Courtesy of Professor J S Mainstone, The University of Queensland.
Courtesy of Professor J S Mainstone, The University of Queensland.

Sometime towards the end of this year, one of the rarest events in science is expected to occur. In a display case in the lobby of the physics department at the University of Queensland in Brisbane, Australia, a small drop of black tar distillate known as pitch will detach itself from the stem of a funnel and fall into a waiting beaker below. It will be the first time a drop has fallen in 13 years, and only the ninth such drop since the experiment was set up 86 years ago.

Thomas Parnell, the university’s first professor of physics, set up the pitch drop experiment to show his students that pitch, which is brittle enough to shatter if hit with a hammer, can flow like a liquid if left to its own devices long enough. Over the course of almost a century, the experiment has survived the relocation of the university campus, extensive renovations to the physics building where it is housed and innumerable changes in university administration and staff. But it serenely carries on, despite the turmoil of the world all around it.

After Parnell’s death in 1948, the pitch drop was stashed in a closet and largely ignored, until physicist John Mainstone, on his second day of work at the university in 1961, was shown the experiment. Mainstone, who had always been interested in the history of physics while working in the Cavendish Lab in Cambridge, was intrigued by the quirky apparatus and decided to take over as custodian, a role he filled enthusiastically until his death last month at the age of 78. He tried to get permission to move it from the closet and put it on public display, but Hugh Webster, the head of the physics department, refused, saying no one would be the slightest bit interested in it.

Years later, Webster’s successor was more amenable to the idea of spreading the word about the pitch drop, and the local media started to take an interest. And when the seventh drop fell while the experiment was on display at World Expo 88 in Brisbane, the explosion of international interest proved Webster definitively wrong. After that Mainstone fielded a steady stream of media enquiries, as magazine journalists and TV crews come across the irresistible hook of such a quirky scientific pursuit.

The pitch drop has inspired artists as well, with sculptures designed and poems composed in its honor. And countless people around the world regularly check in on its glacial progress via the live webcam that streams the experiment 24 hours a day on the university’s website. ‘Quite a number of people have told me they find it extremely soothing to watch it for a while at lunch after they’ve had a busy morning,’ Mainstone told me earlier this year.

But beyond the public enthusiasm, it’s not entirely clear what Parnell had in mind when he started the experiment in 1927. The idea of an apparently brittle solid behaving like a liquid is interesting, and he was probably trying to get his students to think differently about the passage of time by bringing the apparatus out at the beginning and end of an academic year, but Mainstone thinks he was also trying to make a deeper point about the study of physics. In 1927, the nascent field of quantum physics was all the rage, bringing with it the idea that at a fundamental level, nothing was as it seemed. Parnell may have been trying to show that there were still mysteries to be unraveled in the realm of classical physics, Mainstone speculates. ‘Why should these new quantum physicists have it all their own way when it came to things not being what they appear to be?’ he said.

But whatever his motives, Parnell certainly wasn’t doing it to advance his scientific career. In nearly a century, the experiment has produced exactly one paper – a calculation of the viscosity of pitch published in the European Journal of Physics decades after Parnell’s death (the viscosity, by the way, is about 230 billion times that of water). And in 2005 Parnell posthumously shared the IgNobel Prize for physics with Mainstone, though that gently mocking accolade, shared with some genuine Nobel Laureates, certainly hasn’t done any damage to either man’s reputation.

Sadly, Mainstone never got to witness a drop fall with his own eyes. When the eighth drop fell in November 2000, a webcam that was supposed to record the event, malfunctioned at the critical moment. There are now three cameras trained on the apparatus, as Mainstone was keen to finally learn exactly what happens when the drop detaches from the funnel.

But the real value of the experiment has always been its cultural and philosophical significance – the way it allows people to reflect on the passage of time, provides a connection to the history of physics, and demonstrates the importance of careful, patient observation to scientific discovery.

To ensure there is an opportunity for such reflection in the future, Mainstone had already convinced one of his former students, Andrew White, to take over responsibility for the pitch drop after he was gone. ‘‘It’s quite daunting,’’ says White. ‘‘No one will remember any of my other research now, just that I was the third custodian of the pitch drop. But there are worse fates.’’ And with enough pitch left in the funnel to last for another 100 years or so, White will eventually need to find a fourth custodian as well. With such dedication, and a little luck, the pitch drop will continue to inspire for years to come.

You can see the live video stream of the pitch drop at:

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DOI: 10.1016/j.mattod.2013.08.017