The h Bar

ACuriousMind

12:02 AM

@0celo7 The Lie group is a homogeneous space for itself.

0celo7

Indeed.

Your avatar looks like Revolver Ocelot.

ACuriousMind

So what you need to know is that homogeneous spaces are complete, which I have to admit I don't know the proof of

0celo7

@ACuriousMind aha!

You can use isometries to "push" geodesics along and extend them

ACuriousMind

Ah, makes sense

I admittedly didn't think too hard about that one

0celo7

@ACuriousMind Actually, the Killing form metric makes the group into an Einstein manifold!

It's $\mathrm{Ric}=-\frac{1}{4}B$.

ACuriousMind

12:07 AM

Yep

Secret

12:19 AM

Acuriousmind I had a quantum question:
We knew that for an antisymmetric wavefunction, $\psi(x,y)=-\psi(y,x)$ How to obtain information on how the position of two particles are correlated for wavefunction of this type.

I noticed the simplest antisymmetric wavefunction $\alpha (x)\beta (y)-\alpha (y)\beta (x)$ resembles the singlet entangled state $\frac{1}{\sqrt{2}}(\lvert 01\rangle-\lvert 10\rangle)$. While for the latter, I can use the spin operator $\sigma_{A}\otimes \sigma_{B}$ to obtain the conclusion that the electron spins are always anticorrelated, what operator I can use for the $…

ACuriousMind

@Secret I don't think there's any straightforward idea of how the positions are correlated, it depends on the specific form of $\alpha$ and $\beta$.

The position operators are $x\otimes 1$ and $1\otimes y$, but, unlike spin, it's not really clear what the operator $x\otimes y$ would correspond to - multiplying positions has no straightforward physical meaning

Secret

12:53 AM

Hmm I see.

NB The context of this question arise from studying about density functional theory. Basically we tried to model a molecule by solving for its electron density. Because electrons are fermions, the molecular wavefunction must be antisymmetric. We then compute the probability distribution of this wavefunction and integrate it to find the electron density.

As you know, the antisymmetry of the wavefunction result in pauli exclusion principle for electrons with the same spin. However one thing I am trying to understand is how the probability amplitude of electron 1 and electron 2 va…

Short version: I am trying to figure out what spatial symmetry (axial, point group etc.) is being induced by the exchange antisymmetry on the two particle wavefunction, and I intially thought getting all correlations may help on that

Slereah

1:14 AM

I really hate Hawking's notation

Secret

what do they look like, are they similar to the circuit diagrams of penrose's?

Slereah

No, just

$\frac{\partial}{\partial x}$ for basis elements

Secret

lol, I am actually more used to $\partial_x$ being basis elements and $\textrm{d}x$ being (I forgot, coordinates?)

Secret

Hmm:

0

Q: Momentum in static Electric Field?

I know that a static electric field can store energy (which is proportional to $E^2$). I also know that if you have an both an electric field and a magnetic field, there is both linear and angular momentum stored in the fields (both of which can be acquired using the Poynting vector). But, my que...

It's actually an interesting question. Imagine what happens when you boost to another frame. suddenly you have a magnetic field

and we knew all electromagnetic fields has field momentum, thus logically a static field should also have momentum.

Trying to get where that momentum is pointing however is a calculation too long to be done on the back of an envelope, though

ZeroTheHero

1:37 AM

@Secret as pointed out elsewhere: ate.uni-duisburg-essen.de/data/postgraduate_lecture/…

0celo7

@heather Sorry, I got distracted.

heather

@0celo7 not a problem

0celo7

@heather Let $(X,||\cdot||)$ be a normed vector space. We say that a sequence $(x_n)\subset X$ is a Cauchy sequence if $$(\forall\epsilon>0)(\exists N\in\Bbb N):n,m\ge N\implies ||x_n-x_m||<\epsilon.$$

We say that $X$ is a Banach space if Cauchy sequences converge.

heather

what does that dot in the middle of the norm symbol mean?

0celo7

it's the placeholder for a vector

heather

1:49 AM

don't all vector spaces have to follow those rules of norms? so what does it mean to be a normed vector space?

0celo7

Like how we write $f$ to mean the function $f(x)$

$||\cdot||$ is the norm that gives $||x||$

heather

that makes sense ^

0celo7

A vector space by itself has no norm.

The norm is an additional structure.

heather

"if Cauchy sequences converge" - if they have a defined limit?

@0celo7 oh, okay.

0celo7

@Slereah What is wrong with Hawking?

@heather If $(x_n)$ is Cauchy and $X$ is a Banach space, then we can say $\lim x_n$ exists and is in $X$.

Slereah

1:51 AM

@0celo7 $\frac{\partial}{\partial t}$ used as a vector

And then $\frac{D}{\partial t}$ for derivatives

Bernardo Meurer

@0celo7 I think he has ALS

Slereah

It's not a very nice notation

0celo7

it's standard...

heather

@0celo7 that makes sense then.

My internet is cutting in and out for some reason

0celo7

@heather So a Cauchy sequence is one in which the terms get close together as $n$ gets large.

heather

1:55 AM

Okay, that makes sense as well.

0celo7

A converging sequence is one in which the terms get close to some fixed vector in the space.

That these are not the same concept is actually very difficult to see.

Indeed, one can prove that any finite-dimensional vector space with any norm is a Banach space.

So you have to go abstract spaces to see the difference.

@heather The canonical vector space is $\Bbb R$ with the norm given by the absolute value.

This is a Banach space. Let me sketch the proof.

heather

@0celo7 so a Cauchy sequence, the terms themselves get closer to each other, but with a converging sequence, the terms approach a number not necessarily in the sequence.

okay.

0celo7

One first shows that a Cauchy sequence is bounded, that is, it is contained in some sufficiently large interval. Then one uses the Bolzano-Weierstrass theorem, which says a bounded sequence has a convergent subsequence. One then shows that if a Cauchy sequence has a converging subsequence, the full sequence converges.

@heather Not a number, a vector

heather

@0celo7 a vector, excuse me.

0celo7

@BernardoMeurer quavo is so good

heather

1:59 AM

hmm. so is a sequence in $\mathbb{R}^1$ that has a limit not a converging sequence?

0celo7

Yeah, it is. That's what we mean by converging

It has a limit

@Slereah Why are you reading HE anyway?

Slereah

why not

0celo7

just wondering. What section?

Bernardo Meurer

@0celo7 BS

heather

@0celo7 but it must approach a vector, you said...?

Slereah

2:02 AM

(I'm looking at the proof that if $p \leq q$ and $p \not \ll q$ then there's a null geodesic)

0celo7

@heather Please ask the complete question

heather

@0celo7 If a converging sequence must approach a vector, how are sequences in $\mathbb{R}^1$ that have a limit converging sequences?

0celo7

Huh? The thing you're approaching is the limit

heather

yeah.

there are sequences that have a limit where the limit isn't a vector.

0celo7

So I don't see your point

@heather Ah

heather

2:08 AM

how are those sequences converging sequences if they don't approach a vector?

0celo7

Suppose your $X$ is actually in some larger space $Y$

A sequence in $X$ could converge in $Y$

This is related to something very important, called the completion of $X$

we won't talk about completions

@heather Does that answer the question?

heather

sort of. are there sequences that have a limit that aren't converging sequences?

0celo7

No.

heather

okay.

and this completion thing allows the limits to become vectors and it all works out.

0celo7

2:30 AM

No, it allows for Cauchy sequences to all have limits

heather

okay...

okay.

0celo7

Look, a sequence converges means it has a limit. And the other way around too

Secret

3:52 AM

0

Q: Why can't classical computers exploit the same physical asymmetries as quantum ones?

Question: The very ability to use a quantum computer to achieve acceleration over classical ones sounds to me like being able to choose one's optimal universe, which seems to fly in the face of what quantum mechanics is all about. Supposedly this works because not all outcomes are created with e...

quantum-mechanics computer

Well, you cannot really beat the fact that superposition computes all possibilities at the same time, right?

Secret

4:23 AM

Quantum question. Consider $S=\langle i\rvert\frac{1}{r_{ij}} \lvert j\rangle$. Since $\langle i \lvert j\rangle\langle j\rvert i\rangle$ which gives the probability of going from state $\lvert i\rangle$ to state $\lvert j\rangle$, is it correct to interpret $SS*=|S|^2$ as the expectation value of the electrostatic repulsion contributed by the nonzero probability of going from i to j or j to i?

Put it simply, given an observable $\hat{O}$ does $|\langle i\rvert \hat{O} \lvert j\rangle|^2$ have any physical interpretation?

Secret

5:55 AM

Another related question is to consider: $\int |\langle i\rvert\hat{H} \lvert j\rangle|^2 dj =\int \langle i\rvert\hat{H} \lvert j\rangle\langle j\rvert\hat{H} \lvert i\rangle dj=\langle i\rvert\hat{H} \hat{H} \lvert i\rangle$

The result looks like the expectation value of $\hat{H}\hat{H}$ for the ith state. However, while yes the hamitonian is hermitian and it commutes with itself, does powers of hamitonians have any physical meaning?

user228700

6:14 AM

@JohnR: Morning :-) I bought that shirt!!

John Rennie

Shirt?

Ah, I vaguely remember shirts being discussed yesterday ...

yesterday, by Kaumudi. H

user228700

@JohnRennie Yes! :-D I convinced my father and ordered it just now yaay!

John Rennie

It would probably mean more to me if I had ever read CITR :-)

user228700

Yes :-P The protagonist has a habit of calling literally everyone he meets a phony.

John Rennie

I had an idea that he used the term phony as an attack on authority. That is, he labels authority figures he doesn't respect as phony.

But then, I've never read the book.

user228700

6:27 AM

That is also a great way to put it. He has a vehement loathing toward all authority figures. But also, yeah, no, everyone :-P Except like, 2 nuns and his sweet sister.

user228700

The idea is that the shirt will draw in not only nerdfighters but also literature nerds so that's a nice way to make friends in college! :-)

user228700

('cause everyone is a science nerd anyway :-P)

2017

@Kaumudi.H And it will repel away literature anti-nerds like me :P

user228700

@2017 Repel? Surely not repel...

John Rennie

@Kaumudi.H Hmm, we have a few such people in this chat room :-)

user228700

6:31 AM

@JohnRennie :-P Uh huh.

2017

I've no idea who Holden C...is :-D just now google searched...

"phony" is a new word to add to my vocab :D

user228700

Lol, OK.

John Rennie

I've got a few special tee shirts, e.g. from band tours, but I daren't wear them because then they'll need washing and tee shirts always suffer when washed ...

I've got a Stack Exchange tee shirt that is still sitting in a drawer unworn.

user228700

@JohnRennie I have no T-shirts from bands, etc :'-( This one will be my first T-shirt that references a community!

user228700

It shrinks and all, u mean?

John Rennie

6:39 AM

Yes, and the fabric goes slightly fluffy and the picture/writing on the tee shirt peels, etc, etc

user228700

Yeeah :-/ I'll be sure to wear it only once in awhile.

John Rennie

With my regular tee shirts that's not a problem because they're just plain Fruit of the Loom tee shirts, but with a special tee shirt it's a real pain.

user228700

@JohnRennie U wear T-shirts? I've always only imagined you wearing whatever fancy clothes u're wearing in ur avatar :-P

John Rennie

I'm afraid I'm a real slob when at home. At the moment I'm wearing a tee shirt, sweat shirt and jogging bottoms. I only dress up for special occasions.

But then, unless the CIA have hacked my webcam there is no-one around to see me. And if the CIA have hacked my webcam then, well, they deserve everything they see! :-) I imagine they have counselling services available.

user228700

7:49 AM

@JohnRennie x'D Wow, I see.

user228700

(Sorry about that; my mom needed my services urgently :-P)

user228700

@JohnRennie It's just that I guess I expect middle-aged/old people to dress up nicely all the time :-P

John Rennie

@Kaumudi.H sorry to disappoint you. Many of us middle aged men are just teenagers inside a somewhat wrinkled shell.

user228700

Aww, OK :-P I'll reconcile with this one way or another.

user228700

Hang on, so wow u don't even need to dress up for work! U can work in ur sweats! :-o

user228700

7:54 AM

Yeah, no, I guess I just imagined u wearing a formal shirt and pants, sitting real upright multitasking, with a cup of coffee to hand :-P

user228700

Dammit, u've ruined my illusion about old people! :-P

John Rennie

@Kaumudi.H one of the advantages to working from home :-)

I wore suits when I worked for Unilever

user228700

Wow. Man, it would seem that u have a nearly perfect life :-)

John Rennie

I'm probably as close as one can reasonably expect to being a happy man. Or at the least a contented man.

user228700

:-)

John Rennie

8:07 AM

But I had to get middle aged to attain this state, so it's not without its drawbacks.

Jaime Gallego

10:51 AM

Ooowoooowooo exams finished

I can have social life again

Secret

Angkor wat

Danu

You look a bit like Maxime Vachier-Lagrave

Secret

11:10 AM

This is like the most weird thing I saw in maths chat

2

Kenshin

11:31 AM

@JohnRennie better than me I"m naked

Hello @Qmechanic

Qmechanic

@Kenshin : Hello.

Kenshin

Wow you're really here, the famous Qmechanic, you are an inspiration

Qmechanic

@Kenshin : Thx.

Kenshin

If you were born before Einstein, you probably would have discovered relativity and not him

What is the secret to success?

BuddyJohn

12:05 PM

@Qmechanic are you a moderator or admin or whatever of this site? I need some help moving my account to another email address.

Jim

12:29 PM

-1

A: Is it possible for one side of the universe to "meet" the other?

The universe is not expanding, as there was no Big Bang. So the idea that a discussion can be had here based on an expanding universe model is frankly absurd. Have a scientific day!

absurd: wildly unreasonable or illogical

obviously, having a discussion about anything that isn't exactly representative of reality is wildly illogical

Bernardo Meurer

Meh, I understand him

Jim

@BernardoMeurer do you mean it's clear what point he is trying to make or do you mean you see it as a reasonably valid point to make?

Bernardo Meurer

I'm saying I agree with him and that we shouldn't even discuss physics at all

ever

Jim

oh, well that's obvious. No, I totally agree with you

physics is the worst and has no place in polite conversation

Bernardo Meurer

Exactly!

Jim

12:36 PM

have a scientific day

Bernardo Meurer

You too! Have a sciency day! No physics though

skill patrol

what should we discuss?

bridges?

Licorice

unicorns & rainbows

12:37 PM

bees

Secret

@John Rennie. The light cone in GR is invariant, and timelike curves are always timelike in any coordinate system. $ds^2$ is a scalar invariant. CTCs are so for any observer. Maybe I misunderstood what you are saying. Or I'm wrong on some basic GR. Still, the questions does seem to me a little too much, it is not an easy answer. For non local tests, I'm pretty sure it would depend on the size of the universe, whether you'd ever see a CTC effect. And if he's asking from a chat, why doesn't he do some research first? And the face of Zarathustra? I can't tell if he's serious or not — Bob Bee 9 hours ago

Bernardo Meurer

@Qmechanic's true identity

Qmechanic

@BuddyJohn : I'm a mod. You can change your email on 'Edit your profile' page.

Secret

I just quoted directly: Accidental fourier transform has the burden for that piece of info

Also I did google intensively, but none of the articles I found address the temporal topology, only mainly the spatial one

Iam not sure, however how to demonstrate I have done my research for this case

BuddyJohn

@Qmechanic I've been using this computer for quite awhile. I didn't realize til I tried to log in on another computer, that I have forgotten the password and the email address for the account is an old defunct one.

So I am kind of in limbo.

Qmechanic

12:40 PM

@BuddyJohn : Then the mods can't help. You should contact the SE team.

BuddyJohn

:(

Jim

@Secret Are you talking about CTCs, which might not exist at all? Having a discussion about those is frankly absurd. Have a scientific day

BuddyJohn

ok, thanks

how do I contact the SE team?

skill patrol

Have a day of science on pi day.

2

Jim

@skillpatrol darn I was just writing something about pi day when you popped that one out

Secret

12:42 PM

@Jim Indeed, I am talking about experimental detection of a CTC that is of the scale of the universe, because if the whole universe has only one CTC, and it is of this size, then it coul possibly remain undetected for some while

BuddyJohn

ah i see, you already gave me the link. thanks

skill patrol

Should I delete @Jim

Secret

We had some preliminary discussions with Slereah and Accidentalfouriertranform about that

Jim

@Secret my comment was a joke poking fun at an answer I linked a bit earlier

@skillpatrol no sir

skill patrol

:-)

Secret

12:44 PM

Anyway, it still amazes me one of the main drive for me to be so interested in time travel is simply because I want to kill time travel loopholes, unlike most story protagonist who are trying to right something back in the past

because, at least for my life, it is so far ok, and there is nothing need to be righted

Jokingly speaking, sometimes I felt like I am that kind of person who will want to destroy the world simply to prove a trivial point

which is obviously, absurd

BuddyJohn

@secret So if you could go back in time would you immediately try to stab yourself just to make a paradox?

Jim

@Secret all you have to do is change the word "loop" in that sentence to a less polite word and then it's exactly my feelings on the matter

Secret

@BuddyJohn I don't see such necessarily. I can easily rig a more reliable grandfather paradox by having a billiard ball trapped in a time loop, and then it knock itself away to make itself inconsistent

In fact, some months ago I planned to discuss with Slereah about how could one possibly use a wormhole time machine to check what type of time travel model we are living in

BuddyJohn

if quantum mechanics is true, the billard ball thing would not be a paradox

Secret

yeah, you will just branch off to a new timeline, and it will be consistent. Though the tradeoff is that it is hard to track who is really you given there are so many paths to choose (or if you prefer Deutsch's approach, which superposition you will end up in)

One reason I don't like the branching model is because it lacks predictive power on where the time travelers will end up given an action

Jim

12:51 PM

well, local conservation of energy means that we at least know that nothing is going to spontaneously pop into a place in time. It needs to have been within the past light cone of that spacetime coordinate

unless the curvature of a ctc means energy isn't conserved in that region O.o

Secret

Well, according to Slereah, CTC do suffer a lot of field divergences. However I am not sure if the divergence also scales with the size of the CTC

and then in standfold physics website, they can easily calculate that the cauchy problem of CTC is nonunique, which means to anyone outside of it, anything can potentially pop out from it

BuddyJohn

Probably true from both sides

how would someone control the "time machine" destination?

Secret

The destination of wormhole time machine is easy (at least without quantum stuff that might cause it to collapse (maybe?). You just take a mouth, gravitationally time dilate it, then enter from the time dilate mouth ,and you can go back to as far back as when the wormhole is first converted to a time machine CTC

and the point is, the destination is determined by where you place the two mouths

Slereah

1:07 PM

CTCs have a lot of problems but energy conservation isn't one of them

John Rennie

@Kenshin :: John tries, but fails, to suppress a shudder ::

Slereah

Since (local) energy conservation is written into GR anyway

Jim

@Slereah ummmm.... that means any ctc we create doesn't have energy conservation as a problem, but it also means that gr doesn't allow for anything violating local energy conservation, which means it may not be possible to create these ctcs in the first place

Slereah

How do you think a CTC violates energy conservation, exactly?

Jim

one already there doesn't, creating one might

it's complicated and depends on how it comes into existence

Slereah

1:11 PM

I mean it's pretty trivial to show both that in GR, $\nabla_\mu T^{\mu \nu} = 0$ and that it admits acausal metrics

If you create a closed timelike curves, your metric probably doesn't have a timelike Killing vector and there's no global energy conservation

Jim

@Slereah yeah, that's true, it's local that worries me. Global isn't conserved anyway

Slereah

Well unless you're dealing with Misner space

Secret

6

Q: Time travel and entropy

I saw a post on reddit regarding immortality and how it would never be possible due to entropy. That said, assuming time travel was possible, would it not be possible to never reach this state of entropy if you could keep traveling back in time? Say for instance that you couldn't travel back in t...

entropy time-travel

Slereah

But you know

Secret

Except for constant entropy cases, anything else seemed to be a problem

Slereah

1:14 PM

By definition, $\nabla T = 0$

Since that's a Bianchi identity

It will be true for all metrics

Jim

yes, nobody would argue with that

Slereah

what problem do you have, then?

You can have particle creation on the Cauchy horizon, sure

Jim

not a problem, just a concern. I merely think we should pay more attention to how ctcs can be created and less to what happens with the ones that are already created

Slereah

They probably can't be created, either :p

Jim

it seems to me that, if anywhere, gr would more likely prevent ctcs from forming than prevent them from operating

Slereah

1:16 PM

well sure but that's pretty hard to prove

Secret

I recall I have discussed this before with Slereah. It seems GR said nothing on what mechanism can allow the changing of spacetime topology

Slereah

Though I think Kay and Wald's paper on the topic is fairly nice on why they probably can't

but 'course it's quantum business

Jim

and that's why I brought in energy conservation locally. That's hard coded into GR. So if creating ctcs means things will at first kinda pop into existence from another timeline, then forming them is gonna be a problem

Secret

I mean yes, you can have e.g. the trouser spacetime where the topology changes at a certain point in time, but no mechanism is provided on how that change occur except it is built in

Slereah

@Jim what

Jim

1:18 PM

you need to finish the question

Secret

also about CTCs, I am also curious about microscopic CTCs (which is probably something like the size of a proton), because if we can gain access to at least one, we can start doing quantum experiments with them

and I recall causality is quite well behaved for nontrivial spacetimes in the quantum realm

If a CTC is sufficiently microscopic, then the amount of exotic matter needed to make them may be manageable, and the casmir effect might be enough to supply that

(that is assuming that the amunt of exotic matter does scales with the size of the CTCs, but I have not read into the numbers yet, except maybe for van stockum dust it is still unmanegeably high even for CTCs the order of something near planck length

0celo7

What's going on

Slereah

who knows

Usual Secret ramblings

Van Stockum sounds like a bad small scale thing since it is infinitely large

0celo7

Which one is that?

Dust solution?

Slereah

The cylindrically symmetric one, yeah

heather

1:37 PM

IT'S $\pi$ DAY! =)

Happy pi day everyone! Make sure to eat some pie, or pizza pie, or something like that, and try to measure pi =)

0celo7

Nerd.

Slereah

Sorry I use natural math units

ACuriousMind

I don't like $\pi$. Pesky transcendentals, somehow everyone thinks they're better than the rest

Slereah

$\pi = e = 1$

ACuriousMind

I like my numbers integer.

Slereah

1:41 PM

$1^{i\times 1 } = -1$

0celo7

I was watching a podcast with Neil D Tyson yesterday. The fool said there are more transcendental numbers than irrational numbers.

He's so used to talking to people ignorant of math that he doesn't bother actually knowing it

Slereah

I think he meant "than algebraic numbers"

0celo7

No, he said "numbers that cannot be expressed as a ratio of integers"

ACuriousMind

@0celo7 That is sadly all too common for pop-sci :P

0celo7

Also his explanation of the fourth dimension doesn't make sense

He explained what four Riemannian dimensions would look like

ACuriousMind

1:45 PM

To be fair, finding good analogies for 4-dimensional Lorentzian spacetime is hard

obligatory xkcd

0celo7

I don't claim to have an analogy either

But I don't think it's particularly important

Slereah

Well

Just think about space in $n$ dimensions

then set $n = 4$

very simple

0celo7

He did say that it's best to "let the math guide you"

Like a reverse JD

Can't fault him for that.

@ACuriousMind going to shovel snow.

Snow is the worst

ACuriousMind

Have fun

Slereah

I think trying to visualize it is a fool's errand

wasting time on a task that can't be done

and if you finally think you've done it, you just risk deluding yourself

ACuriousMind

2:03 PM

I concur

Secret

Ok it appears I do misremembered some details. I think I am trying to refer to this folllow up calculation by some guys on Mallette's spacetime and they conclude any CTC from this spacetime is larger than the radius of the visible universe, thus suggesting malletees spacetime is not possible

Slereah

lol mallett

that crank

Secret

but what about finite length light cylinders, I cannot seemed to find anything on that, or am I compeltely in the wrong direction in my research?

Slereah

Hawking's theorem.

Can't have a compactly generated time machine without violating the WEC

Or NEC, I forget

Slereah

2:26 PM

the proof for it is pretty shit since it involves Zorn's lemma

Secret

There's a physics review D paper by Kransnikov about a type of time machine with compactly determined cauchy horizon, but unfortunately due to my poor GR background, it reads like alien manuscript to me

arxiv version attached to bypass paywall

(sorry typo)

bolbteppa

Re-express Zorn in terms of turtles (all the way down, or up) and it'll be a bit better ;)

Slereah

Suppose a partially ordered set of turtles $P$ has the property that every chain of turtles has an upper bound in $P$. Then the set $P$ contains at least one maximal turtle.

Emilio Pisanty

oh, boy oh, boy

gotta love this question

originally linked to

https://www.google.co.in/search?q=PICS+OF+THUNDER&client=opera&hs=tR2&tbm=isch&‌imgil=q1E9g4JGmZGMBM%253A%253BlsRV9qixeBASkM%253Bhttp%25253A%25252F%25252Fwww.dog‌ster.com%25252Flifestyle%25252F6-tips-for-soothing-your-dogs-fear-of-thunder&sour‌ce=iu&pf=m&fir=q1E9g4JGmZGMBM%253A%252ClsRV9qixeBASkM%252C_&usg=__qFqTMfQ0e_5V4H5u‌Kna4qtR9NsU%3D

so much to unpack there

AccidentalFourierTransform

PICS OF THUNDER

Emilio Pisanty

2:38 PM

@AccidentalFourierTransform yeah

though in that vein

physics.stackexchange.com/revisions/318648/1

AccidentalFourierTransform

> tips for soothing your dogs fear of thunder

Emilio Pisanty

@AccidentalFourierTransform dogster.com/lifestyle/…

six of them, in fact

also, gotta love the OP's profile pic

heather

my search history just isn't that interesting

unless you count when I'm working hard on a program

then it looks something like "NameError Python; Weird error list Python; dictionary syntax Python; stackoverflow; halp"

Emilio Pisanty

you'd think google would have a search trick if you googled for just "help"

or even better just "halp"

kinda like the barrel roll or the tilt

Slereah

AccidentalFourierTransform

2:47 PM

that's so 2004

Emilio Pisanty

or the good old star wars crawl

Slereah

2

heather

there should be a search trick for searching "gimme teh codez"

ACuriousMind

2:59 PM

@EmilioPisanty I did not know about that

And it only works in Chrome :/

Slereah

There's a nice trick

If you google how to make bombs, the FBI comes to your house

AccidentalFourierTransform

duh, just cover your front camera, your microphone, and your microwave

Secret

[Weird quantum questions from me coming right up]

Consider $S=\langle i\rvert\frac{1}{r_{ij}} \lvert j\rangle$. Since $\langle i \lvert j\rangle\langle j\rvert i\rangle$ which gives the probability of going from state $\lvert i\rangle$ to state $\lvert j\rangle$, is it correct to interpret $SS*=|S|^2$ as the expectation value of the electrostatic repulsion contributed by the nonzero probability of going from i to j or j to i?

Put it simply, given an observable $\hat{O}$ does squaring matrix elements $|\langle i\rvert \hat{O} \lvert j\rangle|^2$ have any physical interpretation?

Q2 is even weirder:

Consider: $\int |\langle i\rvert\hat{H} \lvert j\rangle|^2 dj =\int \langle i\rvert\hat{H} \lvert j\rangle\langle j\rvert\hat{H} \lvert i\rangle dj=\langle i\rvert\hat{H} \hat{H} \lvert i\rangle$

The result looks like the expectation value of $\hat{H}\hat{H}$ for the ith state. However, while yes the hamitonian is hermitian and it commutes with itself, does powers of hamitonians have any physical meaning?

ACuriousMind

3:23 PM

@Secret Not in general, no.

@Secret That's...just a very convoluted way to write $\langle H^2\rangle$, i.e. the expectation of the square of energy in the state $\lvert i\rangle$.

Emilio Pisanty

another one for the archives

0celo7

I need to watch Seinfeld

AccidentalFourierTransform

I have never watched it

Slereah

Seinfeld - George Hates the Jews

►

DanielSank

4:22 PM

I am listening to a talk in which the presenter is showing time domain traces of a quantum state during measurement.

Danu

Hi Daniel

DanielSank

Wave function collapse is neither instantaneous nor magic.

@Danu hi

Danu

Happen to know anything about bikes?

John Rennie

@Danu bicycles or motorbikes?

Danu

bicycles

John Rennie

4:26 PM

I have ridden bicycles. What did you want to know?

Danu

I'm having a small problem with some part and I'd like to identify what it's called.

John Rennie

Picture?

DanielSank

@Danu sure

Danu

Lemme find a good one

John Rennie

The last time I had a problem with my bicycle (a seized v-brake arm) I asked on the cycling stack exchange and they were very helpful.

Danu

4:30 PM

that horizontal bolt

John Rennie

I'm not sure it has a name. I think it's just a bolt.

2017

Stem Bolt

probably

that part is called the stem iirc

aliexpress.com/…

ebay.com/bhp/stem-bolt

Stem (bicycle part)

The stem is the component on a bicycle that connects the handlebars to the steerer tube of the bicycle fork. Sometimes called a goose neck, a stem's design belongs to either a quill or threadless system, and each system is compatible with respective headset and fork designs: Quill: the stem inserts into the steerer tube, which is threaded and does not extend above the headset. Threadless: the stem clamps around the steerer tube, which is not threaded and extends above the headset. == Quill versus threadless == Somewhat counterintuitively, the term threadless derives not from whether the stem itself...

Danu

It's not quite like what the ebay thing has

2017

@Danu You need to search for your specific model

Danu

in The Velodrome, 48 secs ago, by Danu