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	<title>Comments on: What does &#8220;classical&#8221; mean in science?</title>
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		<title>By: Advanced or just obscure? &#8212; The Endeavour</title>
		<link>http://www.johndcook.com/blog/2009/03/27/classical-science/comment-page-1/#comment-103194</link>
		<dc:creator>Advanced or just obscure? &#8212; The Endeavour</dc:creator>
		<pubDate>Tue, 13 Sep 2011 12:02:05 +0000</pubDate>
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		<description>[...] from scratch, but B came first. Teachers and authors tend to present material in the order in which they learned it. They may think of newer material as being more difficult, but a new generation may [...]</description>
		<content:encoded><![CDATA[<p>[...] from scratch, but B came first. Teachers and authors tend to present material in the order in which they learned it. They may think of newer material as being more difficult, but a new generation may [...]</p>
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		<title>By: Neil Gunther</title>
		<link>http://www.johndcook.com/blog/2009/03/27/classical-science/comment-page-1/#comment-16961</link>
		<dc:creator>Neil Gunther</dc:creator>
		<pubDate>Tue, 05 May 2009 05:38:32 +0000</pubDate>
		<guid isPermaLink="false">http://www.johndcook.com/blog/?p=1872#comment-16961</guid>
		<description>The first sentence strikes me as very odd. 

I don&#039;t know about &quot;standard,&quot; but the word &quot;classical&quot; does have an unambiguous definition in physics, viz., anything that is pre-quantum theory. Perhaps this usage is not known outside physics. Historically, it refers to those physical models or theories that existed before the mid-1920s, when a consistent quantum mechanical formalism was finally developed. In the 20 years prior, there was a lot of approximation and guessing going on, as people struggled to accommodate atomic effects into a Newtonian framework, but failed to reach any kind of mathematical consistency. For this reason the term &quot;semi-classical&quot; also appears in the physics literature when referring to those theories. 

An example of a semi-classical theory is the &lt;a href=&quot;http://en.wikipedia.org/wiki/Bohr_model&quot; rel=&quot;nofollow&quot;&gt;Bohr&lt;/a&gt; (planetary) model of the atom c.1913; the one most people have in their heads today. Such an atom would collapse under purely classical laws of physics so, certain ad hoc rules were  imposed (e.g., quantization of  classical angular momentum) to prevent it. Hence, semi-classical. In the later quantum mechanics, these ad hoc rules are unnecessary  because the fundamental physical variables are defined very differently from classical Newtonian variables.

From the modern standpoint, we say that a classical regime is one where the quantum fluctuations (that always exist) are sufficiently small that they can be ignored with respect to the rest of the system; which is how things looked to Newton.</description>
		<content:encoded><![CDATA[<p>The first sentence strikes me as very odd. </p>
<p>I don&#8217;t know about &#8220;standard,&#8221; but the word &#8220;classical&#8221; does have an unambiguous definition in physics, viz., anything that is pre-quantum theory. Perhaps this usage is not known outside physics. Historically, it refers to those physical models or theories that existed before the mid-1920s, when a consistent quantum mechanical formalism was finally developed. In the 20 years prior, there was a lot of approximation and guessing going on, as people struggled to accommodate atomic effects into a Newtonian framework, but failed to reach any kind of mathematical consistency. For this reason the term &#8220;semi-classical&#8221; also appears in the physics literature when referring to those theories. </p>
<p>An example of a semi-classical theory is the <a href="http://en.wikipedia.org/wiki/Bohr_model" rel="nofollow">Bohr</a> (planetary) model of the atom c.1913; the one most people have in their heads today. Such an atom would collapse under purely classical laws of physics so, certain ad hoc rules were  imposed (e.g., quantization of  classical angular momentum) to prevent it. Hence, semi-classical. In the later quantum mechanics, these ad hoc rules are unnecessary  because the fundamental physical variables are defined very differently from classical Newtonian variables.</p>
<p>From the modern standpoint, we say that a classical regime is one where the quantum fluctuations (that always exist) are sufficiently small that they can be ignored with respect to the rest of the system; which is how things looked to Newton.</p>
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