Editing Talk:Particle (section)

== Revert ==
:''Moved from [[User talk:Headbomb]]''
[http://en.wikipedia.org/w/index.php?title=Particle&action=historysubmit&diff=415723380&oldid=415723088 In this revert] you removed some tags that indicate a number of the things written are unsourced.

[http://en.wikipedia.org/w/index.php?title=Particle&action=historysubmit&diff=415723380&oldid=415723088 In this revert] you removed some tags that indicate a number of the things written are unsourced.

Additionally, you may wish to research smooth particle hydrodynamics. The justification for particle number has nothing to do with the scale of stars compared to galaxies. The smallest dwarf galaxies have a very small number of stars and therefore the statement that you can consider a star to be a particle compared to the size of the galaxy alone is a wild overgeneralization.

[[User:IvoryMeerkat|IvoryMeerkat]] ([[User talk:IvoryMeerkat|talk]]) 18:08, 24 February 2011 (UTC)

:It is not. Stars can be considered particles whenever the dimensions of a star is negligible compared to the whole. The number of stars is completely irrelevant. <span style="font-variant:small-caps; whitespace:nowrap;">[[User:Headbomb|Headbomb]] {[[User talk:Headbomb|talk]] / [[Special:Contributions/Headbomb|contribs]] / [[WP:PHYS|physics]] / [[WP:WBOOKS|books]]}</span> 18:15, 24 February 2011 (UTC)
::So, since some galaxies exist where the dimensions of a star are not negligible compared to the whole (e.g. the first galaxies with Pop III stars) then the statement in the article is wrong. [[User:IvoryMeerkat|IvoryMeerkat]] ([[User talk:IvoryMeerkat|talk]]) 18:32, 24 February 2011 (UTC)
:::At worse the statement doesn't apply to a very very very small subset of galaxies at a very specific moment in time, and the article doesn't suggest that stars can always be considered particles, regardless of context. Most galaxies, even the small ones, are several thousands [[light-year]]s wide. The [[hypergiant|largest of stars]] ([[VY Canis Majoris]] for example) are a few thousands [[solar radius|solar radii]] wide a most. That's roughly 8 orders of magnitudes appart. <span style="font-variant:small-caps; whitespace:nowrap;">[[User:Headbomb|Headbomb]] {[[User talk:Headbomb|talk]] / [[Special:Contributions/Headbomb|contribs]] / [[WP:PHYS|physics]] / [[WP:WBOOKS|books]]}</span> 18:48, 24 February 2011 (UTC)
::::The cite you're using, however, is also not really good. SPH doesn't necessarily correspond to stars but more often to packets of gas. More than this, the discussion of stars acting as "particles" is normally attributed in astrophysics literature to the earliest discussion of galactic dynamics (e.g. Binney & Tremaine) where stars are treated as test particles in the potential regardless of whether they are actually that way or not. The problem is that the statements don't really capture the nuance and depth of what we mean when we treat a "star" as a "particle". It's an approximation that is due to observational convenience mostly and happens to be a good approximation when you look at the limiting regimes. In other words, the text is putting the cart before the horse. [[User:IvoryMeerkat|IvoryMeerkat]] ([[User talk:IvoryMeerkat|talk]]) 18:58, 24 February 2011 (UTC)
::::::A better reference could probably be found for that, I just gave it as an example. However, modeling galaxies as being made of particles (aka stars) is hardly putting the horse before the cart. You can even model the universe as being made of particles (galaxy clusters!), and this is indeed done routinely. <span style="font-variant:small-caps; whitespace:nowrap;">[[User:Headbomb|Headbomb]] {[[User talk:Headbomb|talk]] / [[Special:Contributions/Headbomb|contribs]] / [[WP:PHYS|physics]] / [[WP:WBOOKS|books]]}</span> 19:04, 24 February 2011 (UTC)
::::::::But no one models galaxies as being made of particles (aka stars). People start with a particle size that is usually set by the limitations of the computation itself. [[User:IvoryMeerkat|IvoryMeerkat]] ([[User talk:IvoryMeerkat|talk]]) 19:12, 24 February 2011 (UTC)
:::::::::It's done all the time! Haven't you heard of [[N-body simulation]]s before? <span style="font-variant:small-caps; whitespace:nowrap;">[[User:Headbomb|Headbomb]] {[[User talk:Headbomb|talk]] / [[Special:Contributions/Headbomb|contribs]] / [[WP:PHYS|physics]] / [[WP:WBOOKS|books]]}</span> 19:17, 24 February 2011 (UTC)

::::::::::IM is correct, last I checked. The bodies being simulated aren't stars - they're tracer points representing some fraction of the matter in the galaxy (normal or dark, usually tracked separately). You can certainly argue from first principles that stars are essentially particles in gravitational simulations (ditto planets), and I agree with that conclusion, but per [[WP:VNT]], the article should focus on uses of the term "particle" that appear widely in literature. --[[User:Christopher Thomas|Christopher Thomas]] ([[User talk:Christopher Thomas|talk]]) 19:22, 24 February 2011 (UTC)

:::::::::::Well the term is found in literature. I gave a specific citation for that [http://www.cita.utoronto.ca/~dubinski/nbody/] "Galaxies are modelled as a collection of gravitating particles that represent the stars and the mysterious dark matter.", but this is referencing the obvious at this point. <span style="font-variant:small-caps; whitespace:nowrap;">[[User:Headbomb|Headbomb]] {[[User talk:Headbomb|talk]] / [[Special:Contributions/Headbomb|contribs]] / [[WP:PHYS|physics]] / [[WP:WBOOKS|books]]}</span> 19:54, 24 February 2011 (UTC)
::::::::::::Dark matter particles are not stars. [[User:IvoryMeerkat|IvoryMeerkat]] ([[User talk:IvoryMeerkat|talk]]) 20:19, 24 February 2011 (UTC)

::::::::::::Per my initial comment, the "gravitating particles" do not correspond to individual physical objects within the galaxies being simulated. They represent some set fraction of the mass of that galaxy, and nothing more. Read the page you linked: collisions between the Milky Way and Andromeda galaxies was simulated by models containing hundreds of '''millions''' of particles, but the galaxies themselves contain hundreds of '''billions''' of stars ([[Milky Way]]) or more ([[Andromeda Galaxy]]). --[[User:Christopher Thomas|Christopher Thomas]] ([[User talk:Christopher Thomas|talk]]) 23:13, 24 February 2011 (UTC)

:::::::::::::Who said there was, or needs to be, a one to one correspondance? <span style="font-variant:small-caps; whitespace:nowrap;">[[User:Headbomb|Headbomb]] {[[User talk:Headbomb|talk]] / [[Special:Contributions/Headbomb|contribs]] / [[WP:PHYS|physics]] / [[WP:WBOOKS|books]]}</span> 03:20, 25 February 2011 (UTC)

::::::::::::::You did, earlier in this thread, when using that simulation as a citation backing up a "stars can be considered particles" statement. I agree that they can be, and I agree that in some n-body simulations, they are. However, you seem to be a bit hasty in your choice of references, and I think you've been more than a bit hasty when dismissing the various objections that have been raised in this thread and elsewhere.

::::::::::::::That said, this passed my "I'd rather be doing something productive" threshold several hours ago. All I can ask is that you at least consider some of the comments made by myself and others regarding your decision to create this page, as opposed to improving existing pages on this topic. --[[User:Christopher Thomas|Christopher Thomas]] ([[User talk:Christopher Thomas|talk]]) 05:21, 25 February 2011 (UTC)

:::::::::::::::Dont' know if you noticed, but there's a deletion discussion going on. If I'm hasty, it's because I don't have the luxury of time. <span style="font-variant:small-caps; whitespace:nowrap;">[[User:Headbomb|Headbomb]] {[[User talk:Headbomb|talk]] / [[Special:Contributions/Headbomb|contribs]] / [[WP:PHYS|physics]] / [[WP:WBOOKS|books]]}</span> 06:48, 25 February 2011 (UTC)

::::::::::::::::Deletion discussions stay open for a week before being closed. You have plenty of time; relax. If criticisms in the deletion discussion are sufficiently heavy that it looks like it would be deleted, even then there's no rush, as you can copy it to userspace, take your time bringing it to a state where you feel it would answer any reasonable criticism, and then ''convince people'' at [[WT:PHYS]] or elsewhere that the page should be reinstated.

::::::::::::::::The whole reason there's a deletion discussion at all is that the "convince people" stage wasn't fully completed. Enthusiasm is laudable, and you've made very valuable contributions across the board, but every so often there will still be situations where it's worth reconsidering your position. Goodness knows it's happened to me often enough (this AfD is arguably one such case, as it looks like I'm in the minority). --[[User:Christopher Thomas|Christopher Thomas]] ([[User talk:Christopher Thomas|talk]]) 07:30, 25 February 2011 (UTC)