From what I can tell, your ISP likes to assign you different IP addresses more or less by the second – and that's only slight hyperbole. As an example: between 10:02:48 and 10:05:42, a timeframe of just under three minutes, you made requests from fifteen (!) different IP addresses (most of these were heartbeat requests while writing this very question, so they all came from the same computer).
We don't usually require sticky IPs (in particular once you're logged in it shouldn't be an issue), but if during the process of logging in your IP address changes, that's extremely fishy, and that's why we bail out.
So when this happens, all I can tell you is to reload http://physics.stackexchange.com/users/login and click the Google button again, in the hope that your ISP lets you keep the same IP address for a few seconds :)