The h Bar

knzhou

00:35

@vzn There are many productive analogies between general relativity, which is a field theory, and solid mechanics.

I mean, such analogies were why classical fields were invented in the first place.

The reason most physicists aren't going to give this much thought is because we learned very painfully with the ether that giving the "fabric" additional structure (as in done in that paper) doesn't get you anything.

It makes the theory more complex, makes symmetries harder to see, makes the mathematics harder. Plus it's not even directly observable.

Semiclassical

yeah

knzhou

Probably one of the biggest lessons of 20th century physics is that we can formulate our theories independently of "what's really going on" and that actually, this is an extremely good way to make progress.

Semiclassical

this is one reason why, despite my fondness for Bohmian/pilot wave business, I can't exactly call myself a believer

knzhou

@vzn I'm not saying that spacetime definitely isn't a fabric, just saying that this isn't going to be a "physics" question for another couple centuries.

I also wrote a whole essay on this point which I'll send when it gets released.

Semiclassical

I'll defend to the hilt that such interpretations are mathematically coherent and legitimate...but, how productive has it actually been?

the frustrating truth is "not very"

knzhou

00:38

Bohmian mechanics is like the super example... it's so hard to use!

Imagining Bohmian mechanics for fields is just crazy.

Semiclassical

I think that's a bit of an overstatement, at least in the non-relativistic context

knzhou

Yeah, I meant specifically relativistic.

Semiclassical

Gotcha

there was a nice statement about that I saw earlier, let me find it

"It is notable that none of the various extant suggestions for Bohmian quantum field theories — based on associating the hidden variables to fermion number density (Bell 1984; Colin 2003; Colin and Struyve 2007), or particle number (Durr et al 2004), or field-configuration strength (Struyve and Westman (2006, 2007), Struyve 2007) — have discussed renormalisation, or given more than a qualitative verbal plausibility argument for how these theories recover the macroworld. The acid test of such a theory is to demand a full model of how some nontrivial quantum-field-theoretic prediction — say, …

the frustrating truth seems to be this: Within the context of nonrelativistic theory, I really do think Bohmian mech is coherent and interesting. But as a foundation for moving beyond non-relativistic QM? Pretty much entirely unsuccessful.

bolbteppa

Can BM be written in a non-relativistic second quantized formalism

Semiclassical

No idea. Should be easy to google tho

(I don't look at Bohmian field theory stuff in general, since my knowledge of field theory is weak enough that it doesn't seem worth the effort)

From Towler's slides on Bohmian mechanics:

"Second quantization convenient mathematical trick - no new physical information. Is QFT really more fundamental theory than first-quantized QM of particles? If so should be able to reproduce all good results of less fundamental theory, e.g. in relativistic limit Ψ∗Ψ should represent probability in space of particle positions. But this cannot be done. Instead of saying QFT solves problems of relativistic QM, more honest to say it merely sweeps them under the carpet."

:/

bolbteppa

00:49

After equation six here it seems to set up second quantization and say "god does play dice" when you do this, i.e. it seems second quantization of even the non-relativistic Schrodinger equation breaks, might be reading it wrong (just skimmed it)

Semiclassical

saw that one too

bolbteppa

"The jumps are stochastic in nature, i.e., they occur at random times and lead to random destinations. In Bell-type QFTs, God does play dice. There are no hid-den variables which would fully pre-determine the time and destination of a jump."

Semiclassical

that seems pretty definitive

bolbteppa

If the simple act of re-writing the Schrodinger equation in second quantized language completely breaks BM the whole theory is disproven in another way basically

Semiclassical

I mean, Bell-type QFTs is not necessarily the only game in town as far as Bohmian field theory goes

bolbteppa

00:51

I'm sure it's not that simple

Semiclassical

I'll agree, though, that it puts further burden on BM in order to get taken seriously as far as field theory goes

I think it's entirely plausible to say: As a strategy for making sense of non-relativistic wave mechanics, BM is an interesting story. As a guiding principle for how to extend quantum mechanics beyond that context? Pretty damn unproductive.

Which, as someone who does find it interesting, is exceptionally frustrating.

bolbteppa

Like, going to second quantization is basically just a sequence of equalities from the Schrodinger equation to what can equivalently (but not necessarily) be expressed using wave operators, there's really no way non-relativistic second quantized BM can break down without the entire thing completely breaking down

Will try read this properly, only 4 pages :p

Semiclassical

The bit that I quoted Towler on is with citation to this paper, btw:

arxiv.org/pdf/quant-ph/0609163.pdf

page 28

bolbteppa

P1: 'A frequent com-plaint about Bohmian mechanics is that, in the words ofSteven Weinberg [4], “it does not seem possible to ex-tend Bohm’s version of quantum mechanics to theoriesin which particles can be created and destroyed, whichincludes all known relativistic quantum theories.”'

vzn

@knzhou the idea was discarded about a century ago and agree it was the right thing to do at the time. physics has changed radically in 1 century and feel like some others the time is ripe to revisit the idea based on mounting circumstantial evidence/ leads. there is no one killer point to it but think there are numerous signs of an overall coherence/ meshing of the concept to many deep unsolved questions. so instead of a single spark eg with QM past its like dozens of filaments so to speak.

bolbteppa

01:02

The same mathematical language used to describe theories in which particles are 'created and destroyed' is the same mathematical language which describes multi-particle systems where the occupation numbers change, the only real difference is particle number is not conserved in relativistic theories, but the mechanics are pretty much the same, seems like BM just can't be re-written in a way that allows changing occupation numbers (not sure on this, no way could BM be this bad/ridiculous a theory)

vzn

@knzhou defn look fwd to reading it! :)

bolbteppa

Indeed it is absolutely fundamental that one can only predict distributions for possible sets of occupation numbers in solving Schrodinger, that's the whole point, it's completely equivalent to solving the SE for a multi-particle system in coordinate space, no way could BM actually not be able to do this thing which is nothing really but seeking a SE solution in Fourier space

Semiclassical

agreed. my impulse would be to think that you can do it in BM, but that you have to do more work (maybe a lot more work!)

bolbteppa

Yeah I have to be misunderstanding this

Semiclassical

part of the issue, though, could be along these lines

vzn

01:06

@Semiclassical absence of evidence is not the same as evidence of absence. there are some prjs that take a really long time to develop & believe ether theory is one of the longest. not just over a century. its over 2 millenia old...

bolbteppa

If they are claiming that this is why QFT-BM breaks down, I bet I could just do it properly by copying what's in QM books then find a way to merge it with relativity and become an alternative-physics king

Semiclassical

BM is basically an interpretation of wave mechanics i.e. the Schrodinger equation. You say that there's a wave, which evolves according to the schrodinger equation, and that this wave defines the allowed trajectories of the particle that's there.

What about matrix mechanics, e.g. spin? Well, BM can -handle- that: You just have to allow your particle to exist in a larger configuration space

e.g. if you've got a spin-1/2 particle then you'd go from having a particle in 3-dimensional configuration space to 6-dimensional configuration space

vzn

@knzhou alas sticking to theories based on mathematical accessability, taken to extremes, can become like the drunk looking under the streetlight for his keys because... agreed fluid dynamics seems to be mathematically intractable in many ways but think it may be eminently accessible from computer simulations...

Semiclassical

which is fine on the surface. But the basic idea of second quantization is to associate a harmonic oscillator degree of freedom with each point in space

my concern would be that you'd end up needing an infinite dimensional configuration space in order to make the BM story work

which seems like a bad idea

bolbteppa

@vzn literally the only way to get away from the streetlight is the predictive power of the internal logic of the math, the question is the starting point under the streetlight (physical assumptions)

vzn

01:14

@bolbteppa math is powerful. but also starting to run into limitations ala Hossenfelder critique. simulation is made out of math. simulations are probably still underutilized in physics. think there will be some breakthru simulation experiments in the near future if anyone would seriously pursue it.

@Semiclassical nice ref by Nikolic, QM is far less airtight than many of its proponents imagine.

Semiclassical

sigh

vzn

> The fact is that the existence of various theoretical and interpretational ambiguities underlying these myths does not yet allow us to accept them as proven facts. I review the main arguments and counterarguments lying behind these myths and conclude that QM is still a not-yet-completely-understood theory open to further fundamental research.

bolbteppa

@Semiclassical if you start with Schrodinger for some system with stationary states and Fourier transform it you have like a discrete set of possible stationary states a given particle could be in, for a multiparticle system one can instead of simply working with the stationary states they can instead merely ask how many particles are in a given stationary state, this is the occupation number formalism, it's still just working with a finite number of particles in an infinite dimensional space

Semiclassical

Been too long since I did 2nd quantization, frankly. My recollection of Fock space etc is poor

My main frustration, btw

bolbteppa

Basically that's all that's going on really, it's just working in Fourier space but then simply counting how many states are in the existing stationary states

Semiclassical

01:21

When philosophers talk about QM interpretations, they mostly mean "how to understand non-relativistic QM"

in my experience

which is an interesting enough question but uh

very much a narrow vision of the world :/

bolbteppa

It's just called second quantization because the formalism suggests some 'creation and annihilation operators' if you write it out and the way it comes out they end up like the coefficients in the Fourier expansion of the wave function but they're operators so it's a 'second' version of the wave function, but it all goes back to just counting the number of particles in each stationary state, and we have a finite no. of particles which doesn't change in the non-rel version

There's zero chance the BM version of this breaks down unless the interpretation laid on top of the math contradicts itself somehow, which I'd doubt

Semiclassical

yeah

I think there's a story to be told there

bolbteppa

It would be great if it was that simple haha, but I doubt it

Semiclassical

But for Towler to just toss it off as a "mathematical trick" is just kinda dumb

bolbteppa

@vzn @Semiclassical What does that statement mean, this one: "The jumps are stochastic in nature, i.e., they occur at random times and lead to random destinations. In Bell-type QFTs, God does play dice. There are no hid-den variables which would fully pre-determine the time and destination of a jump."

From this P3 after eq. 6

Semiclassical

01:26

"The theories we present are based on the work of Bell [7] and our own recent results [8, 9, 10]; in [8] we study a simple model QFT, and in [9, 10] we give a detailed account of the mathematics needed for treating other QFTs. While Bell replaced physical 3-space by a lattice, we describe directly what presumably is the continuum limit of Bell’s model [9, 10, 11, 12]. Since Bell’s proposal was the first in this direction, we call these models “Bell-type QFTs”"

Might be interesting to go back and see what Bell did exactly

I'd hope he'd be less blithe about the issues involved

@bolbteppa another paper by those same authors on the same topic, but 53 pages instead of 4: arxiv.org/abs/quant-ph/0407116

so if you're looking for a more detailed explanation, that's probably the place to start

bolbteppa

Yeah that looks better

Semiclassical

i'll confess, tho, that my eyes glaze over when i try to read that kind of paper

bolbteppa

Can't find the jumping/dice thing in there from skimming, looks like they do set up second quantization or use it anyway though

Semiclassical

yeah

way too much formalism for me to be willing to dive in tho

bolbteppa

P35 of this BM thing pointing out second quantization could not be the reason BM can't be made relativistic

Semiclassical

01:39

@bolbteppa I can't really judge this paper one way or another, but the figure + caption here is interesting: arxiv.org/pdf/hep-th/0702060.pdf

Figure 1, page 16

the "Bohm + creation/destruction" picture seems to be the Bell-type QFT

(and yes, I do recognize the perversity of a Bohmian interpretation of string theory :P)

bolbteppa

My sense is BM could be made relativistic, it's kind of ridiculous that there are any issues with this, the big problem is the starting point of the whole theory

Semiclassical

It'd be a cosmic joke of the highest order if the most reasonable way to move BM beyond non-relativistic QM was to go to string theory :P

bolbteppa

The statement about second quantization on that slide (P35) is an egregious misunderstanding of QFT

Ah they are by Towler too

Semiclassical

yeah

bolbteppa

Ah we are talking about the same paragraph, I blanked on it earlier sorry

haha

"If so should be able to reproduce all good results of less fundamental theory, e.g. in relativistic limit Ψ∗Ψ should represent probability in space of particle positions. But this cannot be done." that's simply not true

Semiclassical

01:47

No worries. I didn’t link the slides when I referenced them

bolbteppa

But the first sentence is right, but the second brings me back to my suspicion about BM'ers not knowing normal QM/QFT well enough

Spin the best example

If (in BM) spin is not a property of particles, but instead a property of the wave function, how can the wave function not be a real thing since it's affecting the particle and measurable

Semiclassical

I don’t really agree with the “normal QM” side of that claim but I definitely agree with the QFT part

bolbteppa

One couldn't trust BM to predict anything if it can't be blindly trusted to reclaim basic QFT results which really should automatically follow by simply changing the symmetry group and changing to second quantization and using the tools of BM as they stand

Semiclassical

at the very least, there ought to be a solid account of that

Not just someone blithely saying “it’s just a mathematical trick”

An amusing sentence out of an abstract: “ Finally, we discuss the fact that Bohmian mechanics is attractive to philosophers, but not so much to physicists and argue that the Bohmian community is responsible for the latter.”

I have a hard time disagreeing with that

vzn

02:31

QFT? try this! Quantum Field Theory of Fluids / Gripaios, Sutherland 2014 arxiv.org/abs/1406.4422

> Here, we make a different conjecture, which is that quantum fluids are consistent, but that the peculiarities of quantum mechanics make their phenomena completely different to those of classical fluids. If true, there might already exist real-world examples of quantum fluids, without us even realizing it.

> Our results are a strong hint that there exists a consistent quantum theory of fluids. If so, it is of great interest to explore the physical predictions of the theory, and to see whether they are realized in real-world systems.

bolbteppa

@vzn is the electromagnetic field a fluid?

vzn

@bolbteppa EM is a wave in the spacetime fabric. as thought that way over a century ago and discarded, in a near but different form. the different fields are just variations of spacetime fabric waves. even gravity. schroedinger waves are also EM waves. (plz dont crucify me for answering to the best of my ability.) o_O

youre a huge Landau fan right?

bolbteppa

02:50

@vzn is the wave in spacetime a fluid

vzn

> Landau, who was one of the first to attack the problem, tried to bypass the obstruction by arguing [2, 3] that the vortex modes should be gapped in the quantum theory. In doing so, he stumbled not upon a quantum theory of fluids, but rather upon the theory of superfluids.

bolbteppa

Like, when you turn on a light, the light eminates from the bulb, is that thing eminating a fluid?

A fluid is made up of water particles, light is made up of photons, what's the difference

vzn

@bolbteppa spacetime behaves in a fluid way eg pilot wave hydrodynamics, madelung fluid, Tenev + Horstemeyer etc

@bolbteppa we went thru this once. imagine a fluid that is not made up of particles. its hard if youre a physicist but not if youre a mathematical physicist. right? :P actually its the opposite, the particles are made of fluid! o_O

bolbteppa

The difference is for light the particles nearby don't interact, for a fluid the water particles clearly do interact with their neighbors, so when you use the word fluid are you saying the nearby particles interact? Because for the gravitational field they also don't interact with their neighbors

vzn

@bolbteppa "fluid" is a metaphor for a set of related abstract mathematical concepts. our current concept of "fluid" is too limited, and various authors are working to expand it in a coherent way. the answer is on the horizon. its close to soliton dynamics. etc ... its probably at minimum a nonnewtonian fluid... its like a "fluid fabric" etc

bolbteppa

02:55

Fluid refers to a concept like water which is made up of particles that do interact with their neighbors and overall the 'stuff' takes the shape of it's container, basic properties like that, the gravitational or EM field are not made up of particles which interact with their nearest neighbors as they move, like there is a huge difference in behavior

It sounds to me like you're using the word fluid in the way people would use 'field'

vzn

@bolbteppa when you look at the differential eqns eg Navier Stokes etc that govern fluids, they do not presuppose particles. to borrow a software engr analogy, particles are one way to implement a fluid. but spacetime fluid is not "implemented" in this way. its a fluid (apparentliy) not built out of particles. its "implemented" in a "different way"...

bolbteppa

The Navier-Stokes equations have the terms they do in the equations because the particles of the fluid interact with each other, if they didn't the equations would be different, Navier-Stokes are explicitly derived on the assumption of internal friction (viscoscity) due to interactions between the particles making up the fluid, compare to e.g. Maxwell's equations where this doesn't happen

vzn

@bolbteppa yes fluid+ ≈ field... the 1 True Field so to speak... of which there are subfields... o_O

@bolbteppa its possible (for me) to imagine a system that obeys navier stokes eqns but is not built out of particles. its just nearly all our known examples dont seem to fit that. giving us (physicists/ humans) a particle centric bias, which is quite similar to anthropomorphic bias... this is some of the same bias that Bohr sought to remove under different terminology. his program in that regard is not fully complete/ played out yet.

bolbteppa

The Navier-Stokes equations are partial differential equations i.e. the solutions are surfaces or volumes, things people call 'fields', that's why you're seeing all these analogies, because they use the same tools, the word 'fluid' refers to a specific behavior of the points making up those surfaces/volumes that other 'fields' which solve different PDE's do not exhibit

vzn

@bolbteppa yes, "fluid" can be abstractly regarded as a sort of "pattern" found in certain class/ family of differential eqns. eg solitons, navier stokes, pilot wave hydrodynamcs, etc...

bolbteppa

03:04

The word for that is 'field' not 'fluid', i.e. fluids are described by fields, not all fields are interpreted as 'fluids' unless you mean 'fluid' in a way that just means 'field'

vzn

@bolbteppa we are in the process, so to speak, of redefining the words somewhat such that they make the most sense wrt physical reality. some new defns are reqd/ "in the works" etc

bolbteppa

The 'continuity equation' arises everywhere in physics, this originates from fluid mechanics I think, and it just means the stuff passing through the boundary of a surface is the stuff inside flowing out, so anytime you call something a fluid you're calling it a 'field' which is what people do in classical field theory (EM + GR) or quantum field theory

vzn

@bolbteppa the words/ ideas are all close at hand but need to be reformulated slightly. we have to shift somewhat the words/ defns/ povs. this is similar to what Bohr accomplished re "classical vs quantum," "complementarity" etc, and Bohm also struggled with the same task with ideas like implicate vs explicate order, subquantum realm/ space, etc

bolbteppa

Not sure what that means haha, but basically it just sounds like you should be using the word field, but by using the word fluid you get to attach any idea you want to things instead of ideas standard in QFT

vzn

@bolbteppa we have to agree its more than ("mere") semantics at the core, it has to be translated into rigorous/ strict formulas, but semantics can be a rough guide. eg "QM" sure has a lot of meanings too these days, eh?

bolbteppa

03:15

@vzn the word field is the most general thing, 'fluid' can only mean that or less, not that or more, because it already has a specific meaning which means less than 'field' but people already interpret fields like fluids in the loose sense because we're biased to everyday things, but we should do the reverse

vzn

@bolbteppa dude, admit it, the world field has gone thru a lot of shifting over the decades hasnt it! ps so are you a hardcore Landau fan or not? did you know of his cited work in the area? ie, fluid ≈ QFT correspondences...

bolbteppa

Right, he worked on a quantum theory of fluids with the restricted sense of the word fluid (as in 'fluid mechanics')

vzn

@bolbteppa was not aware myself, need to look further. more synchronicity :)

bolbteppa

Landau took classical fluid mechanics and worked on it's quantum version which is basically non-equilibrium multiparticle quantum mechanics

vzn

ok... yes have been thinking/ dreaming along the same lines for quite awhile now...

bolbteppa

03:18

The issue is that when you have such large systems, there are serious restrictions that arise which force things to behave the way they do

vzn

believe there is a relatively straightfwd way to get QM type phenomena out of classical fluids... have sketched it out for years (~2 decades now!), hasnt caught on yet :( :P

bolbteppa

e.g. when you have so many particles, the idea of a 'stationary state' of the whole system is impossible since there are so many energy levels close to each other for the particles so the energies become continuous, this is a really shocking thing if you think about it, and it leads to consequences which are not what you find for bounded particles

vzn

work in progress™/ under construction™/ some assembly reqd™

bolbteppa

The problem is to try to figure out what quantum theory reduces to classical fluid properties, because you can't just blindly apply QM because you've got these difficult details that people usually ignore in normal QM for small systems or QFT which is perturbation theory around free systems etc

vzn

yes, think have figured it out, no one is listening to me yet :( :P

bolbteppa

03:22

We know the starting point, we know the end point, the details are too hard in between

vzn

its a mystery™ ... ps have cited La Cour- Otts work along these lines in here, its been dismissed a few times already...

bolbteppa

The only fluid stuff of your's I remember is stuff that explicitly tries to link to solid mechanics which is kind of close to fluid mechanics and to note similarities in some limited case

vzn

@bolbteppa need to write more than a blog, maybe a paper + need some undergrads with diligence and shoshin :P like Couder-Fort :)

bolbteppa

@vzn I've told you that you're using the word fluid in a way that means 'field', have justified it, even studied one of the papers you use to justify yourself on and pointed out this was a study of a special case which is really no surprise at all, yet it seems like you will just ignore that and keep doing it

vzn

@bolbteppa lol do you want to take the red pill or the blue pill? the red pill is another chat room. the blue pill is this chat room. sometimes think you dont really want to get past the existing framework. theres a going-in-circles feeling around here sometimes.

bolbteppa

03:34

@vzn Things go in circles when it's pointed out to you that you're making a basic mistake yet keep doing it, like this is a first year calculus thing, the use of the word 'field', e.g. slide 2 here

vzn

@bolbteppa am not entirely disagreeing with you, but why dont we figure this out elsewhere? isnt it clear we both think its important to do so? am merely suggesting a different venue/ "shift in surroundings" could help. (my musings/ efforts along the lines are sometimes clearly not welcome here.)

bolbteppa

@vzn I am directly asking you to justify yourself, you can clearly do it as long as you listen and don't ignore points that are made - I think it's been over a year of using the word 'fluid' like it was some miracle answer to everything, the whole time you may as well have used the word field instead of fluid, but doing that you'd ask why doesn't QFT already do what you want, but the point is, based off a pidgin-understanding, piecing random things together,

vzn

@bolbteppa sigh its more than you assert/ devalue & we both know it. but maybe this is the end for the moment.

bolbteppa

the word fluid is mysterious and might answer everything magically, but simply replacing one word shows how flawed this is

vzn

@bolbteppa want to get into the details as you do at times but other times you dont seem to really want to.

bolbteppa

03:40

@vzn happy to go into the details, still waiting on an answer to the point about the Tenev paper I made like a year ago that it was nothing but a special case studying some limit which basically allowed an approximation that would obviously relate to non-relativistic continuum mechanics, that it was unbelievable to generalize this to a 'fluid theory of everything'

Again if you called it a field theory of everything, you'd immediately see you were calling some classical GR stuff a 'field theory of everything' which you immediately know is wrong

But that simple one-word switch has led to how many hours of time pretending classical field theory was some miracle cure

vzn

@bolbteppa "powers that be" around here dont like a mere ref to Tenev+Horstemeyer so can you please engage the topic in another room? think you will find it worthwhile with patience but maybe not instantaneously. (life is like that) how many yrs have we been discussing this topic in various forms now anyway?

bolbteppa

@vzn the problem was not discussing the Tenev paper, it was ignoring the pointing out of the basic/elementary flaws in what you were saying and instead repeating the point, I almost predict the use of the word fluid wont stop after this discussion but am hopeful maybe the point was finally made?

vzn

@bolbteppa not agreeing with your interpretation, but ... what is your objection to another room? can you plz articulate it?

bolbteppa

@vzn what's the point of going to another room, there's no point haha

vzn

@bolbteppa (lol) the point will become clear over time but maybe you prefer to miss the point :)

bolbteppa

03:49

Field (physics)

In physics, a field is a physical quantity, represented by a number or tensor, that has a value for each point in space-time. For example, on a weather map, the surface temperature is described by assigning a real number to each point on a map; the temperature can be considered at a fixed point in time or over some time interval, if one wants to study the dynamics of temperature change. A surface wind map, assigning a vector to each point on a map that describes the wind velocity at that point, would be an example of a 1 dimensional tensor field. Field theories, mathematical descriptions of how...

Fluid

In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress, or external force. Fluids are a phase of matter and include liquids, gases and plasmas. They are substances with zero shear modulus, or, in simpler terms, substances which cannot resist any shear force applied to them. Although the term "fluid" includes both the liquid and gas phases, in common usage, "fluid" is often used as a synonym for "liquid", with no implication that gas could also be present.This colloquial usage of the term is also common in medicine and in nutrition ("take plenty of fluids...

vzn

> In many applications, we do not consider individual vectors or scalars, but functions that give a vector or scalar at every point. Such functions are called vector fields or scalar fields.

bolbteppa

To completely ignore standard terminology is a big issue

vzn

↑ huh! sounds a lot like a fluid o_O :P

Navier Stokes is a set of eqns/ functions that "give a vector at every point". same with maxwells laws. and schroedingers eqn. etc

bolbteppa

@vzn "The mathematical description of the state of a moving fluid is effected by means of functions that give the distribution of the fluid velocity $\mathbf{v} = \mathbf{v}(x,y,z,t)$ and of any two thermodynamic quantities pertaining to the fluid, for instance the pressure $p(x,y,z,t)$ and the density $\rho(x,y,z,t)$. Such quantities are, in general, functions of the coordinates and of time."

Semiclassical

the old example of heat transfer as "caloric fluid" is a good one

bolbteppa

03:57

Here you see a fluid is modelled not just by a vector field for the velocity of each point in the fluid, but also by two scalar fields, the pressure and the density. Multiple 'fields' are needed to build up the notion of a fluid.

vzn

@Semiclassical you brought it up earlier & agree you have some good pt there.

@bolbteppa aha! so you found a correspondence, congratulations :)

bolbteppa

Literally need to know what thermodynamics is to even talk about it, which means you need statistical mechanics to exist

Semiclassical

it's not a perfect analogy, I suspect: i don't know how much "fluid mechanics" went into the idea of caloric fluid

but people did buy into the idea of it as some kind of substance/fluid, and they did so until it became clear that the kinetic theory of heat was more sound

vzn

have you guys heard of this one?

Two-dimensional electron gas

A two-dimensional electron gas (2DEG) is a scientific model in solid-state physics. It is an electron gas that is free to move in two dimensions, but tightly confined in the third. This tight confinement leads to quantized energy levels for motion in the third direction, which can then be ignored for most problems. Thus the electrons appear to be a 2D sheet embedded in a 3D world. The analogous construct of holes is called a two-dimensional hole gas (2DHG), and such systems have many useful and interesting properties. == Realizations == Most 2DEG are found in transistor-like structures made from...

bolbteppa

Right, Maxwell's equations use a four-potential vector field $A^{\mu}$, but fluid mechanics instead uses multiple fields $(\mathbf{v} = v^i,P,\rho)$

Semiclassical

03:59

Yeah. Learned about it in solid state

know a number of experts on it in our department

note: just need bog-standard QM / stat-mech for that

vzn

@bolbteppa right, and if you want a TOE it involves breaking down some of the (artificial! misleading! deceptive!) compartmentalization/ walls of modern physics. its not going to arrive without some.... discomfort aka cognitive dissonance :P

@bolbteppa wow speaking of that, look at this! Feb 2019/ Physicists create exotic electron liquid phys.org/news/2019-02-physicists-exotic-electron-liquid.html ... maybe you better write them ASAP to tell them theyre completely ignoring standard terminology and its a big issue :P

bolbteppa

Fluids use say two scalar fields and one vector field, quantum GR is about quantizing a tensor field, it is ridiculous to say two scalar fields and one vector field are the TOE

Semiclassical

except that, y'know, the phrase "Fermi liquid theory" is not new

hell, what people in condensed matter theory find interesting is precisely "non-Fermi liquid theory"

vzn

as long as we dont call it a caloric fluid its all cool right? :P

Semiclassical

considering there's no actual literal fluid

vzn

04:10

@bolbteppa "spacetime fluid-fabric" ≈ TOE

Semiclassical

quora.com/What-is-a-Fermi-liquid

"tl;dr: A Fermi liquid is a conventional real metal."

bolbteppa

So fluid mechanics is using thermodynamics concepts classically, you want non-equilibrium stat mech to reduce to fluid mechanics in some limit, I'm not sure if non-equilibrium stat mech thermo concepts have even been set up properly

Semiclassical

@bolbteppa there's a lot of stuff on non-equillibrium stat mech. my advisor wrote a whole book on non-eq field theory, even. (not that I went down that route)

vzn

@Semiclassical JR recently quoted site question relating sch eqn to heat transfer eqn... would take me a little time to dig it up...

bolbteppa

Yeah but I'm not sure how like finalized it all is

Semiclassical

04:14

@bolbteppa couldn't tell you

bolbteppa

But it's kind of a problem where people know where to start and where to end but the in between is hard

Semiclassical

@vzn it's simple enough: You get the heat equation from the time-dependent Schrodinger equation and go to imaginary time

so $t=i\tau$. (Or is it the other way around? I forget where the minus sign goes)

vzn

yeah. something like that :)

bolbteppa

What does '"spacetime fluid-fabric" ≈ TOE' mean, '"spacetime field-fabric = quantum field version of GR = quantized gravity = what everybody calls a TOE' I think

vzn

@bolbteppa am working out the details, need assistance/ts instead of mere skeptics or worse opponents... there is a difference :)

bolbteppa

04:17

If you replace the word 'fluid' with 'field' what you said just becomes what everybody wants, when you use the word fluid you get to pretend it's something different when it isn't

So the entire time this has all been about quantizing gravity normally, the Tenev stuff is actually just asking about semi-classical quantum gravity in some limit, this has all just been some test :p

vzn

sigh youre right! its not finished yet!

Semiclassical

going back to the starred comment of mine (the linked pdf)

the paragraph on the top of page 39 makes me smile, if only for the notion that a Bohmian interpretation of string theory may be easier to give than a Bohmian interpretation of QFT :>

I don't know if that's true, but it seems like it'd be a cosmic joke if it were

(I have a hard time trusting Nikolic, though. too many self-citations for me to be confident in him)

though, I do like the look of this recent paper of his: arxiv.org/abs/1811.11643

vzn

04:32

ah here it is

9

Q: Connection between Schrödinger equation and heat equation

If we do the wick rotation such that τ = it, then Schrödinger equation, say of a free particle, does have the same form of heat equation. However, it is clear that it admits the wave solution so it is sensible to call it a wave equation. Whether we should treat it as a wave equation or a heat e...

Semiclassical

my reaction to the second-to-last paragraph on page 13 of the paper I just linked is :S

vzn

EM waves ≈ heat

Semiclassical

basically arguing that the truly fundamental particles in nature ought to be described by non-relativistic QM...boy would that be convenient for the Bohmians

vzn

lol speaking in the 3rd person ofc :P

Secret

04:51

(No I have not read the transcript yet, thus this comment is out of place) I think there is a lot can be learnt from the bohemian ontology. Bohemian mechanics seemed to capture the concept of nonlocality better than the orthodox:

The fact that the trajectories can be influenced by a pointer somewhere "far away" carries a phisophical implication on free will:

We might think we are free to decide what happen (or in more non human terms, the clicking of a detector has nothing to do with anything except the particle of interest), but what if when the whole universe is taken account of, our set of possible choices that can be made is really a restricted subset of the full thing

nonlocal influences, thus to a local observer, is like a set of unpredictable and hard to account for constraint that restricts the possible future evolution of the system in question

The unpredictability is precisely what prevents it from being exploited for superluminal signalling

But what if we look further, and find a way to control these in a statistical sense. The outcomes will still be unpredictable as ever, but the probability that the outcome will fell towards our favour will be higher

So basically, the idea goes like this:

1. Superluminal signalling (which as far we knew, is impossible) is when classically an event A transfer information to another space like separated event B

2. Nonlocality is often established when events A and B interact some time ago in the past, and then the correlation is maintained, or for the slow pointer cases in bohmian, the pointer is coupled in real time with the bohmian particle thus reflecting where the bohmian particle is heading

Thus in terms of outcomes we have:

1. Deterministic: Measurements do not change the dynamics in an unpredictable way

2. Statistical and quantum: Measurement changes the dynamics in a probabilistic way, and/or a huge ensemble is needed to extract probability distributions and spectra of observables

But perhaps, we can go one step further. We are not allowed to physically move information faster than light, nor we can use nonlocal correlation to send a message. But how about:

3. Getting the statistics of the nonlocal influence themselves by modifying the initial conditions of the experiment setup in a systematic fashion, and then try to see which of these, when combined with the nonlocal influence, can restrain the wave function just right to maximise a desired outcome

In the trajectory view, the wavefunction is linear hence it can be resolved into a superposition. By modifying the initial conditions systematically and using some kind of tomography to probe the resulting wavefunction, it might be possible to deconvolute the component that is attributed to the environment. We can then probe this wavefunction that is contributed by the environment, and work out from its structure how to set up an experiment such that the resulting wavefunction,

when superimposed with the contributions from the environment, will be constrained in shape in just the right way so that the probability distributions and spectra will skewed to the desired outcomes

-> In other words, control the predestination itself

In the orthodox view, especially under operationalism, the outcomes and distributions are fully determined by how the experiment is set up, thus in theory we can exploit the nonlocal influence that may be established but not explicitly account for, and tune our experiment that way

If that works, then we can get a greater degree of control of the outcomes without violating relativity, since all that information is already there determined by the initial conditions and the wavefunction

Secret

05:40

But in theory, that should be feasible to study since the wavefunction is completely determined by the experiment setup and its evolution in the schroedinger equation. (3) may fail if on one of the experiment trials, the nonlocal part of the experiment setup is disrupted, thus preventing the extraction of the contribution to the wavefunction from the environment

That "quantum mechanics cannot consistently be applied to the user of itself" paper, in particular the section where they said what happens when C is violated, seemed to suggests experiment of the form similar to (3) can be tried, thus giving us some control on how the environment affect our experiments

dsm

05:53

explanations beyond ch.5 in Sakurai are quite poor. for second quantization, it is stated that "We expect $a_i^\dagger a_j^\dagger|\mathbf{0}\rangle = \pm \ a_j^\dagger a_i^\dagger|\mathbf{0}\rangle$ for a two-particle state." plus for bosons and minus for fermions. is this obvious? I understand the overall antisymmetry of fermionic wavefunctions, butwhy should the order of creating different states change this sign when acting on the vacuum state?

ah, I think I see it.

dsm

06:15

this approach to many-body is amazing.

not ch7 Sakurai, to clarify XD

John Rennie

08:02

This was a surprisingly interesting thing to research:

0

Q: Facts about element 173

I am watching a video about element 173 being the biggest and last element that can ever be assembled. The reason given as the last element that can be created is that the electrons would have to orbit faster than the speed of light for an element 174 to be allowed. I have 2 questions concernin...

atomic-physics atoms elements

I didn't know about Feynman's calculation for the maximum possible nuclear charge in an atom.

Captain Bohemian

08:29

there is such kind of particle! anyon - a particle that occurs in two-dimensional space having characteristics of both fermions and bosons.

I wonder which geometrical object should be used to describe it.

user351417

10:34

3

A: Formatting Sandbox II: please test stuff here

under construction Decision threshold and detection limit General aspects $$\textbf{Quantities and symbols}\\ \begin{array}{ll} \hline \text{Symbol}&\text{Name}\\ \hline y&\text{Estimate of the measurand}\\[-3pt] &\text{(e.g. measurement result of the measurand)}\\ u{\left(y\right)}&\text{Stan...

Abhas Kumar Sinha

11:05

Hello Everyone here

What do you think which is more important? Developments on Quantum Computing, Time Machine, Sci-fi Futuristic projects or Global Warming, climate change, Disturbed Ecosystem, Water imbalance, like stuff?

Kyle Kanos

11:19

Throwing in fake stuff (e.g. time machine) in with other real protects (quantum computing) seems like a bad idea.

Slereah

super rude

Physics Meta

-2

Q: How Perfect are High Impact Physics Journals?

Various physics journals have "impact factors" that indicate how much the articles in those journals are, on average, cited by other physics papers. The journals with the highest impact factors are generally thought to be the most exclusive, but they undoubtedly make mistakes at times. So what i...

discussion

-2

Q: Most Important Physics Paper Never Published

Not all physics ideas come to us through peer reviewed journals. Some come from journals where peer review can be skipped as Julian Schwinger did in publishing his Measurement Algebra papers at the Proceedings of the National Academy of Sciences i.e. Schwinger, J. (1960) Proceedings of the Nation...

discussion

jeea

11:52

I have a question regarding superconductors

In image b, the flux trapped in ring must be quantised so its value will be different from that inside ring in image 'a'

?

Also is there ever a current in bulk of material in case of multiply connected superconductors like ring? or the current only remains on surface (and penetrates from there some distance ~ 100nm or so) ??

Also will flux in the hole of ring be same in case of a, b, c? i guess same in b and c but due to quantisation condition, different in a??

Captain Bohemian

12:55

mails usually come out of expectation. Yesterday I got two such mails, one from my MSc advisor and one from my Quantum Mechanics teacher. I didn't expect they would reply me the mails.

actually I got the latter mail the day before yesterday but I didn't find it until yesterday.

bolbteppa

13:30

@Semiclassical that those BM papers actually entertain the idea all of physics comes from non-relativistic physics is unbelievable, its now not only denying the most basic claims of QM but also even classical physics, denying Einstein's special and general relativity, I can't believe this is taken seriously in the BM field

Good quote from those BM slides '“In recent years the debate on these ideas has reopened, and there are some who question what they call ‘the Copenhagen interpretation’ of quantum mechanics’ - as if there existed more than one possible interpretation of quantum mechanics.”Rudolf Peierls (1979)'

Semiclassical

13:46

@bolbteppa yeah, that seriously undermines Nikolic’s credibility for me

bolbteppa

I can't imagine a non-relativistic string theory as well haha

Semiclassical

I mean, the suggestion that the Bohmian strategy makes sense when applied to particles or strings, but not when applied to fields

That, if nothing else, is an interesting suggestion

But the idea that you could blithely “bypass” relativistic QM

That seems entirely lacking in credibility

Semiclassical

14:03

I find that previous suggestion quite funny, though. BM is an interpretation that mostly philosophers seem to like, whereas string theory is the sort of thing they turn up their noses at

So the notion of Bohmian string theory being more natural than Bohmian field theory? I kinda want that to be true now, simply for how hilarious it’s be

knzhou

14:20

It's honestly strange how popular BM is in philosophy.

I did a bunch of reading on this and it definitely seems to be their #1 favorite by a long shot.

Even though, in a poll of physicists working in quantum foundations, pilot wave was tied for last place!

There are a lot of very angry philosophy papers that say nothing besides, "why have the physicists been so stupid as to deny the reality of the pilot wave?" Of course, not citing any actual physicists.

bolbteppa

Not even all of them agree that the pilot wave is real

For e.g. the beginnings of bosonic string theory you take a classical field and Fourier expand it and promote the modes to operators as you do in QFT, for some reason this causes tons of problems in BM, I really can't see how BM could cover string theory but not QFT haha

Semiclassical

14:42

There’s a bit of a feedback loop there

Philosophers find it appealing, so they work on it...but they work on it as philosophers and not as physicists

So you don’t get people pushing BM in a way that would bring it into contact with contemporary physics practice

bolbteppa

The implication being that BM is internally consistent but physicists just don't appreciate or care about that haha

Semiclassical

I think BM mostly appeals to philosophers because it seems to avoid the measurement problem. But that’s not a criterion most physicists really care about, so that blunts a lot of the appeal

(My own predilection for it, is that it shows that there’s a natural set of trajectories built into the structure of the Schroeder equation.)

Bah, Schrödinger*

I think a plausible reading is that 1) BM is a natural enough interpretation of wave mechanics, but 2) wave mechanics does not teach lessons which are anywhere near as useful as those from matrix mechanics

It’s a funny little fact: when I think of how a typical textbook will present QM. It’s via wave mechanics (Schrödinger picture)

But when it comes to the orthodox interpretation, that’s very much coming from matrix mechanics rather than wave mechanics

So wave mechanics won out as the “face” of QM. But matrix mechanics won as the spirit of QM

@bolbteppa you might appreciate this paper, if only because he very quickly starts talking about first/second class constraints: arxiv.org/pdf/0707.3685.pdf

I trust Struyve more than I trust Nikolic, tbh

bolbteppa

15:16

"Instead he started by arguing that the degree of freedom $A_0$ in the theory of the classical electromagnetic field is a mathematical artifact, arising from the insistence on Lorentz invariance", sentences like this are simply madness tbh

Like this is denying classical electromagnetism very directly

"The pilot-wave models are further formulated with respect to a preferred frame of reference. In the context of a relativistic quantum field theory this will imply pilot-wave models that are not Lorentz invariant." I'm not sure why this is so, in QFT you can do everything non-covariantly but everything still has to be covariant in the end result, i.e. it just is because you're still just doing correct steps at each stage...

Again it just seems so inherently flawed if one has to do things which destroy Lorentz invariance

Semiclassical

Wouldn’t surprise me if it were so, frankly, it’s what I’d expect from Valentini (who he’s referencing in that first quotation)

As for the second: the basic issue, as I have understand it, is that the pilot wave idea reeeeally does not conform well with something as seemingly innocuous as the relativity of simultaneity

You can avoid that in an observational sense, ie a preferred space time foliation is necessary for the story to work, but the nature of the theory can be such that said foliation is not empirically accessible

but that seems a very strong violation of the spirit of relativity theory, even if in an empirical sense you’re protected from visible Lorentz violations

So yeah. This is why, while I’m willing to defend BM in the non-relativistic context

When it comes to relativity? Ughhhhh

A pertinent quotation from Gisin, in the vein that the biggest obstacle for BM is the Bohmian community itself:

“Bohmian mechanics, like quantum theory, is in deep tension with relativity theory. I know of Bohmians who claim that it is obvious that any non-local theory, Bohmian or not, requires a privileged universal reference frame. I also know of Bohmian who claim that it is ob- vious that Bohmian mechanics can be generalized to a relativistic theory
(though, admittedly, I never under- stood their model). But I know of no Bohmians who tries to get inspiration from their theory and its tension with relativity to try to go beyond Bohmian mechanics...”

bolbteppa

15:44

If you're happy to deny all of modern classical physics then sure, that this is not treated as crank stuff is shocking haha

I really just doubt it can't be merged with relativity properly

Loong

15:59

@Chair ?

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