6:04 AM
what's with all the videos about the hilbert hotel lately

1 hour later…
7:32 AM
I woke up this morning with the complete and certain knowledge that I had missed an important meeting. Turns out it's nowhere to be found in my calendar, looks like I just dreamt that there was one. Great job, brain.

@ACuriousMind sounds like one of those "sitting an exam that you haven't prepared for" dreams that brains make up when they're feeling bored.
For extra points you realise halfway through the meeting (or exam) that you're naked :-)

Well, I didn't dream about the meeting - I apparently just dreamt about looking at my calendar and going "oh no that's a very early meeting tomorrow". Then I woke up, looked at the time and was pretty sure I'd actually missed it, only to find out the meeting never existed :P
apparently, dreaming about the exam/meeting itself would have been too exciting

8:18 AM
@ACuriousMind reminds me of Omphalos Hypothesis

8:41 AM
@ACuriousMind have definitely done that with exams, waking up and thinking i missed a final only to remember I already graduated

1 hour later…
9:47 AM
@Qmechanic (CC @JohnRennie) quick word of protest regarding physics.stackexchange.com/questions/635490/the-resistor-cube -- I raised a flag pointing out that it's a good Q&A pair from a new-user OP and that OP needed guidance on getting it into form
straight closure without commentary doesn't help much
this looks like a good new user, and it pays to be friendly =)

10:33 AM
@JohnRennie yes
I got a very bad nightmare
terrible one
@ACuriousMind same... 3 hours before

10:47 AM
this room inspires me to think that I'm never left out

### Dot Chat (AI Chat)

This room is for teenagers and AI enthusiast only. Come here a...

@PM2Ring it's perhaps not surprising that a zoologist by training will emphasize the effect of the environment on a species ;)

my professor introduces phase velocity
and said that its actually not of any signficance at all. Its the group velocity that carries information
well then why have we defined such a thing?

11:29 AM
Because it pops out fairly easily from the equations
Also beware of statements about what "carries the information"
Otherwise you're in for a rollercoaster when you find out about front velocity

@satan29 because it's the "naive" definition of velocity
we need a name for it so that we can say "that thing doesn't actually mean what you might think"

ok...

otherwise people would over and over ask "Shouldn't this (phase velocity) be the "velocity" of a wave?" :P

Also

anything is meaningful if you want it to be

11:40 AM
If you do enough physics you will just realize that "velocity" means like 20 different things

My professor said that there is no such thing as a wave with only a "single wavelength", waves having slightly different wavelengths are in a spread

Gotta watch out for what the author means

so he took $y1= Asin(kx-wt)$ and $y2=Acos((k+dK) x - (w+dw)t)$
and added them up by the superpositon principle, and so y came out to be:
$$2Acos((dk/2) x- (dw/2) t)cos(kx-wt)$$
he then said the amplitude term is $$2Acos((dk/2) x- (dw/2) t)$$ and the phase term is kx-wt
and from the amplitude term he said we can see that the amplitude is moving with a velocity dw/dk
and likewise w/k for the phase term
And thats who he derived the "group" and "phase" velocity
or defined
My question is, out of the two cos() expressions, how does he know whats tha amplitude term, and whats the phase term?

@satan29 you have to think about what this is - it is a "fast" oscillation (the phase term) inside a "slow" oscillation (the "amplitude term", often also called "envelope")
i.e. this function has two periods, and we think of the shorter as an oscillation whose overall maximal amplitude varies over the longer period

11:58 AM
hmm

12:51 PM
@satan29 If you define refractive index(in the visible spectrum) of a linear isotropic medium as $n=c/v$ then that v is the phase velocity of the EM waves in the medium, not the group velocity

1:44 PM
snarxiv.org/vs-arxiv is great fun
snarXiv is a parody of arXiv which randomly generates high energy physics article titles and abstracts
you have to tell the genuine paper from the generated one

Nihar, are you masters student?

I am not

2 hours later…
3:59 PM
Hey there
how's everything going?

4:22 PM
@NiharKarve apparently I oscillate between being a 1st/2nd year grad student
Despite having already got a PhD (though not in HEP)
Still, ~80% is not so bad and I think better than how I did the first time I tried it a few years ago

4:42 PM
@fqq honestly, some real papers have such horrible titles that 100% is unrealistic to achieve :P
you can learn how to tell meaningless technobabble from actual research topics, but every once in a while the two papers it offers you have titles like "General Structure" vs. "A Comment on Certain Particles" and you really have no good way of figuring out which of these is real

among other things the snarXiv generator seems to love "bubbles", "CY n-folds", "deformations" and horrendously long titles that are too awkward even for physicists
the first two times I played it I scored below 40% though
literally worse than picking randomly

for statistical purposes, (and I had no other choice too), I attempted it randomly
20/35 , i.e 57%

but yeah after the first few tries, 80-85% seems to be the average
It's made and maintained by David Simmons-Duffin
big fan of his conformal bootstrap stuff

5:08 PM
Who has read (parts of) all of those books
From this

5:22 PM
0

What current would be measured in the following circuit? There was some commentary about it being a homework question, which it isn't , I genuinely want to understand what happens when the point of intersection moves faster than the speed of light. I actually find this an interesting question.

6:15 PM
I haven't, but one thing I know is that in Iron Man 3, they show off a "Secret formula"
and on the paper, there's a rotation matrix
So I'm not gonna praise his science advising for it

8

I watched Iron Man 3 today, and wonder what this formula is about. Is it an important topic of physics? You know, like Transformers: Dark of the Moon, as I can recall, when an answer of some formula (problem) is got, an amazing weapon is invented. But I don't see any relationship in this movie. T...

Maybe once a simple proof of BCH is found it'll be the first step towards realizing physics is just a big quest to decode where one infinity stone is hidden

6:35 PM
0

Is a question that asks whether something is possible within the laws of physics on-topic here? (Very vague) Example: assuming any level of technology, can you do this within the laws of physics, or would some law(s) of physics prevent it? Would such a question be on-topic here (with more details...

2 hours later…
8:32 PM
Hi All..
How in thermodynamics... Pressure, Temperature and Volume are both state variable and state function?

9:11 PM
Why is thermal equilibrium only talk about temperature (not pressure, volume etc..) as in zeroth law of thermodynamics.
It can also be seen in isothermal process there can be volume and pressure change but temperature constant. Than why zeroth law only talk about temperature in equilibrium. If other properties can change at constant temperature. System should not be in equilibrium.

9:42 PM
why do the W bosons only couple with the left-handed quarks in the Lagrangian for the Standard Model? (why don't W bosons also couple with the right-handed quarks?)

@Bohemianrelativist that's how it is, there are no (mainstream/broadly accepted) theoretical explanations
4

I find the profound asymmetry in the sensitivity of left and right chiral particles to be one of the most remarkable analytical observations captured in the Standard Model. Yet for some, I've not found much in the way of discussions that worry about why of such as truly remarkable fact is true. I...

same as the more general question "why is the SM particle content the way it is"