The h Bar - 2017-07-28 (page 2 of 5)

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user228700

10:05 AM

Sheesh, that took such a long time to upload!

user228700

I'm sorry about the latency;y I was/am speaking with my grandpa.

user228700

10:20 AM

@JohnRennie Nope; will do that tomorrow, I guess. My parents are busy working today.

user228700

@JohnRennie Clearly, yep! :-)

Ah, the whole family has come to Kerala?

user228700

Yep, yep. Even my sisters!

That's not so bad then. At least everyone will be there to help you settle in. How long are they staying?

user228700

Yes! I'm incredibly grateful that they did. Although, they are required to attend the orientation program on Tuesday so they didn't have much of a choice haha :-)

10:23 AM

Your parents have to come to the orientation? That seems a bit odd.

What if some parents refused? Does that mean the student would be kicked out?

user228700

Yes, they do. That is the case in lots of colleges here, actually. God knows what they need orientation for but we'll find out!

I don't think my university ever even cared whether my parents knew I was studying here :P

Nor mine!

Though I believe parents of students under 18 have to sign something...

user228700

@JohnRennie Haha, no, I don't think so :-) I really dunno why they've requested them to be present.

user228700

10:26 AM

@ACuriousMind Lol :-)

Gran's cooking any better? :-)

user228700

@JohnR: They'll leave on Tuesday, after I've given them the illusion of having settled in at the hostel.

At least give the hostel a chance before you decide it's a living hell!

user228700

@JohnRennie Haha, I've only had dosas so far and they were certainly very tasty. I didn't get any sleep last night, because of which I have been sleeping through the day; I woke up from a 5-hour "nap" only an hour ago :-P

user228700

@JohnRennie :-P Right, of course.

10:31 AM

@Slereah why $\pi$?

also, how does that Misner thing look like, topologically?

user228700

@JohnR: I plan to drive around the scooter to farther distances for now, I have a learners' license YES!

Congrats.

@Kaumudi.H Aha! A friend of mine (who doesn't drive) bought a scooter recently and found it was amazing how much difference it made being able to go where he wanted without worrying about buses etc.

user228700

Now would be the ideal time to go but I've forgotten my application number, so I'm having to wait for someone at my driving school to text me the same.

@EmilioPisanty a cylinder

10:35 AM

@Slereah huh

weird

but I guess you wouldn't expect it to not be weird

or actually no, that's about the simplest you could get

it's fairly simple

But if you're editing that Wikipedia page, a Topology section would be rather welcome ;-)

Compared to, say

user image

It has the topology of a cylinder

I'm not sure I can expand much more on that

Unless I get into the whole covering space and bifurcate manifold part

But that's a whole other story

10:47 AM

FUN FACT : if you take the fundamental domain of Misner space and try to get its maximal extension it is not Hausdorff

user228700

11:00 AM

@JohnRennie Ooh, nice :-)

I suppose I felt much the same when I got my first motorbike.

Until then it had been cycling or the bus.

user228700

Ah :-) Well, as it happens, it is illegal for me to drive the scooter without pasting an L sign on the front and rear of the vehicle and being accompanied by a person who does hold a valid license. Now, I'm not sure if they do any checking around these parts but :-/

being accompanied by a person who does hold a valid license - eh?

Does the person with a licence have to ride pillion?

user228700

Yep.

user228700

I only hold a learners' license, you see.

11:04 AM

In the UK learner motorcyclists aren't allowed to take pillion passengers.

How long until you get a regular license?

So when you have a provisional licence you have to ride alone.

user228700

Oh, that's nice :-( I might risk it and go anyway. I'll update you later!

Let us know if you need bail

user228700

(I'm terribly sorry about the latency, really: my internet connection is quite poor here)

user228700

11:08 AM

@JohnRennie Let's hope I don't!

user228700

@skullpatrol At least 2 months.

It wouldn't be the best start to your university career :-)

user228700

Sheesh, certainly not!

Put the learners' license thing on the back burner until you get comfortable in school would be my advice :-)

Prioritize your time and efforts.

@JohnRennie If you aren't caught by the police doing something you have trouble explaining at least once, you're doing university wrong ;)

11:14 AM

@ACuriousMind I got fined £25 for cycling a pushbike after dark with no lights!

Heh, that's pretty common here, too

I know someone who found out the hard way that they will suspend your driver's licence for cycling when absolutely wasted

(I don't endorse drunk cycling, for the record)

I also got stopped by a police car while I was walking down Cambridge high street doing a slalom in between the cat's eyes. I might have had a drink or two and it seemed like a good idea at the time. But all the policeman did was give me a lift home :-)

I suspect the Cambridge are well used to inebriated students and take a lenient view as long as they aren't doing any harm :-)

Is being drunk in public an offense in the UK?

Police here typically don't care at all about drunk people unless they bother someone

Drunk and Incapable

"Drunk and Incapable" is the debut single by from Jamaican dancehall artist Krishane featuring vocals from British singer Melissa Steel. It was released as a digital download on 12 October 2014 in the United Kingdom. The song has peaked to number 27 on the UK Singles Chart. == Legal definition (UK law) == The official definition of being drunk and incapable is when you are so drunk you are unable to stand or walk or unaware of what you are doing or unable to understand what is said to you. If a person is arrested for the offence, and they have no previous record and are not disorderly, they will...

Oops, the bit of the article I wanted is about half way down ...

The UK police won't arrest you for singing Jamaican dancehall tunes :-)

They don't call it "liquid courage" for nothing :P

11:19 AM

Ah, I think we have an "incapable" clause like that, too. I don't know anyone who's ever been arrested for that, though

Not for lack of trying in some cases ;P

Same here. Unless you're making a nuisance of yourself they'll just tell you to go home.

If you're so drunk you can't get up you'll probably earn yourself a night in police custody but you'd be released without charge in the morning.

Actually I'm out for a drink tonight, so I can try the experiment :-)

::Makes me wonder is there a "solid form" of courage?::

Narcotics?

@JohnRennie I look forward to future-Johns hungover lamentations ;)

Pro tip: hangover recovery time increases exponentially with age :-)

I know, I know

it's almost as if alcohol wasn't good for us

Dawood ibn Kareem

11:37 AM

@JohnRennie Perhaps they should.

@DawoodibnKareem :-)

I just spent 15 minutes writing an answer to this then I did a quick search to find references to negative pressure and realised the question was a duplicate.

Dawood ibn Kareem

So copy-paste your answer to the duplicate question.

I had already answered the duplicate but forgotten about it :-)

But then I see I've written 562 answers to questions about GR and I can't remember them all.

Oh well. I'm now about to try installing the Creator Edition feature update to Windows 10 on my laptop. See you all in several hours ...

gregegan.net/ORTHOGONAL/07/QMExtra.html#CQS

the horror

I kinda want to send an email to Greg Egan to propose him a tex version of that page

it's tough to read

Dawood ibn Kareem

@JohnRennie You are allowed to provide two answers to the same question.

And as far as I know, there's no rule against disagreeing with yourself, if you need to.

There is, however, a rule against disagreeing with yourself.

12:18 PM

"Contact details: My email address can be formed from parts of the URL of this web site. Please don’t send messages to my agents or to other third parties to pass on to me; these people have better things to do."

Dammit Greg Egan

Is it gregegan@gregegan.net?

Dawood ibn Kareem

Very helpful. As it happens, my email address can be formed from parts of the alphabet, provided you add the @ and . characters.

Apparently Greg Egan also wrote a book on ultrahyperbolic spacetimes

Called Dichronauts

The absolute madman

12:33 PM

What is the URL of the website?

gregegan.net

What math should I learn today

Propositional logic

Basically half of Egan's books are weird spacetimes

@Slereah nooo

@JohnRennie will they arrest you for being tired and emotional, though?

12:41 PM

Only if you're "incapable" of controlling your emotions :P

It is illegal to break the British Stiff Upper Lip

Tired and emotional

The phrase tired and emotional is a chiefly British euphemism for alcohol intoxication (or drunkenness). It was popularised by the British satirical magazine Private Eye in 1967 after being used in a spoof diplomatic memo to describe the state of Labour Cabinet minister George Brown, but is now used as a stock phrase. The restraints of the parliamentary language also mean it is unacceptable in the House of Commons to accuse an MP of being drunk, but one may use this or other euphemisms such as not quite himself and overwrought. The Guardian describes the phrase as having joined those "that are...

T. I. L.^

Thnx @EmilioPisanty

@skullpatrol I'm not sure why it's described as "British euphemism" though

you could just say "British phrase"

all British phrases are euphemisms

including e.g. "British phrase"

I mean it is the country that referred to a campaign of terrorism as "the troubles"

12:51 PM

lmao

this guy uses $1,2$ to denote spin $-1/2$ and $1/2$

Finally on holiday tonight

I'm gonna have the biggest sleep

Been getting hard continuing the book lately, not enough sleeps

Two weeks?

yes

will be a nice break

@ACuriousMind I have an actual quantum mechanical ~~doubt~~ question now

@0celoñe7 shoot

1:02 PM

don't

QM is a lie

as u well know

@ACuriousMind If $(u_i(z))_{i\ge 1}$ is an o.n.b. for $L^2(\Bbb R^n,\Bbb C^q)$, then why are the "determinantal functions" $$\psi(\mathbf z)=\frac{1}{\sqrt{N!}}\det\{u_i(z_j)\}_{i,j=1}^N$$ a basis for $\bigwedge^N L^2(\Bbb R^n,\Bbb C^q)$?

That's not a QM question :P

What is $L^2(\Bbb R^n, \Bbb C^q)$

@Slereah What do you think it is?

@ACuriousMind Idk, seems to be highly relevant to fermions

Square integrable functions in $\Bbb R^n$ and [something something $\Bbb C^q$]

1:05 PM

Ah, that is a Slater determinant!

Oh wait is it a spinor

@Slereah yes

I see

@ACuriousMind yes

I think the easiest way to see that (if you don't believe it "intuitively") is by induction

1:07 PM

I've been basically getting 6 hours of sleep every day for a few week

I am of the walking dead

"Sir David Frederick Attenborough, OM, CH, CVO, CBE, FRS, FLS, FZS, FSA"

God damn!

That $\Lambda^2$ is spanned by these determinants is straightforward

For you maybe

Is $\bigwedge^N L^2(\Bbb R^n,\Bbb C^q)$ supposed to be the $N$-particle Hilbert space

@Slereah yes

1:10 PM

i c

@0celoñe7 It's linear algebra - the antisymmetrized tensor product of a space with a basis $v_i$ is spanned by $v_i v_j - v_j v_i$, no?

@ACuriousMind Literally no idea.

And this is an analytical "span"

@0celoñe7 That's pretty much the definition of the second exterior power/antisymmetrized tensor product.

@ACuriousMind Hilbert space tensor products are not tensor products though.

They are completions of tensor products

1:12 PM

They are abstract completions thereof.

I don't see how that complicates anything in this case

I guess those determinants would then be dense in the completion, by definition.

Exactly

Ah well, thanks. This is what I get for doing QM before breakfast

"in the literature one sometimes comes across spinless fermions. One can think of this as a situation in which the particles are $q=2$ fermions, but the wave functions are restricted to those in which $\psi=0$ unless all $\sigma_j$ are $1$"

what?

Not sure what that quote says, but outside of relativistic QFT you don't have spin-statistics, so the (anti)symmetrization of wavefunctions has no relation to the spin

I.e. particles with spin-0 obeying Pauli exclusion/antisymmetrization - which is what I'd calla "spinless fermion" - are perfectly fine

1:19 PM

yeah

that's why non-relativistic supersymmetry is a bit confusing

@ACuriousMind perfectly fine on paper you mean?

well in the real world we don't have non-relativistic particles, so yes

@0celoñe7 Yeah. Of course the real world is ultimately relativistic and spin-statistics holds for real systems.

Well...

...actually, if people can get anyons experimentally, I'd not be surprised if someone also manages to get spinless fermion states

now you're gonna tell me some crazy people managed to avoid that

who managed to get anyons?

@ACuriousMind well to some approximation

1:22 PM

@0celoñe7 physics.stackexchange.com/a/342811/50583, apparently, we're not really sure whether we have seen anyons or not :P

@0celoñe7 I think it was just some system where you could neglect one dimension

@ACuriousMind Is there a simple proof for why a density matrix can be written as a convex combination of pure state density matrices?

"I remember reports of radiant AI from "The Elder Scrolls IV: Oblivion' being an example of this vein of behavior originally as well but it had to be dumbed down because of strange unexpected behavior like NPCs killing each other over food."

:D

@ACuriousMind Ah, it's clear.

@Slereah What are you reading?

Game Dev Stack Exchange

Since I be devving a game

1:35 PM

Hmm, maybe not so clear...

user image

this is either genius or it's completely nuts

not fully sure which

skyandtelescope.com/astronomy-news/…

Obviously not a spacecraft

No rocket on it

@Slereah it's meant to solar-sail on laser beams

No sail, either

all the way to alpha centauri

1:39 PM

You're gonna need a better radio if you want to go to Alpha Centauri

I agree

but apparently there's a half-dozen of these things in orbit at the moment

so yeah, they do seem to qualify for spacecraft

More of the payload of a spacecraft

Just being in space doesn't qualify you as a spacecraft, else a Russian skeleton would qualify

There's a Russian skeleton in orbit?

I really hope that's in jest, but it wouldn't surprise me if it there was one.

@JohnRennie It's thought that Yuri Gagarin was not the first man in orbit, simply the first that made it back alive.

1:46 PM

@Slereah that just sounds like desperate word-twisting

@0celoñe7 I think the rest would of made it back by now...

if it's in space and operating autonomously, it's a spacecraft

'Outside of the earths atmosphere, under your own propulsion' ?

@EmilioPisanty Say that to me, in my spaceship

I am currently orbiting the earth

@EmilioPisanty The moon? :D

1:46 PM

Here's a quora discussion of possible Russian space skeletons, though with the caveat that it's quora: quora.com/…

@Slereah more of a payload than a spacecraft

=P

@EmilioPisanty Was Neil Armstrong a spacecraft

Or Laika

user image

The cutest little spacecraft

@Slereah see? that's what I call word twisting

my defintion works, no?

One could count Neil Armstrong as a spacecraft. Just one which wouldn't remain operational by itself for very long :)

1:48 PM

(the moon landing was faked)

Buzz Aldrin is still around, on the other hand

1 min ago, by djsmiley2k

'Outside of the earths atmosphere, under your own propulsion' ?

@djsmiley2k I'm pretty sure several cubesats have no autonomous propulsion

they are satalites, not spacecraft

also, if a spacecraft runs out of fuel, does it stop being a spacecraft?

1:48 PM

@EmilioPisanty depends how far it is away ;)

It has the means to be a spacecraft

The potentiality

spacecraft
ˈspeɪskrɑːft/Submit
noun
a vehicle used for travelling in space.

there we go.

The δύναμις

It must be a vehicle.

If those things carried.... ants...? then they are spacecraft :D

sigh Let's leave it at "Slereah doesn't like the starshot sprites and therefore will endlessly argue against them", then?

1:49 PM

Is a man in a space suit a spacecraft

@Slereah Yes, he just won't remain operational for very long.

@Slereah is that a vehicle?

Though absent more details on these Sprites I'm not inclined to debate them either way.

Hmm, are density matrices compact?

@0celoñe7 probably, right?

1:51 PM

@djsmiley2k he can move

why wouldn't they be

?

@Slereah no he can't

center of mass is on rails

Can't he

@EmilioPisanty Not sure that's the right question to ask in math.

I return after my laptop has finally installed the W10 update to find the chat room dominated by conspiracy theories and wild speculation!

What if he throws a hammer

1:51 PM

lol

@JohnRennie The usual

Reminds me of when I was walking to get lunch one day, and passed a guy with a selfie stick that had a sign saying "Satellites do not exist" on the other side.

a bold statement to put on a phone related device

pretty much.

@0celoñe7 Are there trace-class operators with continuous spectrum?

1:53 PM

@Semiclassical ask him how the GPS in his phone works :-)

@JohnRennie to be fair you could probably do the GPS system using phone towers

Triangulation of your phone using phone towers, then each phone tower knows its own position

I think that's how most GPS works on phones?

@EmilioPisanty Not sure. Wiki says trace class is the dual of compact.

To apply spectral theory I need to know if the density matrix is compact, and it doesn't seem obviously true.

@Slereah No, GPS works using GPS satellites. Simply go into any building and you'll find the GPS lock is lost when you still have a phone signal.

I think their rejoinder to that was that "it's not really satellites, it's just bouncing signals off the atmosphere."

and other stuff of that nature.

@JohnRennie Quite odd

1:56 PM

@Semiclassical Correct.

Why not use the phone tower?

@0celoñe7 What?

Approximate locations are obtained by recording which tower you're connected to, but that's as accurate as the phone signal gets.

but then how does the phone communitate with the satellite

@Slereah Satellites get more funding.

1:58 PM

Or do you only need one way communication

@Slereah the phone only needs to receive signals from the satellite - it doesn't need to send signals back. Signals are received by an aerial in the phone.

I s'ppose

@EmilioPisanty I am imagining some density matrix that takes an o.n.b $\{x_j\}$ and takes it into $\{x_j/2^j\}$. It has trace $1$ but the image of that o.n.b. certainly isn't compact.

I mostly find conspiracy thinking with regard to space annoying because human entry into space is a modern scientific achievement.

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