Thomas Parnell’s The Pitch Drop Experiment Explained

In 1927, an Australian physicist named Thomas Parnell set up a laboratory demonstration at the University of Queensland. He heated a sample of pitch, a tar-like substance once used to waterproof ships, poured it into a glass funnel, and let it settle for three years. Then he cut the stem of the funnel and left the pitch to do whatever it would do.

What it did was almost nothing, for a very long time. It is now the longest-running laboratory experiment in the world.

A solid that flows

Pitch looks like a solid. Hit it with a hammer and it shatters like glass. But Parnell wanted to show that it is actually an extraordinarily thick liquid. Given enough time, it flows.

He was right, though “enough time” turned out to be absurdly long. The first drop fell from the funnel in December 1938, about eight years after he opened the stem. The second took until February 1947. Later drops arrived roughly every eight to nine years, though the intervals changed over time, partly because the lab’s temperature changed once air conditioning was installed.

Measurements eventually put the pitch’s viscosity at roughly 100 billion times that of water.

Slow is not the same as still. Some of the most important processes in the world are simply too gradual for us to notice.

The wait

The experiment was largely forgotten after Parnell’s death in 1948, but his successor, John Mainstone, took over in 1961 and watched over it for decades. The setup sat in a display cabinet in the physics building, and the drops fell on their own schedule, indifferent to anyone’s attention.

The cruel irony is that nobody had ever actually seen a drop fall. Observers would step away for coffee, a meeting, or a long weekend, and return to find the pitch had dropped in the interim.

The universe doesn’t schedule its big moments for when you’re looking.

Mainstone came closest in 2000. A webcam had been set up to film the experiment, but it failed on the day the drop fell. In 2014, a ninth drop was expected, and the team set up three cameras to make sure it would be caught. It ended up falling on a day when nobody was watching the feed in real time, and one camera was not recording properly. Mainstone had died in 2013, so he never got to see it.

Finally, on camera

A drop was finally caught on camera in 2014. Researchers reviewing the footage found that the ninth drop had been successfully recorded, giving the experiment’s custodians the first visual confirmation in its history. The moment itself was undramatic: a bulge of black pitch stretching, thinning, and detaching.

It also resolved a small scientific footnote. The footage showed the drop’s motion in a way that supports the physics behind the measurements, rather than relying on inference from the aftermath.

Patience in science often means waiting decades to confirm what you already suspected.

The Nobel that wasn’t

The experiment earned a different kind of recognition in 2005. Mainstone and Parnell were jointly awarded the Ig Nobel Prize in Physics, a parody award given for research that “first makes people laugh, then makes them think.” It’s a lighthearted honor, but it brought the experiment worldwide attention.

Is glass a liquid too?

The pitch drop is often mentioned alongside a popular myth: that old cathedral windows are thicker at the bottom because glass slowly flows downward over centuries. This is false. Glass at room temperature is a rigid amorphous solid, and the uneven thickness in old panes comes from how they were manufactured, using methods that produced slightly uneven sheets, with the thick edge typically installed at the bottom for stability.

Pitch really does flow at room temperature, but glass does not, at least not on any timescale relevant to human history. The comparison is a fun way of separating the genuine curiosity from the folk belief.

What the drops teach

The pitch drop doesn’t reveal any big secret of physics. It is a demonstration of something that is easy to say and hard to feel: that materials can behave very differently depending on how long you watch them.

It’s also a quiet lesson about commitment. Generations of caretakers have kept watch over a funnel that does almost nothing, because a tradition of curiosity is worth preserving even when the payoff comes once every decade.

The most valuable experiments aren’t always the most exciting ones. Sometimes they’re just the ones someone was willing to keep running.

A final thought

As of this writing, the experiment continues, and the next drop is expected in the coming years. There is plenty of pitch left in the funnel, and no one knows exactly when it will fall.

If you ever visit the University of Queensland, you can stand in front of the display, stare at a bulge of black tar, and feel the odd privilege of being part of a story that began almost a century ago and hasn’t finished yet.