Collision in pitch drop experiment

The University of Queensland’s (UQ) pitch drop experiment marked a milestone last week as the ninth drop of pitch slowly collided with the eighth drop in the bottom of the beaker.

Image showing the ninth drop’s movement between 9 and 14 April 2014. The gap closed quite rapidly during this period, with the drop descending 2 mm in a week, compared to the average of 6 cm over the last 13 years, before it collided with the eighth drop. Image credit: Dr Roy Duncan, UQ.

The experiment was established in 1927 by Professor Thomas Parnell to demonstrate that pitch - a derivative of tar and a solid material - can flow like a liquid. Pitch-drop custodian Professor Andrew White said seven drops fell between 1930 and 1988, at an average of one drop every eight years.

“Two things changed after that - the 2000 (eighth) and 2014 (ninth) drop each took about 13 years to fall, and each collided into the decades-old pile of drops in the beaker before it could break away from the funnel,” he said.

The eighth drop ran into the seventh drop in 2000, but took almost 14 years to tip over. Professor White explained, “It was still connected to the ninth drop but almost broke free this year.

“The connection had become thin, stressed and light grey - but now that the ninth drop has run into it the whole cycle starts again.”

The former custodian of the experiment, Professor John Mainstone, missed observing the drops fall on three occasions - by a day in 1977, by five minutes in 1988 when it was on display at the World Expo in Brisbane, and in 2000 when a webcam that was recording it missed the moment when the drop fell during a 20-minute power outage. Professor Mainstone died in September last year.

The experiment is now under constant surveillance, with three webcams trained on it to capture the ninth drop’s fall. Interested viewers can keep an eye on the ninth drop through the live web stream at www.theninthwatch.com, with nearly 25,000 viewers from 158 countries currently registered.

Those viewers who were watching the live stream when the drop collided will have their names recorded for posterity. Professor White said the video footage will be analysed to determine the actual moment of the collision.

“We look forward to observing what will happen next with the ninth drop,” he said.

“It may tip over quickly, or it might slow right down and take years to break away from the imminent tenth drop.”

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