Pitching for the long term

If you need a less frantic pastime than watching paint dry or grass grow you should head to the St. Lucia campus of the University of Queensland in Brisbane, Australia. Make your way to the second floor of the Parnell Building and behold the world’s longest continuously running laboratory experiment – the pitch drop experiment.

In 1927, Professor Thomas Parnell, the university’s first professor of physics, set up the experiment. Heated pitch (in this case bitumen) was poured into a sealed funnel and allowed to settle and cool for three years (this experiment is nothing if not far sighted). In 1930, the sealed neck of the funnel was cut, allowing the pitch to start flowing.

If you’ve ever seen a road shimmer on a hot day and walked on the sticky, sun-warmed tarmac then you’ll know that bitumen is not a true solid. The pitch experiment sought to determine the viscosity of bitumen – how quickly will tar flow compared with water?

A few months after the funnel was opened a fat, glistening drop of pitch began to slowly bulge from the neck. After a few years, it assumed a rounded form – a droplet perfectly suspended but not ready to drop. And it stayed like this, growing imperceptivity bigger day by day until in December 1938 when the first drop fell.

Fast forward 73 years and another seven drops have fallen with an average interval between drops of 8.75 years. The last drop was on 28 November 2000, so the ninth drop is well overdue. In the 84 years of the experiment no one has actually seen a drop fall. If you fancy your chances, the University of Queensland now operates a webcam – guaranteed to be more exciting than watching paint dry!

The result of the experiment? Physicists have calculated that the bitumen has a viscosity approximately 230 billion times that of water.