« first day (4047 days earlier)      last day (876 days later) » 
05:00 - 23:0023:00 - 00:00

5:12 AM
How do you feel Might Guy didn't kill Madara?
 
5:27 AM
Those nlab diagrams are getting out of hand
 
 
2 hours later…
7:30 AM
apparently the world function goes back to at least Ruse : era.ed.ac.uk/handle/1842/30712?show=full
 
8:02 AM
0
Q: Why does spacetime bend or curve in the downward direction?

Kapish LakhotiaIn relativity , when mass bends spacetime in a downward direction. So ultimately we are coming to the conclusion that any mass body is also going down .

 
 
5 hours later…
1:02 PM
So fields with dotted indices belong to $\overline{V}$ and fields with undotted indices belong to $V$
But why can't I just normally contract between them
As in $V^* \overline{V}$ or something
Where $V^*$ is a vector in the dual space of $V$
 
1:24 PM
"contraction" is the application of some dual vector to a vector
when you write $v_a w^a$, this is $v(w)$ for some $v\in V^\ast$ and $w\in V$. The application $v(w)$ is defined by the definition of what the dual is
there is no such application defined for a conjugate
so it just doesn't mean anything to "try to" contract a vector and its conjugate
 
Okay so I can't have $v^*(w)$ because it's not defined
sorry I mean $\overline{v}(w)$
 
while you might write something like $\bar{v}_a w^a$ for some $\bar{v}\in\bar{V}$ in a particular basis, this isn't "a thing" - there's no basis invariant way to write what is supposed to happen here
 
Ah okay
 
(another reason why coordinate/basis-free notation is great because you can't even write down the things that don't make sense :P)
 
So how come people seem to throw away these rules for $(\sigma_m)_{\alpha \dot{\alpha}}$?
There must be something weird going along with the Pauli matricies
 
1:28 PM
the annoying thing is that people usually work with spinors in a concrete representation
you choose some particular basis in which the $\gamma$ matrices have a particular form, and particular sign conventions
and then you do all sort of manipulations that don't really make sense abstractly but nevertheless lead to useful result in your chosen basis
 
There is no distinction between dotted and undotted indices for $\mathrm{SU}(2)$, only for $\mathrm{SL}(2,\mathbb{C})$
 
What's the reason behind there being no distinction?
 
I rant about that in my question about Majoranas - it's extremely hard to find resources that actually explain what's happening here in a linear algebraic way and not in the physicist's "just write down the matrices and compute this stuff" way
@DIRAC1930 the conjugate representation is isomorphic to the original one for SU(2)
while the conjugate representation of the rep $(s_1,s_2)$ of SL(2) is $(s_2,s_1)$, i.e. not generally isomorphic
 
Okay thanks
Would this be easier just to do it from the Clifford algebra?
 
123
1:56 PM
Hi All...
How Moment of Force $M = r \times F$ equal torque $\tau = I \alpha$ .
In moment of force how $r$ and $F$ take of property rotational inertia.
 
 
1 hour later…
fqq
3:07 PM
@ACuriousMind you can, e.g. the $\overline{v}(w)$ above makes as much sense and is as clearly wrong as \bar{v}_a w^a$
 
@fqq but $\bar{v}(w)$ doesn't tell you what to compute, it's unclear what its value would be. $\bar{v}_aw^a$ directly carries with it the recipe for how to compute it, so you have no chance to notice it's not actually something that was defined
 
fqq
$\overline{v}$ should have a dotted index
so if you parse it correctly, you see it's wrong
if you parse $\bar{v}(w)$ incorrectly you can also assign some computation to it
 
fair enough
 
fqq
but I see your point, many physicist probably would compute the one with indices blindly and would stop at the function notation
 
These susy conventions are hard enough, the difficulty really ramps up when you try to do the sugra version
 
fqq
3:12 PM
to me they are really the same, I take the index notation to also be coordinate free (as in Wald)
 
I still haven't managed to construct a single invariant
Of the form $M_{\alpha \beta}\psi^*{}^\alpha \psi^\beta$
 
fqq
3:29 PM
@DIRAC1930 I really don't think it is, the representation theory of SU(2) is pretty straightforward but maybe I'm missing something
 
I guess the difference with the relativistic version is that since the spinor space and the conjugate one are isomorphic, the conjugate is an automorphism?
So you just have the same basis
Or you can write the isomorphism explicitly if you're bothered
I don't super know why you have to consider the conjugate space in the first place rly
I assume it's related to that conjugate rep business
but I'm not privvy to the esoteric mysteries of group theory
3
Q: Are the complex conjugate representations $e,\bar e$ and $d,\bar d$ for $\mathfrak{sl}(2,\mathbb{C})$ inequivalent?

Edward HughesLet $\mathfrak{sl}(2,\mathbb{C})$ be the complex Lie algebra of $SL(2,\mathbb{C})$ and $\mathfrak{sl}(2,\mathbb{C})_\mathbb{R}$ be its realification; that is $\mathfrak{sl}(2,\mathbb{C})_\mathbb{R}$ is $\mathfrak{sl}(2,\mathbb{C})$ considered as a real Lie algebra. Let $d$ be an irrep of $\mathf...

something about this I assume
also here
 
3:45 PM
I keep getting $M_{\alpha \beta} \psi^*{}^{\alpha}\psi^\beta \rightarrow -M_{\alpha \beta} \psi^*{}^{\alpha}\psi^\beta$ for literally everything and I don't know where I'm going wrong
 
fqq
@Slereah because you can, it's useful to build real lagrangians and the theories you build with are good
 
$\psi^{\dagger} \gamma_0 \psi$ is an invariant under the rotation group, what's wrong with that
 
Yes, I've been trying to get that to work but I'm not doing the transformations correctly or something
 
$M = \gamma_0$
Do you know how to do a rotation of a spinor
 
I guess if you want to write it fancily you can use a map $f : \bar{V} \to V$
 
3:47 PM
$(\gamma_0)_{\alpha \beta} \rightarrow F_{\alpha}{}^\gamma F_\beta{}^\delta (\gamma_0)_{\gamma \delta }$ right?
 
and then you have $f(\psi^{\dot{a}} t_{\dot{a}}) = \psi^{a} t_a$
Or somethibng
idk if the isomorphism from the spinor space to its conjugate has a standard symbol
but you can just write that $*$ is an isomorphism
$^* : V \to V$
 
fqq
@Slereah that looks $\mathbb{C}\to\mathbb{C}$, not $\bar{V} \to V$
 
and then $\psi^*{}^{\alpha} \rightarrow \psi^*{}^\gamma(F^{-1})^*{}_\gamma{}^\alpha$
 
@fqq That's why I included the basis vectors
 
Wait, maybe write out the invariance of $\psi^{\alpha} \psi_{\alpha}$ fully
 
3:50 PM
The $F's$ cancel and produce a $-$ sign for everything I try independently of what I choose the $M$ to be
 
fqq
clearly there's something wrong, but it's right that what you choose for $M$ does not matter, that's the whole point of the indices after all
 
But then I can't get anything invariant unless $M=-M$ which means $M=0$
 
fqq
no, you are doing something wrong, scalars exist
 
Trying to find what I'm doing wrong with all these different conventions is impossible
 
It's certainly a nightmare, you should try to stick to one credible source and learn from that instead of reading around until after you get one down
Doing this non-relativistic stuff is a waste of time, do it the relativistic way first
 
3:54 PM
From what I can see the basic difference between the relativistic and non-relativistic case is that one uses the actual $\mathfrak{su}(2)$ while the other one uses the complexification $\mathfrak{su}_{\mathbb{C}}(2)$
This is why you need the dotted indexes
because the complexified version of the algebra is not isomorphic to its conjugate
 
From a non-relativistic quantum mechanical perspective, one finds $\mathrm{SL}(2,\mathbb{C})$ from the representation theory of angular momentum, but because the probability density $|\psi|^2 = |\psi_1|^2 + |\psi_2|^2$ must be invariant under these transformations, we must restrict to it's real form $\mathrm{SU}(2)$.
From a relativistic perspective, the probability density is just the time component of a four-current vector so there's no restriction, we're stuck with $\mathrm{SL}(2,\mathbb{C})$.
The Weyl representations $J_a = \sigma_a$, $K_a = + i \sigma_a$ and $J_a' = \sigma_a$, $K_a' = - i \sigma_a$ form in-equivalent representations of the Lorentz group because any transform that sent $J_a$ into $J_a'$ couldn't send $K_a$ into $K_a'$.
 
4:12 PM
Anyway I guess the lesson to take is that it's kosher to write your spinor product as $(\psi^a)^* \psi_a$
Now you can stop worrying about that and start worrying about the ordering of your Lagrangian in Grassmann variables
 
$(\psi^a)^* \psi_a$ is not an invariant, I think that may be the issue
 
Lol
 
@bolbteppa Dag nabbit
 
I know what you meant but I held off suggesting it earlier so good opportunity :p
 
What's the lagrangian of the superpoint
 
4:17 PM
The usual $\dot{x}^2 - \psi i \partial_t \psi$ right
 
Does that really change much?
I don't think rotation would change under $\partial_t$
 
That's actually assuming a massless particle through using the whole einbein stuff
 
Although I guess it might for Galilean transforms
 
Here’s something on my mind lately
 
I give up lol
 
4:24 PM
How Kepler orbits are taught in intro courses emphasizes conservation of energy and conservation of angular momentum
Which, within context, entirely makes sense
But one thing that forces upon you is a sense of scale: how far are the various planets from the sun
 
Grosche suggests $i\bar{\eta} \dot{\eta}$ indeed
idk if it's the most general one
 
And the funny part of this to me: historically, distances are exactly what you didn’t know
 
@Semiclassical would you rather they teach the historical reasoning
 
All you could observe was where planets were in the sky, ie, the angular part of spherical coordinates
 
Planets move on those orbits due to esoteric reasons
"the universe itself was an image of God, with the Sun corresponding to the Father, the stellar sphere to the Son, and the intervening space between them to the Holy Spirit."
Teach the controversy
 
4:29 PM
For an intro course, no. But it’s just a bit funny how we we simultaneously emphasize Kepler’s role in history conveniently ignoring how limited their tools actually were
 
I'm guessing you could probably try to work out the most general Lagrangian for non-relativistic (free) spinors the same way as the one for point particles
Just find the most general quadratic Lagrangian that's invariant under the Galilean group
Since the Lagrangian also has to be real I guess it somewhat limits the number of possible terms
Can't have any linear term due to this
I guess you're limited to the bilinear terms of $\psi$ and $\dot{\psi}$
idk what's the space algebra for $SO(3)$ but I'm guessing it's similar-ish to the Lorentz group
Although since you have $\sigma_j \sigma_k = \delta_{j k}I + i\varepsilon_{j k \ell}\,\sigma_\ell$, I'm guessing the algebra's basis is just $I$ and the Pauli matrices?
 
the amount of stuff which astronomers were able to achieve over the centuries does blow my mind at times
 
@Semiclassical it wasn't easy
For a while they just kind of had to look at the sky and figure it out
 
Also this was even before the technology for a notebook
if you had to write something down you had to carve a wax tablet or something
 
4:39 PM
like, how tf did Kepler come up with this picture: researchgate.net/figure/…
 
$I$ and those is the full clifford algebra in 3D,
 
yeah looks like it
@Semiclassical sorcery
 
There's a Schrodinger equation for the non-relativistic case, everything passes through it
 
@Slereah i think this is related to the following fact: you can write down the Dirac equation in 2+1 spacetime using just 2-by-2 matrices rather than having to go through 4-by-4 gamma matrices
 
I'm not even sure how anyone had the patience to notice that the celestial sphere rotates
But I guess this was before television
 
4:41 PM
b/c in that case you only need three gamma matrices rather than the usual 4
 
They still haven't figured out that it's all just a big turtles shell rotating about as it shuffles
 
and the pauli matrices suffice for those (up to factors of i)
though i don't know what the 'best' choice is there. this came up as a HW problem for a course i'm grading recently
 
I don't even know how we can guarantee that $\bar{\psi} \dot{\psi}$ is real
 
e.g. you need $\{\gamma^0,i\gamma^1,i\gamma^2\}$ to be the three Pauli matrices. but what's the nicest choice to assign that?
it can't make a difference at the end, but what's the least tedious choice along the way?
 
I guess it doesn't matter much for the EoM since it turns out linear
But how can we be sure that the kinetic term for spinors is real in the Lagrangian?
Is it always real or only on shell?
Although... I guess the spinor part is all Grassmann stuff so it's all linear
So it's not too hard to work out
it's probably fine
why no $\bar{\psi} \psi$ terms allowed?
is it one of those terms that can just be integrated away
Grosche has some non-relativistic term that's proportional to $[\bar{\psi}, \psi]$
 
4:51 PM
I'd say it is but they usually discuss a massless particle
 
It's non-relativistic, isn't the mass a factor of the kinetic term?
from what I can see there are Lagrangians with $\bar{\psi} \psi$ terms, but those are more for potential terms
like the constant magnetic field Lagrangian
Also there seem to be terms in $\bar{\psi} \sigma^\mu \psi$ for EM stuff?
when currents are needed
 
I'm specifically looking for potential terms
 
Well use the spinor and its derivative and their conjugates as fields, and the two possible bilinears
Also since it is grassmann variables I think you can exhaust every possible polynomial terms
 
rob
5:07 PM
2
A: Anomalous muon magnetic moment and muon catalyzed fusion

Cosmas ZachosWhen cannonading ghosts, it's very hard to tell what you shot down... Theorists do speculate; but, counterintuitively, they have pretty stringent rules of how to do that, and your question is actually quantitative (!!). For the muon magnetic moment anomaly, we know that could be smaller than one ...

> When cannonading ghosts, it's very hard to tell what you shot down.
I appreciate Cosmas's writing so much.
 
@Slereah I cant get anything to work
 
@DIRAC1930 that's physics for you
 
$\psi^\dagger \psi$ should be one I'm assuming because it's used everywhere in non-rel phys
which would be $\delta_{\alpha \beta} \psi^*{}^\alpha \psi^\beta$ right?
 
I s'ppose
Hm
Why are they mixing spacetime and spin indices
 
What is that dot on top of the $x$?
 
5:15 PM
Time derivative
a less mysterious example
"It is real up to a total time derivative"
Dang
 
123
In moment of force $M = r \times F$ formula, how we take care of moment of inertia.
 
complex terms swept under the rug of the De Rham cohomology
 
Question is, does this argument work for QM?
Is $\dot{\bar{\psi}} \dot{\psi}$ illegal
 
If you don't do relativity, you don't have the same kind of constrained thinking
The potential can basically be anything to a first approximation
 
5:22 PM
I don't think I've ever seen that term, though
 
Relativity and covariance just makes everything easier
Also what are you even doing non-relativistically with fermions except looking at multiparticle systems so you can take your multiparticle Hamiltonian and turn it into second quantized form for fermions
 
What would even be the EoM of that term, $\ddot{\psi} = 0$?
Also wait, are we doing the lazy physicist Lagrangian
 
$\psi = \psi_0 + a t + \frac{1}{2} g t^2$ :\
Oh wait, with $g = 0$
 
I thought the proper one was like $$\frac{1}{2} (\bar{\psi} \sigma^\mu \partial_\mu \psi + \psi \sigma^\mu \partial_\mu \bar{\psi})$$
I guess $\ddot{\psi}$ doesn't constrain the equation since it's always zero?
Because Grassmann
although idk if that's true if we add all the other terms
but since $\ddot{\psi}$ is always zero I suspect you can probably get rid of that term via some Grassmann magic
I think technically you need the symmetric one because otherwise the EoM of the spinor and its conjugate will be different?
idk it's one of those things people don't bring up much
$C$ symmetry or whatever
Oh wait, IIRC those two Lagrangians are equivalent under partial integration
nvm
 
5:54 PM
Okay, so far I can get $\epsilon_{\alpha \beta} \psi^\alpha \psi^\beta$ being invariant which I think is a known invariant
according to L&L
 
Seems plausible
 
Now I need to see if I can find one that is real
 
Now you just have to work out uuuh 31 other terms
Well, 16 up to symmetry
Also 8 i guess since psi² = 0
 
Why can't I just have $(\epsilon_{\alpha \beta}\psi^\alpha \psi^\beta + \mathit{c.c.})$ as a real invariant?
 
6:17 PM
To have $M_{\alpha \beta}\psi^*{}^\alpha \psi^\beta$ to be invariant, I effectively need $M$ to satisfy $2M_{\alpha \gamma}(F^T){}^\gamma{}_\beta = M_{\alpha \beta}$
Which doesn't look like is possible
Which I think suggests (in matrix notation) that $2 F^T = \mathbb{I}$ which is impossible
 
Where is the 2 coming from
 
$F{}_\alpha{}^\beta F{}_\beta{}^\gamma = 2 F{}_\alpha{}^\gamma$ I think
Since 2 transformations are being done in succession
 
6:33 PM
All there is to it is noting, for $M \in \mathrm{SU}(2)$, that $\psi'^a \psi_a' = (\psi M^{-1})^a (M \psi)_a = \psi^a \psi_a$. So $(\psi'^a)^* \psi_a' = [(\psi M^{-1})^a]^* (U \psi)_a \neq \psi^a \psi_a$ because $(U^{-1})^* U \neq I$.
 
You're mixing your notation :p
@bolbteppa wouldn't that argument also hold for $\bar{\psi} \dot\psi$?
Also as mentionned some Lagrangians do have $\bar{\psi} \psi$
Also wait why is $\psi^a$ transformed to $\psi^a M^{-1}$
Wouldn't the transformation here be $$\langle \bar{\psi}, \psi \rangle \to \langle \bar{M\psi}, M\psi \rangle = \langle M^*\bar{\psi}, M\psi \rangle = \langle M^{-1}\bar{\psi}, M\psi \rangle = \langle \bar{\psi}, \psi \rangle $$
replace with $\delta_{ab} \psi^a \psi^b$ for appropriate component
Man, looking up how spinors in $R^3$ work again in Landau and I still don't know
bloody non-commuting things
It does seem that the spinor metric is still $\varepsilon$?
But the inner product is $\psi^1 \psi^{1*} + \psi^2 \psi^{2*}$
 
6:56 PM
Oh damn $M = U$ :p
What's $\overline{\psi}$, is it $\psi^{\dagger} \gamma^0$ or $\psi^{\dagger}$
 
Well the conjugate I would assume, but now considering that the musical isomorphism isn't trivial, I'm not sure
 
$U^{\dagger} = U^{-1}$
 
Trying to think of a book that may discuss it
Hall maybe
 
Need to be careful about whether you're using $\mathrm{SU}(2)$ or $\mathrm{SL}(2,\mathbb{C})$, $\psi^{\dagger} \psi$ is invariant under $\mathrm{SU}(2)$ but not $\mathrm{SL}(2,\mathbb{C})$
 
Isn't the whole point that any term with fully contracted indices is invariant? (for $V$ anyway)
 
7:00 PM
Well we're talking SU(2) here, since we're doing non-relativistic QM I assume?
 
There's a discussion of this in L&L in the spinor chapter, they even set up raising and lowering indices for them
 
yeah
 
$M_{\alpha \beta}\psi^\alpha \psi^\beta$ is invariant for any $M$
 
they do seem to imply that the inner product is just the obvious one, but the map between dual spaces makes things a bit trickier to be sure
I thought the metric of a vector space defined its musical isomorphisms
 
When you write $M$ that way you assume it transforms as a 'tensor' under $\mathrm{SU}(2)$ transformations
 
7:07 PM
He defines the bilinear form $\psi^1 \phi^2 - \psi^2 \phi^1$ and the norm $\psi^1 \psi^{1*} + \psi^2 \psi^{2*}$
"$\psi^{1*}$ and $\psi^{2*}$ must be transformed as $\psi^2$ and $-\psi^1$ respectively"
ugh
I need to brush up on my spinors I think
Is the dual space isomorphic to the conjugate space???
 
7:20 PM
What's the conjugate space
 
Complex conjugate representation
nice cover
it seems like the appropriate book for the topic
 
Basically an insane book
It starts by using complex vectors under the Euclidean dot product, fun times
 
Let's find out
 
The approach in that book is very confusing because the way he starts doesn't generalize
In the domain of mathematics known as representation theory, pure spinors (or simple spinors) are spinors that are annihilated under the Clifford action by a maximal isotropic subspace of the space V {\displaystyle V} of vectors with respect to the scalar product determining the Clifford algebra. They were introduced by Élie Cartan in the 1930s to classify complex structures. Pure spinors were a key ingredient in the study of spin geometry and twistor theory, introduced by Roger Penrose in the 1960s. == Definition == Consider a complex vector space...
Buried a hundred pages or so in you find out he's just doing a special case of the normal Clifford thing
 
Can i skip it
 
7:28 PM
Unless you want to get into the weeds of one approach I'd say so
But look at the GR chapters in there
 
How do I construct different $M_{\alpha \beta}$'s?
Can it be anything or do they have to be built out of something specific
like is it built out of spinors $\psi_\alpha$?
 
I would guess it's built from the algebra of physical space
 
Might look in the book Roger Penrose says was impenetrable or something
Introduction to Tensors Spinors and Relativistic Wave Equations
 
7:45 PM
I certainly closed it multiple times
 
Okay so I made a mistake earlier. To have $M_{\alpha \beta}\psi^*{}^\alpha \psi^\beta$ to be invariant, we require $F^T F =\mathbb{I}$ which is impossible
 
"A vector is said to be isotropic if its scalar square is zero."
 
Something weird is going on when we mix up $V$ and $\overline{V}$
 
ah right
Bloody mathematicians
 
A complex vector squared under the Euclidean dot product, madness
 
7:58 PM
Something weird is happening
I think $F^T F = \mathbb{I}$ is the correct constraint which is impossible
Wtf in L&L it says that the way $\psi{}^*{}^\alpha$ transforms is related to $\psi^\alpha$ by arguments involving the change in sign when reversing time.
I think this non-rel stuff is weird
 
8:21 PM
I'm surprised it's difficult to find things to do with all of this online
 
8:51 PM
looking at Chevalley's spinor book
Not the worst but he has terminal mathematician disease
 
If I have $M$ transforming like $M_{\alpha \dot{\alpha}}$ it works I think but that seems like an illegal operation
 
9:08 PM
So I would need a contraction like ${\overline{R}}_{\dot{\alpha}} {S}_\alpha \overline{\psi}{}^\dot{\alpha} \psi^\alpha$ or something which is probably wrong
 
You can tell Chevalley is doing too much because he keeps talking about $\Gamma$-graded algebras
what weirdo does anything but $\mathbb{Z}_n$ graded algebras
also he is terribly wrong because he says "semi-graded" instead of "super"
 
Does the $\dagger$ on the field terms arise when considering the quantum inner product?
As in after cann. quant.
 
 
1 hour later…
10:33 PM
I'm not sure about Landau's argument
So we have $\epsilon_{\alpha \beta}\psi^\alpha \psi^\beta$ being invariant
Then he says $\psi_\beta^*$ transforms like $\psi^\beta$ which is true
And I think that's how he justifies $|\psi^1|^2 + |\psi^2|^2$ being invariant
Actually it works I think
So he has $\psi^\alpha = (\psi^1 ,\psi^2)^\alpha$ essentially
$\psi_\beta^*$ transforms like $\psi^\beta$, so he defines the components of $\psi^\beta$ with the components of $\psi_\beta^*$
 
@ACuriousMind Hey man, may I ask you for your help in thermodynamics regarding something?
 
So $\psi^*_\beta = \psi^\beta= (\psi_1^* ,-\psi_2^*)^\beta$
Sorry that was meant to be $\psi^*_\beta = \psi^\beta= (\psi^*{}^1 ,-\psi^*{}^2)^\beta$ I think
And then when replaced in the $\epsilon_{\alpha \beta}\psi^\alpha \psi^\beta$, you get $\psi^1{}^*\psi^1 + \psi^2{}^* \psi^2$
Does this seem correct?
 
05:00 - 23:0023:00 - 00:00

« first day (4047 days earlier)      last day (876 days later) »