The h Bar

Semiclassical

00:00

A more defensible version of that is in Norsen's spin paper:

bolbteppa

I can ignore these things, but man it really is concerning to see these kinds of sentences, these are the kinds of sentences the iconoclasts should be challenging

Semiclassical

"The reason for this schizophrenia, I suspect, is that orthodox quantum physicsts are, after all, physicists. They cannot just “shut up and calculate” – not completely. They need some sort of visualizable picture of what, physically, the mathematical formalism describes, or they simply cannot keep track of what in the world they are talking about. So they retain the classical picture while simultaneously, out of the other sides of their mouths, rejecting it"

bleh, meant to include the sentences preceding that:

"Nevertheless, there really is a sense – highlighted especially by Townsend’s use of the word “too” in the passage just quoted – in which the orthodox view insists on retaining the classical picture (of a little spinning ball of charge with definite spin angular momentum vector) but simultaneously apologizing for this, by demanding that the picture not be carried over too completely, not be taken too seriously."

bolbteppa

"We propose that the wave function belongs to an altogether different category of existence than that of substantive physical entities, and that its existence is nomological rather than material. We propose, in other words, that the wave function is a component of physical law rather than of the reality described by the law. "

In other words, probability?

Or something else?

Semiclassical

I think the analogy they draw is with the Hamiltonian in classical mechanics?

i.e. that you wouldn't talk about the Hamiltonian being some real, substantial object

rather, it's a function on phase space which dictates the phase space flow

in particular, you'd never worry about particles 'acting on' the Hamiltonian. That's not how it works

the Hamiltonian expresses particle interactions and how they'll influence the 'laws of motion' for the system

I'll also note, though, that they make reference to the 'universal' wavefunction

that's tied up with how they deal with the object-environment distinction along with the measurement process

and it's something I don't know too well

bolbteppa

The only things that really seem to matter are a) determinism, b) normal QM, doesn't matter how you match them up, the goal is to simply do it, re-interpreting things that were settled 90~100 years ago in trying to save a) could go on forever

It's simply beyond the pale to call the wave function anything but probability, seeing how Bohm does it I am actually shocked it was taken seriously

The first 4 pages of his paper are interesting, then section 4 comes and I'm just shocked

Starting from the Schrodinger equation out of thin air is literally no different to starting from atomic theory and then justifying phlogiston theory concepts, there's no way people take it seriously to just take the Schrodinger equation axiomatically

Semiclassical

00:16

questions about the interpretation of psi aren't confined to dBB, tho

bolbteppa

Yeah, my sense is dBB is the most respectable of the alternatives, except maybe MWI

Semiclassical

part of what I like about dBB is that it is fairly precise

the fact that it it's so wedded to the usual formalism means that there's not much wiggle room

whereas MWI seems like a huge mess of wiggle room

@bolbteppa Except that you could actually disprove phlogiston theory concepts experimentally

Eulb

why is pilot wave theory important?

Semiclassical

.......

<_<

tbh, in a lot of ways, it's really not

Eulb

why does everyone talk about it then lol

i see it mentioned everywhere

Semiclassical

00:19

i find it interesting, but unless something changes it's not going to change the world

bolbteppa

Pilot wave theory is brought up all the time because it's an attempt to restore determinism to quantum mechanics on a fundamental level, while still respecting the experimental fact things are random, so it tries to show that only happens 'out of necessity', it's actually directly challenging QM at the very core while mimicking it in all practical respects

Semiclassical

conceptually, I'll agree. Though I very much don't like describing it as 'classical mechanics', given how strongly it has to reject Newtonian mechanics in order to work

Eulb

@Semiclassical 🔁 string theory reposted this

@bolbteppa but it won't change anything in other fields right?

how would it affect QFT?

Semiclassical

something as simple as "if V=0, then objects at rest remain at rest" goes poof

I also find it a bit annoying to describe dBB as 'mimicking' QM. Historically, dB's work was contemporaneous with Schrodinger's

Now, just because something was born in the same garden as QM doesn't mean it's equal to it. But as a historical fact, the guidance equation actually was written down before the Schrodinger equation

bolbteppa

Yeah but deBroglie using random unexplained facts like $E = \hbar \omega$ and getting the right answers is not a theory, he has to justify what he's doing, Schrodinger tried to justify what he was doing iirc

Maybe he did but I would bet it's a waste of time to read anything on dB before 1952

I wouldn't even trust Schrodinger's QM papers from 1926 etc

Semiclassical

00:29

I think the main reason pilot-wave people tend to 'assume the Schrodinger equation' is because they don't feel the need to justify using the same wave equation that everyone else agrees is relevant for a nonrelativistic wavefunction

bolbteppa

I think in his 4th paper he gets the TDSE, first two papers use crazy cool weird ideas to get his equations

Semiclassical

on that note, you might look at slide 21 onwards of tcm.phy.cam.ac.uk/~mdt26/PWT/lectures/bohm2.pdf

in particular the 'heuristic derivation' based on the HJ equation

tbh, though, I'm a little disappointed that I can't find much commentary on 'why should the schrodinger equation govern the pilot wave'

bolbteppa

You can make some of this derivation completely rigorous in QM once you accept no paths, without that, none of this makes any sense

e.g. slide 24 is sort of similar to QM

Semiclassical

I suspect that the justification will be is on the lines of: "What kind of evolution equation should a non-relativistic pilot wave satisfy?"

and then use symmetry etc

user1732

00:45

@bolbteppa have you read Gell-Mann's Quark and The Jaguar?

bolbteppa

Another way to say it:
Question - do electrons obey Newton's laws?
Answer - QM: No. dBB: No.
Question - Why?
Answer - QM: Path's don't exist. dBB: idk...
Question - what the hell is going on?
Answer - QM: merging probability and classical limit leads to everything. dBB: Lets steal a few tools from QM and call them axioms then re-interpret them so that paths do exist but we just can't see them for some reason we'll figure out by playing with the theory.

Semiclassical

Why? "Because the electrons are guided by their wavefunction."

bolbteppa

Am I mischaracterizing it? I really don't think so.

What is a wave function...

Again, out of thin air a tool arrives for no reason from QM

Semiclassical

Meh. It's a wave in configuration space.

bolbteppa

If an electron is guided, is it following a path as it's guided?

Semiclassical

00:50

Yes. That's the trajectory

bolbteppa

Bohm says it follows a hidden path we can never see for practical reasons

Semiclassical

It's a bit stronger than 'for practical reasons'. This is where the whole 'equilibrium' thing comes in

bolbteppa

So yeah, my description is accurate, no idea why classical mechanics fails, no comment on what to do about the fact it should hold, even worse, paths do exist but CM just ignores them except in a classical limit, that is crazy

Semiclassical

i.e. if one weren't in quantum equilibrium then the paths wouldn't be strictly hidden

bolbteppa

and this quantum equilibrium stuff clearly makes sense only because you take the Schrodinger equation as an axiom

Semiclassical

00:53

in dBB, classical mechanics fails because you can't determine the motion of a particle from knowledge of the Hamiltonian alone

bolbteppa

Why not?

Semiclassical

the Hamiltonian dictates the evolution of the wavefunction, and the wavefunction dictates the motion of the particle

but you still need to know the initial wavefunction in order to make that work

bolbteppa

You're not allowed use the words wave function

haha

Semiclassical

meh. "wave-particle duality", wooOOOoooOOO

user1732

wavicle?

bolbteppa

00:55

haha that's really the truth of it though, just assume it will all work out

A lot of it comes down to analyzing cool properties of PDE's and calling it Bohmian mechanics

Semiclassical

hey, why do you think I prefer to think of it as a particular formulation of QM

That does go with my suspicion that you could do a dBB story using most any reasonable wave equation, though

bolbteppa

Yeah I understand, I don't mean to challenge you in particular I think you're very careful with this and being fair etc I'm just hashing the logic of it out here, I could be wrong

Semiclassical

I mean, all you really need is the continuity equation $\partial_t \rho+\nabla\cdot \vec{j}=0$

once you have that, you observe that the flow generated by $d\vec{v}/dt = \vec{j}/\rho$ will preserve the density

user1732

is that how you teach it?

bolbteppa

I don't know why we pick the Schrodinger equation and not another equation, clearly the reason is to match up with QM, everything after that should be applicable to other equations though, except (apparently another huge flaw in B) he uses particular properties of the Schrodinger equation

Semiclassical

00:59

I don't teach it, so

user1732

oh

Semiclassical

@bolbteppa eh, having a continuity equation isn't terribly particular to the Schrodinger equation I think

bolbteppa

In fact, we need it to hold for multiple equations, so we can apply this to Maxwell's theory

Semiclassical

otherwise you wouldn't be able to run the dBB story on the Dirac equation, and you can do that. (there are reasons why it's not a satisfactory story, but at least at first the story is the same)

bolbteppa

People say the Dirac equation doesn't make sense for RQM, you need QFT, but it does make sense for RQM

user1732

01:01

\o @mercio

Semiclassical

no idea

bolbteppa

So I wouldn't be surprised if it makes sense in dBB and people just write it off without thinking

vzn

@vzn ...2nd thought, thinking out loud, or maybe rather bohr told an "anti-ghost" story that everyone believed...

Semiclassical

I mean, Bohm did have a story for N Dirac particles

For N=1 particles, it's even a Lorentz invariant story

vzn

@Semiclassical just skimmed this, remarkable! had never looked closely at bohms own science papers... he cites madelung and brownian motion etc wow o_O

Semiclassical

01:04

the problem comes in once you have more than one particle

bolbteppa

Well the Dirac equation is inherently multi-particle due to anti-particles

Semiclassical

eh, I think it's in a stronger sense than that

could you ever have two electrons at once in the Dirac equation as such?

bolbteppa

I'm sure he commented on all this if he's writing papers from the 50's up to the 90's

Yeah you can, e.g. you can deal with compton scattering etc

Semiclassical

huh

I'd have expected that to be two Dirac electrons, interacting via EM Lagrangian

vzn

@Eulb for 1 theres new measured physical evidence via Vinante + Adler o_O chat.stackexchange.com/transcript/71?m=45949321#45949321

Semiclassical

01:08

anyways, the basic problem asserted for the N>1 model is that of formulating the quantum equillibrium hypothesis

basically, you can't ensure equilibrium in all frames; you have to pick one and ensure equilibrium in that frame

bolbteppa

@user1732 no I haven't read it

Semiclassical

Bohm etc argued that, in terms of the observational content of the theory, that this would be enough

bolbteppa

@user1732 his 200 short interviews e.g. here youtube.com/watch?v=3LU6kbao3vo are amazing, full list: youtube.com/playlist?list=PLVV0r6CmEsFxKFx-0lsQDs6oLP3SZ9BlA

Semiclassical

"Nonetheless, Bohm and coworkers have argued that the observational content of this model is as Lorentz invariant as the covariant formalism of relativistic quantum theory: Since the predictions for results of measurements for this model can be regarded as reflected in the configuration of various devices and registers—and hence can be derived from probabilities for positions given by ̺$\rho=\psi^\dagger \psi$—at a common time in the distinguished frame, these predictions must agree with those of the usual interpretation.

(from here: arxiv.org/pdf/quant-ph/9801070.pdf)

However, this solution isn't one people like: Even if it works in terms of phenomena, it seems to commit oneself to a privileged reference frame, and that seems contrary to the spirit of relativity

There's a more subtle version of this, but regardless it seems like you at least would have to go back to some kind of absolute simultaneity in order to get a Lorentz invariant theory.

That's my reading of it, anyways.

the basic issue could be said as "being relativistic should mean more than just not being able to observe violations of Lorentz invariance"

danielunderwood

I hate to bug you guys with Copenhagen, but I'm a bit confused by measurement causing wavefunction collapse to an eigenstate and how that goes along with position/momentum. There was a discussion in here a couple days ago how position/momentum eigenstates aren't physically valid states and I'm confused how that gets along with the collapse picture.

I found this question, but something seems off to me about the accepted answer.

Semiclassical

01:17

a more recent summary on this is this paper: arxiv.org/abs/1307.1714

bolbteppa

I'm not sure re the whole relativity thing

Semiclassical

@danielunderwood Well, consider how eigenstates of the Hamiltonian work. You have $H\Psi=E\Psi$, where $\Psi$ is a normalizable wavefunction.

Suppose you wanted to do the same thing with the momentum operator. Then in position space you'd do $\hat{p}=-i\hbar\partial_x \Psi = p\Psi$, which you can solve as $\Psi=Ae^{i p x/\hbar}$

Can you normalize that state?

should've labelled the energy eigenstate as $\Psi_E$, bleh

(you want to have normalizable wavefunctions so that you can get probabilities from your wavefunction)

vzn

> However, de Broglie used this approach as a physical basis for his thinking, by proposing that, more generally, this fluid is a space-filling background analogous to ether, but with new qualities (it may be regarded, for example, as the physical basis of the "zero point" fluctuations of quantum mechanical field theories).

Semiclassical

@vzn yeah

I don't go down that route

vzn

> It is significant to note here that this model provides at least a conceptual connection between quantum mechanics and Einstein's attempt at a unified field theory, in which the particle was also treated as a nonlinear singularity that merges with a linear background field (modern soliton theory is also closely related in concept to this approach).

Semiclassical

01:28

yeah. i mean, there is precedent in that lineage for the kind of pilot-wave hydrodynamics that Bush-Couder want

I just don't find it appealing myself

danielunderwood

@Semiclassical but that's only if you have an unbound particle, right? If you had one bound in a potential, it would be normalizable. Then you try to measure either position or momentum, which from everything that I've heard would collapse to $x \lvert x \rangle$ or $p \lvert p \rangle$. But it also seems that those aren't valid states to collapse to

vzn

so uh SC whats the difference between science and propaganda anyway? wanna hear your thoughts on that one o_O :P

danielunderwood

Although I imagine that you'd realistically have to introduce some sort of deltaish potential to measure it

Semiclassical

I dunno a good simple criterion. But interrupting people who are trying to explain their POV in favor of proselytizing for your own counts, in my book

vzn

@Semiclassical lol! pot kettle black

Semiclassical

01:31

@danielunderwood well, in particular you can make sense of momentum eigenstates for a particle on a ring

the case of a particle in a box, by contrast, doesn't work since you need the wavefunction to vanish at the edges.

However, if you're with a particle on a ring then you've got periodic boundary conditions and it works out fine

However, there's a big difference with a particle on a ring and a particle on the real line

namely, while I said that periodic boundary conditions work out fine, they only work out fine for some momenta

namely, those for which $e^{2\pi i p/\hbar}=1$ (over the ring [0,2\pi)

(on a ring your coordinate is an angle and p is therefore angular momentum i.e. $p=n\hbar$ is sensible)

main point is that there's only countably many momenta allowed

However, on the real line, any momentum should be allowed. There's no cutoff

And in that case, you're faced with normalizing a non-normalizable state

So I guess I could sum up the issue as: There's no way to have a momentum eigenfunction with momentum as a continuous variable

danielunderwood

Well my question is more of how a wavefunction can collapse to a position eigenstate if that's not physically allowable by the uncertainty principle.

Semiclassical

Ah

realistically, I'd say it doesn't

danielunderwood

But the Copenhagen interpretation would say that it does, wouldn't it?

Semiclassical

when you make a measurement of position, you do not strictly speaking produce a position eigenstate

in any actual measurement, you'll 'measure position' by localizing the particle to a very small region

What you create will be (in an appropriate sense) a good approximation of a position eigenstate

But you can't measure it with infinite precision

danielunderwood

Yeah I'd agree that you physically couldn't. But on a theoretical level, I've always seen $\hat{A} \lvert \Psi \rangle = a \lvert \Psi \rangle$ regardless of how accurately you're able to measure it

Semiclassical

01:41

Well, typically when you write that you're really talking about some operator with discrete spectrum

bolbteppa

Continuous spectrum is a bit nuts

Semiclassical

And if it's a discrete spectrum, then only specific values are allowed and therefore it makes sense to talk about measuring them precisely

danielunderwood

Ahhh

bolbteppa

Hand-waving about relative probabilities to make sense of it

danielunderwood

I guess I've never really handled a continuous spectrum

Semiclassical

01:42

Now, what it actually collapses to is a tricky question and I don't think you can specify it in a universal way

I think it'd be dependent on how the measurement actually works

That said: because the state is so localized in position space, it's very spread out in momentum space

as such, one expects the wavefunction to immediately spread out and to do so rather quickly

Once it does so, I believe it'll be well-approximated by the evolution of a Gaussian wavepacket

danielunderwood

For a continuous spectrum, I take it that collapse would be to an eigenfunction rather than eigenvalue?

Semiclassical

so in practice one just says "start with a gaussian wavepacket" and be done with it

@danielunderwood it really wouldn't be either. it'd collapse to something like a narrow Gaussian

that'll be 'approximately' a delta function

with the level of approximation basically being the width of the Gaussian i.e. how narrowly you localize it

danielunderwood

Hmmm I get what you're saying. Now I have a different unsettled feeling, but I can't really put my finger on what it is

I suppose I should look into continuous spectra

Semiclassical

(I think this also, I should, creates some complications as far as how one should think about 'position measurements' in the dBB account, i.e one never actually measures $x$ but rather one measures that $x\in(a,b)$ by some means)

danielunderwood

Is it valid to think of an interval as an observable?

Semiclassical

01:50

hmmmmm

I don't believe so, but I'm not sure come to think of it

You can talk about the probability of finding x in an interval, but that's still x being observed

bolbteppa

Free space eigenfunctions of the position operators are delta functions

Which already means you should use rigged Hilbert spaces I think

Semiclassical

sure

my issue is really more with 'what does it mean to measure position'

bolbteppa

Then the whole $e^{i\alpha}$ invariance means it should be projective rigged Hilbert spaces :p

danielunderwood

QM is all rigged :D

Semiclassical

it's all prigged up, apparently

bolbteppa

01:52

danielunderwood

lol yeah I just swapped to another tab and happened to have that open

bolbteppa

Turtles all the way down

Semiclassical

I reiterate that the joke makes just as much sense if you replace that with "lol you thought academia was actually a stable career path lololol"

bolbteppa

and we wont bring in fiber bundles :p

danielunderwood

I barely know about fiber bundles in CM, much less QM

something something product of spaces

Semiclassical

01:57

I understand trivial fiber bundles

them's easy

danielunderwood

Wait would a fiber bundle in QM just be a tensor product of states?

Semiclassical

no

a fiber bundle is a fiber bundle, regardless of QM or CM

they just have different applications

danielunderwood

So a fiber bundle in QM isn't $\mathcal{H_1} \times \cdots \times \mathcal{H_n}$?

Semiclassical

No.

A fiber bundle has a mathematical definition

it's not a physical concept

danielunderwood

Well yes, but I was under the impression that that definition was a product of spaces

Semiclassical

02:01

that's only true in the case of a trivial bundle, in which case it's a product of two spaces

in anything more complicated, you have some more elaborate notion of structure on the product

danielunderwood

Ahh I guess trivial bundles are the only ones I have any familiarity with. And that just kind of an I've heard of them familiarity

Semiclassical

can't say I'm much different

bolbteppa

A wave function is a section of a line bundle or something

danielunderwood

And I heard something about a fiber bundle perspective of getting to EM, but don't have a clue about that

rob

10 mins ago, by bolbteppa

and we wont bring in fiber bundles :p

Translation: let's talk about fiber bundles

Semiclassical

02:05

lol

bolbteppa

Basically, in an action $S = S(x,A) = \int L(x,A(x))dx$ in performing symmetries you need to transform both $A$ and the $x$ that $A$ depends on, most natural way (apparently) to talk about this? fib... I mean...

user1732

@rob your translation skills are incredibly astute , Professor :P

danielunderwood

Sounds like one of my old professors "we don't cover that in this course, though it's an interesting thing for us to talk about now that you brought it up"

That was a lot better than the professors that completely avoided a subject though. When we were first doing Lagrangians, I asked if that had anything to do with the QM path integral and apparently that was a forbidden topic

Semiclassical

02:39

@danielunderwood @EmilioPisanty has a nice answer as regards momentum measurements

7

A: Measurement of observables with continuous spectrum: State of the system afterwards

Your intuition that If, instead, my measurement is only partly accurate and says that the momentum of the particle is in a set $\Delta =(a_x,b_x)\times(a_y,b_y)\times(a_z,b_z)$, will the measurement collapse the wave function into $P\Psi$ (where $P$ is the spectral projector of the momentum o...

and I suspect a similar approach works for position: namely, think of your position detector as a bunch of individual detectors, each with a certain response sensitivity to nearby positions

in which case $P_j=\int d\mathbf{x}f_j(\mathbf{x})|\mathbf{x}\rangle\langle \mathbf{x}|$

and the wavefunction of the system after measuring on detector $j$ would be $$\langle x|P_j \Psi\rangle = \int d\mathbf{x'}f_j(\mathbf{x'})\langle \mathbf{x}|\mathbf{x}'\rangle \langle\mathbf{x'}|\Psi\rangle= \int d\mathbf{x'}f_j(\mathbf{x'})\delta(\mathbf{x}-\mathbf{x'})\Psi(\mathbf{x}')=f_j( \mathbf{x})\Psi(\mathbf{x})$$

That still leaves the specification of the $f_j(\mathbf{x})$ up for grabs, of course. I suspect that, in a realistic scenario, you'd pick $f_j(\mathbf{x})$ to be smooth rather than, say, a box function.

Semiclassical

03:04

"The source for most misunderstandings of Bohmian Mechanics lies in the fact that many physicists have severe difficulties understanding that the ‘measurement’ of the position operator is not, in general, a measurement of the Bohmian particle positions."

...crap. I fall into that category

(I think I get the point, but hoo boy does it complicate my headspace)

user2236

lol

how's the blog going?

Semiclassical

not good yet

it's just as well, since the first thing I was going to talk about was some basic facts about Bohmian stuff

and, as I'm plainly displaying here, there's some big gaps in my understanding

Cows

03:29

TUPAC SHAKUR 1988 HIGH SCHOOL INTERVIEW

►

bolbteppa

Good interview

Cows

yes, I'm watching it right now

bolbteppa

imdb.com/title/tt0343121 is really good too

Semiclassical

03:49

"Thus, while a measurement of [the Bohmian] position is always a measurement of the position operator, a measurement of the position operator is not necessarily a genuine measurement of [the Bohmian] position!"

aaaaaaaagghhhhh

(I think I actually do understand what's being stated, but ugh)

Novalium Company

09:40

http://faculty.bard.edu/belk/math213s14/LinearCombinationsAndSpanRevised.pdf
On example 2, are x1 and x2 equal to 1? (I'm asking because I don't know about row reduction yet)

Actually, I don't know how to solve a system with 3 equations, so these are probably not the answers?

I just treated the 3 eq. system as a 2 eq. so it's probably wrong.

user1732

it is better to start at the Khan Academy, pal :-)

Linear combinations and span | Vectors and spaces | Linear Algebra | Khan Academy

►

Novalium Company

10:26

I watched it already but it doesn't give answer to the example problem :D

Secret

11:31

Quarks, Gluon flux tubes, Strong Nuclear Force, & Quantum Chromodynamics

►

The group structure then organise these gluon interactions in a neat way, conveying that there are 8 gluon states

I wonder what happens when a proton and an antiproton that are at rest wrt each other annihilate, will it still produce jets, or we end up with purely photons since the net charge is 0 and the net color is also white?

Loong

13:21

> I think it's fair to say that explainxkcd.com is the authoritative source for questions regarding xkcd.

sigh

53 upvotes

sigh

user2236

sigh

.media.giphy.com/media/oAfqlSp7990m4/giphy.gif

sigh

Jacob P.J

ibb.co/eMDAWK

Novalium Company

Hello everybody! :-)

Jacob P.J

can sombedoy explain the part about the components of magnetic field outside the solenoid

highlighted in the link above

thank you

philmcole

14:22

Hi, can somebody help me understand the difference between average intensity and average power of a harmonic wave? Like from the equations it seems to me that those two are the same?

There is the energy density $u = \rho A^2 \omega^2 \sin^2(kx-\omega t)$ and the average energy density $\bar u = \frac{1}{2} \rho A^2 \omega^2$. Then there is the power $P = u v$ and the average power $\bar P = \bar u v = \frac{1}{2} \rho A^2 \omega^2 v$. Now for the intensity I've seen often this exact equation $I = \frac{1}{2} \rho A^2 \omega^2 v$ which means $I=\bar P$? But then there is the definition of the intensity as $I = \bar P / \text{area}$. Something doesn't make sense...

philmcole

15:00

Please anybody?

Semiclassical

You’d have to cite an actual source. Otherwise there’s no way to be sure what’s being asserted

philmcole

Source is Ramamurti Shankars book Fundamentals of Physics I if that helps..

But I mean just in the simplest case of a harmonic plane wave.

user2236

Does he have a YouTube video covering this?

philmcole

Yeah there should be a playlist on the YALE channel

user2236

may i suggest you try looking there?

philmcole

15:18

Well this is the source of my confusion. That's why I'm asking here if somebody can it explain better.

Secret

-1

Q: What happens when you create a time paradox?

Let's say that we have a time machine. We take a trip to the past to.. Let's say, kill Hitler. We go back, kill Hitler while his was still a child and return to the present day. But now in this timeline no one knows who was Hitler, and that include us. So if we never knew who was Hitler then we w...

time-travel

It appears killing Hitler will lead to a grandfather paradox what?

user351417

Is anybody familiar with the differences between the Mathematica for Raspbery Pi and the version for Windows?

danielunderwood

15:40

Not really, but from the site

> Full version of the Wolfram Language and Mathematica, including support for notebooks and dynamic constructs like Manipulate and Animate. Additionally includes new Device API to connect to serial devices and the on-board GPIO and RaspiCam.

(wolfram.com/raspberry-pi)

It does say non-commercial use and you'd be running it on the computational limitations of the pi

@Semiclassical thanks for the link to that answer. That helped a bit

user351417

16:07

@danielunderwood Hmmm how demanding are most mathematica scripts? Usually I can run pretty heavy python stuff on my Pi without much trouble...

danielunderwood

I've only used mathematica on my desktop, so I can't really say. But it hasn't had a problem with most things. There is the occasional command that will take a bit, but those are rare from my experience. I'm no mathematica expert though

I've had issues with having enough memory for numerical stuff and I'd imagine that could be pretty bad on a pi. But I'm not sure how that would translate to the typical work you'd do in mathematica

user351417

16:43

I've never used mathematica before and I don't have the time to explore much right now, so I'd probably only get through only basic stuff.

vzn

@bolbteppa still pondering on what you meant by that, "these are the kind of sentences the iconoclasts should be challenging". "iconoclasts" are anti copenhagen. the paper was written from the iconoclast pov. you feel they went too far with their criticism? recent polls/ surveys published support the claim that physicists pay lip service to the copenhagen interpretation...

@Semiclassical you say youre a bohmian but then very fastidiously circumsize parts of his ideas and reject key others with no qualms/ reservations, and then call my direct pointing/ quotes of the excised ones propaganda. then you quote a paper on the schizophrenia of physicists in their attitudes about copenhagen interpretation. touche

Semiclassical

I must definitely do not consider myself a Bohmian. I’m sympathetic to their account, but I’m too conflicted about it too consider myself an out-an-out advocate for it

vzn

@Semiclassical it would seem, youve embraced everything except the heart of it. think its like a sterilized version or something. and BT has a point about "if bohmian mechanics/ theory is just a different pov on schroedinger eqn with no new math, then why bother, its pointless"

Semiclassical

In addition, my citation of Norsen was to present a sentiment similar in quality to the line cited by Goldstein, but in a less polemical way

Yes, and that’s part of why I -don’t- consider myself a Bohmian

As an internally consistent story of how quantum mechanics works, it is interesting

But for all that it’s been remarkably fruitless in terms of actual progress, in particular by comparison with the success of QM without such interpretation

vzn

16:58

nevertheless (now speaking in the spirit of your own unrestrained/ no-holds-barred/ harsh candor) youre the closest to a bohmian in this group/ room seen in years/ ages, guess it will have to do/ suffice for now. maybe someday will meet a real disciple somewhere... our dialogue has led me to striking new insights into his pov, and understanding the nuances/ complications of its community/ society acceptance :( :P

Semiclassical

In one respect I’m definitely not educated enough to be a dBB person: the measurement theory of dBB is something I’m not an expert on

as exposed by the fact the sentences like “measurements of Bohmian position are always measurements of the position operator but not vice versa” can throw me for s loop

vzn

← feeling some frustration verging on exasperation, wanted to publicly articulate it/ "get off chest", now off to other stuff

mercio

is it easy to do a double slit experiment at home ?

Semiclassical

Depends what kind of wave you’re looking at

Double slit diffraction of water waves isn’t too hard

mercio

of light

Semiclassical

17:05

that’s not too bad either: use a laser pointer

mercio

hmm

Semiclassical

Doing a double slit experiment with electrons, by contrast, would probably take a lot more work

mercio

electrons are spooky

vzn

@Semiclassical hard to follow but maybe its saying there are "hidden/ bohmian positions" that are not subject to measurement aka trajectories

Semiclassical

spoooOOOoooOOOky

bolbteppa

17:10

@vzn I'm saying comments like those indicate how psychological all of this is, nearly every paper I have looked at advocating this stuff has some psychological dig at normal QM as an attempt to justify themselves which actually indicates a lack of understanding, if the iconoclasts were interested in the truth they would smell the weakness of such statements and, given how many don't have the math to understand either normal QM or dBB, should be more than equipped to pinpoint the weakness

vzn

@bolbteppa have you "pinpointed the weakness"? what is it? for me its called the copenhagen interpretation exactly as is asserted, sometimes more brashly than others. what exactly is the questionable part of the assertion? (have said many similar things myself here/ in m blog...) anyway its ~½ a sociological observation...

bolbteppa

I can separate those statements from the main arguments and ignore them while evaluating this stuff, but I am surprised the iconoclasts would

Semiclassical

Yeah. It smacks of a certain defensiveness—which, to some extent, I can understand. It’s easy to get defensive about dBB when you see certain common misconceptions showing up again and again

vzn

Bohmians hae a lot of reason to be defensive after being sidelined for spurious, bordering on unprofessional/ antiscience motives ~½ century verging on ¾-1 or longer and yet being right o_O

Semiclassical

Or at least not being wrong for silly reasons

Anonymous

17:14

@BalarkaSen I asked a question about that on Math Overflow. The point really seems to be that all homeomorphic objects share the same topological invariant (in our case, the Euler characteristic), but all objects sharing the same topological invariant are not necessarily homeomorphic.

Semiclassical

I think there are problems with the account, but it’s not as simple as just “lol, silly Bohmians talking about paths”

I do think there is a less defensive argument on the dBB side:: that while we talk about the Copenhagen interpretation as the standard, that doesn’t necessarily translate to how people actually use and gain intuition about QM

This is not to say that the orthodox account is inconsistent either, but it is to say that the orthodox account is not sufficient for the varied ways in which QM is actually used

bolbteppa

@vzn my only point was that the iconoclasts ill-equipped to evaluate either dBB or normal QM on the details should at least sense that such defensiveness indicates a larger problem with defending the subject in general, I'm looking for the best arguments and the best arguments so far seem to be 'assume the Schrodinger equation axiomatically so it's just an alternative interpretation' which I think is a mortal flaw, but lets see

Semiclassical

Having looked at what the dBB people say, I think part of their response would be: the orthodox account is not just the Schrödinger equation, but rather the equation plus the measurement postulates

bolbteppa

@vzn clearly the iconoclasts will put up with anything (they would sieze on in a moment if the other side was doing it) so long as they get to save determinism, but these subtle things are basically the antithesis of science, like really :p

Semiclassical

And within the orthodox account, those are indeed postulates. One doesn’t inquire as to how the wave function goes from a superposition to a single state upon measurement; that’s just how measurement works

bolbteppa

17:23

@Semiclassical here's a subtle one I have just read is the case, but the Schrodinger equation is derived on the assumption of a quasi-classical approximation, and there are quantum systems with no classical limit, so QM can still account for them no matter what modifications one needs to make to Schrodinger, I think this is more based on relativistic thinking, not sure

Semiclassical

@bolbteppa I have to object there: that only works if one already has as a posivist view of science

In particular, it requires more than just “theory X has empirical predictions Y”

bolbteppa

My main issue is really that I don't think theory X is even a theory

I would love it to make sense but it's actually a really bold claim theory X is making

vzn

@bolbteppa what is the difference between determinism and probability? even bohm (just read his paragraph on that) says its subtle. bohmians are not asking for a return to deterministic physics, its a simplistic/ straw man account of the ideology. :(

Semiclassical

I’ll note here the distinction between predictability and determinism

Particle trajectories, in the dBB story, are perfectly deterministic. But that’s not the same as them being predictable

bolbteppa

@vzn there's a huge difference between requiring probability "out of practical necessity" and "an inherent lack of complete determination" forcing probability out of theoretical necessity, my sense is Bohm people want to act like it's an equally valid alternative, they way I see it, it's like claiming 'left is fine and right is a fine direction to go on, down equally valid ways of getting to QM land'

vzn

17:29

@bolbteppa there is much more to the theory than merely reinterpreting the schroedinger eqn, but 1 thing at a time, the foundations have to be grasped before the more advanced concepts are. bohm himself talks about a provisional aspect. he was clearly aiming for something bigger than could be realized at the time and its now increasingly taking shape/ form/ solidifying a few decades after his (heroic! prescient! visionary!) efforts

bolbteppa

@Semiclassical that's why I think dBB is considered one of the most legitimate alternatives since at least a lack of predictability is respected

Semiclassical

@bolbteppa agreed

Now, whether the dBB account, of how ‘absolute uncertainty’ is supposed to arise from determinism, is a satisfying one

That’s a harder question. (In particular, is it a well-motivated account?)

But it is an internally consistent one

bolbteppa

I would be happy leaving dBB alone and saying it's just another way, I would still ignore it because it has serious issues with relativity, qft, it's fine if people want to fix the theory and get a concensus

But the opinion I am slowly coming to, and I'm happy for it to be wrong, is that there's nothing more to it than trying to splice a) determinism with b) normal QM, leading to a potentially infinite number of ways of re-interpreting fundamental randomness as mere "practicality", despite the inherent lack of compatibility

Semiclassical

Getting back to one thing I was going to say earlier: I’ve seen dBB people argue that their story allows them to show how the measurement postulates arise, rather than simply taking them as definitions

bolbteppa

My sense is, if you start from the Schrodinger equation, which allows for the existence of paths, and lets you link e.g. the solutions to probability, you can do anything with it tbh

Semiclassical

17:36

@bolbteppa yeah. In some cases, I think that lack of determination is simply due to the contextuality of quantum systems

vzn

@bolbteppa maybe some mixup of forest vs trees around here lately! a great "argument" is that the pilot wave "anomaly" has now been predicted/ measured by Adler/ Vinante and standard QM has no explanation whatsoever because its incomplete o_O

Semiclassical

Like, you wouldn’t expect to have an account in terms of trajectories for the results of an SG device unless, at the very least, one specifies the design of the SG device

mercio

what is a SG device ?

Semiclassical

Stern-Gerlach

I mean, consider sending two identical electron beams through two SG devices with opposite orientations

If the electrons aren’t spin polarized to begin with, then in both cases you’ll get 50-50 spin up and spin down

But that doesn’t mean that the trajectories leading to spin-up in one SG device need be the same as those leading to spin up in the other

So just knowing the statistics at that level of coarseness isn’t enough

However, that’s the easy kind of “underdeterminism”

bolbteppa

@vzn what did they do

SG is magnetism e.g. relativity, blugh

vzn

17:46

@bolbteppa directly measured subquantum effects roughly predicted by Bohmian theory etc, its only ¾ yr old result & isnt gaining widespread attn yet, think its revolutionary o_O :D

Semiclassical

The harder point is this: If I have a sufficiently-specified measurement device, do I have enough info to uniquely deduce the Bohmian trajectories?

To be sure, the dBB account will give unique trajectories in that case. But what justifies those trajectories?

@bolbteppa If one takes spin as a given, e.g. in rather the same way as one might see in an undergraduate course, with spinor wavefunctions instead of scalar wavefunctions, then the dBB story still works at a technical level

@vzn dBB does not predict subquantum effects. It can be said to make them more plausible, i.e. either through violations of quantum equilibrium or by proposing dBB to emerge as some kind of average over subquantum fluctuations

bolbteppa

@vzn this stuff sciencenews.org/article/… ?

Semiclassical

But to the extent that dBB is an alternative formulation of QM, there's no way (for better or worse) for it to give anything subquantum

bolbteppa

Trying to say that as the wave function 'collapses', it leaves noise?

vzn

@Semiclassical you misread/ limit the theory, it is apparently not directly in the (Bohmian) math but thats where Bohm was leading/ pointing to repeatedly in the later theory, eg in a paper you just cfd, and now realized by descendants. Bohm said himself he thought subquantum effects may someday be measured from further developments...

Semiclassical

17:54

That, I should note, is basically how Bush-Couder want to view dBB vs. QM vs. pilot-wave hydrodynamics

If it's not in the math, it's not in the theory.

It can be in another theory, one which is motivated by certain features of dBB

But it's not in that theory.

vzn

@bolbteppa yes, afaict its significantly based on bohmian ideas/ theory but almost nobody has figured that out yet

bolbteppa

@Vzn this wave function collapse stuff is pretty ridiculous, have you heard the airport argument

Semiclassical

in the walking droplet models, you have paths which, when averaged over many trajectories, give the Bohmian trajectories. as such, the Bohmian trajectories are no longer the trajectories of actual particles; rather, they describe 'typical' paths

bolbteppa

"Heisenberg would often describe his interpretation of the wave function using a story about a guy who fled the city and we don't know where he is but when they tell us at the airport they saw him 10 minutes ago, our wave function describing his position immediately collapses to a smaller volume, and so on. This "collapse" may occur faster than light because no real object is "collapsing": it's just a state of our knowledge in our brain."

motls.blogspot.com/2011/05/…

vzn

@bolbteppa not sure what youre referring to, yes think collapse can probably be "explained" by newer models, not up on all the details but have a rough idea, its indeed key/ gaping part of QM incompleteness...

bolbteppa

17:57

Clearly Adler is trying to say 'the wave function is a real thing (insane), collapse thus has to be a real thing, thus it should leave an imprint of real collapse'

vzn

@bolbteppa yes it seems similar to the concept of conditional probability...

@bolbteppa by "they" do you mean bohmians? it sounds like youre calling both the conventional view "ridiculous" and the bohmian view "insane," not totally following

bolbteppa

edited to clarify

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