The h Bar

DanielSank

00:01

@0celo7 You should be nice to people. I did something nice for some dude and it turns out he works for EA. Now he's offering me arbitrary games :D

I don't even have a computer that can play them XD

Emilio Pisanty

00:26

> The comments section is not for roleplaying as deities. – AncientSwordRage♦ 13 hours ago

^ not a problem we've had, fortunately

FenderLesPaul

@DavidZ although to some extent I disagree

it's true your advisor's status in the field is really important but it's also important to be at a department with a rich physics atmosphere

both in terms of colloquia, caliber of students, number of post-docs etc.

and stuff like that tends to scale with ranking

Mike Miller

00:43

@Danu: Like I said, you'll have to teach me.

DanielSank

@FenderLesPaul In my experience, the quality/motivation/helpfulness of the other students in your PhD program matters an order of magnitude more than anything else.

Specifically, those in your lab.

This is obviously correlated with having a good PI.

But anyway, you'll spend a lot more time with the other students in your lab than with anyone else.

Physics Meta

0

Q: Contributations Lost in Time

What happens to good physic questions and answers lost by people who lost their accounts either which a way?

FenderLesPaul

01:05

@DanielSank I just like being around a variety of really smart physics students

even if they aren't in your group

DanielSank

01:52

@FenderLesPaul Sure. I'm just telling your from experience what I've found actually matters.

Let me put it like this: there are a lot of people who go to highly ranked institutions but are literally miserable because they're in a crappy group with a crappy PI and crappy students.

I would rather be not miserable, instead of being miserable.

TanMath

02:09

@DanielSank:

FenderLesPaul

@DanielSank right I get what you're saying

seems very reasonable

TanMath

@DanielSank ^

@FenderLesPaul you ruined it!

Bernard Meurer

02:33

@DanielSank Up?

TanMath

^

0celo7

03:23

@DanielSank What?

Who was I not nice to

you should give me the games, I'm nice to you

ACuriousMind

@0celo7 That...doesn't sound very nice

GBeau

@FenderLesPaul Harvard Applied Physics, Northwestern, USC, Wisconsin Madison

U Pittsburgh

0celo7

@ACuriousMind whaaaaat

GBeau

Emory

0celo7

you know what's not nice

the length of The Revenant

that's a loooooooooong movie

ACuriousMind

03:30

I take it you just watched it?

0celo7

yes

I liked it. She didn't.

Not sure Leo dying for 2 and a half hours is her thing :/

@ACuriousMind so what's wrong with "just take the maximum" as the proof of "every finite set contains its sup"?

its trivial to verify that if the maximum exists, it must be the sup

@ACuriousMind all of these proofs online use induction

ACuriousMind

@0celo7 Yeah, but how do you know it exists? It's "obvious", but you have to formally prove it once.

0celo7

@ACuriousMind My prof said along the lines of "prove it if you want"

0celo7

03:52

"Tis vain to tell, you wot full well"

Yes I fucking "wot" because I have no clue what you're talking about Mr. Wigglesworth

GBeau

06:01

@FenderLesPaul expect a small number monday/tuesday pick up on wednesday, and raining emails thursday/friday/saturday

particularly thurs/fri

DanielSank

06:12

@0celo7 I never said you're not nice. Just saying, karma works, apparently!

@BernardMeurer yeah

Secret

06:25

@DanielSank @ACuriousMind @DavidZ
http://www.feynmanlectures.caltech.edu/III_03.html

Couple of quantum conceptual questions:

1. Has there been any theoretical mechanisms being proposed by the literature that explains why indistinguishable states can interfere (besides it is an experimental fact from double slits, alpha particle, neutron diffraction etc.)?

2. (Will wrote this up in more detail and formally later) Suppose I have a double slit set up, where the hole detectors and the light source can be receded on the press of a button. Suppose I have increase the voltage and intensity of …

(The questions are based on that from reading the Feynman lecture notes, I understood that if two states are indistinguishable, they can interfere, but they cannot if they are different (hence distinguishable). From the double slit experiments we knew that the interference pattern disappears when the detectors are triggered. But I am wondering it is sensible to ask about
how fast does the setup knows its state (described by $|detector\rangle \otimes |x\rangle$) are being changed, thus changing the way the pattern developed (from interfering to non interfering))

@ChrisWhite I have a question about rotation mechanics

Which radii I should use for the 3rd case?

DanielSank

06:58

@Secret:

> Has there been any theoretical mechanisms being proposed by the literature that explains why indistinguishable states can interfere

I don't know what you mean by "indistinguishable states can interfere".

Secret

In the same page of that feymann lecture feymann said

"Now we would like to emphasize an important point so that you will avoid a common error. Suppose that you only want the amplitude that the electron arrives at xx, regardless of whether the photon was counted at D1D1 or D2D2. Should you add the amplitudes given in Eqs. (3.8) and (3.9)? No! You must never add amplitudes for different and distinct final states. Once the photon is accepted by one of the photon counters, we can always determine which alternative occurred if we want, without any further disturbance to the system. Each alter…

Jan 19 at 18:52, by ACuriousMind

@Secret: You're probably confused about this: When you have the detector information, then you are asking "What's the probability to detect either $\lvert x,D_1\rangle$ or $\lvert x,D_2\rangle$?" Two states, two probabilities, just add them.

Jan 19 at 18:52, by ACuriousMind

When you don't have the detector information, all you're asking is "What's the probability to detect $\lvert x \rangle$?", where now you may in the calculation of this single probability, write $\lvert x\rangle = \lvert x,D_1\rangle+\lvert x,D_2\rangle$, and when you now evaluate $\langle x \vert \text{initial state}\rangle$, this gives the added amplitudes.

and in here
http://www.feynmanlectures.caltech.edu/III_04.html

"Now suppose, however, that aa and bb are identical Bose particles. Then the process of aa going into 11 and bb going into 22 cannot be distinguished from the exchanged process in which aa goes into 22 and bb goes into 11. In this case the amplitudes for the two different processes can interfere. The total amplitude to obtain a particle in each of the two counters is"

actually feymann repeated the same thing many times that indistinguishable particles their amplitudes will interfere from Ch. 3-7 in Vol III

NB: Acuriousmind elaborated Feymann's statement in terms of state vectors instead of amplitudes, thus the principle is the same

So then the interesting question is then on whether there are mechanisms that are proposed that explains why indistinguishable states can interfere

GBeau

10:42

@FenderLesPaul I have to stop checking it...it's tearing me apart

busukxuan

hey guys my custom dark theme is ready
it's on github: https://github.com/busukxuan/Stack-Exchange-TM-Custom-Dark-Theme/blob/master/physics.css
you'll need a User CSS browser extension/add-on to use it, the extension probably even needs to inject CSS rather than just inject a link to the stylesheet, or some stuff might break and look inconsistent
if you love dark themes, please feel free to try it out :-)

the code is pretty messy though...

user116211

11:32

I'm not getting the time-reversal transformation: $t\rightarrow -t$; suppose you are seeing a stroboscopic film with instances: 10, 20, 30, 40, 50. When you rewind the film (in backward direction of time): the instances would be: 50, 40, 30, 20, 10. Now, how can it be reversal of time; as time-reversal means $t\rightarrow -t$ but here only the instances are in reversed order; where is the minus sign of the transformation? Can anyone point me that?

Secret

It's the physical process that happened that is the subject of time reversal

suppose you have some physical process that is described by some relation with the parameter t. Time reversal means you replace t with -t. If you still get the same physics, (e.g. a ball rolling to the right when t is increasing corresponds to a ball rolling to the left when t is decreasing) then you have time reversal symmetry

If your video frames is actually describing the evolution of the system as it move forward in time (where each frame (the 10 20 30 40 50) is a snapshot of the system at that moment in time, then the - sign goes into reversing the order of the video frames, such that the system in the video evolve backwards

For example a ball rolling with constant speed v such that $$S=vt$$ will become $$S=v * (-t)=-vt$$ thus on reversing t it rolls backwards

user116211

13:33

@Secret: Thanks for the reply. Okay, when the film rewinds, would the time sequence be -10,-20,-30,-40,-50 or 50,40,30,20,10 or -50,-40,-30,-20,-10? I could get the physical implication of time reversal but still having problem with $t\rightarrow -t$. So, which of the above would be the correct sequence of reverse time?

John Duffield

14:04

@0celo7 : you know what motion is, a gradual change of position. Moreover, it's empirical. I can hold my hands up a foot apart and show you the gap, the space, between them. And I can waggle my hands to show you motion. But you can't show me spacetime, because it's an abstract "mathematical space" in which there is no motion.

GBeau

14:21

@FenderLesPaul Stony Brook U seems like they're later than normal this year

Secret

@user36790 I am not very sure, I am guessing it will depend on where you set $t=0$

If you are doing a rewind directly at the 50 frame by $t\riaghtarrow -t$, then it will go like 0 -10 -20 -30 -40 -50

But (at least for something linear as indexing each frame with a time), you can always shift the position of the $t=0$ point such as adding 50 for each value of t so that it becomes 50 40 30 20 10

Since it is ultimately like a coordinate system, it does not matter where the origin is

I welcome anyone else that can point out anything I miss in this analysis

yuggib

@GBeau Are you waiting the answers for Ph.D. applications? Do they now interview people before deciding about the scholarships?

user116211

14:43

@Secret: thanks, again; didn't want to post such a dumb question at the site; but want to like to hear others' view also; so willing to post it as a question:/

Secret

The Chat Crowd generally arrived 2-3 hours later, unless it is noon in your place, you might just have t check back later

GBeau

@yuggib some do, some don't

yuggib

@GBeau ah ok I see...SB didn't in my time

(almost ten years ago)

GBeau

@yuggib pretty much everywhere doesn't, but then asks for them on a case by case basis as an exception

but seemingly more people are getting asked every year

yuggib

I see

Secret

15:07

hey guys need some help here
Sicne I don't even know the name of that general formula (i.e. can aloow things like summation, integrations etc.) that relates $r_{eff}$ to v of an arbitrary rigid body, I don't know how to answer this question myself

0

Q: Trying to understand the physical intuition on finding/deriving effective rotation radii of a rigid body

This question is inspired from the answer of this If the rolling is assumed to be without slipping, we can solve the problem by conservation of energy: $$ mg \Delta h = \frac{1}{2}mv^2 + \frac{1}{2} I \omega^2 \tag{*} \,,$$ with the relationship between $v$ and $\omega$ set by the spacing...

rotational-dynamics rotational-kinematics

tpg2114

@ChrisWhite I just had a random thought about that crazy star dimming thing. What is the potential for stars to develop really massive star-spots? Is there an upper bound on the size of star-spots for a given star?

@Secret How can the rods be unequally spaced around a sphere? I'm not sure I understand the setup

Any configuration I can think of will still end up being symmetric around the sphere for some axis, it just might not be the axis aligned vertically

Secret

You can have a track where one end is shorter than the other, then as long it is not too much, the ball will not tip over and will remain on the track

tpg2114

But any 2 points in contact with a sphere will always be symmetric about some axis of the sphere won't it? I am having a really hard time drawing something that isn't symmetric about some axis

And again, the axis of symmetry may not be aligned with the gravity vector in general

Secret

15:38

i.imgur.com/wl4J0iF.png?1

It will look something like this
(Sorry but it seems h bar takes forever for me to upload it directly)

ACuriousMind

@Secret I feel that the issue here is that Feynman talks about processes (probably because he thinks in terms of Feynman diagrams). But, formally, there is no notion of a "process" in quantum mechanics. You have an initial state $| i \rangle$ and a final state $|f\rangle$ and the time evolution operator $U$ and the probability to see the transition $i\to f$ is $| \langle i|U|f\rangle|^2$. There is no notion of a process "$|i\rangle$ goes into $|v_1\rangle$ and then into $|f\rangle$".

The idea of the path integral, however, is that you can sum up the amplitudes of "all possible ways" in which $i\to f$ "can happen" (which is basically just using that the identity can be resolved as $\sum_i|b_i\rangle\langle b_i|$ for an orthonormal basis), and this is what you try to express as "indistinguishable states can interfere".

The problem is that our thinking in terms of "processes" is not a sensible notion in the quantum theory itself - if you are not measuring $|v_1\rangle$ or $|v_2\rangle$, it makes no sense to speak of the processes $i\to v_1\to f$ and $i\to v_2\to f$ as distinct because the transition $i\to v_i$ doesn't happen if $v_i$ is not being measured.

Secret

Hmm... no wonder when I read it it so remind me of path integrals (at least the basic gist, as I have not learnt them formally yet)

But then the issue of measurement became more mysterious
If the notion of process is not defined in quantum mechanics, then how does the setup itself knows it has detected $v_i$ and thus the electrons started to pile up into the non interference pattern

I know that in the maths the difference between the case where $v_i$ is measured (i.e. the detector detects something) and not measured (i.e. the detectors failed to detect something, or fail to determine which hole the electron came from) lies in the information avaliable for us to let us work out what scenarios we are dealing with thus co…

ACuriousMind

15:55

@Secret The setup doesn't "know". The setup is different whether there is a detector present or not. It's not the same setup in the case where we can detect the $v_i$ and in the case where we can't.

There's nothing mysterious here. Adding a detector means there's the interaction between the detector and the particle that wasn't there before, which changes the time evolution operator, and hence the probability of $i\to f$.

0celo7

The setup has feelings too

Secret

hmm, in that case this brought me to the 2nd question

Based on the settings of the voltage of the electron gun and the lamp mentioned earlier (so that the stream of electrons are packed close enough to effectively build up a pattern on the screen within a time interval $t_0$), and (from previous message) that we know that slotting in the detector or out will effectively switch the setup between two different ones. Will we expect no time delay (correct to the nearest $t_0$) that the interference pattern become a non interfering one?

correction: slotting in the detector or out will effectively switch the setup between two different ones (because The setup is different whether there is a detector present or not)

ACuriousMind

@Secret Well, go ahead and do some time-dependent perturbation theory to determine that, given a characteristic function $f(t)$ with $f(t\geq t_\text{switch}) = 0$ for the detector Hamiltonian $H_\text{int}$ and full Hamiltonian $H = H_0 + f(t)H_\text{int}$. It's gonna get ugly, but it's pretty obvious that you should expect the propagation of the change to be $\sim c$ without any calculation, since there's nothing in the system to have memory of the switch.

yuggib

16:26

And the beautiful thing is that in quantum mechanics (in QFT one has to be a bit careful), every observable has a measuring process (at least theoretically) i.e. an apparatus such that the interaction with it measures the observable giving an output on the apparatus pointer, and "collapsing the system" (very bad words) to the proper subspace corresponding to the measured value.

So I don't see anything mysterious in the measuring process, as described à la von Neumann...everything behaves as it should, and it is always theoretically possible to measure an observable (even very "bad" observables such as unbounded ones and with continuous spectrum)

user54412

@tpg2114 You know, I don't know of any formal upper bound, though this would be a lot more extreme than I've ever imagined. Star spots are complex phenomena emerging from strong convection and magnetic fields. At this point, though, I'm kind of hoping the star unexpectedly goes supernova. Then we'd have no choice but to conclude supernovae are weapons used by interstellar civilization.

ACuriousMind

@yuggib The problem is that people don't introduce measurements as von Neumann measurements. Most treatments seem to just state the rule that a measured state collapses into an eigenstate, and never talk about the measurement itself or decoherence, which leads students to believe there's a mystery there.

yuggib

indeed

tpg2114

16:48

@Secret Yeah, so that's still symmetric but it's rotated

FenderLesPaul

@GBeau yeah I'm expecting people to get a flurry of emails thursday/friday

tpg2114

@ChrisWhite I guess you missed the article by some guy who said the reason we haven't found aliens yet is because they are all extinct

FenderLesPaul

it'll be quite a blizzard

Secret

@tpg2114 but isn't the weight pointing striaght down?

FenderLesPaul

@GBeau I'm hoping to be caught in the eye of the storm and not left out of it

@GBeau also stop checking dude

I stopped checking and I've been way more relaxed

still anxious but not nearly so

it's not going to do you any good to check anyways

so just relax

tpg2114

16:49

@Secret Yes, but like I said in my comments about it -- there will always be an axis of symmetry but that axis may not be aligned with gravity

FenderLesPaul

@GBeau at the end of the day your apps are already in so it is what it is, it's out of your hands so just wait it out; good things will come :)

worst case scenario you'll learn to love wherever you end up

tpg2114

The weight will be distributed onto one or the other rail based on the dot product between the gravity vector and the axis of symmetry or something like that. But given two points of contact and a sphere, one can always find a symmetric axis so the notion of an asymmetric set of rails you asked about doesn't exist

Or at the very least is unclear

ACuriousMind

@FenderLesPaul Nice to see you finally adopted reasonable fatalism :)

yuggib

@FenderLesPaul or you can always refuse and try next time if you don't want to compromise...

Secret

So in that case the $r_{eff}$ will be at an angle wrt the weight vector and thus we solve the problem as in the symmetric case?

tpg2114

16:53

@Secret That's the approach I would take. That said, I haven't done it so I can't verify

Secret

I don't even know how to verify because I cannot find the formula for the most general case, unlike how we have the moment of inertia formula as $$I=\int_V \rho r^2dr$$

you approach will clearly gave a different ans to mine, but the issue is I don't know how to check which approach is the correct one

tpg2114

@Secret The answer that agrees with both limiting cases will be the right one. So when the symmetry axis aligns with the gravity vector, you have the original formula. When it is exactly 90 degrees to the gravity vector, you just have a ball rolling down a ramp

If you have a formula that works in both those limits then it is likely to be correct. But not a guarantee

And it will almost definitely involve the dot product of the gravity vector and the symmetry axis unit vector

no_choice99

hi guys

i'm a little bit fan of @ACuriousMind , deep knowledge in so many physics fields always a pleasure to read his answers

Secret

Acuriousmind is especially good at quantum mechanics, a field I love so much but struggle so hard on it

4

no_choice99

i see

i think he's even good at engineering stuffs

Secret

17:04

other quantum guys include yuggib and danielsank (who actually work with quantum information systems)

ACuriousMind

@no_choice99 Thanks ;) (There are many people with deeper knowledge about things though)

no_choice99

yeah danieksank is another physics monstah

3

ACuriousMind

@no_choice99 Definitely not, my whole practical experience consists of obligatory lab courses, and I've never done something that would really be engineering

no_choice99

i see

yuggib

@ACuriousMind @DanielSank ahahah it surely is one of the sock-puppets of the master speaking here.... >:-D

self-promotion...

no_choice99

17:07

who is the master? :)

ACuriousMind

Psssssssst

yuggib

For the uninformed ones, I'm just kidding

tpg2114

Ohh, I forgot about the sock puppet/master post

That was a good one

ACuriousMind

@tpg2114 "the"? There were like, hundred, until that user finally gave up

yuggib

the "supposed" master (obviously it is not true) was DS

and ACM was one of his supposed sock-puppets actually

tpg2114

17:09

@ACuriousMind I just considered them all one.

ACuriousMind

@yuggib Well, have you seen us in the same physical room? ;)

yuggib

@ACuriousMind of course not

ACuriousMind

So you cannot exclude there is one very sophisticated puppet master at work.

yuggib

I can't

ACuriousMind

Perhaps physics.SE has only a dozen of users, all thinking they're the sole puppet master

yuggib

17:11

In theory, on the beginning of february me and @Danu could be on the same room

(but probably he won't attend the seminar)

ACuriousMind

What are you talking about?

yuggib

I am giving a seminar at LMU

ACuriousMind

I meant what is the topic of the seminar...I see "what are you talking about" wasn't the best way to say that :D

yuggib

:D

Bohr's correspondence principle for the Nelson model

that (or a small variation of it) should be the title

ACuriousMind

I know one of those two things ;)

Secret

17:14

@tpg2114

yuggib

but maybe not in its full mathematical majesty

Secret

ACuriousMind

@yuggib I know the correspondence principle as a vague statement, and I have no idea what the Nelson model is

Secret

The limiting cases seemed to agree, it seems we need something more stringent to test them apart

ACuriousMind

Searching for "Nelson model physics" gives surprisingly useless results :P

yuggib

17:16

@ACuriousMind yeah, I figured that out...Nelson model $\simeq$ non-relativistic quantum particles in a phonon/KG field

with linear coupling

because it is a name used mostly by mathematicians

the model was not invented by Nelson, but introduced by him to the mathematical community

(in the paper where he proves that it has a non-perturbative renormalization)

ACuriousMind

I see. Having non-relativistic particles and a quantum field in the same theory seems a bit strange to me, though

0celo7

@ACuriousMind Your definition of dimension was a nerd trap

yuggib

@ACuriousMind that happens often, e.g. in condensed matter or quantum optics and the alike

ACuriousMind

Ah, that explains why I haven't seen that before

Never did QO, and the only condensed matter course I had was the very basic obligatory one

yuggib

originally, it was used also for nuclear physics

e.g. nucleons interacting with a meson field

of course the nucleons are only approximatively non-relativistic

ACuriousMind

17:29

@0celo7 lol, what happened?

@yuggib Ew, filthy effective theories

yuggib

:-D

at least it is possible to define a renormalized self-adjoint dynamics in (3+1)d

the rest is just physical taste, and I am a mathematician now...

0celo7

@ACuriousMind he came up with some crazy counterexample

ACuriousMind

@yuggib Well, that sounds indeed nice.

0celo7

basically showed that the trace need not be defined on the identity

I don't know enough functional analysis to know what exactly happened

ACuriousMind

@0celo7 For a finite-dimensional vector space?

I was aware that the identity on infinite-dimensional spaces is not trace-class

0celo7

17:32

we didn't have a notion of finite vs. infinite

in any case, he proved the thing using induction without the AoC

ACuriousMind

Hm, well, if there was "functional analysis" involved, the c.e. was probably infinite-dimensional.

0celo7

it was

he looked at the vector space $\mathcal{F}(I):=\{f:I\to\mathbb{R}\}$

we also ran out of time

damn physics lecture

ACuriousMind

Well, @yuggib had already said that my definition fails for the infinite case:

Jan 21 at 22:19, by yuggib

I don't think you can define the trace that way for $V$s that are not finite dimensional

0celo7

yes, Denzler proved that

or maybe he didn't, I can't functional analysis

ACuriousMind

Well, the physicist's "proof" is easy: We know that $1 = \mathrm{i}\hbar[x,p]$, but the trace of a commutator is zero, and the trace of the identity can't be zero, so it has to be undefined.

0celo7

17:39

lol

ACuriousMind

eh, modulo constants. I can never remember the sign of the $\mathrm{i}$

0celo7

or the power of $\hbar$ :P

ACuriousMind

Oh, yes. Might be $\hbar^{-1}$

It's 1, anyway :P

0celo7

$(\mathrm{i}\hbar)^{-1}$

ACuriousMind

Well, $\mathrm{i}^{-1}$ is just a sign

0celo7

17:41

@ACuriousMind he was talking about a nuclear theorem or something

crazy stuff

I really need to pick a mentor now

What's not clear is if this person will be the same one overseeing my undergrad thesis

ACuriousMind

What function does this mentor have then?

Don't say "mentoring me" :P

0celo7

Meet once a month for at least 30 minutes and talk about stuff

And oversee the thesis (probably)

So I can throw out all of the algebraists

They're useless

Number theory, too

@ACuriousMind the problem with the guy I originally had in mind is he WAY overestimates my abilities

wtf I can't be expected to know what my thesis will be on right now

I need someone who knows everything about GR, curvature flow, symplectic geometry, gauge theory, K theory, Riemannian geometry, PDE, functional analysis, algebraic topology

that probably captures everything I could write the thing on

oh also differential topology and measure theory

ACuriousMind

@0celo7 I thought the mentor need not supervise your thesis, so why would you decide now?

0celo7

I don't know if they do or not!

I'm meeting with the director tomorrow, he'll tell me

ACuriousMind

@0celo7 that...seems like really relevant information that should be easily accessible

0celo7

17:54

Dude I can't even Google

What makes you think I can look that up??

ACuriousMind

Hm, good point

0celo7

> In addition, every student has a faculty mentor and eventually a thesis advisor.

Ok

FenderLesPaul

@ACuriousMind it's the 5 stages of grief haha

yuggib

so this means the mentor can be whoever

FenderLesPaul

anxiety -> acceptance

ACuriousMind

17:57

@0celo7 See, you can do it!

GBeau

@FenderLesPaul Wisconsin Madison sending out a bunch today

>.>

FenderLesPaul

dude stop checking lol

0celo7

@yuggib no, they should be someone that knows about stuff I enjoy reading about

FenderLesPaul

just wait until the end of the week

did you apply to WM?

GBeau

No

0celo7

17:59

@yuggib also I was told not to ask postdocs to do it

yuggib

yes, but not necessarily someone who would supervise your thesis; so it's not so important

0celo7

well I don't want to get one of these algebraic geometers and start bugging them about Riemannian geometry

yuggib

@0celo7 that seems reasonable...postdocs are not yet able to mentor

FenderLesPaul

@GBeau I hope there aren't any new UMich admits

Secret

18:17

for some reason, this expression reminds me of the transition moment integral...

Secret

18:30

Hmm, an iterative procedure, no wonder they said it is nasty...
Guess I should boot my matlab or mathematica to crunch through it

0celo7

@yuggib and I wouldn't mind learning some category theory...I don't think anyone who knows category theory also knows geometry, twistor theory, etc

So really I need my mentor to be Urs Schreiber

@ACuriousMind ::sigh::

@DanielSank you should pick a mentor for me

DanielSank

@0celo7 k what are my options?

0celo7

also I wrote a few lines on the TeX file, you should take a look overleaf.com/read/rkwznvjqqwwp

@DanielSank math.utk.edu/Research/research.html

from "motivation" to where I start on the tangent space and then the "some review" near the bottom

DanielSank

Ok lemme handle a small crisis in the lab and then I will sit down and read the whole thing again.

Looking forward to it.

0celo7

ok thanks

@ACuriousMind German detected. It's "a hundred."

FenderLesPaul

19:00

what the fuck

you wrote up stuff on index notation?

:p

0celo7

@FenderLesPaul it's not nearly finished

it will be very good once it's done

in 20 years

DanielSank

19:38

Yo @0celo7 I'm reading your thing now.

fyi

El'endia Starman

I woke up thinking about electricity and motors, and soon identified a confusion I had. Photons are the carriers of the electromagnetic force, but I've seen/heard that electrons are excitations of the electromagnetic field (e.g. MinutePhysics) --

DanielSank

@El'endiaStarman That's incorrect.

El'endia Starman

Upon watching that video, he actually says that electrons are excitations of the quantum field, not the electromagnetic field. Confusion (mostly) gone. :P

DanielSank

Electrons are not excitations of the EM field.

Photons are excitations of the EM field.

Electrons are excitations of the electron field.

FenderLesPaul

@El'endiaStarman EM field is a quantum field

DanielSank

19:42

@FenderLesPaul Indeed. All quantum fields are quantum fields :D

El'endia Starman

Ha, okay. Why does it make sense to talk about an "electron field", though?

Are these fields different representations/manifestations of the underlying most-fundamental field (quantum?)?

DanielSank

@El'endiaStarman There is no single thing called "the quantum field".

Anyone who says "the quantum field" is guaranteed to be not a physicist.

FenderLesPaul

@DanielSank :D

DanielSank

There are many types of things in the Universe: electrons, photons, etc.

Mike Miller

@DanielSank: But you just did!

FenderLesPaul

19:44

I am the quantum

I am the field

I am

DanielSank

@MikeMiller ಠ_ಠ

Anyway....

@El'endiaStarman Each of these things was once thought of as little balls flying around.

It's a pretty good picture, but in the end it fails.

We find that these things have two peculiar properties:

1) they exhibit interference

2) They are indistinguishable in a very deep sense.

Item 2 is rather important and very often misunderstood.

El'endia Starman

Indistinguishible between electrons, say? (So only between particles of the same kind.)

DanielSank

@El'endiaStarman Correct.

@El'endiaStarman There was a question on the site about the relation between fields and particles a while back, and actually, I wrote what I thought was a good answer:

40

Q: Which is more fundamental, Fields or Particles?

I hope that I am using appropriate terminology. My confusion about quantum theory (beyond my obvious unfamiliarity with its terminology) is basically twofold: I lack an adequate understanding of how the mathematics of quantum theory is supposed to correspond to phenomena in the physical world ...

quantum-mechanics quantum-field-theory soft-question particles

Mike Miller

I hate trying to find copies of theses.

It's impossible.

DanielSank

@MikeMiller Physical copies?

Mike Miller

19:53

Any copies.

DanielSank

If a group doesn't post their students' theses on their website, I don't know how to save the world from disaster.

privetDruzia

Hello @DanielSank, @MikeMiller, @El'endiaStarman, @FenderLesPaul :)

DanielSank

@privetDruzia Hello.

FenderLesPaul

@privetDruzia Hi

Mike Miller

Remember, everything varies between disciplines. "A group posting theses on their website" barely even parses in math.

DanielSank

19:55

@privetDruzia Kak dela?

privetDruzia

xorosho a y tibya? :)

DanielSank

@MikeMiller Oh :(

@privetDruzia normalno

privetDruzia

hahaha)

DanielSank

@privetDruzia shto?

privetDruzia

nichivo

)

FenderLesPaul

19:56

In retrospect I feel like QFT would be so much better to teach from a CMT point of view

than a particle physics point of view

DanielSank

ya zanimalsya s'ryskim yazikom v'yniversitete

privetDruzia

could anybody tell me:
If is force the cause for a certain potential energy or is it the other way around?

DanielSank

^ Horrible attempt to spell Russian with English characters :(

privetDruzia

@DanielSank kryta

s okay)

DanielSank

@privetDruzia eeehhhhhh that's kind of ambiguous.

I wouldn't really say one causes the other.

privetDruzia

19:58

based on the formula $ \frac{-dU}{dx} = F(x)$

DanielSank

@privetDruzia Yeah, in a sense that's a definition of potential energy.

But you can take different points of view.

@privetDruzia (Ti Rysskui?)

privetDruzia

so is my point of vieuw, based on this formula, incorrect?

niet prosto tam rabotal i zhil

DanielSank

@privetDruzia ehhhhh, I dunno. That equation has an equals sign, right? Equality isn't really directional. It's more saying that those things are "the same".

privetDruzia

ya v evrope zshivy

mhm yes right

but

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