The h Bar

0celóñe7

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.

0celóñe7

@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

Secret

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

Secret

0

Q: Help me to understand how time is relative!

I'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...

spacetime

Kyle Kanos

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?

Secret

2:06 AM

Expecting white dwarf merger soon

arxiv.org/abs/1708.05287

0celóñe7

soon?

like astrophysically soon?

Rumplestillskin

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?

Prathyush Poduval

@AHB Okay thanks!I hope google translate suffices for it :P

0celóñe7

@Rumplestillskin Yes to the first but no to the second

what exactly are you trying to do

Secret

It does not say when. 40 someting minutes period seemed long enough to not merge like e.g. next 5 years

Rumplestillskin

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

Secret

@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

Rumplestillskin

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

0celóñe7

@Rumplestillskin Yeah, but I doubt there's a general framework for such things

In some cases it's obvious I guess

Secret

If you are going to include non gravitational forces, are you talking about fictitious forces or those arise from other interactions like electromagnetism?

0celóñe7

it should also be a linear superposition

Rumplestillskin

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.

0celóñe7

@Rumplestillskin you can add vectors and the RHS of the geodesic equation is a vector

Rumplestillskin

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.

0celóñe7

No, but the standard reference for motion is De Felice and Clarke

they have information about extended modies/nongeodesic motion

maybe look there

Secret

arxiv.org/abs/0901.1606

hmm?

Rumplestillskin

@Secret haha I just began reading that document!!

@0celóñe7 okay I will have a look

Thanks!

Secret

Hmm... I wonder if trying to put in solar radiation pressure as part of the stress energy tensor will be a bit overkill

Rumplestillskin

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.

DanielSank

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.

David Z

6:32 AM

@ACuriousMind my internet connection failed just after I posted :-(

it was going back and forth for a little while

apt45

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.

AccidentalFourierTransform

10:29 AM

@DavidZ are you there?

Slereah

@apt45 You fool

I had to work on this topic for a few months

it's terrible

And it was only for SU(3)

apt45

@Slereah Ok.. the best option is to work with Mathematica?

Slereah

10:54 AM

birdtracks.eu

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. :)

apt45

@Slereah Thanks!

Anonymous

Sorry about yesterday. I got too involved in English prep.

Slereah

11:16 AM

Don't thank me til you read it :p

It's an awful notation

ACuriousMind

11:35 AM

@KyleKanos I think I shouldn't even tell you whether I am unable to discern that or unable to tell you ;)

Secret

0

Q: Uncertainty principle in quantum entangled states

Imagine 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...

quantum-entanglement uncertainty-principle

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

Kyle Kanos

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.

ACuriousMind

@KyleKanos The last of the rep should return in at most a few days. (What do you need the rep for, anyway? :P)

Balarka Sen

@Blue Sorry, I was away. Are you free now?

ACuriousMind

@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.

AccidentalFourierTransform

@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

djsmiley2k

/me looks in

Secret

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

ACuriousMind

@AccidentalFourierTransform Yes.

Secret

Indeed a sign that I need to revise

AccidentalFourierTransform

k thanks

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!?

ACuriousMind

@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

ACuriousMind

Ah

skullpatrol

Hi @Jim

Jim

@skullpatrol hi

ACuriousMind

@Jim But then they should measure the "centripetal force" varying over the course of the motion to begin with, shouldn't they?

skullpatrol

Hi @Jim

Jim

12:10 PM

@ACuriousMind "should" being the key word

ACuriousMind

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

skullpatrol

Oops, sorry about the re-ping. My connection is wacky :-/

ACuriousMind

@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

Kyle Kanos

@ACuriousMind I need rep because I have a deep, psychological need fake internet points.

Jim

12:14 PM

Hi @KyleKanos <3

ACuriousMind

@Jim Then do they tell you at what point in the motion you should read off the "centripetal force"?

skullpatrol

lol

Jim

@ACuriousMind all points, it's collected by computer

ACuriousMind

Because right now I think that the display of the tension measurement should vary continuously

Jim

@ACuriousMind it does

ACuriousMind

12:15 PM

Uh...so they claim the centripetal force varies not only with velocity but with position? :P

skullpatrol

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

ACuriousMind

I guess the actual question here is why you wouldn't design an experiment about the centripetal force with a horizontal motion :P

djsmiley2k

@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"

skullpatrol

12:20 PM

How's the new job position going? @djsmiley2k

djsmiley2k

going well thanks @skullpatrol

that's well remembered!

skullpatrol

Facebook Messenger now supports rendering LaTeX for writing maths.

2

djsmiley2k

o_O

I heard a few days ago it also does code indentation properly too

it's almost usable ;D

ACuriousMind

::sigh:: Another two bounties down the drain.

0celóñe7

stop bountying crap ACM

djsmiley2k

12:30 PM

No answers?

Phase

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

EdinburghDruid

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?

0celóñe7

@ACuriousMind can u put 500 on my qm doubt pls

skullpatrol

Did you watch the eclipse? @0celóñe7

0celóñe7

@skullpatrol yes

ACuriousMind

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?

Phase

@ACuriousMind fair, I did pretty much pull one of the most fringe topics into conversation with little prep

0celóñe7

5

Q: Does mathematical sloppiness in quantum mechanics ever produce incorrect predictions?

Does 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...

quantum-mechanics experimental-physics mathematical-physics

I can revise it if you want

ACuriousMind

@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

Phase

@0celóñe7 the snark in that post

"contrived counterexamples"

0celóñe7

help

skullpatrol

12:41 PM

Where's The Evidence? :P

ACuriousMind

@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

0celóñe7

@ACuriousMind I will accept any satisfactory answer.

EdinburghDruid

@ACuriousMind That would be the same reasoning to say |2,2>=|1,1>|1,1> with a D2 molecule, because each has spin 1?

0celóñe7

And I know you want to know about it to.

ACuriousMind

@EdinburghDruid Yes

@0celóñe7 Is someone answering "I don't know of any such case" satisfactory? :P

Phase

12:45 PM

@EdinburghDruid is this an abuse of notation or what?

ACuriousMind

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

Phase

I dont really get how you can put two kets next to each other and have them do anything useful

ACuriousMind

@Phase There's an omitted tensor product sign, as physicists are wont to do

Phase

Oh fucks sake

0celóñe7

@ACuriousMind An affirmative answer, please no trolling

Phase

12:46 PM

That confused me for a few solid weeks once

ACuriousMind

@0celóñe7 Huh?

0celóñe7

@ACuriousMind hence why I am expecting an affirmative answer

EdinburghDruid

@Phase The question is to construct spin state |0,0>.. Yes about the tensor product :)

ACuriousMind

@0celóñe7 Ah, see, but I am not, hence I'm not willing to offer a bounty for nothing :P

0celóñe7

I can't into kerning

Phase

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

ACuriousMind

@0celóñe7 If you want into kerning, use microtype. Have fun going down the rabbit hole.

Qmechanic

0

Q: What exactly is a Quantum Computer?

What 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?

quantum-mechanics quantum-information quantum-computer

Too broad?

0celóñe7

@ACuriousMind that seems like something for someone with too much time

djsmiley2k

@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

ACuriousMind

1:00 PM

Just leave this SMBC there.

djsmiley2k

lol

Is quantum powered encryption on the cards yet?

Jim

@ACuriousMind are you saying there's no point in trying?

Mithrandir24601

@djsmiley2k there's Quantum Key Distribution (secure data transfer)

That's much closer than fully scaled, error corrected quantum computers

djsmiley2k

hmm

i read about something I think in china

ACuriousMind

1:17 PM

@Jim No, I'm saying Scott and Zach already did it

Jim

excellent

djsmiley2k

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

djsmiley2k

1:33 PM

yeah

Secret

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

djsmiley2k

you did what to it! :O

Secret

1:49 PM

Nah, not the python prgramming language. I am referring this particular program script I have stuck with for nearly 4 weeks

djsmiley2k

ooooo

you're programming a snake?!

Secret

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

0celóñe7

@Slereah yo what is the schwarzschild topology?

$\Bbb R\times(\Bbb R^3\setminus \{0\})$?

Balarka Sen

Wait you are programming a S N E K?

Dayum guy

Transcript for