Wikipedia talk:WikiProject Physics/Archive June 2010

How to get a 'lay person' review of an article

I am working on magnetic field and I am to the point that I can't do too much without some help from a non-technical person to review it. It has a clean-up tag that I want to remove but I want to make certain that it is actually is accessible to non-technical audiences first. Do we have a place to find people who can help with this? If not would it be possible to funnel some from the physics portal? TStein (talk) 03:22, 23 May 2010 (UTC)

How about trying WP:COPYEDITORS ? 70.29.210.155 (talk) 05:30, 23 May 2010 (UTC)

How "lay" do you want it? A person would have to understand more than elementary calculus (ie. simple differentiation) to get it at the moment. 70.29.210.155 (talk) 05:33, 23 May 2010 (UTC)

To get the whole of the article, yes, but it is possible and desirable to write it in a way that a layman can get the gist of it, and to be sure of that we have to have laymen read it. A. di M. (talk) 11:08, 23 May 2010 (UTC)

I think the rewrite is very good. The first part doesn't appear to be over-technical to me. In fact the general explanation (or overview) in the first part seems really clear to me. I reccomend looking up "magnetic field" in other encyclopedias such as Groilers online, or Britannica online, and see if you can get any ideas from there. Keep in mind the whole article doesn't have to be suited for the lay reader. Just move the most technical parts to the last part of the article. But, you probably already know this. Steve Quinn (formerly Ti-30X) (talk) 21:28, 23 May 2010 (UTC)

Thanks to everyone. If we still have peer review, I may put it up there soon. I am going through an old list that I have now to take care of a few more issues.

I am also debating about setting up a "Jargon Patrol" page if there is some demand for it and if we can find a way to make it useful both in funneling people in and in helping them navigate the mazes of WP. There are already too many half-baked ideas for recruiting and retention on WP I don't want to clutter things up even more by including my own, though. TStein (talk) 16:15, 24 May 2010 (UTC)

I overlooked you suggestion about funneling people from the physics portal for "review and comments", about articles, from general readers. This is certainly not a bad idea. I say if you can manage it somehow, go ahead and do it. Personally, I can't think how to do this - maybe put a note at the very top of the page with a link to the article, such as the one being discussed here. Of course then there needs to be some space accessible to the reader who is willing to comment. Maybe you could set up a dummy article with a section for comments in a bottom section. Just an idea or two. ----Steve Quinn (formerly Ti-30X) (talk) 17:12, 24 May 2010 (UTC)

This is an excellent idea. I wound up here because there was a link from the Wardenclyffe article suggesting that it could be split into two pieces. My opinion is that the sections on electricity should be much shorter and written for the general public. In other words, it needs a layperson's review. Perhaps finding a way to list articles in need of review? Kovar 17:53, 2 June 2010 (UTC) —Preceding unsigned comment added by Kovar (talk • contribs)

Weighty issues

You'd think that we'd have a decent physically correct article on weight. You know, the force that pushes DOWN on masses just enough to put them in free-fall, if they weren't supported by something (like the floor or a scale)? This is the NIST definition for weight, but we've had endless problems in the talk pages (TALK:weight and TALK:apparent weight) about the matter. The concept is very close to F = ma where "a" is the proper acceleration, which is (when multiplied by mass) the related force (in the opposite direction) necessary to put a mass, from free-fall or inertial motion, INTO the frame in which it is weighed (and in which it has a "weight"). These are counterforces. However, the article on apparent weight has gotten bogged down in gravitational issues, none of which are relevant, when the only thing it's good for as an article (if anything) is discussing why some scales give inaccurate returns for weight due to buoyancy issues. Your weight on the moon or in an elevator, is your real weight, not your "apparent weight."

Anyway, I invite all people who know their physics to weigh in on these talk pages, so we can get this mess cleaned up. SBHarris 23:05, 30 May 2010 (UTC)

There was an additional (brief) note about this page at Wikipedia talk:WikiProject Physics/Archive April 2010#Alert for Weight. --Christopher Thomas (talk) 01:09, 31 May 2010 (UTC)

The problem is that there are different definitions of the concept of weight: the "gravitational definition" F = mg, due to the 3rd CGPM and given in most physics textbooks, and the ISO/NIST definition, which (if I'm allowed to put it in simple terms) is tantamount to defining weight as what a spring scale measures, thus also taking fictional forces into account, such as the centrifugal force due to the rotation of the Earth (but excluding buoyancy). Several textbooks happen to call this "apparent weight". I would agree that the ISO/NIST definition is the more useful one, but it is not in some sense more "true" than any other definition, and in any case in Wikipedia the criterion is not truth but verifiability. (And if usefulness is the criterion, then defining weight as a scalar is obviously more useful; I've still to see an actual weight measurement recorded in a lab notebook, publication, or anywhere else, that also indicated the direction of the vector.)  --Lambiam 08:56, 31 May 2010 (UTC)

Doesn't general relativity say that the distinction between gravitational and fictional forces is unphysical, depending only on the choice of a frame of reference? AFAICT, g is generally taken to include the centrifugal force, as I reckon by looking the values in this table for cities near the equator. (As the centrifugal force is just 0.3% of the gravitational force at the equator and even smaller at higher latitudes, the difference is typically practically irrelevant.) Anyway, I seldom use the word weight unless what I exactly mean is either obvious or irrelevant (and when I do use it, I most often mean "mass"). A. di M. (talk) 13:29, 31 May 2010 (UTC)

As I see it, the issue is that the apparent weight of a person should be a measure of how heavy the person feels. SBHarris has argued that g-force already captures this concept and also that apparent weight is a term that is not used in physics. But the g-force apparently considers all non-gravitatonal forces as the same, as it considers the propper acceleration.

Clearly, to define apparent weight what you need to do is consider the stress tensor and integrate sigma(n-hat) over the boundary of an object. What you then get is due to all short range forces acting on the body which are transfered into the bulk via internal stresses caused by deformations due to the short range forces only acting on the boundary and not the bulk.

Now there is a clear issue with buoyancy if you were to consider the surface integral. For a person standing on the bottom of a swimming pool, you could consider the integral of sigma(n-hat) over only his feet. This amounts to only considering the normal force. So, you then want apparent weight to be a measure of how much weight your feet carry. Of course, this notion would break down if the person were accelerated in a spacecraft at 1000 g while floating in a heavy liquid; he would still be squashed.

So, my objection to the way the article is written now is that the crucial physics is omitted. The difference between long range forces that act uniformly per unit mass which do not give rise to internal stresses and short range forces which do, is not mentioned at all. This can't be brushed away by saying that you only need to consider proper acceleration, because gravity is not the only possible long range force. A counterexample is mentioned in the article: You can levitate frogs in a strong magnetic field. Count Iblis (talk) 14:48, 31 May 2010 (UTC)

But is there any evidence that people actually refer to the surface integral of sigma n-hat as apparent weight? I, for one, don't see the point of distinguishing contact forces from long-range forces: the levitating frog stands still because its weight is cancelled out by the force of the magnet, just as it would be cancelled out by the elastic force of a table, the only difference being the distance. (Consider one of those toys made of two cylindrical magnets -- both much denser than air so that you can neglect buoyancy -- the bottom one with a prong, and the top one with a hole. Put the hole in the prong: the top magnet will float if the polarity of its bottom face matches that of the top face of the bottom magnet, and it will stick to the bottom magnet otherwise; but in both cases the force you need to exert to support the toy by holding the bottom magnet will be about g times the total mass of the toy.) A. di M. (talk) 20:01, 31 May 2010 (UTC)

You do need to distinguish between gravity and other forces, because you want to be able to say that a floating astronaut in the space station is weightless, even though his/her weight is practically the same as on Earth. You then cannot capture this correctly by ignoring the difference between long range and short range forces. If you just say that the apparent weight of an object resting on the floor is the normal force exerted by the floor on the object, then that would imply that your weight on Earth would become less if I were to pull on your hair.

But making an exception for gravity alone is not good enough. If the space station were to be acclerated at 1 g, but the astronouts inside would be subject to a force that acts uniformly per unit mass equal to

m g in the direction of the acceleration, they would still be floating weighlessly inside the space station.

I dont know if people have defined apparent weigh as the surface integral of sigma(n-hat). I suspect not. When the subject matter is not of interest to theoretical physics, people tend to be focussed on practical situations and are not focussed on getting things rigorously correct from a theoretical point of view. But this then irritates people like me. Count Iblis (talk) 23:35, 31 May 2010 (UTC)

I understand your irritation, but your (or mine) opinion about what should be the definition of weight, apparent weight, etc. Is completely irrelevant to wikipedia, no matter how good our theoretical arguments are. Wikipeda is only concerned with how reliable sources define weight. The debate about the "correct" way to define weight has been ongoing with physics educators since the 1960's, there are basically two mainstream (non-compatible) definitions:

1. Conceptual/gravitational: The weight of an object is the force acting on it due to gravity. (W=mg) With various variants, and some disagreement about whether or not to include centrifugal force due to the Earth's rotation.
2. Operational: Weight is the quantity measure by weighing (with a bathroom scale for example). Again with more rigorous variants available.

The two definitions clearly disagree on issues like the weight of an object in free-fall, which is all well-documented. Proponents of the first definition sometimes use the term "apparent weight" to refer to the second, while proponents of the second definition refer to the first as the force of gravity or gravitational force.

The ISO has produced a third definition, which takes the middle ground, agreeing with both definitions depending on a choice of frame:

"The weight of a body in a specified reference system is that force which, when applied to the body, would give it an acceleration equal to the local acceleration of free fall in that reference system"

This definition agrees with the first, if the specified frame is the rest frame of the Earth (and can or cannot include a centrifugal contribution depending on whether the frame is co-rotating with the Earth). It agrees with the second definition if the specified frame, is the rest frame of the weighing apparatus (which usually also is the rest frame of the body). TimothyRias (talk) 08:18, 1 June 2010 (UTC)

Isn't the ISO definition the same as definition 1, as what g is depends on the frame of reference (per general relativity) anyway? In any event, that's the definition I had always implicitly assumed. A. di M. (talk) 14:12, 1 June 2010 (UTC)

BTW, I think the article weight should discuss all the definitions in common use (I would start from the ISO one and show how it reduces to the others for suitable choices of a reference frame), and so should apparent weight (but, judging at this, it appears to me that most often just means the literal meaning of the phrase, rather than any technical meaning). A. di M. (talk) 14:48, 1 June 2010 (UTC)

Yes, but definition 1 is usually expressed in an explicitly non-relativistic context. But, it (the ISO definition) can definitely be view as a straightforward general relativistic generalization of 1, which immediately includes 2 as well.

Also, currently the weight article start by discussing these various definitions. TimothyRias (talk) 15:16, 1 June 2010 (UTC)

An object in free-fall obeys the equation

${\frac {dp_{\mu }}{dt}}=\Gamma _{\mu \nu }^{\lambda }p_{\lambda }{\frac {dx^{\nu }}{dt}}\,$

where p is the momentum 4-vector and Γ is the Christoffel symbol (which is the gravitational force field). Any separation of the right hand side into a part which is weight and a part which is fictitious (inertial force) must be based on an arbitrary choice of a preferred reference frame where (by definition) there are no fictitious forces. JRSpriggs (talk) 17:59, 1 June 2010 (UTC)

Yes, I agree that we need to stick to the official definitions. However, Wikipedia physics articles are more than mere dictionaries. We do need to describe the relevant physics and explain things in detail. The example of floating frogs is mentioned in the article. But this needs to be explained better. Whatever the official definition of (apparent) weight is, the frogs are "effectively weightless" because of the non-contact nature of the magnetic force exerted on the frogs. Count Iblis (talk) 19:02, 1 June 2010 (UTC)

How so? The magnet still has to bear the weight of the frog, whether the distance between them is 1×10⁻¹⁰ metres, 1×10⁻⁵ metres, or 1×10⁻⁰ metres. A. di M. (talk) 15:31, 2 June 2010 (UTC)

Yes, but look at it from the frog's perspective. It will feel weightless because the force acts to a good approximation uniformly per unit mass throughout the bulk of the frog. Count Iblis (talk) 22:21, 2 June 2010 (UTC)

As illuminating as this is, we need to keep the conversation in one spot so that everyone sees the entire conversation. So please discuss this on the weight talk page. Thanks. TStein (talk) 16:51, 2 June 2010 (UTC)

Ok, but some things discussed here were not raised on the apparent weight talk page. But I agree that the discussions can be continued on the talk pages of the articles. Count Iblis (talk) 22:21, 2 June 2010 (UTC)

Yang-Mills problem solved?

Please see Talk:Yang–Mills existence and mass gap#Proof by Dynin?, to keep the discussion unfragmented. --bender235 (talk) 11:48, 3 June 2010 (UTC)

Hyperbolic coordinates

FYI, Hyperbolic coordinates has been nominated for deletion. 76.66.193.224 (talk) 03:55, 6 June 2010 (UTC)

Links to dark fluid

I've just noticed Bbbl67 (talk · contribs) adding links to Dark fluid (edit | talk | history | protect | delete | links | watch | logs | views) to a few of the cosmology articles (as an apparently good-faith attempt at orphan cleanup). Checking over the dark fluid article, it seems to be fringe work relying on a lot of "unpublished manuscript" reference links, created by a single-edit account (Suraj.kapil.singh (talk · contribs)), with cleanup edits by a handful of users since then. Could someone with expertise in the field please take a quick look at the article? My impression is that it isn't noteworthy enough to have links from other cosmology articles (and should probably wind up on AfD due to poor referencing), but I don't follow the field closely enough to be certain of that. --Christopher Thomas (talk) 20:28, 13 June 2010 (UTC)

Sounds like Original Research to me. JRSpriggs (talk) 21:02, 13 June 2010 (UTC)

There seems to be some activity in this field: [1]. But I cannot assess the quality of the article (certainly better, peer-reviewed references seem to be available). -- Crowsnest (talk) 22:42, 13 June 2010 (UTC)

Universe

FYI, there is a notice at WT:AST about an edit war at Universe , concerning its shape being a dodecahedron.

70.29.212.131 (talk) 04:55, 16 June 2010 (UTC)

I've read the arxiv paper that MC cited, and explained in detail why it doesn't make anything resembling as strong a case as he/she thinks it does. It and the other papers cited (and the fact that similar ideas have made the news on a few occasions) show only that the idea of a finite universe (with various proposed topologies) has been floated within the scientific community, and that people have looked for its signature in the microwave background. So far, nobody's made anything resembling a strong case for any such topology. If we're looking for citations for a statement that such searches have taken place, you can probably find better papers than MC's. --Christopher Thomas (talk) 06:19, 16 June 2010 (UTC)

Guidance about history section in physics articles

I am not asking for a general guideline, nor do I want to systematize everything, but I was wondering how people felt about history sections in physics technical articles. There seems to be three main options:

1. History first
2. History last
3. No history except link to separate article

I can see arguments for all of these which is why I am asking for your thoughts. I often prefer that there is no history, since it distracts from the subject matter and because history is messy with many dead ends and incorrect ideas. On the other hand, a well written and not too historically correct 'history' can be a good not too technical introduction to the subject. It can explain why something is important and why the quantity is called what it is called and it can deal with the inevitable historical artifacts with the quantity. Having it at the beginning means that technical details cannot be discussed, though, without losing most of you readers before they even get into the meat of the article. Any thoughts on this would be greatly appreciated. TStein (talk) 17:12, 14 June 2010 (UTC)

If the history provides motivation for the definitions, then it needs to be first. Otherwise, last. Include major steps forward. Leave out wrong turns, unless they represent errors which are still commonly made. JRSpriggs (talk) 06:33, 15 June 2010 (UTC)

Wherever it fits best, which needn't be section 1 or the last section before "See also". It is section 2 in Quark, section 8 in Sun and section 3 in Photon, and these are all top-priority featured articles among the 500 most viewed in WP:PHYS. A. di M. (talk) 07:31, 15 June 2010 (UTC)

Also, including a history section is often important to round out a subject. A lot of people look stuff up on wikipedia to figure out the who, when and where about a subject. For many physics topics this information is hard to find because most sources focus on the what and why, this makes history sections for physics article hard to write, but very important to include. TimothyRias (talk) 08:07, 15 June 2010 (UTC)

It may be interesting to note that the famously excellent 1911 Britannica had a consistent policy of history last. But I agree that a somewhat chronological sketch can be a good way to motivate a concept at the top of an article. There is no actual reason we can't have both -- a potted history at the top, and then a more detailed presentation at the end or elsewhere. This could be seen to parallel the "spiralling pyramid" that is sometimes used in our more technical articles, starting with a simple explanation and then growing increasingly technical, in the process sometimes revisiting in a more sophisticated or detailed way material that has already been sketched above. Jheald (talk) 09:18, 15 June 2010 (UTC)

It may be best to place the history last, unless it is the overall organizational structure of the article. In that case it is part of the text. I didn't think about it, until it was brought up here, but even in the first section just after the introduction seems to interrupt the flow. So, I think the actual history section should go in the last section. That is - unless there is a smooth flow into some other section. Steve Quinn (talk) 02:33, 17 June 2010 (UTC)

Personally, I find that history should generally be found towards the beginning of the article, especially in complex articles such as quark (per Jheald's "spiralling pyramid"), rather than towards the end. However, what is best should be decided mostly on a per-article basis, rather than in a one size fits all approach. Headbomb {talk / contribs / physics / books} 04:56, 17 June 2010 (UTC)

Unsourced additions to Friedmann equations etc. - more eyes needed

User Kentgen1 (talk · contribs · deleted contribs · logs · filter log · block user · block log) seems to have issues with the Friedmann equations, FLRW metric etc. and some strange ideas regarding the gravitational potential of black holes and dark matter. He has made unsourced and somewhat POV additions to deceleration parameter, dark energy, dark matter, Friedmann equations, Friedmann–Lemaître–Robertson–Walker metric and Big Bang. I have reverted the most dubious of these additions (and other editors have made some reversions too) but review from more expert eyes would be useful. Gandalf61 (talk) 14:28, 17 June 2010 (UTC)

Einstein summation convention

There's been some confusion about the use of abstract index notation in a number of articles. My knowledge of the situation is incomplete, but I do know that Einstein notation was developed to deal with certain non-orthogonal coordinate systems, with upper and lower indices representing contravariant and covariant components respectively. Repeated up/down subscripts imply summation. When the systems are orthogonal they become Cartesian, the covariant/contravariant distinction disappears, and all indices may be lowered. Penrose extended the concept to the abstract index notation. The indices appear the same, but now an indexed variable is considered to be free of any coordinate system. Repeated indices represent a particular operation which is also coordinate free (trace, divergence, etc.) but they do not now generally imply a simple summation. For a coordinate system that is locally orthogonal everywhere, the indices may again all be lowered. Some articles (e.g. linear elasticity imply that the indexed equations given are confined to Cartesian coordinate systems, thereby implying that they somehow have limited validity, which they do, assuming Einstein summation. However, the equations given are nevertheless valid for any orthogonal coordinate system, assuming that abstract index notation is being used. I would like to expand the usefulness of this and other articles by removing the Einstein summation note and making the same article more general by saying that abstract index notation is being used, and, in the case of all-lowered indices, is therefore valid for any locally orthogonal coordinate system. I just want to run this by everybody before going off on a campaign. PAR (talk) 14:28, 19 June 2010 (UTC)

In abstract index notation, you only get a physically meaningful scalar/vector/tensor (i.e. one that is independent of the co-ordinate system in which it is described) if a contraction is over one covariant (i.e. lower) index and one cotravariant (i.e. upper) index. Therefore, when the linear elasticity article produces the expression

$\sigma _{ji,j}+F_{i}=\rho \partial _{tt}u_{i}$

we know that this cannot be in abstract index notation (even though it uses the Einstein summation convention) because it is contracting over a pair of lower indices. And, indeed, the article says that this expression only holds in Cartesian co-ordinates. The correct expression of the equation

${\boldsymbol {\nabla }}\cdot {\boldsymbol {\sigma }}+\mathbf {F} =\rho ~{\ddot {\mathbf {u} }}$

in abstract index notation would be

${\sigma ^{ji}}_{,j}+F^{i}=\rho \partial _{tt}u^{i}$

- note that force, like displacement, is a contravariant vector. In this form the equation is independent of the co-ordinate system. Gandalf61 (talk) 16:17, 19 June 2010 (UTC)

Dark matter lede

There's an ongoing dispute about what the lede to Dark matter (edit | talk | history | protect | delete | links | watch | logs | views) should look like. More eyes would be helpful. The relevant thread is at talk:dark matter#Dubious lead sentence(s). --Christopher Thomas (talk) 19:10, 25 June 2010 (UTC)

Omega Point (Tipler)

Article appears to suggest that Tipler's theories are supported by mainstream physics rather than considered fringe. More eyes needed. - LuckyLouie (talk) 15:24, 26 June 2010 (UTC)

Logger9

Please comment on this edit of Transparent ceramics article at the Talk:Transparent ceramics. Thank you. Materialscientist (talk) 04:25, 28 June 2010 (UTC)

Michelson–Morley experiment - Cahill - Apeiron

Can we have some close looks at this addition? The basic source is Reg Cahill (See [2]), published in fringe journal Apeiron, which I think is a heavily unreliable source, specialized in proving "that the Ether really exists" and that "relativity is wrong". See also Tom Roberts' comments in this, and search for the string "Cahill" in PhysFAQ/Relativity/SR/experiments.html. DVdm (talk) 10:05, 18 June 2010 (UTC)

This addition is now quoted on Usenet where it is immediately refuted in two sentences. Don't we have a Tom Roberts here :-) ?

It looks like D.H. removed material. Thanks D.H. I guess it will not take long before someone will put it back though. DVdm (talk) 08:25, 28 June 2010 (UTC)

Black hole nominated for GA

I've nominated Black hole for GA. All citation needing statements have been referenced now. The article is not perfect, but feedback from a GA review might reveal some specific aspects to work on for FA. TimothyRias (talk) 09:33, 30 June 2010 (UTC)

Combustor A-class review

Hi all -- I'm over here by way of the Aviation WikiProject, and I have an open (since March) A-class review of the combustor article. I know it may not be strictly within your purview, but it's something you might be interested in. I am also looking for reviewers who aren't particularly familiar with the topic to take a look because it is a technical topic that I tried to write at an understandable level. Anyway, I would really appreciate it if anyone could take a look and leave some comments on the review page. Thanks! -SidewinderX (talk) 12:10, 30 June 2010 (UTC)