The h Bar

4:28 AM

He is trying to give too much physical interpretation to the various components of the wavefunction. The phase of the wavefunction (which is usually where the complex part is at play) basically only affects what energy levels the molecule has. In the molecular orbital picture, we can treat the wavefunction as made up of many molecular orbitals interfering with each other. The magnetic properties of the molecule meanwhile depends on the whole wave function, not just the complex part as the author claimed

Semiclassical

Yeah

Secret

Finally, I really won't consider mass and energy as conjugates, as what he tried to claim with $E=mc^2$

though they can be lumped into a covariant quantity called the 4-momentum

Semiclassical

Agreed

There’s a lot there that just seems off

Devolves into gobbledygook pretty fast

Secret

The last line can probably be explained by how our brains like to base our thinking on things we are familiar with and extrapolate from it. It is how we get from simple theorems to more general cases by modelling it using the simpler cases

The commonality between wave particle duality, energy and momentum, electric and magnetic fields, is that they all are dependent pairs of concepts, thus the temptation to get them related is easy

Semiclassical

I guess

As a mode of poetry, I don’t mind it

Secret

4:37 AM

This phenomenon is particularly common in religious, esoteric and magical communities (though I might actually need to spend some time to dig up a sociology paper on belief attitudes someday to support my point here) it is the basis of how the concept of "family resemblance" lead to magical thinking, thus eventually leading to the cosmology in those circles that everything all stems from some same underlying thing

Semiclassical

But as a note in the “Journal of Chemical Education”, it’s rather WTF

Secret

Myself is also guilty of that kind of slippage, which is why having some physics knowledge and a physics community to keep track of my thinking can be helpful when the mind tries to take shortcuts on nonintuitive concepts.

On the other hand, it makes me extremely creative in problem solving

And yeah, sometimes, crackpot level material do slip through into legitimate journals, and is quite common especially in the past since we don't understood as much

Semiclassical

I have no idea what the standard for that journal is

So it’s hard to make any guesses

Secret

It's significant, given the large number of papers on it, and is part of ACS (hence have to have some quality assurance). I sometimes saw experiments on chemical education published there. But if my memory serves, it is not the most prestigious of the chemical education journals

pubs.acs.org/journal/jceda8

Semiclassical

Gotcha

Secret

4:43 AM

I forgot the name of the most popular since I am not in that field and only catch up with that knowledge by interacting with my collegues

::::::::::

As for the ontology of the wavefunction, if one insists to give one for it, I think most people nowadays will probably say it is really information. But accepting that ontology will end up with some pretty strange philosophical consequences such as it will imply everything in our world is made up of information, and not only that, that somehow by measuring it, you get something physical from it. (and if the maths is over interpreted, then you have this bizarre "why" problem
on why linear operations on this information somehow give you values of physical quantities

While information does has some kind of physical nature that we still don't understood as shown by a paper that used mutual information in entangled states to cool a quantum wire (to be dug) and also to use measurement to control a cooling engine:

journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.070603

It is still a bit early to say that everything, field and energy-momentum is really information.

Secret

5:04 AM

This one: journals.aps.org/pre/abstract/10.1103/PhysRevE.93.032147

Also an unrelated article stumbled upon when googling for the above:

nature.com/articles/s41586-019-0918-8

It basically say that to cool something, you can do it more literally by exploiting the chemical potential of a material so that infrared photons preferentially tunnel away to the material and hence drawing heat away

Captain Bohemian

6:20 AM

I don't like my Facebook posts to be arbitrarily liked.

I prefer my scientific posts to be given knowledgeable comment than being innocently liked.

Mauro Giliberti

6:52 AM

Hello everyone! Is the h bar accessible from the app?

If so, how?

John Rennie

@MauroGiliberti I get the impression the app has been abandoned. Just use the browser. The mobile site has been optimised for phones.

Mauro Giliberti

7:33 AM

That's unfortunate. Thanks!

Akash. B

7:51 AM

@JohnRennie I have a question

John Rennie

@Akash.B hi :-)

What's the question?

Akash. B

@JohnRennie I once asked you a question

about a ball kept on the surface of earth

John Rennie

@Akash.B yes, I remember

Akash. B

@JohnRennie if we keep the ball on an inclined surface ,it rolls

John Rennie

Yes

Akash. B

7:54 AM

is there any external force applied on it?

John Rennie

Yes, the external force is the gravitational field of the Earth.

Akash. B

@JohnRennie the direction of gravitational field is downwards ,right?

then how can the ball move towards the side?

@JohnRennie

John Rennie

Let me draw a quick diagram:

Akash. B

@JohnRennie yeah go on

John Rennie

Suppose we put the ball on a perfectly horizontal surface like this:

Gravity exerts a downwards force $mg$ on the ball. The surface exerts a normal force upwards on the ball.

Akash. B

8:06 AM

@JohnRennie okay

John Rennie

The two forces are exactly lined up so they cancel each other out and there is no net force on the ball. So the ball doesn't move.

But now let's draw the ball on an incline:

Gravity (the blue arrow) always acts straight down, and the normal force (red arrow) acts normal to the surface. So when the surface is not horizontal the two forces are not lined up and they don't cancel.

Akash. B

@JohnRennie oh

John Rennie

When you add up the two forces they partially cancel but not completely. So you're left with a force pointing down the slope. That's why the ball moves down the slope.

Akash. B

@JohnRennie so doesn't the surface of earth exert a force?

isn't the surface of earth curved?

John Rennie

Because if we assume the Earth is a perfect sphere then at every point on the surface the surface is locally perfectly horizontal.

Akash. B

8:12 AM

@JohnRennie how's it possible ?

is it an illusion?

John Rennie

Consider the point at the top in my diagram. Gravity always points towards the centre of the Earth so the gravity points straight down.

At the point at the top the angle of the ground is the tangent to the sphere, which I've drawn as the dashed line. The normal force is normal to this line.

The dashed line is horizontal so the normal force (red arrow) points straight up i.e. exactly opposite to gravity (blue arrow) pointing straight down.

Akash. B

@JohnRennie oh

John Rennie

So the two forces exactly line up and cancel each other out.

Akash. B

so its perpendicular

John Rennie

Yes

Akash. B

8:22 AM

@JohnRennie is the ball really siting on a horizontal surface?

ahh this question makes my head ache

John Rennie

They got the length contraction wrong (it would be Terrell rotated) but we can't have everything :-)

Secret

10:47 AM

lol at that diagram

Physics Meta

12:37 PM

0

Q: Where am I supposed ask questions regarding Physics education/research?

I asked this as a soft question How to explore Physics research as an undergraduate, keeping in mind the ever increasing weightage on publications? I have explictly asked if this is the right place to post this and if not please suggest an alternative. One of the contributors suggested "Academia...

Akash. B

@JohnRennie terrell rotation?

Semiclassical

2:00 PM

Simplest version is this: suppose you were to fly past a spherical object at relativistic speed

You might expect to see length contraction, ie the sphere would appear to be compressed along the direction of your motion

But actually you’d still see a spherical object. The surface of said sphere would look different, but the overall shape in that case wouldn’t

vzn

4:28 PM

@Semiclassical did you see this? so far bohmian mechanics hasnt been linked to QM scarring but suspect its just a matter of time. Probing many-body dynamics on a 51-atom quantum simulator nature.com/articles/nature24622 while not in abstract this paper is linkked to scarring in this article

Mar 20 at 21:45, by vzn

Quantum Machine Appears to Defy Universe’s Push for Disorder / quanta mag

Semiclassical

hmm

i don't know about the newer experiment

but that bit on quantum chaos is one where a Bohmian perspective could be useful

that's a good article

reminds me of nothing so much as

Quantum revival

In quantum mechanics, the quantum revival is a periodic recurrence of the quantum wave function from its original form during the time evolution either many times in space as the multiple scaled fractions in the form of the initial wave function (fractional revival) or approximately or exactly to its original form from the beginning (full revival). The quantum wave function periodic in time exhibits therefore the full revival every period. The phenomenon of revivals is most readily observable for the wave functions being well localized wave packets at the beginning of the time evolution for...

but that reflects a case where the energy levels are rational

which seems strange to expect in this context

vzn

@Semiclassical am having trouble following the nature article from just the abstract vs the popsci writeup. Bernien et al say its a simulator but Woo writes about it as if its a physical experiment. anyway my vague handwaving idea is that pilot waves could be linked to scarring somehow.

Semiclassical

4:44 PM

well, note that if what they're doing is a simulation, then it's already incorporating QM

so it won't be a scenario where BM says one thing experimentally and QM says something else experimentally

that said, it'd be interesting if the Bohmian formulation of the story is more intuitive etc

vzn

@Semiclassical to me simulation typically refers to software/ algorithm(s). but maybe here term is used in the sense that an artificial/ contrived but real atomic arrangement can in a sense simulate other types of interactions.

Semiclassical

my read so far is that it's based on numerical/analytical calculations

but i can't quite figure that out either

I think the point is that the Turner et al is numerical/analytical

whereas the Bernien paper is about an actual experimental ssystem

vzn

its cutting edge stuff and can see they are themselves maybe struggling somewhat with new vocabulary/ concepts etc., prob half my confusion would be cleared by reading the article. only $9 o_O

Semiclassical

which paper?

the Bernien paper is on arxiv: arxiv.org/abs/1707.04344

bolbteppa

How does BM explain the whole instability of atoms thing, the whole electrons radiating all their energy almost immediately (Larmor)

Larmor formula

The Larmor formula is used to calculate the total power radiated by a non relativistic point charge as it accelerates or decelerates. This is used in the branch of physics known as electrodynamics and is not to be confused with the Larmor precession from classical nuclear magnetic resonance. It was first derived by J. J. Larmor in 1897, in the context of the wave theory of light. When any charged particle (such as an electron, a proton, or an ion) accelerates, it radiates away energy in the form of electromagnetic waves. For velocities that are small relative to the speed of light, the total...

vzn

4:54 PM

@Semiclassical (thx!) Bernien Nature 2017 doesnt even use word "scar" in abstract.

Semiclassical

@vzn yeah, the Turner et al paper seems to be where that's introduced

bolbteppa

"A classical electron orbiting a nucleus experiences acceleration and should radiate. Consequently, the electron loses energy and the electron should eventually spiral into the nucleus. Atoms, according to classical mechanics, are consequently unstable. This classical prediction is violated by the observation of stable electron orbits. The problem is resolved with a quantum mechanical description of atomic physics, initially provided by the Bohr model. "

vzn

@Semiclassical just searched paper and scar is not inside it either.

Semiclassical

@vzn arxiv.org/abs/1711.03528 is what I have in mind

vzn

it was maybe feynman who introduced idea that a QC is much like a quantum simulator or can be utilized as such.

Semiclassical

4:58 PM

@bolbteppa my naive guess would be that, in the bohmian story, the electron doesn't radiate

hmm

vzn

@bolbteppa @Semiclassical since youre both here had a proposition. re your recent conversations about QM vs classical systems analogies. wanted to start a longterm investigation along these lines. think it is in line with your bkg/ interests/ questions/ talents. however feel it could only be carried out "elsewhere" in some other (chat) room.

bolbteppa

Seems like the true path which actually exists but that we can't observe would have to not radiate energy continuously for some reason but reduce to a theory where paths now exist which do radiate energy

Semiclassical

yeah, the issue is not so much "why doesn't an electron radiate in this situation" but "why would it radiate in some contexts but not others"

bolbteppa

@vzn you can discuss such things here as we have been doing for a while now periodically

Semiclassical

I'm rather disturbed by the fact that I"m not seeing a discussion in the literature of it

bolbteppa

5:05 PM

One weird thing is this was dealt with before QFT was even developed properly

Semiclassical

yeah

this is why I say that, within the context it was developed in, the Bohmian story is internally consistent

but "internally consistent" and "satisfactory" are not the same thing

bolbteppa

I think this is one of the stronger arguments for needing QM too

@Semiclassical as you know I completely disagree it is even legitimate let alone consistent :p

Semiclassical

yeah, well, that's like, your opinion man

this argument about larmor radiation does bother me tho

bolbteppa

I bet one could copy the standard QM Bohr thing to deal with Larmor

Semiclassical

I'm not so bothered by the lack of Larmor radiation for an electron in orbit around an atom---it's easy enough to say "larmor radiation is a property of classical charged particles, not of Bohmian charged particles"

but that line of reasoning doesn't really work, since there are certainly cases where we do talk about larmor radiation from electrons

ACuriousMind

5:11 PM

I don't think there really is a "problem" as such with the radiation: Standard QM also essentially waves a magic wand and says "classical ideas of radiation are invalid here", so BM may do so, too. However, this seems to hurt the Bohmian metaphysics much more, since it makes evident that the "classicality" of the paths taken by the guided particles is questionable.

Semiclassical

if I was to guess, the out is that when you observe larmor radiation from electrons

you're really talking about a collective phenomenon rather than that of a single particle

vzn

@bolbteppa what is your obj to another room?

Semiclassical

But I'm not sure.

ACuriousMind

In particular, QM has not much of an issue if it says that both classical paths and Larmor radiation emerge only in its classical limits. But it is harder to see how BM is going to argue that while it had classical paths for the particles all along, Larmor radiation only emerges in the classical limit. To argue this, you'd need a full Bohmian description of how the EM field couples to charged particles.

Semiclassical

yeah

vzn

5:14 PM

@bolbteppa there is a classical experiment that nobodys done, suspect it will reveal electron orbits as an emergent property of a fairly straightfwd wave + interacting particle system. similar to the Couder-Bush oildrop experiments. undergraduates could potentially carry it out.

Semiclassical

@ACuriousMind yeah

I mean, that's not so bad in certain respects

If you can write down the appropriate wave equation, then getting the Bohmian story out of that is doable

But I'm always a bit anxious when it comes to electromagnetism, since that's wrapped up with special relativity

and BM definitely has issues there

Not going to lie, this does bother me

bolbteppa

Basically paths exist in BM, and the claim is that non-relativistic classical mechanics just doesn't apply to them, fine, so clearly relativistic mechanics isn't going to apply to them either, and electromagnetism is the interaction of relativistic particles with (relativistic) EM Fields, so there is no reason to expect the particles following those hidden paths satisfy any classical theories...

But here the claim is we are measuring systems which move along continuous paths which reduce to a theory of a different set of continuous paths now predicting the instability of matter only when our measurements are less and less accurate, i.e. we go from a theory predicting stability of matter to instability of matter depending on our level of measurement accuracy, waving my hands I can pretend it's like some Taylor approximation to see it :p, but it's not pleasant

Semiclassical

It's definitely something which somebody should have talked about in the literature

but I'm not seeing that so far, and that doesn't sit well with me at all

bolbteppa

It seems they were able to partially resolve this (Bohr model) before QM (let alone QFT) was even fully developed because QFT seamlessly reduces to QM in the non-relativistic limit which is kind of nuts given how bizarre QFT is

@vzn what is your objection to talking about physics theories in the physics chat :p

I really can't get over how shocking it is that simply changing the group (i.e. working with the Galilean subgroup of Poincare instead of Poincare) leads to such huge problems, you could analyze the issues with BM using cohomology (group extension) theory :p

vzn

5:29 PM

@bolbteppa will explain at length elsewhere but so many in here seem to be allergic to leaving this room, its apparently like a security blanket for some o_O

bolbteppa

Of course going from a subgroup to the larger group does in general lead to madness, but the physics tames the bigger group to be related to the subgroup in a manageable way, the way we get from QM to QFT, so it shouldn't cause such big issues for BM which should be a real motivation for relativistic BM'ers

vzn

holy @#%& stop the presses missed this just found this randomly googling, am in a bit of shock, couder-bush electron slit experiment supposedly debunked (late 2018). these are fighting words lol — Famous Experiment Dooms Alternative to Quantum Weirdness quantamagazine.org/…

bolbteppa

(and cohomologists :p)

vzn

> After perfecting their experimental setups, getting rid of air currents, and setting oil droplets bouncing on pilot waves toward two slits, none of the teams saw the interference-like pattern reported by Couder and Fort. Droplets went through the slits in almost straight lines, and no stripes appeared. The French pair’s earlier mistake is now attributed to noise, faulty methodology and insufficient statistics.

Semiclassical

yikes

I mean, I object to saying that that "dooms" Bohmian mechanics. It just means that this supposed experimental analogue is not valid

But if it's true, it makes me very annoyed at Couder et al

vzn

5:37 PM

feel this supposed debunking must be bogus on some level. think the correct interpretation is that they simply have a different experimental setup and didnt isolate the exact differences. expect a counter-rebuttal from Couder-Fort or et al... :|

Semiclassical

It's worth remembering that the Couder et al analogy was not intended to show that BM is right and QM is wrong

if anything, it was to argue that both were wrong and there's some fast time-scale dynamics

with pilot wave emerging when you look at moderate times and QM when you look at long times

(that's a gloss of course)

vzn

@Semiclassical the intentions have changed over time. the original intention was just purely experimental investigation. aka curiosity/ no agenda/ shoshin "beginners mind". there was never any double slit prediction. it was an idea they tried out after trying earlier simpler experiments. afaik original experiments were not even motivated by BM at all. etc

Semiclassical

So if the experimental results hold up, I'd say it undermines the idea that short-time scale dynamics are a credible explanation

@vzn true enough, but that's the story they got out of it

they may not have come to the experiment with that story, but that's the story they tell when discussing that experiment

vzn

there are different stories and they dont line up yet. dont think this is the final verdict at all. think it only shows how subtle and tricky it can be to study emergent dynamics. think not all the variables are being correctly controlled/ isolated yet.

Semiclassical

the following paragraph from one of T.Bohr's papers is pertinent to the point I"m making

bolbteppa

5:47 PM

"Classically, the electron should fall in. Charged particles emit Larmor radiation when they accelerate, so classically the electron radiates as it orbits and fall into the nucleus. This is a huge part of why we developed QM, it was known very well right away that there were serious foundational problems either with the Rutherford model or with electromagnetism.

Gravitational orbits, in the strictest sense, really shouldn't be possible either, since an accelerating mass emits gravitational waves for the same reason that a charged particle emits EM. However, due to the energy scales involve…

Semiclassical

"In the work of de Broglie a theoretical picture is presented, which is surprisingly close to the experiments on walkers. In his double solution theory, de Broglie proposes that, aside from the Schrodinger wave function, which only has statistical significance and thus is normalized, there might be a “real” wave, which acts like a guiding wave for a “real” particle, while the particle is responsible for exciting the wave [21]. This picture is very different from Bohm’s interpretation of quantum mechanics, where one solves the Schrodinger equation for the problem at hand and then lets the pa…

bolbteppa

I wonder if gravitational waves are ever factored into the whole moon coming closer to the earth thing

Nobel prize winning idea right here

Semiclassical

So I think T. Bohr's read on the situation would be that their experiments cast doubt on de Broglie's double-solution approach, while leaving the Bohm interpretation in the same place as it was (i.e., an interpretation of the physical predictions, not a modification of such)

bolbteppa

6:02 PM

researchgate.net/publication/…

"It explains how Bohmian particles need not radiate electromagnetically. It makes an experimentally testable prediction involving bremsstrahlung as well [19]"

"It is shown that the hydrodynamic interpretation of a charged quantum particle leads to a different theoretical prediction for low energy bremsstrahlung than does quantum electrodynamics (QED). In the calculations, the electromagnetic fields are treated classically in the hydrodynamic case, but are quantized in QED. "

Conflicts with QED though

Semiclassical

I'd be suspicious of equating a hydrodynamic interpretation with the Bohm interpretation

for rather the same reasons as the paragraph I cited

I think it's worth remembering that the Bohm interpretation is pretty minimal in its original context. the schrodinger equation dictates the time evolution of a given wavefunction, and that wavefunction tells the particle what trajectories are allowed

you can add additional stuff to that, and that's how I'd view hydrodynamic stuff

But I'm overall rather dissatisfied by my inability to find a discussion of this by people I trusst

vzn

6:30 PM

@bolbteppa hydrodynamic formulas must be mere approximations of a complex phenomena. so think its not so so accurate to refer to "calculations". as have long advocated what is needed are simulations...

Semiclassical

"must"

[needs citation]

vzn

but generally agree that the hydrodynamic model might make some different predictions than QED in extreme cases.

@Semiclassical this is a basic concept in fluid dynamics. many formulas used in it are understood to be approximations. eg think Navier-Stokes etc

Semiclassical

@ACuriousMind one partial objection here: While BM does have classical paths, it doesn't have Newtonian mechanics save in the classical limit

that's a statement of how they arise in those systems, yes

doesn't mean that, if a hydrodynamic-looking formula shows up, that it must have that origin

I think it's credible to say that their appearance suggests a "approximation to complex phenomena" interpretation

but I don't think it's logically sound to say that it demands it

@ACuriousMind a partial objection to this: While Bohmian particles do have classical paths (in the sense that you suppose that position as a function of time makes sense) it doesn't have Newtonian dynamics

so while Bohmian mechanics doesn't need to invoke the classical limit to get paths, it does need to do so if it wants to recover Newtonian mechanics

so I don't think it's quite so bad for BM. But it does bother me

vzn

@Semiclassical not an expert on this but eg navier stokes has key assumptions about viscosity and compressibility that are locally reasonable but break down with eg turbulent flow. en.wikipedia.org/wiki/Navier%E2%80%93Stokes_equations the general idea of comparison of hydrodynamics with QED is great, highly endorse it, but think the predictions of both are extremely complex to compare in detail and that any such projects must be regarded as tentative and not definitive aka work-in-progress.

Semiclassical

I'm not trying to say that a hydrodynamic interpretation of QED is absurd on the face of it, to be clear. (I can't say I'm terribly fond of it, but that's another matter.) But I hardly think that QED must be hydrodynamic

vzn

6:42 PM

@Semiclassical the paper is a clearcut case of the fluid paradigm slowly "emerging" as a research program but then trying to reject/ refute/ rule out it early on. think its a decades-long prj at the minimum.

Semiclassical

could be. could not be

until it actually does emerge, you're not in a position to draw that conclusion

vzn

@Semiclassical (lol!) ("expert") surfers can judge the emergence of waves :P

Semiclassical

expert surfers, yes

vzn

@Semiclassical the signs of its emergence are everywhere, its still building into critical mass, just looking at last few hrs of chat transcript is littered with aspects/ leads. its a very slow building wave.

ACuriousMind

@Semiclassical I don't think we need to defer to Lisi on this ;P

Semiclassical

6:51 PM

@ACuriousMind Going back to the earlier point, I wonder what the simplest treatment possible is in order to have a single quantum-mechanical particle which produces Larmor radiation

ACuriousMind

@Semiclassical Neglecting quantum corrections to the Larmor formula itself, you just need a non-stationary state whose expectation value for the time-derivative of momentum (the "acceleration") is non-zero, no?

Semiclassical

sounds right

ACuriousMind

I mean, at its core, the reason why the energy states of electrons in an atom don't radiate is that they are stationary states, right?

Semiclassical

ya

ACuriousMind

Of course, anything using the Larmor formula will necessarily be semi-classical in the sense that it does not quantize the EM field, but I guess this is appropriate given your username ;)

Semiclassical

6:56 PM

lol

that's a good way to put it, yeah

one is looking for a semiclassical treatment of Larmor radiation

my intuition would be that, if you try to do this with the free Schrodinger equation, you won't succeed

and that you'll need to do something more extreme, such as introduce a nonlinearity into your wave equation

ACuriousMind

@Semiclassical No solution to the time-independent Schrödinger equation will ever radiate, regardless of the system.

Semiclassical

yeah

ACuriousMind

If you want an example, this is just an exercise in finding an initial state such that the expectation value of acceleration is non-zero for the evolved state.

You won't find this for the free equation indeed, because all the momenta spread out in the same fashion.

Semiclassical

right

ACuriousMind

Although...

What about a travelling wavepacket, but in momentum space?

Semiclassical

7:02 PM

Hmm

What would a travelling Gaussian wavepacket in momentum space look like in position space?

and could you have that in the absence of a potential?

i mean, the free Schrodinger equation in momentum space would be $(p^2/2m)\tilde{\Psi}(p,t) = i\hbar \partial_t \tilde{\Psi}$

ACuriousMind

Probably not. After all, the Hamiltionian is $p^2$, making the situation asymmetric between position and momentum

Semiclassical

Right

an interesting case might be a particle in a harmonic potential

in that case you can prepare a coherent state and have it oscillate in both position and momentum space

ACuriousMind

should work

Semiclassical

yeah

and should be tractable

though, the expectation value for dp/dt should still be governed by Ehrenfest

and that'd seemingly preclude getting anything for <dp/dt> besides the usual force from the potential energy

ACuriousMind

@Semiclassical Ehrenfest says that for d/dt <p>, not <dp/dt>.

Semiclassical

7:08 PM

hmm, true

ACuriousMind

Though that latter expression is problematic since it only makes sense in the Heisenberg picture

the two expressions shouldn't be different, the former is Schrödinger, the latter is Heisenberg

Semiclassical

right

ACuriousMind

Ah! The catch is that Ehrenfest says that that expression is equal to the Expectation value of the (derivative of the) potential, not the potential

Semiclassical

oh, true

ACuriousMind

so you don't get the "usual force"

Otherwise this would be an argument that the stationary states in an electron radiate :P

Semiclassical

7:11 PM

hah

yeah

I dunno. I feel like we'd need to invoke something more than just the one-particle Schrodinger equation in order to get radiation

ACuriousMind

@Semiclassical Why? We don't have to invoke anything more than a single classical particle path to get radiation, either!

(+Larmor formula, in both cases)

Semiclassical

hmm

ACuriousMind

(I'm a bit distracted; I'm only applying half brainpower to this train of thoughts because I'm rewatching Trek's TNG after about a decade since I last watched it and it's very interesting how different it is from what I remember)

Semiclassical

lol

my impulse, i guess, would be to replace a single particle with a many-particle "lump" of such particles

with the idea being that, for a macroscopic particle, you certainly should see radiation

ACuriousMind

@Semiclassical But the classical limit is not "many quantum particles", so why do you want to do that?

Semiclassical

7:21 PM

yeah, that doesn't really hold up does it

ACuriousMind

The same sort of reasoning would suggest that quantum condensed matter theory shouldn't be a distinct field of study ;)

Semiclassical

lol

ACuriousMind

7:42 PM

lol, this TNG episode just used Fermat's theorem as an example for a puzzle we may never solve. (It was written before Wiles' proof)

Semiclassical

Ah, the Royale

funnily, they made a reference in DS9 to retcon that

ACuriousMind

Truly the most important plothole

Semiclassical

Quite

bolbteppa

8:15 PM

Can't find anything on QM corrections to Larmor in my normal sources anyway, lots of other EM things have QFT analogues

@vzn QED is probably the most successful theory in history right, different results to that are clearly a big issue

Semiclassical

Yeah. If you’re going to have a story about something underlying QED, your story had better explain why QED nevertheless works so well

ACuriousMind

@bolbteppa One issue I see is that you won't get any Larmor effects from a scattering-like treatment. I suppose the QFT corrections, if any, are not easily computed because you have to have control over the full evolution of the state, not just the asymptotic past/futire

bolbteppa

@ACuriousMind was literally just thinking that, you do get things like Rayleigh and Mie scattering analogues and was thinking ahh

Semiclassical

In that vein

2

Q: accelerated charge in QED

Classical electromagnetism says an accelerated charge would radiate. Is this understood in QED? Is there QED derivation of Larmor's formula?

electromagnetic-radiation quantum-electrodynamics

bolbteppa

I'm not sure though, it seems like you could get something along the lines of Larmor

One can look at relativistic electrons in external fields for which first order processes of emission and absorption can occur and in a sense it'd just be that so I'm not sure

Semiclassical

8:28 PM

A major gap in my understanding, I should note

ACuriousMind

@Semiclassical The answer there is semi-classical in exactly the opposite way that we were before - it treats the EM field as quantized but not the charged current.

Semiclassical

Yeah, which is interesting

ACuriousMind

This may be a sign that asking for something like the Larmor formula in a fully quantum theory doesn't really make sense

Semiclassical

My recollection of QFT is weak, and my knowledge of Bohmian attempts at field theory even more so

ACuriousMind

When you neither have straightforward radiation to emit (but photons) nor something that possesses a straightforward acceleration (but instead a nebulous quantum state), what exactly would we expect such a fully-quantum Larmor formula to apply to?

Semiclassical

8:31 PM

if you need to go to field theory to get a story for quantum Larmlr formula, then my expertise pretty much guarantees I can’t say much

ACuriousMind

In the end, the semi-classical treatments are only insufficient if we can construct an actual situation where they fail

bolbteppa

I think you could only get a Larmor formula analogue by including an external field

Radiation like this would be like emission or absorption which is first order which is banned in QED proper (without an external field)

Semiclassical

It’s a reasonable hypothesis

ACuriousMind

@bolbteppa fair enough

bolbteppa

But I'm kind of shocked the power distribution formula has no direct QM analogue

Student404Mus

8:45 PM

I'm confused with entanglement measure.

Since entanglement measures are LOCC invariants, then why local actions cause entanglement degradation

bolbteppa

Found it!!!

Student404Mus

Or we say, the thing that measures something changes has not to change?

bolbteppa

Eq. 45.8 of Landau vol 4

Dipole radiation in an external field gives the distribution

Semiclassical

@bolbteppa the closest approach I’ve seen is this paper: aflb.ensmp.fr/AFLB-303/aflb303m305.pdf

Oh nice

bolbteppa

One thing I wonder about is calling this stuff fundamental, because you are using an external field and so approximations, but at the same time you're using wave functions solved in that external field which is considered weak, is it like saying emission and absorption are like fake processes to say this isn't fundamental...

ACuriousMind

8:56 PM

@bolbteppa "Emission" and "absorption" are not "fundamental" because they are discrete processes: When we say that X "emits" something, we're always thinking about a process where, in one moment, X is in an excited state, and one moment later, X has emitted something and is now in a lower state.

But that's not how "real" processes work, there's always a messy intermediate state in whose asymptotic past you find the excited state of X and in whose asymptotic future you find the lower state of X and that something which it has "emitted".

That messy intermediate state does not lend itself to identify a discrete moment in time where the single entity X splits into another state of the entity X and the emitted thing.

bolbteppa

I kind of get what you mean I think :p

One thing I will say is this derivation looks simpler than the effort involved in going from the Lienard-Weichert potential to the EM field (which is just like taking derivatives :p)

ACuriousMind

It's the same with generic scattering. But there the messiness is confounded by people taking Feynman diagrams too literally and thinking the vertices in the diagram are "where/when the magic happens".

bolbteppa

'the messiness' seems to be this integral $\hat{V}_{fi}(t) = e \int \hat{j}_{fi} A d^3 x$ or it's conjugate

ACuriousMind

Without knowing what exactly the symbols there are, I'm gonna say yes because that looks like an interaction, and the asymptotic future/past are exactly the places where we turn the interaction off to be able to identify free particles again.

bolbteppa

So all the matter you see around you, you would say is just a collection of instantaneous free particle asymptotic qft interactions occurring all the time :p

ACuriousMind

9:08 PM

Most of the matter around me doesn't seem to consist of free particles

bolbteppa

There is a lot of bound state stuff in QCD which I really don't know how to square with the whole 'free particle scattering processes are all we can do in qft' thing

ACuriousMind

Condensed matter probably has much more to say about that "real world matter" than the type of QFT I'm familiar with

cows

11:49 PM

Just pooped out some symetries in a condensed matter system then slung out some code. Currently at the beach listening to migos while gulping sparkiling water and walking. Great things are coming I swear provided it all works out. Hehe

Some errors, . . . corresponding via phone

Was going to upload a photo but might not have that capability now.

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