The h Bar

DanielSank

1:17 AM

@Danu ::bows::

Kyle Kanos

Bow....which type?

ACuriousMind

@KyleKanos I...don't think the verb is ambiguous, is it?

Although you could construe "::bows::" as the plural of bow, granted.

Kyle Kanos

@ACuriousMind I was trying to be funny

Apparently my humour hath failedeth me :(

Also: getting the first figure was hard because the 2nd and 3rd images were appearing in the search

I had to type bowing to get it

ACuriousMind

1:33 AM

Well, I know you were trying to be funny, I was trying to uphold the image of the pedantic German. ;)

In the spirit of: "How many Germans does it take to change a lightbulb? One. We're efficient and don't have a sense of humor."

Kyle Kanos

That's funny

Hadn't heard that one before

I had my bank interviews today.

ACuriousMind

And? How did they go?

Kyle Kanos

It was suggested (by a mathematician) that my model for my PhD might not be correct because time is distorted by mass

i.e., I didn't account for GR

In my supernova remnant

With a number density on the order of 10-100 particles per cubic centimeter

But that was the only, um, hiccup, I guess, in the day

ACuriousMind

Well, I have no idea about the scale here - do you think the effect the mathematician pointed out might be significant?

Kyle Kanos

Let's put it this way, your drinking water has a density of 1 g per cc; I'm talking about 1e-24 (ish) g/cc.

ACuriousMind

1:41 AM

Sooo...probably not significant? :D

Kyle Kanos

Not probably not, definitely not

I had to say that to him too

He also suggested that QM is about solving for $|\psi|^2$ and not $E$.

ACuriousMind

Well, that's not that wrong

Kyle Kanos

But at the level of undergrad & grad level courses I have had, the emphasis is more on getting E

ACuriousMind

Yeah, because it's easier getting $E$ than the wave function of states which aren't energy eigenstates

$\lvert\psi\rvert^2$ isn't the whole story, either, though

Only $\psi$ itself or the density matrix $\rho$ or some equivalent really have all the information we can have about a system

Kyle Kanos

He made that statement in the context that "probability theory is a requirement for physics because you need it to study quantum mechanics"

ACuriousMind

1:45 AM

@KyleKanos ...he obviously never has taken a physics course on QM, then

Kyle Kanos

I don't think he's ever taken a GR course either

Or a hydrodynamics course

He also made the statement that he didn't understand what was so difficult about doing my research in 2 dimensions (because no one else is doing it in 2d)

ACuriousMind

Seems like such a nice guy. Will you have to work with him?

Kyle Kanos

Yep. He'll be a senior quantitative analyst whom I would have to confer with every now & then

Stan Shunpike

2:20 AM

@ACuriousMind Question: So let $|\psi\rangle = \alpha |\uparrow \rangle +\beta |\downarrow \rangle$. If I apply $$\begin{bmatrix}0&1\\
1&0\\\end{bmatrix}$$ to $|\psi\rangle$ does this act as the NOT operation on the quantum state? I know if $|\psi\rangle$ is an eigen state, then it does. But I was wondering if that still applies when we have a superposition.

Note: $|\alpha|^2 +|\beta|^2 =1$

ACuriousMind

@StanShunpike You are mixing quite a lot of things. The kets are given in basis-free notation, but the operator is given as an explicit matrix - how am I supposed to know how it acts on the kets? Also, for me (or you) to say whether or not it is a "NOT operation", you'll have to define a "NOT operation" first, since that is not a general QM concept. (It is in quantum logic, probably, but I am not a quantum logic guy, and hence not familiar with their vernacular)

But, in general, if an operator acts on the basis of a state as a certain operation, then it acts on the whole of the space as that operation, since operators are defined by their action on a basis.

Stan Shunpike

2:35 AM

Okay, so I just arbitrarily defined the vectors while learning this stuff. I said

Let's suppose $$|\uparrow\rangle =\begin{bmatrix}1\\
0\\\end{bmatrix}$$ and $$|\downarrow\rangle =\begin{bmatrix}0\\
1\\\end{bmatrix}$$.

I learned in my information theory class that NOT(0)= 1 and NOT(1) = 0. And for this example, you would agree that applying this matrix has that effect.

ACuriousMind

Yes

Stan Shunpike

So what I am asking is, if I defined my vectors that way, then does that operator act as a NOT operator for any given superposition? In other words, for any given $|\psi\rangle$ does there exists a $|\phi\rangle = NOT\left(|\psi\rangle\right)$ that can described as some linear combination of the basis vectors?

or does that only work for the eigenstates where either $\alpha$ or $\beta$ equals 1?

ACuriousMind

@StanShunpike How do you define a "NOT operator"?

Also, $1$ and $0$ are not eigenstates of the NOT!

Stan Shunpike

I thought that part my be wrong

Might be

Oh your right.

That was idiotic

Wait no, I dont get it.

$A\vec{v}=\lambda\vec{v}$ is the general equation for an eigenvector of an operator

ACuriousMind

Yep. $\mathrm{NOT}(0) = 1$ means that $0$ is not an eigenstate, since the vector representing $1$ is not a multiple of the vector representing $0$.

Stan Shunpike

2:55 AM

I see your point. I can't define it. I looked it up on Wikipedia. Suppose I had an interval $S = [a,b]\in\Bbb{R}$, then NOT(S) = $\lbrace (-\infty,a), (b,\infty)\rbrace$. So that makes sense because its just about sets. But if I consider $$|\downarrow\rangle =\begin{bmatrix}0\\
1\\\end{bmatrix}$$

Then $\mathrm{NOT}(|\downarrow\rangle)$ would be a set of points not $$|\uparrow\rangle =\begin{bmatrix}1\\
0\\\end{bmatrix}$$

So now I'm confused.

And definitionless

Lol

ACuriousMind

Yeah. I guess the best or usual definition of the "quantum not" would indeed be the operator that turns $\uparrow$ into $\downarrow$ and vice versa. Since operators are defined by their action on a basis, that already completely defines the operator.

But, more importantly, unless you are doing quantum logic, I don't see why we would need a NOT at all.

Stan Shunpike

3:10 AM

Yeah, that's what I am asking about.

Because for a set $\lbrace 0,1\rbrace$, there are only two values.

But for $|\psi\rangle = \alpha |\uparrow \rangle +\beta |\downarrow \rangle$, defining a NOT operation as I did for the clasdical case would yield a range of values...

Alfred Centauri

Aha! I finally got the ChatJax

Stan Shunpike

@ACuriousMind So you are right to question my definition of NOT because it doesn't seem to make sense.

Stan Shunpike

3:34 AM

@DanielSank physics.stackexchange.com/q/172197/66165 if you get a chance, peep my question here.

DanielSank

4:43 AM

@StanShunpike I edited the TeX heavily. I suggest looking at what I did because it will save you a lot of typing throughout your life.

user54412

5:15 AM

@DanielSank Every single strawberry season here (and they grow quite well in NJ) I do the exact same thing. And then I get home and realize I have nothing resembling a food processor. So I eat non-lemonaded strawberries and drink hand-squeezed, decidedly non-strawberry lemonade.

user54412

I make this mistake every. single. year.

Stan Shunpike

5:33 AM

@ChrisWhite why don't you set up a calendar alert or something

user54412

an alert to remind me to not do something over the course of a month?

Stan Shunpike

Yeah, something like that. I mean, wouldn't that solve the problem?

@DanielSank WOW. I can't believe I've been doing it that way O.O

Thanks for the tip. SO much time saved. :: facepalm ::

user54412

call me a Luddite, but I just don't see how a calendar can do such magic

user54412

::adjusts abacus::

Stan Shunpike

The problem is you don't have a food processor wherever you are right?

user54412

5:38 AM

Yeah

user54412

wait, google calendar functions as a food processor too? ah, now I'm good to go

user54412

;)

Stan Shunpike

Caltech collaboration in action :: does victory dance::

user54412

I remember quite a number of late-night collaborations that made less sense than this one. A surprising number of students there lost all coherency past midnight, yet insisted on trying to work.

Stan Shunpike

Those are the worst. But I've gotten really good at that.

But some people can't handle it. They just become raving fools lol

Are the strawberries really good?

user54412

5:43 AM

They are really good here -- berries in general I think are better in the northeast than in California

Stan Shunpike

Huh, I wonder if it's the climate.

I wonder what factors influence food.

user54412

well, climate for anything not grown in a greenhouse

user54412

also soil quality

user54412

supposedly (not sure if I buy it) wines from parts of Pennsylvania have unusually low? sulphite concentrations due to acid rain, and as a result lead to less of a hangover

user54412

at least I've seen this used as a marketing point

Stan Shunpike

5:51 AM

Is soil quality hard to control for non greenhouse conditions? I suppose there's the quantity of light, the composition of the rain, the types of organisms that live there and affect it....I dunno what else might be a factor

user54412

and the type of rock/silt/sand that's there already

user54412

plus how you choose to fertilize it

user54412

which might be related to what you last grew there

Stan Shunpike

@ChrisWhite So what's the deal with this MathJax thing? What's happening?

user54412

I did a fair amount of gardening growing up. If I ever get a lawn again, I'd like to do more, but with a scientific approach.

Stan Shunpike

5:55 AM

@ChrisWhite people did a lot of gardening during world war ii. I listen to radio shows from that period and people grew "victory gardens" to help reduce the demand for food.

user54412

@StanShunpike What thing? Some people seem to be having problems I guess, but that's the CDN's issue. Works find for me.

Stan Shunpike

I see. Okay. I didn't understand what was going on.

What would you include in your garden?

user54412

depends where I live

user54412

tomatoes, eggplant, bell & paprika peppers I've had lots of success with in NJ

user54412

(berries are too hard to keep birds away -- you need netting, and then you need to remove dead birds from the netting)

user54412

6:01 AM

in California I suppose it would be citrus fruits and avocados, but those seem like big investments

Stan Shunpike

Do you use instruments to measure properties about the soil? or is it more low key and you just pour water on stuff casually

user54412

in the past it was somewhere in between -- pay attention to fertilizer NPK values, but don't actually test conditions

user54412

but like I said, I'd like to be more scientific

Stan Shunpike

You can be GR theorist by day and Mendel by night.

JamalS

6:32 AM

@KyleKanos What is your model?

DanielSank

6:53 AM

Protip: put your strawberries in a pot. Heat 'em up and mash 'em good. Press the soft mushy berries through a strainer, collecting the juice. Dump the juice into a pitcher with some club soda, powdered sugar, and cream. Add the pulp back in and stir.
The resulting beverage is sublime.

Stan Shunpike

@DanielSank does \ket require a package? It doesn't show up on my mobile.

DanielSank

@StanShunpike No problem. Writing a PhD thesis teaches you an incredible amount of LaTeX trickery.

@StanShunpike Look at that question of yours which I edited. Note the very first line.

@StanShunpike Why the @*$% would you define a NOT gate using set theory?

This is not Math.SE!

And @ACuriousMind, shame on you!

Set theory for a NOT gate. Now I've heard everything.

Stan Shunpike

LOL true. I felt that way. but @ACuriousMind had every right to demand a definition if he wanted one and I could not provide. DavidZ has been reminding me I must always provide definitions even when talking to experts

Oh, you will like this.

So I was explaining how Qubits work to my sister.

She's 14.

And she found the up and down, yes no, boring.

So we now use Heart and Butt as our basis vectors.

David Z

If you need some help with the math, let me know, but that should be enough to get you started! Huh? No, I don't need to read your thesis, I can imagine roughly what it says.

I can't believe nobody else picked up on that "reference". For shame. :-P

@StanShunpike it seems like you're trying to learn everything at once...

Stan Shunpike

@DavidZ well, I've now partioned my studies. Everyday I spend time now watching Susskind lectures in the way recommended by bolbteppa alarge and Danu. And the remainder of my time I read whatever I feel like. But as alarge pointed out, I do seem to have a bad habit of jumping around

David Z

7:07 AM

Oh, I just mean it seems like you were just trying to understand QFT and the standard model, and general relativity, and then now you're asking about quantum information (which is totally different)

Stan Shunpike

Well, here's why....I found qubits were a nice way to think about superpositions

And that helps me understand wave functions defined using eigenfunctions

I am also trying to teach my sister things I learn. And simple examples work better. And qubits are relatively simple. But I like to clarify points of confusion before I give her wrong info.

David Z

That's true. My recommendation, in that case, is go through quantum information (to whatever extent you want) before getting into QFT.

Stan Shunpike

Okay, that's good to know.

Should I do that before "group theory"?

David Z

Incidentally this stuff you're talking about, except for the mention of the NOT operator, is mostly basic QM. So it is good practice as a prerequisite for QFT

Yes, I'd say go through qubits etc. before fully studying group theory. Though you'll run into some little bits of group theory as you go.

Stan Shunpike

@DanielSank @DavidZ what is a good introductory book to quantum information?

David Z

7:11 AM

For example no study of introductory QM would be complete without mentioning SU(2) as it relates to particles' spin.

Nielsen & Chuang is the classic reference, and pretty well written, though that's for if you really want to get into quantum information for its own sake (and not just as an example of how to use quantum mechanics)

Stan Shunpike

Were you recommending to read something like that before the group theory or did u just mean practicing my QM? I just got Landau's QM book. It is decent. One thing I really like is his exercises have answers. So I can do the problems see where I went wrong and try again

David Z

Well, my point is, why do you want to study quantum information? Are you interested in understanding quantum computing algorithms?

Or do you see this as just a step on your road toward understanding QFT?

Stan Shunpike

I love information theory. Its useful for neuro and I am minoring in neuroscience.

But really, I just thought it was easier to understand superpositions

I don't have much desire to do Quantum IT

My interests are QFT, GR

David Z

Yeah, I was just thinking that, quantum information theory doesn't seem terribly useful for neuroscience.

N&C (or any quantum information book) is probably more detail than you need, then.

Stan Shunpike

Griffiths QM book is nice, but the math isn't really very formal. He doesn't use Hilbert spaces and he doesn't really discuss unitary operators very well. I haven't read much of Landaus book yet but I am hoping it is better in this regard.

David Z

7:18 AM

Probably is. I mean, Griffiths is unparalleled as an introductory book, but once you get beyond that level it's good to have something a little more detailed.

Stan Shunpike

Yeah, I am really impressed. He's very good at organizing his books I think.

Do you teach? I find QM is hard to explain to people. qubits is a nice way to get people used to the idea of something being not an average but a superposition.

So that was the approach I used explaining the initial idea of what a superposition is.

David Z

Not really. But I do try to explain things to people from time to time.

Stan Shunpike

GR has been much easier to learn because I have been able to assess what tools I need (eg Riemannian geometry, understanding the different metrics, etc). I picked good books, so its much more about just dedication and studying. In contrast, I still don't feel like I have a grasp on what basic tools I will need for QFT.

I like my textbook, but its more of a teaser than a textbook. My new one Ryder is much harder than QFT for the Gifted Amateur.

But QFT for the Gifted Amateur at least made me realize I needed to master tools like CM, ClassFT, gauge transformations, etc

But likely clearly there are things its weak on because your comments about group theory caught me by surprise.

David Z

You need to understand nonrelativistic quantum mechanics first. Very well.

Sabre Tooth

7:33 AM

my impression also

-5

Q: Is this about Physics, or just theoretical armchair physics?

Because when it comes to real practical advice on specialist experimental techniques I may as well be talking to myself. It seems to be singularly useless.

discussion

David Z

My recommendation (#2) is not to even think about what else you will need for QFT until you have gone through the first half of Griffiths (or equivalent), possibly more than once, and are quite comfortable with it.

Stan Shunpike

Yeah, I can tell. I have basically stopped trying to study QFT and devoted all my extra time to QM. But I am still making my list of stuff I need to know to learn QFT.

David Z

OK, but that's my point: don't bother making a list until you understand basic quantum mechanics.

I mean, you can have the list, but don't go out and look for things to put on the list.

Just, if you find out that something will be necessary for learning QFT, you can stick it on the list and then not think about it any more.

Stan Shunpike

That's what I am doing

David Z

e.g. the group theory

Stan Shunpike

7:36 AM

Yeah, exactly

David Z

OK cool

Honestly, the most important thing you need for quantum field theory is linear algebra, which is also the basis (no pun intended) of nonrelativistic QM

And in particular, you need a good understanding of how to treat differential equations using linear algebra. Which is also essential to nonrelativistic QM.

Stan Shunpike

Yeah, I noticed that too. I am using Linear Algebra by Shilov. I can't tell if its written with QM in mind or not. But he at least uses the term operator.

David Z

To be honest I learned ~~all~~ most of the linear algebra I needed from my quantum class.

pretend I used proper strikeout markup :-P

Stan Shunpike

Haha

David Z

there we go

triple hyphen

Stan Shunpike

7:44 AM

I think I need to read less and do more problems. My uncle keeps telling me you only learn math by doing proofs. I think that would help my QM. Its nice Landau provides answers. Griffiths doesn't.

David Z

That's true. Physics is very much learn-by-doing.

It's like the mental equivalent of building muscle memory. (Don't say "memory" :-P)

Stan Shunpike

Yeah, I need to make it a habit. Like set a quota. I dunno. Some subjects you can learn by reading, but I think I've reached a point where to learn more problems would be best.

I'm still figuring out my physics study habits.

I have develoed a nice one

David Z

Sometimes I think this is the main benefit of taking a class over self-studying, that it gives you homework problems and an incentive to do them

As well as an excuse to do them instead of whatever work somebody else wants you to do

Stan Shunpike

I have elective courses I can take. I think I will try to do the QM sequence. There is also a Coursera one I could try. Ordinarily, I have found books at enough, but QM seems to be very important to understand thoroughly

David Z

Yeah, I'd say so

I think it's really a function of how much practice you get, whether that comes as part of a course or from self-study

Stan Shunpike

8:00 AM

I'm good at being diligent. The last course I took on physics was kinematics. Everything else I pretty much learned on my own. But even so, I feel more exposure to QM cant hurt. Its gotta be engrained in my mind like addition and multiplication. Second nature.

Ugh, I gotta sleep. Good #TimeOfDay! And check out my new track on my profile. I'm still addicted to it. Its a constant cycle. Write a new song, listen for days on end until I'm bored of it. Repeat. Anyways, see ya later moder-ator! Thanks for the advice :D

David Z

no problem

Mew

sup peeplz

physics and mathematics is procedural memory

whereas other fields such as biology invoke declarative memory

physiologically different mechanisms are behind these two different forms of memory.

infinitesimal simplicio

8:42 AM

JamalS

Speaking of interfaces and buttons, this is an amusing talk:

Scott Meyers – The Most Important Design Guideline

►

Physics Meta

0

Q: Homework - the view from the chat session

During yesterday's Physics Chat we had an, erm, lively discussion about the evergreen subject of homework questions, and I think we reached as near a consensus as a motley crew of physics nerds is ever likely to achieve. I thought it would be worth posting (what I think is) the consensus view her...