This is just an hypothetical example of what future defense system would like, to combat terrorism. Potential defenses range from advanced electronic surveillance system that could intercept would be terrorists before they get onto a plane. But experts say that just being prepared to stop 9/11 type of attack is not enough. 9/11 was a unique attack in its own times. Future attacks will likely to use different methods to maintain the terror and surprise. Researchers are also searching for a technology to defend aircraft against a wider range of threats including shoulder fired missiles. It's probably a matter of time before such defense mechanisms are tested again.
Listing some of the newly technical advancements that have come into picture recently.
These in theory are the same ones which we usually play with. Boeing has actually developed remote-control technology for flying an airliner which in theory would have prevented the attacks on the world trade center.
The best way to stop an attack in theory would be to identify the terrorists in the airport and arrest them. Already, U.S airports have been equipped with explosive detection devices and imaging scanners that would allow security personnel to look under passenger's clothing for concealed weapons or explosives. But researchers are envisioning sophisticated video surveillance that has a software which analyzes facial expressions for signs of lying. Other devices would be the remote scan for irregular blood pressures and heart rates.
Onboard video monitoring
European researchers are developing onboard surveillance systems in the cabin. The setup might include the software to analyze passenger's facial expressions and movements for suspicious behavior.
Light guns to dazzle attackers
U.S govt researchers are looking into the possibility of equipping air marshals with non lethal weapons similar to the laser beam of light that could temporarily blind a person and disorient an attacker. Such weapons might disable hijackers without endangering other passengers.
However, many experts warn against spending too much time and money preparing for a repeat of 9/11 attacks since its unlikely that the future attacks would involve flying planes into the buildings. Also, today the airline crews and passengers would fight back. For eg. in the 9/11 attack, one of the flight that was about to hit capitol building was grounded to shanksville Pennsylvania. In a second incident there was a passenger who was a "shoe bomber" but his attempts were subdued by the passengers.
So, when was the last time we see such plane attack, it is sept 11 attack. So, the chances are we are not going to see terrorists trying the same thing.
Background information - National Geography