The h Bar

Sir Cumference

00:15

@enumaris Hmmm? ;)

6

danielunderwood

Must be a man of habit

danielunderwood

02:35

So when you want the probability density from the Schrodinger equation to be constant for a time-independent potential, you get something like $\left[ \Psi^* \nabla \Psi - \Psi \nabla \Psi^* \right]_{- \infty}^\infty = 0$ right? Is there some pathological wavefunction that satisfies this without $\Psi = \nabla \Psi = 0$?

I'm terribly sorry if $\Psi = \nabla \Psi$ offends someone

rob

03:10

@SirCumference I have a cat that I have had since 1999. My daughter has a new kitten. It is very strange to wonder whether she will still have this kitten in 2037, which seems unfathomably distant, even though 1999 sometimes feels like last year.

Sir Cumference

@rob Dude, time is weird

I'm 20. I've already lived a fourth of my life.

Things like that freak me out

rob

@SirCumference And yet yesterday I wore a tee shirt that is older than you are.

Sir Cumference

Lol

NormalsNotFar

05:25

@bolbteppa which text book are you using/did you use to learn Supersymmetry?

bolbteppa

05:36

@NormalsNotFar the question I posted (and then removed, got it!) is set up in the first 5 pages here books.google.com/…

@NormalsNotFar pdf page 9 here library01.ictp.it/exlibris/aleph/a22_2/apache_media/… too

@NormalsNotFar Basically going back to the early papers, can recommend something more modern for a first go, what book are you using

NormalsNotFar

@bolbteppa Thanks for the links, glad you solved your problem! I am using Buchbinder and Kuzenko as my main text, I am on page 127 at the moment.

bolbteppa

@NormalsNotFar That book is pretty crazy, too formal for me :p

The start is really really nice

NormalsNotFar

@bolbteppa I agree, I have found it pretty difficult. It has taken me quite a long time to prove all of the results up to page 127, but it has been very rewarding I feel. My supervisor wants my to learn from this book

bolbteppa

Section 1.6 was scary enough to put it down :(

NormalsNotFar

Section 1.6 was the most difficult section of the first chapter in my opinion, but after having done it I felt like I learned a lot. I am lucky because I my supervisor has done all of the proofs in the book, so if I get really stuck I can ask him

bolbteppa

05:50

Yeah, if you have him/her to ask questions of, that changes things

My worry with a book like this is I'd pick up an easy book and not be able to do anything after all that effort, e.g. the susy demystified book

NormalsNotFar

You don't have a supervisor/mentor?

And why would you not be able to do anything?

The main thing problem is switching between notations and conventions

for me at least

Is that textbook good?

bolbteppa

My sense was Buchbinder does things in more complicated ways than other books because it's supposedly very complete and it's confusing enough matching up conventions and getting the basics right, I'd say it's better if you get his approach

user2236

06:08

@bolbteppa do you study independently?

bolbteppa

@NormalsNotFar the susy demystified is supposed to be easy

I find it really hard to make sense of different sources approaches to the same material which always differ

It works out in the end (a long time later) :p

NormalsNotFar

I find myself thinking the same thing, I usually try to formulate the problem in the language that I feel most comfortable with and then attack the problem from there

Although sometimes its hopeless. Like when I was trying to compare Buchbinder's superanalysis to De Witt's

Their notation is so vastly different that I just gave up lol

bolbteppa

06:25

Yeah, I'd say stick religiously to that book if you're expected to know that

NormalsNotFar

Well that's the plan atm :P

have you studied QFT ?

bolbteppa

I have studied it, still trying to make sense of it :p

NormalsNotFar

Hahaha well that's more than what i've done. I will start my QFT studies very soon, I'm going to use the book by Ryder. Which did you use?

bolbteppa

I went through chunks of Ryder, Peskin and a few more

With QFT there is literally no book you can trust

I would read Amazon reviews for some qft books saying 'this is the 7 qft book I have tried', boy were they right :p

Chapter 2 of Ryder will sync up nicely with Buchbinder's stuff on spinors, other books like Peskin will wash over it, or Srednicki will make it all look a bit crazy

But then chapter 5 of Peskin (the meat of QFT) you wont find in Ryder etc...

Captain Bohemian

06:46

it's like the book of Ryder has some mistakes.

bolbteppa

Like what

Captain Bohemian

I only read the first several chapters in it, but have found some mistakes, like in a derivation it makes some mistakes in which it claims something obviously not a total derivative to be a total derivative but can still get the right result in the end. I have pointed out that problem to my advisor, who agreed what I said but was also baffled there and couldn't clarify it for me. I afterwards (after a long period, like over years) resolved it on my own, but didn't tell my advisor.

NormalsNotFar

07:05

I don't think there are many books that are without mistakes, even Buchbinders has a lot of errors in it. Half of the learning process is identifying and correcting those errors/typos :P

@bolbteppa Yeah it seems people just take there favourite sections from all of the literature available, and never really just use one text

Secret

[Random]

Winding up something:

A(BC),k(AB)C,kA(BC),kk(BC)A,kkB(CA),kkk(CA)B,kkkC(AB)

Captain Bohemian

but I found the book of Ryder contains mistakes among derivations, not just usual errors, like typos which are common in textbooks of graduate level. That let me once consider that book was written by nonexperts and may not be trusted completely.

Afterwards I still continued reading it because I didn't know other books of qft to read. Nevergtheless, I always kept dubious attitude throughout my reading it. That book was introduced to me by my collaborators who took some related course whose professor used that book.

NormalsNotFar

07:36

Well thank you for the warning, I'll keep an eye out for them!

Emilio Pisanty

09:37

@Chair Minor comment regarding this imgur.com/a/2GMbNP7

I would strongly discourage you from adding curved trend lines, unless they are a fit that you report separately

I imagine that you did this in Excel, and it is likely using a spline interpolation for the data points

that's nice for the business people, but it's bad science

user351417

Yeah, I think it's pretty useless... I can't remember why I chose that one

user351417

I imagine those curved stuff make sense when you're actually dealing with something that won't have sudden changes but would actually show smooth changes.

Emilio Pisanty

@Chair no

user351417

Really?

user351417

Then why would people use those?

user351417

09:43

I guess they are a bit prettier... but that seems like insufficient justification

Emilio Pisanty

if you want a curved line like that, then it needs to be a fit to a model that's well justified (and then it probably won't go exactly through the points themselves)

5 mins ago, by Emilio Pisanty

that's nice for the business people, but it's bad science

user351417

@EmilioPisanty Any idea why business people would do that?

John Rennie

@Chair if the scatter in the points is due to experimental or sampling error then the true curve won't pass through the points.

But a spline curve by definition passes through all the points.

Emilio Pisanty

@Chair because it looks pretty

user351417

@JohnRennie Hmmm I didn't think of that....

user351417

09:46

@EmilioPisanty Wow... I guess... priorities?

user351417

Anyways, the more important question is... why does my chart show peaks in April-August while @EmilioPisanty's shows a drop at that time?

user2236

14:30

\o @nitsua60

nitsua60

o/

Semiclassical

\o/

danielunderwood

/o\

user2236

How goes it guys?

nitsua60

We're doing calisthenics

=D

user2236

14:31

YMCA

^calisthenics to music

I think aerobics classes have gone out of fashion actually.

Semiclassical

I'd make a sarcastic objection to the M in YMCA being chauvinistic, but I suspect assuming that the C applies here is probably more unrealistic

danielunderwood

And the Y

I think aerobics may have just changed. My college had a bunch of dance and zoomba classes if you'd call those aerobics

user2236

yeah, dancers have taken over

danielunderwood

15:01

Well I just realized something silly. On the whole minimize vs extremize the action thing, I always wondered of an example where you'd maximize the action. Aside from the physical example, $L$ and $cL$ lead to the same EOM, so you could just use $c = -1$ to get trivial examples it seems

Unless there's something like an inflection point for functionals

Avnish Kabaj

15:21

@danielunderwood it's zumba

danielunderwood

Oops that's right

zoomba could be an interesting thing from the zoom bit though

vzn

@NormalsNotFar Supersymmetry Fails Test, Forcing Physics to Seek New Ideas / With the Large Hadron Collider unable to find the particles that the theory says must exist, the field of particle physics is back to its "nightmare scenario" / SciAm 2012!

Semiclassical

@vzn oh hey, Mikhail Shifman

i know that guy (albeit not well)

one point of clarification: that's not an article written by SciAm, but rather an article from Simons Science News that they've reprinted with permission

actually, looks like it was originally a Quanta article.

user2236

@AvnishKabaj "Zoomba" isn't a recognized word in English or Spanish. It may be a homonym for "Zumba," a trademarked name for an exercise program. The name seems to be an invented portmanteau word, a combination of the words "zoom" (carrying the implication of rapid movement or speed) and "rhumba" (or rumba, the Cuban dance; The name "rhumba" itself combines connotations of rum and Latin dances such as the samba).

John Rennie

@vzn in a way the failure to see supersymmetry (so far) makes things more exciting. Supersymmetry would have been a nice explanation for lots of things, an an obvious pointer towards string theory, but if it genuinely isn't there it means the universe is (even) stranger than we thought.

Semiclassical

15:34

It's also worth distinguishing SUSY as algebra from various realizations of SUSY in field theory

the former is a mathematical idea and can't actually be tested

what can be tested are various ways that SUSY could show up in nature, and is based on certain model-building assumptions

for that reason one doesn't speak of 'falsifying' SUSY but rather that one shows that certain models don't work

(and even there, the freedom of parameters means that one doesn't eliminate the model altogether but rather rules out possible regions of parameter space)

danielunderwood

15:55

Are there models of SUSY that would result in superpartners all with a higher mass than what we've been able to test?

To me it would seem more likely that the superpartners would be scattered around in mass just like what we've observed. But I don't really know much of anything about SUSY

And is there something that prevents a massless superpartner? Or would stop one from being created at current energies?

John Rennie

@danielunderwood have a read through Shifman's paper. He discusses the various possibilities.

2

danielunderwood

Awesome thanks!

John Rennie

It's rather general, but it still makes for an interesting read.

vzn

@JohnRennie lol making lemonade out of lemons™ & ps what were you just saying about fringe theories? :P

John Rennie

@vzn we clearly don't understand everything about fundamental physics so we need some new insights. My problem is when we get suggested approaches that don't seem to be motivated by anything that seems sensible and/or don't seem to further our understanding.

vzn

16:06

@JohnRennie yes its the baby + bathwater problem but notice we all have our own personal babies so to speak (& sometimes reject others babies)... there is also that expr in english writing, "kill your darlings/ babies" o_O

Captain Bohemian

what on earth is non-local gravity? What does "non-local" therein mean?

John Rennie

@CaptainBohemian context?

vzn

@CaptainBohemian there are new theories of "emergent gravity"... maybe connected?

ACuriousMind

@JohnRennie Maybe it is more boring than we thought: Maybe we've hit the bottom of the reductionism barrel, and our "effective" theories are all we get.

John Rennie

@ACuriousMind it wouldn't be the end of the world. I still don't think we have the best possible formulation of QFT so there's lots to discover there. But I have to concede it would be disappointing.

vzn

16:12

@Semiclassical there have been a lot of refs to falsifiability by the hardcore physicists in this room wrt other proposed theories (but not SUSY). your words are reasonable and yet also sound like a hedge verging on groupthink.

Captain Bohemian

@JohnRennie I have seen the term nonlocal gravity many times but have never seen there is a definition for it anywhere. So I keep guessing what it means.

John Rennie

@CaptainBohemian OK I think this stems from the fact that Einstein's equation is local i.e. the various tensors involved are functions of position on spacetime.

ACuriousMind

@CaptainBohemian Unless you give us a more specific context, it's just a theory of gravity with a non-local action.

John Rennie

A nonlocal theory would be a theory where this is no longer true.

ACuriousMind

A non-local action is basically any that can not be written as the integral of a Lagrangian that is just a function of the fields at a single spacetime point.

vzn

16:17

nonlocality is/ can be a very subtle concept eg wrt bell experiments. tightly coupled with QM entanglement... there it is a ref to "lightcones" and "actions" (cause vs effect) wrt them.

ACuriousMind

@vzn That is an entirely different notion of "action", and an entirely different notion of "non-locality".

vzn

@ACuriousMind its a different (now large) domain but the ideas interrelate.

ACuriousMind

@vzn No, they do not. The "non-locality" in the Bell context is about "spooky action at a distance", the "non-locality" in the context I was talking about is about the mathematical form of the action. Non-local actions in my sense can have perfectly local dynamics in the other sense.

Captain Bohemian

@JohnRennie so nonlocal gravity is a quantum theory of gravity?

vzn

← lol had feeling youd disagree... one of my fave exprs is "find a way to agree" not widely held around here :|

enumaris

16:26

@SirCumference hmmm

vzn

googling turns up lots of refs presumably the term has evolved a lot since its origins ~1922.
> A few theories have an action but not a Lagrangian density. A good example is Whitehead (1922), the action there is termed non-local. _Alternatives to GR_ https://en.wikipedia.org/wiki/Alternatives_to_general_relativity

John Rennie

@CaptainBohemian in gravity nonlocality can arise in two ways. From nonlocal quantum interactions like vzn suggests or from nonlocal terms in the Lagrangian like ACM suggests.

These are quite different meanings of nonlocal

ACuriousMind

@vzn My point is a purely technical one: In both contexts, "non-locality" has a precise but different definition, and one does not imply the other. That is a fact. You can either claim that I am factually wrong and provide evidence for that, or you can admit that you tried to draw a connection between two different things because they use the same word and you didn't actually know the technical definitions.

But there's no room to "agree to disagree" here, just room for one side deciding that this discussion is not worth continuing.

Avnish Kabaj

@user2236 Now that's something new which I learnt today.

ACuriousMind

:46010199 Which he copy-pasted from quora.com/What-does-zoomba-means-etymologically without attribution :P

vzn

16:30

@ACuriousMind think even the math defns likely interrelate in various ways. but maybe nobody has explored that in detail. the same word shows up in two different contexts not merely at random.

Abcd

exposed

Semiclassical

i vaguely remember that nonlocality in the sense of terms in a Lagrangian have to do with the number of derivatives involved?

(I could be entirely wrong on that, of course)

philmcole

Hello, I have two non-conducting spheres with surface charge density $\sigma_1, \sigma_2$ and in between a volume charge density $\rho(r)$. I am given that the electric field between the spheres is constant $E = E_0 e_r$ and radially outwards and that it is zero inside and outside. How can I compute the charge densities?

John Rennie

A quick one for the panel. Is paramagnetism dominated by spin angular momentum, orbital angular momentum or do both make equal contributions?

ACuriousMind

@Semiclassical Ah, right, people also use "non-local" to talk about Lagrangians that depend on an infinite number of derivatives. That's a third meaning of the term.

philmcole

16:34

I know Gauss law and for the inner charge density I would choose as Gauss surface the inner sphere. This would give me $\sigma_1 = \varepsilon_0 E_0$. But how would I go about the outer charge density?

Semiclassical

😑

2

vzn

re (non)locality in QM vs GR, googling, Nomura seems to be looking at a big(ger) picture + interrelations here. citing Bell nonlocality in the paper at least once. Quantum Mechanics, Spacetime Locality, and Gravity arxiv.org/abs/1110.4630 47p

ACuriousMind

@Semiclassical There are only so many words for so many concepts :P

Semiclassical

Fair enough

ACuriousMind

Natural language unfortunately does not force people to use unique identifiers when defining something new :P

Semiclassical

16:38

Ugh

@philmcole you won’t be able to do this problem by doing just one Gaussian sphere of fixed radius

Rather, you should consider how the charge enclosed and the electric flux vary as the radius is varied

bolbteppa

@CaptainBohemian interesting, if you remember the part of Ryder that had that would like to see it, maybe it's just standard physicist hand-waving :p

Semiclassical

@vzn full disclosure: I suspect I’m parroting Peter Woits POV as far as SUSY goes

vzn

@Semiclassical lol gotta read his book. hes one of the worlds most vocal string theory critics. think he would come down more harshly on SUSY because it seems to be connected.

danielunderwood

Hmmm no wonder I had some confusion on what locality actually meant. It seems different people mean different things

bolbteppa

susy could have been found in the 30's it's that closely related to the ideas of RQM/QFT, it will be a real shocker if it doesn't arise in some form

Semiclassical

16:42

Though for him I’d add the view that, while it is hard to falsify SUSY in an absolute sense, it is not hard to argue that it hasn’t been successful in practice

vzn

@danielunderwood it would be helpful if someone surveyed it. its an important crosscutting concept in physics. einstein redefined it with GR (euclidean space vs noneuclidean). etc ps think its not a coincidence that 2 major realms it shows up in (QM + GR) are in urgent need of unification.

philmcole

Thanks. My problem is I don't understand my material because it goes like "if we take an infitesimal sphere as Gaussian surface which contains a piece of area $dS$ inside, then we get $\Delta E_\perp \, dS = \frac{\sigma \, dS}{\varepsilon_0}$ and therefore $\sigma = \varepsilon_0 \Delta E_\perp$. For the inner sphere therefore

$\sigma_1 = \varepsilon_0 (\Delta E_0 - 0) = \varepsilon_0 \Delta E_0$

and for the outer

$\sigma_2 = \varepsilon_0 (0 - \Delta E_0) = -\varepsilon_0 \Delta E_0$"

They somehow use Gauss but I can't follow this argument. I don't understand what they take as Gauss…

Captain Bohemian

@ACuriousMind indeed I seem to have seen such an example for which "nonlocal" is used to disignate, but I have seen "nonlocal" elsewhre which seems to to mean something different based on their contexts. That' why I keep guessing what on earth it means.

Semiclassical

They’re not actually taking a Gaussian sphere, is the thing

Rather, they’re taking a Gaussian shel

philmcole

Sorry, to late to edit but the last two equations should be without the Deltas, i.e.

$\sigma_1 = \varepsilon_0 (E_0 - 0) = \varepsilon_0 E_0$

and for the outer

$\sigma_2 = \varepsilon_0 (0 - E_0) = -\varepsilon_0 E_0$"

ACuriousMind

16:50

@CaptainBohemian Many words have different meanings in different contexts. I feel this is not the first discussion we've had where you were looking for a "general" definition but there often just isn't one. Just think about how, despite all connections between the two, "the Hamiltonian" is a totally different mathematical object depending on whether we're talking about classical physics or operator QM.

Semiclassical

Ie inner radius r1 and outer radius r2

philmcole

@Semiclassical How does this work? Because wouldn't you enclose a volume charge density in between the two radii instead of a surface charge density $\sigma$?

Semiclassical

You would, yes. But if you take the thickness of the shell to be infinitesimal, then the volume charge density will be approximately constant

In which case the charge enclosed will be rho(r) * 4pi r^2 * thickness

philmcole

And then you take sigma instead of rho?

danielunderwood

@ACuriousMind but they're very closely related physical objects, correct? From the discussion of locality, it kind of sounds like the definitions of locality are physically different.

vzn

16:58

@CaptainBohemian think about it this way, if you had the exact/ correct answer youd be a 21st century einstein :P

Semiclassical

Phone ran out of battery momentarily

ACuriousMind

@danielunderwood Yes and no. The Hamiltonians are physically related but mathematically very distinct. The notions of "non-locality" in "non-local gravity" are also likely physically related (I don't know for sure because I don't know much about any of them), but rather distinct from the "non-locality" in "Bell's theorem says physics is either non-local or non-realist".

philmcole

no problem

Semiclassical

@philmcole well, the point is that the charge per unit area of such a shell would be rho(r)*thickness

However, if the charge is concentrated onto the boundary of a sphere, what happens to rho(r)?

vzn

@ACuriousMind there are some new papers that prefer contextuality to so called nonlocality in the EPRBB context...

danielunderwood

17:02

Ahh. Side question: does locality imply a finite speed of propagation of information? As in does it imply something similar to SR?

Semiclassical

Contextuality is different from nonlocality

philmcole

@Semiclassical hmm it becomes sigma? I don't know..

Semiclassical

@philmcole it’s not that it becomes sigma, it’s that the volume charge density diverges

You’d be dividing the charge by the volume of the surface, and a surface has zero volume

bolbteppa

These are still buzzwords to me

"Quantum contextuality is just another fancy word for Bohr's complementarity

Quantum contextuality is a fancy word for the fact that quantum mechanics doesn't allow you to assume that the quantities you measure objectively had (in the classical sense) the sharp values you ultimately measured before the measurement.

Because of the quantum contextuality, what the measurements reveal depends on the character of the measurements, and not just some would-be objective reality that exists independently of the measurements."

Locality is confusing

Semiclassical

However, if you assume said boundary actually has some thickness, then the volume is area*thickness

philmcole

17:05

@Semiclassical Okay I understand that. But what's used here is that over small volumes one can approximate the volume charge density $\rho$ * area * width by $\sigma$ * area * width?

@Semiclassical I think this is just a simple boundary without thinkness. They didn't specify any

bolbteppa

"The word "contextuality" is just a word, one that doesn't mean any new insight whatsoever. The word was introduced to the popular-level physics discourse by John Bell in the 1960s while he was trying to undo the quantum revolution." :p

Captain Bohemian

@ACuriousMind Ok, you are right. I know the example of Hamiltonian you refer to. I also have thought of some examples whererin the same term means different in different contexts, like superpotential, whose meaning I have learnt in GR, but I afterwards know it has different meaning in the supersymmetry context. I am fine with many terms I am familiar with being used differently in different contexts, but I just have uncertainity about the meanings of the terms I am not familiar with;

Semiclassical

Whenever I hear people talking about undoing the quantum revolution I want to shake them

Captain Bohemian

@ACuriousMind that's probably why you feel "this is not the first discussion we've had where you were looking for a "general" definition but there often just isn't one."

Semiclassical

At a practical, empirical level there’s nothing to undo

vzn

17:08

@Semiclassical lol keep in mind Bell was a big Bohm fan :P

bolbteppa

haha

Semiclassical

And treating the various interpretations as being reversions to classical physics is only possible if you classify anything other than Copenhagen as classical

mercio

hi, I wonder what happens if I do a double slit experiment where the last screen is tilted ?

vzn

maybe was thinking of this, googling, there is new research into nonlocality vs contextuality & shows theres a tradeoff, ie tightly coupled somehow. Two defining features of quantum mechanics never appear together phys.org/news/2016-03-features-quantum-mechanics.html

Semiclassical

You’ll get bars whose width is changing

mercio

17:10

is there a meaning to ask "when" is the wavefunction collapsing and how ?

bolbteppa

It's amazing how he sneaks in the entire 60's

philmcole

@Semiclassical Is this because you reduce one dimension of rho by multiplying by the thickness so it becomes a charge density per area?

Semiclassical

Pretty much.

@bolbteppa is that LM

bolbteppa

Yeah motls.blogspot.com/2014/06/…

Semiclassical

If so, he can kindly [CENSORED]

vzn

17:13

@bolbteppa (lol Lumo) Bohm + Bell are the natural "intellectual descendants/ later devlelopers" of Einstein + his contrarian ideas on QM.

Semiclassical

@mercio to be more precise: in a double slit experiment, you’ll get bars of roughly equal width in the center of the intensity pattern

danielunderwood

@Semiclassical but don't you want to hear about the revolutionary quantum leap of understanding we got with the paradigm shift of the quantum revolution

Semiclassical

If you tilt the screen, you’ll still get bars but the width will vary along the screen

actually, hmm. Am I right about that? Now I’m not sure

mercio

moving the screen around will just show you the amplitude of the wave function ?

bolbteppa

@vzn I guess it's infuriating for people to hear that that's a bad thing :p

Semiclassical

17:17

I think conflating Bohm/Bell with Einstein isn’t a great idea

vzn

@Semiclassical EPRBB

Semiclassical

If only because the conclusion Bell drew from his inequality was that nature was nonlocal, a conclusion that Einstein wouldn’t have liked

mercio

(if my name doesn't start with a B does it mean I can't be a physicist)

vzn

@Semiclassical there is a clear/ unmistakable intellectual thread with the sprinkling of individual differences

bolbteppa

Bohr Born Bohm Bell

danielunderwood

17:20

Quantum mechanics -> B-theory

Semiclassical

To call it a mere sprinkling is irritatingly reductive

vzn

lol

danielunderwood

And can't forget Beisenberg, Bhrodinger, and Bauli

bolbteppa

Ballentine, Blumenhagen, Broullian

Semiclassical

I get very tired of people who prefer their nice tidy diagrams of history rather than the full messiness of it. That’s true regardless of which ‘side’ one takes

Brillouin

bolbteppa

17:22

Yeah that dude

Bemmy Boether

Someday Bvzn

vzn

← more anti reductionism than almost anyone in physics, have blogged on it repeatedly :P

ACuriousMind

@bolbteppa Please do not post derogatory statements about people, even as a quotation.

@bolbteppa "anti-scientific delusion" is clearly derogatory. This is not up for debate.

John Rennie

Was Motl slagging Bell? That surprises me. What is controversial about Bell's work?

vzn

@JohnRennie lol oh geez the question is more who doesnt Lumo slag :P

Semiclassical

Bell was sympathetic to Bohm, and Bohm was a delusive anti-quantum communist

John Rennie

17:25

@Semiclassical :-)

Semiclassical

Ergo, Bell must also be deluded and inferior

Secret

bolbteppa

@ACuriousMind oh that quote, that's a Motl quote, clearly posted as a joke (from an article on contextuality which was under discussion) it's so ridiculous, get rid of it if you want

Semiclassical

Then why aren’t I laughing

vzn

?!? confused. the Lumo quote re Einstein vs Bell/ Bohm was not nice but its not allowed?

Captain Bohemian

17:28

@ACuriousMind I also have seen this kind of example for which "nonlocal" is used to designate. I have seen these different kinds of contexts you and vzn and Semiclassical meantioned wherein "nonlocal" is used. But I have not studied any of these contexts in detail; that's why I consider "nonlocal" have a uniform definition in all these contexts. But now thanks to your clarification, I know they mean different.

Secret

easy politics 101

(spoiler alert: Politics is NOT easy)

vzn

@CaptainBohemian (finding a way to agree with ACM...) its a highly technical concept developed relatively independently in 2 different physics realms, GR + QM, and the meanings generally havent been unified yet, but you can see hints of it in eg Nomura.

Semiclassical

The tricky thing about nonlocality in QM is that it's properly "nonlocal correlations", not "nonlocal signalling"

bolbteppa

One argument for why the position-space wave function solution of Schrodinger's equation in the case of the free Maxwell's equations does not exist in the usual sense of a coordinate space probability is because you need to invert the square root of a differential operator in momentum space, the result being 'non-local'

ACuriousMind

@CaptainBohemian It gets worse! Sometimes not even the people writing it know which definition they are using! (I kid, but only a little :P)

bolbteppa

17:34

Similar problem with simply working with the square root of $E^2 = \mathbf{p}^2 + m^2$ as a Hamiltonian I think

Captain Bohemian

@ACuriousMind really? That's why I have never seen the exact definitions of some terms laid out somewhere?

Semiclassical

My own viewpoint re: nonlocality goes something like this

ACuriousMind

@CaptainBohemian Sometimes. Often it is just because the authors are so immersed in their topic that they expect the reader to understand it as well as they do, but occasionally they will just have a vague idea with no formal definition behind it. Not even mathematicians define every word they use rigorously, and physicists are worse than mathematicians in that regard

Semiclassical

Suppose someone gives you a QM system like the Stern-Gerlach experiment. If all you're provided with is the outcomes of measurements, labelled as spin-up/down for various orientations, can you give a more detailed/complete description of what's happening?

bolbteppa

6

Q: Non-Locality of Space - QFT (Srednicki's book)

I was going through Mark Srednicki's book on QFT. It says in the relativistic limit the Schrodinger equation becomes something like : $$ i\hbar\frac{\partial}{\partial t} \psi(\vec x,t) = \sqrt{-\hbar^2c^2\nabla^2+m^2c^4}\psi(\vec x,t) $$ Now he says that if I expand the square root (say binomi...

vzn

17:39

@Semiclassical yes

Semiclassical

Actually, the answer is no, for at least one reason: Can you tell the difference, from such results alone, between a Stern-Gerlach device which is oriented upwards versus downwards?

bolbteppa

Do not appreciate/get the answer given there

Semiclassical

And you really can't. The difference between the two orientations is what label you assign to spin up vs. spin down

vzn

@Semiclassical QM says its not possible with combinations of SG devices but to rule it out entirely is a (anthropological) cognitive bias... aka Copenhagen interpretation

Semiclassical

If all you're told is the measurement results themselves, with no further details about the measurement systems, then you are not in a position to give a detailed description of what's going on

vzn

17:42

@Semiclassical if there exists 1 ("local") hidden variable theory then it disproves the assertion that none exist (even if it cant be experimentally detected).

Semiclassical

My point is that, in the scenario I’m laying out, the results alone do not completely characterize the system

vzn

@Semiclassical it only shows your "measuring system/ detectors" are limited. its the sensible conclusion. and that is not in contradiction to (most) QM ideology...

Semiclassical

Eh. What it shows, at minimum, is that you need to know the experimental context before you try to give an account of what’s going on

vzn

@Semiclassical you should take that small )( further step, take the red pill, fully embrace Bohmianism, both formally/ ideologically, it says theres a subquantum realm called the implicate order & the higher explicate order is emergent... that subquantum realm has now been measured by Vinante exactly as predicted by Adler with his ("neoclassical!") emergent theory. o_O

Semiclassical

Kindly stop interrupting with propaganda

vzn

17:48

@Semiclassical lol direct quotes from Bohm + his descendants who is interrupting who :P

Semiclassical

now, suppose you are provided such a description of your apparatus. For instance, you’re shown a configuration of magnets generating a specific field gradient

With the spin direction being inferred by which of two directions an electron can exit along

If you’re given such info, then in the dBB picture you can provide such an account in terms of trajectories

bolbteppa

"Another difficulty with quantum gravity is the absence of local observables. Heuristically, due to the possibility of making general coordinate transformations, we can move spacetime points around... Thus, quantum gravity cannot be based on local observables, but instead has to be built out of nonlocal observables. This statement provides one of the starting points of the approach to quantum gravity known as loop quantum gravity...

I am very much troubled by the thought that physics may be ultimately nonlocal, even if it is only on the cosmological scale."

From Zee's GR

Semiclassical

Cf Norsen’s paper/calculations on spin from a pilot-wave perspective

vzn

@Semiclassical maybe a better term than trajectories is flows

Semiclassical

Not according to dBB. One can construct theories which go beyond that, but for dBB proper that’s not an option

bolbteppa

17:53

"People sometimes say, rather sloppily, that in an observable $\mathcal{O}(x)$, the $x$ cannot be specified"

Novalium Company

Guys, where can I read about why does the water stay in the bucket when rotating over head? Is there a simple explanation? What is the force that pushes the water to the bucket? Or maybe there is no force, just the water is thrown in a circular motion an the bucket "rides" with it, creating the illusion?

vzn

@Semiclassical dBB died decades/ ~century before Couder-Bush experiments/ theory...

Semiclassical

Again, would you kindly desist from propaganda?

vzn

sigh, BYE

Semiclassical

In any case, my main point is that—given an experimental context—dBB is perfectly capable of giving a ‘complete’ account of any particular measurement outcome

That completeness is only in a conceptual sense, as once you make such a measurement you’re no longer on a position to make further measurements on that original account

Loong

17:58

Bob Hoover Barrel Roll

►

Semiclassical

But you can, for instance, attribute a trajectory to a given outgoing electron

bolbteppa

Why does the Water stay in this Bucket?!

►

Semiclassical

Now, that was for a single SG device

You can do the same for any particular orientation. The hard question is to what extent you can combine these accounts

The Norsen paper talks about that in more detail

(Ugh, why is my laptop freezing up so much)

Novalium Company

18:14

Is there a formula telling us what the bucket's angular velocity has to be above, in order for the water not to fall? I suppose it will depend on the radius/diameter of the rope/hand and the mass?

Semiclassical

it's easiest to think about if you replace the water with, say, a solid object of the same mass

as the bucket moves along the arc, the object is forced to move along the same circular arc

as such, it'll be undergoing centripetal acceleration a=v^2/r where v is the angular velocity

that's true just as much at the top of the arc, where that acceleration will be down (towards the center of the arc)

as such, you'll need to have the forces be such that Fnet = ma=mv^2/r

Novalium Company

Ok, but why would there be an angular acceleration? Because the bucket starts wity

h

ups sorry

Semiclassical

angular acceleration != centripetal acceleration

Novalium Company

Yes i know?

oh sorry

I misread

Semiclassical

the fact that you've got centripetal acceleration is just the fact that, near the top of the arc, the trajectory of the bucket (and therefore the object inside of the bucket) is approximately circular

(and one neglects angular acceleration in order to get a calculation which isn't horrible)

bolbteppa

18:21

@vzn apparently Bell had real issues with relativity too motls.blogspot.com/2015/05/…

Novalium Company

Ok thanks @Semiclassical. It's almost clear. I'll go review the rotational mechanics and be back to the problem. See you for now, and again, thanks :-)

bolbteppa

So calling him an 'intellectual descendant' of Einstein given him apparently having problems with SR probably needs some justification :p

Semiclassical

and yet another instance of why I don't read LM. whatever point he may have is utterly overwhelmed with polemic and irrelevant invective

Secret

Pure quantum woo. Can people just stop misunderstanding what entanglement is:

quantum entanglement =/= magic because

Semiclassical

@Secret /triggered

Secret

18:24

quantum entanglment >>>>>>>>>>> magic

bolbteppa

If you wade through the polemic, I would be surprised if he makes errors on the physics

Semiclassical

On the physics calculations, perhaps

On the interpretation of such, I basically have no confidence

Interpretation is so much a matter of understanding various perspectives. when you're reading someone for whom such concepts automatically qualify one as a deluded scoundrel (who is probably also a dirty commie)

I basically don't expect to learn anything useful.

bolbteppa

My understanding is he takes the consistent histories approach to QM, which is basically just Copenhagen but going a step further and facing up to some issues like 'wave function of the universe' which Copenhagen doesn't face up to, I'm not sure if you can justify going beyond Copenhagen but haven't really thought about it yet

Semiclassical

consistent histories is something i don't really know about either

Anonymous

A formatting question: If one defines a new term (say "xyz") in an article and the first time when it is mentioned in the article it is put in quotes, is it necessary that even during the following usages of the term in the article the term has to be put within quotes? That is, do I have to write it as "xyz" whenever I use it later on?

bolbteppa

18:29

One point, my sense is he is so vicious against LQG because he tried to learn it, even wrote a paper on it in the early days, and found it was ridiculous

Semiclassical

None so zealous as the convert.

Captain Bohemian

@vzn seems to be a good example to interrelate nonlocal quantum interaction with nonlocal gravity though I have not got a definite idea of what the paper is going to discuss by just reading its abstract.

Semiclassical

I'm forgetting. Is there a way to see what pages link to a given page on Wikipedia?

e.g. which pages have page X as a link

@bolbteppa the most I've seen re: consistent histories that I felt like I understood was this bit on the counterfactual definiteness page: en.wikipedia.org/wiki/…

Some of the calculations I did re: Bell's theorem gave me an impression (albeit very limited) of how such a story might work

vzn

@bolbteppa Lumo is a cantankerous reactionary curmudgeon uninformed on cutting edge recent developments, fixated on decades-old pseudo-dramas, aka living in the past

@CaptainBohemian are you a physics major?

Captain Bohemian

@vzn yes.

Semiclassical

18:41

my main frustration is that there's an easy way to dismiss dBB and a hard way

the easy way is to replace dBB with a strawman, argue against that, and declare victory

the hard way is to actually understand what it is---and is not---claiming, what arguments its proponents make use of, and what one can expect from it

vzn

@bolbteppa maybe this is not widely recognized/ acknowledged in the compartmentalized physics community but try to look at the bigger picture/ pov! think its really an uncontroversial assertion. can accept minor )( quibbles.

Semiclassical

once one has gone and done that, I think there are indeed good reasons to be skeptical about dBB. But there are some very silly objections out there

vzn

@CaptainBohemian what year? your questions are getting into very advanced topics at the edge of current research. locality is well understood in some ways and not in others. its a sort of open zen question.

@bolbteppa its an intellectual/ philosophical/ ideological lineage/ ~1 century long research program etc

bolbteppa

I don't think it's a strawman to do what I am doing in quoting Bohm's paper and showing he explicitly says he is trying to fundamentally bring back determinism (with randomness arising only out of necessity so you still get QM) and pointing out not only does this contradict the first claim of QM,

it seems (not 100% on this yet) like it should ban you from using things like the Schrodinger equation, but even if does let you use them, it should definitely ban you from re-interpreting a probability wave function as a real force, which it seems Bohm does and is his only way of justifying why the classical variables can be hidden

Maybe deBroglie fixes it or re-does it etc idk yet, but given all the claims about how similar dB is to B (while fixing the flaws), idk if these are flaws that can be fixed

vzn

@bolbteppa dont recall him talking much about determinism can you cf that?

Semiclassical

18:48

eh, determinism is definitely there in dBB

it's not an operational kind of determinism, i.e. you can't measure a particle in one place and make a stronger prediction than QM allows about its location a moment later

vzn

@bolbteppa its a revision but not a radical revision and to get further you have to go to the "next generation" after dBB which is already taking shape, its a stepping stone. dBB is/ will be seen as provisional/ temporary... hint: Sch eqn can be derived from fluid mechanics/ dynamics & has been done so by misc authorities...

Semiclassical

but it is deterministic in that, if you measure the location of a particle and you know what the wavefunction was up to that point, then you can deduce what trajectory it had up to then

the fact that knowing the final point (and the wavefunction) tells you about the entire prior history of the particle is definitely deterministic

by contrast, you don't hold onto that determinism in various theories that go beyond dBB

bolbteppa

@vzn re determinism: "In contrast to the usual interpretation, this alternative interpretation permits us to conceive of each individual system as being in a precisely definable state... Quantum-mechanical probabilities are regarded (like their counterparts in classical statistical mechanics) as only a practical necessity and not as a manifestation of an inherent lack of complete determination in the properties of matter at the quantum level" cqi.inf.usi.ch/qic/bohm1.pdf

In QM they are a "manifestation of an inherent lack of complete determination in the properties of matter at the quantum level"

vzn

← feel word determinism/ deterministic is highly overloaded, slippery, should be used with great care

Semiclassical

@bolbteppa The argument you seem to be aiming for is this: "Various authors derive the Schrodinger equation (conclusion Q) from the assumption that paths do not exist (premise P). Bohmians accept that the Schrodinger equation is valid. Therefore Bohmians should not think about paths as existing."

Secret

18:52

Determinisim is soooooooo overated

Semiclassical

or, in more simple form: "P implies Q, and Bohmians believe Q, therefore Bohmians should also believe P."

bolbteppa

It's literally an on or off switch, either the paths exist or they don't, they don't to Copenhagen, they do to Bohm (who then proposes reasons why we just can't measure them so we still get the results of QM)

Semiclassical

The thing is, there's a name for that kind of reasoning: It's called "affirming the consequent" and it's a basic fallacy of logic.

Even if I accept that you can derive the Schrodinger equation from the assumption of no paths, that is not logically the same as being able to derive that assumption from the Schrodinger equation

And the fact that, via the dBB approach, I can give trajectories to particles in a consistent manner, means that this derivation can't exist

or, at the very least, it can't exist without some additional premises

Now, it may in fact be the case that the assumption of no paths is in fact the better one, that it generalizes beyond the non-relativistic Schrodinger equation

and I think there's a genuine argument there

vzn

@bolbteppa maybe Bohm didnt refer much to "copenhagen interpretation" but think all his ref to determination is really an attempt to address/ refute it. think part of the huge slipperiness of determinism is how it relates to probability etc

bolbteppa

@Semiclassical how is that a fallacy? The fallacy is clearly on the Bohm side who take Q from thin air (we know the reason they take it, because they need it to re-produce the consequences of P while trying to deny P) and then re-interpret the components of Q in a way the explicitly contradicts the premises of P

Semiclassical

18:56

But within the context of non-relativistic Schrodinger, the lack of any kind of trajectory is not a forced conclusion

@bolbteppa that'd be lovely history, were it true. but de broglie came up with his ideas prior to schrodinger writing down the schrodinger equation

And, again, that P implies Q does not mean that Q implies P

vzn

@bolbteppa its not a bug its a feature™, its a new theory that reinterprets the schroedinger eqn, just as new theories reinterpret/ revise prior equations, many historical examples of that

bolbteppa

@Semiclassical I can grant that deBroglie does something different for now, but sticking with Bohm, this is very clearly what his paper is doing

Semiclassical

Yeah, and it's why I don't stick with Bohm

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