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vzn
vzn
2:46 AM
in theory salon, 3 mins ago, by vzn
hi all thx for all the great chats. great to see this & some other SE chat rooms livening up recently. inspired to come up with some ideas from 3yr experience/ participation in misc SE chat rooms. suggestions/ feedback welcome — cool chatting session tips / stackexchange profile tips
 
 
2 hours later…
dmckee
dmckee
4:42 AM
I always wonder what people are thinking when they chose a picture of a famous physicist for their avatar. Do they think we're going to be impressed? That we'll read it as an expression of admiration and spontaneously flood with fellow feelings for them? Or ... what?
 
 
2 hours later…
Slereah
Slereah
6:45 AM
I picked a fancy lawyer for legal protection
 
John Rennie
John Rennie
@dmckee Recently I was asked what famous physicist was depicted in my avatar :-)
 
David Z
David Z
7:34 AM
@dmckee Yeah I was thinking the same.
It could probably contribute a point or two on the crackpot index, actually.
 
John Duffield
John Duffield
8:34 AM
@Slereah : and what do you mean by that?
 
 
1 hour later…
Shing
Shing
9:52 AM
what exactly do we mean, when we say "spin is intrinsic"?
simply no related classical analog? or some properties that particles always have?
 
Slereah
Slereah
10:15 AM
Spin has a classical analogue
What he means is that spin isn't because particles spin on themselves
 
ACuriousMind
ACuriousMind
@Shing The classical angular momentum is always $\vec x \times \vec p$, i.e. it is extrinsic in the sense it is caused by motion w.r.t. the reference point. Spin is not - although it is angular momentum, it has no motion associated to it, it is intrinsic in the sense that you cannot write it in terms of other properties of the particle.
 
Shing
Shing
That solved my confusion, thanks, guys.
 
 
1 hour later…
John Duffield
John Duffield
11:36 AM
@Shing : ask a question on the main site and I'll give you a detailed answer. Meanwhile check out the Einstein-de Haas effect.
 
 
2 hours later…
ACuriousMind
ACuriousMind
1:34 PM
Is there an easy way to see what happens to $\partial_\mu \tau$ if I transform $\tau\mapsto \frac{a\tau + b}{c\tau + d}$ (i.e. do a Möbius transformation, $\tau$ is a complex function).
 
David Z
David Z
2:20 PM
I would think that if anyone here would know, you would ;-)
 
vzn
vzn
2:33 PM
via reddit
 
David Z
David Z
I suppose they're talking about the 750 GeV diphoton resonance
but they never explicitly say so in the article
 
vzn
vzn
@DavidZ maybe its the weasel... "pop goes the weasel" :P
 
vzn
vzn
3:08 PM
we had some discussion here of peer review & there are those interested in science/ math history, & there is some commentary on my blog. via blog aggregator/ david eppstein blog
 
 
1 hour later…
John Rennie
John Rennie
4:15 PM
→ 1 message moved to Trash
→ 1 message moved to Trash
 
3075
3075
lol
 
Ali Caglayan
Ali Caglayan
4:32 PM
When it is stated that QED is a relativistic field theory, are they referring to special relativity?
 
FenderLesPaul
FenderLesPaul
5:24 PM
@AliCaglayan Yes
 
David V
David V
I'm reading about inflation. Anyone know why they consider the universe as having a decelerated parameter, when we know this is observationally false?
(decelerated parameter after inflation, that is)
 
Ali Caglayan
Ali Caglayan
6:02 PM
@FenderLesPaul Then can they have accelerating particles if special relativity says everything is a constant speed?
 
AccidentalFourierTransform
AccidentalFourierTransform
6:36 PM
@AliCaglayan that's a common misconception. In SR accelerating particles work just fine! just like in Newtonian Mechanics. Acceleration is not a flaw of neither SR nor Newtonian Mechanics, but is a fundamental ingredient instead!
 
Ali Caglayan
Ali Caglayan
@AccidentalFourierTransform How do you have acceleration in SR then?
 
AccidentalFourierTransform
AccidentalFourierTransform
in the same way you have acceleration in NM: with a force field, like an electromagnetic field
 
Ali Caglayan
Ali Caglayan
@AccidentalFourierTransform I seem to remember that SR emphasised that frames are not accelerating. What is it trying to say?
Am I misinterpreting
Or is it just talking about the frame
I take it that when it is said that acceleration is allowed in SR it is literally the flat space time solution in GR
As in Lorentzian boosts don't make any sense anymore
 
AccidentalFourierTransform
AccidentalFourierTransform
6:59 PM
@AliCaglayan accelerating frames of reference or accelerating particles? its not the same
 
Ali Caglayan
Ali Caglayan
@AccidentalFourierTransform Yeah i think thats where my confusion was. Thanks
 
AccidentalFourierTransform
AccidentalFourierTransform
@AliCaglayan it is a very common confusion :-P
anyway, you're welcome! see you around :-)
 
Ali Caglayan
Ali Caglayan
@AccidentalFourierTransform thank you again :)
 
 
2 hours later…
user54412
9:11 PM
@DavidV Before we had any relevant cosmological observations, it was assumed the universe must be decelerating, since that's what matter and dark matter do. Combine the strong energy condition with the Friedmann equation to see this.
 
user54412
If I understood what you're asking.
 
user54412
By the way, do I know you @DavidV ?
 
Bernard Meurer
Bernard Meurer
There's a David V and a David Z now?
 
user54412
@BernardMeurer Admit it, you're a David B on the inside
 
Bernard Meurer
Bernard Meurer
Hahahaha I wish! David is a nice person's name
David is your cool friend that has a truck and helps you move
Or that guy who you don't really know at work but always says good morning to you
Okay, nevermind that I just got imaginative
 
kevin Tah N.
kevin Tah N.
9:23 PM
oh hey bar mates!
 
 
2 hours later…
Sir Cumference
Sir Cumference
11:04 PM
@ChrisWhite Er, you know astronomy, right?
Could you answer this?
0
Q: Why does the Chandrasekhar limit affect white dwarfs differently?

Sir CumferenceThe Chandrasekhar limit is the maximum mass of a stable white dwarf star. Beyond this, a carbon-oxygen white dwarf will typically explode in a type 1a supernova, due to the nuclear reactions at those temperatures. I've heard that oxygen-neon-magnesium white dwarfs, on the other hand, will not ig...

supernova mass neutron-star white-dwarf degenerate-matter
White dwarfs and neutron stars are friggin complicated
 
dmckee
dmckee
11:31 PM
@SirCumference Anything with energies high enough to make nuclear physics part of the picture is complicated. The current round of mass/metalicity dependent predictions for stellar end-states are diverse and surprisingly complicated.
 
Sir Cumference
Sir Cumference
Excellent...
 
ACuriousMind
ACuriousMind
Newsflash: There are no simple answers :P
 
Sir Cumference
Sir Cumference
I'm trying my best to comprehend the stuff. After reading about Fermi gases, I think I've got a good idea of how neutron stars and white dwarfs work. Still, I've got questions like the one above
 
ACuriousMind
ACuriousMind
@SirCumference Well, there's nothing bad about having such questions!
 
user54412
@SirCumference Nobody knows what really happens when you pile mass onto a white dwarf of any type. We also don't know what a Type Ia supernova actually is physically.
 
user54412
11:40 PM
There are thousands of speculative papers, some more believable than others, but the physics is extremely complicated.
 
user54412
Core-collapse SNe, though covering a larger range of scales, are easier in that you don't really worry about nuclear physics, beyond neutrino cross sections. And even there we haven't succeeded in ab initio simulations after 40 years. Thermonuclear SNe are even worse.
 
user54412
10 years ago, everyone would have told you that most Ia's are slowly overloaded CO white dwarfs. Now 30-70% of people (depending on the week) will tell you most of them are actually mergers of two white dwarfs, going way beyond Chandrasekhar all at once.
 
Sir Cumference
Sir Cumference
@ChrisWhite You should post all this as an answer, in case anyone else has this question.
 
user54412
It's more of a non-answer.
 
user54412
Also, I'm not a member of that site.
 
Sir Cumference
Sir Cumference
11:47 PM
Well it would still tell people that we don't really have answers
Ya don't wanna make an account?
 
ACuriousMind
ACuriousMind
@ChrisWhite I think "we don't know" is a perfectly fine answer if that is what the state of the art objectively is.
It's far better than throwing your own personal speculation out there as an answer, which I see far too often.
2
 
user54412
@SirCumference No. I feel the site's formation was a hostile attempt to annex astrophysics from here, and no one from that site has ever justified its scope.
 
user54412
There is a lot of meta discussion on this, here and at Area 51, where multiple people called them out and they never responded.
 
Sir Cumference
Sir Cumference
@ChrisWhite You think we shouldn't separate astronomy from physics in general?
 
ACuriousMind
ACuriousMind
^that is true. I never understood why astronomy is its own site when almost all of its questions would be in-topic here.
 
Sir Cumference
Sir Cumference
11:50 PM
Astronomy has its own branches, like cosmology
 
user54412
@ACuriousMind I kinda feel what I said was my own speculation. At least, there are people who would object to what I said. (They would object with all sorts of conflicting reasons, proving my point about not knowing the truth, but still.)
 
Sir Cumference
Sir Cumference
That's like saying Chemistry should be merged with Physics
 
ACuriousMind
ACuriousMind
See also:
10
Q: How can we encourage more Astrophysicists on physics stack exchange?

OrcaI'm a relatively new user and have already noticed a lack of users with interest in Astrophysics, compared to Quantum Mechanics or Quantum Field Theory. I have interest in these more popular areas as well, so I am still enjoying the site, but it would be great to see more Astrophysicists on here!...

discussion
@SirCumference Not at all. Astrophysics is manifestly a discipline within physics. You can't study "astronomy" in most places, you study physics with a focus on astrophysics.
 
user54412
@SirCumference I would be fine with separating amateur astronomy from physics, like the old astro SE. But all professional astronomers and astrophysicists have degrees in physics.
 
Abhishek Bhatia
Abhishek Bhatia
hey guys!
 
user54412
11:52 PM
Astro is a specialization of physics, like plasma physics or condensed matter.
 
Abhishek Bhatia
Abhishek Bhatia
This is might be silly question, how to I do calculate average speed?
 
Sir Cumference
Sir Cumference
Hm, lots of sites with the same concepts are split up
StackOverflow and Programming
 
user54412
@AbhishekBhatia Total distance over total time?
 
Sir Cumference
Sir Cumference
Computer Science and Theoretical Computer Science
^These are literally the same concepts, but different focuses
 
Abhishek Bhatia
Abhishek Bhatia
I have data from some simulations. For a given particle I have x and y coordinates and velocity in x and y directions for each time-instant.
 
ACuriousMind
ACuriousMind
11:53 PM
@SirCumference The people at programming.SE are very angry if you insinuate the two sites have similar scopes.
 
user54412
Programming was designed to hold all the opinion questions people asked on SO.
 
Sir Cumference
Sir Cumference
"Computer Science and Theoretical Computer Science"
 
ACuriousMind
ACuriousMind
For one, programming.SE is not about actual code, while SO is demonstrably mostly about specific code.
 
Sir Cumference
Sir Cumference
again, same concepts, different focuses
 
user54412
CS and TCS is about homework help versus ivory tower isolation.
 
user54412
11:54 PM
not about subject differences really
 
Abhishek Bhatia
Abhishek Bhatia
@ChrisWhite or could I compute the magnitude of the velocity at each time instant and take a mean of it?
 
ACuriousMind
ACuriousMind
@SirCumference It's like math.SE and MO, difference in level, not in scope.
 
Abhishek Bhatia
Abhishek Bhatia
does that make sense?
 
Sir Cumference
Sir Cumference
Not to mention, not all astronomy is physics
 
ACuriousMind
ACuriousMind
But physics.SE has no level constraints, so it doesn't make sense for another site with overlapping scope to exist.
 
Sir Cumference
Sir Cumference
11:55 PM
You get questions like this
4
Q: What would the night sky look like if the Milky Way were the only galaxy in the universe?

ViziionaryI'm curious to know what the sky would look like without any other galaxies out there. How much do other galaxies factor into the stars we see? Does the Milky Way account for most of them? Would the night sky look normal? Or would it be very empty?

fundamental-astronomy
 
user54412
@AbhishekBhatia that works, as long as your time intervals are uniform
 
Abhishek Bhatia
Abhishek Bhatia
@ChrisWhite so I will get the same the answer either way?
 
user54412
well, physics better work the same
 
Sir Cumference
Sir Cumference
Astronomy has an array of different concepts, from concepts like K-corrections to observations to opposition surges
 
ACuriousMind
ACuriousMind
@SirCumference Just like every other subfield in physics.
 
HDE 226868
HDE 226868
11:56 PM
@ChrisWhite (cc @ACuriousMind) I think that the current issue is that astronomy/astrophysics questions - and there are certainly way less of the latter - get lost quickly in Physics, whereas they get more attention on Astronomy. So it's kind of a vicious circle, and given that there are a lot of newer users that weren't around when Astronomy started, it's harder and harder to bring the sites together again.
 
ACuriousMind
ACuriousMind
I literally cannot understand someone specializing in hydrodynamics, and I doubt they could understand me, either.
 
user54412
@AbhishekBhatia there is the subtlety that some methods give you average of speed (which is equal to total distance traveled over time), while others give you average velocity (which is total displacement from start to finish over time)
 
HDE 226868
HDE 226868
The whole thing does make me hate that we can't close questions as cross-site duplicates, though.
 
ACuriousMind
ACuriousMind
@HDE226868 Well, that's a ridiculous reason. Every specific subfield "quickly gets lost" here unless it is homework or quantum mechanics :P
 
Abhishek Bhatia
Abhishek Bhatia
@ChrisWhite I wish for average speed only.
 
Sir Cumference
Sir Cumference
11:58 PM
Again, not all astronomy is physics. A lot of these questions involve basic phenomena and observations.
7
Q: Lack of objects between heliopause and Oort cloud?

Sir CumferenceIf we take a look at a logarithmic scale of the Solar System, there appears to be a large gap between the heliopause and the Oort cloud: Why is that? There are some objects in the gap, but why are most of the Oort cloud objects in one area? Shouldn't it also fill the large gap in the picture ju...

orbit solar-system gravity oort-cloud interstellar-medium
 
user54412
@AbhishekBhatia averaging magnitude of velocity is probably the easiest way to do that
 
Sir Cumference
Sir Cumference
Like this
Or this
4
Q: Why are stars more metallic closer as you move closer to the galactic bulge?

Sir CumferenceAs I see it, most of the stars in the galactic bulge are Population I stars. However, as one moves farther from the galactic bulge, star metallicity decreases. In fact, halo stars are almost entirely Population II stars. Why is this?

star galaxy galactic-dynamics galaxy-center metal
 
ACuriousMind
ACuriousMind
@SirCumference I would not close this question as off-topic here.
 
user54412
Those are both physics.
 
ACuriousMind
ACuriousMind
@SirCumference That is also an on-topic astrophysics question here.
 
HDE 226868
HDE 226868
11:59 PM
@ACuriousMind Perhaps. It could just be observation bias on my part; It's true that I'm not monitoring question rates of, say, fluid mechanics.
 
user54412
Physics is broader than just blocks on ramps and particles in boxes.
 
Sir Cumference
Sir Cumference
@ACuriousMind These questions are clearly focused on astronomy
Not simply physics
 

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