The h Bar

skullpatrol

12:04 AM

::recalls Aristotle saying something similar::

Ryan Unger

@ACuriousMind yes it is

skullpatrol

in The Ivory Tower, Jun 1 at 14:59, by skullpatrol

Aristotle’s Lyceum

►

dmckee

4:45 AM

@skullpatrol When he was 22. But he was ROTC, so he went to OCS and off to the Navy.

skullpatrol

5:26 AM

cool

Jake Rose

5:42 AM

@JohnRennie just came out of Magdalene may ball

I love may balls

They are beautiful

John Rennie

@JakeRose you sound suspiciously high :-)

Just oxytocin, or might alcohol be involved as well? :-)

Slereah

7:58 AM

mornin

Captain Bohemian

8:35 AM

@MoreAnonymous are positions in condensed matter physics easier to get?

@bolbteppa are these theses funded positions for applications? If funding is still uncertain, then even if one applies for one of the theses and gets acceptance of the principal investigator, whether they can work with that thesis still depends on whether funding is available.

Jake Rose

9:17 AM

@JohnRennie I am drug free if we don’t include caffeine and ethanol ;) never seen the value tbh

John Rennie

@JakeRose I did security at a few May Balls, but I never went as a paying guest. They aren't really my thing. I have no romance in my soul :-)

More Anonymous

9:31 AM

@CaptainBohemian depends where I guess ... but I knew a prof in durham university how thought I'd b good at it because he liked my insightful questions I'd ask ... Compared to that I couldn;t get anywhere for quantum gravity ... So in my experience yes

but to be fair networking can play a massive role in these kind of things

Slereah

9:44 AM

Hm

Trying to find a timelike curve in my dumb spacetime is trickier than expected

More Anonymous

@Slereah since ur a boss in GR ... Is there an explicit calculation showing the equivalence principle for stress energy tensors? (the reason I ask for this is instead of the usual line-elements is that fields cannot be thought like this)

Slereah

Who told you I was a boss in GR

I am awful

More Anonymous

really ?? I thought u were good at it ... L

(I presumed myself)

Any hints on how to approach this problem I posed?

John Rennie

10:01 AM

@MoreAnonymous what do you mean by the equivalence principle for stress energy tensors?

More Anonymous

Equivalence princple = It is a physical principle in which given Einstein's thought experiment setup: One cannot distinguish if the lift is under the influence of gravity or acceleration using any experiment within the lift

John Rennie

The equivalence principle is just the statement that the four acceleration is the sum of a coordinate term and the curvature term.

More Anonymous

I wanna somehow show a relation between the stress energy tensors of both cases

John Rennie

The four acceleration is:

$$ A^\alpha = \frac{\mathrm d^2x^\alpha}{\mathrm d\tau^2} + \Gamma^\alpha{}_{\mu\nu}U^\mu U^\nu \tag{1} $$

But neither of the two terms on the right are tensors so both are coordinate dependent.

That is either term can be made zero by an appropriate choise of coordinates.

More Anonymous

yea ... but I'm not sure if u can apply this for a field ...

John Rennie

10:05 AM

@MoreAnonymous Well the coordinate term is zero in the rest frame of the observer and the curvature term is zero in the Fermi normal coordinates. So I guess you need to write your SE tensor in both coordinates and compare the two versions.

More Anonymous

Alright ... I think it's best to just ask the question on PSE ... I think it's not a dumb question?

sigh ... Im really rusty with GR

PM 2Ring

@JohnRennie That's almost an answer to this question, but it might be too technical for the OP.

Slereah

samuel-lereah.com/articles/Physics/dumb-metric

Working on a more rigorous proof for that dumb metric

More Anonymous

hahaha

Any bookk recommendations for someone who wants to revise GR? like I'd prefer it to complete yet fast paced ...

John Rennie

@PM2Ring I'll have a look ...

PM 2Ring

10:18 AM

@JohnRennie Thanks. I added a related question, but I don't know if it's close enough to be a dupe target.

Secret

I am thinking about liquid spacetime

skullpatrol

@MoreAnonymous if you want fast paced try Relativity: the special and general theory by Einstein

Secret

Worse than chaotic spacetimes where the metric varies chaotically from event to event, the metric itself changes in such a way such that it behaves like a fluid

More Anonymous

Ugh ... He should put a warning out on his Boltzman brains

Sean Carroll - Extracting the Universe from the Wave Function

►

@skullpatrol I was hoping for something with more exercises (plenty of examples would b ideal) and more mathematical (something a physicist can still digest though)

And more modern as well

(Skimmed past the book)

PM 2Ring

10:46 AM

The worst thing about being a Boltzman brain? Boltzman zombies!

Physics Meta

0

Q: Ashamed of my own answer. Should I delete it?

I have been on physics stack exchange for about four years. Some of my answers I am proud of some of them not so much. There is one particular answer where I did my best at the time but I lacked the knowledge to understand what OP was asking. Some people however liked the answer and nobody else r...

Captain Bohemian

11:34 AM

@JohnRennie isn't this just the covariant derivative of the 4-velocity of an object? I didn't think of it can be an illustration of equivalence principle.

@MoreAnonymous I think that the gravitational energy-momentum is not a tensor but a pseudotensor is just an illustration of equivalence principle.

More Anonymous

Feel free to answer here

0

Q: Equivalence principle for stress energy tensors?

Is there an explicit calculation showing the equivalence principle* for stress energy tensors? (the reason I ask for this is instead of the usual line-elements is that fields cannot be thought like this). $*$ =It is a physical principle in which given Einstein's thought experiment setup: One c...

general-relativity field-theory stress-energy-momentum-tensor

@CaptainBohemian

Captain Bohemian

@MoreAnonymous is writing to researchers who work in topics similar to my MSc thesis one kind of network? but usually they would just reply me that have no funding.

ACuriousMind

@MoreAnonymous I don't understand what "equivalence principle for stress-energy tensors" is supposed to mean

More Anonymous

well knowing ppl helps ... Like the prof in durham was my lecturer for stat mech. ..

@ACuriousMind I mean if we are in einsitein's life the stress energy tensor of the lift should have some kind of relationship with the stress energy tensor of the life in a graviataional field

ACuriousMind

What's the "stress-energy tensor of the lift"?

More Anonymous

11:49 AM

well we know kinda a line element since it's accelerating ... right? and then I can go EFE and find the stress energy tensor ... though I wanted to know if u could do something similar for fields ?

Captain Bohemian

a pseudotensor is coordinate related and you may choose a coordinate to make it vanish.

Slereah

I mean you know

Energy itself is coordinate related

it's not that weird that it would be

ACuriousMind

Particularly when involving gravity, you need to be careful to define what you mean by "stress-energy tensor".

Are you deriving it from an action? Are you defining it operationally through 4-momentum fluxes? Is it a tensor or a pseudo-tensor? Etc...

John Rennie

@CaptainBohemian the equivalence principle is the fact that by choosing appropriate coordinates you can make the Christoffel symbols zero or the coordinate acceleration zero. The former is what Newton would call gravity and the latter acceleration. So the two are interchangeable.

Slereah

Also there's the whole stress energy tensor v. canonical stress energy tensor v. Beliphantes tensor business

quite dreadful really

Captain Bohemian

11:53 AM

but usual energy is a tensor, which follows usual transformation law when you make coordinate transformation, and if it vanishes in a coordinate system, it vanishes in all coordinate systems.

Slereah

Well no

Energy isn't a tensor

and you can indeed have no energy in a frame and energy in another frame

Just consider the case of classical mechanics

rest frame v. Galilean boost

Although things are more complex with GR involved certaily

Captain Bohemian

gravitational energy-momentum is a pseudotensor which follows some physics principles but don't follow the usual coordinate transformation rules for tensors.

Slereah

true

Since it is basically derived from Christoffel symbols

The basic idea being to have $\partial_t (T_M + T_g) = 0$

Since you have $\nabla T_M = 0$ you can sort of work out how it's derived

More Anonymous

@ACuriousMind Im not really well versed in the technicalities .. I was thinking about the following: (forget about fields for the moment) .. If I think in terms of line elements ... The line element of the lift in presence of acceleration enables me to get a stress energy tensor ... Now the measuring apparatus is inside lift is also inside the lift undergoing acceleration ...

So using these I should be able to make a statement about the possible measurments such as energy difference, etc. On the other had I have the lift in the presense of a graviations field ...

Now re-enter fields ... Is it possible to do the above analysis in terms of fields (since they have a different starting point (no-line element))

?

ACuriousMind

I don't understand what you mean by "the line element of the lift", either

"Line element" is usually just another name for the metric

Slereah

12:10 PM

It's also a section of the projective vector bundle

though I doubt that it is here

John Rennie

@MoreAnonymous but suppose you're in space near the Earth, then the stress-energy tensor at your position is zero.

More Anonymous

wait i think u guys re right and Im confusing myself

John Rennie

You can't determine the SE tensor by measuring proper acceleration. You have to do some curvature measurement then you can get the SE from the Ricci tensor.

More Anonymous

but can't u gain the chirstoffel symbols by knowing the proper accelreation

Slereah

you may if you know the acceleration of a geodesic congruence, I think

Since knowing the Christoffel symbol at a point won't help you much

since you need to know the derivatives too

And knowing a geodesic congruence is basically what curvature measurement is, anyway

just the spread and shear and twist of the congruence

More Anonymous

12:18 PM

alright ... I kinda get ur point

sigh ,.. I really need to revise GR

getting outta touch

any book recommendation?

Slereah

44

A: Books for general relativity

This list is extensive, but not exhaustive. I am aware that there are more standard GR books out there such as Hartle and Schutz, but I don’t think these are worth mentioning. Books with stars are, in my opinion, “must have” books. (I) denotes introductory, (IA) denotes advanced introductory, i.e...

More Anonymous

thannks

PM 2Ring

Speaking of GR, nice work on physics.stackexchange.com/a/487103/123208 John. Thanks! It could make a useful dupe target, but I guess we need the OP to clarify the question a little more.

Ryan Unger

12:40 PM

@Slereah doing god's work

I need to update that list

Slereah

Also we need to make a list of worst books for GR

More Anonymous

hahaa ... any popular books out there which should join the list :P

" worst books for GR"

Slereah

amazon.fr/RELATIVITY-Theory-Everything-John-Duffield/dp/…

Ryan Unger

@Slereah sachs-wu

@Slereah @ACuriousMind please yeet this into the shadow realm

I will not have my friend assaulted

Slereah

You traitor

Sachs-Wu isn't that bad

Certainly not the best introduction

Hm

what's a really awful one

Besse certainly isn't great as far as notation goes

More Anonymous

12:51 PM

wow!

This book claims that matter, energy and space are the same thing, with the particles we observe being 'knots' of space. This book is self-published, after having been rejected by a number of publishers. I would like to say that Deepak Chopra would turn down this author.

Ryan Unger

besse is a good book

More Anonymous

savage

Slereah

If I am paid enough I am willing to write a GR book entirely in Penrose notation

Ryan Unger

If I do GR for my PhD I'll write a GR book one day

More Anonymous

I actually met Penrose once in Nottingham

Slereah

12:52 PM

Any specific topic?

Ryan Unger

the book or the PhD

Slereah

Ironically I don't think Penrose ever made any extensive use of penrose notation

it was just an idea he threw around

@RyanUnger Both I'd suppose

More Anonymous

haha ... thats funny

Ryan Unger

I think there's a real lack of a book that addresses a good cross-section of GR topics that mathematicians could find interesting

Choquet-Bruhat talks about a lot of them but the book has so many issues

JMac

@MoreAnonymous That's the type of book I would write, except I wouldn't try to get it published, I would just self-publish for laughs to see if anyone actually tried to understand the word-salad. It's always seemed really alluring to me to try to write a nonsense "theory" just to get a bunch of parody out of my system.

Slereah

12:54 PM

what would a mathematician find interesting

Ryan Unger

AFAIK none of the minimal surfaces stuff has made it into books

More Anonymous

@JMac ... ur actions would seem synonymous with evil

Slereah

Minimal surface stuff is as far as I know more used for the study of extended objects than GR proper

although I don't actually know

I am trying to write a general thing on extended objects currently

JMac

@MoreAnonymous Just Poe's law in action. It's basically impossible to tell who is serious or not online

Slereah

although right now I have a much nobler goal

which is to make up a really stupid spacetime

samuel-lereah.com/articles/Physics/dumb-metric

The dumbest

There are errors I'm aware, didn't fix all the computations

But it's the law : every counterexample must use $\sin(1/x)$ somewhere

3

extended objects will be here soon, inch'allah : samuel-lereah.com/articles/Physics/extended-objects

More Anonymous

12:59 PM

Anyone read this book

amazon.com/dp/3642214517/?tag=stackoverfl08-20

?

Seems ambitious

Slereah

I have not

More Anonymous

got it from the least voted answer in book recommendation post

Slereah

it's hard to know what the worst GR book is because it's probably some crank book that nobody's ever heard of rly

More Anonymous

it might b a worthwhile goal: "find the worst GR book"

:P)

Slereah

I don't think I have a book that is really bad

More Anonymous

1:04 PM

I feel like now upvoting that post ... So someone can actually tell me how much baloney is in that book

I wonder if it would fall under the ethical use of upvote

Now, on a more serious note (some QM) ... how does the program where they have wavefunction of the universe . How do they go about deifning a measurment

?

Slereah

Jesus takes the measurement

More Anonymous

I was actually being serious here ...

Slereah

as thus

More Anonymous

noooooo

I mean like how they overcome this kind of objection

2

Q: Energy cost of the measurement without perturbing the system?

Background Let's say I have a Hamiltonian $\hat H$ (assume the Schrodinger equation) and it be in an arbitrary eigen-energy state: $$ \hat H_{\text{system}} | m \rangle = E_m |m \rangle$$ And I want to measure the momentum of the system without perturbing the Hamiltonian. We know this will...

quantum-mechanics quantum-information measurements

Slereah

not a clue

More Anonymous

1:09 PM

@Slereah I notice u have a stash of memes available instanteous

-ly

Slereah

I use the ancient art of google image search

More Anonymous

theres a meme with lubos on it as well on google search top result

?

Slereah

no, I made that one

it is the official logo of PSE

Ryan Unger

I remain unconvinced that QM is not just due to physicists being bad at experiments

Slereah

Or at least so I would like to think

Feel free to try to prove it wrong

More Anonymous

1:11 PM

oh I thought those google algorithm was really amazing for a moment

@RyanUnger if u have a seperate way to resolve the conundrum in my post ... please answer :)

Slereah

There is kind of a Thing where it's kind of hard to do experiments wrt a model without some assumptions

Because you don't actually know the initial conditions properly

and measuring them is impossible without some assumptions beforehand

like how you have to assume that distances are roughly euclidian over a short enough time when you're doing the light clock thing

since the metric could be arbitrarily complicated in the way

ACuriousMind

@MoreAnonymous Sorry, but I don't understand that question, either. "energy cost of the measurement" is not a standard term, as glS points out. For energy to be conserved, of course changes in the expectation value of energy of the measured system result in changes of the expectation value of energy of the measurement apparatus. If you do not consider the measurement apparatus, measurement is never a unitary time evolution, hence will never conserve conserved quantities.

More Anonymous

So can I conclude the: the measurement only makes sense if there is more than one system?

Slereah

It's that Epistemology Thing where a theory's interaction with the real world has to include the definition of the measurement

otherwise it's not worth a damn

ACuriousMind

@MoreAnonymous Sure, a measurement needs at least two systems: A measured system and a measurement apparatus.

But you don't need to do the confusing computation you do there at all to conclude that - that's what measurement means

More Anonymous

1:18 PM

The how does the wavefunction of the universe make sense

?>

ACuriousMind

@MoreAnonymous The "wavefunction of the universe" is never measured.

More Anonymous

does it ever make contact with experiment

?

ACuriousMind

Only subsystems of the universe are measured, with one subsystem being the measured system and another being the measurement apparatus

More Anonymous

So ur bsaically sayin I'm right ... I wasnt sure ... since the measurement was kinda a taboo in physics class

Slereah

A long time ago, a friend of mine made a joke

Where he said that, if asked that ifthe universe is closed, then what is inside

you should answer Azathoth

ACuriousMind

1:20 PM

Whether you believe that there is a "wavefunction of the universe" or not is a matter of your chosen quantum interpretation, in particular whether it believes in true collapse or not. The formalism really doesn't care about "the universe", it just describes quantum systems.

Slereah

and it is still what I do to this day

More Anonymous

@ACuriousMind this one is a bit more involved but since u agree with the prevoius post .. I;d love ur thoughts?

1

Q: Is this analysis contradictory with the $2$'nd measurment?

Summary and Motivation "The below idea is about making a mathematical statement on system $2$ which induces a measurement on system $1$ while $1+2$ obeys unitary evolution." Basically, I'm modelling the measurement (occurring at time $t$) as an interaction and that I have some constraints based...

quantum-mechanics quantum-information measurements

More Anonymous

1:36 PM

It follows the idea of a subsystem performing a measuremnt on another subsytem but the whole system following Unitary evolution

PM 2Ring

1:50 PM

@MoreAnonymous That reminds me of Wigner's Friend; sorry, I should have mentioned that the other day.

More Anonymous

@PM2Ring ... thanks .. To be fair I dont go into the case where $\tilde \epsilon_\pm \to 0$ so i sidestep the discontinous

Ryan Unger

@Slereah someone one needs to merge Wald, HE, and Straumann

They all have boring bits that can be cut out

Like do we really need to see the computation of geodesics

Slereah

@RyanUnger Depends I suppose

Gotta target your audience

Ryan Unger

I’m going for big brain

Slereah

I'm guessing most people who read HE know the geodesic formula

Ryan Unger

2:00 PM

Straumann has way too much stuff in the appendix

Slereah

Ryan Unger

Just need to fix sign conventions really

I can feel his energy around here

Slereah

Does he teach

Ryan Unger

Unclear

I’ll check when the course reg opens in the fall

There’s very little overlap between physics and math at Princeton

Slereah

Witten's resume is quite to the point

But I guess that when you're Ed Witten

you don't need to put bells and whistles on it

Ryan Unger

2:04 PM

I don’t know how many courses I can take in the fall

Have to prepare for the generals

Slereah

he is listed as "Visiting Lecturer"

Ryan Unger

He’s not a professor at Princeton

He’s faculty at the IAS

Slereah

just go find him in the hall and yell at him that string theory is a sham

More Anonymous

hahhaha

tnf Einstein said the same for religion

Ryan Unger

@Slereah I wouldn’t be the first to do that

Problem is he’s so smart that it’s impossible to argue with him

More Anonymous

2:16 PM

@RyanUnger Next time this happens plz use ur cellphone to show footage of the person being intellectually pawned

I think we should have a big brother show where ppl of different camps come stay in the same house and discuss physics

though if u wanted to reach a more general audience replace camps with religion

Slereah

Sorry @RyanUnger

you don't have the $\mathbb{PHENOTYPE}$

More Anonymous

2:36 PM

@PM2Ring did anyone take the

Sorry accidently pressed eneter

but did anyone do the math behind Wigner's friend?

More Anonymous

2:57 PM

nevermind just saw the wiki link

bolbteppa

@CaptainBohemian "This page presents the thesis proposals submitted to the ED, irrespective of whether or not they are the subject of an application to the funding competition."

@MoreAnonymous I'm not sure the equivalence principle question makes sense

More Anonymous

k . .. I sadly do not get Wigner's friend's paradox ... Like how does the friend measure the system without an interaction hamiltonian? If there is an interaction hamiltonaian the tensor product comes under question

@bolbteppa yea it doesnt i have to delete it

thanks for remindin me

bolbteppa

3:12 PM

'Thus the properties of the motion in a non-inertial system are the same as those in an inertial system in the presence of a gravitational field. In other words, a non-inertial reference system is equivalent to a certain gravitational field. This is called the principle of equivalence.' Not sure what it means to apply this to the stress-energy tensor which is basically the derivative of the action with respect to the metric...

More Anonymous

yea .. i was confused about some stuff ...

But I was thinking that the stress energy of both them must be related in some way ...

On a related note are there any fields which can be expressed in tensor notation but do not respect the equivalence princple?

ACuriousMind

@MoreAnonymous I don't know what it's supposed to mean for a field to "respect the equivalence principle".

Any field can be "expressed in tensor notation"

Gyromagnetic

Hi, first time in chat :]

More Anonymous

Like if I have stress energy tensor of lets say the EM field between a capcitor in a lift

accelrating

ACuriousMind

@Gyromagnetic Welcome!

Gyromagnetic

3:23 PM

Please help me out with this: physics.stackexchange.com/questions/487094/… Im sure its trivial <3, but I want someone that knows their QM to say its right

More Anonymous

then would it behave the same when in gravitational field?

Gyromagnetic

Thanx ;)

More Anonymous

Can I construct a field that does not behave like tis is the ("related note question")

ACuriousMind

@Gyromagnetic You need not guess the answer there and think this is a "conceptual" question - you can actually straightforwardly compute the answer! Write down the time evolution for spinning particles in a constant magnetic field, then put in those two states as the initial conditions and see how differently they evolve!

More Anonymous

I actually envy those who have good physical intuition and can tell u stuff about a physical system without calculating it

ACuriousMind

3:27 PM

You get that intuition by having done these calculations so often that you no longer need to actually do them to make a stab at the result

3

@MoreAnonymous You still seem to think the "equivalence principle" is somehow a property of specific fields or something. It's not. It's a property of the theory of general relativity.

More Anonymous

I really need to revise my GR (im muddled in the head right now)

Gyromagnetic

Kind of embarrassing to admit but I dunno if I could even do it... I mainly build experiments, been years since I actually evolved a free particle hahaha... I guess I'll open my dusty Landau & Lifshitz and try to figure it out haha

bolbteppa

The point of the equivalence principle is just that the motion of a body in a non-inertial system is indistinguishable (locally) from the motion of a body in an inertial system in a potential where all bodies move the same independently of their masses provided the initial conditions are the same, i.e. a gravitational field.

A tensor transformation represents going to a different coordinate system in the same gravitational field, so if your tensor transformation in a gravitational field is not a change of coordinates from one system to another using the line element of the GR field it's not respecting the equivalence principle I think

More Anonymous

got it! (i think)

Any experts on Wigner's friend now?

"Like how does the friend measure the system without an interaction hamiltonian? If there is an interaction hamiltonaian the tensor product comes under question"

requoting myself

ACuriousMind

@Gyromagnetic By the way, I'd agree that the phase difference is what you write. But there isn't really any way to back up that claim without actually doing the computation, so we end up with at least one of us doing it either way ;)

enumaris

4:45 PM

weez

danielunderwood

5:26 PM

hmmm

enumaris

correct number of m's

good man

PM 2Ring

8:10 PM

@JMac Walsby's at it again: physics.stackexchange.com/a/487166/123208 At least this time the OP is knowledgeable enough to see instantly that that answer is baloney.

More Anonymous

10:54 PM

Am I being super dense or maybe my brain is dead ... But I can;t get what he's saying

There is no energy cost here like you have defined above. See, in classical example, when your interacation doesn't matter, you still get different values for every measurements as such you can do what you have done here for classical case and still argue that this formula is "energy cost" but it isn't! We clearly know that in classical case there is nothing such as adding energy to the system and etc. If you write something similar for classical case, it would only mean error of each data relative to average, i am suggesting that same logic applies for quantum case, for that formula of course — Paradoxy 8 mins ago

2

Q: Energy cost of the measurement without perturbing the system?

Background Let's say I have a Hamiltonian $\hat H$ (assume the Schrodinger equation) and it be in an arbitrary eigen-energy state: $$ \hat H_{\text{system}} | m \rangle = E_m |m \rangle$$ And I want to measure the momentum of the system without perturbing the Hamiltonian. We know this will...

quantum-mechanics quantum-information measurements

JMac

11:32 PM

@PM2Ring I noticed this gem today physics.stackexchange.com/questions/487049/… and decided to look at his self published works on amazon, exact same type of rambling. It's like the gish gallop of physics questions, just so much nonsense that it takes longer to explain whats wrong than it does to come up with the rambling.

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