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	<title>Comments on: N Bodies</title>
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	<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/</link>
	<description>Random samplings from a universe of ideas.</description>
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		<title>By: Napka&#8217;s Top Science &#187; The N Body Problem</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18994</link>
		<dc:creator>Napka&#8217;s Top Science &#187; The N Body Problem</dc:creator>
		<pubDate>Fri, 16 Nov 2007 15:41:18 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18994</guid>
		<description>[...] read more &#124; digg story [...]</description>
		<content:encoded><![CDATA[<p>[...] read more | digg story [...]</p>
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		<title>By: General Sciences &#187; The N Body Problem</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18969</link>
		<dc:creator>General Sciences &#187; The N Body Problem</dc:creator>
		<pubDate>Wed, 16 Aug 2006 13:45:44 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18969</guid>
		<description>[...] The N-body problem is one of the most famous, and easily stated, problems in mathematical physics: find exact solutions to point masses moving under their mutual Newtonian gravitational forces (i.e. the inverse-square law). But let N=3 and chaos breaks loose, quite literally.read more&#160;&#124;&#160;digg story [...]</description>
		<content:encoded><![CDATA[<p>[...] The N-body problem is one of the most famous, and easily stated, problems in mathematical physics: find exact solutions to point masses moving under their mutual Newtonian gravitational forces (i.e. the inverse-square law). But let N=3 and chaos breaks loose, quite literally.read more&nbsp;|&nbsp;digg story [...]</p>
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		<title>By: Dynamics of Cats</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18965</link>
		<dc:creator>Dynamics of Cats</dc:creator>
		<pubDate>Wed, 26 Jul 2006 15:46:35 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18965</guid>
		<description>&lt;strong&gt;Experimental Astrophysics&lt;/strong&gt;

 Doug Hamilton as a nifty Astronomy Workshop web page, with lots of fun little tools....</description>
		<content:encoded><![CDATA[<p><strong>Experimental Astrophysics</strong></p>
<p> Doug Hamilton as a nifty Astronomy Workshop web page, with lots of fun little tools&#8230;.</p>
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		<title>By: Tony</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18968</link>
		<dc:creator>Tony</dc:creator>
		<pubDate>Wed, 26 Jul 2006 08:49:41 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18968</guid>
		<description>bobocopy:

I can guess why anyone with limited time might not want to risk editing Wikipedia. The finer the sculpture, the more likely it is to have its nose knocked off by drunken 16-year-old vandals. The &quot;no original research&quot; policy is infinitely regressive. There&#039;s no mechanism to identify authority whatsoever.

You might consider adding a taunting footnote, in hopes that a seasoned warrior will see it and brave the gauntlet.</description>
		<content:encoded><![CDATA[<p>bobocopy:</p>
<p>I can guess why anyone with limited time might not want to risk editing Wikipedia. The finer the sculpture, the more likely it is to have its nose knocked off by drunken 16-year-old vandals. The &#8220;no original research&#8221; policy is infinitely regressive. There&#8217;s no mechanism to identify authority whatsoever.</p>
<p>You might consider adding a taunting footnote, in hopes that a seasoned warrior will see it and brave the gauntlet.</p>
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		<title>By: Jawad Shuaib</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18966</link>
		<dc:creator>Jawad Shuaib</dc:creator>
		<pubDate>Tue, 25 Jul 2006 16:50:52 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18966</guid>
		<description>owww this is amazing. Though, a lil&#039; bit of mathematics would&#039;ve been appreciated. But beautiful work nonetheless, thank you!</description>
		<content:encoded><![CDATA[<p>owww this is amazing. Though, a lil&#8217; bit of mathematics would&#8217;ve been appreciated. But beautiful work nonetheless, thank you!</p>
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		<title>By: bobocopy</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18967</link>
		<dc:creator>bobocopy</dc:creator>
		<pubDate>Tue, 25 Jul 2006 13:56:19 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18967</guid>
		<description>You mention that, &quot;The Wikipedia entry on PoincarÃ© tells a much less interesting, and less accurate, version of the story.&quot;  May I ask why if the story is so woefully inadequate 1) you link to it and 2) you have yet to fix the article?</description>
		<content:encoded><![CDATA[<p>You mention that, &#8220;The Wikipedia entry on PoincarÃ© tells a much less interesting, and less accurate, version of the story.&#8221;  May I ask why if the story is so woefully inadequate 1) you link to it and 2) you have yet to fix the article?</p>
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		<title>By: Steinn Sigurdsson</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18992</link>
		<dc:creator>Steinn Sigurdsson</dc:creator>
		<pubDate>Mon, 24 Jul 2006 15:24:42 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18992</guid>
		<description>There has been some exploration of the N-body problem for potentials other than the inverse square law - a 1/r^D-1 generlization follows from extending Gauss&#039;s law to extra dimensions (standard grad problem is to consider &quot;the problem&quot; in arbitary dimensions).
Most orbits are unstable in D != 3, so the problem is not as interesting is the short answer. The long answer is that the problem has not been studied as hard.

Not a lot of people know that there is an exact perturbation expansion solution to the three body problem, but it has rather poor convergence...</description>
		<content:encoded><![CDATA[<p>There has been some exploration of the N-body problem for potentials other than the inverse square law &#8211; a 1/r^D-1 generlization follows from extending Gauss&#8217;s law to extra dimensions (standard grad problem is to consider &#8220;the problem&#8221; in arbitary dimensions).<br />
Most orbits are unstable in D != 3, so the problem is not as interesting is the short answer. The long answer is that the problem has not been studied as hard.</p>
<p>Not a lot of people know that there is an exact perturbation expansion solution to the three body problem, but it has rather poor convergence&#8230;</p>
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		<title>By: nigel cook</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18993</link>
		<dc:creator>nigel cook</dc:creator>
		<pubDate>Mon, 24 Jul 2006 09:07:56 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18993</guid>
		<description>&#039;... the &#039;inexorable laws of physics&#039; ... were never really there ... Newton could not predict the behaviour of three balls ... In retrospect we can see that the determinism of pre-quantum physics kept itself from ideological bankruptcy only by keeping the three balls of the pawnbroker apart.&#039; â€&quot; Tim Poston and Ian Stewart, Analog, November 1981.

It isn&#039;t quantum physics that is the oddity, but actually classical physics! The normal teaching of Newtonian physics (at least at low levels) falsely claims/indoctrinates the persistent lie that it allows the positions of the planets to be exactly calculated (determinism) when it does not if you have 3+ bodies, which you do.  Richard P. Feynman conceded this in his book QED:

&#039;when the space through which a photon moves becomes too small (such as the tiny holes in the screen) ... we discover that ... there are interferences created by the two holes, and so on. The same situation exists with electrons: when seen on a large scale, they travel like particles, on definite paths. But on a small scale, &lt;strong&gt;such as inside an atom, the space is so small&lt;/strong&gt; that ... interference becomes very important.&#039;

The interference is due to many vacuum virtual charges:

&#039;All charges are surrounded by clouds of virtual photons, which spend part of their existence dissociated into fermion-antifermion pairs.&#039; â€&quot; I. Levine, D. Koltick, et al., Physical Review Letters, v.78, 1997, no.3, p.424.

The duration and maximum range of these charges is easily estimated: take the energy-time form of Heisenberg&#039;s uncertainty principle and put in the energy of an electron-positron pair and you find it can exist for ~10^-21 second; the maximum possible range is therefore this time multiplied by c, or 10^-12 metre.  This is far enough to deflect electrons but not enough to be observed as vacuum radioactivity.  Like Brownian motion, it introduces chaos on small scales, not lare ones:

&#039;... the Heisenberg formulae can be most naturally interpreted as statistical scatter relations, as I proposed [in the 1934 book &#039;The Logic of Scientific Discovery&#039;]. ... There is, therefore, no reason whatever to accept either Heisenberg&#039;s or Bohr&#039;s subjectivist interpretation ...&#039; â€&quot; Sir Karl R. Popper, Objective Knowledge, Oxford University Press, 1979, p. 303.

The Schroedinger wave equation arises naturally from a sea of particles because we know that you get waves in particle-based fluids: http://feynman137.tripod.com/#b</description>
		<content:encoded><![CDATA[<p>&#8216;&#8230; the &#8216;inexorable laws of physics&#8217; &#8230; were never really there &#8230; Newton could not predict the behaviour of three balls &#8230; In retrospect we can see that the determinism of pre-quantum physics kept itself from ideological bankruptcy only by keeping the three balls of the pawnbroker apart.&#8217; â€&#8221; Tim Poston and Ian Stewart, Analog, November 1981.</p>
<p>It isn&#8217;t quantum physics that is the oddity, but actually classical physics! The normal teaching of Newtonian physics (at least at low levels) falsely claims/indoctrinates the persistent lie that it allows the positions of the planets to be exactly calculated (determinism) when it does not if you have 3+ bodies, which you do.  Richard P. Feynman conceded this in his book QED:</p>
<p>&#8216;when the space through which a photon moves becomes too small (such as the tiny holes in the screen) &#8230; we discover that &#8230; there are interferences created by the two holes, and so on. The same situation exists with electrons: when seen on a large scale, they travel like particles, on definite paths. But on a small scale, <strong>such as inside an atom, the space is so small</strong> that &#8230; interference becomes very important.&#8217;</p>
<p>The interference is due to many vacuum virtual charges:</p>
<p>&#8216;All charges are surrounded by clouds of virtual photons, which spend part of their existence dissociated into fermion-antifermion pairs.&#8217; â€&#8221; I. Levine, D. Koltick, et al., Physical Review Letters, v.78, 1997, no.3, p.424.</p>
<p>The duration and maximum range of these charges is easily estimated: take the energy-time form of Heisenberg&#8217;s uncertainty principle and put in the energy of an electron-positron pair and you find it can exist for ~10^-21 second; the maximum possible range is therefore this time multiplied by c, or 10^-12 metre.  This is far enough to deflect electrons but not enough to be observed as vacuum radioactivity.  Like Brownian motion, it introduces chaos on small scales, not lare ones:</p>
<p>&#8216;&#8230; the Heisenberg formulae can be most naturally interpreted as statistical scatter relations, as I proposed [in the 1934 book 'The Logic of Scientific Discovery']. &#8230; There is, therefore, no reason whatever to accept either Heisenberg&#8217;s or Bohr&#8217;s subjectivist interpretation &#8230;&#8217; â€&#8221; Sir Karl R. Popper, Objective Knowledge, Oxford University Press, 1979, p. 303.</p>
<p>The Schroedinger wave equation arises naturally from a sea of particles because we know that you get waves in particle-based fluids: <a href="http://feynman137.tripod.com/#b" rel="nofollow">http://feynman137.tripod.com/#b</a></p>
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		<title>By: Ambitwistor</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18970</link>
		<dc:creator>Ambitwistor</dc:creator>
		<pubDate>Mon, 24 Jul 2006 02:56:14 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18970</guid>
		<description>See the bottom of &lt;a href=&quot;http://www.burtleburtle.net/bob/physics/kempler.html&quot; rel=&quot;nofollow&quot;&gt;this page&lt;/a&gt; for an animation of the &lt;a href=&quot;http://en.wikipedia.org/wiki/Klemperer_rosette&quot; rel=&quot;nofollow&quot;&gt;Klemperer rosette&lt;/a&gt; of the puppeteer Fleet of Worlds in the Larry Niven novel &lt;i&gt;Ringworld&lt;/i&gt;.</description>
		<content:encoded><![CDATA[<p>See the bottom of <a href="http://www.burtleburtle.net/bob/physics/kempler.html" rel="nofollow">this page</a> for an animation of the <a href="http://en.wikipedia.org/wiki/Klemperer_rosette" rel="nofollow">Klemperer rosette</a> of the puppeteer Fleet of Worlds in the Larry Niven novel <i>Ringworld</i>.</p>
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		<title>By: bittergradstudent</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18971</link>
		<dc:creator>bittergradstudent</dc:creator>
		<pubDate>Mon, 24 Jul 2006 01:52:47 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18971</guid>
		<description>chris #6--

nuclear physics is inherently quantum mechanical, so you can&#039;t get away with using the cold determinism of classical mechanics and hope to get answers corresponding to what experimentalists observe--you run into all sorts of odd probleems having to do with the indistinguishability of particles inside the nucleus, as well as the probabilistic nature of quantum mechanics.</description>
		<content:encoded><![CDATA[<p>chris #6&#8211;</p>
<p>nuclear physics is inherently quantum mechanical, so you can&#8217;t get away with using the cold determinism of classical mechanics and hope to get answers corresponding to what experimentalists observe&#8211;you run into all sorts of odd probleems having to do with the indistinguishability of particles inside the nucleus, as well as the probabilistic nature of quantum mechanics.</p>
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		<title>By: bittergradstudent</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18972</link>
		<dc:creator>bittergradstudent</dc:creator>
		<pubDate>Mon, 24 Jul 2006 01:46:37 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18972</guid>
		<description>Nicholas #2:


This is a slightly different issue than the one above:  Einstein was looking for a universe that was neither expanding nor contracting.  By adding a cosmological constant, and fine-tuning the mass in the universe, this can be done pretty easily within GR--simply write down the standard cosmology that prefers no direction nor any position (as modern cosmologists do when taking their first, most basic run through things), and then set all the time derivatives equal to zero.  This ends up giving you a set of relationships betwene the CC, the density of the universe, and the pressure of the matter in the universe.

It is a perfectly legitimate solution in the presence of a CC.  However, a slight bump of &lt;b&gt;this&lt;/b&gt; system leads not to a transition to chaos, but rather will cause the universe to either contract or to expand.  Therefore, the Einstein static universe ends up beng akin to blaancing an egg on the top of a basketball--not &lt;i&gt;impossible&lt;/i&gt;, per se, but it is essentially impossible to do perfectly enough to get the system to stay like that.

But you wouldn&#039;t really consider the system chaotic--it would still remain a well-defined robertson-walker-desitter universe and would still be relatively close to the original system in the parameter space, at least initially.  Late observers will be able to deduce the initial conditions of the system.  Chaotic systems are much more uppity and difficult to manage.</description>
		<content:encoded><![CDATA[<p>Nicholas #2:</p>
<p>This is a slightly different issue than the one above:  Einstein was looking for a universe that was neither expanding nor contracting.  By adding a cosmological constant, and fine-tuning the mass in the universe, this can be done pretty easily within GR&#8211;simply write down the standard cosmology that prefers no direction nor any position (as modern cosmologists do when taking their first, most basic run through things), and then set all the time derivatives equal to zero.  This ends up giving you a set of relationships betwene the CC, the density of the universe, and the pressure of the matter in the universe.</p>
<p>It is a perfectly legitimate solution in the presence of a CC.  However, a slight bump of <b>this</b> system leads not to a transition to chaos, but rather will cause the universe to either contract or to expand.  Therefore, the Einstein static universe ends up beng akin to blaancing an egg on the top of a basketball&#8211;not <i>impossible</i>, per se, but it is essentially impossible to do perfectly enough to get the system to stay like that.</p>
<p>But you wouldn&#8217;t really consider the system chaotic&#8211;it would still remain a well-defined robertson-walker-desitter universe and would still be relatively close to the original system in the parameter space, at least initially.  Late observers will be able to deduce the initial conditions of the system.  Chaotic systems are much more uppity and difficult to manage.</p>
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		<title>By: Count Iblis</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18974</link>
		<dc:creator>Count Iblis</dc:creator>
		<pubDate>Mon, 24 Jul 2006 01:20:14 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18974</guid>
		<description>In general relativity the two body problem can&#039;t be solved exactly. And in QFT even the zero body problem (vacuum) can&#039;t be solved. :)</description>
		<content:encoded><![CDATA[<p>In general relativity the two body problem can&#8217;t be solved exactly. And in QFT even the zero body problem (vacuum) can&#8217;t be solved. <img src='http://blogs.discovermagazine.com/cosmicvariance/wp-includes/images/smilies/icon_smile.gif' alt=':)' class='wp-smiley' /> </p>
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		<title>By: Doug</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18973</link>
		<dc:creator>Doug</dc:creator>
		<pubDate>Mon, 24 Jul 2006 00:49:28 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18973</guid>
		<description>This dynamic n-body curve may be a [complex?] Spatial-3D, Time-1D [or more?] version of the Lemniscate of Bernoulli. A 3 star system like Centauri may slightly differ from a black hole dominated galaxy, solar dominated planetary system or a planet dominated lunar system.
http://curvebank.calstatela.edu/lemniscate/lemniscate.htm

The latter three tend to resemble a logarithmic spiral that may relate to the &#039;Mice Problem&#039;
http://mathworld.wolfram.com/MiceProblem.html

Yet all the multiple body systems would appear to likely have some type of radial symmetry that may resemble a &#039;Superellipse&#039;.
http://mathworld.wolfram.com/Superellipse.html</description>
		<content:encoded><![CDATA[<p>This dynamic n-body curve may be a [complex?] Spatial-3D, Time-1D [or more?] version of the Lemniscate of Bernoulli. A 3 star system like Centauri may slightly differ from a black hole dominated galaxy, solar dominated planetary system or a planet dominated lunar system.<br />
<a href="http://curvebank.calstatela.edu/lemniscate/lemniscate.htm" rel="nofollow">http://curvebank.calstatela.edu/lemniscate/lemniscate.htm</a></p>
<p>The latter three tend to resemble a logarithmic spiral that may relate to the &#8216;Mice Problem&#8217;<br />
<a href="http://mathworld.wolfram.com/MiceProblem.html" rel="nofollow">http://mathworld.wolfram.com/MiceProblem.html</a></p>
<p>Yet all the multiple body systems would appear to likely have some type of radial symmetry that may resemble a &#8216;Superellipse&#8217;.<br />
<a href="http://mathworld.wolfram.com/Superellipse.html" rel="nofollow">http://mathworld.wolfram.com/Superellipse.html</a></p>
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		<title>By: Amara</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18991</link>
		<dc:creator>Amara</dc:creator>
		<pubDate>Sun, 23 Jul 2006 21:11:08 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18991</guid>
		<description>Navneeth: That gravit program implements a &lt;a href=&quot;http://www.ifa.hawaii.edu/~barnes/treecode/treeguide.html&quot; rel=&quot;nofollow&quot;&gt;Tree code&lt;/a&gt; and with the usual inverse square law force, so you would need to modify the source code first. Tree methods are a deep leap into N-body methods (a heads up, if you&#039;re not aware). I couldn&#039;t find the module that calculates the forces in my cursory look of the gravit source, but then C is not my strength, and I have never implemented a Tree-code before. (Particle-particle and symplectic schemes are my only N-body coding experience.)</description>
		<content:encoded><![CDATA[<p>Navneeth: That gravit program implements a <a href="http://www.ifa.hawaii.edu/~barnes/treecode/treeguide.html" rel="nofollow">Tree code</a> and with the usual inverse square law force, so you would need to modify the source code first. Tree methods are a deep leap into N-body methods (a heads up, if you&#8217;re not aware). I couldn&#8217;t find the module that calculates the forces in my cursory look of the gravit source, but then C is not my strength, and I have never implemented a Tree-code before. (Particle-particle and symplectic schemes are my only N-body coding experience.)</p>
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		<title>By: Sean</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18976</link>
		<dc:creator>Sean</dc:creator>
		<pubDate>Sun, 23 Jul 2006 20:03:09 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18976</guid>
		<description>Steve-- thanks, fixed.

Nami--  I don&#039;t know anything about work with non-inverse-square force laws, but then again I wouldn&#039;t.

None of these solutions is at all stable!  Or realistic. But they are interesting and fun.</description>
		<content:encoded><![CDATA[<p>Steve&#8211; thanks, fixed.</p>
<p>Nami&#8211;  I don&#8217;t know anything about work with non-inverse-square force laws, but then again I wouldn&#8217;t.</p>
<p>None of these solutions is at all stable!  Or realistic. But they are interesting and fun.</p>
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		<title>By: Navneeth</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18975</link>
		<dc:creator>Navneeth</dc:creator>
		<pubDate>Sun, 23 Jul 2006 19:55:48 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18975</guid>
		<description>Re: Amara

I just found this simulator program that &quot;looks&quot; great. I haven&#039;t yet downloaded it, but will be doing soon. I came across this while searching for the program you mentioned. I&#039;d really love to a non-inverse square universe.

&lt;a href=&quot;http://gravit.slowchop.com/&quot; rel=&quot;nofollow&quot;&gt;http://gravit.slowchop.com/&lt;/a&gt;</description>
		<content:encoded><![CDATA[<p>Re: Amara</p>
<p>I just found this simulator program that &#8220;looks&#8221; great. I haven&#8217;t yet downloaded it, but will be doing soon. I came across this while searching for the program you mentioned. I&#8217;d really love to a non-inverse square universe.</p>
<p><a href="http://gravit.slowchop.com/" rel="nofollow">http://gravit.slowchop.com/</a></p>
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		<title>By: Paul Valletta</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18978</link>
		<dc:creator>Paul Valletta</dc:creator>
		<pubDate>Sun, 23 Jul 2006 19:54:05 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18978</guid>
		<description>My interest in the N-Body problem as stated initially at Baez site, was relative to the last box in Sean&#039;s above post.

If one adds a simplistic directional value to the system, then it cannot surely evolve ? all the above solutions are not really ideal?, for instatnce the first &quot;figure-of-eight&quot; is background independant.

If one was to find 21 point masses evolving thus, anywhere other than in a computer generated program, I would be amazed!

Their mutual Newtonian gravitational attraction, is realistically to &quot;clinical&quot;? . Example, any Cosmic background Expansion paramiter would nullify the systems evolution?

Even a simplistic 2-body system cannot evolve as an isolated system indefinate ?

Again, the input of the constrained orbits in the last box is neat, and is based on the unchanging Mathematics of Isolated Systems, which are not &quot;Relative&quot; to change !

Still cool to look at though.</description>
		<content:encoded><![CDATA[<p>My interest in the N-Body problem as stated initially at Baez site, was relative to the last box in Sean&#8217;s above post.</p>
<p>If one adds a simplistic directional value to the system, then it cannot surely evolve ? all the above solutions are not really ideal?, for instatnce the first &#8220;figure-of-eight&#8221; is background independant.</p>
<p>If one was to find 21 point masses evolving thus, anywhere other than in a computer generated program, I would be amazed!</p>
<p>Their mutual Newtonian gravitational attraction, is realistically to &#8220;clinical&#8221;? . Example, any Cosmic background Expansion paramiter would nullify the systems evolution?</p>
<p>Even a simplistic 2-body system cannot evolve as an isolated system indefinate ?</p>
<p>Again, the input of the constrained orbits in the last box is neat, and is based on the unchanging Mathematics of Isolated Systems, which are not &#8220;Relative&#8221; to change !</p>
<p>Still cool to look at though.</p>
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		<title>By: Amara</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18977</link>
		<dc:creator>Amara</dc:creator>
		<pubDate>Sun, 23 Jul 2006 19:47:03 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18977</guid>
		<description>I should clarify that the exact solutions described in Grossman&#039;s book are for limited cases, while the N-body methods on the web page are numerical and inexact solutions. A good site for details about writing numerical N-body codes &lt;a href=&quot;http://www.artcompsci.org/kali/vol/n_body_problem/title.html&quot; rel=&quot;nofollow&quot;&gt;here&lt;/a&gt;.</description>
		<content:encoded><![CDATA[<p>I should clarify that the exact solutions described in Grossman&#8217;s book are for limited cases, while the N-body methods on the web page are numerical and inexact solutions. A good site for details about writing numerical N-body codes <a href="http://www.artcompsci.org/kali/vol/n_body_problem/title.html" rel="nofollow">here</a>.</p>
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		<title>By: Amara</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18980</link>
		<dc:creator>Amara</dc:creator>
		<pubDate>Sun, 23 Jul 2006 18:46:13 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18980</guid>
		<description>Nathaniel Grossman&#039;s delightful text: &lt;a href=&quot;http://www.amazon.com/gp/product/0817638326/&quot; rel=&quot;nofollow&quot;&gt;The Sheer Joy of Celestial Mechanics&lt;/a&gt; chapter 2, section 3 describes other exact solutions to a non-inverse square law. For example, if it is cubed you get solutions called &quot;Cote&#039;s spirals. It seems to me I&#039;ve seen N-body shareware software that lets you play with the exponent of the inverse force, but I can&#039;t locate it now (I would be interested to know, if you find it). Such a study would be simple-enough to program yourself however, look at the &quot;particle-particle&quot; numerical method of my old  &lt;a href=&quot;http://www.amara.com/papers/nbody.html&quot; rel=&quot;nofollow&quot;&gt;N-body Methods&lt;/a&gt; web page. Perhaps you can find existing studies on the &lt;a href=&quot;http://it.arxiv.org/find/astro-ph/1/N-body/0/1/0/all/7/0&quot; rel=&quot;nofollow&quot;&gt;ArXiV&lt;/a&gt; archive too.</description>
		<content:encoded><![CDATA[<p>Nathaniel Grossman&#8217;s delightful text: <a href="http://www.amazon.com/gp/product/0817638326/" rel="nofollow">The Sheer Joy of Celestial Mechanics</a> chapter 2, section 3 describes other exact solutions to a non-inverse square law. For example, if it is cubed you get solutions called &#8220;Cote&#8217;s spirals. It seems to me I&#8217;ve seen N-body shareware software that lets you play with the exponent of the inverse force, but I can&#8217;t locate it now (I would be interested to know, if you find it). Such a study would be simple-enough to program yourself however, look at the &#8220;particle-particle&#8221; numerical method of my old  <a href="http://www.amara.com/papers/nbody.html" rel="nofollow">N-body Methods</a> web page. Perhaps you can find existing studies on the <a href="http://it.arxiv.org/find/astro-ph/1/N-body/0/1/0/all/7/0" rel="nofollow">ArXiV</a> archive too.</p>
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		<title>By: Arun Madhav</title>
		<link>http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/comment-page-1/#comment-18979</link>
		<dc:creator>Arun Madhav</dc:creator>
		<pubDate>Sun, 23 Jul 2006 18:08:33 +0000</pubDate>
		<guid isPermaLink="false">http://blogs.discovermagazine.com/cosmicvariance/2006/07/23/n-bodies/#comment-18979</guid>
		<description>Made very interesting reading.  I am interested in the answer to Nami&#039;s question too..regarding N-body problems with a non inverse square law.</description>
		<content:encoded><![CDATA[<p>Made very interesting reading.  I am interested in the answer to Nami&#8217;s question too..regarding N-body problems with a non inverse square law.</p>
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