The Millennium Bridge is the first bridge to be built across the Thames for a hundred years. It links St Paul's Cathedral and the City of London on the north bank with the new Tate Modern and Shakespeare Globe on the south bank.

Spanning some 320 metres the Millennium Bridge is the longest pedestrian suspension bridge in the world. The designers call it a marriage of architecture, sculpture and engineering and it cost eighteen million pounds.

But after all the excitement on the first day of opening it wobbled! Its engineers had carried out computer simulations, wind tunnel tests, and mathematical calculations and were still caught out.

Its wobble has a psychological and social explanation as well as a purely mechanical one. This is why:

People synchronise their step in small crowded spaces so they don't bump into each other. This is an automatically action we do without thinking but was not well known among engineers before the bridge opened. The engineers had not made adequate allowances for this action, so when a few people started walking in step the bridge started swaying. This is because as well as producing a downward force the pedestrians were producing a sideways force as their weight transferred from foot to foot. Once a sway starts to develop, other people readjusted their step to walk in time with the sway. This further exacerbates the sideways movement.

All structures are designed to move and engineers had made allowances for the bridge moving. They had also made sure that the natural frequency the bridge would move at - called the resonance - was different to the frequency caused by people walking on the bridge. But the bridge still wobbled.