The h Bar

user351417

12:47 AM

@EmilioPisanty Sure. I've always flagged homework with NotAnAnswer. I'll raise mod flags next time.

user351417

(I'm assuming that it was a homework answer because I think I remember something of the sort on that question and there isn't anything particularly remarkable over there right now.)

David Z

2:04 AM

@Chair Oh, yeah, if the reason you're flagging is that the post is an answer to the underlying problem in a homework-like question, use a mod flag for that, like Emilio said. The NAA flag generally doesn't apply to those.

ZeroTheHero

3:57 AM

@Qmechanic man... do you ever sleep? physics.stackexchange.com/questions/430687/…

user351417

6:35 AM

@DavidZ @EmilioPisanty I don't object or anything, but aren't these discussions more on-topic on the Physics Meta chat?

Mohan

Do physists and engineers really understand mathematics and then apply it or do they just apply it?Iam not sure whether this is the right chat to ask it.If not please direct me.

David Z

@Chair No. The other room is named Physics Meta only because it's associated with our meta site and nobody ever bothered to change the default name. (Which we should probably do...) It's not meant to imply that meta discussion should be held in that room. The scope of both rooms is the same; the other one is just a backup for when this one is busy.

Though that could certainly change if someone makes a proposal on Physics Meta that we change the scopes and the community approves of that proposal.

John Rennie

@Mohan I guess it depends what you mean by understand mathematics

Slereah

6:53 AM

Nobody understands mathematics

Mohan

I tried to understand the operators like curl, divergence,etc..(but still now I couldnt understand them clearly).I think, these operators are widely used in physics and engineering.Without understanding them how can we apply them?I did undergrad in engineering.But my teachers didnt realy insist on understandin math, they just taught me to apply it.(by understanding I meant to learn how and why these operators give the results they do.)

Mohan

7:33 AM

My doubt is whether this is the right way to learn or should I try and understand these things?

John Rennie

@Mohan what areas of physics are you studying. Basic stuff or the more advanced areas like QFT or general relativity?

Slereah

Divergence isn't too hard to understand

It's the infinitesimal flux

Pick an infinitesimally small volume, the divergence is the flux of your vector field in that volume

Mohan

Just the basic stuff.From your answers it seems, understanding things is the right way to learn.Ill try to understand it.Thanks for helping me guys.

Anonymous

11:19 AM

@Mohan Well, if you're not understanding what you're doing then that's definitely not the right way. But, perhaps we should not forget that engineers and physicists have different priorities compared to mathematicians. If they spent all their time trying to understand the math better, they'd not have much time for anything else :P Anyway, as far as the basic differential operators are concerned, Khan Academy covers them really well in an intutive fashion.

user 2646

1:04 PM

@Slereah they just get used to it?

Avantgarde

1:32 PM

There's now this pesky sidebar on PSE. "Home", "Questions", etc. It takes up a lot of space of the page. Is there a way to hide it?

Why was this changed anyway? The bar was perfect when it was at the top of the page.

user 2646

Sep 19 at 12:03, by John Rennie

@bolbteppa go into your profile, then the Edit Profile & Settings tab, then click Preferences on the left then tick the option Hide Left Navigation. That removes the wasted space on the left. You can still get the left menu by clicking on the hamburger icon.

Avantgarde

@user2646 Thank you.

user 2646

np

Avantgarde

I have another question. How do I see the "new" questions.

I can't see that option anymore on the main page.

user 2646

physics.stackexchange.com/questions

Questions ---> Newest

Avantgarde

1:43 PM

@user2646 Thank you.

user 2646

:-)

Avantgarde

But why don't I see that option on the main page? physics.stackexchange.com

user 2646

It is under "Home" on the left panel.

Avantgarde

Aha, but I removed the left panel.

user 2646

:-/

Avantgarde

1:45 PM

Guess I'll have to stick with the panel

I don't like the new layout.

Thank you for the help

user 2646

you're very welcome

John Rennie

2:30 PM

@Avantgarde Hide the left panel (you know it makes sense). Click on the hamburger at the top left:

This opens a menu. Click on Questions to see new questions.

Avantgarde

3:02 PM

@JohnRennie Done! Thanks!

John Rennie

@Avantgarde I wasn't sure about the new layout at first, but now I've got used to it I really like it.

It's mildly annoying that it now takes two clicks not one to get the new questions page, but only mildly annoying.

Anonymous

3:16 PM

@JohnRennie I never really understood why the "Active" page is set as default (that's the page that is linked when you click on the site logo) rather than the "Questions" page, given that most of the active users care more about the new questions (which they can answer). Also, there are just 5 options on the hamburger menu which could easily have fitted on the top of the page too!

Anonymous

To avoid the annoying double-clicks though I saved the "Newest Questions" tab as a bookmark on the bookmarks toolbars :P

John Rennie

@Blue I agree. But then the design is really targetted to Stack Overflow, which is where the SE make all their money.

And the new questions page is next to useless on SO because there are so many new questions per second (or whatever unit time)

@Blue I've done the same :-)

Anonymous

@JohnRennie The active questions page would be equally useless :P

Anonymous

Anyway, whatever works for them I guess

Anonymous

I like that they're doing some work on "being able to easily find answerable questions" front

John Rennie

3:19 PM

I'm disinclined to make a fuss. After all the PSE has been a great benefit for me and ... it's free! :-)

Anonymous

Hehe, that's true of course :)

Anonymous

The SE sites are really unique

2

ACuriousMind

3:40 PM

@Blue "Most of the active users" != "Most of the users looking at the site"

We shouldn't forget that we active users are a tiny fraction of the total traffic

Also, I always looked at the active tab, even when it wasn't the default. But maybe I'm weird :P

John Rennie

@ACuriousMind I will at least glance at every question posted on the PSE, and for that I need the questions in time order.

Anonymous

@ACuriousMind Don't we already have a "Top Questions" page for external visitors? That is, the page you get when you visit https://physics.stackexchange.com/

Anonymous

I'm not sure what purpose "modified 2 mins", etc ago would serve for the visitors

ACuriousMind

@JohnRennie Ah, well. Especially since I've become a moderator, I'm at least equally interested in every other activity that goes on, too.

@Blue Yeah, good point. Maybe it's just the personal preference of the dev who coded it :P

Anonymous

Another thing I just noticed: Even the Top Questions page contains downvoted questions....that's weird

Anonymous

3:46 PM

Do we really want to display the (objectively) bad questions to the visitors?

Anonymous

4:02 PM

23

Q: A Jim for all seasons

Note: This contest is currently inactive. It may be resumed again in the future. So there has been a recent influx of new Jims (meaning maybe 2 in a while). Now, we all know that I am the one true Jim, lord of all other Jims, but nonetheless it gets confusing when someone pings Jim and Jim gets ...

discussion

Anonymous

LOL

Anonymous

I never saw this

ACuriousMind

Ah, good times

It was a jimteresting idea

The Dark Side

"Don't ask about asking, just ask"

Alright.

Here we go.

The site is populated with multiple questions regarding the premise: a free electron cannot absorb a photon, i.e. we can only have Compton scattering and not complete absorption.

The same can be demonstrated via conservation laws, most notably in this answer, quite explicitly.

So, for free electrons, it is always Compton scattering, never a complete absorption event.

But, there are also several questions on the site, e.g. this one

... which concern Photoelectric effect instead.

In PE, the electrons are bound, not free.

enumaris

3

A: How can the Alcubierre drive itself move faster than light?

You're making a mistake that is very easy to make when transitioning from understanding special relativity to general relativity - which is, you aren't thinking of things purely locally. When we go to GR from SR we lose a very important part of physics - the existence of global inertial reference...

this safesphere dude just comes in and puts words in my mouth to criticize my answer...uncool...

The Dark Side

4:12 PM

The only part I do not get is, if we think in terms of conservation laws, how is the aforementioned answer "cured", i.e. how is the inconsistency resolved, by the fact that the electrons aren't free, but bound?

Offhand, the fact that the electron isn't free but bound, would only indicate that we have something else to share the momentum transferred by the incident photon.

John Rennie

@TheDarkSide because the electron can exchange momentum with whatever it is bound to.

enumaris

When the electron is bound, it means there's "other stuff" around it. The momentum that couldn't be conserved in a 2-body problem can be conserved by this "other stuff"

The Dark Side

Yes. Exactly. But ...

enumaris

I think you answered your own question with your second statement.

Avantgarde

@JohnRennie Yeah it's annoying to find your way through to the new questions.

The Dark Side

4:15 PM

Perhaps. But let's think of it in terms of Rob's answer, or a simpler version of the same answer where we consider ourselves in the rest frame of the electron originally.

Avantgarde

In any case, I don't see much of a (good) difference with the new layout.

The Dark Side

How does this other stuff enter the derivation, and mitigate the inconsistency, is something I haven't been able to resolve.

ACuriousMind

@TheDarkSide It's there, so it can participate in the scattering. Quantum scattering events are not collisions between billard balls.

The Dark Side

e.g. consider a simple picture where the electron is bound to the nucleus.

@ACuriousMind True. (But returning to the derivation ...)

enumaris

Electrons which are bound share an electro-magnetic interaction with the atoms it's bound to. This electro-magnetic interaction will mean that the atoms the electron is bound to can share in the interaction since if you move the electron around with the photon, you are also moving the atoms around.

@TheDarkSide are you asking for a rigorous mathematical calculation to show how momentum and energy are conserved in a 3-body collision?

ACuriousMind

4:19 PM

@TheDarkSide In the derivation, you just add a term $p_\text{stuff,i}$ to both sides of the momentum conversation equation.

The Dark Side

@enumaris Yes. qualitatively I understand that, that's why I wrote that sentence.

ACuriousMind

And since there aren't any other constraints on it, it'll be able to absorb all "excess" momentum.

The Dark Side

@ACuriousMind Yes. I am only trying to construct the argument. So, in energy conservation, I'll have an extra $M_N c^2$ term, and nuclear recoil KE can be neglected due to the large mass, right?

ACuriousMind

Why would you neglect it? The recoil is the very thing that makes the absorption possible!

enumaris

To see that momentum can be conserved is easy - just do what @ACuriousMind said. But to actually calculate the E.O.M. for a given 3-body collision (given some impact parameter) seems quite hard to me. I'm not sure I've seen it done in the past. However, energy and momentum conservation will both go into these equations as constraint equations. In the 2-body case, it was just to show that such constraint equations cannot be simultaneously satisfied if the photon is totally absorbed.

The Dark Side

4:22 PM

@ACuriousMind True, but for whatever momentum is exchanged, aren't we dividing by mass of a nucleus which is substantially smaller, so the terms shrinks considerably.

@enumaris Right.

@enumaris But maybe just showing it can be satisfied in this case would be sufficient consolation ...

enumaris

@TheDarkSide just follow @ACuriousMind's advise then :)

The Dark Side

Alright. I am onto the maths now. Will return in a few minutes.

enumaris

@TheDarkSide chat.stackexchange.com/transcript/message/46889698#46889698

The Dark Side

Yes.

Just one point.

@ACuriousMind So, basically, aren't we claiming that it can eat some appreciable amount of momentum, but eats up very little of energy owing to large denominator?

Is that correct?

vzn

8:17 PM

@TheDarkSide think you have some deep questions & think they point toward a new idea/ understanding. there are some new ideas about energy dissipation in atoms leading to quanta... can discuss this more at length with the a priori understanding that its a new research area.

Lozansky

8:54 PM

@Semiclassical So it's Griffith's 4.18

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