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12:03 AM
@0celo7 raider nation would never forget those who would go into the prison system to teach
 
@LasVegasRaiders wot
 
12:28 AM
That is what Chris White did @0celo7
Not only that but he was the master of "terse one-liners."
 
1:28 AM
@Justwinbaby we don't know why he left
But it certainly wasn't to teach
 
1:40 AM
@GPhys Small world, indeed. That's cool.
 
@dmckee It's certainly a very strange answer. I'm trying to imagine what the universe would look like if electrons were exempt from gravity. Basically everything we know about would be a little bit off.
 
2:11 AM
@dmckee I (think) I have a fairly concrete question now. If not an answer maybe you can let me know if it's at least concrete and nontrivial enough to ask on the site.
 
@GPhys You might want a second opinion either way.
 
@dmckee Do all measurements I make privately that I describe with wavefunction collapse lead to a state whose later measurement by someone else can be consistently described with a wavefunction that didn't collapse over my measurement span?
This isn't as trivial as, say, with entangled particles where collapsing my particle's wavefunction doesn't make a real difference on the a priori expected rate of each outcome for the second measurer (i.e. if we repeat the setup over and over.)
and now the concrete part
In particular, imagine the Schrödinger's cat setup as mediated by a radioactive decay triggering the poison. Now consider the cats I buy are always trained (unknown to me) to run a quantum Zeno effect experiment on the radioisotopes. Inexplicably to me, a binned distribution of cats alive over time seems to indicate the cats live much longer than is suggested by the half life of the radioisotope I chose.
For the cat, this is no problem. He describes the situation by the usual quantum Zeno effect where his repeated measurements cause the time since his last measurement/wave function collapse to be small enough to slow the decay as previously described.
How do I describe the situation? If I am later told by my cat supplier about the laser pulsing device the cats smuggled into my box, can I sufficiently describe the situation by perturbing the radioisotope Hamiltonian with the full rapid laser pulses? Is this an equivalent description to the cat's? (to the extent it matters with measurements)
(sorry for so much text)
and also - what is the relationship of all of this with quantum decoherence? I see it thrown around in some questions and it seems possibly related, but its wiki page seems obtuse.
 
2:28 AM
I love how you describe cats smuggling laser devices to measure quantum effects as "the concrete part".
 
sorry you don't really need to give a complete answer, but maybe you can at least let me know if I'm asking something well formed enough to make a new Phys.SE question
This gets at the heart of my questions from before
To make sure I was clear: my (proposed) description with the perturbed radioisotope Hamiltonian is meant to only be collapsed when I make my final measurement (in particular to not collapse repeatedly like the cat's description)
 
2:51 AM
if the general question is not so so incredibly dumb or a dupe I can add details and post it on the site
 
3:23 AM
@GPhys Not necessarily. Consider a system that oscillates like neutrinos. You could, in principle, make a flavor measurement at a time when the wave-function is $(|e\rangle> + |\mu\rangle)/\sqrt{2}$ and get, say, $|e\rangle>$ then the subsequent observer could make a measurement at a time when ther oscillating wave-function should have been $|\mu|rangle>$.
I you had not measured the system he should only be able to get a mu flavor, but because your measurement projected the system into a new state it might have the possibility of showing $\tau$ when the unperturbed system should not have been able to do that.
(Not that we can rig neutrinos with such precision, but it is a simple system to talk about.)
That said, the situation I describe above requires that we allow the wavefunction to evolve undisturbed for a time after the moment of your putative measurement. If we do it immediately afterward the evolution is infinitesimal and the odds are unlikely to have drifted by far enough to create a no-go situation.
 
@dmckee to clarify in the context of the original question (which I think is more clear in the example I gave after it), I mean to allow that the wave function as described by the second person have additional coupling added to it
@dmckee in the second person's description it doesn't actually collapse at the time when I say I measured it, but only when he measures it later
 
I think I'm lost.
 
oops meant to post that new, but
 
I'm getting further and further out of my depth here, but I have a suspicion that if you ask and expert the phrase "density matrix" is going to come up. And maybe the notion of mixed states.
 
@dmckee So with the neutrinos, the question becomes can the subsequent observer create a wavefunction of the system that adds a new coupled (environmental) potential when the first observer says it collapsed that creates a situation consistent with what he observed (yet notably doesn't say the system collapsed when I said it did)
i.e., is the subsequent observer really forced to say the wavefunction collapsed earlier? My proposition is that he's not forced into this - he can (I propose) just model the situation as a wave function traveling as I saw it and then becoming coupled with a measurement device like I used before reaching him
and then if we repeated the whole setup over and over again, this description of his should give a situation consistent with what he observes without invoking the first observer's collapse (?)
my rambling about the cats meant to describe a similar situation to the neutrinos, but for a quantum zeno experiment for which the effect of the measurement isn't a change in the second observer's measured neutrino flavor but instead his measured decay rate. And then I again ask - is the second observer who isn't doing the collapsing forced to make a description invoking prior collapse, or can he again describe the situation with a potential added to the radioisotope hamiltonian
(that collapses only when he measures it)
 
4:26 AM
@0celo7 I meant he volunteered his time to go into a prison to teach physics to inmates.
 
1
Q: How to delete my account?

AntHow do I delete my account? I don't use this site, and I just want it to be deleted. I can't figure out how to delete it. Thank you

 
4:39 AM
Does anyone fancy themselves to be a bit of a whiz at variational principles? If so, I could really use your help
 
4:55 AM
@Kaumudi.H Choco chunkies eh? :-)
 
@dmckee I think maybe I will go ahead and ask an on-site question since I think in particular the context of the QZE experiment is nice. I actually think the work (and proposition) I've put into it is most probably a step toward the correct description, and it would help me a lot to get any remaining questions answered
 
@Kaumudi.H: it's a bit sweeping to say I don't like biscuits when there are so many different types of biscuit. I don't like all biscuits, but I quite like most types of biscuit and really, really like a few types. Melting moments are one of those few very special types :-)
 
@ACuriousMind Ok, I can see why you wouldn't want me to post a picture.
 
user228700
5:12 AM
@JohnRennie No, no, that whole range!
 
For someone who doesn't like biscuits you seem to like biscuits :-)
 
user228700
 
user228700
^ These. I haven't tasted Choco chunkies yet!
 
I do like biscuits with nuts in. The flavour of toasted nuts goes really well with the biscuit.
 
user228700
@JohnRennie I realise this, yes. In all these years tho, I have come to like only these and a few types of cookies.
 
5:13 AM
Though I don't think I've eaten biscuits with cashew nuts in.
 
user228700
You must! Yummmm.
 
I don't think I've ever seen biscuits with cashews in the UK ...
It's generally hazelnuts or peanuts.
 
user228700
Ah, I see.
 
And I really like cake with walnuts
... or pecans (walnut and pecans are similar)
 
user228700
Hmm, I haven't ever tasted pecans but I see.
 
5:16 AM
Did you finish portal?
 
user228700
No, I'm still on level 18.
 
user228700
I don't feel well today so I might not be able to continue right away...
 
I watched a few minutes of a walkthrough on YouTube, just to see what it was like. Is it all working out how to put the cube on the button?
@Kaumudi.H that doesn't seem fair, to be ill in the holidays!
Nothing serious I hope?
 
user228700
Sometimes, yes. Other times, there is a ball of alpha particles or something, which has to be directed into a vessel of sorts, which then opens up a lift.
 
Hi chat
 
5:19 AM
Morning
 
user228700
...and there are many other obstructions along the way.
 
user228700
@JohnRennie Nah, just intense cramps :-)
 
user228700
Have u seen this, BTW:
 
user228700
 
user228700
@LasVegasRaiders Hello :-)
 
5:21 AM
The nice thing about HP is that the characters aren't one sided.
 
Hi @Kaumudi.H @JohnRennie
 
I think coffeeinanebula is being harsh on Dumbledore though
 
user228700
@JohnRennie One sided? What do u mean?
 
One sided means they only have one defining feature. So for example a one sided bad guy is just bad and we never worry about why they are bad, whether they loved their family, etc.
 
user228700
Ah, synonymous with one dimensional :-) Yes, yes definitely. This might actually be my favorite thing about HP.
 
5:24 AM
Bad to the bone :-)
as they say
 
I'm not sure I agree about McGonagall
 
user228700
@JohnRennie My memory fails me and I cannot paint a correct picture of what Dumbledore was like in the books...
 
user228700
@JohnRennie Why not?
 
She is fundamentally a good person, and she is competent, but she isn't a charismatic leader like Dumbledore.
If I was going to fight a half-snake psychopath I'd rather have Dumbledore next to me than McGonagall
 
user228700
She may not have been charismatic but I do believe that she was able to rise to the occasion and wage her wars as well as anybody else. Dumbledore may have been the better wizard and of that, there is absolutely no doubt but I don't think that's what the post is talking about...
 
user228700
5:29 AM
@JohnRennie Speaking of half-snake, have you seen this?
 
user228700
Actually, I can't be too sure that she wasn't charismatic:
 
user228700
 
@Kaumudi.H Hmm, 15 minutes long. I'll have to set aside a time to watch that.
 
vzn
@Jim dont have a solution to the problem of when to do a mtg, its challenging. any time would tend to exclude someone at some time in the world. suggest doing either early or late US time. not sure if wknds would be better than wkdays. tried to solve problem with the recurring mtg set up by DZ. we did have one AMA with DS at a nonstandard time & it was well attended.
 
user228700
It's great! It is about how, each time Voldy splits his soul in half, he is left with far less than actually half. So the first time he does it, he has half left but the next time, he only has one quarter and so on.
 
5:33 AM
"Have a biscuit, Potter" cracked me up everytime
 
user228700
:-)
 
@Kaumudi.H Hmm. Does it say anywhere that creating a Horcrux splits your soul in half?
 
user228700
Watch the video!
 
vzn
@DavidZ can always use whatever assistance is available, consider it a collective effort, mod(s) tend to at least show up during the scheduled sessions etc
 
@Kaumudi.H If the point of the video is that $2^{-n}$ gets small fast with increasing $n$ that's not terribly exciting.
Nor, since JKR is famously slapdash about anything mathematical, is it terribly significant.
 
user228700
5:36 AM
@JohnRennie :-) No, of course not. This fact is used by him to reveal some key points that might leave you wondering...
 
user228700
(him being MatPat, the creator of the video)
 
I'll watch the video. Even if I'm not impressed by the conclusions the presenting style is entertaining :-)
 
user228700
:-) Haha YES!
 
But not right now.
 
user228700
Wokay.
 
5:38 AM
I've thought of a game that might interest you ...
 
user228700
And my dad, after giving me Rs. 56 to buy Portals 2, told me last night that I shouldn't spend any more money on games :-P
 
Hexen: Beyond Heretic is a dark fantasy first-person shooter video game developed by Raven Software and published by id Software through GT Interactive Software on October 30, 1995. It is the sequel to 1994's Heretic, and the second game in Raven Software's "Serpent Riders" series. The title comes from the German noun Hexen, which means "witches", and/or the verb hexen, which means "to cast a spell". Game producer John Romero stated that a third game in this series which was never released was to be called Hecatomb. == Plot == Following the tale of D'Sparil's defeat in Heretic, Hexen takes place...
There is still a lot of killing things in it, but it's much more puzzle orientated than Doom.
 
spend money on arcane and artistic movies instead and justify the spending with convoluted philosophical theorization
 
@Kaumudi.H Ah, you did buy Portal 2 :-)
 
"yeah, you know, this film talks about the epistemology of the dodecahedral socialism which Marx and Engels described in their four volumes on communism"
 
5:41 AM
dodecahedral socialism?
 
yeah it has many faces
 
:: John slaps forehead ::
 
user228700
@JohnRennie Hmm, OK, I see.
 
I'm pretty certain Hexen is open source now, so it's free.
 
user228700
@BalarkaSen :-P Yeah, my parents wouldn't be in favor of that either. They already think that I'm messed up because I watch a lot of YouTube.
 
5:47 AM
All parents think their children are messed up :-)
 
user228700
@Balarka: BTW, you realise that this is what immediately pops up if I Google you?
 
@the harry potter soul thing butbut.... theres no telling that mathematics can even describe how the soul works; what if its a substance that when split becomes two wholes? and even with that; theres no conclusion to be had; the..
. final battle... it goes completely different in the books the movie ruins it because the point is that its supposed to be anticlimatic; voldemort is supposed to be just a normal person; with a normal death; no longer the undefeatable inconquerable ghost of a legend... its..
 
oh yeah that
i'm supposed to go there
 
user228700
@BalarkaSen ...you're literally the only one still in High school, man.
 
eh sure
i am mostly going there out of peer pressure
 
user228700
5:50 AM
From where? The Math room?
 
no i mean i know the conveners in real life
well two of them
 
user228700
Ohhh, I see. Nice!
 
user228700
@BalarkaSen It says "Class 11". I figured it had already happened.
 
when i applied i was in 11th
it's in 20th July
 
user228700
Oh, I see.
 
user228700
5:52 AM
What was the application process like? (Not that I plan on applying; just curious as to whether there was an exam etc.)
 
There was no exam. I just filled in stuff; of course you need to be a 1st year PhD student or something, or an undergrad with good qualifications, and you need a recommendation letter. I was taken in because the ones organizing the workshop know me (and indeed the ones who recommended me to go there)
 
user228700
Ohh, I see. That was nice of them to take u in. Have fun!
 
meh, i am not super enthusiastic about it. thanks.
 
user228700
Oh :-P OK...
 
6:17 AM
Watched the video. Entertainingly presented but rubbish.
@Kaumudi.H Well I've been through the books and I can't see anywhere where it says making a horcrux splits the soul in half. When Dumbledore and harry are talking about the diary Dumbledore uses the phrase a fragment of his soul.
 
@BalarkaSen how long does it last?
 
a week
 
7:18 AM
cool
will you have internet access? @BalarkaSen
 
Morning
Hm
Is there a conformal diagram of the Gödel metric
Usually they're reserved for causal spacetimes
 
7:56 AM
Hi all, is there any expert of AdS/CFT?
 
@ACuriousMind maybe
 
When one starts studying AdS/CFT, the coordinate $z$ of AdS in Poincarè coordinates is often identified with an energy scale for a CFT. I don't quite understand this identification... can anyone clarify this point? I wanted also to know If at each point $z$ we have a CFT at a different energy, for example z=0 CFT in UV and z=+\infinity a CFT in IR
 
8:19 AM
enjoying wimbledon @JohnRennie?
 
bs has invaded my fb feed :'(
 
@Kenshin I'm not much of a sports fan. I follow motor racing and rugby to some extent, but not tennis.
 
:'D
 
Ladies aren't all innocent, either
Superman needs a lead Burqah
"As far as higher education is concerned, there are too many people who shouldn't be there. It's not that most of them are even incapable but rather that their previous education was so substandard that they're going to get stuck on the basics or just barely keeping up instead of progressing further.
No where is this more obvious than in any field that requires some college level math without even considering proof-based mathematics. You can see the students struggling because of their poor grasp of this subject.
 
8:58 AM
Nope, this is an invalid model as otherwise we would have seen lorentz transformation violation in the propagation of light a long time ago
0
Q: Saleh Theory - real revolution?

Jirka KopřivaI found this article: http://www.saleh-theory.com/ Sounds good for me, nicely done. But i have not found other references nor mentions. Can it be taken seriously?

VTC as non mainstream
But suppose this guy did managed to somehow incoporate special relativity properly in his future articles, then the best he will end up is recovering some of the results of circularly polarised light, and he would then find his FTL proposal contradict with relativity unless it is tachyonic
Actually, that brings out an interesting tangent question:
Let the spacetime be flat. suppose there are two spacelike trajectories with different slopes. Now suppose we interpret them as valid observers, will causality be preserved between these two observers since neither trajectory can pass through the light speed barrier and become timelike?
NB Due to the runaway free associations, it took me a longer time to find a crackpot theory crackpot given the numerous free association will often find a sensible way to interpret otherwise word salad.

O and it is even worse if the crackpot is high level enough to ensure the formulae are dimensionally consistent and partially physically meaningful (even if it is the wrong way to put the physics together)
 
9:25 AM
@Yashas I weep when I read that kind of thing.
 
9:39 AM
@Slereah who are you quoting?
 
A dude
 
1
Q: Is there either a Lagrangian or a Hamiltonian formulation of electromagnetism with continuous distributions of magnetic monopoles?

tparkerMaxwell's equations generalize very nicely if we add in magnetic monopoles: we get $$\begin{align*} \partial_\mu F^{\mu \nu} &= J^\nu \\ \partial_\mu \tilde{F}^{\mu \nu} &= \tilde{J}^\nu, \end{align*}$$ where $F_{\mu \nu}$ is the electromagnetic tensor, $\tilde{F}_{\mu \nu} := \epsilon_{\mu \nu \...

This will be useful to my scifi
(since elementary monopoles are a major thing in my scifi)
 
9:55 AM
@DawoodibnKareem You do realize that so long as this answer is present and with upvotes, the question will not be removed by the automated cleanup systems?
Also, if OP chooses to accept it, you will then be unable to delete it even if you wanted.
I do not want to pressure you into deleting content, just keep those facts in mind.
 
@EmilioPisanty But if enough users vote to close and then delete the question, then the question will be deleted. And if there aren't enough users who want to close and delete the question, then I'm not sure why there's a problem.
@EmilioPisanty No, it sounds like you do want to pressure me into deleting content.
 
@DawoodibnKareem I want to suggest that you delete the content, but it is indeed your decision and if you want to keep it that's fine
 
I've been using Stack Exchange sites for a long time. The voting rules are there for a reason, and they work well. I don't think you should try to circumvent them by pressuring individual content authors.
 
@DawoodibnKareem Can closed questions be deleted even if answers are upvoted?
 
10:01 AM
@EmilioPisanty I believe they can, yes. Please correct me if I'm wrong.
Are you able to cast a delete vote on the question? If so, then do so.
 
Either way, community deletion works differently to how it does on SO simply because there isn't a critical mass of >10k users actively using delete votes (and actively patrolling the 10k tools for undue deletions)
@DawoodibnKareem and no, I do not at present have the option to vote-to-delete
 
OK, sure, I admit that the majority of my SE experience has been on SO. I'm prepared to accept that the rules are different here.
So is pressuring authors in chat rooms a legitimate thing to be doing? It seems skodey to me.
I'm reading the meta-post about non-mainstream physics. I'll get back to you when I've read it.
 
@DawoodibnKareem the rules are the same, but if you're basing decisions on an assumption of the form "if enough people think the question should be deleted then it will be deleted by community vote", then the community dynamics don't currently do that due to lack of numbers
@DawoodibnKareem I apologize for the tone on my initial message. It was information and a suggestion but nothing more.
 
OK, the meta post clearly says that answers to non-mainstream questions are not welcome, so I have deleted my answer as per your request.
I still feel that you have bullied me, but I guess we got to the right result in the end.
 
@DawoodibnKareem Again, apologies for the tone.
 
10:06 AM
Apology accepted, and no need to discuss this further.
 
@DawoodibnKareem a'ight =).
 
And while we're on non-mainstream, @ACuriousMind @DavidZ @Qmechanic @dmckee, would this be a good candidate for faq or at least [faq-proposed]?
9
Q: Is Physics SE an appropriate location for peer review?

GPhys What peer review style questions are acceptable to ask on Physics SE and what actions, if any, should be taken against unacceptable questions? When I originally encountered this question and its accompanying comments (note the question has since been edited), the asker appeared to be making ...

 
@EmilioPisanty You have to wait for 2 days until you can vote to delete closed questions, unless you are >20k and the question scores -3 or lower.
 
@ACuriousMind yeah, so I figured
 
10:11 AM
@AccidentalFourierTransform Any chance you're gonna tire of that joke any time soon? :P
 
@EmilioPisanty From my personal experience of how often I link it, yes, absolutely
 
@ACuriousMind I know extrapolation is a risky business but in this case the trend looks pretty clear to me
@ACuriousMind should I tag it faq-proposed myself?
the faq tag is mod-only
 
@ACuriousMind Ah, it was the -3 part I didn't know about. On Stack Overflow, almost all questions score -3 or lower.
 
@EmilioPisanty I'd wait and see if the lord of the tags Qmechanic has any objections
 
10:16 AM
@ACuriousMind sure
 
@DawoodibnKareem And people complain voting here is harsh... :P
(although I presume you meant "almost all closed questions")
 
Do they? I've found people are really lovely with their votes here.
And it seems to me that almost all questions do get closed on SO.
 
@ACuriousMind I complain about the happy-go-lucky do-gooder everyone-gets-a-lollipop voting culture on tex and mma =P
@DawoodibnKareem oh, they do
lemme dig around meta for a bit
 
Why does every book on distributions on manifold start with a long ass intro on distributions in $\Bbb R^n$
Get on with it Hormander
 
@DawoodibnKareem Yeah, see e.g. this
 
10:18 AM
well a naive guess, all manifolds are locally $\Bbb{R}^n$
 
-14
Q: The downvote option for questions should be removed?

Ajinkya NaikThis is not related to Physics obviously, but what I think is that the downvote option to questions on this website should be removed, as on asking a dumb question out of curiosity, the question is generally bombarded with dozens of downvotes which then decreases the reputation and kills the curi...

 
thus to get ditributions on manifolds, one must understood distributions on flat spaces
 
@ACuriousMind wow, I'm absolutely staggered. My opinion is that people upvote bad answers far too readily here.
 
though i do agree it gets boring once it is alreadty read a lot of times
 
I've seen high numbers of upvotes on answers that are just entirely incorrect.
I tend not to vote on questions so much, because I'm less aware of what the community considers a good question or a bad question. Answers are more clear cut.
 
10:19 AM
@DawoodibnKareem You and me both :P
 
I also don't see the point of the "accept" vote here. And I've said that about other SE sites too.
 
@ACuriousMind have you gone back to that query recently?
 
@DawoodibnKareem Yeah, "accept" carries much less usefulness here than on the sites where users have actual problems that need to be solved and then can mark the answer that actually solved the problem
 
> not counting the Community bot
not true anymore
 
Yeah, I know, the numbers are out of date (and I don't really like the trend my voting ratio is going)
 
10:23 AM
plus your monthly downvote rate has about doubled from that reported in the answer
 
@ACuriousMind Yup. The original querent is the one user who has admitted not knowing the answer. So why give them the ability to give 25 unicorn points to an answer, when everyone else can only give 10? If anything, they should get fewer unicorn points to distribute, not more.
It makes sense on Stack Overflow, and other sites that are about "how do I do X" type questions. But not on sites like this one that are about facts.
 
@ACuriousMind What, don't I get to say anything? ;-) (not that I really have anything to add, except that is fine for anybody to add)
 
@DavidZ Imagine how @rob must feel, @EmilioPisanty didn't ping him to begin with ;)
 
Fair point
 
@ACuriousMind imagine how @ManishEarth must feel, even you forgot to add him when complaining about mods getting left out
 
10:32 AM
Fair point
 
Hi chat
 
why is being a mod a permanent position here? shouldnt they be re-elected or something?
 
@LasVegasRaiders Viva Las Vegas
 
@AccidentalFourierTransform On the off chance you don't know the answer to that, here's the canonical mother meta thread.
 
:-D @DawoodibnKareem
 
10:40 AM
I was thinking that "hi chat" would be quite funny in a room for francophone catlovers.
 
Interesting thought pal
 
@ACuriousMind thx
Feature request: downvote bounties — Michael Mrozek Aug 9 '12 at 20:37
 
Negative bounties? So I can increase my reputation by awarding big negative scores to other people? Cool!
 
@DawoodibnKareem I think the comment means rather that one can downvote a bounty that's offered
 
10:44 AM
@ACuriousMind Oh, that would be less fun.
So someone can offer, say, +100, and I can devalue it to +98? Is that how it would work?
 
No idea, I'm guessing the author was just miffed about the description text of some bounty that was on that question at that time
 
Not enough people offer bounties. Perhaps I could post a meta question about that.
 
@DawoodibnKareem People like their fake internet points - if I had offered as many bounties as @EmilioPisanty, I'd still be behind him! (You're actually right, though)
 
@ACuriousMind as would @DavidZ, @dmckee and Rod Vance
and yes, I agree, not enough people offer bounties
 
only "Reward an existing answer" is useful IMO
 
10:50 AM
I reckon @JohnRennie sees plenty of bounty-worthy questions in his traversals over the site
@AccidentalFourierTransform I'm not sure
8
Q: The spin-orbit interaction for a classical magnetic dipole moving in an electric field

Emilio PisantySpin-orbit coupling is one component of the fine structure of atoms, which is explicitly concerned with the interaction of the electrons' spin with their orbital angular momentum. It can be explicitly derived from the Dirac equation by taking the nonrelativistic limit to subleading order in $1/c$...

 
@AccidentalFourierTransform I think "this question has not received enough attention" is also useful once
 
wouldn't have gotten an answer otherwise
 
Maybe I should start giving out bounties...
@EmilioPisanty Also that question that I answered... About those zilch things!
 
9
Q: Are non-zero-area pulses of electromagnetic radiation possible?

Emilio PisantyThis is a bit of a long-standing question / bone of contention that I've seen floating around, and which I would like to ask here in the hopes of getting some outside perspectives from a broader community. My neck of the woods centers around pulsed lasers: these can be long or short pulses, at...

 
@Danu You answered a question about zilch, that's...nothing!
 
10:52 AM
ditto
@Danu yep
 
@EmilioPisanty Increasingly I find that the questions to which I'd like to know the answer have already been answered, and the questions that are unanswered are of little interest to me. When I got to 200K I thought I'd start giving away surplus rep as bounties, but it hasn't worked that way.
 
@JohnRennie huh
fair enough
 
You use bounties as a way of encouraging participation (or at least it seems that way) and I thoroughly approve. But that takes quite a bit of effort locating and bountying worthy questions, and just recently I haven't had the time.
 
I occasionally go through my favourites list and offer some bounties on the questions I liked there, but the track record of my bounties actually getting answers is abysmal :P
Although...going through them it's not so bad
I guess my perception is skewed
 
I think the more experienced answerers are already pretty good at getting answers, either by Googling or here in the chat. So if the only motivation is that you'd like to know the answer that's likely to mean you'll only bounty the really hard questions.
Of course if you feel like bountying questions on colloid science or buying laptops ... :-)
 
11:05 AM
@JohnRennie yeah, but that's not the only motivation
"I think this is a good question (though it's not my area and I don't know the answer) and I think the site would benefit from having more like it"
"I think this is a good answer and I think the site would benefit from having more like it"
are my prime motivators
 
Agreed. That's an excellent use of bounties and the fact you do it so much is no doubt stacking up the karma points (do it enough and you'll be reincarnated as a mathematician :-).
 
@JohnRennie y u wish me such bad luck?
 
@EmilioPisanty :-)
 
11:31 AM
Can what I describe as wavefunction collapse always be described by by somebody else without invoking wavefunction collapse by instead considering a wavefunction that includes more of my environment?
 
wot
 
@GPhys Well, choose an interpretation without collapse and then even you can describe what you see without "collapse". I don't quite understand the question
 
@ACuriousMind The VLQ which was discussed yesterday still survived! :D
 
11:47 AM
@Yashas It seems to be going for an unusually controversial review, it's had 6 already.
 
@ACuriousMind I was thinking about a quantum zeno experiment
i can describe the effect by repeated measurement and wavefunction collapse
is describing it by a continuous interaction with my measurement device (e.g. a pulsed laser) an equally good description?
 
@GPhys No. If it was, there'd be no measurement problem now, would there?
 
@ACuriousMind ah, but this is why i meant to invoke somebody else who didn't know what I measured
what is the difference to him? if we did the same setup every morning, is the distribution of possible results the other person measures predicted the same by him supposing all of my possible measurements or alternatively supposing the continuous interaction before he measured?
is there a difference?
 
@GPhys Ah. Well, then it depends on whether your interpretation believes in objective collapse or not, but sure, as long as they don't interact with you, they can model you + the system you measure just as a unitarily evolving quantum system.
I mean, that's essentially just describing a measurement a la von Neumann, isn't it?
 
I spent quite a lot of time to come to the conclusion that these are probably equivalent descriptions
 
11:54 AM
You'll have to be precise what you mean by "equivalent" there
 
(I'm not so well read on the particulars of various measurement ontologies)
 
Everett's thesis is pretty nice on the measurement problem, btw
 
I mean, clearly the two descriptions are not equivalent: You have a collapsed state, they don't. However once they measure you + the system, their results will be the same as yours. How you explain exactly that they get the same result is a matter of interpretation.
 
@ACuriousMind by equivalent I mean there is no experimentally realizable difference between the description given by him supposing a wavefunction that collapsed many times (to states he does not know yet) before he showed up and a second description given by him supposing a wavefunction that continuously interacted with my measurement device
 
If an experiment has two results, then $| \text{Obs}\rangle \otimes | \psi \rangle$ has two evolutions
Either $| \text{Obs}_2\rangle \otimes | \psi_1 \rangle$ or $| \text{Obs}_2\rangle \otimes | \psi_2 \rangle$
 
11:59 AM
@GPhys Ah, yes, the outside observer's time evolution will of course be engineered to produce exactly the same probability distribution
 
Then you can "measure" the observer by asking about him
 
If I measure the quantum system, by von neumann, the result is the same as I become entangled with the quantum system. But when another party B then measure me (and the quanutm system) they two beome entangled with us and thus we have agreement in the outcomes we get from the quantum system. But how can there be three systems (me, the quanutm system and B) entangled together all at the same time, isn't monogamy prevent more than two subsystem to be maximally entangled?
 
@ACuriousMind This seems very not obvious to me in the case of e.g. the pulsed laser quantum Zeeman effect. i.e. where by making extremely fast measurements you slow down an exponential decay
when I calculate this in time dependent perturbation theory, expand for low t
it feels extremely tied to an actual measurement, but in the other point of view you couple the system with a pulsed laser
 
@GPhys Well, there's a general theory of measurement that says that for every measurement you do on a Hilbert space $H$, there is an apparatus Hilbert space $K$ and a time evolution $U$ such that on the combined space $H\otimes K$, applying $U$ and then measuring the apparatus yields the same result as measuring the system directly (where "measuring" means some generalization of projective measurements).
You can just carry out that procedure $n$ times in succession to model your case of $n$ repeated measurements
I mean, the pitfall here is that you really can't realistically get the evolution $U$
 
What are you talking about?
 
12:07 PM
You saying that it's just the laser and the system is incomplete, there must also be something "reading out" the laser, some kind of apparatus with a pointer state that records the outcome of the measurement (otherwise you don't get the Zeno effect, quantum eraser style)
 
only Jesus measures the state
 
@GPhys that depends on whether you're Wigner or his friend.
 
@EmilioPisanty Wow!
 
Is the combined system in $$H \otimes \underbrace{K \otimes \cdots \otimes K}_{\text{n times}}$$ that result will always be an entangled state?
 
@EmilioPisanty I wrote up almost this exact scenario when trying to reconcile my confusions this morning
or rather, last night
 
12:10 PM
@EmilioPisanty I always wondered why that's a "thought experiment", it seems perfectly feasibly to carry out in reality (just the measurable results won't tell you anything about the question that's being investigated). Is it hypothetical because Wigner didn't have friends? ;P
 
Wigner's a big old prick
 
@GPhys I think you don't suffer from "confusions", you just suffer from plain old measurement problem ;)
I'd prescribe a healthy dose of "shut up and calculate", but tastes vary.
 
"was the state of the system a superposition of "dead cat/sad friend" and "live cat/happy friend,""
 
@ACuriousMind it's a thought experiment because it's useless unless and until you can perform coherent operations on Wigner's friend to determine whether s/he is in a superposition state or not
 
Maybe it's hypothetical because all of his friends enjoy killing cats
 
12:11 PM
@ACuriousMind the motivation of all this is I was trying to calculate how possible it was to realize the quantum zeno effect for some particle decays when I realized I didn't understand what constituted a measurement in the sense that mattered
 
@ACuriousMind @GPhys yeah, that
 
How is having 3 subsystems (Wigner, friend and cat) maximally entangled possible, I thought monogamy only allows two subsystems to be maximally entangled at the same time?
 
@EmilioPisanty Well, but is that in principle possible? What sort of operation could I in principle - in thought - perform on the friend to determine that?
@Secret No one is talking about "maximal" entanglement, I don't know what you're talking about.
 
In physics you don't understand anything; you just get used to it.
 
@ACuriousMind my measurement device seemed clear enough, but then I thought the thermal environment of my sample must be measuring (again, in the sense that matters for the rate change observable by the a zeno effect)
 
12:14 PM
@GPhys What does "the environment must be measuring" mean?
Is that einselection?
 
@ACuriousMind it's easier to think of in terms of the cat
say you have a box that can turn your cat from |alive> to |alive>+|dead>
where obviously <alive|dead>=0
 
Well, Wigner friend experiment, in the mathematical formulation, the cat begins as a superposition of 0 and 1 states. Then Wigner friend came along and he then become happy or sad, thus we have the bell state 0/sad + 1/happy. Then Wigner came along and the resulting state is 0/sad/sad + 1/happy/happy, which is also a bell state. But if I recall monogamy correctly, if you have two maximally entangled subsystems and then it tries to entangle with the 3rd, then the resulting
3 subsystem system cannot be a maximally entangled state, thus it either has less entanglement shared among the 3 or the
 
@ACuriousMind to clarify I'm happy with it now, I'm just describing what led me to consider an alternate description of measurement in the first place
 
then if you feed that box an |alive>+|dead> cat, it will always make it |dead>
but if you feed it a |dead> cat
then it should prepare an |alive>-|dead> one
and if you feed it an |alive> - |dead> cat, then it will come back |alive> 100% of the time
 
@EmilioPisanty Never forget the $+\epsilon\lvert\text{angry monkey}\rangle$
4
 
12:18 PM
and if it hasn't gotten unsettling by that point, then you need to talk to a shrink
@ACuriousMind ah, yes, of course
 
@ACuriousMind is what I called a continuous interaction with my measurement device decoherence, basically?
 
@EmilioPisanty I was gonna ask if this has the primary purpose of making me feel like a mad scientist ;)
So, say I have such a box (let alone something that can preserve the superposed cat state for long enough). How does that help me resolve Wigner's friend?
 
Also: a bell state is always maximally entangled, so...
 
@ACuriousMind I'm not convinced of this right now
 
@GPhys No, decoherence includes interaction of the environment, the interaction with the measurement device is what a von Neumann measurement scheme would call "premeasurement"
 
12:25 PM
"Unclear what you're asking" should not be a close reason. All it means is "I don't understand your question, but somebody else might, so I want to prevent that somebody else from helping you".
2
 
@ACuriousMind you make a box that can produce your friend in a superposition of the relevant states, and you put your friend in the box multiple times
@DawoodibnKareem I disagree, but to each their own.
 
@GPhys Well, there has to be something doing "the measurement" - you're not just having a pulsed laser and the system and nothing else there and can still claim to be making "measurements", right?
 
11
Q: What is the "monogamy principle" in entanglement?

Jus12What does the "monogamy" principle imply? At a superficial level, it seems to say that a particle can be entangled with at most one other particle. However, I keep reading that several particles are entangled. For example: Quantum entanglement can reach into the past. Here they describe an ex...

ok that solves it, the 3 subsystems are not maximally entangled all at the same time in the GHZ state
only every possible pair in the system is
 
@ACuriousMind I think no measurements are exactly how the experimental realizations in the literature are saying they did it
let me grab the reference
 
@DawoodibnKareem To me, it can also mean "There are multiple possible ways to interpret this question, please narrow it down to what you really want to know so we don'T waste each other's time", "This is completely unintellegible", "You seem to already quote the answer in your question, so it's not clear what you want to know", "You throw around a lot of jargon and notation without explaining the context", and some more, so it's far from useless.
It's emphatically not meant to be used for "I don't know the area you're asking about, so I'll close your question because I don't know the answer"
 
12:31 PM
"your question is missing key bits of information, whose absence makes the question unanswerable"
 
@GPhys My point is that when youi want to describe a measurement by a continuous evolution, you typically want to include "the measurement apparatus" as a quantum object there. But "The apparatus" will not be something like just a laser beam, but also some sort of "pointer" (that's why the apparatus has "pointer states" in several approaches to such descriptions). Such systems are to me so complex that you really have no hope of describing their quantum dynamics from first principles.
 
@ACuriousMind oh I see what you mean
 
12:45 PM
@ACuriousMind what is a reference for this general measurement theory? Is this what you referred to as von Neumann measurement?
 
@GPhys I was thinking of Ozawa's work
 

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