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12:01 AM
I also randomly insert (using an if-else block with a randomized condition) minus signs in code for fun...I like to watch the world burn.
 
12:12 AM
@ACuriousMind since you seem to know of every language under the sun, have you heard of elixir? I've heard it a couple times this week and now I'm curious
 
12:25 AM
@enumaris Yeah apparently! There have been a lot of changes in the recruiting team since I applied. Anyway your application is still in the system. I'll email you if I find any info to share.
 
12:36 AM
@DavidZ cool thanks! :D
 
Actually the only complaint of the team I interviewed with today was that the HR process took too long. Maybe it's a common thing these days
 
1:40 AM
@danielunderwood Huh, I was thinking of portal when I opened the chat
Spooky
 
Always gotta be thinking with portals
 
 
2 hours later…
3:15 AM
I don't just think in terms of portals. I think in terms of bundles of timelines
 
 
1 hour later…
4:29 AM
Hi. I am trying to understand the actual degrees of freedom that the metric has in relation to the fact that the graviton is a spin 2 particle. In my understanding, subtracting the Bianchi identities from Einstein equations, we are left with 6 independent equations which determine the 6 physical degrees of freedom of the metric because out of the 10 independent metric elements, 4 are not physical as they correspond to diffeomorphisms.
But this answer seems to suggest that the redundancies due to the Bianchi identities and the diffeomorphism invariance should be added for some reason and thus, we end up with only 2 actual degrees of freedom. This doesn't sound right to me. physics.stackexchange.com/a/74330/20427
Also, I would expect 5 actual degrees of freedom due to spin 2 nature of graviton--neither 6 nor 2. I realize that due to the masslessness, we should actually get only 4 helicities instead of 5 spins but I expect to get to 5 degrees of freedom classically just like we get to 3 independent degrees of freedom of vector potential for electrodynamics classically despite the fact that the photon has only 2 helicities. I am sure I am making a lot of mistakes here so any suggestions would greatly help!
 
5:05 AM
@DvijMankad slide 2 particle.physics.ucdavis.edu/modernsusy/slides/… shows how to get from 16 to 2 through vielbein's
Page 2 of arxiv.org/pdf/1706.08140.pdf also brings up this constraint + eom argument and compares to the massive case
 
vzn
5:26 AM
> However, this new study suggests that at high enough energies, the tiniest particles in the universe can behave and flow like an idealized, near-frictionless liquid.
 
6:01 AM
Hello, I'm currently studying the effective action in QFT and I don't think I fully understand what is called "classical field", aka the VEV of the field in the presence of the sources. Does anyone know a good resource/book for this?
 
 
2 hours later…
7:53 AM
0
Q: Question about notifications

WillOAlice posts a question. Bob posts an answer. I want to comment on Bob's answer, and I want both Alice and Bob to be notified of the comment. Therefore I type something like "@Alice: As @Bob said....". I then get a pop-up notice that says I can have only one @-sign per comment. Fine. The...

 
8:29 AM
new hat
 
I only have the one hat so far - for reviewing. But then I hardly ever answer questions these days ...
 
8:47 AM
I spoke too soon, I just earned a waffle :-)
 
perfect for breakfast
 
@JohnRennie Ooh, sounds like a secret hat
 
@Mithrandir24601 it is :-) I have absolutely no idea what I did to get it.
 
@JohnRennie The namesake is presumably Waffles
 
9:24 AM
The secret hat list is here but so far no-one has worked out what the waffles hat is for.
 
9:37 AM
I have a simple question about the effect of some operator
 
@danielunderwood the Windows market was (actually still is) much, much larger than the *nix market so in general Windows programmers weren't fussed about getting their code to compile on other systems.
Where you need your app to run on multiple OSs you'd use some ifdefs but mostly you'd use a development environment that allowed the platform independent code to be shared and the platform dependent bits to be implemented in seperate source files.
 
I don't see how the second equality holds, i.e. before and after the second equals sign??
Is this normal?
 
@enumaris it is just the latest step in our government's attempts to make British a synonym for farcically incompetent.
 
It's not so much about the way the operators work and give the eigenvalue $\xi_k$, it's more about how i only have the operator before the $a^\dagger_k$?
 
9:58 AM
in winter the worst weather circumstance is persistent raining and overcast sky, which makes no stars to watch at night and brings depressing air. The lowest air temperature I have experienced here is 4 C, but it doesn't occur every winter. The only storm here is typhoon, which mostly occurs in summer.
 
10:13 AM
@JohnRennie last time you told me about excitation of fields, which recalled me the creation and annihilation operators, which I learnt in nonrelativistic quantum mechanics. Now I found in quantum field theory there are also creation and annihilation operators in en.wikipedia.org/wiki/Normal_order.
 
@CaptainBohemian I was talking about quantum field theory ...
 
There is
a Link
between the two
 
@Slereah that's simply harmonic
 
yes
 
Helpful comments ' r ' us
2
 
10:28 AM
I mean it's in every QFT book
 
 
2 hours later…
12:11 PM
what is up with the special conformal transformation anyway
Every other conformal transformation is like
fine
then you see the special conformal transformation and what the hell is that
 
0
Q: Physics mystery : why does it rotate?

mick In The beginning this thing was floating still and the candle was out. Then someone lit the candle and the flame somehow Made the object spin on the water. See the picture. I know a little bit about thermodynamics and a bit about electromagnetism. But not The combination. Is this due to ma...

 
12:32 PM
0
A: Physics mystery : why does it rotate?

John RennieWhat your GIF doesn't show is that the bottom ends of the copper tube are angled: The toy is known as a tea-light engine and Googling this will find you various sites describing it. The copper tube is full of water. When the candle heats the tube the water at the point where the heat is applie...

 
 
1 hour later…
1:57 PM
I think it should be harder for mathematicians than physicists to find a friend outside school.
 
2:12 PM
@CaptainBohemian Should I infer that the contrary holds? :D
 
He wants a friend
 
@user55789 maybe it's so, but I don't really know because I haven't had much experience with interacting with mathematicians in real world. But I have an experience contrary to my imagination--people from society department are very unsocialized in that they didn't want science people to use their equipments.
 
You mean the soft sciences?
 
no
physical sciences
I used to be in an applied science research institute which borrowed the building of society department as our office because this institute has no their own building, and the people from society department were very unfriendly to us like they were unhappy with our using their space. For example, when their food in refrigerator disappeared, they suspected it's us who stole it, but not their own people. After their food disappeared for several times, they forbad us to use their refrigerator.
 
2:42 PM
How does this relate to science people making friends ouside school?
 
I just feel mathematicians study so abstract issues which have no much relation with what happen in real world so that most people outside math fields don't understand those issues, so how could they find a person outside school understanding what they are talking about?
 
It's no different for physicists, is it?
 
the topics studied by physicists are less abstract than issues in maths and sometimes some phenomena occurred in daily world can be explained in physics laws; I have seen the media invite physicists to explain something for the general public.
 
it depends a lot on the field you study
if you're making nanomachines it's fine
if you're trying to compute the VAV of some string theory it's trickier
 
 
2 hours later…
4:47 PM
hmmm
welp, the NHL event was pretty nice...apparently SAP is in the process of building an innovation center here in LA
the private suite was not as fancy as in the movies though...lol
 
5:04 PM
The one I was in wasn't particularly fancy, but it had food and alcohol provided which is about all I can ask for in a stadium
 
same...but I don't drink alcohol so I guess it was much cheaper to host me lol
I just got a ginger ale
 
Well unfortunately I was under 21 at the time anyway
I do approve of the ginger ale though. That's about the only soda I drink
 
XD
I felt like a freaking salesman peddling my NLP wares yesterday lol
2
gave the whole "I work on NLP" spiel like 4 times last night
 
I made a new Why are images of text, code and mathematical expressions discouraged? thread on Meta SE. In case anyone has points specifically relevant to the math/science sites - like Physics SE, in mind, please go ahead and add it there. (cc @EmilioPisanty)
0
Q: Why are images of text, code and mathematical expressions discouraged?

BlueOften I see new users posting images, containing the text of the question they were intending to ask, or a part of it. This is especially true for the science and math based sites where new users don't know how to use MathJax. I tried searching through the previous questions on Meta SE, but could...

 
Brave man talking about ML in public
They may start asking you questions about AI
 
5:28 PM
The head of the center seemed interested in following up
I followed up by sending him a message on linkedin
we'll see if anything comes of it lol
 
Of the SAP center?
Maybe you could become a COBOL NLP guy
 
yeah head of the SAP Center
 
You could be the first and last one
 
it seemed like they were mostly interested in software engineers though that's true
but they wanted to do "research into the next 5-10 years" and their current goal is a "smart hospital"
head of this Newport Beach Innovation Center
It sounded like their current openings are very much software based. The guy said he's looking for software engineers and "architects"
 
Hey some of my work stuff is in the direction of IoT devices in medicine...it's just being done in a way that's less than ideal
 
5:31 PM
maybe you can apply lol
They did say they have 1 data scientist and 1 more was joining soon
but they said the data scientist was very heavily math/stats focused
not really ML focused
 
We should all join SAP and start a physics uprising
Also you may be past this level, but it's neat to me github.com/GokuMohandas/practicalAI
 
lol
 
Also jax has had a lot of documentation added since I linked it
I'm going through all the newsletter emails I've ignored over the past week...fun times
 
I should just tell that VP to make me the head of strategy at that innovation center
I'll build him a deep learning system that reproduces duck quacking sounds
 
lol there was a kaggle competition about turkey sounds at Thanksgiving
 
5:41 PM
I think it kind of depends on how out of the box thinking they want to do
Cus obviously their current thought process is "get a bunch of software engineers and we'll figure out how to build some software"
but to want to hire me, they'd have to shift a little bit more to the pure research side I think
 
Well that's better than the approach of "get a couple interns and keep saying IoT"
Replace IoT with ML, NLP, AI, or your favorite thing
 
the Pharm company was kinda doing that lol
during my interview, he was like "for pure research stuff, we might get a few interns to do a POC and then after POC we'll get a full time resource on it" or something like that
 
6:11 PM
@danielunderwood ::ominous chanting:: One of us! One of us!
 
vzn
6:27 PM
@enumaris lol welcome to capitalism + science, sometimes an uneasy mix to say the least o_O
 
:D
 
7:05 PM
Hi , I was wondering if someone could help me. THe question i am working on seems ill defined
It was put on hold, don't know why.
I am looking just for a direction.
-2
Q: Orbit velocity satellite question, is this question ill defined

johnI am attempting to solve this question but I feel that the question may be ill defined. The question is in two images, the first image and second image. I have tried many attemps at this but nothing seems to make sense. The question seems to be talking about entering an orbit starting from the su...

 
SAP dude says he wants to connect first week of January cus he's taking time off...I'm gonna be in negotiations w/ my current employer soon though...dunno how all this is gonna work out...
feels like timing everything to coincide is pretty much impossible lol
 
I think the teacher who assigned this problem is some kind of moron, thats what im concerned about.
It doesn't make any sense.
 
I can't view images so I dunno what the question is unfortunately
 
can i embed them here
 
probably I still won't be able to see them lol
hopefully someone else can help you :)
 
7:17 PM
<img>https://imgur.com/a/TlbG02D</img>
it looks like no one will, my question got put on hold
 
@john 1. Please don't call people morons. 2. The question is closed because we don't do homework-like problems here. 3. I agree that the question doesn't seem to make much sense, especially since it is rather unclear what "stay in orbit for X seconds" is supposed to mean. Something is either in orbit or it isn't.
 
im not sure how to contact the person who put the question on hold
its not a homework question
not my homework anyway
any question can potentially be someone's homework question, i dont thats a good reason to make it off topic
i dont think
what should i call the teacher. a genius?
 
"homework-like" here means that it's essentially asking us to solve a computational exercise. Whether it is your exercise (or indeed an actual exercise at all) isn't relevant.
 
i didnt ask that
i asked, is the question ill defined
 
And how would one show that it is well-defined except by explaining how to solve it?
 
7:20 PM
wow you are making a lot of assumptions
well for example, like you did above
"I agree that the question doesn't seem to make much sense"
 
@john No, that's pointing out that it's ill-defined. It's easy to show that an exercise is nonsense without solving it. But nothing except the solution could really show that it is well-defined.
 
i know how to find the speed of a satellite
i disagree
you could say simply that the question doesn't make sense.
 
Sure - in the case where the question indeed makes no sense. But in the case where it made sense and you simply didn't see it, the answer to "Is this question ill-defined?" would be to show that it can indeed be solved.
 
for a satellite rotating earth $v=\sqrt{ \frac{Gm_E}{r}$
ok i see your point
but you could also leave a comment, 'the question does make sense, please attempt it , or show your work'
 
It's usually helpful if you rephrase the question to be more specifically about why you think it's ill-defined. But it's still not a very good fit for the site because it will always remain more about trying to interpret the intent of the person writing this specific question and less about actual physics.
 
7:26 PM
i did show my work though
i did try
one sec let me see
Yes I showed my reasoning why it seems ill defined.
Were you able to see both images @ACuriousMind
i dont mean to be a nag, i just want to be clear
 
@john It's not really clear why you think that the orbital speed is too high. 38 kph are not very fast, e.g. a satellite at 200 km altitude needs 27400 kph to stay in orbit.
@john Yes
@enumaris just can't see them because his sysadmin doesn't let him see images :P
 
woah woah woah, let's not bring the "s" word in here
 
the reason why it didnt seem to make sense
which gave me sqrt( 6.67e-11 * 5.92e24 / 6.38e6 ) = 7867 m/s
which is about 8 kph?
 
You're trying to compute orbital velocity at Earth's surface? According to Wikipedia the correct result would be 7.9 km/s, so that's correct.
 
err, kps
 
7:33 PM
Forget about what I said earlier, I did the units wrong.
 
right, since the canon in the problem is on the earth's surface, or very close to it
 
I think that's not what your questions wants you to compute, though
I suspect that "stay in orbit for X seconds" is supposed to mean the same as "not touch the ground for X seconds" in this case
 
right, so what i did was, assuming it hits the ground after 1 second, y = 1/2 (9.8)*(1)^2
the vertical displacement is 4.9 meters
 
So the 7.9 km/s would be the answer to "How fast does the object need to stay in orbit indefinitely?" in this context.
 
yes, indefinitely , ie in orbit
but to stay in orbit for 1 second, thats the part that is confusing me
 
7:37 PM
<--transplant from rpg.se
physics question that we're struggling with
 
also to stay in orbit around earth, it would probably be a feet or so above the idealized sphere of earth, so we can avoid friction on the surface (and neglect air resistance as well)
 
how would one generate 100 pounds of force with a 10ft pole? One may apply weight to the pole.

This is for the purpose of setting off pressure traps.
 
can you use the law of a lever
 
yes?
Though, I'm not 100% sure how that would help
 
@goodguy5 Put the pole vertically onto the target, then put something weighing 100 pounds on top of the pole :P
 
7:39 PM
If you have a force of 100 lb acting at a distance 10 feet from the fulcrum or pivot point, the torque is 100 lb times 10 feet = 1000 foot-pounds
 
@ACuriousMind that defeats the purpose of being far away!
 
as an example, if you put the fulcrum 2ft from the end of the pole, and apply 25 pounds of force perpendicular one end, you'll get 100 pounds of force on the other
(2ft -> 2:8 = 1:4 ratio)
 
oh, that makes sense.
what about gravity?
 
what is a 'pressure trap'
 
@john think like a plate under a brick. When a certain amount of weight is applied, a bad thing happens.
 
7:42 PM
oh
"This is for the purpose of setting off pressure traps."
you want to release the pressure of bricks , etc
 
Well, I want to push down on the trap from a "safe" distance of 10 feet so that it sets the trap off and doesn't hurt my friends
again, this is for D&D
 
oh thats interesting
so this is a real world situation
 
welll....... it's a "real world" D&D situation
Dungeons and Dragons, if you don't know
 
and this trap is a real pain inducing trap?
like a bear trap
 
it could be anything. It could be an explosion
or a bunch of bees
 
7:51 PM
To clarify: Are you looking for a tool you could use to disarm a 100-pound pressure trap from 10 feet away?
(I imagine "attach a 100 pound weight to the end of the pole, then pick up the pole and maneuver it so that the weight rests on the trap" is too simple-minded)
 
I hope this teacher gets decertified.
 
@Semiclassical No, that would work, but it seems very hard to hold 100 pounds at the end of a 10 foot pole.
 
that's probably true.
 
My first thought was

"from how high would you have to drop an object to generate 100 pounds of impact force"
but I don't know the physics words to google it.
afk
 
i mean, once it's lying there, then it'll still be generating 100 pounds of impact force
 
7:59 PM
I mean of a less weight.
 
ah. fair enough
 
like, how high would I need to drop something 50 pounds, so that it lands with 100 pounds
 
the trouble with that approach, I think, is that it'll depend on the time frame for the collision
I mean, imagine a stunt person jumping off a building into a big airbag
the reason why they survive it is because the airbag ensures that the collision lasts long and therefore the max force is never too large
conversely, if you drop a very stiff object so that it bounces, then the max force will be rather high
 
@john I have found your error: You cannot assume that the curvature of the earth is linear, it introduces too large an error here. If you use the proper Pythagorean approach to compute the horizontal distance necessary for 4.9 m difference in height, you get about 8 km
 
You should do the totally unexpected and make it a trap with a good outcome
 
8:03 PM
checking
@ACuriousMind are you sure you are using pythagorean theorem
 
@goodguy5 That's very difficult to tell because the force depends on how quickly the substance is decelerated on impact, which makes it a subtle question about the elastic properties of the material
@john Yes. As a consistency check, If you put "8 inches" instead of the "4.9 meters", you get 1 mile.
 
ok checking, brb
 
Just fling a rock and hope you get a 20
 
@AvnishKabaj Well, I'd argue that if your GM is actually specifying the exact amount of force you need to trigger a trap, you're probably no longer playing a game in which dice rolls are all that important :P
 
Well said
 
8:10 PM
just a clarification you are using 6371 km for the radius of earth
and the 4.9 meters is your initial height above surface of earth
 
@john Basically, yes. Although it's less "initial height" than "final height" here.
 
or are you shooting the canon as close to the ground as possible
 
You could throw dynamite
If your party has some
 
@john Yeah, that. I'm computing the horizontal distance between the point where the cannon is fired and the point where the projectile is 4.9 meters above the Earth if we imagine it just going straight without gravity
 
I can't come up with anything better than dynamite
 
8:14 PM
@AvnishKabaj Just throw a henchman
Or some corpses. Adventuring parties exploring dungeons are rarely in want of corpses :P
 
Just get a dragon and throw it
Though I honestly don't know if most D&D games include a dragon
 
interesting, yes i just checked using radius 3959 miles
(3959 mi) ^2 + (1 mi)^2 ≈ (3959 mi + 8 inches)^2
so that is a good estimate of the curvature
of earth
 
@john "Curves 8 inches for every mile" is a good estimate. It's just that you can't linearly extrapolate from that how far it curves for 10 miles because it's a curve, not a slope
 
i'd forgotten how headache-inducing array manipulations can be when you've got more than one index
 
8:34 PM
ah yes good point
i extrapolated as if it was a straight line, but its a curve. the change in y over change in x is not constant
that explains why my orbital speed was so high, which disagreed with the formula v = √Gm/r
@ACuriousMind someone attempted to solve it answers.yahoo.com/question/…
but it looks like they did something similiar to what i did, extrapolating the curvature linearly
 
8:50 PM
reminds me of a calculus project I had. I don't remember the details---it was about a decade ago---but I think the upshot was to take four cities in the US and work out what the optimal location between them was, in the sense of minimizing the distances to each city
I think the problem I ultimately ran into was precisely the curvature of the earth :)
 
:D
 
to be more specific: I had grabbed the distances between the cities using an online tool
and I tried to act as though those distances were between points in a plane
which, of course, they're not---they're distances between points on a sphere
 
My course with GR has given me an instant reaction to people saying curvature. I don't think it was ever mentioned that most people in the real world would use Mathematica
 
"most"
"""""""""most""""""""
@Semiclassical spherical geometry is the bread and butter of astrophysicists
 
I mean people in GR don't sit there calculating Christoffel symbols and curvature tensors by hand, do they?
 
8:55 PM
@enumaris can't say I'm shocked there
 
"most people in the real world would use Mathematica"
 
"most people in the real world would use a CAS", maybe
 
most people in the real world don't give 2 cents about Christoffel symbols
 
Well I didn't mean of all the people in the world!
 
8:56 PM
"most GR people would use a CAS", maybe
 
"I would also like to allude to a point regarding..."
 
I mean I could go up to random people on the street and go "How would you like to learn about Christoffel symbols?"
 
I'm not very comfortable with this (allude to a point) but can't find a decent alternative. Any suggestions?
 
I may have glossed over the "would" in that sentence lol
@Mostafa refer?
 
@enumaris hmm, I feel "refer" doesn't exactly give the same sense of speaking briefly about something
 
9:00 PM
mention?
 
"As an aside,..."
hmm. maybe i've been thinking about that phrase wrong myself
 
"I would also like to allude...well not allude, but you know what I mean right? like when you just talk about something briefly? anyways...I would like to allude to a point regarding..."
 
lol
@enumaris I think mention is good. thx
 
np :D
 
 
2 hours later…
10:39 PM
hmmm the only 3d modeling courses I could find on coursera are for the Unity 3D engine specifically...
I suppose any course that actually does any hands on exercises (rather than pure theory) would require you to use some particular engine though
 
11:05 PM
I think 3d modeling on a technical level may be a bit outside of what Coursera would offer
It may be like finding a theoretical classical mech course on there I think
 
probably I'll play around in Blender for a bit first
then look into other resources
 
Actually this may be something edx.org/course/computer-graphics
 
I'll keep that one in mind :D
 
Actually I'm kind of interested in that one myself now
 
lol
 
11:53 PM
If there’s one thing that drives me crazy about Uber it’s when the ETA changes
App says: oh, you want a ride from there? Ok, should be 10 minutes
Confirm request, Uber says: ok great it’ll be there in 20 minutes
 

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