In topology, can someone please describe why the sphere $S^2$ is not contactable? Surely it can just 'shrink' to a point?

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    $\begingroup$ But imagine trying to shrink $S^2$ to a point while trying to stay inside the sphere itself. Of course you can let it shrink to a point if you let it move through $\mathbb{R}^3$. But the homotopy must be between the identity and a constant map and lie inside the sphere for all $t$. $\endgroup$ – Zach L. Dec 14 '12 at 12:10

The issue with your logic is that you must shrink your space while staying within your original space. For example, your idea is roughly to consider the balls of radius $t$ around $0$ and let $t\to 0$. Now, the definition of the homotopy is a map $H:[0,1]\times S^2\to S^2$--your spaces (maps rigorously) don't lie on (map into) $S^2$ after $t=1$.

  • $\begingroup$ What you mean 'after $t=1'$? Do you mean $t$ can be more than $1$? $\endgroup$ – Idonknow Apr 2 '14 at 17:35

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