I fear that I got a bit ahead of myself at the end of the last post on the spectral lines of hydrogen. To fully close the circle between dark matter and everything I've been talking about in the last few entries, we do need to cover Newton's theory of gravity. Therefore, I will try to do so now, so that we can put this particularly sequence to rest.
First, on talking with a friend earlier today, I was asked, "I always hear the term "Newtonian gravity: is there any other kind?" This is exactly the kind of question I need to be asked, because otherwise I forget that I've been studying this stuff for 10 years. The answer is yes, there is another kind of gravity; more precisely, the answer is that there is a more complete form of gravity, namely Einstein's Theory of General Relativity (and possible string theory is a yet more complete form, although I'll leave that argument to Brian Greene's The Elegant Universe). Newton observed the effects of gravity and described those effects using math. However, he didn't describe how gravity works. To borrow an analogy directly from Greene's book, when my mom (and no, Greene didn't refer explicitly to my mom in his book [he did address himself directly to "you," but I don't think my mom has read it]) uses her computer, she doesn't know how it actually works (i.e. little electrical signals flashing through a chip). She only knows how to use it to write an article or to read this blog (sometimes she is unsure even how to do those tasks, at which times, coincidentally enough, she often checks in with her favorite son). Newton gave us a personal computer (gravity) and told us how to use it to write articles and check email (the equations), but he didn't tell us how it actually works (how is gravity transmitted, or how does the apple "know" it has to fall to the ground?).
In addition, Newton's formulation was only an approximation that was incorrect on certain scales (which are largely inaccessible to us and unobservable in our day to day lives). For example, I'll poach another commonly used illustration; from far away, a Seurat painting looks like a smooth picture. Newton's theory of gravity is accurate when viewed "from far away," which more or less is the point from which we all experience gravity. Up close, however, and all the dots become clear. While Newton got the big picture right, he did not describe the dots. General relativity can handle both the smooth "far away" view, but also the rough "up close" view.
That's why we can talk about "Newtonian gravity." For all intents and purposes, we could just talk about gravity and we'd all be referring to Newton, but since this is a physics blog, I figure I should try to be more precise in my language. In the next post, I'll actually use Newton's equations to look at the rotation of galaxies.
The image of Seurat's Sunday Afternoon on the Island of La Grande Jatte was scanned by Mark Harden