The h Bar - 2017-02-23 (page 8 of 8)

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9:01 PM

@ACuriousMind I obsessively feel I should know every topic in a standard physics curriculum in a way that if someone wakes me up at midnight and asks me a random question I can answer :/

a charged particle accelerating radiates EM waves. The equivalence principle says there is no local experiment that can distinguish between a uniform gravitational field and a uniform acceleration. So shouldn't, therefore, a charged particle sitting still in a uniform gravitational field radiate? Where is this energy coming from?

@Mostafa I hope your knowledge isn't like this guy's $100 :-)

@Jim gulp

@0celo7 I don't suppose you want to use Riemann mapping?

where do you physicists come up with these things

@ACuriousMind No, I already know that proof though.

9:05 PM

@Jim Why should it radiate while sitting still?

If it's sitting still something is counteracting the acceleration the gravitational field exerts

@ACuriousMind if it's sitting still, then the equivalence principle says it's the same as accelerating

this is the elevator thought experiment

The equivalence principle tends to make knots into my brain. What exactly do you think it says?

Einstein was wrong...

The local effects of motion in a curved spacetime (gravitation) are indistinguishable from those of an accelerated observer in flat spacetime, without exception.

I knew all that evidence was fishy!

9:10 PM

if you go back to the elevator thing, sitting in an elevator on Earth that's not moving is equivalent to being in an elevator in empty space that is accelerating

@Jim Do you know that in full GR, accelerating charges radiate?

@Jim Right

Yeah, I see what the issue is. I think that would be a nice question for the site, actually!

Paradox of a charge in a gravitational field

The special theory of relativity (SR) is known for its paradoxes: the twin paradox and the ladder-in-barn paradox, for example. Neither are true paradoxes; they merely expose flaws in our understanding, and point the way toward deeper understanding of nature. The ladder paradox exposes the breakdown of simultaneity, while the twin paradox highlights the distinctions of accelerated frames of reference. In SR any paradox can be mapped on a geometrical 4-dimensional euclidean problem to be studied. In general relativity (GR) things are not so easy. Among others, there is the paradox of a charg...

@Jim Did this not help?

> a charged particle and a neutral particle fall equally fast in a gravitational field, despite the fact that the charged one loses energy by radiation

wat

@0celo7 that'll probably help

9:15 PM

The point about non-inertial frames is valid, but the above statement seems very strange to me nevertheless

Oh god what's radiation in one frame is a slight deviation from the 1/r^2 Coulomb law in the other?

That's...unsettling

@ACuriousMind what?

@0celo7 It's in the article you linked

> When that is done, one finds no radiation in the supported frame from a supported charge, because the magnetic field is zero in this frame. Rohrlich does note that the gravitational field slightly distorts the Coulomb field of the supported charge, but too small to be observable.

@ACuriousMind I'm sitting in a QM lecture

none of this GR nonsense

This is very intriguing

The JEE Launch Pad

6

Do you mind pinning it?

9:21 PM

@0celo7 Nonsense? I have a crush on Classical Physics!

@skillpatrol Why would I pin it? I already mentioned that room above, btw

@Mostafa Oh, I mean physics in general is nonsense

general relativity is general nonsense

Wait a moment

Just as a gentle reminder @ACuriousMind

By the equivalence principle, we are all accelerated observers, right?

Where's our Rindler horizon/Unruh radiation?

9:23 PM

oh god

stop

@Jim ^see above

@ACuriousMind Can the observer see his own Unruh radiation? Or is it infinitesimally small?

@0celo7 The radiation is probably so cold as to be undetectable, yeah

@ACuriousMind I mean, everything would radiate, no?

Everything moving, basically.

Or would 'see' radiation, whatever that means.

I don't remember the derivation of Unruh

Hm, by the equivalence principle, is the Rindler horizon fully equivalent to the Schwarzschild horizon? Has the frame of every one of us a Rindler horizon sitting deep inside the earth?

9:26 PM

German alert...

@0celo7 ?

@ACuriousMind "has the frame of us"

Ah

yeah

it happens sometimes

Hi @heather

@0celo7 Unruh says that every accelerated observer sees radiation emitted from the Rindler horizon

9:28 PM

@ACuriousMind yeah, I know that

What does "sees" mean?

@skillpatrol, hello

@0celo7 Well...it means that the "vacuum state" - the state a non-accelerating observer determines to be empty - is a thermal state for an accelerated observer

@heather Don't bother reading the stranscript.

@ACuriousMind More words! What kind of radiation? What temperature?

"seeing" means that if you have means to detect those particles, you will see them, and since it's thermal it will look like blackbody radiation

@0celo7 that just makes me want to read it.

9:31 PM

@0celo7 Every possible sort of radiation/particles, the derivation of the effect is not particular to a specific field

@ACuriousMind I knew all of that.

But what temperature?

@0celo7 It's proportional to the acceleration. I'm not sure what you mean by "what temperature".

@ACuriousMind If it's 1K for our acceleration (whatever that is), then I don't consider it an issue.

How do we even measure our acceleration?

Relative to what?

@0celo7 It's probably below nano-Kelvin

Already said that:

9 mins ago, by ACuriousMind

@0celo7 The radiation is probably so cold as to be undetectable, yeah

I'm more interested in where the horizon is

@ACuriousMind now hold on

9:34 PM

what the heck

why is the chatroom getting so divided over JEE questions?

girl I told you not to do that!

Fun fact: Unruh radiation of 1K is equivalent to an acceleration of $10^{20} \mathrm{m}/\mathrm{s}$.

you said not to do something. if i said that to you you'd do it @0celo7

@ACuriousMind RELATIVE TO WHAT

@0celo7 Huh? Since when is acceleration relative?

I think the quantity called acceleration here is formally the magnitude of the 4-acceleration

9:37 PM

@ACuriousMind We measure acceleration on Earth relative to ourselves, but we're accelerating!

@0celo7 Oh god GR is so confusing :P

@ACuriousMind yeah, screw it

Let's do topology instead

Hm...I guess you need some sort of preferred frame that acts as the one that defines the quantum vacuum

@ACuriousMind CMB I guess.

Then the acceleration relevant to the Unruh effect is the one as measured in that frame

@0celo7 Probably, yeah.

9:39 PM

Let's do some JEE questions :P

Still one would need to come up with some abstract reason for that frame, no? Maybe semi-classical QFT is just non-sense, though

@ACuriousMind Also don't forget that the Earth itself is accelerating around the SMBH in the middle of the galaxy.

That's also accelerating, etc.

Ah, wait. Technically the Unruh effect just tells you what temperature the vacuum of one observer has as seen by another. So one needs to determine which vacuum the state of the universe corresponds to

@ACuriousMind Yeah, I thought that was clear.

I said CMB because it seems reasonable.

Which is probably reasonably the CMB, as you said

9:42 PM

@ACuriousMind I miss Chris.

He would have enjoyed this.

I do, too.

@DavidZ I was told that you'd be interested by a fork of Chemobot here. If so, what should I name it ? PhysicsBot just doesn't have the same ring to it imo

@ACuriousMind This $U\subset\Bbb R^2$ problem is still stumping me. If we remove a point from $S^2$, it is still simply connected. Why does that fail for a subset of the plane?

@Hippalectryon I still don't really have an idea what functionality that bot would offer. Is there an explanation somewhere?

@ACuriousMind Well, it's up to you to suggest commands that could be relevant here. For instance in the chem chatroom the bot can give a picture of a molecule with the img/molecule_name,identifier,... command

9:54 PM

@Hippalectryon Can it do commutative diagrams?

@0celo7 Well, i suppose I could add that

@ACuriousMind :)

pleeeeeeeeease

Hi all, I have a peak in some data with an assumed exponential background. I have fit the data to a gaussian + background model. Now I want to get an estimate of how likely this peak would be from chance, i.e. background only. What is a test I could do to figure this?

@0celo7 How would you expect the command to work ? Like, what arguments would you give to the bot ?

the tikz-cd commands

mathjax does not support that package

9:57 PM

Oh well it's a python bot, so if the command already exists I can just ask python to generate the image and upload it here

Surely there's a LaTeX python package

@Hippalectryon Well, I'm not the one who thought we needed a bot...I kinda thought it came with stuff it could do

Not really sure what functionality we could use

@ACuriousMind It was originally made for the chem chatroom, which enjoys its more fun-oriented commands (meta.chemistry.stackexchange.com/questions/2723/…) but I don't think you want those here

How about we call it The hBar Tender?

4

@ACuriousMind I would become an algebraist just to put beautiful diagrams on the chat

@Hippalectryon Hmm, doi and xkcd sound useful at first but then I realize the doi or the number of the xkcd is usually info I only have when I could just post the link myself

10:03 PM

@ACuriousMind arctic one boxing

@ACuriousMind doi is useful when people reference dois in their answers without direct links for instance

Ooooh, if it can onebox stuff that normally doesn't onebox, that'd be awesome

Though I'll have to admit that it's not used a lot

@Hippalectryon I think I see that about once per 1000 posts, if that often

@ACuriousMind Tbh I don't remember the last time I saw that command used :( but I made it because someone initially asked me, so I guess it was useful at some point

10:05 PM

@Hippalectryon 4chan oneboxing please

@0celo7 oneboxing ?

@0celo7 vetoed :P

@Hippalectryon You know how chat auto-formats Wiki and amazon links?

We've long wanted that for arXiv links, could the bot do that if told?

:o I didn't know it auto formatted amazon links

Well the issue is the bot, like any other user, can't do what you can't, i.e. he can only post markdown text and images

No CSS

But it could definitely make an arXiv command that provides links to arXiv's pdfs for a given ID or whatever

@Hippalectryon no, people always give links

But you have to click on the damned things

Very hard to do!

It would be nice if you could at least post the author and abstract

Oh sure that's easy

10:09 PM

Date, institutions

@ACuriousMind ??

It has a welcome for new users to the chat also

Shows them the guidelines etc

@0celo7 Yeah, title, author, where published, maybe the first few sentences of the abstract

The hBar Tender

1 1

:D

Now I just need to find a LHF to get 20 rep

10:14 PM

I think I'd prefer "The h Bartender", but that's details

LHF?

Low hanging fruit

Easy question to get rep

Homework question

Yup

Can't find an easy unanswered one >.>

10:19 PM

@Hippalectryon I've been meaning to write on one wave constraints for strong G waves

@ACuriousMind Chris would have been able to answer it :(

@Hippalectryon are you an astrophysicist

@0celo7 I'm not :(

Why @ACuriousMind do you prefer more emphasis on Planck's constant?

@0celo7 What about the bot's acc posts the answer and you take care of the OP's eventual comments ? :D

@skillpatrol The room is named "The h Bar", I find it grating to not have the space between h and Bar in the bot's name

The room is misspelled

I've been saying this for (holy crap) years

@ACuriousMind we've known each other for over two years

10:23 PM

I guess that's true

@0celo7 How would you spell it?

hBy2Py, Dayton, OH

8.5k 2 24 54

homework inorganic-chemistry electrochemistry physical-chemistry organic-chemistry computational-chemistry thermodynamics

How about that^ avatar for the bot?

@skillpatrol done

Thanks pal @Hippalectryon

Need 20 rep now >.>

Ask a question

10:38 PM

I don't have any question :( got one ?

Why does the earth spin east to west and not the other way?

Surely that's been asked before :(

If I'm asking a questions I might as well ask a good one

Yup on earth science

I recall you asked a good question awhile back @Hippalectryon

@skillpatrol can't transfer reputation though :(

2 hours ago, by Jim

a charged particle accelerating radiates EM waves. The equivalence principle says there is no local experiment that can distinguish between a uniform gravitational field and a uniform acceleration. So shouldn't, therefore, a charged particle sitting still in a uniform gravitational field radiate? Where is this energy coming from?

Ask that^

10:54 PM

What title should I give ?

Where does the energy come from?

@skillpatrol That's not a good title, it's very non-descript

That sounds like a clickbait questions >.<

Where does the energy come from HELP URGENT [plz]

If you want to ask that, choose a specific title, like "Does the equivalence principle imply that a charge particle resting on earth radiates?"

7

Q: Equivalence principle and radiation from falling particle

I am currently having a hard time solving a problem of GR from Lasenby's book. I can't make it more clear than by quoting the exercise: 7.2 A charged object held stationary in a laboratory on the surface of the Earth does not emit electromagnetic radiation. If the object is then dropped so that ...

homework-and-exercises general-relativity equivalence-principle free-fall

:(

10:58 PM

Hm, could use a better answer, but yeah, it's a duplicate

@Jim ^

All we need is the 20 rep :P

2 upvotes

@skillpatrol We're not in a hurry, are we? No point in posting crap/dupes for that.

Yes, master.

How about "The Role of Rigourous Argument in Physics"

Too soft?

76

Q: The Role of Rigor

The purpose of this question is to ask about the role of mathematical rigor in physics. In order to formulate a question that can be answered, and not just discussed, I divided this large issue into five specific questions. 1) What are the most important and the oldest insights (notions, results...

mathematical-physics mathematics research-level

Yes, master.

11:06 PM

It's probably easier to give the bot a good answer

Jul 17 '15 at 10:22, by Hippalectryon

13 hours ago, by Hippalectryon

Hi! I'm studying a usual linear accelerator for electrons and I'm trying to get the force that corresponds to the power radiated ($P=m\tau \ddot{x}^2$). I've tried calculating the associated work which gives me $W=m\tau\int_0^T\ddot{x}^2\mathrm{d}t=\int_0^T\vec{F_r}\cdot\vec{\dot{x}}\mathrm‌{d}t$, how do I continue ?

Remember that^ @Hippalectryon

Yeah :D

No clue how to solve that though

That's why you asked.

I haven't done any physics/maths in at least 6 months :(

And in chat we didn't find an answer :c

Sure we did

Let the bot ask

It's a good question

11:18 PM

@skillpatrol It's a homework-like question and will likely be closed.

Yes, master.

Jul 17 '15 at 10:23, by Hippalectryon

Sounds too homeworky for PSE. And it should be an easy question :/ I'm just somehow stuck

Also, is the answer not just the Abraham-Lorentz force?

Well iirc the answer was related to that, yeah

11:42 PM

0

Q: Lab journal best practices - on topic?

I had a question about best practices for keeping a lab journal (specifically, should one only record experiments, or should one also include research for experiments, or time spent looking around on say McMaster Carr for parts). Is this on topic or off topic? It would seem to fall under experime...

discussion scope

hello

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Feb '1723

The h Bar

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