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## [Computational Complexity] Axioms: What should we believe?

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• Some misc thoughts on set theory inspired by yesterdays comments and other things. - Geometry: Use Euclidean Geometry when appropriate, for example if you are
Message 1 of 1 , Jan 5, 2010
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Some misc thoughts on set theory inspired by yesterdays comments and other things.
1. Geometry: Use Euclidean Geometry when appropriate, for example if you are designing a bridge, use Riemannian geometry when looking at space time, and use geometries when they are appropriate. So there is no correct geometry, its more of a right tool for the right job thing. So far Set Theory does not seem to have a strong enough connection to the real world for this to make sense. I supposed you use ZFC when dealing with most of mathematics, but I doubt you would ever say something like: When dealing with Quantum Mechanics its best to assume AD. So what can you use to decide what axioms to use? You may decide what axioms to use based on your tastes. For example see this prior blog posting. This is good for an individual but will not really work for the whole community. For example, I happen to like AD since I like a world where the Banach Tarski paradox is false. But that's just me.
2. People concerned with these issues in the early 1900's were much more passionate then we are today. They had strong opinions on foundations and on non-constructive proofs. Mathematicians commonly carried firearms. We are far less passionate today on these issues. As an example, there are today people who study constructive proofs and prefer them, but I doubt anyone today would reject a theorem that was proven nonconstructively. Why the change of heart? Possibly Godel's theorem, but also the fact that people in different parts of math can't talk to each other so they can't argue.
3. Another axiom of interest: The existence of Inaccessible cardinals. MOTIVATION: Take omega. If |X| < omega then |powerset(X)| < omega. Does any other cardinal have this property? Why should omega be so unique? Kappa is an inaccessible cardinal is such that if |X| < Kappa then |powerset(X)| < Kappa. Do such cardinals exist? The existence of an inaccessible cardinal large than omega cannot be proven in ZFC. An inaccessible cardinal would be a model of ZFC and hence would prove that ZFC is consistent (omega does not prove ZFC consistent since no proper subset of omega is infinite). It is known that ZFC cannot prove its own consistency (I think that's true of any theory but there may be some conditions.)
4. Penelope Maddy has two nice articles on why mathematicians believe what they do: believing the axioms I believing the axioms II Also good to read: Shelah's Logical Dreams

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Posted By GASARCH to Computational Complexity at 1/05/2010 11:20:00 AM
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