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Gravity - The Love Story I: Black Holes Are Not 'Universal' Drains

28 Sep 2012, 00:47 UTC
Gravity - The Love Story I: Black Holes Are Not 'Universal' Drains
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This semester I am teaching a conceptual physics class at LSU that uses minimal mathematics to understand how the Universe works. Yesterday, we covered the chapter on gravity and my closing question to my students was, "What would happen to the Earth's orbit if the Sun were to become a black hole instantly?" Assume that it simply changes in size from what it is now to how big a black hole with the same mass would be and the center of mass never changes.I'm not going to make you wait... Nothing would happen to the Earth's orbit!This is one of the most dramatic examples of simply using an equation to tell a story that I have come across. I suspect that much of the drama comes from the misconception that black holes WILL consume EVERYTHING, turning most people's mental picture of a black hole into a universal drain.(I know the following analogy is a bit corny, but it makes the point that equations can tell stories and aren't just recipes to combine numbers into new numbers...)EQUATIONS TELL A STORYIn order to resolve this misconception, consider Newton's law of universal gravitation: Now, don't worry overly that this is an equation because ...

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