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03:11
@Kaumudi.H So wait, why did you set your gravatar as yourself from ~13 years ago?
user228700
@SirCumference ~14. I'll turn 18 this April :-)
@Kaumudi.H Woah wait, so I'm older than you?
Dang, now I feel old D:
I turned 18 this december
user228700
@SirCumference Only by a smidge! I skipped one year of elementary school.
rob
rob
@Kaumudi.H Dang, now I feel old. I barely even remember elementary school.
user228700
Anyway, I'm not very photogenic, you see. I decided that my old picture wasn't good so I changed it this, which is a better picture, albeit, not um, representative of reality :-P
user228700
03:15
@rob :-) I don't remember too much of elementary school either. I only know from my mother that I used to be a badass--one time, my class teacher was absent and I wasn't very fond of the substitute so I refused to enter class and played in the playground all day x'D Gosh, I'm not even kidding.
Oye, in my photos I always have a creepy smile on my face...
I hate using them
user228700
@SirCumference Lol :-P I look slightly better in my old pictures.
user228700
@Sir: I'd be a freshman in college too, were it not for this exam that I decided to write again, so don't fret, you're not that old, after all :-P
@Kaumudi.H Oh, wait, you're in high school?
user228700
@SirCumference No, I graduated last year.
03:19
Oh, so you're in between graduation and starting college
user228700
Precisely.
@rob "Getting" is a bit euphemistic here
rob
rob
@BernardoMeurer "Getting" as in "getting older"?
rob
rob
@BernardoMeurer How would you say it less euphemistically?
03:22
@rob "I am old"
rob
rob
@BernardoMeurer No, the progressive tense conveys important information to me. I don't feel like I've crossed an "old threshold" and am different than some other point in my past. I feel my life stretching out, with old memories gradually getting displaced by more recent ones, and I can see how that stretching has been gradually happening over several years and will continue over several more.
There are grey hairs in my beard, but I won't get mistaken for Santa for many more years.
@rob What's a 10 or less year radius of your current age?
If "A downward electric force of 8.0 N is exerted on a -8.5 μC charge" then by $E = F/q$, shouldn't the answer be 8 / 8.5e-9? I'm getting it marked wrong on a hw site. If you guys have time, of course.
rob
rob
@BernardoMeurer Apparently I was about @Kaumudi.H's age when she was born.
So she's like 15, so you're around 30?
user228700
03:30
@BernardoMeurer I'll be 18 in 3 months.
rob
rob
Scroll up for a better estimate, but that's within the error bar you requested.
@Kaumudi.H Oops
@rob So ehhh, soon to be 36?
OR WAIT
We recently passed your 35th birthday?
rob
rob
You're suffering from spurious precision.
You asked for a ten-year window.
I know, I know
rob
rob
Assuming the middle of the window exactly is a noob mistake.
03:32
I got excited?
Oh, @rob, can I ask you some GR questions?
Ugh...you guys ever stub your toe so hard it bleeds?
rob
rob
I was thirty-one for two consecutive years. Drove my kids crazy. "You're not allowed to do that, Daddy!"
@SirCumference That sounds like a GRRRR question.
2
@rob XD ow....
user228700
@SirCumference Damn, that sucks, man :-/
You're birthday is on February 29th?
@SirCumference I have fractured my toe 5 times
03:35
@BernardoMeurer Ooff. I fractured three of my left fingers, my left wrist, and my left toe.
@SirCumference It's not a competition you know...
I RIPPED ALL THE LIGAMENTS IN MY RIGHT FOOT
LOSER
@BernardoMeurer Huh? No, I'm just relating with you...
rob
rob
@BernardoMeurer No, thirty-one was just a good year. I aged non-monotonically for a while after that. I forget how old I decided to be this year.
@BernardoMeurer Damn, you sure proved me wrong
03:35
I thought micro (1e-6) was nano (1e-9) it seems.
@SirCumference lol, you reacted before you got the joke
@BernardoMeurer XD
rob
rob
@Anon234_4521 That happens to me much more often than I care to admit.
Well I saw the "It's not a competition, you know" and then quickly replied
@Anon234_4521 Micro is tiny. Nano is super tiny.
Just remember that.
Femto is teeny
@Kaumudi.H So you know what you're going to major in?
rob
rob
@SirCumference Once I fell off a bicycle and jammed my toe on the pavement Nail turned black for a few weeks, then fell off to reveal a new one underneath.
03:40
@rob Woah...
Suddenly my toe doesn't hurt so much anymore...
user228700
@SirCumference I have heard that in foreign universities, you're allowed the liberty to change your major midway through the course, is that correct?
@Kaumudi.H Yeah
My sister changed it like 5 times
user228700
This is not allowed here. If u sign up for a degree in physics, that's all you can do. The concept of a "major" is almost non-existent here.
user228700
Anyhoo, that's besides the point.
user228700
03:43
The long and short of it is that it depends on how well I do in my exams this year.
In a lot of the schools in the U.S., you only have to declare your major at the end of sophomore year
rob
rob
@Kaumudi.H I wonder if that's a difference between a "liberal arts (and sciences)" curriculum and a "technical school" curriculum.
Most US programs have several "general education" requirements, each of which might serve as an entry point into some other major
@Kaumudi.H Well can you double major, drop majors or minor?
user228700
@rob Hmm...
user228700
@SirCumference Yes, I can do a double major.
03:45
So then can you drop one of those majors?
user228700
@SirCumference .__. What's the point of that?
I dunno, if the difficulty gets to you
I'm barely making it right now, double majoring
user228700
No, you can't drop it--you'd fail, that's all, I guess.
Ok, can you minor?
user228700
What does "minor" even mean?
03:46
Oh...
user228700
?
Yep, it's an American term...
"a college student's subsidiary subject or area of concentration" -Dictionary
user228700
And that goes on your CV?
Huh, thought it'd be a worldwide thing
@Kaumudi.H Yep
For example, I could minor in biology
I'd take some classes in it, but not really focus on it like a major
Meanwhile, I'd major in something else
user228700
It works a little bit differently here. When u join college, you sign up for just one course--"BSc. Physics", "X engineering" etc. This doesn't, of course, mean that it's the only subject we'll be studying but no, nothing else gets to go on ur C.V because the concept of a minor doesn't exist here.
03:50
@Kaumudi.H Dang. That's like a different concept altogether
In the U.S. many people just explore the classes in college before declaring their major
user228700
The structure of foreign universities has always fascinated me...
user228700
@SirCumference It sounds wonderful :-(
They often try out tons of different classes before finding what they like
Most of my friends had/have no idea what they want to major in
user228700
^ This certainly takes the pressure off high school children.
A lot of colleges also have pass/fail classes for that reason. If you just want to explore a subject, you take a pass/fail class. It's not letter-graded, but you either get by or you fail it.
That lets you know whether that subject is a good major, or if you're not good at it
@Kaumudi.H Yep. Actually, very few high school kids know what they want to major in
I just happened to know I wanted astronomy
user228700
03:53
We were all so angsty and scared during senior year of high school--not only did we need to prepare for these effed up exams, but we also needed to know exactly what to do with the rest of our lives.
@Kaumudi.H Damn...that sounds harsh
user228700
It really was. To answer your original question, I was intent on doing an integrated 5-year (BSc. + MSc.) course in physics but after what went down last summer, I changed my mind.
@Kaumudi.H The only legit test we have to take for college is the ACT or SAT (sometimes the SAT II as well)
A lot of non-rigorous colleges don't even require those
You take the SAT and ACT?
@Kaumudi.H That was my goal too, regarding getting into the best colleges.
But darn, that sounds 2x harder where you are.
user228700
Let me phrase that in a better way.
@Kaumudi.H Hmm, you have to take the JEE too?
user228700
03:59
What I meant was that there is a college in the North of India which offers a "double major" of sorts--it's a five year course in Electronics engineering and Physics. Out of all the places, that's where I'm hoping to be by August of this year.
@Kaumudi.H Five year? Most people here only go to college for four years
user228700
Same here, but this course will be long--Masters in Physics and a Bachelors in electronics engineering, don't you think it will take some time?
Oh, then 5 years sounds pretty good
user228700
Yes. That's where I want to be in August. It's incredibly difficult to get in but I'm hoping for the best.
Well, good luck
You'll probably be fine
user228700
04:02
Thanks very much :-) And yes, we have to take JEE--only if we want to tho.
@Kaumudi.H Wikipedia says "It is regarded internationally as one of the most challenging engineering admission tests."
Jeez
user228700
@SirCumference Yeah :-/
user228700
Some of the rich kids write exams like SAT, ACT and IIELTS and join foreign colleges, BTW.
@Kaumudi.H :/
Well, I need to head to sleep
'Night, and good luck
user228700
Thanks :-) 'Night.
04:41
Does anybody know why an electric field points down if you have this configuration? s28.postimg.org/vrvuxs5j1/Untitled.png
Electron is at (-1, 0). Proton is at (1, 0). Both are the same charge. Why would the E-field point down at (0, 5)?
user228700
Wonder what's up with this:
user228700
user228700
I haven't been upvoted in a terribly long time and now, bam, so many upvotes within the span of just one week.
05:00
Sounds like a serial upvote. It's going to auto-undo it all if that's the case.
rob
rob
@Anon234_4521 It's a dipole field. Consider the $x$- and $y$-components of the field from each charge separately, then use superposition.
I looked at a diagram after your post and the electric field out from the proton goes in the electron. Maybe I should just math-it-out instead of relying on intuition, since mine seems to malfunction a lot.
rob
rob
05:27
@Anon234_4521 Yes, that's what it does.
@Anon234_4521 Part of doing physics is realizing how much your intuition sucks. You have to go back and forth between mathing it out and asking yourself "why does this make sense?"
My first quarter (mechanics) put less emphasis on the conceptual aspects. I appreciate that this one is trying to nail down the concepts more than "can I do it if everything is on a slope?"
05:41
@rob So, er, do you know GR?
rob
rob
@SirCumference In a hand-waving sort of a way. What's on your mind?
@rob Cosmological confusion
rob
rob
@SirCumference Sounds like a big problem
@rob Do you know about phantom energy?
rob
rob
@SirCumference Doesn't ring a bell, but perhaps you'll remind me.
05:43
@rob Well, its equation of state $w$ is $< -1$
Whereas dark energy's equation of state is $-1$
And normal matter is $\approx 0$
@rob It's also responsible for the Big Rip
rob
rob
@SirCumference No, I haven't encountered this before
Keep going.
I thought dark matter and dark energy would be related or something.
@Anon234_4521 Two different things
Dark matter has nothing to do with dark energy
@rob Hmm....well its pressure decreases as its volume increases, and its volume increases as its pressure decreases
Whereas dark energy's pressure is constant
By the way, it also has a negative pressure, like dark energy
rob
rob
So the pressure becomes more negative as the volume increases?
@rob Yep, and the volume increases as the pressure becomes more negative
This feedback loop causes the scale factor to reach infinity in a finite time, known as the Big Rip
rob
rob
05:48
@SirCumference Should I sell my house?
@rob If you do, someone buying it would experience a Big Ripoff
OK, jokes aside :)
Ever heard of it?
He already said it was new, I think. I know I haven't researched that.
@Anon234_4521 Hmm
rob
rob
Big Rip is vaguely familiar, but from a time before I knew about equations of state for dark matter.
"phantom energy" is a new term.
I remembered "big rip" as a consequence of inflation.
or something.
OK, thanks
Oh, @JamalS, you good with cosmology?
user228700
06:37
@JohnR: Morning! :-)
Morning
user228700
@Sir: Thought you were going to bed .__.
@Kaumudi.H Can't sleep :/
@Kaumudi.H Is the avatar you as a young girl?
Decided to just study instead
user228700
06:38
@SirCumference Oh, OK :-|
user228700
@JohnRennie Yes :-) I was about 4 or 5 yo when that picture was taken.
Did you see the picture of me as a young boy?
@JohnRennie Wait, is that you in your profile pic?
user228700
@JohnRennie I think so--weren't you eating a chicken-leg or something?
user228700
@SirCumference I am starting to suspect that you may be blind .__.
06:39
@Kaumudi.H Well, uh...Slereah made me question everything
Aha you did see it then :-) It was a lamb leg.
user228700
@SirCumference Lol :-P
Howdy @SwapnilDas
user228700
@JohnRennie Ah, yes :-)
@Kaumudi.H Now I'm having an existential crisis
user228700
06:40
@SirCumference Why, what's up?
If the picture was higher quality I'd be tempted to use it as an avatar, but it has faded over the years.
@Kaumudi.H Kidding :P
Regarding the pics
Howdy @SirCumference
@SirCumference I can confirm you exist
@JohnRennie Can you?
06:41
I think therefore you are
user228700
@JohnRennie Huh, really? I don't remember it being of low quality...
@JohnRennie Poor bird...
It was a lamb
user228700
06:42
My picture is also not of good quality:
Poor lamb
That's life for lambs I guess. Oh well :-)
user228700
user228700
See?
Ah yes, with the full picture you can see how it's been damaged over the years
user228700
06:43
Exactly.
@JohnRennie I could try to un-damage that for you. I have some graphic skills and I'm bored enough
user228700
Interestingly, for a very long time after that picture was taken, my face was always changing. Then 2 years ago, I chopped off my hair and then I took another picture and bam, I looked exactly the same! It was very weird. I didn't think I'd ever look like that again but seriously, in that other picture, I look almost exactly like the person in this picture.
Sadly I now have too many wrinkles ever to look like the photos of me as a boy :-)
user228700
Aw :-(
user228700
It was probably the hair that made it obvious again. I got literally the same haircut that I have in that picture.
user228700
06:47
@JohnR: I see that u had the evil eyes even as a child :-P
I was always destined to be a supervillain/dark lord!
@JohnRennie Say, do you remind if I ask you a (probably dumb/obvious) question?
Again about cosmology...
Go ahead ...
user228700
@JohnRennie It's so sad then, that you don't even know what you're doing with your life--I see u as a saint, not as a super-villain. Seriously, get it together :-P
Well, is dark energy have an equivalent form of matter?
06:50
@SirCumference do you mean is there a form of matter with the same equation of state? If so then I don't know of any form of matter whose energy density is unaffected by expansion.
@JohnRennie Well, Slereah brought up the idea to me
Jan 8 at 13:56, by Slereah
Most forms of energy are associated to a matter field
I just need verification as to whether that applies to dark energy
By matter field I assume he means a quantum field
He was using "phantom energy" and "phantom matter" interchangeably
I don't think his statement is necessarily true.
For example zero point energy could, in principle, act in the same way as dark energy but zero point energy is a complicated property that results from the interaction of many different quantum fields.
@JohnRennie OK, might sound dumb, but can you elaborate? How is zero point energy related to dark energy?
Or rather, a particle at its zero point energy
06:55
We should start by saying that zero point energy doesn't gravitate, because if it did then its gravitational field would be so immense that it would quickly inflate the universe to nothing.
Why ZPE doesn't gravitate is one of the things we don't understand.
@JohnRennie Er, what? Why?
user228700
@JohnR: Whenever u're done discussing, say, dyou like songs like these:
So particles at their zero point energies don't gravitate?
You know that a quantum oscillator has a zero point energy (of $\tfrac{1}{2}h\nu$ ?
06:56
@Kaumudi.H Sounds like country...
@JohnRennie Sounds familiar
user228700
@SirCumference It's Ed but only kinda country, his new music. It's great.
Familiar isn't really good enough ...
@JohnRennie Well, I probably have it in my notes
Regardless, I probably have the general idea
Hmm, OK. Well in quantum field theory we write down the states of a quantum field. There is a ground state and excited states.
Ok then, this is QFT
06:58
Yes
@JohnRennie Is there any particular difference b/w QFT and QT?
@JohnRennie Right
@SwapnilDas QT?
Quantum Theory.
I guess QT is just QM or QM + QFT
06:59
Not sure if that's a formal term or just a generalization of quantum stuffs
So you need QFT in astrophysics?
@SwapnilDas Maybe?
@SirCumference When we write down the ground state of an interacting quantum field, like the electromagnetic field, we find that it has a zero point energy i.e. the ground state of the field is not zero energy.
@JohnRennie Yep
So for every cubic metre of space the quantum field(s) occupying that cubic metre have some non-zero energy due to this zero point energy.
07:01
Wait, the ground state =/= ZPE?
ZPE is the energy density of the ground state
i.e. the energy per unit volume
Yeah, got it
I'm confusing myself, it's early in the morning
I always mix up the states of the quantum field and the quantum state itself in QFT. In brief what are their difference other than one is operator valued and the other is like a vector?
Suppose we take our cubic metre and expand it to two cubic metres.
@JohnRennie Yep
07:02
The ZPE is a constant energy density per unit volume, so the energy inside the cube has doubled.
So as we expand the cube the energy inside it increases, and that's exactly what we mean by negative pressure, just like dark energy.
But ...
Epiphany
@JohnRennie ok...?
07:04
If we work out the cosmological constant that we would get from the ZPE it turns out to be $10^{129}$ times greater than the observed cosmological constant.
So first, let me see if my broken mind understands this all. A quantum field at its zero point energy will exert a negative pressure far stronger than that of dark energy?
So the conclusion is that the ZPE doesn't act like dark energy, because if it did the cosmological constant would be so big the universe would almost instantly inflate away to nothing.
@SirCumference Yes
Woah
Don't remember learning that...
So how about particles at their zero point energies in a degenerate gas? Will they act similarly to this?
@JohnRennie 86 pages...
07:07
Or Google vacuum energy cosmological constant for similar articles
OK, I need to be more awake to pull my thoughts together. Dark energy is the vacuum energy density, while ZPE is the energy density of a ground state quantum field
Both exert a negative pressure
Both exert a negative pressure - in principle yes, though ZPE doesn't seem to have a negative pressure and we don't understand why it doesn't.
Huh...
OK, now do particles at their ZPE act similarly to this?
No
@SirCumference you should be able to answer this yourself.
@JohnRennie I'm not going to even try right now
@JohnRennie Well, I'll head off. Thanks for the help, btw
07:12
@SirCumference Off to sleep? :-)
Hopefully
I haven't slept in a few days, so it might be easier
'Night (or morning)
@Kaumudi.H Ed Sheeran is a bit too mellow for me
@Kaumudi.H Nice song btw
I mostly just listen to soft rock, so that was a bit refreshing
@JohnRennie More specifically, I want to know about the difference between an interacting quantum field and the quantum state it act on?
@SwapnilDas You definitely need QM in astrophysics though
user228700
07:15
@JohnRennie I see...
user228700
@SirCumference Yep, it's nice :-)
user228700
What about this:
Well, I guess I'll head to sleep in 2 mins or so
@Kaumudi.H Listening
Btw, ever listened to Van Morison or Cat Stevens?
@Secret quantum fields don't act on quantum states. I don't understand what you're asking.
user228700
07:17
@SirCumference Nope, neither .__.
I remember Cat Stevens
Kind of like a British Simon and Garfunkle.
@JohnRennie In QFT we usually focused on the quantum fields and not so much on the quantum state. What are their difference. I seemed to mix up quantum fields with quantum states as in QFT the quantum fields seemed to play like the role of a quantum state?
user228700
@SirCumference I'll listen to it and tell u what I think by the time u wake up tomorrow.
All right
user228700
07:19
Fanfarlo?
@Secret Quantum fields have states. For a non-interacting field these are the Fock states. For interacting field we don't know what the states are because we only have a perturbative treatment.
user228700
@JohnR: Kodaline?
So what we knew as state vectors in relativistic quantum mechanics get promoted to quantum fields, or state vectors still exists in some form in QFT?
I got a bit confused on that because in the 2 scalar field scattering example I read in the lecture note, they get the fock states by acting the quantum field on the vacuum state, which is a state vector?
(Non-interacting) QFT states live in a Fock space not a Hilbert space
@Kaumudi.H I've heard of them, and I recall vaguely liking the songs I've heard on the radio.
Ah I see, I was not made explicit of this, I guess that's why I mixed them up
07:25
I think you need to take a step back and start QFT from the beginning again ...
You seem to be missing some basic concepts
yeah, I think I will grab an actual text this time instead of relying on lecture notes
@Kaumudi.H Oh wait, so do you need to know calc for engineering?
Wait, what kind of dumb question was that...
Working now. Back in half an hour or so ...
user228700
07:46
@JohnRennie OK. I like them a lot :-)
user228700
@SirCumference :-P Does it still call for an answer?
user228700
Also, seriously dude, being on the computer is not going to help you get to sleep any faster.
@Kaumudi.H Nah :P
@Kaumudi.H I'm trying to listen to relaxing music while I go to sleep
Hopefully that'll work
user228700
@SirCumference ...and yet, you're still here :-P
user228700
Also, Fanfarlo! Did u like the song?
07:48
@Kaumudi.H Well yeah, I get distracted easily :P
user228700
?
user228700
@Sir: U fell asleep, I guess :-P
@Kaumudi.H Nope...
user228700
@Sir: Have u tried listening to podcasts?
@Kaumudi.H Like what?
user228700
07:55
Good question :-P Arguably there are better alternatives but when I can't get myself to fall asleep, I usually listen to either Dear Hank & John/Ear Biscuits. Don't concentrate on the stuff they're saying--have them just be there in the background and then whatever conversation is happening sort of paints pictures in my brain and then before I know it, I'm kinda dreaming and can't remember what they're talking about and bam, I'm asleep.
user228700
Right before I fall asleep, I unplug my earphones so it stops and then I can just sleep.
user228700
I listened to this last night and it helped a lot:
user228700
Those guys have a super slow cadence. They're perfect to fall asleep to :-P
user228700
Obviously, I can't guarantee that it will work for you.

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