User talk:David s graff

Thanks for experimenting with Wikipedia. Your test worked, and has been reverted or removed. Please use the sandbox for any other tests you want to do. Take a look at the welcome page if you would like to learn more about contributing to our encyclopedia. Thanks. --Mel Etitis ( Μελ Ετητης ) 11:02, 16 March 2006 (UTC)


 * My message concerned an edit (a vandalism warning) that you made to User:Jagged 85; you probably meant to add it to his Talk page, but there was no indication as to what it was about. My apologies if I mistook a mistake for mischief. --Mel Etitis  ( Μελ Ετητης ) 23:29, 16 March 2006 (UTC)

Welcome
Others (usu. Admins) will tell you at the top of their talk page what they want. (e.g. Some will be glad if you only bottom post, others won't mind if you interject so a thread stays together. Varying the Indenting level on Interjections is good manners. A quick 'diff' from your watchlist will usually show whether the answer was to you, or not. It's easy to click an item to remove it when the time has expired. Experment.
 * I was just getting embarrassed by Mel, saw your query, and snooped. But then thought someone ought to say Hi, welcome to Wiki. They normally (must be running short handed) would have an admin post a mixed boilerplate plus some stuff the individual found useful in a message, so I'll spare you the clutter.  You might want to poke around my user page for some useful links, that's my quick file for things I'd rather not forget.  Snooping around is not considered impolite, but almost expected in Wikiculture. Took me some getting used to, but that's the way it is. Most people expect a response in the same place the original message was posted, so if you leave a question, the answer may only appear where you asked... So add such talks to your watchlist (clickbox), and check it regularly and frequently after leaving a question.
 * But again, feel free to snoop most anywhere. Try email if you think things should be private, but be advised that many people leave it blocked. Wiki-ideals are that everything should be out in the open, so thats' the way the old-timers work all the time.
 * Last, spend some time hoovering in WP:AFD, WP:RFC, and WP:PR and thouroughly browse the 'HELP' link to get a sense of the culture and it's tools. Hope this helps. If you mention you're new, you should be fine. (The only dumb question is the one unasked)
 * Fra nkB 20:56, 17 March 2006 (UTC)

Two more Tips

 * Just two more thoughts.
 * Try to make it a habit to check an articles history to see how actively it is being edited by others BEFORE clicking and edit button, even if it's a section edit. If it hasn't changed much recently, you won't be stepping into the middle of controversy or an edit war (which I advise you to give a wide bearth!)
 * Always check the talk page and it's history tab as well. Whatever you spotted may be embroiled in controversy... etc., or someone may have left a note about the deficiency and that they plan on handling it, or whatever. If things are quiet, edit away.
 * Here's some boilerplate greetings: Standard_user_greeting (results follow) Fra nkB 21:13, 17 March 2006 (UTC)

Welcome!

Hello,, and welcome to Wikipedia! Thank you for your contributions. I hope you like the place and decide to stay. Here are a few good links for newcomers: I hope you enjoy editing here and being a Wikipedian! Please sign your name on talk pages using four tildes (~&#126;); this will automatically produce your name and the date. If you need help, check out Questions, ask me on my talk page, or place  on your talk page and someone will show up shortly to answer your questions. Again, welcome! Best wishes!Fra nkB 21:13, 17 March 2006 (UTC)
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Chinatown Edits
In reply to the message you sent me in User_talk:Jagged_85, I think you've got the wrong person. If you check the edits before mine, you'll notice that the vandalism was already there, though I didn't notice it myself earlier. I've already given a clear edit summary for each edit I made in that article, which you can see for yourself in the Chinatown history page. If you need to find the real vandal, you'll need to look back even further. Jagged 85 07:35, 18 March 2006 (UTC)


 * templates substituted by a bot as per Template substitution Pegasusbot 22:15, 25 March 2006 (UTC)

(IDF) Purity of Arms
I took up your early August comment to cite the instance of the IDF's use of ground troops in southern Lebanon during the 2006 Israel-Lebanon Conflict. Having been a noncombattant in the latter, I'm perhaps a little too close to the topic to be entirely objective, nor do I have time at present to search and provide the supportive references required to add this material to the article. So I noted my point on the Discussion page and would be interested in your response. -- Thanks, Deborahjay 14:55, 9 March 2007 (UTC)

History of Iran
Hi, You've put this comment last year in the talk page of the article. I tried to improve it. Would you please check it now? Thanks a lot. -- Sa.vakilian(t-c) 05:40, 27 June 2007 (UTC)

Thanks for your changes. I worked on this article again today. Please review my changes and let me know what you think. David s graff 17:33, 21 August 2007 (UTC)

Lost you
I think,you are the one,i met in berlin some years ago, we wrote letters to eachother.At that time,i lived in romania,i hope you can remember.we visited berlin,we were 9 children,we slept at a church.now i live in germany,and tried manny times to get into contact with you,but i didn-t mannage that.now,if you remember me,you told me edy,than,please write me a short mail,my adress is *****@yahoo.de. thank you,and i hope,you are the david i know. —Preceding unsigned comment added by 172.173.209.36 (talk) 18:19, 12 October 2007 (UTC)

Re:Islamicization of Persia
Salam David, Thanks for your comment. I'm too busy and can't be active wikipedia.-- Seyyed(t-c) 05:02, 1 July 2009 (UTC)

Paracel islands
Thank you for your reply. I understand your good faith and I want to add that I don't use my biases or personal opinions in writing wikipedia. I would agree that adding chinese and Vietnamese names for every time the islands are mentioned makes the article longer than neccessary. I did that because there is one guy who only added chinese names, and it was also very nice of him to ask me to add the Vietnamese names. We can eliminate those chinese and Vietnamese names, only use English names outside of the table, which means the table with English, chinese, Vietnamese names should be kept. I also notice that the English names you got from CIA map, some of which are originated from Vietnamese names, some are from Chinese names. Finally, the neutrality of wikipedia cannot ignore the fact that China seized the Paracels from Vietnam in 1974, which you may have heard of that. Any dispute is just a tactic used to justify China's claim after everything is in their hands. Regards! Trananh1980 (talk) 08:41, 20 March 2011 (UTC)

Susanna Cox and Reading
Hi, David s graff. Question about this edit: by "20,000 viewers" do you mean the number of people attending her trial or viewing her execution or what? I think that should probably be clarified, since "viewers" usually means something rather different in the present day. Rivertorch (talk) 10:55, 12 January 2013 (UTC)

Sorry for not being clear, 20,000 people came to the execution, pretty remarkable since, at the time, the population of Reading was about 3,000. See e.g. http://news.google.com/newspapers?id=jIkxAAAAIBAJ&sjid=jKYFAAAAIBAJ&pg=4103%2C9915630 David s graff (talk) 19:56, 12 January 2013 (UTC)


 * Fascinating stuff. Thanks for clarifying! Rivertorch (talk) 22:42, 12 January 2013 (UTC)

on spin
i was reading thru the spin article, and i found an unfortunate confusion that you appear to have added in the article.

i note also on your talk page that you claim to be a physicist -- tho it's difficult for me to believe how you might have made such a fundamental error if that claim were true -- and i understand that those who refer to themselves as "wikipaedians" take poorly to others' edits of their additions, so upon discovering who had made the error, i thought to preempt any attempt to re-insert your error.

that quantity known as "spin" is definitely not a spinor.

(in any case, in the abstract and purest sense, even a spinor is a vector as a member of a vector space (a hilbert space) carrying a representation of a covering group of the special orthogonal group. in fact, the vector space that spinors inhabit must represent a DOUBLE cover of SO(3), which is precisely whence spinors get their behaviour of sign reversal upon being subjected to 360-degree spatial rotation: the "double cover" means that there is a 2:1 surjective homomorphism ("2 spinors per vector"), so the first rotation by 360 degrees maps the element of the spinor space into the same elemet of the vector space that the second 360-degree rotation does.  for strictly geometric purposes, spinors are not tensors (vectors are tensors of the first rank), so in the geometric picture, spinors are often dissociated from vectors.  but in the algebraic picture, they are certainly vectors, which is why we speak of the state vector of a system even when referring to spin-1/2 spin states...tho this is a mathematical digression.)

i'm afraid to tell you that you have completely misread whatever you thought tomonaga was telling you; surely such an eminent theorist would not so easily have led a reasonably informed physicist astray, so perhaps you can understand my scepsis! what a spinor is used for in terms of quantum mechanical spin is to represent the eigenstate of a spin-1/2-carrying quantum state. now, if you were confused by tomonaga's book for the layman, i may not be the right person to distill the concept any more clearly, but i'll try my best: the spinor, while carrying information on the possible results of a spin measurement, is not spin itself -- it merely represents of the spin state of a system (e.g. electron) that has the property of spin of magnitude ħ/2.

spin itself is an observable and therefore represented in the quantum formalism as something called an "hermitian operator" and in fact is a vector! more precisely, it is an axial vector ("pseudovector"), like all angular momenta, and therefore changes sign under parity transformations, but this distinction is rather trivial. the eigenvalues (the spin measurements) that result upon the application of the spin operators to a spin eigenstate are the +1/2 and -1/2 is the value of the spin state, along the axis of measurement, into which the measurement has "collapsed" the wavefunction. in essence, we premultiply a spinor by a spin operator (i.e., a scaled pauli matrix) to represent the physical act of measuring the spin of a spin-1/2 particle.

the formalism looks something like this:

$$\hat{S}_z |\chi\rangle = \pm{\hbar\over 2} |\pm\rangle$$

where we have applied the spin-z operator to a random spin state χ, returning the value of the measurement (plus or minus ħ/2) and "collapsing" the state χ into an eigenstate of the spin-z operator, here labelled either spin up (+) or spin-down (-).

i'm not sure if this helps at all. i'm notable for my poor explanations. ._.

the end result is that the spin itself is a vector...an axial vector to be sure, but all angular momenta are axial vectors, so this subtlety should not present additional confusion. however, to call spin itself a "spinor" is an error: it is the spin wavefunction -- the particle's state -- and not the spin itself, which is represented by the spinor.

i hope you will not take this last exhortation with offence; i only mean to improve both wikipaedia and your own contributions here. i'm not sure why you are calling yourself a physicist on here, and while i certainly appreciate your interest in physical theory, it's probably advisable to avoid editing physics pages unless you are entirely certain that your edits are correct! i sincerely do appreciate the obvious interest in even relatively obscure laymen's texts like the tomonaga book, which i had never heard of before. (by the way, you may want to return to the article and remove the reference citation itself. i don't know how to do that.)  however, they say about wikipaedia that all the pages are wrong, because experts don't go to the pages in their areas of expertise, and they can't identify errors in others! in fact, it's only by luck that i happened to notice your error this evening; it had been present for over a year!

to ensure that this project maintains an encyclopaedic level of accuracy, it's best to only edit when certain. after all, being interested in physics as you are, i am sure there are times that you rely on this resource for high-quality information: surely you would prefer the information be accurate! no offence meant and sorry to leave such a long message, but i wanted to ensure you knew it was done in good faith; i understand that registered editors can become quite defencive about their contributions being edited away. o.o 72.179.38.56 (talk) 07:19, 17 April 2014 (UTC)


 * Hi, whoever you are,


 * First off, I think that you should sign up for a Wikipedia account, even a pseudonymous one. Its difficult having a discussion with an anonymous edit only labelled with a URL.


 * Despite what you say, I am indeed a physicist. It shouldn't be too hard to guess my real name from my Wikipedia handle and a little googling will show that I hold a PhD in Physics from the University of Michigan, and have more than 50 publications in journals such as PRL, ApJ, and A&A.  Of course, there are many different types of physicists with different specialities.  I've worked in more different fields than most, mostly in Astronomy, especially in Gravitational Microlensing, most recently in non-linear ultrasound and non-linear diffusion, but have not had to work with quantum spin in my professional career since I took a graduate Quantum Mechanics class over 20 years ago.  If you look through my Wikipedia contributions, you will find that I have made many contributions to some areas where I am (or was at the time of writing) a world-class expert, such as Gravitational Microlensing, but also, I think, useful contributions into areas where I have no particular expertise.  I think that the spirit of Wikipedia is that amateurs can often make useful contributions.  In particular, an amateur may have a better understanding of what non-expert readers will understand.


 * But you can't deny that Tomonaga had in his book a chapter called "The quantity that is neither vector nor tensor" What quantity was he referring to?  There is a link to this book at http://books.google.com/books/about/The_Story_of_Spin.html?id=he9sANipCj8C and you can search the chapter there at http://books.google.com/books?id=he9sANipCj8C&pg=PA113&source=gbs_toc_r&cad=3#v=onepage&q&f=false .  If you look through this chapter, you will see that it is not designed for laypeople, but is a technical lecture for specialists.


 * I submit that what I wrote is not really different from what you wrote, though I was trying to distill Tomonaga's argument for a lay audience and might have used a different formalism. You wrote, "so in the geometric picture, spinors are often dissociated from vectors.  but in the algebraic picture, they are certainly vectors".  I wrote, "However, in a technical sense, spins are not strictly vectors, and they are instead described as a related quantity: a spinor.  In particular, unlike a Euclidean vector, a spin when rotated by 360 degrees can have its sign reversed."  By referring to "rotation", I was referring to the "geometric picture" that you mentioned.  By referring to "related quantity", I was referring to the many ways that spinors are like vectors, in particular the algebraic relations that you referred to, and by "technical sense", I was trying to indicate that the differences are subtle and only important to specialists.


 * Here is Tomonaga, p. 118 available at the Google Books site: "According to this definition, we can consider a vector as a quantity that transforms A(1) = A, corresponding to the rotation A of the coordinate system..." It was this definition that I was trying to relate by "rotation", and I think that you were trying to relate by "geometric picture".  I think that you will agree that under this picture spin 1/2 particles do not behave like vector fields.


 * I think that this is the reason why spin 0 fields are often referred to as "scalar fields" and spin 1 fields like photons as "vector fields" and spin 2 fields like gravity as "tensor fields".


 * I think that we can try to distinguish between the rotational properties of the field and the rotational properties of the observables. The - sign introduced by rotation of spin 1/2 fields is of course only a phase and vanishes when we make an observation.


 * I think we both agree with the introduction to the article: "In some ways spin is like a vector quantity". This language implies that in *some* ways spin is *not* like a vector quantity, otherwise, we should write "In *all* was, spin is like a vector quantity" or, simpler, "spin *is* a vector quantity".  I think that if we want to say that in some ways spin is like a vector, we should also say in what ways spin is not like a vector.  That is all I was trying to do.


 * The article does actually say in what ways spin is not like a vector. Later on, it says "Mathematically, quantum mechanical spin states are described by vector-like objects known as spinors. There are subtle differences between the behavior of spinors and vectors under coordinate rotations. For example, rotating a spin-1/2 particle by 360 degrees does not bring it back to the same quantum state, but to the state with the opposite quantum phase; this is detectable, in principle, with interference experiments."  I think that my sentence "However, in a technical sense, spins are not strictly vectors, and they are instead described as a related quantity: a spinor.  In particular, unlike a Euclidean vector, a spin when rotated by 360 degrees can have its sign reversed."  is completely consistent with this.  Instead of saying that "spinor" is vector-like, I say it is a "related quantity" -- means the same thing to me.  Instead of saying "Mathematically, quantum mechanical spin states", I say "In a technical sense".  And instead of saying "there are subtle differences under coordinate rotations", I specify what the difference is: the sign flip of the complex phase.  And instead of just stating the fact, I provide a reference, which I think if you read it, you will see is an appropriate reference written for a technical audience by an obvious expert.


 * I think that the only problem is when I wrote "have its sign reversed". This is poorly written I agree.  I should have written "have the sign of its quantum wave function reversed".  It does look as if I was implying that a spin up particle could become spin down after a 360 degree rotation.  Obviously, I know thats not true.  I'm going to undo your edit, but fix what I wrote to reflect this. David s graff (talk) 15:29, 18 April 2014 (UTC)


 * I think the IP's tone is unfortunate – there is no need to attack your credentials or contributions, and I believe doing so goes against WP policy. However (though I'm no expert myself), the point that spin and spin state should not be confused seems to be valid: the article's "spin states are described by vector-like objects known as spinors" and your "spins are not strictly vectors" are referring to entirely different things, whereas above you imply that they are the same by your juxtaposition of the statements. This edit apprears to retain this confusion. I suspect that the statement "In some ways spin is like a vector quantity" should be reworded to "Spin is a vector quantity". There are other potentially confusing subtleties: "a spin-n field" evidently refers to a spinor field, not to a spin field. —Quondum 03:55, 19 April 2014 (UTC)


 * Hi Quondum, I moved this discussion to the spin talk page where I took the liberty of quoting you. Can you look at that page and respond? David s graff (talk) 22:53, 20 April 2014 (UTC)


 * I do not see this at Talk:Spin (physics), which is where I assume you are referring to. Perhaps your edit didn't get saved? You're welcome to quote me there. —Quondum 02:25, 21 April 2014 (UTC)


 * Yeah, I had problems saving and had to put my kids to bed before I could finish. Anyway, I'm still confused by this.  I don't understand the difference between spin and spin state.  Doesn't that reflect classical thinking about spin?


 * I'm going back and rereading Feynman Lectures about spin. In V III ch 5, he derives how spin 1 systems transform as classical vectors and emphasises that this reflects how spin 1 particles like Photons are referred to as "Vector Fields".  I always thought that this is why the E&M field is a vector field, and the tensor Gravity field would be carried by spin 2 Gravitons.  In contrast, if we are going to confine "Vector" to spin 1, then spin 1/2 particles must not be vector particles. David s graff (talk) 02:46, 21 April 2014 (UTC)


 * Somewhat confusing, I know. Strictly speaking, the EM field is an order-2 antisymmetric tensor field (and transforms accordingly), not a vector field. If one were to derive a spin field from the EM field, one would end up with a field (probably representing "angular momentum density" in the classical case), which in some sense is the "square" of the EM field. A similar "squaring" allows one to derive the EM stress–energy tensor, which similarly is not the original field, but a derived quantity. Thus, although they can be thought of as interdependent facets of the same field, they are distinct in terms of what the fields describe, emphasized by the different units. I'll repeat my disclaimer that I'm no expert, but the need to distinguish does come through quite clearly to me. On second thoughts, since angular momentum is a bivector and not a vector quantity, I expect spin itself would also be a bivector rather than a vector field. Only in 3 dimensions can one associate a vector with angular momentum (via the Hodge dual). In spacetime, there may be a trick for translating this into a vector field, but how this would transform is beyond me. —Quondum 03:44, 21 April 2014 (UTC)

I know that the EM field is described by the stress energy tensor, but there is an equivalent description as a 4-vector vector potential. Thats what I meant when I was describing the E&M field as a vector field. But you are right that if we want to play that game with spin, we need to make a space-time description of angular momentum. I guess that that would lead to the Dirac equation and the four gamma matrix version of spin.

Photons do carry angular momentum, and so the E&M field can carry angular momentum, even classically, e.g., as circularly-polarized light. So I think that it is perfectly appropriate to talk about the angular momentum density of an E&M field without having to put it in quotes. I'm not sure that thinking about angular momentum as a bi-vector is really helpful. When studied classically, we think of angular momentum as a derived quantity, as x × p. But I think that the whole point of spin is that angular momentum really is a fundamental quantum phenomenon with its own set of transformations. And of course, Wikipedia is our source for how to find out how angular momentum behaves relativistically, "as an antisymmetric second order tensor" Angular_momentum.

The more I think about this, the more I think I was right with my edit. There are some relations between spin and the classical vector angular momentum. But there are technical defferences which should not really be discussed in detail in the introduction to the spin article. Rather, we should just mention that there are differences, discuss them more fully in the body of the article (which we do) and link to pages and external references that go into more detail for people who want to delve into the differences (which I do with my link to spinor and to Tomonaga's article). Since even non-quantum angular momentum stops behaving like a vector relativistically, I think its sufficient to just warn the reader that they should not accept the vector representation of spin as more than a useful metaphor in some instances, and warn them of possible complexities with links.

I'm totally confused by what you and the IP commentor mean by "Spin Itself". I think we all completely understand how spin transforms under rotation (spin 1/2 by Pauli matrices classically, by gamma matrices relativistically, spin 1 as a Vector, e.g., by standard rotation matrices). I think that has to be sufficient.


 * We're probably tripping over each other's language. When you say "I know that the EM field is described by the stress energy tensor", one might be tempted to infer that its stress–energy tensor gives a complete description of the EM filed, which it doesn't. Similarly, when you say "there is an equivalent description as a 4-vector vector potential", it is true that we can determine the EM field (as well as the current 4-vector) from this vector potential, we do not call this vector field the EM field. It is precisely this type of distinction that is being drawn between the spin and the spinor.
 * I put angular momentum density in quotes because there are subtleties I'll trip myself up on, such as distinctions between intrinsic and orbital angular momentum, and how these depend upon the choice of origin, even in the classical case; I was not trying to say that it is not real angular momentum distributed over the field. Though I would be interested to see how one can derive an angular momentum of a classical circularly polarized plane-wave EM field; I had expected to find some, but the 3-d Poynting vector appears to give a uniform momentum in the direction of propagation only. This is something that I have yet to fathom, I guess.  An electrically charged stationary magnet does carry angular momentum though, much like an electron's EM field carries angular momentum (for which reason I like to think of the spin of the electron as residing in its EM field, not in the point particle itself).
 * Back to the original point: spin is essentially (intrinsic) angular momentum, and it transforms into itself under a 360° rotation. Flip an electron right around, and it again has the same angular momentum, but its spinor wave function is negated.  So, when I say the "spin itself", I mean the intrinsic angular momentum, measured in units of ħ.  Spin is a property of the wavefunction, but is not the wavefunction. The two transform differently under rotation. And while you can represent (nonrelativistic) spin by a vector, you cannot represent the electron spinor wavefunction by a vector. —Quondum 00:58, 22 April 2014 (UTC)

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