How much energy would it take to blow up the Earth? There is a common statement running around that we as a species has enough nuclear weapons to blow up the earth several times over. What I want to know is: by how many orders of magnitude  is that a wrong statement? (Just getting a lower bound would be fine.)
 A: The most obvious interpretation of the phrase blow up the Earth is to dismantle it into tiny particles headed off to infinity. If you're prepared to accept this definition then the calculation is easy because it's (approximately) the gravitational binding energy for matter with the mass of the Earth falling into a sphere the size of the Earth. I say approximately because I'm ignoring the ellipticity of the Earth and the fact it's rotating, and I'm assuming it's a uniform density throughout.
The gravitational binding energy of a uniform sphere is:
$$ U = \frac{3GM^2}{5r} $$
and for the Earth this works out as 2.24 $\times$ 10$^{32}$J.
According to the Wikipedia article on nuclear weapons, the Federation of American Scientists estimates there are more than 17,000 nuclear warheads in the world as of 2012, with around 4,300 of them considered "operational", ready for use. Let's take the average yield to be one megaton (almost certainly an overestimate), which is 4.184 $\times$ 10$^{15}$J. In that case the total energy of all 17,000 bombs is about 7 $\times$ 10$^{19}$J or about a factor of 3 $\times$ 10$^{-13}$J smaller than the gravitational binding energy.
I suppose another interpretation of blow up the Earth would be to render it uninhabitable. An obvious reference point for this is the meteor collision that caused the extinction of the dinosaurs. Wikipedia estimates this as 4.2 $\times$ 10$^{23}$J, or about a factor of ten thousand greater than all the current nuclear bombs.
