A (Massachusetts Institutue of Technology (MIT) Physicist Spells It Out About the Collapse of the Twin Towers...
Jeff King, PhD (MIT Physicist)
In the case of the twin towers even a rough calculation of the amount of energy needed to pulverize all the concrete and gypsum to the very fine dust observed (and without including the energy needed to crumple and deform steel) indicates that it would have far exceeded the entire gravitational potential energy of the structures.
As measurements by Cahill have shown, dust particles were mostly <30 microns, with a sizeable fraction smaller than 0.25 micron. Not only is an immense amount of energy needed to break most of the chemical bonds holding the concrete together, but there is no mechanism that can be postulated with only a gravitational collapse that might account for such rapid and complete pulverization. Such a process is almost by definition an explosive event: the almost instantaneous conversion of a slab of concrete into a rapidly expanding cloud of dust.
The obvious question is: what does it mean that there was a controlled demolition. At the simplest level, it means that someone had a lot access to the buildings over a long enough period of time to set this up. It implies, as many other things, that the people who had effective control of the site had an interest in having it scrubbed and making sure that no information was available; that a forensic reconstruction couldn't be done.
Even in much smaller catastrophes we typically will reconstruct things as completely as possible. For example with TWA Flight 800 pieces were dredged off the bottom of the sea. A complete reconstruction was done to allow a detailed analysis.
In this case, just the opposite was done.