The h Bar

Semiclassical

01:58

@Blue I had a communications prof (a guy I liked, by the way) who brought up the snowflake "experiments" featured in "What The Bleep Do We Know" in a non-ironic way

that was painful

vzn

03:09

there was some discussion of the poincare-perelman proof in here awhile back esp wrt the new yorker Nasar-Gruber article. fyi reddit recently revisited/ rehashed some of the same angles reddit.com/r/math/comments/98i9j7/…

Avnish Kabaj

05:00

Can I ask the fan ball question on the main site?

Or would it be closed

David Z

05:14

@AvnishKabaj Which question is that?

(I haven't been following the discussion)

Avnish Kabaj

10 hours ago, by Avnish Kabaj

If I have a fan and I throw a ball at it

10 hours ago, by Avnish Kabaj

If I keep on increasing the speed of the fan

David Z

Ah I see, thanks

Avnish Kabaj

10 hours ago, by Avnish Kabaj

1. What happens to the probability of the ball getting sliced

10 hours ago, by Avnish Kabaj

2. Bouncing back

David Z

Kind of an interesting problem but I don't think it's suitable to post just that problem itself on the main site. But if you were trying to figure that out and you got stuck on something, it might be okay to ask about just the part you got stuck on. (Basically the same rules as with our homework policy.)

Avnish Kabaj

Ok sure

Thanks

John Rennie

05:54

@AvnishKabaj the probability of the ball bouncing back only remains constant in an ideal bounce i.e. a bounce taking zero time.

In a real bounce the ball deforms as it hits the blade and the bounce takes a non-zero time. So if the ball hits near the trailing edge of the blade it may skid off the trailing edge of the blade and be sliced by the next blade.

The probability of this happening increases with increasing fan speed.

Avnish Kabaj

06:22

@JohnRennie wheeeee! That's what I thought

Thanks

Akash. B

07:24

@JohnRennie is there a time dilation near blackhole?

John Rennie

← 1 message moved from Problem Solving Strategies

Akash. B

okay

John Rennie

@Akash.B any gravitational field is always associated with time dilation.

Even the Earth's gravitational field causes a very small time dilation.

Akash. B

@JohnRennie what are the factors determining gravitational field?

is it only mass?

John Rennie

@Akash.B to a first approximation, i.e. in Newtonian gravity, a gravitational field is due only to mass.

Akash. B

07:33

@JohnRennie so time moves slower for objects who have speed more than light as well as objects with mass of a black hole Right? sir

John Rennie

Yes, both gravity and relative motion cause time dilation. If you're interested there is a description of how and why time dilation happens at:

56

Q: What is time dilation really?

Please will someone explain what time dilation really is and how it occurs. There are lots of questions and answers going into how to calculate time dilation, but none that give an intuitive feel for how it happens.

Akash. B

@JohnRennie why that line bends in the graph?

John Rennie

This graph?

Akash. B

@JohnRennie yes sir

John Rennie

07:48

That shows an observer starting at $x=0$ at time $t=0$, then moving out to some maximum value of $x$ before turning round and returning to $x=0$.

Akash. B

oh i see

John Rennie

For example suppose you throw a stone into the air. It starts at ground level, moves up to some maximum height then falls back down to the ground again.

The graph might look a little confusing because it has time on the vertical axis. That's the way we usually draw graphs like that in relativity.

Akash. B

08:01

@JohnRennie Ahhhhhhhh! mathematics!

@JohnRennie can you explain it to me without mathematics?

John Rennie

@Akash.B no, I'm afraid you need to understand the maths to have any idea what's going on.

6

Akash. B

@JohnRennie do not delete that question until I am a physicist

5

Secret

11:32

::Initially frustrated for seeing a comment that said no mathematics, but the final response above is so funny that alright, I will let that slip::

Novalium Company

12:44

Hello physics masters, a question:

danielunderwood

13:08

Is there a sort of abstract version of a tensor that you could call type (a, b, ..., k) with k types of indices? I can't see how it would come out of the geometric definition of a tensor, but I wondered if there was a way to create such a thing.

Semiclassical

@danielunderwood I’m not sure what you mean by ‘type’ of index

Naively, at least, I’d say what you’ve got is a rank-k tensor

(Eg a matrix is a rank 2 tensor)

peterh

Finally I collected all the 6 steward badges on the site!

user 2646

Congrats

peterh

I am the 4th.

danielunderwood

13:27

@Semiclassical Well like you have covariant and contravariant indices and you'd call a normal tensor type (p, q). Is there a way to expand those. I don't know of any way to put that in from the geometric definitions and can't really think of a type of index that wouldn't just be increasing the number of co(ntra)variant components, but there are all sorts of weird objects in math

But I guess I'm also trying to create an object without a real need to, which probably isn't the best way to do things

Maybe say that tensor is an abstraction of scalars, vectors, and such depending on rank. Would there be a corresponding abstraction of tensors based on type?

Semiclassical

so you're looking for a 'type' of index beyond co- and contra-variant?

danielunderwood

Yeah more or less

Or if such a thing is even possible

Semiclassical

can't say I know of any such generalization

Adam Uraynar

13:55

If I tag a user in this chat room, who is not in the room, will they be notified?

danielunderwood

If they've been active in the past so many hours (24 or 48 maybe?)

Adam Uraynar

So if they're active with anything?

(They don't have to be active in the chat room?)

danielunderwood

I think they have to be active in chat, but I'm not sure if it's just this chat or any SE chat

Adam Uraynar

I asked around on StackOverflow, they need to be active in the actual chat.

That's one of the few sites where I can consistently grow my reputation just by editing answers that use "i" instead of "I" ;)

~ cuz the activity is gonzo there.

Anonymous

@AdamUraynar Yes, they will, irrespective of whether they're active or not

Anonymous

14:07

Also, users are not questions. You don't tag them. You "ping" them. :P

Adam Uraynar

Wait wait, if I ping Joe Shome, they'll get a notification even if they've never set foot in the chat room?

(Because I was told the opposite in the StackOverflow chat room.)

Anonymous

@AdamUraynar You won't be able to ping people who haven't visited the chat room, in the first place.

Anonymous

Try it out

ACuriousMind

@AdamUraynar No, only if they have been in the room sometime in the last two days

They don't need to have talked, though

Adam Uraynar

Ahh, I see now.

Thanks a bunch!

Anonymous

14:13

The "two days" feature doesn't sound that great to me though.

Anonymous

Users should have an option to opt out of chat. But there shouldn't be any time limit like that :/

Semiclassical

eh, given how many duplicate names there are

I think that sort of limitation is necessary

Anonymous

@Semiclassical Well, you could always rank the most recent/active users to be the top of the list

Anonymous

The less frequent visitors could be in an additional drop down list

Semiclassical

maybe. I imagine there's some technical issues at play

danielunderwood

14:18

there are duplicate names?

Anonymous

I guess the main issue is that chat isn't something profitable for SE, which is why they're reluctant to spend much dev time on this.

Anonymous

One option would be to make chat open source, so that people who do have the time can add bug fixes and additional features.

ACuriousMind

@danielunderwood Sure, usernames are not unique since users are identified by an ID

danielunderwood

I dunno. Looks like a giant space for an ad below the stars list and a bundle of text data

I would like to see the chat open source though

And ahh I didn't know that. Could always pull a discord and do @user#1234

ACuriousMind

@Semiclassical @Blue the two days limit is a deliberate design decision - moderators are able to ping all users everywhere, so it's not as if the system can't do it.

Anonymous

14:25

@ACuriousMind SE is so racist!!!!!!!!

ACuriousMind

... what?

Anonymous

On a serious more serious note, how does the mod superping work? :P

Anonymous

@ACuriousMind That was a joke based on the assumption that "mods" and "non-mods" are two races in the SEdom. ;)

danielunderwood

They ping you and an army of robots comes to find you

Anonymous

You may delete it lest people find it offensive XD

ACuriousMind

14:28

@Blue I'm not sure what you mean - it works just like a normal ping without the restrictions, and one can also ping users by ID instead of only by name

Anonymous

@ACuriousMind Ah, and what if there are 20 users having exactly the same name? Like say "John"

Anonymous

In that case does it show a drop down list of all the 20 avatars?

ACuriousMind

@Blue Yes, although fully loading the list can take ages, so pinging by ID is usually faster

Anonymous

Ah, I see

danielunderwood

Just see what it does when you @user

Though none of those have the same name

Anonymous

14:36

44

A: What is a 'superping'?

Superpings are a chatroom feature only available to diamond moderators, where they can reach a user irrespective of past interaction. Compare that to a normal ping, where you can @name someone in comments or in a chatroom only if they have been active on a post or in that chatroom. Superpings h...

Anonymous

What happens if normal users use the @@<site-userid>@<sitename>.stackexchange.com format? Gotta try

Anonymous

@@[email protected]

Anonymous

@@user:[email protected]

Anonymous

@@<user:24839>@<physics>.stackexchange.com

Anonymous

Doesn't work

Anonymous

14:43

Lol

Avnish Kabaj

Hey blue

Anonymous

Lemme try the chat id thing

Avnish Kabaj

@blue

Let's make

A new beta site

And become mods there

Hmmmmmmm

Anonymous

@AvnishKabaj I could run for mod in the QCSE election when it graduates :P But I probably won't. Being a mod comes with struggles like users abusing you for petty stuff XD

Avnish Kabaj

@Blue how do I become a mod tho

Anonymous

14:46

I did participate in the pro-tem mod thingy though. But they choose James Wotton over me. Looking back, that was a good decision. I really don't have the patience to deal with grown up trolls

Avnish Kabaj

Age and wisdom do go hand in hand

danielunderwood

I don't even have the patience to listen to grown up trolls

Anonymous

Even without becoming a mod, I had users attacking me for improving their posts ^ :P

Anonymous

There were 4 of these iirc. Over-zealous editing by Blue(Part I), (Part II), (Part III), (Part IV)

Secret

14:50

grown up trolls = nuke them

Avnish Kabaj

@Blue guess you're the Gaurang Tandon of QCE

Sid

@Secret Moderators aren't a certain person who talks about nuking endlessly...

(Also, hello people.)

Secret

Well I am not a moderator, and I cannot be a moderator because I will just want to nuke the world if I gain power

Avnish Kabaj

Too much effort being a moderator

First of all you can't say anything rude

danielunderwood

I'm one of the admins in a discord chat and it can get fairly tough. There's a "this guy is really annoying, but I can't ban him just for being a dick"

Fawad

14:56

Don’t use d word

danielunderwood

can't ban him for being a <favorite light insult here>

Anonymous

@Fawad Dick Feynman....I'm surprised there aren't any jokes about that

Anonymous

Or are there?

danielunderwood

There have to be

Sid

What is a neck rope?

Anonymous

15:00

That's preferable to writing lab reports for tomorrow :(

Sid

@Blue Ha! Fortunately(or Unfortunately) my lab reports submission are only on Monday and Tuesday. So, although my weekend is wasted writing lab reports, I have a relatively light weekdays

ACuriousMind

@Fawad Telling other users to commit suicide is not acceptable, even in jest.

6

Fawad

@ACuriousMind and about using d word?

@Blue it wasn’t meant for you

Anonymous

@Fawad Slangs by themselves aren't abusive :P

ACuriousMind

@Fawad What about it? It's a mild insult, but it wasn't directed at anyone in particular.

John Rennie

15:05

If you bring someone back to life after they have been hung would that be neckropemancy ?

ACuriousMind

@JohnRennie I...have no words for that pun

John Rennie

My mission is complete :-)

danielunderwood

It made me laugh-sigh or whatever that expression is

Anonymous

Not yet. Put it up on Urban Dictionary. And only then your mission will be truly complete ;)

danielunderwood

Hopefully you guys know what a laugh-sigh is and I'm not just weird

Avnish Kabaj

15:09

Ha ha hmmmm

danielunderwood

A sigh with a slight pinch of chuckle...or something

Anonymous

Or a chuckle with a pinch of sigh

Anonymous

Or a pinch of both

Fawad

@ACuriousMind I don’t if it’s insult of what. But googling it in front of my parents will make me block from this “knowledge gaining website”

Anonymous

Always lock your door when you're on the internet :P

Anonymous

15:21

(that's what locks are for after all)

Anonymous

And freaking delete your history

Fawad

Let acm answer it

Anonymous

Kk

ACuriousMind

@Fawad I'm not responsible for what you decide to google in front of your parents. If you have a problem with the word, you can ask people not to use it, which you already did, and no one complained about that. I'm not sure what more you want from me.

Anonymous

Anyhow, if your parents freak out seeing you reading certain mildly offensive things on the internet, that's a bit concerning :)

ACuriousMind

15:27

In unrelated news: Are the reputation and bounty tabs on profiles gone for anyone else?

Fawad

@ACuriousMind delete texts which include bad words

ACuriousMind

All other SE sites have it, but they don't show up on Physics for me anymore.

Anonymous

I can still see the bounty tab: physics.stackexchange.com/users/1325/john-rennie?tab=bounties

Anonymous

@Fawad Dude, just stop. Moderators are not your personal censoring machine. If you ask nicely, we'll avoid certain words in your presence, but really, don't make it a demand. This is an informal chat after all.

ACuriousMind

@Fawad We do delete blatantly offensive words. "Dick" is not really all that offensive. I can understand if you don't want to see it, but I'm not going out of my way to punish users for using it once innocently

Physics Meta

15:34

0

Q: Where is the downvote?

Now and then I get a downvote that shows up in "recent achievements". I go to the associated question, and there are no downvotes shown at all, on answers or comments. What am I missing?

discussion

Anonymous

And if you're that sensitive, learn web scripting and make your own bookmarklets to blacken out all slangs. That would be a more productive use of your time.

danielunderwood

@Blue maybe your comment was like Beetlejuice

ACuriousMind

@Blue Ah, nevermind - I was looking at the meta site!

Anonymous

Yeah, that's the meta site :P

Novalium Company

Hello everybuddy, a question:

Anonymous

15:36

@danielunderwood Which one? Also I don't know what Beetlejuice is :P

ACuriousMind

@NovaliumCompany actio = reactio - the same amount of momentum that the propeller imparts to the air is imparted in the opposite direction on the propeller, and hence the boat.

So after the air then imparts the momentum to the sail and hence the boat, the net change in momentum is zero.

danielunderwood

It's a movie. I don't really remember the details, but you say the guy's name and he appears

Novalium Company

@ACuriousMind The total momentum in a closed system is conserved, is that it?

So in order to change the momentum, we need to change it externally, so the system won't be closed?

ACuriousMind

@NovaliumCompany Yes. If you look at the boat+sail+propeller system, there's no external force acting on it, so its total momentum cannot change

Novalium Company

@ACuriousMind Ok, thanks. I was just reading about it in my book, how didn't I think about it :D

nitsua60

15:43

@NovaliumCompany Related:

12

Q: While sailing can Gust of Wind be used to speed up your boat?

We are about to sail to arctic seas to check out a boat that has been attacked. I am trying to figure out an escape if we get attacked. What I am wondering is: if my cleric is in the back of the boat can he cast Gust of Wind directed at the sails successively in an attempt to escape an ambush? ...

dnd-5e spells

John Rennie

31

Q: Blowing your own sail?

How it this possible? Even if the gif is fake, the Mythbusters did it and with a large sail it really moves forward. What is the explanation?

classical-mechanics popular-science

nitsua60

(If only more physics questions could be answered with "because magic." It would have made those Jackson E&M problems so much easier....)

danielunderwood

Hey mods I just wanted to tell you guys thanks

Novalium Company

I was wondering, how do rocket engines work in space since Newton's 3rd law won't work because there is no air for the exhaust gases to produce the desired trust?

Avnish Kabaj

@Blue prolly one exists

John Rennie

15:52

13

Q: What is a rocket engine thrusting against in space?

I know Newton's third law of motion might be the answer for this but still I am wondering how the rockets could thrust in the empty space and move in the opposite direction. I guess an astronaut wouldn't be able to push in the empty space with his hands or legs to move himself, but with a rocket ...

newtonian-mechanics general-relativity momentum conservation-laws rocket-science

Anonymous

@NovaliumCompany Why do you even need air?

danielunderwood

Rockets use conservation of momentum too! You eject mass and give it momentum and the rocket has to gain momentum in the opposite direction. I'm not entirely sure how that relates to an atmosphere though

Anonymous

reddit.com/r/askscience/comments/3569v1/…

Anonymous

/r posts are best for these kind of stuff...lol

Novalium Company

Hmm, cool. I didn't know that thinking in terms of conservation of momentum can be so useful!

Anonymous

15:54

If you ever become a space engineer or astronaut, Newtonian mechanics is what you'll be learning, for years

danielunderwood

Just wait until you think about conservation of 4-momentum

Jokes on you. I bought the game Space Engineers and it didn't require years of Newtonian mechanics

Anonymous

@danielunderwood lol

ACuriousMind

@NovaliumCompany Conservation laws in general are exceedingly useful. Physicists' love of symmetries is essentially because symmetries determine conservation laws through Noether's theorem.

Anonymous

Astronauts I guess come from a bit diverse applicant pool

Anonymous

Compared to space engineers

Anonymous

15:58

Space engineers either have an electrical or mechanical or an aerospace background

danielunderwood

Like engineers for things to go to space or engineers in space?

Like build the ISS or live on the ISS?

Anonymous

@danielunderwood Like engineers who build rockets

Anonymous

And work on communication in space

danielunderwood

Speaking of which, thanks to whoever said something about First Man a couple weeks ago. I just finished the audiobook and it was pretty good

Anonymous

We have an undergraduate institute in India, very close to (and associated with) ISRO. So all the students there get to see rocket launches regularly, which is sorta cool

Anonymous

16:01

The main three subjects they offer are aerospace, avionics and engineering iirc

Anonymous

I don't know how well ISRO is doing these days though

Anonymous

They did launch a few satellites last year

Anonymous

*engineering physics

John Rennie

@Blue weren't tey talking about putting a man on the Moon?

Anonymous

@JohnRennie A rover probably

John Rennie

16:09

(Google, Google) ah, just putting a man in space and the (optimistic) target date is 2022.

Anonymous

@JohnRennie Ah, that was a long time ago. Haven't heard of it since. But there's this:

Anonymous

Indian Human Spaceflight Programme

The Indian Human Spaceflight Programme (HSP) was created by the Indian Space Research Organisation (ISRO) to develop the technology needed to launch crewed orbital spacecraft into low Earth orbit. The first crewed flight is planned with a spacecraft called Gaganyaan for 2022 on a home-grown GSLV-III rocket.Previously, human spaceflight was not the priority for ISRO, though most of the capability for the same had been realised. ISRO has already developed most of the technologies for crewed flight and in 2018 it performed a Crew Module Atmospheric Re-entry Experiment and a Pad Abort Test for the...

John Rennie

@Blue yes, the 2022 mission is what I was thinking of.

Anonymous

"The project will cost less than Rs. 10,000 crore."

Anonymous

One good thing about them is that they're being able to cut down on the costs significantly

Anonymous

16:11

At least that's what I had heard about their Mars mission

Anonymous

Mars Orbiter Mission

The Mars Orbiter Mission (MOM), also called Mangalyaan ("Mars-craft", from Sanskrit: मंगल mangala, "Mars" and यान yāna, "craft, vehicle"), is a space probe orbiting Mars since 24 September 2014. It was launched on 5 November 2013 by the Indian Space Research Organisation (ISRO). It is India's first interplanetary mission and ISRO has also become the fourth space agency to reach Mars, after the Soviet space program, NASA, and the European Space Agency. It is the first Asian nation to reach Mars orbit, and the first nation in the world to do so in its first attempt.The Mars Orbiter Mission probe...

Anonymous

Yeah, the Indian news channels went gaga over the mangalyaan mission.

Anonymous

That was just US$73 million, which is quite cool tbh

Avnish Kabaj

A new meme format came up

Anonymous

Meme format. Meme format?

Avnish Kabaj

16:15

Indians can put up a rover on Mars but [insert insult]

@IceInkberry ?¿

John Rennie

Just seen this on Facebook. Seems like good advice:

Anonymous

@AvnishKabaj Indians didn't put up a rover on Mars yet.

John Rennie

3

Avnish Kabaj

@IceInkberry eh?

ACuriousMind

@JohnRennie No Youth Thunderdome? :(

Semiclassical

16:18

@JohnRennie How long until the hammer drops?

John Rennie

@ACuriousMind the children will all be too busy working down the mines or cleaning chimneys.

Avnish Kabaj

My my

Is it that bad

Anonymous

@AvnishKabaj '__'

John Rennie

@Semiclassical not sure actually. It's disturbingly close ...

@Semiclassical March 29th next year. And there's still basically no agreement with the EU.

Semiclassical

I suppose the American equivalent of "local vigilante unit" would be "militia"

John Rennie

16:20

A "no deal" Brexit is almost inevitable now.

Semiclassical

I don't really fathom how the situation got to this point. I mean, "human stupidity" goes a long way

but still

Curio

I'm going to ask a question on the main site but I'm having problems finding the title.
The question is about a problem. In this problem the issue is about the forces of a body, I mean my decomposition of the force brings to a wrong result. Any ideas?

Pulley problem, wrong decomposition of forces?

John Rennie

@Semiclassical the trouble is that there exists a degree of discontent with the EU in many of the countries. If the UK is seen as benefiting from leaving the EU there is a real risk that referenda will be demanded in other countries.

tatan

Imagine having a spherical conductor with some charge ON the surface itself. If I now use Gauss's Law and argue that although the net charge enclosed in zero, there is non zero field on the surface am I correct?

Semiclassical

right. hence why the EU has a strong motivation to make Brexit hurt

John Rennie

16:34

But on the other hand restricting trade between the UK and the EU will hurt both sides, so it's a delicate balance.

Semiclassical

and "maybe Brexit wasn't a good idea after all" isn't a politically viable option

John Rennie

And the UK could make life very difficult for the EU if it wanted. For example it could refuse to do anything about the Irish border, in which case there would be massive smuggling across that border between the EU and the UK and that would undermine the EU's position.

Semiclassical

Right

It does seem like this is a mess of the UK’s own making, tho

John Rennie

Then there's the forty billion pounds that the UK was committed to pay to the EU as part of long term budgets. If there's no deal the UK is likely to refuse to pay it and losing £40bn from the EU budget is going to sting.

@Semiclassical it's not as simple as that

The European commission is seen as arrogant in many countries. This is partly their own fault since they do have a tendency to assume they know best and everyone will fall in line. This has stirred up a lot of resentment. If you held referenda in all the EU countries I suspect many of them would be ucomfortably close run contents.

Semiclassical

Hmm

The closest US equivalent to this is the election of Trump

John Rennie

16:42

Basically the UK was the only country stupid enough to hold a referendum, and that was done by the Conservatives purely for the sake of holding the Conservative party together, not out of any desire to let the people have their say.

enumaris

bam, right in the kisser

John Rennie

@Semiclassical well yes. And look how well that has turned out. Now Trump is threatening a 25% tariff on cars imported from the EU. Which of course means the EU will retaliate.

Semiclassical

setting aside the considerable difference between American and English conservatives

enumaris

politics

Semiclassical

That sounds a lot like trump’s election

enumaris

16:45

blood boils

Semiclassical

Particularly since a vote against Trump was seen as a vote against the Washington establishment

John Rennie

I do sometimes wonder if it was worth climbing down from the trees.

2

enumaris

seems like it wasn't worth it

Semiclassical

Use Physics know-how to build treehouses

Problem solved?

enumaris

17:21

maybe

vzn

17:37

@JohnRennie lol democracy was a stupid idea? o_O (ok guess churchill said nearly that himself over ½ century ago...)

Ryan Thorngren

check this out unicornriot.ninja/2018/…

vzn

@RyanThorngren o_O similar to this An 11-year-old hacked a replica of Florida’s voting system in 10 minutes vox.com/policy-and-politics/2018/8/13/17683666/…

Ryan Thorngren

incredible

the guy in this vid has some good things to say, but it's pretty scary

Anonymous

17:54

@AvnishKabaj I think about your ball bounce thing, it will depend on a lot of factors like precentage area of the fan, velocity of the fan, time of bounce, etc. Assuming that the fan occupies A fraction of it's circle, I think the probability that it will hit the fan and bounce off will be A .

Anonymous

Now, talking about velocity. It will slice only if it remains in the blades' plane. If you ideally throw the ball so that it just either touches the fan's plane then it will be down immediately with no opportunity to get sliced.

Anonymous

Another thing here is the size of the ball. The above assumptions are true when the ball has negligible size. Assuming that the ball remains in the fan's plane for time t and the velocity of blades is v then the probability that blade will hit the blade... wait... will be also dependent upon the fact that how many blades the fan has.

Avnish Kabaj

@IceInkberry I didn't ask how to calculate it

Just wether it will increase or decrease

Anonymous

Assuming that the fan has n blades having A/n area each. Also, assuming that the blades are rectangular in shape(circular at ends). The angle of the circular region of the fan free of blades will be 2π - 2πA/πr^2

Anonymous

Don't message in b/w the calcs o_o

Anonymous

17:58

I am just trying.

Avnish Kabaj

43 secs ago, by Avnish Kabaj

Just wether it will increase or decrease

Anonymous

18 secs ago, by Ice Inkberry

I am just trying.

Anonymous

Nvm. I will try on a paper.

Avnish Kabaj

Sure

Anonymous

People who want to see a different site logo for QCSE, go and vote: quantumcomputing.meta.stackexchange.com/questions/89/…

Anonymous

17:59

Anonymous

This one looks pretty good

Anonymous

(As mentioned, colors will be added later)

Avnish Kabaj

@Blue is quantum computing related to physics

Or does quantum have some other significance

Semiclassical

it's computing predicated on exploiting certain features of quantum mechanics

Avnish Kabaj

Oh

Semiclassical

18:01

so it is based on physics. that said, a lot of it is about how to design programs/algorithms

and that's not necessarily going to involve a lot of physics-thinking as such

Anonymous

@AvnishKabaj Sure, it's an extremely versatile field. You even have pure mathematicians (for example Gil Kalai), chemists (like Kais, Aspuru Guzik, etc) and of course computer scientists working on it

Avnish Kabaj

@Blue o.O

Anonymous

And it's also a very nascent field. So lots of scope for new discoveries

Anonymous

Unlike the more developed fields in physics

enumaris

physicssss

Avnish Kabaj

18:05

21

A: Quantum Computing Logo

This one does look good

It's a qubit?

Anonymous

It's a Bloch sphere

Anonymous

@AvnishKabaj Indeed. I like it!

Avnish Kabaj

The logo should be something that captures the essence

The being

Something that even makes illiterates like me go

"That's quantum computing, yo"

Anonymous

An image of a pie in the sky would be suitable :P

Anonymous

18:12

Yo, that's a quantum computer for ya ^

Anonymous

For those who didn't get the joke

Avnish Kabaj

a pair of qubits can be in any quantum superposition of 4 states,[10]:16 and three qubits in any superposition of 8 states.

Anonymous

π in clouds.

Avnish Kabaj

How does the superpostion work

Is it like the average

1010

0000

½½½½

This can't be it

ACuriousMind

@AvnishKabaj No. It works like nothing you know from the classical world.

Semiclassical

18:19

You can have superposition in classical physics, of course, e.g. water waves

ACuriousMind

Quantum objects can be in states that have no definite value for some observables. In this case, the qubit can occupy states that have neither definitely the value 0 nor definitely the value 1, merely certain probabilites to return one or the other when you measure it.

Avnish Kabaj

@ACuriousMind does that have an impact on the accuracy

?

Anonymous

@AvnishKabaj Simple example: An electron can be in a superposition of its two spin states i.e. it's state is something like $a|\text{up}\rangle + b|\text{down}\rangle$. That's a state in itself. The closest thing which would probably be intuitive to you is: Draw the graph of $\sin(x)+\sin(2x)$. The resulting graph which gets is a linear superposition of those two graphs: $\sin(x)$ and $\sin(2x)$

Avnish Kabaj

Nevermind

Anonymous

@AvnishKabaj Hahah

Avnish Kabaj

18:21

I know nil about quantum physics so there's no point in learning to read if I don't know the alphabet

@IceInkberry :'(

Anonymous

The only thing I know of superposition is the Schroedinger's cat experiment which I don't even know.

Anonymous

I don't even know what I know.

Anonymous

I don't know anything.

Anonymous

It's meaningless to ask a question like: What is the real state of the $\sin(x)+\sin(2x)$ graph? Is it actually $\sin(x)$ or is it actually $\sin(2x)$ ?

Anonymous

Anyhow, that's just an analogy

Avnish Kabaj

18:22

I see

Seems very interesting

Making a quantum computer must be very difficult though

How do you represent all of that

Anonymous

It sure is

Semiclassical

there's a variety of challenges involved

enumaris

|$\uparrow\downarrow\rangle$

gateprep

The integral 2
1 P dV in (2.27) is not an ordinary integral. For a closed system of
fixed composition, the system’s pressure P is a function of its temperature and volume:
P P(T, V). To calculate wrev, we must evaluate the negative of
(2.28)
The integrand P(T, V) is a function of two independent variables T and V. In an ordinary
definite integral, the integrand is a function of one variable, and the value of the ordinary
definite integral b
a f(x) dx is determined once the function f and the limits a and b are

Semiclassical

a big one is to create devices which can sustain a quantum state for a long enough time, while still being able to do useful operations on it

Anonymous

18:24

@AvnishKabaj I wrote an answer about that

Anonymous

In the pie in the sky question

gateprep

Why do we need line inrgral for a tupe of integration P(T,V)dV

Semiclassical

there's been work on that, of course, but it's still in progress

gateprep

Should we have two limit values?

enumaris

maybe

gateprep

18:26

What about the line integral why path dependant this case

In single variable calculus this was not the issue

@enumaris

enumaris

In single variable calculus there is still path dependence...it just changes the sign of the integral depending on if you integrate left to right or right to left

but in 1-D that's the only degree of freedom you have

in 2-D or higher D there's more freedom in how to choose the path

gateprep

In case there is more freedom why should it be path dependant

enumaris

1-D: only options left or right. 2-D: up down left right are all options now, 3-D: up down left right, forward backward, etc...

gateprep

It should rather be path independant

enumaris

o.O

Avnish Kabaj

18:29

Is there a "father" of QC yet?

enumaris

why's that

Anonymous

David Deutsch

David Elieser Deutsch (; born 18 May 1953) is an Israeli-born British physicist at the University of Oxford. He is a Visiting Professor in the Department of Atomic and Laser Physics at the Centre for Quantum Computation (CQC) in the Clarendon Laboratory of the University of Oxford. He pioneered the field of quantum computation by formulating a description for a quantum Turing machine, as well as specifying an algorithm designed to run on a quantum computer. He is a proponent of the many-worlds interpretation of quantum mechanics. == Early life and education == Deutsch was born in Haifa in Israel...

gateprep

sorry

Semiclassical

I think Feynman was another person to glimpse that possibility

Anonymous

That's a local file...

Avnish Kabaj

18:31

@Blue nice

Anonymous

@Semiclassical Indeed. He was the one who suggested that Hamiltonian simulation could be used to replicate quantum systems

gateprep

@enumaris But then the path depandance would only mean about taking the direction of the arrows...

This has infinite number of paths between two points

Anonymous

Quantum simulator

Quantum simulators permit the study of quantum systems that are difficult to study in the laboratory and impossible to model with a supercomputer. In this instance, simulators are special purpose devices designed to provide insight about specific physics problems.A universal quantum simulator is a quantum computer proposed by Yuri Manin in 1980 and Richard Feynman in 1982. Feynman showed that a classical Turing machine would experience an exponential slowdown when simulating quantum phenomena, while his hypothetical universal quantum simulator would not. David Deutsch in 1985, took the ideas further...

Anonymous

BTW I learned just yesterday that Ed Witten was a history major. Wtf

Anonymous

Witten attended the Park School of Baltimore (class of '68), and received his Bachelor of Arts with a major in history and minor in linguistics from Brandeis University in 1971. He published articles in The New Republic and The Nation. He worked briefly for George McGovern's presidential campaign.

Witten attended the University of Wisconsin–Madison for one semester as an economics graduate student before dropping out.[2] He returned to academia, enrolling in applied mathematics at Princeton University in 1973, then shifting departments and receiving a Ph.D. in physics in 1976 under David G…

Semiclassical

18:36

yeah

witten is weird

Anonymous

He got an effing PhD in 3 years?!?!?

Anonymous

And that too without any undegrad physics degree

Anonymous

Is he even human? Lol

Semiclassical

the world wonders

enumaris

@gateprep what?

I'm having a hard time following your logic

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