The towers fell at or near free fall speeds, a possible sign of controlled demolition.
Stage one in establishing this claim is to calculate the actual time it took for the towers to fall, but dust clouds obscuring the end of the collapse make this difficult.
Coming up with a final figure involves a degree of estimation, which is probably why the times you’ll find online range from 8.4 to 15 seconds..
The rate of free fall in a vacuum, at least, is easier to define. The towers were around 417 metres tall (excluding the spire), giving 417 = 0.5 gt^2, so with g = 9.8m/s^2 that gives a time of about 9.22 seconds. So if you dropped a ball off the roof, and there were no air resistance, then that’s the time it would take to reach the ground.
Now we have a basis for comparison. If the towers really did fall completely in 8.4 seconds, then that would actually be faster than gravity, requiring some major additional force to push from above (or pull from below). We’ve seen it suggested that explosives created a “powerful vacuum”, for instance, but that’s not apparent from the collapse videos and images. Like this one, for instance.
Large chunks of rubble, which are in free fall, are clearly falling faster than the rest of the building. The base of the massive chunk lower left is, what, 20 storeys lower than the top of the right-hand corner of the building? (And there may be rubble below that, and the building may be intact higher higher still). This suggests we should be looking at a collapse time greater than our 9.22 second freefall figure, not less.
How much greater? If the video evidence gives such a great ranges of guesses, then maybe another approach is required, at least as a crosscheck. We tried looking at the audio of each collapse, and came up with a minimum of 14 seconds in each case (see our South Tower and North Tower pages for more), and the potential for them to have taken several seconds longer. Calculating these times involves far too many judgement calls for us to claim proof of anything, but we do think it adds significantly more support to the 15+ seconds collapse time, and makes the 8.4 second end of the spectrum look particularly unlikely.
We can cross-check this by looking at the seismic evidence. Although often presented as supporting the shortest 8-point-something time, in our view there’s a case for arguing that this, too, indicates the collapse time was much, much longer.
And if you look carefully, then you will find some videos that also back us up. Here’s one indicating to us that the first collapse took more than 12.5 seconds.
Where people have quantified the collapse time they thought should have arisen, it’s not always helpful to the conspiracy case. D.P. Grimmer, for instance, believes the towers demonstrably fell in around 10 seconds, and has this to say about the time it should have taken in one scenario (if 30% of the gravitational energy of the collapse was lost in pulverising the concrete):
Now the observed time t = 10 seconds (a free fall time, the fastest possible time under g = 9.8 m/sec/sec = 32 ft/sec/sec = 32 ft/s exp2). For the cloud debris creation to absorb 30% of the gravitational energy, the observed time of fall would be 10s x 1.195, or almost 12 seconds. This long a collapse time was observed by no one. Clearly, there are serious flaws in the official explanation/conspiracy theory.
So Grimmer thinks a 12 second time might be more reasonable, in the case he describes? Yet we (and others) suggest a collapse time of 15 seconds or more is more accurate, significantly longer still.
Of course the main issue is still whether each tower fell faster than it should have done in air, not a vacuum. Read more on this in an extremely detailed and interesting paper from Dr Frank Greening, which he’s kindly agreed to let us host here.
And in the interests of balance, check out the “Refutation of the Official Collapse Theory”. Be sure to pay attention to their calculations of collapse time, and the way the pancaking towers are assumed to come to a dead stop as each floor is hit.