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12:04 AM
::recalls Aristotle saying something similar::
 
@ACuriousMind yes it is
 
 
5 hours later…
4:45 AM
@skullpatrol When he was 22. But he was ROTC, so he went to OCS and off to the Navy.
 
5:26 AM
cool
 
5:42 AM
@JohnRennie just came out of Magdalene may ball
I love may balls
They are beautiful
 
@JakeRose you sound suspiciously high :-)
Just oxytocin, or might alcohol be involved as well? :-)
 
 
2 hours later…
7:58 AM
mornin
 
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.
 
9:17 AM
@JohnRennie I am drug free if we don’t include caffeine and ethanol ;) never seen the value tbh
 
@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 :-)
 
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
 
9:44 AM
Hm
Trying to find a timelike curve in my dumb spacetime is trickier than expected
 
@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)
 
Who told you I was a boss in GR
I am awful
 
really ?? I thought u were good at it ... L
(I presumed myself)
Any hints on how to approach this problem I posed?
 
10:01 AM
@MoreAnonymous what do you mean by the equivalence principle for stress energy tensors?
 
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
 
The equivalence principle is just the statement that the four acceleration is the sum of a coordinate term and the curvature term.
 
I wanna somehow show a relation between the stress energy tensors of both cases
 
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.
 
yea ... but I'm not sure if u can apply this for a field ...
 
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.
 
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
 
@JohnRennie That's almost an answer to this question, but it might be too technical for the OP.
 
Working on a more rigorous proof for that dumb metric
 
hahaha
Any bookk recommendations for someone who wants to revise GR? like I'd prefer it to complete yet fast paced ...
 
@PM2Ring I'll have a look ...
 
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.
 
I am thinking about liquid spacetime
 
@MoreAnonymous if you want fast paced try Relativity: the special and general theory by Einstein
 
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
 
Ugh ... He should put a warning out on his Boltzman brains
@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)
 
10:46 AM
The worst thing about being a Boltzman brain? Boltzman zombies!
 
0
Q: Ashamed of my own answer. Should I delete it?

Puzzled studentI 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...

 
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.
 
Feel free to answer here
0
Q: Equivalence principle for stress energy tensors?

More AnonymousIs 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...

@CaptainBohemian
 
@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.
 
@MoreAnonymous I don't understand what "equivalence principle for stress-energy tensors" is supposed to mean
 
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
 
What's the "stress-energy tensor of the lift"?
 
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 ?
 
a pseudotensor is coordinate related and you may choose a coordinate to make it vanish.
 
I mean you know
Energy itself is coordinate related
it's not that weird that it would be
 
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...
 
@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.
 
Also there's the whole stress energy tensor v. canonical stress energy tensor v. Beliphantes tensor business
quite dreadful really
 
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.
 
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
 
gravitational energy-momentum is a pseudotensor which follows some physics principles but don't follow the usual coordinate transformation rules for tensors.
 
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
 
@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))
?
 
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
 
12:10 PM
It's also a section of the projective vector bundle
though I doubt that it is here
 
@MoreAnonymous but suppose you're in space near the Earth, then the stress-energy tensor at your position is zero.
 
wait i think u guys re right and Im confusing myself
 
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.
 
but can't u gain the chirstoffel symbols by knowing the proper accelreation
 
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
 
12:18 PM
alright ... I kinda get ur point
sigh ,.. I really need to revise GR
getting outta touch
any book recommendation?
 
44
A: Books for general relativity

Ryan UngerThis 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...

 
thannks
 
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.
 
12:40 PM
@Slereah doing god's work
I need to update that list
 
Also we need to make a list of worst books for GR
 
hahaa ... any popular books out there which should join the list :P
" worst books for GR"
 
@Slereah sachs-wu
@Slereah @ACuriousMind please yeet this into the shadow realm
I will not have my friend assaulted
 
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
 
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.
 
besse is a good book
 
savage
 
If I am paid enough I am willing to write a GR book entirely in Penrose notation
 
If I do GR for my PhD I'll write a GR book one day
 
I actually met Penrose once in Nottingham
 
12:52 PM
Any specific topic?
 
the book or the PhD
 
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
 
haha ... thats funny
 
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
 
@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.
 
12:54 PM
what would a mathematician find interesting
 
AFAIK none of the minimal surfaces stuff has made it into books
 
@JMac ... ur actions would seem synonymous with evil
 
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
 
@MoreAnonymous Just Poe's law in action. It's basically impossible to tell who is serious or not online
 
although right now I have a much nobler goal
which is to make up a really stupid spacetime
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
 
12:59 PM
Anyone read this book
?
Seems ambitious
 
I have not
 
got it from the least voted answer in book recommendation post
 
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
 
it might b a worthwhile goal: "find the worst GR book"
:P)
 
I don't think I have a book that is really bad
 
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
?
 
Jesus takes the measurement
 
I was actually being serious here ...
 
as thus
 
noooooo
I mean like how they overcome this kind of objection
2
Q: Energy cost of the measurement without perturbing the system?

More AnonymousBackground 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...

 
not a clue
 
1:09 PM
@Slereah I notice u have a stash of memes available instanteous
-ly
 
I use the ancient art of google image search
 
theres a meme with lubos on it as well on google search top result
?
 
no, I made that one
it is the official logo of PSE
 
I remain unconvinced that QM is not just due to physicists being bad at experiments
 
Or at least so I would like to think
Feel free to try to prove it wrong
 
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 :)
 
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
 
@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.
 
So can I conclude the: the measurement only makes sense if there is more than one system?
 
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
 
@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
 
1:18 PM
The how does the wavefunction of the universe make sense
?>
 
@MoreAnonymous The "wavefunction of the universe" is never measured.
 
does it ever make contact with experiment
?
 
Only subsystems of the universe are measured, with one subsystem being the measured system and another being the measurement apparatus
 
So ur bsaically sayin I'm right ... I wasnt sure ... since the measurement was kinda a taboo in physics class
 
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
 
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.
 
and it is still what I do to this day
 
@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?

More AnonymousSummary 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...

 
1:36 PM
It follows the idea of a subsystem performing a measuremnt on another subsytem but the whole system following Unitary evolution
 
1:50 PM
@MoreAnonymous That reminds me of Wigner's Friend; sorry, I should have mentioned that the other day.
 
@PM2Ring ... thanks .. To be fair I dont go into the case where $\tilde \epsilon_\pm \to 0$ so i sidestep the discontinous
 
@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
 
@RyanUnger Depends I suppose
Gotta target your audience
 
I’m going for big brain
 
I'm guessing most people who read HE know the geodesic formula
 
2:00 PM
Straumann has way too much stuff in the appendix
 
Just need to fix sign conventions really
I can feel his energy around here
 
Does he teach
 
Unclear
I’ll check when the course reg opens in the fall
There’s very little overlap between physics and math at Princeton
 
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
 
2:04 PM
I don’t know how many courses I can take in the fall
Have to prepare for the generals
 
he is listed as "Visiting Lecturer"
 
He’s not a professor at Princeton
He’s faculty at the IAS
 
just go find him in the hall and yell at him that string theory is a sham
 
hahhaha
tnf Einstein said the same for religion
 
@Slereah I wouldn’t be the first to do that
Problem is he’s so smart that it’s impossible to argue with him
 
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
 
Sorry @RyanUnger
you don't have the $\mathbb{PHENOTYPE}$
 
2:36 PM
@PM2Ring did anyone take the
Sorry accidently pressed eneter
but did anyone do the math behind Wigner's friend?
 
2:57 PM
nevermind just saw the wiki link
 
@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
 
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
 
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...
 
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?
 
@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"
 
Hi, first time in chat :]
 
Like if I have stress energy tensor of lets say the EM field between a capcitor in a lift
accelrating
 
@Gyromagnetic Welcome!
 
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
 
then would it behave the same when in gravitational field?
 
Thanx ;)
 
Can I construct a field that does not behave like tis is the ("related note question")
 
@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!
 
I actually envy those who have good physical intuition and can tell u stuff about a physical system without calculating it
 
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.
 
I really need to revise my GR (im muddled in the head right now)
 
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
 
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
 
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
 
@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 ;)
 
 
1 hour later…
4:45 PM
weez
 
5:26 PM
hmmm
 
correct number of m's
good man
 
 
3 hours later…
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.
 
 
3 hours later…
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?

More AnonymousBackground 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...

 
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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