The THES gauges reactions to the attacks and experts assess what might happen next.
AMERICA AND THE WORLD
Fred Halliday, professor of international relations, London School of Economics
"This won't weaken America, but it will humiliate and punish it. It will recruit people to the cause of whoever did it.
"It is part of the long-term US neglect of Palestine. President Bush has ridden away from the Palestinian state implied by President Clinton.
"It also goes back to the cold war. Osama bin Laden was set up by the Americans to fight the Russians. He is the sorcerer's apprentice, an ex-CIA client.
"In terms of response, how does America manoeuvre against a non-state group? It is very difficult. Pakistan funded bin Laden and ran him, and it funds Afghanistan as a colony, although it pretends it doesn't."
Youcef Bouandel, senior lecturer in politics and international relations, Lincoln University
Bouandel advised against western governments rushing to bomb the usual suspects. "The risk is that any retaliation will make the victims into martyrs and escalate the conflict still further."
It was unclear who was responsible, but the Middle East situation was likely to be a factor, he said. "While this is a delicate time and the US will not want to be seen to give any ground to terrorists, it does seem that a new outlook towards the Middle East is needed from America."
Peter Lawler, senior lecturer in international relations, Manchester University
"This attack raises questions about whether our expenditure on defence is rightly placed. Interstate war is a declining likelihood, yet we still prepare for it.
"This is a new sort of enemy that may consist of a very small group. That is a difficult thing for national security systems to beat.
"The mechanics of how to go about it are impossible to imagine. Our lives will either have to be so tightly secure as to be unbelievable or very open to security threats such as these."
Mary Kaldor, expert on organised violence, Centre for the Study of Global Governance, London School of Economics
"This is very much a new kind of war. It's not between states, it cannot be contained, and it spreads.
"The worst response would be to start a war against the terrorists. If someone commits a crime, you don't shoot the criminal, you try him. America should be using the war crimes legislation.
"If Afghanistan says it is not them, it should cooperate in finding those who did it. If it doesn't, there should be sanctions - but not retributions and bombing of Afghanistan citizens.
"We will have to increase policing and intelligence and think about why people become suicide bombers."
David Carlton, senior lecturer in international studies, expert on terrorism, Warwick University
"There is not much study of terrorism in the UK, and maybe one result of the attacks will be more interest. In some senses, the whole international system has been changed. Although this kind of attack is not new, what has made the difference is the numbers killed.
"It could be we have to look at a new principal mode of conflict for the coming century.
"Organised terrorist groups and states are unable to compete with the US in international warfare. Some states may see this as a method of equalising things. The United States should be seen as a Gulliver plagued by a number of small tormenters."
Andrew Oswald, professor of economics, University of Warwick
"The danger will be one of over-reaction. I don't think this will have a major effect economically.
"Oil prices are absolutely crucial to the world economy. Although the market responded badly and oil prices shot up, they came down immediately once it was known that it was not an Iraqi plot that would threaten supply.
"Although America is frightened, and frightened customers won't spend, it will wear off. The Federal Reserve will cut interest rates to increase spending. For a short time, people will cease to spend but the panic will subside."
BUILDING AND FIRE SAFETY
Ed Galea, professor of mathematical modelling, director of the fire safety centre, University of Greenwich
"It is extremely difficult to design a structure able to withstand this sort of assault. It would not be considered a credible design scenario to protect against it.
"But there are a number of things we should look at. Perhaps we need to look at how technology can help the evacuating process. We could do more to make lifts usable in the event of fire.
"We also need to look at developing models to study people's response to terrorist acts, which differs from other situations."
PSYCHOANALYSIS OF TRAUMA
Martin Page, expert in the psychoanalysis of tragedy, University of Lincoln
Page was at a conference on dealing with "extreme circumstances" when the news of the attacks arrived.
He believes showing solidarity with the victims is crucial to their recovery. "Nations can extend a sort of vital form of family integrity at times like this."
The attacks have reset the boundaries of trauma management, he said.
"We have to go back to basics. A new culture - which in Europe was born out of the second world war when people pulled together with almighty weight - will start to be seen in the US."
Join our debate by emailing email@example.com