“ There are several things that you need to understand. *First*, most isotopes are unstable, that is, they decay into something that is then stable.... (read full comment) ” 

Posted on December 8, 2011 at 12:21 PM EST

+ 3 more comments posted in this discussion

“ so does it push the spaceship forward, or space back? Relatively speaking, yes. The two have no meaningful difference. (read full comment) ” 

Posted on December 11, 2009 at 4:47 PM EST