He is not.
Potential energy is calculated in physics with
E = m*h*g
where
m=total mass
h=height of the center of mass
g=gravity's acceleration
=9.805 m/s2 in NYC
So let's find values to plug into the formula:
1. MASS
If you do real research, you will quickly find quotes for the mass of one of the twin towers given in the range from 250,000 to 500,000 tons (metric). Let's quickly check if this order of magnitude is plausible:
A building is made of steel and concrete, but contains mostly air. Much like a ship. The building would swim. if you could keep it water thight. We know that much of the volume of a ship is above water, and only a fraction below: maybe 1/5th of it. That means average density of ship/building can be estimated as 0.2 tons/m
3.
We know the dimensions of the towers:
416m*63m*63m = 1,651,104m
3
Now lets apply the sensity here, and we get the mass of the tower:
1,651,104m3 * 0.2 tons/m3 = 330,220.8 tons = 330,220,800 kg[/B]
2. HIGHT
The towers were 416 meters high. The center of mass would be at half that height, if mass was evenly distributet. But it isn't, because in the lower floors, core columns are stronger. A reasonable estimate is 40% of the hight. So
h = 416m * 40% = 166,4m
ENERGY:
We now have all the numbers and can compute potential energy:
E = m*h*g = 166,4m * 330,220,800 kg * 9.805 m/s2 = 538,772,406,681.6 Joules, or 538,772MJ
TNT:
How much TNT is that? Well, you can look it up everywhere: 1kg of TNT contains 4.7MJ
So our TNT-equivalent m(TNT) for what gravity does to a twin tower is:
m(TNT) = 538,772MJ / 4.7kg/MJ = 114,632kg (rounded) or
114.6 tons
This is a little less than beachnut's 130 tons, but close, and of course we estimated mass quite roughly; some people give higher numbers, some give lower numbers (I usually only go with 100 tons, preferring the most conservative values), but the bottom line is true: You have many dozend tons of TNT equivalent to work with from gravity alone.
There is not the tinies shred of evidence that explosives reaching that energy even remotely were used.