The h Bar

Physiks lover

00:00

@AndreasBonini power is the rate at which energy is produced i.e how much energy is produced every one second of time

So power = (energy produced)/ time

but energy is defined as force x distance moved

So power = (force x distance moved)/time

Andreas Bonini

Yeah I think I got it now..

9.8 * 500 * 10 / 50

= 980 ~ 1000

Physiks lover

yep

Andreas Bonini

Seems easy now =p

Physiks lover

how did you know how to get the force right?

well obviously you know that force = mass x acceleration = weight

Anonymous

01:45

@Mostafa I added it back giving him a comment saying why.

Anonymous

@AsphirDom No...

Crazy Buddy

15:10

@ManishEarth: I got 51 comments totally... (Not wholly encouraging though) :P

Anonymous

I just changed my parent site to Area 51 so that I can ask in chat if I'm ever suspended again . . .

Crazy Buddy

What..? o_O

Anonymous

@CrazyBuddy Such comments are generally all crappy nonsense spam. On my blog, ; I had once thought they were genuine, now I'm individually deleting each of them .

Anonymous

...

Anonymous

...

Crazy Buddy

15:13

At least some spammer has noticed my blog...

All those 49 were spammed yesterday :D

Anonymous

@CrazyBuddy Pretty slowly, though. I got like 70 nonsense spam on my first days... 49 is very little.

Anonymous

@CrazyBuddy Why, will there be any limitation on the number of messages I can post in hbar or something ? .

Anonymous

By 70, I may mean 140.

Anonymous

or more.

Crazy Buddy

@DImension10AbhimanyuPS Chat is network-wide.. You can't talk in chat when you're suspended.

(I don't know about meta.SO)

Anonymous

15:16

@CrazyBuddy Really? When suspended, I was told "Your account is suspended on the parent site and cannot chat for 7 days.".

Crazy Buddy

Because your suspension was for 7 days :)

Anonymous

@CrazyBuddy I know that.

Anonymous

I meant that

Anonymous

the account could not

Anonymous

chat

Anonymous

15:17

because of the fact

Anonymous

that

Anonymous

I was suspended

Anonymous

on Physics.SE

Anonymous

and not

Anonymous

if I were suspended on, say,

Anonymous

15:17

TeX.SE

Anonymous

or A51.SE.

Anonymous

or Math.SE.

Anonymous

or MO.SE.

Crazy Buddy

I don't think you can chat on other places .... Hmm... O_O

Anonymous

@CrazyBuddy I know that too.

Anonymous

15:18

That I couldn't be able to

Anonymous

chat in say, the chat room for

Anonymous

Math . SE , say, because

Anonymous

I was suspended on Physics.SE, which is (Was) my parent site.

Anonymous

I changed it to A51 so that I'd still be able to chat everywhere.

Anonymous

Except, probably the hbar. But I could still be

Anonymous

15:19

able to ping the mods

Anonymous

in another chat room.

Crazy Buddy

If I were you, I won't allow another suspension... I know you'd take that the other way.

At least I'd try not to heat things up :D

Anonymous

@CrazyBuddy I'd try too.

Anonymous

Buut it may not

Anonymous

be possible.

Anonymous

15:25

...

Anonymous

c jcndjcnx x jncnsdjc v xcjn jc xcn jnddidie2!###@43j3 cicd&*

David Z

16:00

So, who's here for the chat session?

Crazy Buddy

I'm here...

Welcome to our Chat Session people :)

user54412

mememe

David Z

woot

Crazy Buddy

I was actually searching my dad on facebook :D

David Z

I don't remember if there was anything specific to discuss today

Crazy Buddy

16:04

I see someone suspended on Space.SE yesterday (for the first time) :D

Oops... Sorry... :)

18 hours ago, by David Z

By the way guys, even just discussing your lack of knowledge about suspensions like this can subtly lead other people to ask questions that they're not equipped to answer, and that in turn leads to facts being made up. (Not that I'm saying you're making anything up, but as I'm sure you know, there are people out there who would.) That's why I ask (but don't require) that community members also refrain from discussing specific suspensions.

Okay.... H-bar is gonna be silent for the first time -_-

user54412

hmmm, nothing too interesting on the astro side of things

David Z

Not the first time

Crazy Buddy

Oooohhhh...

David Z

It used to be quiet all the time in the early days, before we started having chat sessions

There are a few active discussions on meta we could discuss, if anyone has anything to say about them

Crazy Buddy

ahh...

David Z

16:09

@Chris same with particle physics, I haven't heard of much new for a while

user54412

random minor topic (less controversial than meta :P): did anyone else see this on academia?

user54412

8

A: How many arXiv papers are uploaded in their final (refereed) versions?

It is a lot of work to find out which paper in arXiv has actually the same content as a published paper. However, taking arXiv's journal-ref field as a surrogate indicating which paper has a journal version out, I arrive at these fraction of papers: So it looks as if the gut feeling that this ...

David Z

Oh yeah, I did have a look at that

user54412

lots of graphs - I didn't realize that arxiv basically started out as astro + condensed matter

user54412

before mathematicians took it over ;)

David Z

16:11

Huh, I thought it started with hep-ph - according to one of Sean Carroll's recent blog posts

but maybe not

user54412

I mean, astro-ph and cond-mat had the most papers initially

David Z

Ah, gotcha

I'm not sure if the answer there really quite addressed what the question was asking

It seems like the graphs are for papers that were reuploaded with revisions, not papers which are uploaded in their final form

ManishEarth

Hey guys, sorry I'm late :s

user54412

@ManishEarth And you should be sorry :P

David Z

No problem, there's not much going on anyway

Crazy Buddy

16:20

@ManishEarth Are you not busy now? :P

ManishEarth

I was

David Z

Nobody seems to have much to talk about

ManishEarth

Well, I realized something the other day

there exist real numbers (an uncountably infinite number of them) that cannot be expressed mathematically

David Z

How so?

ManishEarth

To put it formally:

I allow you to define your mathematical language and symbols. You can even use English, if you can keep out the ambiguity

We already have a way of conveying mathematical concepts formally, anyway.

Now, assuming the contrary: Every real number can be expressed as a (finite) string of these symbols. This is basically saying that we can express all numbers mathematically.

David Z

16:25

Oh, well if you specify finite strings then it makes sense

ManishEarth

Well, this is a finite string of symbols. It can be converted to a number (just encode the symbols). Using this you can prove that if the assumption is true, then the cardinality of real numbers must be less than or equal to that of the natural numbers. Contradiction. Boom.

@DavidZ To me, if something is an infinite string, it's not really "expressible"

David Z

Yeah, we differ on that point.

user54412

I'm gonna side with @ManishEarth here

user54412

Definable real number

A real number a is first-order definable in the language of set theory, without parameters, if there is a formula φ in the language of set theory, with one free variable, such that a is the unique real number such that φ(a) holds in the standard model of set theory (see Kunen 1980:153). For the purposes of this article, such reals will be called simply definable numbers. This should not be understood to be standard terminology. Note that this definition cannot be expressed in the language of set theory itself. General facts Assuming they form a set, the definable numbers form a field...

David Z

I'm not talking about the mathematical definition of "expressable" (if there is one) but my intuitive sense of what is expressible vs. not

ManishEarth

16:28

This brings up two questions. The first has an obvious flaw:

user54412

Where rationals ⊂ algebraic numbers ⊂ computable numbers ⊂ definable numbers ⊂ reals

ManishEarth

'What about the string ''The first non-definable number"? That itself is a definition of a number'

There's a flaw there, but it's fun to confuse other people with :P

The other question is umm ... meta-mathematical? "If a number can't be defined, does it exist?"(just like the tree falling in a forest question)

user54412

@ManishEarth You're probably stepping outside the bounds of your own logic - what finite string of first-order symbols can reference all definable numbers?

Kazi arafat Ahmed

can any tell me, why we put complex number in front of ket vectors(say with two component) instead if putting real in front of a ket vector having four component?

ManishEarth

@ChrisWhite Yeah, recursion breaks things. Here, the entity "non definable number" wasn't defined in my initial system of symbols. I can't just feedback loop it and watch the world burn.

Nor can it be derived.

Its fun to think of such things, though.

David Z

16:33

@Kazi it might have something to do with orthogonality of states

user54412

English is richer (and probably less consistent) than first-order logic :P

ManishEarth

@KaziarafatAhmed Because of the way the conjugate behaves

user54412

@KaziarafatAhmed Agreeing with David Z and ManishEarth - the "natural" inner product for vectors in R^4 is different from those in C^2

ManishEarth

I think Dirac calls these the "conjugate imaginary" (i--> -i) and the "conjugate complex" (ket --> bra, or transposition)

Kazi arafat Ahmed

could you help me explaining further..

ManishEarth

16:34

But both of these conjugates behave like the same conjugate in the sense that applying one makes the other appear. Or something like that.

user54412

you use complex numbers so you naturally incorporate the sign changes you want - you can often imitate things with twice as many reals, but then you will have to check other constraints that would otherwise tacitly just go along for the ride

ManishEarth

Hmm, yeah

Kazi arafat Ahmed

can you give me any external link explaining this..

I have heard from susskind's video that we need more parameter to explain two electron system than we need in one electron system. what does that mean? @ChrisWhite

user54412

@KaziarafatAhmed Umm, I don't know enough of the context there. Certainly adding more stuff to a system will require more parameters to describe it. Even classically, 1 electron has 6 parameters (3 position, 3 momentum), while 2 electrons have 12

David Z

16:53

Anyway, it's quiet enough that we're probably done for this week

One thing:

This fall after this week I'm not going to be around for the start of chat sessions. So someone else will have to kick them off. (Or people can always just start talking)

user54412

@DavidZ @CrazyBuddy can do that ;)

Crazy Buddy

Thought of typing the same...

ManishEarth

@DavidZ I can try. But can't be certain :s

Crazy Buddy

I'll do that :)

user54412

20

A: Are some numbers more irrational than others?

The other answers and comments are fascinating, particularly about the irrationality measure, but allow me to give a little more information along the lines of Mark Sapir's answer by mentioning that there are several very large, intensely studied hierarchies of complexity for reals numbers. After...

Kazi arafat Ahmed

16:56

he said we need two parameters to indicate an electron(say), we don't need four variables to indicate a two electron system, we need six parameter. @ChrisWhite

user54412

@ManishEarth ^ ^ pick your degree of definability :P

user54412

@KaziarafatAhmed hmmm, i don't really know what he means by that - it all depends on his specific assumptions (how many dimensions we're in, what variables he's measuring, etc.)

Gugg

17:38

@ChrisWhite Thanks for the book suggestion. I think it is a very good read.

Crazy Buddy

17:51

@Gugg Whaaaa..? You read the whole book? O_O

Gugg

@CrazyBuddy p. 186, not there yet...

Crazy Buddy

ahh... :)

twistor59

19:16

@CrazyBuddy chief chatter! The honour. The responsibility. I have faith in you......!

Crazy Buddy

@twistor59 Of course... Be my guest :)

Hmm... Nowadays, ~~storm~~ twistor is calm :P

twistor59

@CrazyBuddy Twistor has had too much work to do - hardly any time for PSE (sad face)

Gugg

@twistor59 The horror.

Crazy Buddy

@twistor59 Whaa? You've already quitted ~~mathematical~~ physics, now you're quitting Phys.SE..? What about Tachyons & Gravitons? They would definitely be angry on you... :)

twistor59

19:52

@CrazyBuddy LOL Not quitting, just slowing down a bit....

Crazy Buddy

@twistor59 Seems like my question has made you lazy... :D

5

Q: Is it true that Physics.SE gets bored often?

I have a thought that this question is off-topic. But... From the day I joined here, I was active (though I had low-quality answers, I was still actively answering) for almost 8 months. Then, I had a vacation (can't use internet that much) when I stopped answering. I (am) still visited (visiting...

Physics Meta

21:42

1

Q: Merging duplicates?

When I flag a duplicate it's usually because I've found an older question with substantial topical overlap and an answer that covers the new question completely. Sometimes I hesitate because the new question is actually asking the same thing as the old question, but much more clearly. So I'm conf...

discussion close-reasons duplicates

Physics Meta

22:27

0

Q: Bug - Asking question anomally and then registering problem

This bug happened to me when I created an account, and it went like this: I already had a Google subscription in another site affiliate to the StackExchange network. I posted a question in physics anonimally, and posted using a hotmail address. I then realized the ocurred and tried registering ...

bug user-accounts

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