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1:51 AM
@SillyGoose $\int\ddot x\mathrm dx=\int\dot x\frac{\mathrm d\dot x}{\mathrm dx}\mathrm dx=\int\dot x\mathrm d\dot x=\int\mathrm d\left(\frac12\dot x^2\right)$
 
2:21 AM
hello -- i have read that the Aharonov–Bohm effect tells us that a particle can actually be influenced by the vector potential even where the magnetic field is zero. if the vector potential is not unique and can even sometimes be zero, how is this possible?
 
@Relativisticcucumber That is somewhat easy to explain: While at any point A is not uniquely defined, over a loop the integral of A, which corresponds by Stokes's theorem to the area integral of curl A, is gauge-invariant, and if you remember that curl A = B, then obviously it will pick up B field effects even along loops that B fields are zero, as long as B field is enclosed.
jigglypuff~
H O N K
meow
 
@naturallyInconsistent but how is the area integral invariant? if we have a constant A, then the curl is zero but the loop integral in this case will not be zero?
 
@Relativisticcucumber You can only arbitrarily alter A on one point. Everywhere else there are some constraints. You cannot make A be constant over the entire loop, or zero curl over the entire loop.
 
i thought if i the field $B = (0,0,B)$ (which tong pervertedly loves to use) then i can make $A$ anything that satisfies this?
isnt that what it means to choose a gauge?
 
@Relativisticcucumber Yes, and that A will not have zero curl
 
2:34 AM
bah but outside a solenoid it should if this is mag field due to solenoid?
 
$\vec B=(0,0,B)$ is not perverted in any way, shape, or form. When you have a spherically symmetric system and you do not choose for your first symmetry breaking perturbation to be in the z direction, you are very strongly M
 
@naturallyInconsistent oh no it was a joke
bc it looks like
"boob"
sorry im five
 
@Relativisticcucumber The B field outside the solenoid is zero, but the A field, while having zero curl outside the solenoid, is still carrying the information that the curl A is nonzero inside the solenoid.
@Relativisticcucumber smirks
Look, the A field that has zero curl outside the solenoid can be visualised in the symmetric gauge as doing constant rotation. In that way, the loop integral obviously isn't zero.
 
i think im thinking that the A needs to be different in the solenoid and outside it. is this incorrect?
 
This is incorrect. As with all Maxwellian fields, you must have one single field that satisfies continuity equations at boundaries.
And IIRC A field does not have to decay at infinity.
Anyway, I gtg, doc's appt, bye for nao
 
2:45 AM
@naturallyInconsistent i see. time to freshen up on e&m. bye and thank you
 
 
5 hours later…
7:16 AM
0
Q: Users > Voters on Meta Page: I cannot be the only one voting this month!

HokonI just noticed something extremely strange: on my end, I appear to be the only person that has voted on any Physices.SE Meta Posts this entire month. I'll post a screenshot since this is unbelievable even to myself. This is impossible, as I've clearly observed certain questions being upvoted and...

 
@PM2Ring very nice. she's so happy
 
@RyderRude hey Rider. going through about proving why L can't depend on $x$ with the proof, it got me thinking about what naturallyinconsistent said. chat.stackexchange.com/transcript/message/64449620#64449620
 
@GiorgiLagidze hi. perhaps @naturallyInconsistent can elaborate that. I am not sure about the logic
 
i said that $L(x+a) - L(x) = a \frac{\partial L}{\partial x}$, but now I'm not so sure why this is valid. the more correct way is: $L(x+a) - L(x) = a \frac{\partial L}{\partial x} + \frac{a^2}{2!} \frac{\partial^2 L}{\partial x^2} + \frac{a^3}{3!} a \frac{\partial^3 L}{\partial x^3}$.
 
I think naturallyinconsistent's is close to the proof that @bolbteppa gave
 
7:27 AM
what he said was: "you are missing that he is pointing out a much stronger criterion, namely that this relation is true for arbitrarily large a and not just the infinitesimal version. Then all higher derivatives of L by x must also vanish."
what makes us make higher derivatives vanish ?
 
@GiorgiLagidze that's what i was talking when i said $L(x+a)-L(x)=a\frac{dL}{dx}$ is only true for linear functions. but your argument may still hold i think because there's a total derivative on the RHS
 
what's RHS ?
 
@GiorgiLagidze the right hand side of the eqn. consider the special case where it is 0. u get $L(x+a)-L(x)=0$. This indeed implies $\frac{\partial L}{\partial x}=0$
but u want to take on the more general case where the rhs is a total derivative. in this case, it cannot generally be proved that $\frac{\partial L}{\partial x}=0$ because of some pathological counter examples like $L=x\dot{x}$
$L=x\dot{x}$ produces a translationally invariant EL eqn, but $L$ nevertheless depends on $x$
 
@RyderRude ibb.co/F7tm6dW this is what was told to me
 
@GiorgiLagidze they are assuming $a$ to be very small. in this case $L(x+a)-L(x)=a\frac{dL}{dx}$ is valid
@GiorgiLagidze they are just ignoring the other terms because $a$ is small
 
7:35 AM
yes. in that case, true, but it's said on the image that "that must work also to the approximation.."
note the word: "also"
which means they definitely mean that for even large values of $a$
and naturally inconsistent also said the same thing. chat.stackexchange.com/transcript/message/64462270#64462270
so we must be missing something
 
no, they mean that the eqn that came before must holds for all $a$ (hence for small $a$ too)
the eqn that comes after that sentence is for small $a$
@GiorgiLagidze yes. but im not sure about what his derivation is here. i too think it should be true but im not sure
 
"the eqn that comes after that sentence is for small a" - if so, then the proof is not correct
because it should prove it for large a as well
 
can you share the full proof
im sure there is at least one incorrect step in the proof where they check for terms proportional to $\dot {x}$ on both sides
 
@GiorgiLagidze i think u r correct. this proof can only be true if that step is valid for all $a$, which i cant see why it would be
 
7:43 AM
yeah. for small a, true, but for large a, i don't know
 
@GiorgiLagidze and even if that step works out, there is an additional assumption needed here that $\frac{\partial L}{\partial x}$ does not have terms proportional to $\dot {x}$
@naturallyInconsistent can you please why that step works for all $a$?
 
maybe the idea is this.

so what we know for sure is the following.
$L(x+a) - L(x) = a\frac{\partial L}{\partial x} + \frac{a^2}{2!}\frac{\partial^2 L}{\partial x^2} + \frac{a^3}{3!}\frac{\partial^3 L}{\partial x^3} + ... = \dot x \frac{\partial f}{\partial x} + \frac{\partial f}{\partial t}$

because of homogeneity, the equation for all a must hold true. I think this is criterion.
 
that step does work for all $a$ when the rhs is 0
 
I think in order for the criterion to hold true, that's why higher orders vanish
because if higher orders are present, no chance the equation would hold true in terms of equality
 
i am also thinking that but idk any concrete derivation
and your source is explicitly saying small $a$. is this from a book?
 
7:47 AM
nope, i asked this on a forum
 
i don't think he says explicitly to small
in my opinion, he means for all a. :P but anyway, can you come up with such L that the our equation would still hold true if we allow second order derivative not to be 0 ?
 
yes. thats what i am looking for
 
I think I get it.

imagine you found such L and f such that https://chat.stackexchange.com/transcript/message/64462327#64462327 holds true for a=2.
now, don't chage L and f(just have them the same) and plug in a=3. ofc, it's not gonna be equal because of $\frac{a^2}{2!}$
so they must definitely be 0
makes sense ?
will be back in 5 min.,
@RyderRude i'm back
 
8:14 AM
@GiorgiLagidze ok here is a proof. first assume the Taylor series doesnt have any terms proportional to $\dot {x}$. So, this means we ignore the $\dot {x}$ term on the rhs. This means the RHS can only be a function of $t$. so the Taylor series is also independent of $x$. This means the $L$ can at most be linear in $x$, and hence the higher derivatives vanish
 
why do you assume?
 
it's not true in general. e.g. for $L=x\dot{x}$
 
i prove it differently
follow me . i will type now
so this is something we both agree on:

$L(x+a) - L(x) = a\frac{\partial L}{\partial x} + \frac{a^2}{2!}\frac{\partial^2 L}{\partial x^2} + \frac{a^3}{3!}\frac{\partial^3 L}{\partial x^3} + ... = \dot x \frac{\partial f}{\partial x} + \frac{\partial f}{\partial t}$

The idea is that it must work for every a. what this means is that for the same L and f, you change/plug in different $a$, but same L and f and it still should work. That's the beauty of homogeneity criterion. What this means is that unless higher order derivatives are zero, our criterion won't hold true. why ? because once you
 
Sigh, the correct behaviour is that $\forall a\in\mathbb R\ :\quad L(x+a,\dot x,t)-L(x,\dot x,t)=0\qquad\implies\qquad\forall n\in\mathbb Z^+\ :\quad\frac{\partial^nL}{\partial x^n}=0$
 
yeah, That's what I say in my latest reply. @naturallyInconsistent do you find anything in it incorrect ? chat.stackexchange.com/transcript/message/64462547#64462547
 
8:22 AM
@naturallyInconsistent yes, the step works when the RHS is 0. but does it work when the rhs is a total derivative?
 
yes, but again, ACM gave a perfect reason why f should simply not be there
 
@naturallyInconsistent i am looking for the proof of why it works for the total derivative general case
 
well, even if it is, i don't find anything wrong. i still get to the same answer as everyone
@RyderRude and what don't you like in my proof ? :P
3 mins ago, by Giorgi Lagidze
so this is something we both agree on:

$L(x+a) - L(x) = a\frac{\partial L}{\partial x} + \frac{a^2}{2!}\frac{\partial^2 L}{\partial x^2} + \frac{a^3}{3!}\frac{\partial^3 L}{\partial x^3} + ... = \dot x \frac{\partial f}{\partial x} + \frac{\partial f}{\partial t}$

The idea is that it must work for every a. what this means is that for the same L and f, you change/plug in different $a$, but same L and f and it still should work. That's the beauty of homogeneity criterion. What this means is that unless higher order derivatives are zero, our criterion won't hold true. why ? because once you
 
@RyderRude ACM gave it
 
@GiorgiLagidze i dont understand the reasoning :P
@naturallyInconsistent can u give some keywords so i can search
or link it
 
8:25 AM
It is faster if you just scroll back to the conversation yesterday or so. It is on you that you did not read what ACM is saying and instead focused upon chatting when you were there yesterday
 
@RyderRude can you find such L and f that you plug them in the equation, and you only change $a$ ? you might find that for specific $a$, equation holds true, but once you switch $a$, it fails
 
@naturallyInconsistent i did read it but i dont remember reading this proof. i remember ACM pointing out that the original proof was incorrect because of the step where u look for terms proportional to dot x
 
yesterday, by ACuriousMind
the silly thing here is to expect that one could somehow derive constancy of $L$ while simultaneous considering the pseudo-symmetry case of changing by a total time derivative; of course you can't, but what is true is that among all these pseudo-symmetric Lagrangians there is one that is constant and that's the one we pick
 
@GiorgiLagidze i cant find it. but im also not sure that it doesnt exist :P
@GiorgiLagidze oh ACM is just saying that it cant be proved for the general total derivative case, which i completely agree with
 
@naturallyInconsistent you also don't understand what I'm saying here - ? chat.stackexchange.com/transcript/message/64462547#64462547
 
8:28 AM
@GiorgiLagidze the simplest counter example is $L=x\dot {x}$. the EL eqn is translationally invariant but $L$ is not
 
@RyderRude You are reading this wrong. ACM is saying that this proves the general case because it works for the special case, and the special case is sufficient to prove the general case
 
Ok, so here's my answer to this.
I was born during this this era and independently of my ideas about moral - be it artificial or natural, absolute
or relative - I do believe that decent human beings should follow a moral code in order to live in a functional society, you may call it an "effective moral" if you will.
Now, we've discussed enough to make clear that I consider morals to be strictly dependent on the context and devoid of any
absolute connotation.
Even then, my ideas of those "effective moral" I've mentioned is mostly aligned with what we'd call "good" or "right" today. To make an example, since that's
what you've mentioned: I, the person who you're talking to now, obviously consider genocides wrong. If this society's "New Thing" appeared, my effective moral would not change and
I'd still deem it as wrong (in that effective sense). Or, to consider a different scenario, if this version of me that is writing were sent to a far past with different ideas, I would still
In a sense, this effective moral I've discussed about originates instinctually and emotionally (e.g. I would by horrified by a murder), while this nihilist/relativist position comes later after a rational
arrangent of thoughts and a complete detachment from my feelings about it.
 
@naturallyInconsistent ok i got it now
 
@GiorgiLagidze It is not about not understanding. It is that I do not intend to expend mental effort on something that is not elegant when you already understand the much more elegant solution.
 
@naturallyInconsistent Meow
 
8:30 AM
the proof is simply incorrect for the non zero total derivative case there exist counter examples where $\frac{\partial L}{\partial x}\neq 0$. like ACM said, this cannot be proved
but if u choose L such that the total derivative is 0, the proof works
 
@Mr.Feynman 😻
 
okay you got $L = x\dot x.$ which means you got $a\frac{\partial L}{\partial x} + 0 = \dot x \frac{\partial f}{\partial x} + \frac{\partial f}{\partial t}$ then from it: $a \dot x = \dot x \frac{\partial f}{\partial x} + \frac{\partial f}{\partial t}$
 
@naturallyInconsistent Somebody is typing from their phone :P
 
@Mr.Feynman not at all. just copy-pasta
 
@RyderRude now, from this - chat.stackexchange.com/transcript/message/64462594#64462594, find $f$ such that it holds true
can you ? :P
 
8:31 AM
@naturallyInconsistent i dont think we can choose such a Lagrangian for the Galilean transform symmetry, can we, where the total derivative term is 0?
@GiorgiLagidze yes. this was not a counter example specifically to that. i am saying that this is a counter example to the entire proof
 
@RyderRude take it up with ACM. Yall are impossible. I just came back from the hospital having fasted for a procedure and yall just cannot take an simple mathematical / physical argument and instead want to dig random complicated holes. I need my late lunch.
 
yall who? :P
 
have some lunch.
 
@GiorgiLagidze do you notice that a step in the proof is "since no terms proportional to dot x exist on the lhs"? this is the step thats not true for general L
 
@Mr.Feynman just them two. you are a fine man miehehehe
 
8:35 AM
@RyderRude ok, do you agree that $a\dot x = \dot x \frac{\partial f}{\partial x} + \frac{\partial f}{\partial t}$ ? in your provided L ?
 
@GiorgiLagidze yes. that wasnt a counter example to this, becuz of course the higher $x$ derivatives of this Lagrangian are 0 :P
 
okay show me any kind of L you want
and I will still prove to you
 
i cant. i havent found :P. Im just saying that another step in the proof has counter examples
 
i think you're forgetting the stuff that the equation should work for any $a$. if your equation works for $a=2$, once you plug in $a=3$, how would it still work ? it would fail
that's why the RHS on the right side becomes 0
 
so this proof doesnt work anyway. u can just follow ACM and choose a $L$ which stays constant under the symmetry. then the proof is trivial
 
8:37 AM
i'm just trying to see where I'm making a mistake :/
 
exactly. why are you not picking the trivial route?
 
@GiorgiLagidze ok i concede here :(. But i dont understand this "plugging-in" argument
 
when we have $L(x+a) - L(x) = a\frac{\partial L}{\partial x} + \frac{a^2}{2!}\frac{\partial^2 L}{\partial x^2} + \frac{a^3}{3!}\frac{\partial^3 L}{\partial x^3} + ... = \dot x \frac{\partial f}{\partial x} + \frac{\partial f}{\partial t}$
, shouldn't this work for every $a$ ? meaning that the equation equality must hold true if $a=3$, if $a=2$, if $a=5$, so by pliugging in, if I put $a=2$ in the equation, LHS and RHS must be equal, if I put $a=3$ in the equation, LHS and RHS must again be equal
am I wrong here ?
 
@GiorgiLagidze no this is correct
 
@RyderRude, ok then it should work such that even when you change $a$'s values, you still leave L and f the same in the equation and it still should work
 
8:42 AM
but idk why you're trying to fix this proof when it has another invalud step anyway. just choose an $L$ which stays constant
@GiorgiLagidze yes
 
okay, if so, if you got some L and f that works for $a=2$, now, if you put $a=3$ in the equation where L and f stay the same, equality will break because $a^2, a^3$ of the higher order terms
so, higher orders must vanish
and you're left with $a\frac{\partial L}{\partial x} = \dot x \frac{\partial f}{\partial x} + \frac{\partial f}{\partial t}$, do you agree with me till now ?
 
idk why you say that. the RHS also has $a$ dependence generally
so $a$ changes on both sides. whatever u plug in
 
yes, but homogeneity should work even for $f$ that doesn't have $a$ in it, but only $x,t$
 
so you're assuming the form of $f$
 
if you got L and you add total time derivative to it to make sure EOM doesn't change, you choose such f that doesn't have $a$, because EOM shouldn't change whatever form of $f$ you choose as long as it's total time derivative
 
8:46 AM
if $f$ doesnt have $a$ in it, then how can the Taylor series even contain the linear term?
with this assumption, u prove dL/dx=0 in one stroke
 
yep
and that's how it is
 
ok but idk how this assumption is justified :P. it's not true for general symmetries like the Galilean transform
 
which assumption ?
that f has form that doesn't have $a$ in it ?
 
that the total derivative term does not depend on the symmetry parameter
this is an assumption
 
the point is imagine you got some specific L. you know adding total time derivative doesn't change the EOM. so even if you add $d/dt 2xt$ or $d/dt 2xat$, EOM still won't change for either case.
 
8:48 AM
yes
 
basically, it shouldn't change for either case
 
so u r just assuming the $a$ dependence is such that u can factor it out
 
so whatever answer you get for d/dt 2xt, the same answer you must get for d/dt 2xat
you can try to choose d/dt 2xat, but you realize that you can't prove it that way
even though in that way, the result will be the same, you can't just prove it
so if you know that 2 things have the same result, you can just choose "thing 1" to prove, because if "thing1 result = thing2 result", it doesn't matter for which one you prove
 
idk. this assumption just makes for a weird proof
 
i think this is what ACM was saying all along
 
8:50 AM
but if it works for u, then good :)
 
about special and general case
@RyderRude it's not rigorous proof, but it's a solid one
 
he meant choosing a Lagrangian which stays constant under the symmetry. so the rhs is 0
 
yeah, and with my proof, i get to his point soon. because i also prove that rhs has to be 0
he immediatelly chooses rhs to be 0, while I don't
 
then it is true :)
 
so he makes 2 assumptions, while I make 1 assumption
 
8:53 AM
@GiorgiLagidze that is not how it works, but if you are happy, we are not going to dig this up
@GiorgiLagidze this is not true
 
well, I'm sure I'm right in the part that I can choose $f$ to not contain $a$
i might be wrong about ACM's comments
 
@GiorgiLagidze well, since ACM's point is that you can choose f = 0, then obviously you can choose f to not contain a, since ACM's point is stricter.
 
yeah, we're pretty much close to each other's points
@naturallyInconsistent @RyderRude can the same type of proof be used to show that $L$ can't depend on the $\vec v$ (velocity direction) ?
 
but im not sure if ACM's point applies for Galilean symmetry. can i get please get a proof that this can be done for any symmetry?
@GiorgiLagidze u can prove this using isotropy symmetry
 
yeah, @RyderRude, tried it but have a hard time with rotations :(
 
8:57 AM
Maybe this stuff is a bit advanced right now, really you don't need any of this, it's just saying $L(x+a) = L(x)$ must hold so that $L$ is independent of $x$, nothing more is needed, from this you can then argue some $df/dt$ stuff but you will end u invoking $L(x+a) = L(x)$ to get rid of the extra terms in the Taylor expansion of $L(x+a)$
 
Ur $L$ must be an invariant of rotation. $|v|^2$ is one such thing. u cant obviously make direction dependent quantities invariant under rotation
 
Like $f(x,y,z) = y - x^2 = 0$ is independent of $f(z+a)$ shifts
 
@bolbteppa with your way, you say that L1 = L2 in the exact way and not different by anything while they can still be different and yield the same results
 
And you can always trust blobteppa to come in and talk to himself. This time, about stuff that we have already moved on from.
 
but now, I get it
 
8:59 AM
Yes they are exactly equal
 
@RyderRude i wanted to prove with the same idea, such as you got $L(x, \vec v, t)$ and now, L2 is $L2(x, \vec v + ..., t)$
what would you put instead of ... ?
 
we need to prove that the only rotationally invariant scalar functions of $\vec{v}$ are of the form $f(\vec{v})=g(|v|)$
@bolbteppa is there a proof of this
 
@RyderRude that doesn't show the same proof idea. what I'm looking for is $L(x, \vec v+ ..., t) = L(x, \vec v, t) + \frac{d}{dt} f(x,t)$
 
$\mathbf{v}$ is a vector, it points in different directions when rotated, its norm is invariant under rotations, it's obvious by the formulation
 
@bolbteppa yes. i think so too. unless there are other tensors u can contract it with in ur physical situation, contracting with the metric is the only way to make a scalar
 
9:04 AM
mainly, after rotation, what does new vector equal to
 
@GiorgiLagidze u r again considering the total derivative to be non zero -_-
 
okay make it zero. what I am asking is $L(x, \vec v+..., t) = L(x, \vec v, t)$
 
@GiorgiLagidze it will be the matrix multi of the velocity vector and the rotation matrix
 
what would you put instead of $...$
 
@GiorgiLagidze i think it is best to switch to polar co ordinates first in 2D and consider translation of the angle
 
9:08 AM
@RyderRude something like: $L(x, R\vec v, t) = L(x, \vec v, t)$ where $R$ is a matrix?
 
The only rotational invariant functions of $\vec v$ are functions of $v^2$, even if you want to pick square roots of that.
 
and i can have R matrix with cos\theta, -sin\theta and so on
 
write L as $L(\rho, \theta, \rho ', \theta ')$ and just consider translations of \theta
then u have the same proof as before
 
let me write it down, think and then and ask a better question.
 
@naturallyInconsistent yes. but if u contract $v$ with some other tensor than $g$. then u can get other functions too. but these functions would change their form under rotations because of the transformation of the tensor's components
so these would not really be rotationally invariant functions
 
9:12 AM
Why are you digging a hole again? Just accept that only $v^2$ is rotational invariant.
 
yes. sorry.
multiples of kronecker delta are the only tensors which dont change their components under rotation
so contracting with this is the only way to make form-invariant rotation-invariant scalar functions
 
No, there is also the Levi-Civita
But yes, since contracting with Levi-Civita more than once leads to zero, the only way to get the rotational invariants from one single vector is to use the metric.
 
thanks.
 
9:30 AM
I asked my prof whats gauge theory (I saw the word in jdjackson when he says $A \mapsto A+\nabla \phi $ and thhought whats the big fuss) and wat are the prerequisites to learn it. he just gave me this arxiv.org/abs/math-ph/9902027 . Is my prof trolling
 
@nickbros123 ooo, i might snatch that
 
@GiorgiLagidze hello. $L=x^2\dot {x}$ is a counter example to the idea that the higher derivatives vanish. u have $L(x+a)-L(x)=2ax\dot {x} + a^2\dot {x}$, which is a total derivative of, $f(x,t)= ax^2+a^2x$. nevertheless, the Taylor expansion contains the $a^2$ term
 
@RyderRude yeah, true. i think the point is that f must not contain a
that's what I was aiming for
and why I said what I said
 
You may as well assume $L$ remains constant tho :P
 
yep. :D
 
9:38 AM
@GiorgiLagidze this assumption does not hold e.g. for my counter example. and there is no way to factor out $a$ out of the total derivative
 
@GiorgiLagidze and RR is just giving you a direct example of an L such that the f will automatically contain a.
 
I get that, but the point is even if f doesn't contain $a$, homogeneity assumption still must hold true
and I consider that exact case
 
@GiorgiLagidze I think ur source really mean to write that equation for very small "a" (like they said), but they concluded that their proof worked for all $a$
 
@RyderRude no, it is supposed to be true for all a
 
you can't tell me that when $f$ doesn't contain $a$, homogeneity must break.
because homogeneity doesn't care what you do with $f$ :p
 
9:41 AM
@naturallyInconsistent the proof in Giorgi's sources uses a step that's not true for all $a$
 
@GiorgiLagidze look, you are not exactly free to pick f if you picked that L.
 
the proof is working with the total derivative general case @naturallyInconsistent
 
@RyderRude It is Giorgi who came up with the RHS that has f. Not the source.
 
it is in the source they linked me
 
@naturallyInconsistent and why am I not free to pick such $f$ ?
homogeneity is homogeneity and that's just it. why does picking up $L$ define what kind of $f$ i should choose ?
 
9:43 AM
the source has two mistakes : assuming that that step works for all $a$ in the total derivative case and assuming that the the derivative of L wrt x cant contain $\dot {x}$
 
@GiorgiLagidze Because manifestly if you picked RR's choice of L, then if you picked an f that does not have a, then you do not have equality at all.
 
@GiorgiLagidze in my counter example, can u pick sich a $f$
 
Look, there is no chance of you guys learning any good physics out of digging this hole. You already understand the correct argument. If you want to continue digging this hole, please take it to another chat room and continue there.
 
@naturallyInconsistent at this point, me and @GiorgiLagidze just want to see the proof that the Lagrangian can be chosen to stay constant for any symmetry of the EoM
then we can forever get rid of the total derivative case
can you link that proof
 
@RyderRude I don't think that is what is being demonstrated.
 
9:48 AM
what do you mean
 
ok $L = x^2\dot x$. Now, I need to choose such $f$ that EOM doesn't change. make $f = 2xt$ . try adding $d/dt 2xt$ and you will see EOM doesn't change
 
@GiorgiLagidze are not interested in that proof
 
I do not think people are claiming that "Lagrangian can be chosen to stay constant for any symmetry of the EoM"
 
Hmm
then what was ACM's point again
 
I am quite sure Landau did not use the wording "symmetry of the EoM"
I think you two made it up and then asked us to prove it
 
9:50 AM
@GiorgiLagidze the total derivative term must be derived from the translational symmetry
 
What people are saying is that there are some basic properties of the Lagrangian mechanics system, for example, that two Lagrangians that differ by a total time derivative will yield the same EoM, and thus are equivalent in some sense.
 
for the proof to work, u must start with $L(x+a)-L(x)$ and work with whatever u get
u cant just choose the total derivative term
 
that's where our opinions differ. If you got any kind of $L$, the point is that whatever $d/dt f(x,t)$, I add to it, EOM won't change. so even in your chosen $L$, i can just choose whatever $f$ i want
and show that EOM won't change and from that, I can show that L is independent of $x$
 
And independently, we know that our universe satisfies translational symmetry, and thus there must be Lagrangians whereby L(x+a)=L(x) for any a
 
you are correct but that's not relevant to the proof :P @GiorgiLagidze
 
9:52 AM
I don't know. to me, it seems it is :D
 
the proof starts with $L(x+a)-L(x)$ doesnt it?
 
yes and I don't make higher orders vanish yet
 
@GiorgiLagidze no, that is just mathematically wrong and unsalvageable
 
@naturallyInconsistent but that's exactly what i said. do these lagrangians exist for all symmetries of the universe?
 
hmm... Maybe what's happening here is you guys don't like my mathematical proof because it's not bullet proof or solid.
 
9:54 AM
@RyderRude yes? that is a statement on the restrictions on the type of Lagrangians we consider, i.e. you cannot pick $L=x^2\dot x$ or anything like that.
@GiorgiLagidze What we are saying is that it is not even a mathematical proof at all.
 
though, i don't only look at it mathematically, with my assumptions and thinking process, it makes me believe in it.
 
@naturallyInconsistent so this means there must be some Galilean invariant Lagrangian even if the one we use in books isnt Galilean invariant
 
@GiorgiLagidze how is this a scientific way of thinking, again?
 
no, no. I'm not saying that. With rigorous mathematics proof, I can't show you bullet proof
i'm not at that level :P
 
@RyderRude what are you talking about? the usual Lagrangians we use prior to SR are usually Galilean invariant
 
9:56 AM
@naturallyInconsistent $\frac{1}{2} mv^2 isnt :P
the EoM are invariant tho
 
@GiorgiLagidze I was in a irl philo meetup one day, and one guy stood up and said that "When we look in the mirror, you just see yourself, but I see more than just what the mirror looks like." He definitely "don't only look at it mathematically, with my assumptions and thinking process, it makes me believe in it". I told him yes, nodded, and just shut up. After this outburst of his, he was satisfied that he gave his piece, and left within 5 minutes. The moment he left, the entire room burst laughing
@RyderRude That is what we think of as an Galilean invariant Lagrangian...
It is definitely translationally and rotationally invariant. On boosts, it is actually more of covariant than invariant, but that is a brokenness of physics terminology that you should take up with history and not with us
 
@naturallyInconsistent look, I tried my best to understand this proof and somehow be sure in it. With my assumptions, it seems to hold. If I knew the exact bullet proof, i'd mention it here. Am I feeling good that I made myself believe in it ? no, hell no, but at least it lets me move on to next subject
 
@GiorgiLagidze why don't you think of it better that you now understood what ACM was saying, and thus understand what Landau's proof is actually saying, and thus actually made an understanding of a rather important and beautiful and elegant proof of a part of our universe?
Why should you fixate on this particular proof, rather than the bigger picture that you have made progress upon?
 
@naturallyInconsistent because currently, I've been looking at this for the last 3 hours and I even forgot what ACM said or what's the bigger picture. Need to look at it with clear head
 
@GiorgiLagidze And in most of learning, the most important part is to keep that dayum clear head. Not to dig into rabbit holes.
 
10:07 AM
I agree. I'm a beginner and self learner, so that's expected I'd say that i am digging rabbit holes
it will change though after some moment
when we're beginners in any field, we either always dig rabbit holes or we say we understand it while we don't know that we don't understand it :P
once you get better and better, this changes and you at least realize more what you don't know or what exactly you need to focus on
@naturallyInconsistent if you don't mind, could you say with few words a summary of what I should have learned from this huge discussion as you point out "bigger picture" ?
 
I feel that you should first have learnt that the prof who asked you to start from Landau is just the wrong guy to look for. He might be competent at physics, but he is not competent in education. That is the biggest picture. Then you look at the huge discussion, and you should realise that the reason why you went on that discussion is that you wanted to learn a bit about Lagrangian mechanics in the service of building non-relativistic classical mechanics.
 
point 1. the professor is not my professor anymore. I quit. :P
and after this, I'm quitting Landau as well for a while
 
@naturallyInconsistent hmm but energy is not a covariant entity i think. it is the time component of the four vector
 
The point to focus upon, here, is why Landau can narrow down the types of Lagrangians to pay attention to, to the specific type that we always choose in reality; i.e. from the infinitely large space of Lagrangians, why should we only consider that one type? Landau is making a tremendously brilliant point, and it is definitely not an easy feat to even follow what he is arguing about.
 
that lagrangian is just the energy
 
10:14 AM
by specific type, you mean when L stays exactly constant ?
 
@RyderRude You yourself know that you were looking for Galilean invariance. Not Lorentz. You cannot be talking about 4-vectors if you still have a clear mind about this. In super old physics, energy is a scalar.
 
@naturallyInconsistent my main point is that that lgrangian does not satsfy L(v+v')= L(v) . so we have to look for another lagrangian if we want the total derivative to be zero
 
@GiorgiLagidze No, the whole part of the Landau's chapter one is to motivate why the only Lagrangian form that we wish to study, are $L=\frac12mv^2-V$ and no other, at least in the context of old non-relativistic physics
 
@naturallyInconsistent four vectors are there in the spacetime formulation of newtonian mechanics
 
@RyderRude I do not see why what you think is important, is even important, let alone true. The task at hand is something else, and I know that because I read Landau.
 
10:17 AM
if u didnt mean four vectors, then in what sense did u mean covariance
 
@naturallyInconsistent yes I know that, but to fully grasp it, it's not easy
 
@RyderRude You are perfectly free to make your life difficult, but I am not at all intending to force Landau to do stuff that he does not wish to do; after all, he should be resting in peace.
@GiorgiLagidze That is why it is called tremendously brilliant. And why I said that you should be going for other texts instead.
 
to be honest, other texts don't explain why L = K -U
 
@RyderRude what the word actually means? That it varies in a specific way when a change is being made?
 
it means space and time treated in a similar way but whatever
 
10:20 AM
@GiorgiLagidze To be honest, other texts don't start with Lagrangians, and when they finally do, they just say that if you pick Lagrangians of this form, then EL equations are exactly the same as N2L, and thus must be physically relevant and correct.
 
true. when you started learning physics, after what time did you read Landau ?
 
@GiorgiLagidze year 1 or early year 2 of uni. I already finished the basic classical mechanics by then.
 
i found this modification of the free particle Lagrangian which is Galilean invariant : physics.stackexchange.com/a/14928
so this choice of Lagrangian can be made for the boost symmetry
it needs to introduce Lagrange multipliers tho
 
Note that it starts with Theorem-Schmorem.
 
this seems a non trivial choice though
so idk if it can be proved in general that this choice can be made for any symmetry of the EL eqns
chat.stackexchange.com/transcript/71?m=64449772#64449772 Hi. is there a proof that this can always be done @ACuriousMind
@GiorgiLagidze i think you should finish at least this chapter :). These rabbit holes do not seem like Landau's fault to me
 
10:39 AM
@RyderRude yes, same here.. definitely will and then move on with other resources
 
you are just over-thinking some physics which is understandable when you are new @GiorgiLagidze
 
@RyderRude unfortunately true.
 
i think even experienced physicists do this stuff
this is a short Daniel Denette video on consciousness @GiorgiLagidze
 
watching now
 
I dont know why i think he is fumbling as if he somewhat doubts his viewpoint
but he mentions a good experiment to demonstrate his viewpoint
 
10:47 AM
problem is
he explains "how", but not "why"
 
oh
"why is consciousness"?
 
consciousness in my opinion is something that must be explained with "why" , not "how"
and until it's explained with "why", any explanation is meaningless to me
 
what could a sample answer look like to you
 
if you ask that Daniel, why it works like that, he will say: "i don't know" :D
to me, consciesnouss needs to be approached in a psychological way
 
10:50 AM
in that field, I only like peterson's thought process
 
oh. but lets not mention him here
 
:D forgot
 
we can stick to consciousness philosophers like Chalmers and Denette
 
i think the important question to get closer to what consciesnouss is, ask yourself: do you believe in evolution that happened from micro to macro ?
we all understand micro evolution and macro evolution, but transformation from micro to macro, how sure can we be that that's what happened
stephen mayer has good thinking process as well on Joe rogan's show about this
 
i think there's no binary classification of micro and macro. they form a spectrum
 
10:53 AM
@RyderRude Why is it that I am so not surprised, after the Peterson that ACM pointed out would have violated SE rules so much as to be banned, that you would bring in the Chalmers of being taken down from TED infamy? Why is it that your intellectuals are so impossible to work with?
 
@naturallyInconsistent what do you mean
 
Can anyone though tell me why mentioning Pet..... would get us banned ? that means that any name I mention that's not related to physics should get me banned. ?
for example, I mentioned stephen mayer. do I deserve to be banned ? just asking so for future, I know what I'm allowed and what not
 
i think we should avoid religion philosophers. consciousness philosophers are fine
 
if I mention peterson relating to his consciusness analysis (which doesn't relate to religion), why should I be banned ?
it just doesn't make sense
 
ACM didnt yet say you would get banned, but we should avoid people who say controversial things (although im not familiar with Peterson)
 
10:57 AM
+1
 
Denette i think just talks philosophy. im not much familiar with Chalmers yet except his Ted Talk
Chalmers introduced the hard problem @GiorgiLagidze
 

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