The h Bar

Sir Cumference

00:00

Anyone know why some beta sites are suddenly getting designs?

E.g. artificial intellignece

skullpetrol

00:20

I noticed that too, perhaps they're trying to attract new users?

Sir Cumference

Well both AI and QC are sponsored, so I guess they bypass some of the beta limitations

Which kind of sucks for other sites

skullpetrol

doesn't sponsorship mean they'll have advertising from that sponsore on their site?

skullpetrol

00:41

I always thought StackOverflow sponsored StackExchange @SirCumference

Sir Cumference

@skullpetrol StackOverflow is owned by StackExchange

skullpetrol

Not vice versa?

Sir Cumference

Nope

I mean SO as a site predates the SE network, but the company has always been called SE afaik

Or at least changed its name to SE Inc. once it began the whole network

skullpetrol

interesting

skullpatrol

02:29

(removed)

^just like my star on your message about Chair's account being deleted @peterh

12 hours ago, by peterh

Chairs' account is now deleted!

peterh

@skullpatrol Sad. Talking about induvidual suspensions is forbidden, but talking about induvidual deletions is not. As far I know, mods can't delete such a high-rep user, so either it was a self-deletion, or it was a CM deletion. The first is more likely.

skullpatrol

02:47

perhaps a suspension triggered a self-deletion

I've seen suspended users change their username

peterh

03:00

@skullpatrol That is a very sub-optimal solution. I only hide that account in the case of a suspension, but even this I do only because it often creates a fake argument in debates on other sites.

skullpatrol

03:28

yeah, when a mod steps in it usually generates more heat than light

pss 1

03:42

@peterh why isn’t ur name in the list of users..

peterh

@pss1 Where, in which list?

pss 1

@peterh In The list of users.....in physics stack exchange (arranged in order of reputation if chosen!)...

peterh

@pss1 I am there. How did you search for me?

@pss1 Ok, I check

@pss1 I am on the 7th page, in the top row: physics.stackexchange.com/…

@pss1 Btw, as far I know, I have no way to make myself invisible in this list (and I also don't have an intent to do)

pss 1

@peterh Sorry ur profile here showed ...u had 43.9k rep..is it Total of all sites (of SE) rep...so I thought u should be close to the top..considering 43.9k rep as ur PSE’s rep

XD

Luyw

Hello, to vulgarize light being a wave, does it mean that atoms in space are being "disturbed"?

I mean surely there are no wave like "beings" going through space like snakes, those are just field values right?

peterh

04:25

@pss1 I am active on a lot of SE sites

@pss1 Please believe me, netspeak looks really not very well. Would you please read this?

pss 1

04:44

@peterh yeah!thanks for that! I was neglecting such things here.thought it’s just a chat room after all .Sure I’ll take that into consideration

peterh

@Luyw Afaik it is a question like football, everybody has an opinion :-)

@Luyw However, in the case of enough low frequency and enough strong and enough coherent light, probably you can measure it as a sinusoidally changing EM field.

John Rennie

@Luyw the wave is an oscillating amplitude of the electromagnetic field.

The EM field exists everywhere, though its value can be zero. A light wave is just a travelling oscillation in the field value.

peterh

05:03

@pss1 No prob, my pleasure.

Abhas Kumar Sinha

05:42

@PM2Ring ah! I quit, how it is to be done?

John Rennie

@AbhasKumarSinha how is what to be done?

Abhas Kumar Sinha

@JohnRennie It's a question, to find the smallest possible cone that can hold a unit size sphere in itself

@JohnRennie I used Lagrange multipliers, but it's giving answers in form of equations

I doubt 30 deg will be the correct answer

John Rennie

Ah OK. Don't know :-)

Abhas Kumar Sinha

@JohnRennie What you don't know?

John Rennie

Don't know how to use Lagrange multipliers to answer the question.

Abhas Kumar Sinha

05:48

@JohnRennie Lagrange multipliers are not necessary, do you know other methods?

John Rennie

I'd probably just do it using basic geometry. Draw a cross section and you can relate the cone base and height to the cone angle. Then just calculate the volume and find the minimum.

Abhas Kumar Sinha

@JohnRennie I lost everything after cross section.... ?

John Rennie

You know the radius of the sphere is 1 so you can calculate the cone height and the radius of the base for the cone angle $\theta$

(I now realise I didn't draw $\theta$ on the diagram ... )

Abhas Kumar Sinha

@JohnRennie Radius of Sphere is independent of Cone height

John Rennie

@AbhasKumarSinha no it isn't, because you want the minimum cone volume and for that the cone edge will be a tangent to the sphere.

i.e. the slanted line in my diagram will be a tangent to the sphere.

Abhas Kumar Sinha

06:00

@JohnRennie observe that the height will vary in case the angle theta decreases and the height increases and volume decreases too to some extent

John Rennie

Correct. So you can calculate the volume as a function of theta. Or $h$ or $r$. Whatever is most convenient.

Then differentiate and set to zero to find the minimum volume.

Abhas Kumar Sinha

Ah okay

PM 2Ring

06:22

Jun 5 at 23:44, by PM 2Ring

Here's a fun little calculus problem. Find the cone of minimum volume that contains a unit sphere. Bonus points for determining if it's also the cone of minimum (curved) surface area.

Note that the whole sphere is entirely contained within the cone.

John Rennie

There was a time when I did calculus problems for fun. I dimly remember it. It was a long time ago :-)

PM 2Ring

I rendered that using POV-Ray a few years ago. That's the correct solution, although the proportions are slightly distorted by the perspective.

PM 2Ring

06:43

@AbhasKumarSinha Don't forget that the tangent of a circle is perpendicular to the radius. So we get a collection of similar right triangles.

PM 2Ring

06:55

@AbhasKumarSinha Also, John's diagrams are a little misleading. The axis of the cone should pass through the centre of the sphere.

PM 2Ring

07:36

There's a fairly minimal version. You can see it in 3D by looking at it cross-eyed.

John Rennie

08:01

@PM2Ring I thought it was fairly obvious that I had only drawn half the cone ...

PM 2Ring

@JohnRennie Sure, but I just wanted to make it crystal clear for @AbhasKumarSinha

This site gives the following formulae for centripetal acceleration in SR: $a_c = \frac{\gamma^2v^2}{r} = \frac{c^2(\gamma^2-1)}{r}$. How do I go about proving that?

PM 2Ring

08:24

Ah. Greg Egan has written an article on relativistic disks, rings, and hoops. gregegan.net/SCIENCE/Rings/Rings.html That should be helpful. ;)

Student404Mus

09:13

Hello.

I have a question that concerns a research paper "Creation of scalar and Dirac particles in the presence of a time varying electric field
in an anisotropic Bianchi type I universe" by Villalba et al.

The author(s) uses a quasiclassical approach to calculate the denity (probability) of particle creation. This method is based on two main steps, solving the Hamilton-Jacobi equation to obtain the positive and negative frequency modes and solving the Klein-Gordon equation to identify which are positive and which are negative frequency states. I want to know what is the importance of sol…

What I have noticed when he solved K-G eq., is the superposition of two solutions each of which is defined in one asymptotic region that is different from the other. And in mathematics, this solution is standard in being able to be decomposed in terms of positive and negative modes.

Student404Mus

09:33

The second thing is H-J eq. gives an approximate solution, i.e., in the extremals, while the K-G eq. predicts its solutions in the exact picture.

John Rennie

@PM2Ring just calculate the four acceleration and take the norm. You can do the calculation in the ground frame since the norm is a scalar invariant. You'll need the Christoffel symbols for polar coordinates in Minkowski spacetime.

PM 2Ring

@JohnRennie Thanks.

cattt