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12:01 AM
these are allegedly the same gray
formatting is too hard
 
rob
@0celóñe7 Yeah, if you have two different oranges and two different grays no one is going to judge you for it.
 
@rob the black is so close to the gray, that would be less noticeable than the light gray text
there's no good reason why it should be so light
I have to make some food though
 
12:15 AM
@DanielSank For that, I literally copy it line by line in the interpreter. That way, I get to chose which chunk of code to test
Still annoying to copy paste so many times though
 
0
Q: Help me to understand how time is relative!

ChrisI'm a bit confused about how time can be relative. Example: if a cosmic event happened that could be observed from earth, multiple times, because light from this event reached earth at different times because of timespace distortion due to a planets partial obstruction. Does it not just mean that...

 
 
2 hours later…
1:55 AM
@ACuriousMind Thanks ACM. Outta curiosity, is it the same person who's been DV'ing me? Or are you unable to tell me or discern that?
 
2:06 AM
Expecting white dwarf merger soon
 
soon?
like astrophysically soon?
 
2:20 AM
Does any one have any experience with general relativity and incorporating non-gravitational forces in equations of motion?
Or even any idea on where to start?
 
@AHB Okay thanks!I hope google translate suffices for it :P
 
@Rumplestillskin Yes to the first but no to the second
what exactly are you trying to do
 
It does not say when. 40 someting minutes period seemed long enough to not merge like e.g. next 5 years
 
Just interested to read about non-grav forces in the general relativistic framework. I mean in newtonian gravity we express forces as linear superpositions. I am interested how this is achieved in GR @0celóñe7
 
@rob Python is so far ok to me as long it does not need to communicate with the bash shell and PBS pro, then is where it gets VERY ANNOYING
 
2:30 AM
So as an concrete example: Compare the orbit of a satellite in the newtonian regime and relativistic regime subject to X, Y, Z non-gravitational forces. Or at least something to that effect. For the relativistic regime I imagine it is a modification to the geodesic equation to have a non-zero RHS? @0celóñe7
 
@Rumplestillskin Yeah, but I doubt there's a general framework for such things
In some cases it's obvious I guess
 
If you are going to include non gravitational forces, are you talking about fictitious forces or those arise from other interactions like electromagnetism?
 
it should also be a linear superposition
 
a linear superposition? GR is fully non-linear so how could that work? @0celóñe7
@Secret when I say non-gravitational forces I mean those forces perturbing the acceleration of a satellite like solar radiation pressure etc.
 
@Rumplestillskin you can add vectors and the RHS of the geodesic equation is a vector
 
2:36 AM
@0celóñe7 have you seen this done any where before with a concrete example? I would like to see how they do it.
 
No, but the standard reference for motion is De Felice and Clarke
they have information about extended modies/nongeodesic motion
maybe look there
 
@Secret haha I just began reading that document!!
@0celóñe7 okay I will have a look
Thanks!
 
Hmm... I wonder if trying to put in solar radiation pressure as part of the stress energy tensor will be a bit overkill
 
Well I just used SRP as an example. Ideally I am interested in several non-gravitational forces and of course standard geodesic motion all being modelled in the framework of GR.
 
 
3 hours later…
5:13 AM
I need a dummy's guide to voltage regulators.
@rob Yes "time to solution" is cited as a good reason to use Python to solve problems.
 
 
1 hour later…
6:32 AM
@ACuriousMind my internet connection failed just after I posted :-(
it was going back and forth for a little while
 
 
3 hours later…
9:15 AM
Hi, everyone! Do you know what is the general procedure to decompose a representation of a Lie Group into representations of some of its subgroups? For example, SU(4) contains SU(3)xU(1) and the 6-rep decomposes as $ 6 = 3_{2/3} + \bar{3}_{-2/3}$
 
user84215
10:03 AM
Please visit Planning MSE University ; it has been updated with new posts and conversations.
 
10:29 AM
@DavidZ are you there?
 
@apt45 You fool
I had to work on this topic for a few months
it's terrible
And it was only for SU(3)
 
@Slereah Ok.. the best option is to work with Mathematica?
 
10:54 AM
I used this book for it
It's in Penrose notation
 
Anonymous
@BalarkaSen Are you free now? We can get back to learning LA then. I'm free for an hour or so. :)
 
@Slereah Thanks!
 
Anonymous
Sorry about yesterday. I got too involved in English prep.
 
11:16 AM
Don't thank me til you read it :p
It's an awful notation
 
11:35 AM
@KyleKanos I think I shouldn't even tell you whether I am unable to discern that or unable to tell you ;)
 
0
Q: Uncertainty principle in quantum entangled states

Diego F MedinaImagine that you could create quantum entanglement for two particles for both position and momentum. Furthermore, you are able to measure at exactly the same time position (for one of them) and momentum (for the other). How would the uncertainty principle apply here? Is such experiment even poss...

I predict the more certain the position measurements are, the more uncertain the momentum measurements are even if they are correlated
In practice, I don't know and I need to do the maths to see
 
11:54 AM
@ACuriousMind okay, fair enough. As long as I get that rep back, don't bother me much. Just shocking that it seemed to have come from nowhere.
 
@KyleKanos The last of the rep should return in at most a few days. (What do you need the rep for, anyway? :P)
 
@Blue Sorry, I was away. Are you free now?
 
@Secret No, you need to revise your understanding of the uncertainty principle: The uncertainties occuring in the uncertainty principle are not about the "precision" of the measurement devices.
 
@ACuriousMind a user has been making a lot of trivial edits to old questions on meta. I wanted to tell them to stop but I dont know how to -- will they get notified if I leave a comment in a post they edited?
anyway, you mods might want to look into that
 
/me looks in
 
12:03 PM
I tried to said something about variance and how the observables are related by fourier transform so that as one become more narrow the other spread out and vise versa, but I guess I have not practiced it much recently thus I forgot how to express it in standard terminology
 
@AccidentalFourierTransform Yes.
 
Indeed a sign that I need to revise
 
Jim
So I recently bought a set of centripetal force experiments for my first-years. The vendor sells complete experiments with the manual and everything, which makes my job infinitely easier, so naturally I go with them quite often.
This company makes a lot of university physics experiments and usually, they're pretty good.... except this time. The sensor that comes with this experiment actually measures tension in a rod (there's a mass on the end of the rod that moves in a circle), but they refer to that as the centripetal force.
It doesn't sound bad but at some point in the experiment, they ask the student to identify the point where the centripetal force is zero and then ask them to find the non-zero velocity of the mass on the rod at that point..... $v\ne0$... And they don't see how telling students the centripetal force is zero when velocity is not is bad!?
 
@Secret I don't know what you mean by that. I didn't mean you should explain the HUP to me (and the uncertainty principle also holds for observables that are not conjugate!). I meant that if you think that the HUP is about the precision of measurement devices, then you don't understand what the uncertainties involved in the HUP mean to begin with.
For any observable, the "uncertainty" is just the standard deviation of measuring that observable with an idealized, infinitely precise measurement device. It's a property of the quantum state, not of any particular measurement procedure. There are versions for precisions of measurement devices, but they are not what the standard HUP is about.
@Jim ...how can the tension in the rod be zero when the centripetal force is not? What other force is transmitted through the rod that cancels against the centripetal force?
 
Jim
12:09 PM
@ACuriousMind gravity, it's vertically oriented
 
Hi @Jim
 
Jim
@skullpatrol hi
 
@Jim But then they should measure the "centripetal force" varying over the course of the motion to begin with, shouldn't they?
 
Hi @Jim
 
Jim
12:10 PM
@ACuriousMind "should" being the key word
 
Because gravity is always oriented in the same direction, while the centripetal force isn't
At the top they will add to each other, at the bottom they will cancel
 
Oops, sorry about the re-ping. My connection is wacky :-/
 
@Jim What I mean is that nobody could actually try to carry out this experiment because they will find that the tension is zero at one point in the motion but not at another
Which makes me wonder if anyone at that vendor actually tried to perform the experiment :P
 
Jim
@ACuriousMind there's this plot they make of force vs angle. It sows tension in the rod goes to zero at some angle where the mass is clearly still moving. That should be clear indication that it's not centripetal force
And they had to perform it because they wrote their own manual WITH example data
 
@ACuriousMind I need rep because I have a deep, psychological need fake internet points.
 
Jim
12:14 PM
Hi @KyleKanos <3
 
@Jim Then do they tell you at what point in the motion you should read off the "centripetal force"?
 
lol
 
Jim
@ACuriousMind all points, it's collected by computer
 
Because right now I think that the display of the tension measurement should vary continuously
 
Jim
@ACuriousMind it does
 
12:15 PM
Uh...so they claim the centripetal force varies not only with velocity but with position? :P
 
Hi @KyleKanos
 
Jim
@ACuriousMind well, we already figured they're wrong. But the velocity technically does depend on position because we release the mass from the top position, so the velocity accelerates and decelerates in a position-dependent way
 
I guess the actual question here is why you wouldn't design an experiment about the centripetal force with a horizontal motion :P
 
@ACuriousMind yey I was thinking this \o/
Now I feel cleverrerrrerrerer.
 
Jim
or why you wouldn't realize the first-year level problem of saying "find the velocity when the centripetal force is zero"
 
12:20 PM
How's the new job position going? @djsmiley2k
 
going well thanks @skullpatrol
that's well remembered!
 
Facebook Messenger now supports rendering LaTeX for writing maths.
2
 
o_O
I heard a few days ago it also does code indentation properly too
it's almost usable ;D
 
::sigh:: Another two bounties down the drain.
 
stop bountying crap ACM
 
12:30 PM
No answers?
 
Hey @ACuriousMind I think it was you who explained to me about Negative mass right? I just looked it up again on Wiki out of curiosity but I can't actually find a 'criticism' section, and the relativity section seems to at least acknowledge that it could exist
 
Hey guys. I'm going over some QM questions for an exam. In the notes I have, the start of a solution says " |3/2,3/2> = |1,1>|1/2,1/2>". Where would I find out why that is true?
 
@ACuriousMind can u put 500 on my qm doubt pls
 
Did you watch the eclipse? @0celóñe7
 
@skullpatrol yes
 
12:38 PM
@Phase Thinking about it, I was too quick to dismiss it. There is, technically, room for "negative mass" in our models. Alas, it has never been observed.
@0celóñe7 what?
 
@ACuriousMind fair, I did pretty much pull one of the most fringe topics into conversation with little prep
 
5
Q: Does mathematical sloppiness in quantum mechanics ever produce incorrect predictions?

0celóñe7Does mathematical sloppiness in standard quantum mechanics ever produce predictions that don't pan out? I'm not talking about things like the WKB approximation, but instead subtle functional analytic issues, such as assuming every Hamiltonian is self-adjoint, has an eigenbasis of bound states, do...

I can revise it if you want
 
@EdinburghDruid Clebsch-Gordan coefficients in general, but in your case you could simply observe that if you have a spin-1 and a spin-1/2 state, the only way to make a spin-3/2 state with $m_s = 3/2$ is to have both the spin-1 and the spin-1/2 aligned
 
@0celóñe7 the snark in that post
"contrived counterexamples"
 
help
 
12:41 PM
Where's The Evidence? :P
 
@0celóñe7 Potentially, there exists no correct answer (if no such case is known, it's impossible to demonstrate that). Not gonna bounty a question to which an answer might not even exist :P
 
@ACuriousMind I will accept any satisfactory answer.
 
@ACuriousMind That would be the same reasoning to say |2,2>=|1,1>|1,1> with a D2 molecule, because each has spin 1?
 
And I know you want to know about it to.
 
@EdinburghDruid Yes
@0celóñe7 Is someone answering "I don't know of any such case" satisfactory? :P
 
12:45 PM
@EdinburghDruid is this an abuse of notation or what?
 
I mean, nobody who understands the question will deny it is hypothetically possible, and will therefore be wary to declare that this never has happened
 
I dont really get how you can put two kets next to each other and have them do anything useful
 
@Phase There's an omitted tensor product sign, as physicists are wont to do
 
Oh fucks sake
 
@ACuriousMind An affirmative answer, please no trolling
 
12:46 PM
That confused me for a few solid weeks once
 
@0celóñe7 Huh?
 
@ACuriousMind hence why I am expecting an affirmative answer
 
@Phase The question is to construct spin state |0,0>.. Yes about the tensor product :)
 
@0celóñe7 Ah, see, but I am not, hence I'm not willing to offer a bounty for nothing :P
 
I can't into kerning
 
12:51 PM
wow im really dim I think I only just realised that the reason that heisenberg's principle is the way it is is because only operators that commute share an eigenbasis
 
@0celóñe7 If you want into kerning, use microtype. Have fun going down the rabbit hole.
 
0
Q: What exactly is a Quantum Computer?

Shivay VadheraWhat exactly is a Quantum Computer? I mean, I've seen a lot of PopSci BS on this topic, saying how qubits are actually both "0" and "1" at the same time, for instance. Could someone plaese explain what physicists/ Computer Scientists actually mean by Quantum Computers?

Too broad?
 
@ACuriousMind that seems like something for someone with too much time
 
@Qmechanic very much so
imo.
qubits are an indeterminate state.
 
Jim
@Qmechanic not sure. On one hand, one could write a book about what a quantum computer is. On the other hand, a simplistic answer could likely be given in a short, succinct way that expresses the basic idea on how a quantum computer is different from a normal computer
I'd say leave it for the community to decide
 
1:00 PM
Just leave this SMBC there.
 
lol
Is quantum powered encryption on the cards yet?
 
Jim
@ACuriousMind are you saying there's no point in trying?
 
@djsmiley2k there's Quantum Key Distribution (secure data transfer)
That's much closer than fully scaled, error corrected quantum computers
 
hmm
i read about something I think in china
 
1:17 PM
@Jim No, I'm saying Scott and Zach already did it
 
Jim
excellent
 
is this like how a rocket and a bike are both modes of transport....?
 
Jim
@djsmiley2k yes, in that quantum and classical computers are both computers
 
1:33 PM
yeah
 
British tone Bloody quantum mysticists, mixing the two domains when they should not be mixed, and their models does not even work!
37 mins ago, by ACuriousMind
Just leave this SMBC there.
NB, having said that, my quantum is still not very good, this gap of mine will be fixed soon after I f*** this python program
 
you did what to it! :O
 
1:49 PM
Nah, not the python prgramming language. I am referring this particular program script I have stuck with for nearly 4 weeks
 
ooooo
you're programming a snake?!
 
It's not really a snake, but it does look like a two headed snake in terms of the flow chart structure
Script 1 has to feed into sciprt 2, which then feeds simutaneously into scripts 3 and 4
trying to make all the pieces communicate in the required sequence is the hard and confusing part because there are so many modules
 
@Slereah yo what is the schwarzschild topology?
$\Bbb R\times(\Bbb R^3\setminus \{0\})$?
 
Wait you are programming a S N E K?
Dayum guy
 

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