4:52 AM
@fqq Thanks for the link ... I'll read it up. But $t_c$ is not an arbitrary time but when the molecules collide

5:17 AM
@JohnRennie I have heard of a project about a Large Particle Acceleraror that was going to be made, but ended up falling through due to funds and arguments between governments and scientists. But it was going to output 40 trillion electron volts. What could 40 trillion electron volts do theoretically, based on your knowledge of physics?
Could 40 trillion electron volts create antimatter much more efficiently than the 14 trillion electron volts the LHC creates now?
And what are your opinions on a 40 trillion electron volt Particle Accelerator? Could it potentially have enough energy to create the feared black hole? I know when the LHC was being made there was fear that a black hole could be made. But that was debunked to a certain extent.

Are you thinking of the SSC?
The Superconducting Super Collider (SSC) (also nicknamed the desertron) was a particle accelerator complex under construction in the vicinity of Waxahachie, Texas. Its planned ring circumference was 87.1 kilometers (54.1 mi) with an energy of 20 TeV per proton and was set to be the world's largest and most energetic. The project's director was Roy Schwitters, a physicist at the University of Texas at Austin. Dr. Louis Ianniello served as its first Project Director for 15 months. After 22.5 km (14 mi) of tunnel were bored and nearly two billion dollars were spent, the project was cancelled in 1993...

6:09 AM
I've just heard someone saying "the U(1)_A problem is equivalent to strong-CP"
isn't this a little strong? - you can include the theta term in the Lagrangian without looking at the axial anomaly, since it has the same dimensions and symmetries as the F^2 term

7 hours later…
1:12 PM
This seems to me to be a straightforward check-my-work question, which is off-topic. Am I missing something?

1:26 PM
seems to be fall under the "physics of everyday life " type questions

2:17 PM
I would tend to agree with "satan"
off-topic questions have been known to become satanically hot

2 hours later…
4:07 PM
I don't understand why that post would gain 11 votes and the answer 17 votes; this just seems absurd to me

@VincentThacker It probably got onto the Hot Network Questions list, and that means lots of people from other SEs who aren't physicists will see it and upvote it.

yeah, it's HNQ

Though full marks to the OP for doing experiments levitating mosquitoes :-)

I'm still waiting for one of my answers to go onto the HNQ list

@VincentThacker I'm pretty sure physics.stackexchange.com/a/427837/50583 was HNQ, no? (but it's older than that being logged in the question history, so we can't be sure)

4:17 PM
It says "tweeted" in the edit history, but does not have "Became HNQ"
and that was almost 3 years ago
but guessing by the number of votes, it probably was
@ACuriousMind I feel that mosquito post is on the edge between everyday life and homework
It does have a specific computation but was probably not a textbook example
I would be hard-pressed whether to VTC

1 hour later…
5:24 PM
Has a Hot Network Question ever been successfully Closed by Votes, or by the mods?

5:36 PM
\o @rob

Yes, many times
Moderators are also at liberty to remove questions from the HNQ list

Thanks.

5:54 PM
@user178758 Ahoy

How's it going me matie?

Head still on shoulders, can't complain

6:14 PM
Have you ever spelt your name as Robbie or Robby?

1 hour later…
7:33 PM
@MoreAnonymous ok then it's a complicated function of the trajectories. It's much more straightforward to write the interaction explicitly (sorry I only skimmed through your Q&A)
(not so complicated, but still it's a bit cumbersome to use part of the solution to write the Lagrangian)

7:53 PM
@JohnRennie Yes I think that is the one

4 hours later…
11:24 PM
a density matrix isn't necessarily a matrix - it can have more than two ranks - so it's better to call it a density operator?

a matrix can have any rank (up to its dimension), so not sure what you mean

sorry, maybe I should say it can have a rank which is larger than 2.

technically there is a density operator, and a density matrix representing it in any given basis (in finite dimension)
in practice the two are used interchangeably
@Bohemianrelativist I don't see what's the problem with a rank 3 density matrix, are you sure you know what the rank of a matrix is?

@fqq the rank of a tensor is the number of the tensor's indices.

uh ok there's a bit of nomenclature clash here, the rank of a matrix is a different thing
anyway a density operator is always rank 2 in your sense (on the appropriate space)
i.e. it's always a linear operator on the Hilbert space $\mathcal{H}$ on which it lives, $\rho: \mathcal{H}\to \mathcal{H}$

11:51 PM
@fqq I am reading a paper in which I am not sure it uses rank to refer to the number of indices of the density matrix (here the system considered has 4L indices) or the dimension of the matrix as you mean.