« first day (4970 days earlier)      last day (33 days later) » 

1:46 AM
@lucabtz oh i wish i could find a good reference for the theoretical framework of these spinor BECs.
this question is motivated by something @Relativisticcucumber said. I am wondering if every single two level system can be interpreted as a spin-1/2 system.
In particular, let the two levels be $\lvert a \rangle, \lvert b \rangle$. Then, we can define $\sigma_z = \lvert a \rangle \langle a \lvert - \lvert b \rangle \langle b \lvert$ and analogous define $\sigma_x$ and $\sigma_y$
supposing that the algebra of operators is induced by experimental choices (choices of what things to measure), then it seems we can perhaps always construct some choices of operators corresponding to the above definitions to treat our system like a spin-1/2 system
2:36 AM
hehe i have finally finished my chern simons notes
 
1 hour later…
3:55 AM
h o n k 👍
 
2 hours later…
5:36 AM
H O N K ~
@SillyGoose you can; however, $\sigma_x$ and $\sigma_y$ might be meaningless in such a system.
6:36 AM
@Sanjana hi. i meant ur recent post about symmetries. and {} is the Poisson bracket in my comment
@Sanjana one can make H time dependent to make it commute with boost (e.g. H=mX-Pt)$ but it wudnt b very interesting becuz the eigenfunctions become time dependent
7:03 AM
@SillyGoose after including the identity matrix, Pauli matrices form a basis for 2x2 Hermitian matrices. so u can write the Hamiltonian of any 2x2 quantum system using them
but the coefficients must b real to span Hermitian matrices
 
2 hours later…
9:07 AM
I just made a polar plot of the distance of the centre of the Sun from the SSB (Solar System barycentre). astronomy.stackexchange.com/a/44903/16685
> In this plot, the radius gives the Sun-SSB distance (in millions of km), the angle (anticlockwise, from the +X axis) gives the date. The date wraps around with a period of 65295.5 days, so each degree corresponds to ~2 years. Each cycle is plotted in a different colour of the rainbow, starting at red.
user image
2
that's a pretty butterfly~
9:23 AM
I was tempted to do more cycles, but it gets a bit messy. The Sun's speed relative to the SSB is actually fairly small. From astronomy.stackexchange.com/a/28036/16685
oh, that is unexpectedly human-relateable
about a bike ride's speed?
9:43 AM
@naturallyInconsistent Indeed! It's positively glacial compared to usual solar system orbit speeds.
@RyderRude ACM said that this might not be possible even for time dependent Hamiltonians because $[H,J]=-\partial_t J$ is non-zero, for boost invariant systems.
Somehow, his words manipulated me to stop thinking about systems not invariant under boosts :p
@RyderRude Yes. But the formulation I am looking for can be more general. See SillyGoose's comments on the post and discussions in this chat.
@Sanjana oh
makes sense. Valter Moretti's definition of time dependent symmetries isnt satisfied either if we set H=J
 
2 hours later…
12:16 PM
@SillyGoose What does "treat like a spin-1/2 system" mean? All you're really doing here is observing that the space of 2-by-2 matrices is spanned by the Pauli matrices and the identity, nothing to do with spin as such.
12:52 PM
in this song, Beatles deviate from the popular song structure by cutting things short
but kind of hypocritical of Lennon to write this stuff when he was abusing his family around that time
1:59 PM
user image
2
the @SillyGoose mobile
 
2 hours later…
3:50 PM
what do u think about the philosophy where all humans are the same organism
 
5 hours later…
8:23 PM
and I got stuck with commutators again :P
is the following result correct: $$ [S_i,S_{1j}S_{2j}] = [S_{1i} + S_{2i}, S_{1j}S_{2j}] = \cdots = i\hbar( \epsilon_{ijk} S_{1k} \otimes S_{2j} ) + i\hbar( S_{1j} \otimes \epsilon_{ijk}S_{2k} )$$
Indices 1,2 refer to the subspaces of two particles $\mathcal{E}_1, \mathcal{E}_2$ and $i,j,k$ are the ordinary cartesian components $x,y,z$
Wait, I have a hunch it should be zero
forgot to point out that $\mathbf{S} = \mathbf{S}_1 + \mathbf{S}_2$ altough it is highly inferable :P
This comes from trying to prove $[\mathbf{J},\mathbf{S}_1 \cdot \mathbf{S}_2] = 0$ via the fundamental commutation relations
and not by noting that that dot product is a scalar quantity which obv commutes with $\mathbf{J}$
 
2 hours later…
10:06 PM
Why does deriving Maxwell equation from $\mathcal{L}=-\frac{1}{4}F_{\mu \nu}(\partial^ \mu A^\nu-\partial^ \nu A^\mu)$ where $A$ and $F$ are independent variables, does not work? E.g. I get the correct Maxwell equation by varying the action w.r.t. $A$ but variation w.r.t. $F$ gives $\partial_\mu A_\nu=\partial_\nu A_\mu$.
A similar exercise from a different Lagrangian density $\mathcal{L}=\frac{1}{4} F_{\mu \nu} F^{\mu \nu} -\frac{1}{2}(\partial^\mu A^\nu-\partial^\nu A^\mu)$ seems to work all fine. It gives Maxwell's equation+$F_{\mu \nu}=\partial_\mu A_\nu-\partial_\nu A_\mu$

« first day (4970 days earlier)      last day (33 days later) »