6:52 AM
morning

gninrom

5 hours later…
12:08 PM
Can a plasma cutter work without having to connect the metal to the positive side of the power source?
What are those sparks that come out of the plasma cutter? Are they super hot cut metal?

1 hour later…
1:35 PM
May 2 at 9:18, by JingleBells
Nvm I can google all questions

1 hour later…
2:40 PM
Hi everyone!
A diagram similar to the following one was given in my textbook in the section 'Zone refining' in the chapter 'metallurgy':
Image description: Cross-sectional view of the metal rod.
I understood what is zone refining and how it works. But I don't have any idea why one of the above cross section is 'correct' and the other is 'incorrect'. Do anyone have any ideas on what the author means?
Is that central hole in the cylinder to allow some space for thermal expansion to take place? If yes shouldn't the labels be opposite of what is given?

3:00 PM
Zone refining involves melting the rod. Wouldn't the molten metal pour out through the hole?

3:40 PM
That sounds reasonable. Even though we can't expect the molten metal to flow like water in a pipe, it does deform in the second case. I think as we heat only a small section of the rod, the surface tension is large enough to prevent significant deformation. Will that be dependant on the way crystallisation takes place? For example, will the presence of hole causes defects of some kind in the lattice?

3:50 PM
how many independent Killing vectors do deSitter space and anti-deSitter space have?

@GuruVishnu I have no idea. Sorry :-(

guys
I want to create a room with ja72

4:10 PM
Assuming ja72 doesn't mind, and you should probably check first, just create the room and invite ja72.

I considered ja72 is some kind of project.

4:24 PM
I heard by solving some equation for metric, one can get 3 maximally symmetric Lorentzian metrics, the Minkowski space, deSitter space and anti-deSitter space. The Minkowski space has 10 independent Killing vectors. Do deSitter space and anti-deSitter space also each have 10 independent Killing vectors?

Thats what i wish to know (total noob alert) how do I create a room, and how do i invite users @JohnRennie

@satan29 Go here and click the Create a new room button.

ah
done

Now if I click your name I see:

how do i invite?

4:30 PM

now ill have to find ja72's profile on chatstackex?

Don't know

got it, nvm

How does one define autocorrelation in diagrammatic Monte Carlo?
More generally, how does one define autocorrelation in Markov chain Monte Carlo, when each state in configurational space lacks a corresponding observable?

4:54 PM
If I need to sketch the light cone structure of a schwarzschild-like metric around some $r_0$ point, how do I even begin to do that?
How can I know how much the light cones need to be tilted at a particular point?

5:10 PM
I want to say that I need to look at the $dr^2$ component of the metric and determine something about the trajectory needed to travel radially outwards
I guess if the radial spacetime interval is zero, the lightcones are tilted to that of the event horizon

@Charlie The light cone is just the collection of all lightlike geodesic passing through a point. You can compute these (solve the geodesic equation).

5:25 PM
hmm
Ty ACM

5:40 PM
This might be a silly question, but why does the potential in orbital mechanics depend on the angular momentum?
The plots I see for the effective potential in GR are plotted for different angular momentum values, why does a particle orbiting more quickly feel a stronger potential?

4 hours later…
9:33 PM
@Charlie It is the definition of "effective potential" that it's essentially angular momentum plus the usual potential energy, cf. en.wikipedia.org/wiki/Effective_potential
The angular momentum term represents a "fake" potential energy associated with the centrifugal force

1 hour later…
10:37 PM
Hey @DavidZ, do you mind taking a look into this post? I believe it is a complete solution to a blatant homework question
-1

With $F = m\cdot a$ we know that if we want a constant velocity, the acceleration a has to be zero. This gives us $F=0$ which means that the sum of all the forces should be 0. In the picture I've drawn all the forces. $\vec F$ is the force applied, $G$ is the gravity force, $\vec N$ is the normal...

11:01 PM
@PM2Ring hi again. You previously said electrons are fundamental particles, but I just know they can be change into different objects when they're experiencing processes such as pair annihilation
@PM2Ring sorry I mean the electrons are destroyed
@PM2Ring here another process involving destroying electrons phys.org/news/2015-05-electron.html

11:18 PM
> However, in a phenomenon called electron fractionalization, in certain materials an electron can be broken down into smaller "charge pulses," each of which carries a fraction of the electron's charge.
> The experiment reveals that, when a single electron fractionalizes into two pulses, the final state cannot be described as a single-particle state, but rather as a collective state composed of several excitations.
Doesn't that make the breaking of electrons into e/2 supposed to be an emergent property then?

11:28 PM
@Thormund seems to. If elementary particles have no substructure at all, does that means they are 99% void?

I make no comment until experimental evidence is conclusive.

@Thormund do you have any assumptions? Personally, I think there are substructures in them but it is simply I can't imagine them being 99% space or void.

You know how in classical electromagnetism, we treat the electron as a point-like particle?
(somewhat)
What does that really mean?
In quantum mechanics, we say that the electron has a wavefunction
What does that really mean?
Even before the idea of a fundamental particle, what does it even mean by having a spatial coordinate and associated volume?

@Thormund that's one of those things that many people find very confusing. Point particles are just vibrations in spacetime.
So, is spacetime the real fundamental object out there?

Is a mathematical model reality? Or a reflection of?

11:37 PM
But even fundamental object can't be in a state of isolation, right? Objects always exist in multiplicity. That's why fundamental particles can't exist in isolation.

Let's flip the question around. What makes the proton not a fundamental particle?

Honestly I have no clue.
I know they're composed of quarks on deeper questions on why they're not fundamental particles while the electrons are, have no clue
On a deeper level, is spacetime the most elementary of all of them?
Then, what composed the spacetime?
It is very mind boggling isn't it
Maybe spacetime is composed of more elementary particles
Like an EM field

🤔hmmm questions that I did consider before but I'm definetly ill-qualified to answer

What we can do right now is just assuming.

Personal speculation and interpretation: Fields, whatever they may truly be, were created to preserve the idea of locality. Correct me if I am wrong

11:47 PM
Are you saying a field is just a group of particles clumping together? That's what I'm thinking previously too

Not at all.