Does the “theory” of the destruction of the World Trade Center by “Muslim pilots” who crashed hijacked planes into the towers defy the laws of physics?
I don’t see why it is incumbent on me to do the debunking, but the kind of people who subscribe to these and other such theories tend to think that if you’re silent, it’s not because you think these things are beneath you, but rather, it’s because you are either willfully ignorant of the “facts” — or are part of some sinister Osama Bush coverup. This web site seems to be as good a starting point as any:

The official story has it that the towers collapsed because (a) the only connection between the outer perimeter wall and the central core were flimsy lightweight trusses, (b) the plane impact weakened these trusses and the heat of the fires caused them to buckle until (c) the trusses at the impact floors gave way and (d) the floors above lost their support and fell upon the lower floors causing all floors to pancake.
That this “truss theory” is false has been demonstrated in The World Trade Center Demolition.
Firstly, there must have been strong connections between the perimeter wall and the central core so that the wind load on the towers could be transmitted to the central core. If this wind load were not transmitted then the perimeter wall would move several feet in a strong wind and the central core would not have moved, so the floors would have buckled, which never happened. Thus there must have been strong steel girders connecting the perimeter wall to the central core, not merely trusses. These girders would not have suffered catastrophic failure as a result either of the impact or the fires.
Secondly, the assumption that there were only lightweight trusses connecting the perimeter walls with the central core leads to a calculation of the amount of steel in the towers which is only 2/3rds of the amount known to have gone into their construction, leaving 32,000 tons of steel unaccounted for. Thus the assumption is false. Those 32,000 tons are accounted for by steel girders connecting the perimeter wall to the central core.
Thirdly, there is photographic evidence of these, officially non-existent, horizontal beams.
This truss theory is a fabrication which has been spread about to give an appearance of plausibility to the official story as to how the towers collapsed. There have even been a couple of made-for-TV “documentaries”, complete with “experts”, promoting the truss theory, and suggesting that, because of the trusses, the design of the Twin Towers was fatally flawed, and that the trusses were not properly fire-proofed. The refutation of the truss theory is a refutation of the official “explanation” as to “how the towers fell”.
Another problem with the official story is the fact that both the Twin Towers collapsed evenly and smoothly.
If the fire melted the floor joints so that the collapse began from the 60th floor downward, the upper floors would be left hanging in the air, supported only by the central columns. This situation would soon become unstable and the top 30 floors would topple over … How was it that the upper floors simply disappeared instead of crashing to the earth as a block of thousands of tons of concrete and steel? …
When the platters [the floors] fell, those quarter-mile high central steel columns (at least from the ground to the fire) should have been left standing naked and unsupported in the air, and then they should have fallen intact or in sections to the ground below, clobbering buildings hundreds of feet from the WTC site like giant trees falling in the forest. But I haven’t seen any pictures showing those columns standing, falling, or lying on the ground. Nor have I heard of damage caused by them. ? Muslims Suspend Laws of Physics! Part I
Whatever damage the fires did would not have been evenly distributed (especially in the case of the South Tower, where the jet struck a corner of the building). If the collapse was due to the fires then it too would be irregular, with parts of the Twin Towers remaining intact and connected while other parts fell. But both towers collapsed completely symmetrically, with the floors pancaking upon themselves, exactly as we have seen in other cases of controlled demolition of tall buildings.
It is interesting to note that the contractor whose people were the first on the WTC collapse scene ? to cart away the rubble that remains ? is the same contractor who demolished and hauled away the shell of the bombed Oklahoma City Murrah building. The name of the contractor is Controlled Demolition! ? The Blockbuster
Could there be more of a connection between these two building collapses than the identity of the contractor who supervised the removal of the debris?

Similar theories are explored here, and here.
To read about this in incredible detail, try googling the phrase “Muslims suspend laws of physics.” (I got 702 results.)
Googling “World Trade Center” and “controlled demolition” brought over 2000 hits.
Almost every one of these pages argues for a conspiracy theory.
Well, there’s this. In the New Scientist, an engineer explains why it looked like a controlled demolition. But (yawn….) it wasn’t:

The explosions at the higher floors enable the collapse to gain downward momentum as gravity pulls the full weight of unsupported higher floors down into lower floors in a snowballing effect.
On Tuesday, the impacts of aeroplanes on the higher floors replaced the explosives. The collapse of the higher floors caused the floors below to be crushed. “It cascaded down like an implosion,” says Taylor.
The lack of collapse in higher stories was one reason why the 454 kilogram bomb detonated in the underground garage of the World Trade Center in 1993 failed to destroy the building.

That’s not interesting, of course….
I couldn’t help but notice that the proponents of the controlled demolition theory invariably display pictures showing that the buildings fell neatly down, in straight lines. You have to look elsewhere to see pictures like this. And here’s the accompanying story:

Reports indicate that the impact of each plane compromised the structural integrity of each tower, knocking out perimeter columns and the interior structure. The explosions then caused further damage, sweeping through several floors. “These were airliners scheduled for long flights, full of fuel, causing massive explosions,” says Richard M. Kielar, a Tishman senior vice president. “No structure could have sustained this kind of assault,” says Kielar.
As the fires burned, the structural steel on the breached floors and above would have softened and warped because of the intense heat, say sources. Fireproofed steel is only rated to resist 1,500 to 1,600? F. As the structure warped and weakened at the top of each tower, the frame, along with concrete slabs, furniture, file cabinets, and other materials, became an enormous, consolidated weight that eventually crushed the lower portions of the frame below.
Jon D. Magnusson, chairman-CEO of Skilling Ward Magnusson Barkshire Inc., Seattle, one of the successor firms of Skilling Ward Christiansen Robertson, structural engineer for the original World Trade Center, agrees: “From what I observed on TV, it appeared that the floor diaphragm, necessary to brace the exterior columns, had lost connection to the exterior wall.”
When the stability was lost, the exterior columns buckled outward, allowing the floors above to drop down onto floors below, overloading and failing each one as it went down, he says.

Pay no attention to the original structural engineers!
Then there’s this report, showing that the towers did not collapse in the identical manner these conspiracy sites claim:

A big question for implosion expert Mark Loizeaux, president of Controlled Demolition Inc., the Phoenix, Md., is why the twin towers appeared to have collapsed in such different ways.
Observing the collapses on television news, Loizeaux says the 1,362-ft-tall south tower, which was hit at about the 60th floor, failed much as one would like fell a tree. That is what was expected, says Loizeaux. But the 1,368-ft-tall north tower, similarly hit but at about the 90th floor, “telescoped,” says Loizeaux. It failed vertically, he adds, rather than falling over. “I don’t have a clue,” says Loizeaux, regarding the cause of the telescoping.
The twin towers were part of a seven-building complex designed by architect Minoru Yamasaki that covers eight city blocks. An 800 x 400-ft foundation box, 65-ft-deep and with 3-ft-thick retaining walls, is under more than half the complex, including the twin towers and the adjacent hotel. The complex was completed in phases beginning in 1970 (ENR 7/9/64 p. 36). The 1.8-million-sq-ft Seven World Trade Center, constructed in the mid-80s, also had a steel moment frame from the seventh story up (ENR 11/28/85 p. 30).

(These latter links come from an Australian engineering firm which apparently isn’t in the conspiracy business.)
Bill Herbert does a pretty good job of debunking these theories too. But the debunkers are far, far outnumbered by the proponents.
If you don’t believe me, try Googling around.