The h Bar - 2017-06-22 (page 1 of 4)

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12:01 AM

@ACuriousMind Is my analysis there at all healthy? For the situation there where you start with state $|0 \rangle $ and make evolution $H = \hbar a \sigma_y$ for repeated intervals of $\Delta t$.

12:57 AM

@JohnJack correct

@JohnJack incorrect

The probabilities of transferring and not transferring had better sum to 1.

1 hour later…

2:16 AM

@0celouvskyopoulo7 I can help if you want.

1 hour later…

0celouvskyopoulo7

3:44 AM

@Slereah "this example, which is due to Sierpinski and rests on the continuum hypothesis..."

seems like a bad example

4:43 AM

The usual way to deal with update woes is to wipe the update directory:
1. open a command prompt (Windows-R and type cmd)
2. cd \windows
3. net stop "windows update"
4. rd /s /q SoftwareDistribution
5. net start "windows update"

C:\Windows\SoftwareDistribution is the directory where Windows keeps all the update files and deleting it causes the update to run as if Windows had never been updated. However while the update service is running that directory is locked and can't be deleted. Hence you stop the service then delete the directory.

I don't know about Win8, but on Win7 the first update after you've deleted SoftwareDistribution can take an extraordinarily long time. I've seen it take 30 hours! And as you say it uses 100% CPU while it's doing it.

Just start th update and put the laptop in a corner somewhere to get on with it.

But I must echo the comments others have made. Either downgrade to Win7 or upgrade to Win10. In both cases wipe the laptop disk first so it's a clean install not an install over the current OS.

Even though the free upgrade to Win10 has allegedly stopped, I've found that a Win7 key will still work to activate Win10. Well, it works with a Win7 Pro key on my Dells.

BTW if you have the firewall on you can't be infected by the ransomware virus because the virus has to be able to connect to your laptop to infect it and the firrewall will prevent that.

The chaos the virus caused in large companies was because in large companies the firewalls are usually turned off to facilitate remote management of the PCs.

5:08 AM

Finally: you can download a Win10 install image from your laptop. MS have a tool that will build you a customised Win10 installer based on what you currently have installed. You can either burn the image to a DVD or my preference is to use a USB key.

Use this web page and use the Create Windows 10 installation media link.

1 hour later…

6:30 AM

@0celouvskyopoulo7 which example

7:02 AM

user image

Is this appropriate for a bibliography

7:25 AM

@JohnRennie What is the basic idea

16

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@Slereah That any function $F(t,\mathbf r)$ can be Fourier analysed to express it as a sum of plane waves. Quantise those plane waves to turn them into Fock states and bingo you have an expression for the quantum field.

The rest is just details :-)

@JohnRennie That's not true at all!

Isn't it?

You can't decompose non-linear PDEs into plane wave solutions

Well you can, but they're pretty meaningless

7:29 AM

@Slereah I didn't say you could

I said any function $F(t,\mathbf r)$ can be Fourier analysed to express it as a sum of plane waves

That's true (if it's $L^2$ anyway) but that's far from the basis of QFT

Actually that is also wrong if you're not in flat space

It feels like you're being picky now. QFT generally assumes the Minkowski metric.

I am :p

Well you know

That idea is fine for free fields QFT in flat space

But that's usually not the hard part of QFT

@Slereah Bing!

the old infinite oscillator business

7:34 AM

The point is that as someone unwilling to put in the hard work to actually learn QFT it's very hard to get even a basic idea of how it works.

I'm sure it seems trivially obvious to you how the scalar free field theory works, but few books take the trouble to give the simple explanation I outlined above.

QFT books generally dive straight into the maths.

It tends to be hard because often people jump to QFT without having done all the proper prep work before

Like when I had my QFT class, we never did classical field theory before

Obviously if you're going to learn QFT properly that's the best approach, but for the spectator it isn't very helpful.

Well, we did EM, but we never did like the Poisson bracket of field theories

One thing I find lacking in a lot of QFT books is examples

I'd like to see some real examples in QFT books

Hydrogen atom, field in a box, coherent state, etc

usually they only do single excitations which aren't even states

Oh also thermal states would be nice

But aren't those really hard to do with QFT? My understanding is that handling bound states like a hydrogen atom in QFT is really hard. In practice you start with conventional QM and treat QFT effects as perturbations.

Hydrogen atom if you consider a classical potential isn't too different from QM

Field in a box is slightly weirder than the QM one but it's not too hard mathematically

Coherent states are basically like QM coherent states

7:59 AM

@JohnRennie I thought you didn't knwo QFT

@Kenshin I don't. I know various bits and pieces about QFT but my knowledge is that of an interested bystander not a practioner.

@JohnRennie do you ever intend to master QFT?

No

why not

I simply don't have the time (or if I'm honest the motivation)

To learn QFT properly would require me to learn a great deal of mathematics first. Since I was an experimental scientist rather than a theoretician I never learned all the maths that theoreticians take for granted.

8:02 AM

@JohnRennie do you think more people know QFT than required for progress on the open problems in QFT or is there a demand for people who fully understand it?

I'd say a lot of physicists don't understand QFT

Or nobody, possibly

Yeah that's my impression too

QFT is kind of a weird Frankenstein monster

I think the QFT field is in a generally healthy state.

QFT is fine for predictions, yes

8:04 AM

@JohnRennie what do you mean healthy state? As in there are enough people who know it to make the required progress?

@JohnRennie do you believe QFT will be unified with GR soon?

It's found applications all over the place in physics and even in apparently unrelated areas like finance (or so I'm told). As a tool for doing calculations it works very well.

But there are fundamental problems with QFT that we still have no idea how to approach.

We already have unifications of QFT and GR

we just don't know if they're correct

@JohnRennie so we are waiting on a revolutionary idea?

the real question is when we will have some experimental evidence

Or are we talking the problems bit by bit

8:06 AM

we already have plenty of revolutionary ideas, too

For example we don't know how to write down the states for an interacting quantum field. The best we can do is use perturbation theory to describe their properties. If you think about this seems an amazing state of affairs.

@JohnRennie is there reason to think we would ever be able to tho?

or is it just the nature of the universe that we must use perturbative

just like some pde can't be solved analytically

You get people like Nima Arkani-Hamed coming up with all sorts of ideas for how we migh approach fundamental questions, but so far none have come to anything.

Sounds like we're waiting for the next Einstein then

I'll volunteer to learn QFT and help everyone out

Maybe ...

8:08 AM

How far off are we from downloading material directly to the mind

QFT is probably not correct anyway

Pretty much all quantum gravity theories modify QFT in some important way

@Slereah then why is it so accurate? coincidence?

QFT works so well (as an effective theory at least) that no-one things it's basically wrong, but the question is have we taken the best route to formulate it? And right now no-one knows.

Same reason continuum mechanics is correct

it's an approximation

But it breaks down at some scale

I think quantum physics is horribly formulated

8:10 AM

Most quantum gravity either assumes that spacetime is discrete or that interactions don't happen on a vertex

@Slereah hang on a sec. QFT is a mathematical model. As a representation of reality most of us think it's an approximation, but as a mathematical model there's no reason to think it has fundamental flaws.

@JohnRennie as a mathematical model it is a bit questionable

@Slereah our current formulation of it seems to have fundamental difficulties ...

QFT mathematically runs on band aids to cover big flaws

really

is QFT patchwork?

8:11 AM

It is

I think the most scandalous one is that $\phi^4$ theory is actually trivial

@Kenshin The trouble is that while the basic ideas are nice the model is mathematically completely intractable for interacting fields. Hence the patchwork approach.

Right

so the patchwork isn't the theory tho

well there are several theories of QFT

@Kenshin I guess that's a matter of terminology ...

@JohnRennie well if I have a pde, I would consider the pde the theory, not the numerical methods used to solve it

so by analogy, the patch work of QFT is analgous to the numerical methods used to solve a pde right?

8:14 AM

It's a it more complicated than that.

A free field can be Fourier analysed to get its states - the Fock states.

But an interacting field cannot be Fourier analysed in the same way to get its states. Just as any non-linear system can't be (usefully) decomposed into a sum of Fourier components.

there are ways to get data out of an interacting theory without Fourier transforms

Not all is Fourier in QFT

@Slereah but we're back to the patchwork approach

Initially people believed in many particles

Now people believe particles are excitations of a single field

I believe fields are excitations of a single entity

sot here is no real "interaction"

this is how to solve qft

@JohnRennie There are some exact solutions for interacting fields

@Kenshin: unless you can state that as a mathematical model it us just useless speculation.

8:18 AM

As well as some exact results

@Kenshin: that's the problem with the crackpot end of the physics spectrum. Anyone can come up with crazy ideas, but unless you can write them as a mathematical model those ideas are just science fiction not physics.

Except that now we're back on particles

Except they're strings now

@JohnRennie I agree the idea isn't science

but intuitively I don't think the world has interacting bits

the world just is

@Slereah are there any examples of this that would be comprehensible to the interested bystander?

@JohnRennie The simplest example is the Sine Gordon model

8:20 AM

So I think as physicists learn more and more, eventually this conclusion will be reached on a scientific basis

You can look at the classical solutions

@JohnRennie have you heard elon musks' idea that we are a simulation? Do yo uagree

Here's a solution of the KdV soliton :

user image

you can decompose it as two solutions + an interaction term

user image

It's a non-linear theory that has a superposition principle of some sort

Though a nonlinear one

interesting

(though the decomposition isn't unique)

8:24 AM

@JohnRennie If I come up with a theory that makes all the same predictions QFT but has a simpler framework, is this "crackpot" science?

Do you know QFT and its predictions?

user228700

Hi, everyone :-)

No step 1 is to learn that dude

Hi @Kaumudi.H how is u

That might take a while

Na i'm a quick learner

user228700

8:25 AM

@Kenshin I is OK. You?

@Slereah do you have any tips hto

tho

@Kaumudi.H me gud

why only ok tho?

@Slereah Hmm, we were talking about fundamental insights into QFT. I'm not sure understanding Sine-Gordon helps with that.

user228700

@Kenshin It is OK to be only OK :-)

@Kaumudi.H Morning. Don't mind us, we're babbling about QFT but I think we're running out of useful things to say :-)

oh i see :)

8:27 AM

user image

@Kenshin plz solve

@Kaumudi.H Don't take life too seriously. You'll never get out of it alive anyway.

@JohnRennie speak for urself pls

For learning QFT I would recommend Schwarz

lol @Slereah wtf is that question

user228700

@JohnRennie Haha OK :-) Sorry I never returned after breakfast yesterday; I was caught up finishing 13 Reasons Why.

8:28 AM

@Slereah yeah but more like what are the prerequisites

user228700

And then I went to driving school!

@Kaumudi.H Coooooooooooooooool! :-) How did you get on?

user image

the prerequisits

@Slereah it does look like patchwork tho looking at that quesiton

@Slereah compare that equation to Einstein's GR equation, or his special relativity equation of E = mc^2

much more beautiful

user228700

@JohnRennie It was great! :-) I'm getting on poorly but I'm learning quickly.

8:30 AM

GR has a much narrower scope

user228700

...steering properly is proving to be trouble.

Anonymous

@Slereah Whoa. Best mathematical mess I've ever seen :'D.

@Kaumudi.H Most people I know found learning to drive really stressful and not a pleasant experience. You struggle through it, pass the test, and only then can you relax and start enjoying driving.

String theory is much cleaner but on the other hand it hides a lot of complexity

did you crash @Kaumudi.H

Anonymous

8:32 AM

Is there any "QFT for dummies" ? XD

user228700

@JohnRennie It was a little stressful but only a little. I enjoyed it!

I don't want QFT for dummies

I want to master it so that I can help u physicists out

user228700

@Kenshin Thankfully not yesterday :-P I go back in...Jesus Christ, I've to go back in less than 2 hours.

>Jesus Christ

Praying won't help with your driving lessons :-)

@Kenshin Then get Glimm and Jaffe

user228700

8:33 AM

@JohnRennie Really, I'm getting on just fine :-)

amazon.fr/Quantum-Physics-Functional-Integral-Point/dp/…

user228700

@Sir: Hi! :-) How's it going?

@Slereah but what about those pesky prerequisites

oh

that is the prereqauiisites?

It's not :p

Prerequisite is Hilbert spaces

Linear operators on Banach spaces

Distribution theory

yeah ?I understand hilbetert space

8:34 AM

Wavefront set theory

r all those really reqauired ?

Fiber bundles

Anonymous

@Kenshin Yesterday we had someone in h-bar who claimed he could solve all the unsolved problems in Physics without even learning Physics. :)

for QFT not string theory

If you want to understand modern QFT fully, yes

8:34 AM

@Blue it's possible

Group theory

@Kaumudi.H have you been out and braved the Chennai traffic yet?

Topology

Differential geometry

@Slereah but wouldn't you recommend me learning QFT first at least approximately before delving in that deep

yes, that's why I recommend Schwarz

user228700

8:35 AM

@JohnRennie Yeah, yeah, we drive through some of the busier streets of Chennai!

ok bro

for which you'll need QM and special relativity

Fourier analysis

and group theory

yeap I know all that

what about complex analysis?

also yes

anythign else?

8:36 AM

@Kaumudi.H now is a great time to learn. I missed out as a teenager and didn't start learning to drive until my mid twenties, and I think that's a lot harder.

I think I'll need to brush up on these topcis over the next week then get stuck in to Scwarz

user228700

@JohnRennie Ah, hmm, I see...

@Kaumudi.H: in the end I started riding a motorcycle and passed the test on that. Once I'd got used to riding a bike I went back to cars and managed to pass the car test as well.

@Kenshin that should be fine

user228700

Ah, nice. The class is only 15 days long but I will have 15 more days of practice (on my own) until the test.

8:38 AM

oh also distributions

@Kaumudi.H Well, good luck! :-)

user228700

Thanks! :-) How's everything on ur side?

@Kaumudi.H Quiet

The UK's run of hot weather seems to be coming to an end.

Hot for the UK that is - you'd probably think it was pleasantly cool :-)

user228700

@JohnRennie Ah...

user228700

In the 20s now, is it?

8:40 AM

21C in my living room right now, though it's still only 09:40 here and it will get hotter during the day.

user228700

Yeah, no, that's cold :-)

user228700

It's 36C here.

I like to cycle round Chester, in principle to go food shopping but mostly for the exercise, and the last few days when I got home the sweat was literally dropping off me. As in running down my face and dripping off the end of my nose. Lovely :-)

user228700

Wow. You'd melt, then, were u in Chennai...

I think the temperature topped out in the high 20s - possibly creeping into the low 30s at the hottest part of the day.

8:44 AM

39 C here

It's 300 here

(kelvins)

@Kaumudi.H I find you adapt to the temperatures, but it takes a while. So at the beginning of summer it seems very hot but by the end of the summer it just feels normal.

user228700

@JohnRennie Oh, wow, I see.

that's cool

Anonymous

@JohnRennie lol, buy one of these: amazon.com/Solaration-7001-White-Baseball-Golf/dp/B004C2AQJW/… :D

8:45 AM

Likewise winter. As winter starts it feels really cold, but by February sub-zero temperatures just feel pleasant.

@Blue :-)

user228700

@JohnRennie Right, right. Especially since you're also used to very low temperatures...

@SBM where is here?

user228700

@Blue Ohhh. Blue. Hi :-)

Anonymous

@Kaumudi.H Holla!

@Kaumudi.H that's the problem with the UK. We're geared for a cool climate so all houses have central heating but few have air conditioning.

user228700

8:47 AM

Ah, right...

user228700

We only have A.C :-)

@Kaumudi.H no central heating in Chennai??? What do you do when it falls below 20C? :-)

user228700

:-) Haha very funny.

There's like 5 different ways to do QFT, really

at least

Canonical quantization, path integral, axiomatic, deformation and what's his name action principle

Schwinger

Most QFT books do a weird mixture of 3 of those

Because some proofs are hard to do in a single formalism

@Blue "Blue, blue, 'lectric blue, that's the color of my room / where I will live"

I think I have a Bowie track repository on my head, which gets triggered by certain words lol

8:59 AM

oh and quantum wavefunctional

But that one's pretty rare

because it's a bitch to define

you're forgetting AQFT and CQFT

I put them as axiomatic :p

Although I guess there's a bunch of them

what about infinity-TQFT or whatever that Urs Schreiber is onto

Oh right

no love for nlab?

9:01 AM

that is not QFT

I have literally never seen TQFT used outside of nlab

only crazy people do TQFT

that is interesting (?) mathematics

Anonymous

@BalarkaSen Hehe...my study room walls are already filled with different shades of BLUE! I'm obsessed :D

And I guess there's that Hilbert bundle formalism

But only that one insane russian guy does it

Also Jacob Lurie

Anonymous

9:02 AM

I don't understand Bowie's songs though...:P

Anonymous

Don't know what he says XD

It's easy

you mean you don't get how he spells?

Ground control to Major Tom x2

it's british spelling

9:03 AM

Take your protein pills and put your helmet on

etc etc

For some weird reason I have heard a few foreign version of Space Oddity

I've heard a portugese and French version

I have seen the space guy sing it in outer space. But that's it.

Gérard Palaprat - Un Homme A Disparu Dans le Ciel (1971)

French version

Seu Jorge - Space Oddity

Portugese

jesus man

The 70's and 80's had a lot of french versions of pop songs

9:23 AM

fugg it's too hot to work

what is le temperature

28°C

Anonymous

289 K in my room

that's relatively ok on this side of the world

It is not here

9:25 AM

@blue 16C? huh?

Anonymous

@BalarkaSen AC :P

well i'd probably fall sick if i set my a/c to 16

i have it on 25 or something on average

Anonymous

I wish my AC had 0 degree celsius or something even below that

1) empty your fridge 2) fit yourself in the fridge 3) lock it from the outside

done

ACs are pretty rare in France

Anonymous

9:28 AM

My fridge is too small :P

Since we usually don't have too high temperatures

Damn global warming

@Blue haha

user image

I AM FREE

I AM FREE FROM THE SHACKLES OF PRIMERO DE BACHILLERATO

@JaimeGallego That's what they want you to think.

9:33 AM

1984 Apple's Macintosh Commercial (HD)

Whole Lotta Rosie is playing on the radio!!

@DanielSank The probabilities for each measurement do not sum to $1$. Initially being in state $|0\rangle$ and then undergoing evolution $H = a \hbar \sigma_y$, the probability of being in state $|1\rangle$ after the evolution $\Delta t$ is $(at)^2$, then subsequent measurements in short interval $\Delta t$ would I guess not give the system sufficient time to evolve

Hence we get approximate probability of $1$ of being in state $|1 \rangle$ for all subsequent measurements within time interval $\Delta t$ per measurement. Then the total probability after $N$ measurements of being in state $|1 \rangle$ would be $(at)^2(1)...(1) = (at)^2$. What's the prob with that reasoning?

I think this is the quantum zeno effect.

@JaimeGallego lol

Quite hilarious in hindsight

@DanielSank I think the Quantum Zeno effect would follow since if let $t \to 0$, then the probability of measuring $|1 \rangle$ after $N$ measurements, which I have evaluated above to be $(at)^2$, would be $0$.

10:12 AM

Is there an equivalent of the mass and energy definitions in GR for angular momentum?

Like a specific definition of it if you have rotational symmetry

Mithrandir24601

11:05 AM

@JohnJack yeah, I'm just going home for a week to take some time off - just handed in the essay I've spent way too much time on! :) so will spend the next week relaxing and reading papers so I can talk to potential PhD supervisors when I get back

11:19 AM

@ACuriousMind is link rot to Wikipedia actually a problem?

@EmilioPisanty Less total rot of the link but more that the article gets edited and a year later there's pretty different content at the other end, I guess

@Mithrandir24601 Sounds good. Going on vac in about a month as well, recharge a bit...Yeah PhD topic and supervisor choice seems like quite a scary decision, good luck with that.

0celouvskyopoulo7

@Slereah example?

11:34 AM

of what

@Mithrandir24601 pardon me, I was unaware of the "context." :)

@ACuriousMind yeah, well, it was marked "I just wrote this" so I'd say that comes with the territory

the real issue is that it just wasn't an answer, but water under the bridge and all that

@Kaumudi.H Good, you? :)

0celouvskyopoulo7

@Slereah you said something about an example

I have no clue what you mean

oh, the example due to Serpinski

@Slereah it's a pretty awful example

yeah, that

0celouvskyopoulo7

11:48 AM

It's a function $f:[0,1]^2\to [0,1]$ such that $f(x,\cdot)$ and $f(\cdot,x)$ are measurable, $x\mapsto f(x,\cdot)\in C([0,1],L^2[0,1])$, but $f$ itself is not measurable.

it does sound awful

0celouvskyopoulo7

And it requires CH

which seems rather strange

"Let's consider the Einstein equations in all of their glory:"

the GR tag is pretty cringy

People like to write those damn equations

the "glory" is found in the gory details

Why are people's goals so difficult to understand?

what do you mean?

11:59 AM

Like if you try and figure out why people do what they do

it's pretty hard to figure out what is an end, and what is a means to an end

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