit out of my depth, but it depends how they sell their shares, if someone else has bought their shares they can't force them to sell them back

Yes, alright, thanks :--)

1 hour later…
12:07 PM
I unknowingly made my ans a community wiki . Can any of you please undo it?

2 hours later…
1:48 PM
@ACuriousMind @DavidZ I seem to recall that there was a recent question on meta asking why the Related sidebar was missing, but I cannot find it any more.
Did something happen to it?

@EmilioPisanty yes, the OP deleted it, cf. physics.meta.stackexchange.com/q/13035/50583

That is odd. Why isn't it displayed in physics.meta.stackexchange.com/tools?tab=delete&daterange= then?
shouldn't that display all deleted posts?
(dunno if you have access to the same page that I do)
huh
31

I noticed on Retrocomputing that self-deleted posts do not show up in the Deleted section of the 10k Tools. Why is this? It seems like it would be useful to have all deleted posts show up in this list.

known problem, no SE response
O.o

2:38 PM
Hi guys! I was wondering if any of you used Wikipedia recently? Does it ask you to donate some charity to it?

@HrishabhNayal The foundation behind it does that periodically, but not at the same time for all languages/locations. Why?

I am feeling sick today :(

@ACuriousMind I have recently got a lot of them and I wish I could help.

hey guys

Btw has anyone here donated yet?

2:45 PM
is any-one here willing to discuss thermodynamics?
I tend to get pedantic, about certain things which are very casually stated in most physics textbooks....

@BioPhysicist I'll hope you get better then :)
@satan29 that's called "being a mathematician" :P

indeed XD

Thanks. I might look into Covid testing if I feel worse tomorrow. I don't feel too bad right now, but it just started
@satan29 I am willing, but I can't guarantee I will be able to help :P

i really have trouble visualising how reversible processes are carried out

Do you have a specific system in mind?

2:49 PM
consider a typical system: gas in a vessel, which has a piston.
suppose the outside pressure is atmospheric, $p_{0}$
and suppose we heat the gas

Hello
So I'm reading an article on a basic implementation of this system
In it, the author derives all the physics for the system down to the Euler-Lagrange equations
There's one thing that's tripping me up
He claims the moment of inertia for each rod should be the type used for a thin rod
with the rotation point around the COM

Now do we simply conclude that the expansion is going to be isobaric??

why would that be?
It seems to me, for example, for rod 1 in the picture
the rotation point should not be about the middle of the rod
but about the end of the rod
here's the paper in question

@satan29 There isn't any other force on the piston other from the inside / outside gas?

no

2:54 PM
@StanShunpike You can calculate moments of inertia about any point. It doesn't necessarily have to be about stationary points of rotation.
@satan29 I am not confident in this... but I would assume that if the piston can respond instantaneously to changes of the internal gas, then yes the process would be isobaric

@BioPhysicist can you elaborate on that? I'm super confused. So the rod spins about one end, why wouldn't I use the formula that has the picture matching that?

I guess you would also have to assume that the piston doesn't have enough inertia to sort of "overshoot"?

suppose the piston is massless

To me, the diagram looks much more like the bottom picture
if it was the top picture, then couldn't one end of the rod break off???

@satan29 Then yes, I think it would be an isobaric process; the piston would always move to balance internal / external pressure. If the external pressure is assumed to be constant, then the internal pressure would have to be constant too.

2:57 PM
if we are speaking of the rod attached to point O

@StanShunpike Like I said earlier, you can calculate moments of inertia about any point. Choosing a point to do this doesn't necessarily mean that the object will indeed rotate about that point.

@satan29 If you add heat "infinitely slow" so the system is in equilibrium with surroundings. Wikipedia

@BioPhysicist being massless kind of enforces that since $F1-F2= 0*a$, right?

Yes

now a reversible process is one in which the system is in equilibrium with its surroundings at all instants.

3:04 PM
yes

so why do we need to heat slowly ? the piston being massless already enforces mechanical equilibrium with the surroundings

My guess is that is relates to the massless piston assumption

hmm

In general you want the system to change much faster than the thing applying the change
In this case, the change of you system is determined by the piston movement
The change is adding energy through heat
So you need to add heat slow enough so that the piston moves fast enough

i see
i had a second question as well

3:10 PM
So yeah, I guess if you have a completely massless piston then the rate of heat transfer would be irrelevant in terms of having a reversible process

when is it appropriate to assume that the system and surroundings are at the same temperature?

Do you have a context for this?

@BioPhysicist I remember reading in a book that a reversible process also means that the system and surroundings are in thermal equilibrium

Yes, that is true

which basically means that they have the same temperature, correct?
(at each instant)

3:19 PM
If energy can be exchanged, yes

so consider a reversible expansion , in which the system's Temperature increases
how exactly will the temperature of the surrounding also increase?

You could also assume an ideal barrier that cannot transfer energy between the system and the environment. Then the thermal equilibrium condition isn't important.
I think

hmmm
one final question
how do we carry out an isothermal process? In the context of my system, temperature doesnt seem to be a quantity that we can control

If you hold the environment at a constant temperature, and you do changes slowly enough so that energy transfer is fast enough to keep the system at the same temperature, then the process will be isothermal
It is just like your first mechanical equilibrium question before

ah! fast enough
yes, it kinda makes sense now

3:28 PM
Good!

thanks a lot for your help! I feel more comfortable with reversible processes now.

Me too! haha

XD
guess its a bit confusing for everyone

Yeah, for some reason Thermodynamics is one of those areas that I enjoy when it clicks for me, but it is also one of the areas that I seem to forget more rapidly.

3:56 PM
Why government gives subsidies? Is it only in the case of a natural disaster so farmers can be incentivized to continue doing their jobs?

@JingleBells Once again, Wiki is your friend.

4:12 PM
@ACuriousMind My question was why, I don't need the different types of subsidies. Thanks anyways tho :P
Like, when and why does the government give money to a sector or business? In the case of clean energy, I understand that the goal is to push everything away from CO2 emissions. What other examples are there?

@JingleBells If you actually read the types, there is usually an explanation of the goal one typically intends to achieve with the respective type of subsidy

The goal of subsidies is to fund something the government wants to happen, exactly what they want to happen in each case depends on the type of subsidy.

@ACuriousMind Sorry, I'm hungry, I'm eating now. I'll read the types.
@Charlie Got it, thanks!

@JingleBells This is still something that Economics SE would be able to give better answers about

@Charlie sure but I always have questions and I tend to get answers faster here (something for which I am extremely thankful)

2 hours later…
5:56 PM
Is the 1-form analog of a smooth vector field one which resembles a series of smooth, nested surfaces? Kind of like a topographic map in $n$ dimensions?

I wouldn't try to visualize p-forms, really

:C
does the analogy not even hold for $p=1$?

@Charlie Yah I need to change my pic too

@Charlie Well, we first have to agree how we visualize the p-form in the first place. Say I give you a 1-form $f_x \mathrm{d}x + f_y \mathrm{d}y + f_z \mathrm{d}z$. How do you draw your surfaces from that?

I was thinking a series of 3D nested surfaces, which makes the inner product with vectors easy to imagine
actually no

6:05 PM
But where do these surfaces come from?

you can't even do that can you
I was going for the "the inner product equals the number of suraces the vector pierces" visual thing given in textbooks sometimes

For a vector field, I can tell you how to draw it: Embed the manifold it lives on in some $\mathbb{R}^n$, then literally just draw the vectors at each point

but we also have to specify direction

But I don't know how to draw a 1-form

ok the nested surfaces thing doesn't work

6:07 PM
@Charlie have a look at this and this for differential forms in terms of pictures

hmm I actually think I see what you mean
@bolbteppa That's the kind of picture I've seen a lot before, it seems like it's only really nice to visualise a single 1-form, not a 1-form field

Yes, all these pictures fail when the coefficients are not constants or very simple functions

What's a 1-form field

That's because the 1-form is really something local (it has values at points), but you're trying to represent it by drawing extended objects (the surfaces)

wait is a 1-form field not a thing?
is it not basically a vector field?

6:14 PM
@Charlie No, I suspect you're mixing up terminology here

I've seriously been living a lie if that's not a real thing
oh god

At a single point, a 1-form's value is a co-vector

A 1 form is a co-vector field

A "co-vector field" is what we call a 1-form

ohhhh

6:15 PM
don't worry, though, I've written "1-form" plenty of times myself when I meant to say co-vector :P

I've literally never seen that distinction made, 1-form and covector are always introduced as synonyms, good to know though

So the surfaces thing is how you can represent a covector, i.e. the value of a 1-form at a point
But if the 1-form is not constant, there's no good way to draw the surfaces "at each point", so there's no good visualization for an arbitrary 1-form

The fact that the pictures are curved surfaces represents the fact that at each point in space the coefficients are different, at any given point it sets up a constant set of sheets in the tangent space

But this shouldn't be surprising since you can't draw a $n-1$-tensor field either!

(Just as in calculus a tangent space at a point is a vector space, but it still generates a curved surface overall)
This is the approach in MTW as well

6:19 PM
honestly, I think one can trick oneself into thinking the drawings are helpful, but I've always found it more helpful to just work with the actual math :P

Yeah the pictures are only so useful, but it's good to be able to think about stuff this way

when you're looking at p-forms, they're just things you can integrate over p-surfaces to get values, i.e. they're sort of densities over p-dimensional volumes. Why try to draw that? :P

This perspective gives a direct visual interpretation of integrable vs. non-integrable forms
it's the basis of:
In mathematics, contact geometry is the study of a geometric structure on smooth manifolds given by a hyperplane distribution in the tangent bundle satisfying a condition called 'complete non-integrability'. Equivalently, such a distribution may be given (at least locally) as the kernel of a differential one-form, and the non-integrability condition translates into a maximal non-degeneracy condition on the form. These conditions are opposite to two equivalent conditions for 'complete integrability' of a hyperplane distribution, i.e. that it be tangent to a codimension one foliation on the manifold...

ah, another place where worlds change their meaning - "integrable" doesn't mean "can be integrated" but "has a potential" in this context!

Hello.. i am reading this article about a problem of calculating potential of two parallel charge wires. i have a problem understanding something.. could anyonehelp
My problem is equation 9 and 10.. i dont understand why the x component is not being considered..
you know that moment when you ask the question and literally after one second you understand the answer? well this iso ne of them .. thanks... i got it.

2 hours later…
8:30 PM
@ACuriousMind this is why applying the name "indefinite integral" to potentials/primitives/whatever is troublesome :P

9:11 PM
Are there coordinates for the maximally-extended Schwarzschild solution that are simpler than Kruskal coordinates (which requires the Lambert W function)? GP coordinates, unfortunately, fail to be regular across the past horizon.

10:00 PM
Does anyone have a suggestion for a fun little 10 minute talk about an application of particle physics? I came up with en.wikipedia.org/wiki/Muon_tomography so far
Ideally a technical application. Doesn't have to be cutting edge particle physics

I think there's plenty of other imaging (and treatment) methods in medicinal physics, but I don't know enough about them to suggest one as uniquely suited for such a talk

This is great, thanks!
Probably gonna talk about hadron therapy in medicine ("Hadron therapy uses protons and other ions to treat tumours, rather than X-rays as in conventional radiation therapy." CERN) and Myon tomography in archeology now. thanks for the suggestions

1 hour later…
11:17 PM
@TheoreticalMinimum how about bells experiment? :) mermin has a lot of nice writing on it, much at a basic level.

That's neither an application nor particle physics.

lol disagree but dont feel like arguing about it.