I've been angered and sickened to see so many 9/11 conspiracy theorists poking their heads out of the woodwork with last weekend's 20th anniversary of the tragedy. No matter how many scientific, evidence-based, practical explanations are offered, they still insist on their nonsensical speculation that the World Trade Center towers were brought down by explosives planted by some nefarious insider, or that other buildings were dynamited to hide evidence, or that the Pentagon was not hit by an aircraft, or whatever.
It's all stuff and nonsense, of course. I've come to use these conspiracy theories as a litmus test for whether someone's trustworthy or not. If I hear a single word out of them in support of such lies, I know they're not worth my time or attention. To quote one of the 9/11 rescuers who survived, "they're looney tunes".
In case you ever have to debunk their nonsense (I suggest not arguing with them openly, because they won't listen to even the most scientific of evidence - they prefer not to think), I recommend the outstanding analysis published by Popular Mechanics addressing the conspiracy theories. It's worth bookmarking it for future reference. Here's just one example of their thoroughness.
CLAIM: "We have been lied to," announces the website AttackOnAmerica.net. "The first lie was that the load of fuel from the aircraft was the cause of structural failure. No kerosene fire can burn hot enough to melt steel." The posting is entitled "Proof Of Controlled Demolition At The WTC."
FACT: Jet fuel burns at 800 to 1500 degrees Fahrenheit, not hot enough to melt steel (2750 degrees Fahrenheit). However, experts agree that for the towers to collapse, their steel frames didn't need to melt, they just had to lose some of their structural strength—and that required exposure to much less heat. "I have never seen melted steel in a building fire," says retired New York deputy fire chief Vincent Dunn, author of The Collapse Of Burning Buildings: A Guide To Fireground Safety. "But I've seen a lot of twisted, warped, bent and sagging steel. What happens is that the steel tries to expand at both ends, but when it can no longer expand, it sags and the surrounding concrete cracks."
"Steel loses about 50 percent of its strength at 1100 [degrees Fahrenheit]," notes senior engineer Farid Alfawak-hiri of the American Institute of Steel Construction. "And at 1800 [degrees] it is probably at less than 10 percent." NIST also believes that a great deal of the spray-on fireproofing insulation was likely knocked off the steel beams that were in the path of the crashing jets, leaving the metal more vulnerable to the heat.
But jet fuel wasn't the only thing burning, notes Forman Williams, a professor of engineering at the University of California, San Diego, and one of seven structural engineers and fire experts that Popular Mechanics consulted. He says that while the jet fuel was the catalyst for the WTC fires, the resulting inferno was intensified by the combustible material inside the buildings, including rugs, curtains, furniture and paper. NIST reports that pockets of fire hit 1832 degrees Fahrenheit.
"The jet fuel was the ignition source," Williams tells Popular Mechanics. "It burned for maybe 10 minutes, and [the towers] were still standing in 10 minutes. It was the rest of the stuff burning afterward that was responsible for the heat transfer that eventually brought them down."
There's much more at the link. Very useful and highly recommended reading. The article contains links to other articles and essays addressing different aspects of the 9/11 tragedy, which are also worth following.