The World’s Longest-Running Experiment Is Still Teaching Us Physics

We're nearing the experiment's 100th year and 10th drop.

In 1927, physicist Thomas Parnell poured hot pitch into a funnel at the University of Queensland. In this context, “pitch” is a name or asphalt or bitumen, substances that appear solid at room temperature, but are actually flowing extremely slowly.

This is what Parnell wanted to prove: that the substance really does flow. It looks like a lump of tar doing nothing, but in reality, the pitch is moving. It’s been moving ever since Parnell cut the sealed stem of the funnel and gravity started its work. But it’s moving so slowly, only nine drops have fallen until now. But a tenth one is forming.

The Longest Experiment in the World

The Pitch Drop Experiment holds the Guinness World Record for the longest-running laboratory experiment. Parnell prepared the experiment by heating pitch until it softened enough to pour into a glass funnel. He then sealed the funnel and left the material to cool down and settle for three years.

Parnell wanted students to understand that “solid” and “liquid” are not always obvious categories. He set up the experiment not necessarily to prove new things, but rather to teach his students something. Asphalt was perfect for the job. At room temperature, it behaves like a brittle solid. Strike it and it can fracture. Heat it, however, and it softens enough to pour.

The first drop fell in December 1938. The second followed in February 1947. For decades, the drops arrived roughly once every eight or nine years. That rhythm eventually changed.

The eighth drop fell in 2000, more than 12 years after the seventh. The ninth did not separate until 2014, and the tenth one is forming, but hasn’t separated yet. The longer intervals can teach us some important physics. As pitch drains from the funnel, the pressure pushing it downward gradually decreases. But there’s another parameter: air conditioning, installed in the building in the 1980s, also cooled and stabilized the surrounding air, slowing the flow. For a substance as viscous as pitch, a few degrees matter.

The Human Side of Physics

The pitch drop experiment is famous not only because it is slow, but because it has been so difficult to witness. Several drops have been missed.

Parnell showed the experiment to generations of children, but after Parnell’s death in 1948, the apparatus was briefly forgotten. It might have remained a curiosity in storage had it not been rediscovered by physicist John Mainstone, who became its long-serving custodian.

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Mainstone cared for the experiment for more than half a century. He moved it into public view and turned it into one of the world’s most beloved pieces of laboratory equipment.

Yet he never saw a drop fall.

One fell over a weekend, when he wasn’t in. Another dropped when he had gone to get a drink during an exhibition of the experiment. In 2000, a webcam had been trained on the eighth drop, ready to make history. A thunderstorm caused a technical failure at the critical moment.

An Honor and Great Responsibility

In 2005, Parnell and Mainstone were awarded the Ig Nobel prize in physics. This is a parody of the Nobel Prizes, meant for studies and experiments that make you laugh, and then make you think. At the time, Mainstone expressed his satisfaction at the prize:

“I am sure that Thomas Parnell would have been flattered to know that Mark Henderson considers him worthy to become a recipient of an Ig Nobel prize. Professor Parnell’s award citation would of course have to applaud the new record he had thereby established for the longest lead-time between the performance of a seminal scientific experiment and the conferral of such an award, be it a Nobel or an Ig Nobel prize.”

Mainstone died in 2013, eight months before the ninth drop separated. Custodianship passed to professor Andrew White. White made an important decision, replacing the beaker holding the previous eight drops. If the ninth drop had fused to them, it would have permanently affected the ability of further drops to form.

As of this writing, the experiment is still ongoing.

The Legacy of a Long Experiment

The individual drops that form and fall down are interesting and can help researchers finesse calculations about viscosity. But the real value lies in the way it has expanded our imagination of science.

What started as a simple classroom demonstration has become an exercise in patience and continuity, a reminder that scientific vision and demonstrations can transcend generations and individual careers.

Other institutions have their own slow-flowing pitch. Trinity College Dublin began a similar experiment in 1944; its drop was recorded falling in 2013, the first pitch drop captured on camera. Aberystwyth University has a pitch experiment dating to 1914, older than Queensland’s, but it has yet to produce a drop. St. Andrews has pitch that flows more like a slow stream than discrete drops. A demonstration in Edinburgh may date to 1902 but hasn’t been confirmed.

We’ve learned a lot about viscosity since.

There is still enough pitch left for the experiment to continue for many decades, perhaps another century. But a milestone is just around the corner.

The year 2027 will mark 100 years since Parnell first poured the pitch into the funnel. The official flow began in 1930, when the stem was cut, but the centennial still matters. It reminds us that the experiment was built by one generation, preserved by another, and now watched by a global audience that may never meet the people who see its final drops.

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