The h Bar

Slereah

06:50

It is hard to find any details on Einstein's attempt at quantum theory

You'd think people would talk about it more

But maybe he just didn't do that much of it

Slereah

07:19

I can't even find Einstein's original paper for it

"Bietet die Feldtheorie Moglichkeiten fur die Losung des Quantenproblems"

einsteinpapers.press.princeton.edu/vol14-trans/193

ah, there it is

"My arguments will have already reached their goal if they cause mathematicians to collaborate and if they convince them that the path embarked upon is pursuable and absolutely must be thought through to the finish. "

He couldn't be bothered to do it himself

"I willingly concede that the derivation of equations (12) is not as compelling as one would like."

Slereah

07:49

"More specifically, it had been Gustav Mie who, in 1912, had proposed a way to overcome this dualism of particles and fields that was attractive to many physicists at the time (Mie 1912). Mie’s idea was to look for non-linear modifications or generalizations of Maxwell’s equations that would allow for particle-like solutions of the electromagnetic field. We should be able to interpret a solution as a particle if it is spherically symmetric, if the field intensity is very high only in a finite region of space, and if the field equations imply sensible equations of motion for those localized …

Neat

More Anonymous

10:51

@JohnRennie hey were you able to have a proper look?

(at what I sent)

More Anonymous

12:00

Also if someone can explain for me:

Where did $\dot z_q^0$ in the denominator come from?

Slereah

That is the Lorentz factor, IIRC

More Anonymous

@Slereah Wait isn't $\dot z_q^0$ velocity?

Slereah

Well the velocity of the timelike component is something like $\gamma \tau$

Or $\tau / \gamma$, I forget

More Anonymous

I get it's the time component of velocity ...

Slereah

Yes

More Anonymous

12:06

But Im confused how did you get that by integrating proper time

in the first line

Slereah

I think it is this : en.wikipedia.org/wiki/…

More Anonymous

Ah I see now

Thanks

Forgot tabout that

*about

More Anonymous

12:24

@Slereah also one can't apply the idea of Komar mass to FLRW metric? Can he?

Slereah

Well it's not stationary

More Anonymous

@Slereah Yea ... :/

Is there some book which complies and explains the various types of mass?

More Anonymous

13:54

@Slereah Or more precisely is there a nice way to define the mass of a fluid?

Slereah

14:08

The usual definition is fine, as long as you don't mind the mass varying

More Anonymous

I see :/

rb3652

Hi all

I have a basic physics question.

It's about a cable-car system

The maximum permissible mass of each car with occupants is 2800 kg. The cars, riding on a support cable, are pulled by a second cable attached to the support tower on each car. Assume that the cables are taut and inclined at angle θ = 35°. What is the difference in tension between adjacent sections of pull cable if the cars are at the maximum permissible mass and are being accelerated up the incline at 0.81 m/s^2?

I'm confused why the tensions would be in opposing directions.

The whole system is accelerating the same way, so why aren't the tensions aligned?

rb3652

14:49

Ah I figured it out.

cOnnectOrTR 12

I want to ask can we cold weld two aluminum foils?

William John

15:44

At first I wasn't surprised that electric field on by infinite sheet of plane is independent of distance. Now I am bit surprised after second time. Looking back at coulomb law electric field strength decrease as it moves very far away. Even after deriving from my hand it feels strange.

ACuriousMind

@WilliamJohn thing is, Coulomb's law doesn't say anything about infinite charge

an infinite sheet has, of course, infinite charge, and so you can't really argue about it like you can about more realistic finite arrangements of charges

you probably shouldn't expect completely unintuitive situations like having an infinite amount of charge to make intuitive sense :P

William John

sounds satisfying

cOnnectOrTR 12

Hi @ACuriousMind Can you answer?

Semiclassical

16:27

yet another perturbation theory question

a recent HW question i graded was as follows: Start with a hydrogen atom and introduce a perturbation of the form $$V=V_0+\beta_1 (3x^2-r^2)+\beta_2 (3y^2-r^2)+\beta_3(3z^2-r^2)=V_0+\sum_k (3\beta_k-\sum_j \beta_j)x_k^2$$ Problem was to compute the first-order correction to the four-fold degenerate first excited state

so one plunges into doing degenerate perturbation theory, working in the subspace $|200\rangle, |211\rangle, |210\rangle, |21-1\rangle$

all of the shifts contain $V_0$ so i'll ignore it. then one finds the first-order shifts to be 0, $2\beta_1-\beta_2-\beta_3,2\beta_2-\beta_3-\beta_1,2\beta_3-\beta_1-\beta_2$ up to multiplicative constants

my question is whether there was an easier way to see that the shifts had to be of that form.

the first one is relatively easy: $|200\rangle$ is $l=0$ thus spherically symmetric, and this is enough to rule out the terms besides $V_0$ in the interaction

but with the others i'm less sure. it seems like there should be more to say

especially since the states one obtains end up having rotational symmetry around the $x$-, $y$-, and $z$-axes respectively

best i can think of right now is that the usual $nlm$ basis for the hydrogen atom isn't the most useful basis in this context, since it privileges the $z$ axis

Aamirshah SHAIK

16:49

is potential energy the energy an object would gain if a scenario would happen

So is it real

John Rennie

@AamirshahSHAIK Suppose you lift a 1kg mass up by a distance of one metre, then you do work of W = 1kg × g × 1m. Yes?

Aamirshah SHAIK

yes

John Rennie

(g is Earth's gravitational acceleration)

Aamirshah SHAIK

but it's when it would happen ; for potential energy it didn't happen yet

John Rennie

That work went into increasing the potential energy of the mass i.e. after you finished lifting the mass its PE had increased by 1kg × g × 1m.

So that's real energy. Yes?

Aamirshah SHAIK

16:54

but it's not a guarantee that you'll get that energy back into kinetic : the earth might dissapear ; explode

i know but what if like ; you get my point

John Rennie

The Earth can't just disappear. Mass is a conserved quantity so it can't just vanish.

You are guaranteed that you can get the energy back.

Aamirshah SHAIK

not scientifically speaking ; like philosophically speaking if it dissapear would you get it back

the moon might hit the earth ; cause a black hole ; and it goes away , and the object is miraculously not attracted in the black hole

John Rennie

> the object is miraculously not attracted in the black hole

"miracles"?

That's religion not physics :-)

Aamirshah SHAIK

imagine ,

+

John Rennie

You mean imagine something that isn't physics, then expect physical laws to apply to it?

Aamirshah SHAIK

17:00

So potential energy is just a convention that i'll get it bakc ;so theoritically if the earth dissapear i wouldn't get it back ?

John Rennie

In physics we classify forces as conservative or non-conservative.

With a conservative force you are guaranteed to get the PE back. With non-conservative forces you don't have this guarantee.

And gravity is a conservative force, so you will always get the PE back.

If you allow the Earth to disappear you would have to reclassify gravity as non-conservative, and then you would not necessarily get the PE back.

But this is science fiction not physics.

Aamirshah SHAIK

iget it but ; if the earth dissapear or the object is teleported in deep space ; you wouldn't get it right

get the Energy put into it

John Rennie

Well yes, but that can't happen in our universe so I am not lying awake at night worrying about whether I'll get the PE back or not.

Aamirshah SHAIK

😃finally

John Rennie

Finally I've admitted that if you break the laws of physics the results don't agree with the laws of physics? :-)

Aamirshah SHAIK

17:08

i guess so it's an imaginary energy that we expect ; right so i get your point but 99% we'll get it back

So i wanted to know about why energy is negative in electrons shell ; since we add it's real kinetic energy to an imaginary ; that why i wanted to be sure that potoential energy is what i think its

John Rennie

It's because the absolute value of the potential energy is not defined. We can only ever measure changes in potential energy not its absolute value.

That means we can choose the zero point for the PE to be anywhere we want.

Aamirshah SHAIK

What about relative to center of earth

because when it comes to electrons ; coulomb's law it's mesured from the center of proton & electron; but when it comes to earth we approximate it by taking ground as 0 PE

Semiclassical

i wonder if what a good book would be that combines group theory with degenerate perturbation theory

Aamirshah SHAIK

@Semiclassical Google it ; i found several pdfs

Semiclassical

hmm

Aamirshah SHAIK

17:23

i just copied what you wrote

ACuriousMind

just because a book contains the words "group theory" and "perturbation theory" doesn't mean it is about either of that, nor that it's any good

@Semiclassical what kind of "combination" are you thinking of there?

most physics texts already butcher "group theory" (representation theory, really) when they're not trying to also teach something else :P

Semiclassical

lol

something along the lines of being able to guess which basis to use, given the symmetries before/after perturbation

you can always just diagonalize, of course

ACuriousMind

I'm not sure there's more to it than that, in general

More Anonymous

17:53

Does this definition of mass density vary in FRW metric? (I think not).

$ p^\mu = T^{\mu \nu} u_\nu$

So: $ m^2 = p^\mu \p_\mu$

where $m$ is the FRW metric

Anyone else can also help

Slereah

I can't say looking at it like that, but probably?

More Anonymous

I mean the covariant derivative of both $4$ velocity and the stress energy tensor is 0

Also why do people say there's no local energy density in gravity

?

raf

18:35

In special relativity: If for S and S' frame are moving along x axis, we can write $v'_x = \frac{dx'}{dt'}$, right?
But what does $\frac{dx'}{dt}$ signify in this context?

bolbteppa

19:32

@Slereah What about his epr paper

Slereah

does it talk about overdetermination?

bolbteppa

What is overdetermination

I guess in one sense it is about that maybe

But he was trying to say it's a paradox

I'm not sure that he ever tried to reformulate QM in a new way, instead trying to find gaps in it

Slereah

20:07

He did, but I can't find any details about it

except that one paper but he's being a bit vague

Basically his idea was that the discreteness of QM experiments was because the laws of physics also imposed restrictions on boundary conditions

And he thought that it might be due to overdetermined EoM, ie more PDEs than degrees of freedom

So that only specific solutions worked

I can't find any more papers on it, and I can't find any paper that even cites it except for papers on the history of Einstein

So it was probably a very stupid idea

bolbteppa

21:01

This one looks pretty good

> Another idea, which he considered repeatedly, was that quantum restrictions arise due to overdetermination in the system of differential equations, if the number of equations is larger than the number of independent variables. Although unable to develop this idea very far, Einstein mentioned it in a published paper (Einstein 1923), as well as in his later attempts at a unified field theory.

Slereah

21:30

yeah he never really gives an actual example

It's just a vague idea

Slereah

23:35

but I guess if you're Einstein, you can just throw around ideas like that

bolbteppa

23:51

Having skimmed some things, I don't get what he was trying to say

also not every quantum system is discrete so

Slereah

This was 1923

I will forgive him for having vague ideas

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