The h Bar - 2015-09-08 (page 3 of 6)

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4:00 PM

@TerryBollinger never seen that, that's interesting

OK so, chat session time!

Who's here?

Terry Bollinger

@ACuriousMind I'm not convinced that the formalisms most commonly used are necessarily optimal. That's the computer side of me: I know how arbitrarily the human mind can make subtle presentation decisions even within precise and fully correct mathematical formulations. Intuition and formalism don't always match.

@TerryBollinger The whole teaching of the Lorentz group and spinors is so messed up because physicists think they can do group theory when they...can't :P

@JohnDuffield I have question for you

Terry Bollinger

@ACuriousMind Dirac! His intuition on that... hard won over I think a couple of winter months, wasn't it? "Acrobatic" indeed. Everything since, e.g. sadly twistors, I think has gotten off the mark a bit from that amazing start...

Aw, damn, I have to go, dinner's ready.

4:02 PM

I would be interested on the panel's opinion of ...

7

Q: Geometric meaning of the black hole horizon

It is widely accepted that the singularity of the Schwarzschild metric at the event horizon is purely an artifact of the coordinates and no physical singularity exists at the horizon. However, as Karlhede had shown in 1982, the Karlhede's scalar $R^{ijkl;m}R_{ijkl;m}$ (the square of the covariant...

reference-request dg.differential-geometry general-relativity

The first time I've answered anything on Math Overflow :-)

Terry Bollinger

@ACuriousMind Group theory can hide as well as elucidate. I'm kind of glad Mendelev never had it as a tool.

@TerryBollinger : think of spinors like this.

@Secret : shoot.

Go here
http://chat.stackexchange.com/rooms/28896/room-for-secret-and-john-duffield

@yuggib : I don't have strong opinions. I have Einstein and the evidence. Evidence is something to do with something called "science".

Terry Bollinger

@JohnRennie That is a very interesting answer you gave...

4:05 PM

@JohnRennie first thought: that question should totally be ours ;-) nah, but seriously it would be well on topic here and a very good question for this site

@ACuriousMind It's like... 6?!

Your answer seems to make sense, although I haven't worked through it myself to verify... I'll take your word for it

@JohnRennie A bold move!

Terry Bollinger

@JohnDuffield I have this inherent disposition to think of as living in a spin 1 space, whatever that means (and Lisi Garrett seems to have such an idea). The 1/2 spin particles just don't quite fit into it, and "gear down" to half speed in order to fit. Nothing but images, but they stick in my mind and do seem to help a bit.

@Danu I have no rep on MO to lose :-)

Terry Bollinger

4:07 PM

@DavidZ I'm certainly still working through it... you think deep @JohnRennie.

@JohnRennie I think your answer doesn't address the main question: What's the geometric meaning of that scalar?

oh, yes, that was my other thought

@Danu true. The main point of my answer was that there is no special physical meaning to its sign change at the event horizon.

You (@JohnRennie) do kind of dodge the question, but in a useful way because the scalar isn't actually important to resolving the apparent inconsistency

@TerryBollinger : I have more mundane views, wherein an electron is just an electromagnetic wave displacing its own path into a closed twisting turning spin ½ path. In atomic orbitals "electrons exist as standing waves". Standing wave, standing field.

4:09 PM

I'm not sure whether the OP is asking for physical intuition

Terry Bollinger

@JohnRennie Do you accept that charged particles cannot cross the event horizon?

@TerryBollinger No.

@JohnDuffield "more mundane" lol

@TerryBollinger Why would anyone accept that?

@JohnDuffield The dictionary says otherwise. However, I am (now) a mathematician so I am hardly concerned with evidence anymore.

Terry Bollinger

@JohnRennie Is there an explicit error in the logic of those who propose it?

4:10 PM

@TerryBollinger They are being coordinatist

i.e. believing that their coordinate system is somehow special

@Danu I'd expect that on a mathematical site, one is normally looking for more mathematical intuition. But sometimes the answer to a question is simply a physical one.

Terry Bollinger

@Danu Sorry, lost the "that" ref!

@DavidZ If you follow the Arxiv link the paper is about contradicting the argument that the event horizon is locally nothing special.

Terry Bollinger

@JohnRennie Yet the EH does seem to have current flows? At least above it?...

That was the spirit in which I answered

4:12 PM

yeah, I gathered that - I didn't read the paper of course, but the point was suggested by the context

@TerryBollinger Eh?

@DavidZ I think that "one of us" could give a good answer to the part that asks about intuition about black hole horizons, but I'm not sure about that scalar

Aha, I see one of you has upvoted me on MO. Thank you kind sir whoever you are :-)

@JohnRennie If you meant "what does EH mean?" then the answer is "event horizon"

...but you probably just meant "Eh?"

@Danu Eh? :-)

Terry Bollinger

4:14 PM

@Danu My that was a bad acronym choice, eh?

@TerryBollinger Eh?

Terry Bollinger

I did mean event horizon, some of the Kip Thorne discussion e.g.

Eh??

Giant humongous black hole, almost flat... is that event horizon sharper than a razor or fuzzed out and ambiguous? I feel stupid even asking, surely its sharp, yet...

@TerryBollinger Nobody knooooows

@TerryBollinger Classically it's sharp

@TerryBollinger in classical GR it's a 2D surface and has to be flat. But with quantum effects, maybe things get weird?

Terry Bollinger

4:17 PM

@Danu Which is the history I get from books like Kip Thorne's: A sort of meander that even now seems to be uncomfortably non-conclusive...

@TerryBollinger Though be careful about horizons. Remember that a true horizon is defined only by consdiering the entire spacetime out to infinity. In the real universe true horizons probably don't exist.

Terry Bollinger

@DavidZ Really it's the distance and flatness that bothers me greatly, not the quantum. It just seems like a sharpness that doesn't have the right physical parameters of mass and energy around it to justify such a precise position. Not so small ones, where curvature implies sharpness.

I'm not so sure what you mean by that...

Terry Bollinger

@JohnRennie That's an interesting assertion "they may not exist..."

@DavidZ Neither am I, so we're good...

:-P

4:20 PM

108

Q: Why does Stephen Hawking say black holes don't exist?

Recently, I read in the journal Nature that Stephen Hawking wrote a paper claiming that black holes do not exist. How is this possible? Please explain it to me because I didn't understand what he said. References: Article in Nature News: Stephen Hawking: 'There are no black holes' (Zeeya Mera...

black-holes quantum-gravity hawking-radiation event-horizon

Terry Bollinger

@JohnRennie So, if a photon launches perfectly vertically from... well, whatever is in a black hole... is it stopped, or does it eventually exit? That is, does the apparent closure of the horizon never quite close at the top? Call it classical Hawking radiation, no quantum needed...

13

Q: Why is a black hole black?

In general relativity (ignoring Hawking radiation), why is a black hole black? Why nothing, not even light, can escape from inside a black hole? To make the question simpler, say, why is a Schwarzschild black hole black?

general-relativity gravity black-holes speed-of-light

Terry Bollinger

@Danu (Still chillin' to that great sound track...)

The velocity of an outgoing light ray is precisely zero at the event horizon

Coordinate velocity

Terry Bollinger

4:24 PM

Hi @0celo7

@DavidZ Oops, yes :-) Now it's me being coordinatist!

Terry Bollinger

@JohnRennie So, a simple straightforward "no"... though of course that seems to be the very point that Hawking has backtracked from. No matter how I read his sort-of paper, I keep coming back to exactly that image: The apparent horizon never quite closes up, and remains slightly open at perfect vertical. I can't come up with any other geometry that matches what Hawking seems to be asserting.

hm, I may have to read this

@TerryBollinger I love it :D

@TerryBollinger I'm not entirely clear what you're asking, but light can escape from an apparent horizon if you wait long enough.

4:27 PM

@TerryBollinger hello

That's why it only appears to be a horizon

Terry Bollinger

@JohnRennie Classically or quantum-ly?

@TerryBollinger Classically

@ACuriousMind uh every string theory text tells you that

Apparent horizon

In general relativity, an apparent horizon is a surface that is the boundary between light rays that are directed outwards and moving outwards, and those directed outward but moving inward. Apparent horizons are not invariant properties of a spacetime. They are observer-dependent, and in particular they are distinct from absolute horizons. See, however, the articles on ergosphere, Cauchy horizon, the Reissner–Nordström solution, photon sphere, Killing horizon and naked singularity; the notion of a horizon in general relativity is subtle, and depends on fine distinctions. == Definition == The notion...

Terry Bollinger

4:29 PM

@JohnRennie But... isn't that the new Hawking position? I thought the classic was that you can't? Hooboy now I'm confused...

Whatever Hawking is doing is of little relevance to basic black hole physics

Terry Bollinger

@JohnRennie ah, you are distinguishing the apparent horizon case...

#rekt

the case of a classical black hole is well studied

Hawking seemed to change his position quite frequently,

From what I am aware, he goes from Firewalls to apparent horizons and now, saying that information can escape from it

I have not read much in detail, thus this is all I can say

Terry Bollinger

4:31 PM

@Slereah No denying that, it's just not all that study seems to land in the same place...

@Secret Dunno if 30 years is that quick of a switch... :)

@Secret Have you watched Perry's talk on Youtube?

that's because Hawking deals with hypothetical things

He can change position because there isn't much to contradict it

@JohnRennie what's with it?

I don't think Einstein and the evidence say much about black hole information

I thus have no position.

@Secret It covers the ideas about extra information encoded on the horizon, and it does so in a really clear way

Terry Bollinger

4:33 PM

Must leave a bit early today. Fun discussion, thanks, see everyone late!

Just paid for muh summer school

@JohnRennie this one? youtube.com/watch?v=p1k3XKfl0CQ

summer school?

bit early/late for that, no?

@Secret Yes

@0celo7 It starts in 11 days

They weren't very fussy about paying in time :P

4:35 PM

ok I see, should have a watch

European schools are crazy

why are you doing summer school in the Fall

after the civilized world has started the fall semester

German holidays = end of July -- mid-October

@Danu ...so?

I eat at 7 or sometimes 7:50

7:50 is awesome because the dining hall closes behind you

you get to serve yourself

none of this "reasonable portion" bullshit

@0celo7 The content of string theory texts is not what I would call "well-known" :P

4:37 PM

@ACuriousMind indeed. I had to remind Lang what QCD is

Eating after 8 is best eating

so he probably does not know any strings

I usually eat far later, but my parents are early eaters, so I have to adjust while I'm home

What does he work on?

@ACuriousMind Ah you're at your parents'

I'm already back home

(before going to Greece :D)

@Danu I'm not exactly sure...

I'm attending the research meeting today though

4:38 PM

@Danu Yep, came here last week, gonna stay two more or so before returning to HD

@ACuriousMind Coolbeans

HD?

HeiDelberg?

poor acronym

But it's HD

@0celo7 Heidelberg (HD because it's HD on the licence plates)

I guess HB could be Heilbronn

that might have been a language thing

his PhD work was on heavy ions

no way he doesn't know what QCD is

@ACuriousMind what's the German word for antiderivative

4:41 PM

@Danu : yes, more mundane. My view of physics is that there is no magic, no mysticism, no spookiness, and nothing that "isn't classical and surpasseth all human understanding".

Stammfunktion?

@0celo7 Stammfunktion or unbestimmtes Integral.

@yuggib : unfortunately there are physicists who are hardly concerned with evidence anymore.

@JohnDuffield Screw the evidence.

Evidence is for chumps who can't do math.

If the theory is correct, evidence is irrelevant~~

@0celo7 Why does this word "antiderivative" even exist :P

4:43 PM

@0celo7 I must learn some maths one day ...

@JohnRennie Start with algebra: It's a lot of fun

(and it relates reasonably directly to physics)

especially group theory

@Danu I thought for a moment you meant C* algebra, Phew.

I can solve quadratic equations - is that a good start?

Functional analysis and other quantum-related math is blurghhhh

@JohnRennie Maybe

@Danu Do you mean "group theory" (representation theory) or group theory (Sylow's theorems, permutations groups,...)

@ACuriousMind The latter

(but not in great depth)

4:45 PM

How does the latter relate to physics?

@JohnRennie You can go directly with universal algebra en.wikipedia.org/wiki/Universal_algebra

@TerryBollinger : I am not a fan of Kip Thorne.

@ACuriousMind Permutation groups

@yuggib Now why'd you do that? :P

@Danu Well, yes, they occur, but what actual group theory can you use?

@yuggib the trouble is that now I'm old and feeble I find it hard to get interested in maths unless I can see an immediate physical application.

4:46 PM

@yuggib I approve.

@Danu You suggested algebra.... :-P

Maths gets hard when you get over 50 you know :-)

@JohnRennie Another possibility is something like this:

amazon.de/Foundations-Classical-Mechanics-Fundamental-Theories/…

Classical mechanics using differential forms and such from the beginning

@TerryBollinger : the speed of the upward photon is zero. That's why the light doesn't get out. Shine a laser beam straight up from the surface of the Earth, and the speed of the upward photon increases as it ascends.

although it may be a bit too boring physics-wise

4:47 PM

Actually I've just bought myself a copy of QFT for the gifted amateur.

@JohnRennie Not harder than physics, just different

I've had the PDF for a while, but PDFs aren't a good way to read textbooks.

Other option: em.groups.et.byu.net/pdfs/publications/formsj.pdf

Very interesting

@JohnRennie It's the only way I do it :D

@JohnRennie You learnt GR, I don't think most maths is really much harder than that.

amazon.co.uk/gp/product/019969933X

4:48 PM

@0celo7 : screw the evidence? Oh dear.

@yuggib Very hard if you're in a physicist frame of mind

@JohnRennie I feel this may be a little too low-level for you

@JohnDuffield I have clarified my question with context, hopefully you can help me on that

@ACuriousMind you have to be kidding. The maths in GR is pretty straightforward.

QFT is muuuuuuuuch harder

It really isn't :P

Just more vague and ill-defined

GR is so crisp

QFT is filled with often-repeated "well-known" things that nobody really explains

@JohnRennie Not really kidding. I have struggled with abstract algebra less than I did with the handwaving in QFT and the "almost-math" in GR

4:50 PM

GR is, and yes I feel pretentious saying this, beautiful!

Then again, people keep telling me I'm a mathematician :P

@JohnDuffield chat.stackexchange.com/rooms/28896/…

@ACuriousMind I don't think your experiences generalize very well to most of us (no offense, it's a compliment!)

QFT is ill-defined because few people in math phys work on it nowadays

Well we'll see. I had an Amazon gift card hanging around begging to be spent and I thought the QFT book would be an interesting read.

4:51 PM

@ACuriousMind Ah, you noticed that too :P

and indeed it is difficult

@yuggib I don't think so. I think it's just much harder to formalize

I feel we haven't found the right formalism yet

Soem of my peers said I approach problems with a mindset like a mathematician, but only havig the tools of an experimentalist avaiable

>only havig the tools of an experimentalist avaiable - no problem, there's a lot you can do with a hammer

@yuggib If you don't like it feel free to write a well defined QFT :p

4:53 PM

@Slereah :-D few people is still trying

and there is still hope to succeed

approaching physics with the mind of a mathematician sounds horrible

@JohnRennie Indeed. One really feels like an idiot when Bajoran starts going on about p-adic cohomological sheaf ring bundles on Kahler groups.

unless you're Ed Witten and have a super powerful mind

@yuggib I think it's time for a breakthrough in mathematics first, to find a new type of idea that is better suited to QFT

@0celo7 Aliens try to teach you sheaves?

4:55 PM

@yuggib isn't that what Arkani-Hamed's work on the Amplituhedron is about?

yeah, my professors says it is very easy to lose track of reality if always think of doign problems by just solvign the general case and baingnign through algebra

banging*

@JohnRennie that's my favorite QFT book!

@yuggib There's plenty!

@JohnRennie That is one very interesting thing that I'd like to understand

@JohnDuffield I'm done arguing.

4:55 PM

it's just that it's not that appealing to most QFT people

@Danu I think it is maybe needed some new stuff

I guess in a year or so I'll have enough diffgeo knowledge to start thinking about it

Screw the evidence, I am not bound by such things.

or maybe we are just very close

my friends often solve problems A LOT EFFICIENT than me because they know how to take shortcuts while I am very bad a things I cannto derive formo first principles

4:56 PM

and a small leap will be sufficient

@JohnRennie I like how in the beginning he says he almost named it 50 Shades of QFT

@0celo7 But if the evidence is not enough for you, @0celo7, what about Einstein

Einstein and the Evidence

hence my physics intuition sucks

(that is my band's name)

@secret it's just practice

4:56 PM

@Danu What?

stop thinking like a mathematician

and you'll be solving physics problems much more efficiently

Why do all of you want to well-define QFT if QFT can't quantize GR? String theory is obviously the future!

@Slereah Einstein was a punk who did not know how to do homological perturbation theory.

@StanShunpike I glad it gets at least one recommendation :-)

Anyone who can't do that is a punk.

4:57 PM

@ACuriousMind Top kek

@0celo7 : if you're saying screw the evidence, you aren't done arguing, you're done with physics. But hey, that's up to you.

@JohnRennie I haven't even read it, don't take my criticism too seriously ;)

Great. 4chan has invaded.

@JohnDuffield I'm done with you.

It is hopeful that one day, if we have a good quantum gravity theory, we won't have to deal with fucking renormalization

I can still do physics all day long.

4:58 PM

@Secret taking shortcuts i mostly a matter of experience. Unless you're Ed Witten most of us spot shortcuts because we've seen something like it before. It's just a matter of experience.

2

@Slereah I had a dream

In the final theory

W-theory

the path integral is exactly soluble

no perturbation theory

it would be nice

As Mukhanov said many-a-time in his lectures:

You know what would be shit tho

@Slereah Shit?

4:59 PM

If the problem of solving it turned out to be uncomputable

Everything reduces to the harmonic oscillator, and the harmonic oscillator reduces to the free particle. So what is the problem?

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