The h Bar - 2016-04-04 (page 1 of 2)

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12:28 AM

@Obliv I'm learning from artin right now.

then I'll read hungerford.

12:49 AM

@ACuriousMind citation provided ;)

4 hours later…

4:38 AM

New link between quantum computing and black hole may solve information loss problem / Hossenfelder

@FenderLesPaul ↑ 1 for you (via reddit)

5:03 AM

@0celo7 yes, I don't see any problem with this.

2 hours later…

Priyadarshi Paul

6:46 AM

I am facing an indicial equation including dirac delta function for deriving magnetic field from the vector potential's 2nd term in its multipole expansion . Can you suggest anything?

7:40 AM

Hello

7:54 AM

@PriyadarshiPaul can you show a formula?

2 hours later…

9:37 AM

@skillpatrol Uh, the whole article is about there not being a reliable citation.

Valentin Tihomirov

10:15 AM

What is so paradoxial in EPR? Why cannot you just interpret the instant communication as sending two complementary particles to both Alice and Bob? If you know that Alice received White then you know that Bob have received Black and vice-versa. What is more tricky than that?

@ValentinTihomirov for quantum entanglement, that's all there is to it if you ask me

well, that's 90% of it

(and I probably shouldn't have replied since I have to run)

There is indeed a classical analogue of the EPR paradox - sending correlated classical information to two people. E.g. sending heads to Alice and tails to Bob

Valentin Tihomirov

But what is there so spooky that bothered Einstein?

I think the "paradox" is that people who don't like that quantum mechanics only predicts probabilities for measurement results see it as a paradox that the results of two measurements can be perfectly correlated although no information is exchanged. The classical case is not a "paradox" because the classical objects had well-defined properties all along.

But there is more to it. In the classical case, eg Alice had a definite but unknown value prior to her looking at it. In the quantum case, that isn't so.

Valentin Tihomirov

10:21 AM

I do not see what Einstein tried to achieve with EPR measurement.

He wanted to show that QM is incomplete, i.e. must have hidden variables. "God does not play dice" and all that.

He was trying to demonstrate that QM contradicted what he thought of as common sense (local realism) and thus must be wrong or incomplete

Valentin Tihomirov

Please do not pay with standard phrases. I am asking in concrete terms. How do you challenge the incompleteness with splitting a message into Bob's and Alice parts?

You say that this is QM analog of classical truism. Why should it help the Einstein?

Bah read wiki

Valentin Tihomirov

and why do we need superdeterminism/faster-than-light communication if we can resolve the paradox by simply assuming that there is a classical determinism at the moment of message pair creation?

10:32 AM

Uh...because "assuming classical determinism" is exactly saying that "there are hidden variables"?

11:09 AM

WHERE ARE THEY HIDDEN

Where have you hidden the variables, @ACuriousMind

I, uh, appear to have dropped them under the sofa. Sorry about that.

What can be done in chat,any link to read from? any restrictions?

you have to Be Nice

questions can be asked?

related to physics

sure

11:18 AM

ok thanx for your time :)

I find the topic magnetism a bit tough, can someone help me? :P

Valentin Tihomirov

You need to pretend to be nice. For instance, you cannot say "your wife is a shitty whore". You have to be civil and say "she sleeps around prolficirsously".

Ask and see what happens

Is that the crazy guy that complains about Stack Exchange

@JimmyKudo We don't like commiting to answering a question before actually hearing the question here. Just ask about what you want to know

Sorry I mean the fellow with unorthodox views

@Slereah Yeah, it's @JohnRennie's fan

11:27 AM

@ACuriousMind what's a "talk" in German

An academic talk

Vortrag

Oh, presentation

I thought there was a separate word

We've got a German postdoc appearing today

@ACuriousMind chances are she's from Hberg too

I need some damn UH stories

May the experts correct me if I'm wrong, but there's more to the EPR paradox than just sending two "complimentary" particles to Alice and Bob. Just sending White to Alice and Black to Bob is something completely classical.

The point of the EPR paradox is that quantum mechanics says that the particles are *entangled*, which means that in the moment *right before* Alice and Bob receive their particles, it's not yet determined who receives White and who gets Black. All options are still open. The only thing that is determined is that *if* Alice gets White, *then* Bob gets Black. This is true …

Well

local hidden variables

@Slereah Right. Too late to edit :p

Valentin Tihomirov

11:52 AM

@Bass I do not see the paradoxical part here. Everything looks classical. Physics knows that at the time you "prepare" the system. Hidden variables allow you to do that. I do not need

Sᴋᴜʟʟ ᴘᴇᴛʀᴏʟ

@ValentinTihomirov that particular comment, no matter how it is worded, is intended to instigate a response.

0

Q: How to get explanation why question is unclear?

I have had a big problem over many months with a question. I try to tell the story as factually as possible. Help is appreciated. Here we go... I asked a question on Jan 27 at 14:29. It was closed as unclear on Jan 29 at 23:20. I edited for clarity on Jan 30 at 15:45, messaging all who closed ...

discussion close-reasons specific-question vote-to-close

12:09 PM

@ValentinTihomirov How do you explain classically that before the measurement, it's not determined who gets White and who gets Black, but when they receive the particles, one gets White and the other one Black? I mean, what's the exact classical situation before they receive the particles? Where is the white and where the black one?

12:41 PM

@DavidZ hey can we like triple underline the word "concept" in the homework close thing

maybe make it bright pink or something

So do you think I can just say "if a curve is timelike in the hood, its image in the tangent space is also timelike, because of the Gauss lemma"

@Slereah No, but I am working on a proof of that

It's prop. 4.5.1 in HE

I'm going to rewrite that proof.

So say "because of a corollary to the Gauss lemma"

1:23 PM

Why can't people actually write down real proofs

user116211

In reflection, when a transverse pulse arrives a fixed end, it exerts an upward force on the fixed end; by Newton's Third Law, the fixed support exerts a downward force generating an inverted transverse wave....

user116211

But when the same transverse pulse arrives a free end, 'the arrival of the pulse will exert an upward reaction force back on the string, generating a pulse of same polarity'.....

user116211

My question is: What happened to Newton's Third Law in the later case?

user116211

Why didn't the free end exert an opposite reaction force i.e. downward force on the pulse?

2:02 PM

@Slereah do you understand the HE proof?

I already rewrote one part in the gauss lemma post

No :(

Hey everyone! I have a quick question. After reading something on your own from a textbook and have worked out the problems, what else do you do in order to consolidate what you’ve learned? Although I do have an education in physics, I’d like to refine and rehash my own study and self-study habits. Any suggestions?

user116211

@MattSteinberg Answer questions of others?

@0celo7 nah, we only get bold

Can you add some blingees to it

2:15 PM

lol blingees

user image

Something like that

user116211

@Slereah I'm blind.

user116211

I'd love watching 0celot's sleeping cat than this pinky sparkles....

@MAFIA36790 I read that wrong.

2:30 PM

@Slereah Do you get my Gauss lemma proof

The major thing that distinguish between a classical correlated system and entanglement is that while in both cases the information are correlated, in the entanglement case which of the two correlated outcomes you measure is determined only by probability (i.e. there is a random chance of measuring A or B)

Therefore, if Alice or bob only look at the series of measurements they made individually on an ensemble, all they get is a string of e.g. ABBABABBABBBBBAAABABAA in their log book. Thus without actually sending their measurement results to the other party, they will never see that there …

Who are Alice and Bob, anyway

Really just A and B

"These names were used by Ron Rivest in the 1978 Communications of the ACM article presenting the RSA cryptosystem"

I wonder if they are older

There's also a C, forgot his full name

2:34 PM

I remember an old timey relativity book that used...

Mob and

Something else

Mob for mobile

All I remember C is always the evasdopper

C is Carol btw

The eavesdropper is Eve

oops

Something that Chrome users who view papers on arXiv might like: chrome.google.com/webstore/detail/arxiv-title-fixer/…

@DavidZ meh.

I have 30+ tabs open all the time

I never see my tab titles anyway

user116211

2:53 PM

@0celo7 My Opera browser slows down after 20 tabs....

@MAFIA36790 get a better browser?

Chrome gets funny around 80

user116211

@0celo7 o_O

user116211

@0celo7: It becomes difficult to recognise which tab is which if the page has same logo after 30 tabs, although.... don't you face that problem?

I sometimes have over 300 tabs open

I do.

3:02 PM

(My firefox crashes pretty often)

I just memorize which tab is where.

@Slereah Pics or it didn't happen

Eh

Don't care enough

Well I care.

user116211

@0celo7 oh.

@Slereah what's the line that GR books should start with?

LET M BE A TOPOLOGICAL PARACOMPACT HAUSDORFF SPACE LOCALLY HOMEOMORPHIC TO R^N

@Slereah is Steenrod really a topology book

3:14 PM

That is the classic line, yes

It's a book very focused on bundles

@Slereah currently writing the canonical answer on GR books

minus the shitty ones like Hartle.

@Slereah Do you know if Carroll talks about the Hamiltonian formalism

@ACuriousMind Do you know if QoGS can be used for GR at all? Not the quantum stuff, obviously, but the constrained Hamiltonian systems stuff.

@0celo7 Isn't that what the ADM formalism does?

@ACuriousMind Yes.

But ADM is usually done by dirty physicists like @ChrisWhite

Then what's the question?

QoGS is supposed to be nice.

3:25 PM

It's also written by dirty physicists ;P

Dirty physicists are a subset of physicists

There are some like Straumann who are clean

Pure

Well, since its goal is to quantize the constrained system, it's not interested in solving the classical e.o.m. or something, so I'm not sure what might be of interest to you.

@ACuriousMind I don't think you're a dirty physicist

3:59 PM

@JohnRennie I plan to put this into an edit of my question "@JohnRennie believes that changing the time of t=0 may allow us to do this [solve this problem without using unpopulated reference frames], more details if that pans out.". I assume you don't object, but, if you do, let me know and/or edit it out after I post it.

@barrycarter no don't do that, you're attributing more to me than I've said.

Setting the clocks to zero at the meeting is just a convenient computational device.

Let me sketch out my argument and mail you the link, then you can use it as you wish

I still don't get what's bad about using "unpopulated" reference frames. It's the result and the correctness of the derivation that matters, not your philosophical issues with relativity.

@JohnRennie Got it. I'll edit the statement before posting.

@ACuriousMind I tend to agree with you, but a solution wo unpopulated reference frames would be cleaner, if one exists.

@barrycarter which question? Can you post a link?

@ACuriousMind you have this magical ability to separate philosophy from physics

4:03 PM

physics.stackexchange.com/questions/246165 -- I have not yet posted my update.

@barrycarter It would only be "cleaner" if there was something physically significant about the "population" of a frame. There's not, as far as physics is concerned, a frame is a coordinate choice, not something you can populate or inhabit.

@ACuriousMind I think this is an aesthetic issue. I agree with you that the right answer is more important than aesthetics (ie, beauty is not truth). However, 1) I'm pretty sure I've worked problems like this before without having to introduce "if I had been in this frame 30 years ago...", and 2) if there is a solution that doesn't use hypothetical statements like that one, I would prefer it to one that does use hypothetical statements.

Oh well, if that's your sense of aesthetic, stay away from quantum mechanics ;)

@0celo7 The times when one could confuse the two are several hundred years past...

@ACuriousMind for you

I'm still not convinced one can actually make measurements in GR

@ACuriousMind I like quantum mechanics. The concept that the universe is running an non-deterministic probabilistic reality is kind of cool.

4:09 PM

it doesn't make sense

See what Einstein said, god doesn't play dice

Are you saying you're smarter than Einstein

Quantum Mechanics: When even God is a gambling addict.

@0celo7 Einstein's a wonderful guy, but he never liked quantum mechanics, although he understood it.

oh, so you're smarter than him

Corrected my statement.

@0celo7 I realize you're being silly, so I won't answer.

I just grew an octopus in my garden.

Today's xkcd is strange.

4:12 PM

what

An octopus of lights, its the latest thing, everyone has one.

@ACuriousMind if i join all anti-nodes in a 2-D wave, will it be my wavefront? (sorry if pinging irritates)

user image

^ line is wavefront, correct?

Looks good to me, but I'm not @ACuriousMind

What irritates me is why you ping me with that question.

Ah

You're the same one who asked about the wavefronts the other day, right?

yes, that's the reason i asked you , sorry

4:17 PM

Well...not every antinode belongs to the same wavefront

@ACuriousMind Because people think you're friendly!

Little do they know that you eat souls

@0celo7 And they are delicious.

my canonical answer on GR books is getting long.

**Special Relativity**

- E. Gourgoulhon (2013), *Special Relativity in General Frames.*

This is a rigorous and encyclopedic treatment of special relativity. It contains pretty much everything you will ever need in special relativity, like the Lorentz factor for a rotating, accelerating observer. It is not an introduction, the author does not bother to motivate spacetime at all.

**Introductory General Relativity**

These books are "introductory" because they assume no knowledge of relativity, special or general. Additionally, they do not require the reader to have any knowledge of topolog…

I still haven't done half of the books!

@ramsay You have to know whether the nodes have the same phase to conclude they belong to the same wavefront - that they have the same amplitude is necessary, but not sufficient for such a plane/spherical wave.

thank you :-)

can anyone show me diagram of this statement (please) :
> "the direction of propagation of wave at a point is perpendicular to the wavefront through that point"

4:27 PM

If we define wavefront as we just did, propagation is parallel to that line, not perpendicular.

@ACuriousMind Do you want to pay for my uni application?

@BernardMeurer he refuses to Skype with you

I think you know the answer already.

I have to pay this bloody application but I can't

it keeps refusing my credit card

@ramsay
http://astarmathsandphysics.com/gcse-physics-notes/903-waves-and-wavefronts.html

and the deadline is in two bloody days

4:29 PM

@barrycarter but this statement is in my book

@Secret thank you, i really needed that :-D

@ramsay Right, I'm saying that perhaps your book uses a different definition of "wavefront".

@barrycarter Huh? Why do you say that?

@ramsay For example, it may define a wavefront as where the wave crosses from negative to positive. @ACuriousMind Did you read the perpendicular statement above?

@0celo7 Will yopu ask Bob to pay it for me?

Or am I totally misunderstanding that sentence? I thought propagation meant "the direction the wave is moving".

4:31 PM

I bet he would, I'm like the son he never had

Or wanted

@barrycarter "a surface on which the wave disturbance is in same phase at all points is called a wavefront" this is the definition in my book.

So you have an infinite number of wavefronts for a given wave? One for each phase?

yes..i think

@BernardMeurer No.

Michelle is going to be home for a week

Ask her to take a selfie with him

4:34 PM

She probably would, I'm like the brother she never had

@BernardMeurer He has two sons

@BernardMeurer She has two brothers

@ramsay So the perpendiculars would not be the direction the wave is moving? I give up, let @ACuriousMind answer :)

Tsc

dammit

I keep forgetting you have a brother

user image

ACM's handwriting makes me happy

4:35 PM

@barrycarter Yes, there are an infinity of wavefronts

@ACuriousMind Your handwriting is so cute

@ACuriousMind I get it now. I was thinking of a two dimensional wave along the x axis.

Uh...thanks :) I guess.

I'm about to seriously annoy the crap out of @WillO

thank you ACuriousMind

4:39 PM

@barrycarter just be careful

@DavidZ I will be using the Hammer of Truth! Plus, I have a nice diagram, though not as cute as @ACuriousMind's

@ACuriousMind you should draw a manga

I'm crap at drawing actual objects/persons :(

A manga where the characters are all physics entities.

I would totally read that

@ACuriousMind can you draw what exactly an Ehresmann connection does

on a torus

4:45 PM

@0celo7 No, because I can't draw the 4D tangent bundle :P

user image

@ACuriousMind d'aww I wanted to see an attempt

I drew this once to illustrate how a horizontal lift can differ for two connections, though

@ACuriousMind Can you draw them naked?

what is everything there

@barrycarter The physics entities? Who says they're clothed to begin with?

4:46 PM

@ACuriousMind gasp!

what are the blue lines

@0celo7 The blue lines are the "horizontal spaces" at the blue points, the red dotted lines are the fibers

I knew what the read lines are

What would the vertical spaces be

Parallel to the fibers

Well, "tangent" to the fiber is the better word

Why are there two curves on each bundle

4:49 PM

They are two horizontal lifts of the curve $\gamma$ for different starting points

No reason why two specifically

Jeez, I've forgotten everything about connections from this approach

I like the Koszul one much better

This is supposed to be on a principal bundle. Probably should've tried to draw the bundle as a cylinder to show it's not supposed to be a vector bundle.

the fibers are $S^1\cong \mathrm{U}(1)$?

I don't think I ever decided what exactly the fibers here are. Right now they look like (some part of) $\mathbb{R}$

well you can have a vector bundle over $M=S^1$...

shrug I never need principal bundles

4:56 PM

Man fiber bundles are easy to draw man

user image

there

> - M. Visser (1996), *Lorentzian Wormholes*.

A fun book on wormholes. Look here for spacetimes with weird topologies, like handles and non-Hausdorff parts.

@Slereah Any else to add?

I haven't read it yet

Visser doesn't really have a lot about non-hausdorff spaces

It's mostly wormholes and CTCs

hmm

write the thing then

I think the only book to discuss somewhat non hausdorff spaces is like...

Hawking Israel?

write the thing then!

Or I'll remove it from the list

It's not really a GR book

4:59 PM

Well remove it if you want jeez

I'm not your mom

I should get some good topic specific GR books

Like black holes and cosmology

I lack a good cosmology book

I think I'm booked out for the moment, besides Petersen.

But I can't get that one.

Most GR books are the same anyway really

ain't no need to get that many

I think we have all of the GR books

Well all the big ones

yes

neither of us has MTW

5:04 PM

Though there are probably a bunch with proofs we need

SOMEWHERE

I could steal it from one of my prof's offices

Isn't MTW like a million bucks

sure is

Have you ever looked at your copy of HE

On the back cover of mine, it's written that the book costs like $10

@Slereah no

I have never looked at it

I have the book and just read a PDF

5:06 PM

I kinda wonder what would have happened to Hawking if he never got Lou Garig's disease

Pretty sure he would have remained some obscure GR guy

About as well known as Penrose

Gehrig

gagarine

Valentin Tihomirov

5:36 PM

Is it right that Copenhagen school teaches that Heisenberg uncertainties are fundamental but imply non-locality, the dubious thing challenged by hidden parameters in EPR whereas Bell's inequality chooses between the two interpretations?

I wonder how is non-locality implied? Is it what Bell is involved to explain?

5:56 PM

@Slereah 511 KeV just appeared in a talk I'm sitting in.

on the dirac belt?

I was the only one who knew the energy that photons are at after an annihilation process :D

@Slereah no, this is a real talk by an actual physicist.

MonaLisaOverdrive

Hello

@MonaLisaOverdrive Greetings

MonaLisaOverdrive

I'm back with the same question, i.e. is there a "layman's experience" chat in the Physics site.

For those that have a strong interest in, but not-so-strong knowledge of, physics

6:06 PM

I think you could chat here, but I'll let others answer more formally.

MonaLisaOverdrive

Sounds good

FYI, by "not-so-strong knowledge of physics" I mean "struggling through Quantum Mechanics: The Theoretical Minimum"

@MonaLisaOverdrive We don't have any continuous chat except for this one

MonaLisaOverdrive

Ah

Would this be someplace that I could ask...or refer to...what will probably look like a very basic QM question?

Probably

ha ha

MonaLisaOverdrive

Heh

6:11 PM

there are no restrictions on the topics in this room (as long as you respect the general Be Nice policy of SE)

MonaLisaOverdrive

Okay

Well...here goes.

For background reference, see this question: physics.stackexchange.com/questions/244496/…

@MonaLisaOverdrive Yep! (just for the record)

MonaLisaOverdrive

I'm still wrapping my head around the two answers and various comments, but I had a follow-up question that I posted in the comments: When speaking of the representations of |l⟩|l⟩ and |r⟩|r⟩ in terms of |u⟩|u⟩ and |d⟩|d⟩, is it still related to generic state |A⟩|A⟩?

Yikes. That should've been "representations of $|l\rangle$ and $|r\rangle$ in terms of $|u\rangle$ and $|d\rangle$"

...and apparently MathJax does not work in chat...

It does, you just need a separate script. Look for the link at the top right.

MonaLisaOverdrive

...top right...in the sidebar?

6:17 PM

@vzn Thanks that was a great read!

I saw that paper on arxiv a while ago but it was impossible to understand

I thought it was too confusingly written

> General chat for Physics Stack Exchange (physics.stackexchange.com). For MathJax use meta.math.stackexchange.com/a/3297#3297

Sabine's blog post about it made it way clearer what the heck Dvali et al were doing

@MonaLisaOverdrive here's the link. It's the part that @Ixrec quoted, way up top above the sidebar, just under where it says The h Bar.

MonaLisaOverdrive

Very, very nice

I'm guessing the bookmarks need to be clicked on the same tab from which you are running chat…

aye

MonaLisaOverdrive

6:25 PM

Hmm. The mobile bookmark too?

Apparently yes.

Now I need to figure out how to execute the mobile bookmark while in mobile chat

And we're in business.

MathJax rendered

If anyone has any answers to my previous question, which I can't quite figure out how to re-reference, fire away

I actually didn't understand that one. What do you mean by "being related to the generic state"?

MonaLisaOverdrive

I mean, is Susskind still referring to that generic state $|A\rangle$ that he set up initially

$\lvert r \rangle$ and $\lvert l \rangle$ are specific states. They have $\alpha_u = \frac{1}{\sqrt{2}}$ and $\alpha_d = \frac{1}{\sqrt{2}}$ (resp. $\alpha_d = -\frac{1}{\sqrt{2}}$ for $\lvert l\rangle$).

@FenderLesPaul sure, find it an interesting/ leading/ emerging area intersecting CS/ physics. are you a grad student? working on research, in the area?

6:45 PM

@vzn I'll be a grad student in a few months (August); I'm currently an undergrad

and yes the intersection between quantum information/entanglement entropy and quantum gravity (specifically black holes) is what I'll be working on

@dmckee Currently in a talk by a GSI physicist. Powerpoint.

MonaLisaOverdrive

@ACuriousMind okay.

So when Susskind set up the original equation

@FenderLesPaul do you know this professor?
N. David Mermin (born March 30, 1935, in New Haven, Connecticut, USA) is Horace White Professor of Physics Emeritus at Cornell University

MonaLisaOverdrive

$|A\rangle = \alpha_u |u\rangle + \alpha_d | d\rangle$

and then went on to define $|r\rangle$ in terms of $|u\rangle$ and $|d\rangle$...

$|A\rangle$ is a specific state separate from $|r\rangle$ and $|l\rangle$?

7:03 PM

No, $\lvert A \rangle$ is not any specific state. It's what any state of that system looks like. Every state of that system corresponds to a specific choice of values for $\alpha_u,\alpha_d$.

MonaLisaOverdrive

o...k...

Could you call $|r\rangle$ and $|l\rangle$ subsets of $|A\rangle$?

No, none of those three is a set

MonaLisaOverdrive

Right...

Are they a part of that generic state $|A\rangle$?

...similar to how E,F,G are part of the alphabet?

No, I wouldn't say it like that at all. Consider this analogy: If I tell you that a non-prime number is any number $n$ that can be written as $n = pq$ where p and q are numbers greater than 1, would you say that 4 and 15 are "parts" of $n$?

@FenderLesPaul I talked to the librarian

7:08 PM

@skillpatrol yeah I know him

She said that she can't even buy the book yet

MonaLisaOverdrive

@ACuriousMind: no I would not

Wait...

No, I wouldn't. They fulfill the rule you've established

@FenderLesPaul he wrote this article some 12 years ago and I was wondering how the search turned out?

What search?

Did Feynman actually say "shut up and calculate"?

7:15 PM

@skillpatrol I think it was one of those out-of-context cases.

Probably

MonaLisaOverdrive

I may be overthinking this

If you're interested @FenderLesPaul

1

Q: Who was the first to say "Shut up and calculate!"?

The best thing I could find on the internet was this apparently forgotten aricle from 12 years ago.

reference-request philosophy-of-science theoretical-physics quantum-mechanics

Yeah idk what became of the search

I always thought it was a stupid and close-minded motto anyways

Yup

Tough luv back in the olden days

7:21 PM

0

Q: How to access badges in phone app?

I can't find how to see the badge counts etc on the phone app. is it not an implemented feature or am I blind?

two hour talk...I'm dying of hunger

Get something to eat, skeletons can't think :P

Trying so hard not to scream PROOF every 5 seconds

it's hard

lol

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