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12:28 AM
@ACuriousMind I was reading your answer regarding the density matrix. I have one initial question. The density matrix is needed when we study a sub-system, simply because we don't have full information, and that makes it looks like the sub-system,that can very well be in a pure state,is in a mixed state?
1:09 AM
@ACuriousMind careful, that's not how it works!
you cannot get back the whole density matrix from the density matrices of the subsystems
2 hours later…
2:59 AM
@ACuriousMind Damn, SE making all the changes nobody asked for
> This change makes we want vomit. It's absolutely horrible. I would rather remove my teeth with a screwdriver then look at it. Please revert and throw this garbage change into the trash bin. I wish I had something nice to say, about the change, but the simple fact is I actually hate it
^ least dramatic Meta user
3 hours later…
5:42 AM
This actually looks like a really good question for S.E.


A quick google search returns a lot of over simplified pictures with most of the physics taken out. It would be nice to have a first principals picture on Physics S.E.
1 hour later…
6:53 AM
7:44 AM
@ACuriousMind here is one for you Disco Elysium fans
8:29 AM
My next priviledge is that I get to protect questions
I do not wish to protect, only destroy
8:42 AM
@fqq @imbAF right, what I should have said is that if your total density matrix is of the form$\rho_A\otimes \rho_B$, then this works
@imbAF No, the point is that if the full system in an entangled (but pure) state, then there is no pure state for the subsystems (that's the only correct definition of what an entangled state is!)
i.e. in that case the density matrix does not model "missing information", it models the lack of a definite pure state for a subsystem in an entangled state
@Slereah accurate :D
I should do a replay of DE
Except this time instead of being nice I just do all the drugs
@NiharKarve I have to say it does look better on my smaller screen devices, but I hate how it looks on my actual desktop, which is what I use most of the time
The Serpico playthrough
I believe in game it's referred to as the "rockstar"
I mean, you are referring to yourself as a rockstar :P
other people have...different words for that
electrochemistry is a pretty amusing voice to have in your head all the time though, so yes, the drugs playthrough is worth it!
@ACuriousMind bit weird considering I am a disco fan
Shouldn't I be the
8:49 AM
@Slereah internal consistency is never HDB's strong suit, really :P
@ACuriousMind Perhaps I should take some LSD before playing so as to not worry about it
A disco wafer
i mean
that's not not in the spirit of the game
@ACuriousMind Is there a name for the space of matrices that are the sum of a skew symmetric matrix and a symmetric matrix of signature $(p,q)$
well, for (n,0) the name is just "matrix"
I'm not sure for the other signatures
that was the kind of matrices that Einstein used for his unified field theory
But nobody really tried to write about it since the 1930's so it's a bit hard to find a lot of rigorous details
9:32 AM
In his book, Roland Winkler says that the substitution $\vec{p} = -i\hbar\nabla + e\vec{A}$ for an infinite-dimensional Hamiltonian. This is in a specific context. Is it possible to understand in more general terms?
@B.Brekke What do you mean by "substitution"?
assuming you're talking about the particle coupled to an EM field, it's fact that you have $\vec \pi = \vec p - e\vec A$, where $\pi$ is the canonical momentum (i.e. $\partial_{\dot{x}}L$) and $p$ the actual spatial momentum. Canonical quantization gives you that you must represent $\pi$ as $-\mathrm{i}\hbar\nabla$ (cf. Stone-von Neumann theorem), and so the spatial ("kinematic") momentum ends up as what you write there
9:54 AM
Yes, I follow. I re-read his claim and I understand better what he means now
10:09 AM
What is Einstein's unified theory even invariant under, a mix of the Poincaré group and U(1)???
4 hours later…
2:24 PM
Hello dear "theoretical" physicsts..
I am interested in absolving a lecture about LIE groups and LIE algebras. However i heard that the professor who is helding this, will conncentrate mainly on LIE algebras.
Is this good for the physics and the understanding of it? or is it more important to learn about LIE groups
It is good to know both, but algebras are pretty important yes
the magic of Lie theory is that the algebra contains almost all information about the group
So i should def do it ?
2:39 PM
i.e. "focussing on the algebra" is a very normal thing for a lecture on Lie groups to do
Alright, thanks for your opnions! :3
(and btw "absolving" is a German-English false friend, it doesn't mean absolvieren in German but rather something like freisprechen or erlassen)
Damn, the potato in me got caught .
My English got ruiniert the moment i learnt german. but whatever :D
Well we don't know, maybe that lecture is so terrible it needs its sins cast out
1 hour later…
3:58 PM
If we have a current loop that is opened and the ends (1 and 2) are connected to a voltage source, the induced voltage is : $U_{ind}=\int_{(F)}\vec E(\vec r,t)d\vec r= \int_{(F')}\vec E(\vec r,t)d\vec r + U_{12}$ ?
Where does the induce voltage comes from?
Then if we have an ideal conductor with an infinitely high electric conductivity disappears
the electric field.
Can someone explain this ?
First of all where does this induced voltage comes from? I am aware that a change in the magnetic flux generates that, but here we simply have a wire connected with a voltage source
4:52 PM
Q: Help me to improve my answer

HulksterHow to improve the following answer: https://physics.stackexchange.com/a/681503/92708 The background: Due to some speculative questions, I have 'reached my question limit'. Now I try to improve my previous contributions, but it seems that things are going into the opposite direction.

2 hours later…
7:06 PM
Why do we get an induced voltage when we connect a wire loop with a voltage source ?
2 hours later…
9:15 PM
how is one supposed to find the invariants of a Cartan geometry
Or is there no systematic way and that's what the Erlangen program is

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