Magnetic forces to blame for 9/11 tower collapse

Scientists can finally explain why the Twin Towers collapsed on September 11, despite the temperature of the fires being well below the 1,500C melting point of the steel girders holding up the buildings.

The discovery that unusual magnetic forces within the girders made them weak at temperatures of about 500C explains away the conspiracy theories that have spread like wildfire since the disaster.

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Sergei Dudarev, of the UK Atomic Energy Agency, found that steel loses its strength above 500C because its molecules undergo a physical transition from one state to another due to magnetic fluctuations. "The steel didn't melt, it just became soft. It is an unusual state and the temperatures in the Twin Towers were high enough to cause it because the thermal insulation was knocked off the girders through the impact with the aircraft," he said.

"Understanding how materials behave means we can find the right 'medicine' to make steel stronger at high temperatures... and if our work can be used for other applications, such as safeguarding tall buildings against disasters, so much the better," he said.