12:09 AM
the rank of a matrix is not its dimension - en.wikipedia.org/wiki/Rank_(linear_algebra)
I think your questions comes from the following
if $\mathcal{H} = V^{\otimes n}$, you can see $\rho: \mathcal{H}\to\mathcal{H}$ as rank $2n$ tensor on $V$
but that's not really the point, it's still a "simple" linear operator on $\mathcal{H}$

1:06 AM
@Bohemianrelativist can you provide an example of a density matrix that has rank >2? Are you thinking of multiparticle density matrices? It is a matrix, so it’s a 2x2 array.
The entries may have additional n-tuples of indices in the case of multiparticle system, but it’s still a matrix.
of course it is also an operator…

@ZeroTheHero yes, I am reading a paper which talks about quantum many-body systems.
@ZeroTheHero it says the system can be described in terms of a tensor network for which the many-body system state can be written as $|\Psi>=\sum_{\overline{\alpha}=1}^{D^{4L}}|in(\overline{\alpha})>\otimes|out(\overline{\alpha)}>$.

2 hours later…
3:25 AM
@fqq How would you write the action in your case?

5 hours later…
8:22 AM
If I've supplied the fifth close vote on a question then repented there doesn't seem to be a way to retract my vote. I can only vote to reopen, which doesn't reopen the question as it counts as only the first of the required reopen votes.

8:36 AM
@JohnRennie Time fore meta? And which question is this?

0

If you throw 2 same particles into mass-spectrometer, 1 relativisticly slow and 1 relativisticly fast, would you get same result?

@MoreAnonymous this one.
I'm not sure it's worth a meta post. I think the moral is to engage your brain before pressing the close link.
2

@JohnRennie aGREED
Sorry caps lock was on

2 hours later…
10:47 AM
@JohnRennie If the fifth vote were retractable it's unclear how this should interact with the usual aging away of close votes - potentially someone could vote to close a question, then retract the vote a few days later to effectively unilaterally reopen it and reset it to 0 close votes (because the earlier votes had aged away but theirs hadn't)

1 hour later…
12:17 PM
1

Someone asked this question: Why linear displacement is length of the arc? which is similar to this question: Confused about linear displacement in circular motion. The second question has been published earlier. So when I tried to flag the new question as a duplicate, the old one didn't appear o...

3 hours later…
2:58 PM
why are the moduli spaces of the vector- and hyper-multiplets called the Coulomb and Higgs branches?

3:51 PM
what does this notation mean?
what exactly are 2 unit vectors doing there?

4:05 PM
@ACuriousMind why are so many of your questions about M-theory on G2 manifolds

@NiharKarve because it was my master's topic :P

@satan29 outer product

almost all of my questions are stuff I stumbled over while having to write something up for my studies and realizing I didn't actually understand how that worked

Wow, it only took 10 hours for this to hit the top of the HNQ.

wow, thats a nice question

4:17 PM
yesterday, by user178758
off-topic questions have been known to become satanically hot

But how is this off topic?

it depends on a lot of things which are mod regulated

such as..?

as a mod

I have no idea what you're talking about, that's an excellent example of a question that is both about an exercise and on-topic since it doesn't ask us to check any work or solve the exercise itself

4:22 PM
Precisely

@ACuriousMind I love how none of the M-theory questions have been answered except one, and that's Urs Schreiber offering a hint

why would you love that?

0

My question is simple and specific and is obviously regarding homework questions (which are viewed as off-topic). If I am having trouble computing an integral, am I disencouraged to ask what the next step should be in its evaluation or, at least, asking for a pointer? Will the post be closed if I...

@user178758 "love" = "intrigued by"
I just rediscovered Tetris today, it's so fun

@NiharKarve it's not a large field and the questions are close to the boundary of research (even if many texts like to pretend we understand what's going on :P), I honestly didn't have high hopes to get actual answers after the first few

4:35 PM
There are at least a bunch of comments, so I'm glad that it's not totally lonely over there :P