The Pitch Drop Experiment — alaricstephen.com

Pitch is a very gloopy sort of tar. To say that again with a bit more scientific vigour, pitch is a highly viscous fluid. You probably already have a sense of what viscosity is if you think about the way that liquid honey or syrup flows, but the take home message is that if a high viscosity fluid has something happen to it quickly then it acts like a solid, but over a long time frame its true liquid properties are revealed. For instance, if you hit something like custard with enough force then it will provide an instant reaction force, so you can run over the top of a swimming pool filled with the stuff, but as soon as you stand still on it then you will start to sink.

In reality fluids have some viscosity and all of this is just an argument of scale; water isn't very viscous, but it is more viscous than air. The exact relative speed at which things switch from being liquid like to solid like is governed by a constant called the Reynold’s Number. An example of this comes in air resistance, where below a certain speed the resistance is proportional to the speed of the object moving through it, but above that limit (which depends on temperature and pressure) the resistance jumps to being proportional to the square of the speed instead. Air resistance is a continuous function, but not a smooth one as we vary the speed.

A straight line up until a certain point then quadratic thereafter

In 1927 there was an experiment held at the University of Queensland which put a large pile of pitch it on a funnel and let it slowly drip through. The emphasis here being very much on the slowly. As it flowed the drops would form at the bottom, just like water droplets forming on a mostly turned off tap, before gathering enough critical mass to finally drop. The pitch was allowed to settle for three years before the bottom of the funnel was opened and over the last 86 years the there have only been 9 drops.

Here’s a table of the drops so far:

Air conditioning was installed between the 7th and 8th drops which we think may have lengthened the process, but it should be slowing anyway.

The last two are worth mentioning because they both happened in a world where webcams exist. In 2000 there were several webcams trained on the pitch, ready to catch the eighth drop, but an unfortunate glitch in the software meant that they failed to capture the moment. With the ninth, the pile up of the previous drops meant that the ninth touched the top of the eighth on the 17th April 2014 without having to have a distinct snap. A week later a decision was made to change the beaker at the bottom, but the ninth snapped as a result on the 24th April. This leaves the exact date of that job as questionable and it means that no one has yet seen an actual drop happen yet. Many eyes will be watching the live stream in 2027-2029.

Other pitch drop experiments have been started, but it recently emerged that there was one that began in 1914 (so predating the famous one) in Aberystwyth University in Wales which has yet to produce a single drop due to a higher viscosity pitch being used. The role of custodian of the main pitch experiment has been passed on, with the third to hold the position, Professor Andrew White being appointed in 2013.

More on very long term projects and experiments to follow soon.