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5:01 PM
How do we know that we have the correct inner product?
What if there is another invariant tensor?
 
@DIRAC1930 apply a rotation
It should be invariant under it
 
What if there is another one?
I'm guessing there isn't
 
If it acts as a scalar under a rotation
 
But what if there is another invariant tensor in addition to $\epsilon_{\alpha \beta}$?
There probably isn't in this case, but in general, is there always 1?
 
$2 \varepsilon_{\alpha \beta}$ is another one
I would suggest thinking in terms of the $\gamma_0$ stuff, if you've seen the Dirac equation where they derive scalar, pseudo-scalar vector, etc invariants after showing the Dirac equation is Lorentz invariant that's probably the first time most people see this stuff
 
5:13 PM
Or do everything from Clifford algebras I guess
Valid in all dimensions!
 
This is something I may look into but I think it's above my level for now
 
It's easier than looking at this non-relativistic stuff that's for sure
 
So $|\psi^1|^2 + |\psi^2|^2$ is also a scalar
 
fqq
@DIRAC1930 the answer is once again $1/2 \otimes 1/2 = 0 \oplus 1$
there's an invariant tensor of a certain type iff the trivial representation appears in the corresponding decomposition
 
Okay so the Dirac Lagrangian has a term $\psi^\dagger \gamma^0 \psi$.
 
fqq
5:20 PM
since the trivial representation 0 appears once in $1/2 \otimes 1/2 = 0 \oplus 1$, there is one invariant tensor with two indices
 
Are Pauli spinors Weyl spinors?
 
fqq
sort of, the adjoint does not really mean the same thing
 
How is the Hermitian adjoint defined for vectors?
Its this right $\langle y, x \rangle = y^\dagger \mathbf{M} x$
 
fqq
again, depends on the notation you are using. it's right in the right notation
you keep asking about stuff without defining much
 
5:35 PM
Okay, so let start from the basics. Our starting point is that $\psi$ is an $SU(2)$ spinor with an inner product $\epsilon_{\alpha \beta}\psi^\alpha \psi^\beta$. We want to define invariants of SU(2) so that out interacting Lagrangian is real and invariant under $SU(2)$
One invariant that satisfies this is $\epsilon_{\alpha \beta}\psi^*{}^\alpha \psi^\beta$
Which I'm just going to check again
 
5:50 PM
From the beginning, you can start from spinors of $\mathrm{SU}(2)$. You then note that a determinant whose columns are two different spinors is invariant under conjugation by $\mathrm{SU}(2)$ which is just an $\mathrm{SU}(2)$ transform of the spinors. This gives you a way to form scalars from two spinors. Of course this should agree with standard Clifford algebra terminology, so you can realize it can be related to the usual $\gamma_0$ thing familiar from the Dirac equation
 
6:07 PM
If I have $F_{\alpha \beta}$ and transpose it, is it $-F^T{}_{ \beta \alpha}$ because the objects are Grassmann in their indices I think
I'm not sure, I keep getting $\epsilon _{\alpha \beta} \psi^*{}^\alpha \psi^\beta \rightarrow - \epsilon_{\alpha \beta}\psi^*{}^\alpha \psi^\beta$ as the transformation but I'm not sure if Ive made a mistake
$\epsilon_{\alpha \beta} \psi^*{}^\alpha \psi^\beta = -\psi^*{}_\alpha \psi^\alpha$
$ \epsilon_{\alpha \beta} F^*{}^\alpha{}_\gamma \psi^*{}^\gamma F^\beta{}_\delta \psi^\delta$
$\psi^*{}^\gamma \epsilon_{\alpha \beta} F^*{}^\alpha{}_\gamma F^\beta{}_\delta \psi^\delta$
$-\psi^*{}^\gamma F^*{}_\beta{}_\gamma F^\beta{}_\delta \psi^\delta$
$-\psi^*{}^\gamma F^\dagger{}_\gamma{}_\beta F^\beta{}_\delta \psi^\delta$
$\psi^*{}_\gamma F^\dagger{}^\gamma{}_\beta F^\beta{}_\delta \psi^\delta$
$\psi^*{}_\gamma \delta^\gamma{}_\delta \psi^\delta$
 
 
1 hour later…
7:33 PM
Is there a word for like
a co-foliation
If I have a manifold that can be foliated into hypersurfaces, is there a dual foliation into curves
most probably if it's $\mathbb{R} \times \Sigma$, but what about in general
does it involve the word "integrable", I suspect it might
 
Did anyone spot a mistake I made?
 
@Slereah I'm not sure what you mean by "dual" there, but if you pass from a foliation to the associated distribution (its set of tangent vectors, essentially), you can ask for an orthogonal distribution. If this orthogonal distribution is integrable, then there's also a foliation associated with it that will intersect the original foliation "orthogonally". Is that what you're looking for?
 
most probably, yes
does this have a specific name?
 
not more specific than "orthogonal complement", I think
it's only a good operation on distributions, I think the complement will fail to be integrable quite "often"
 
7:49 PM
From what I've read it's probably already true for what I'm looking into [foliation of globally hyperbolic spacetimes]
 
doesn't globally hyperbolic mean you have a non-zero time-like vector field orthogonal to the spacelike foliation everywhere?
 
If you have some timelike vector field on a Cauchy surface and then do the Hamiltonian flow it gives you an appropriate foliation of the manifold into timelike curve
 
in that case your complementing foliation would just be the integral curves of that vector field
yeah
 
@ACuriousMind The time function gives such a field yeah
it's unfortunate that they are called "distributions", because it's not a very easy name to google
lots of things with that name
Is Ryan still around
 
not currently pingable, but I see him drop in from time to time
@Slereah the problem is likely that most of these names were made up by people that couldn't google whether someone else already named their stuff the same :P
 
8:02 PM
@ACuriousMind I mean it's distribution
I'm sure they heard of probabilities, at least
Ryan get here you lazy bum
I have to ask you for free labour
 
@Slereah but do you know when someone first called the function that assigned probabilities a distribution?
 
Let's find out
 
I don't, but it wouldn't surprise me if it was more recent that you'd expect
I tried a quick search but didn't find anything useful, perhaps a question for History of Science and Mathematics
 
Kolmogorov used it in 1933 at least
 
8:55 PM
You know all that measurement thing makes me wonder
Is there a simpler example of what I'm doing?
Like if I consider the set of all triangles on a Riemannian manifolds, does it also determine the LC connection on it
If I have the set of all sets of 3 points and their associated angles/lengths, does that determine the connection
 
@Slereah probably it determines at least the curvature, since curvature "is infinitesimal holonomy" and the angles/lengths tell you how parallel transport along that triangle works
 
what's an example of two different LC connections that have the same triangles?
 
9:13 PM
Can someone help me with a question in electromagnetics
 
How does $\overline{\psi}{}^\dot{\alpha}$ transform?
 
If this equation is true $D = \Epsilon_0 E + P$. Ofc in order for polarisation to exist in an object you need to expose it in an external electric field, lets call it E_0. Now if the electric field
Inside the material is the one in the initial equation
then if we go outside the material the P vanishes
leaving D=epislon_0 E
And E is the electric field inside
but outside the material E=D
Then D=E but the field outside shouldn't be the same with E_0 ?
 
9:27 PM
If anyones interested, the correct invariant is $\delta{}_{ \alpha \dot{\alpha}} \overline{\psi}{}^{\dot{\alpha}} \psi^\alpha$ I believe
 
9:39 PM
Hmm maybe I've used the $\delta$ tensor wrong
It's of mixed indices so I don't know what exactly it is
However I've defined it so $\delta^\alpha{}_{\dot{\alpha}}\overline{\psi}^\dot{\alpha} =\overline{\psi}^\alpha$ which seems to perhaps not be right
 
$\delta_{\alpha \dot{\alpha}}$ doesn't make sense
The invariant is $\overline{\psi} \chi = \psi^{\dagger \alpha} (\gamma_0)_{\alpha}^{\beta} \chi_{\beta}$
 
Okay, yeah mine seemed to not be right
Wait, your's is for Dirac spinors in rel. systems right?
I'm after it for 2 component non. rel systems
 
The big difference is whether you transform them under $\mathrm{SU}(2)$ vs $\mathrm{SL}(2,\mathbb{C})$ so they are going to look very similar
 
In these handwritten notes I've found, first he defines $\overline{\psi}{}_{\dot{\alpha}} = (\psi_{\alpha})^*$ and then he says now that we have Grassmann fields we define $\overline{\psi}{}_{\dot{\alpha}} = (\psi_\alpha)^\dagger$
Can you decipher what any of this means
 
For complex numbers you just take the complex conjugate, when you quantize you promote them to operators so you now use the Hermitian conjugate rather than just complex conjugation
 
9:54 PM
@Slereah I said "at least the curvature", but I now realize (see e.g. mathoverflow.net/a/243517, if I'm reading it right) that "LC" also fixes the torsion and so there probably generically isn't more than one LC connection with the same curvature
so your triangles have a decent chance at fixing the LC connection in the generic case
 
I'm trying to think of additional measurements one can do physically but probably not much you can do without
uuuuh
 
But we have $(\sigma_m)_{\alpha \dot{\alpha}}$ and for $m=0$, this is essentially $\mathbb{I}_{\alpha \dot{\alpha}}$ so why cant we have $\delta_{\alpha \dot{\alpha}}$?
 
Inside a dielectric is polarisation and electric field in opposite direction ?
 
You can't set $m = 0$ in the non-relativistic case
 
Okay thanks
I don't see how your expression will reduce to $\psi^\dagger \psi$ is matrix notation
 
10:08 PM
anyone can help me regarding the direction of polarisation and electric field in a dielectric?
 
I should say, yeah for $m = 0$ in the relativistic case that's right, but it's not the invariant Kronecker delta, which is a bit confusing
 
Do I need the $\overline{\psi}$ terms for non-rel systems?
Man I suck at this
 
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