Coffee quakes

ripples on coffee at Rosslyn, the City
From ripples on the surface, to listening to the sound your coffee makes. What links a coffee to an earthquake?

What do you hear when you listen to your coffee? Or a related question, what links your coffee to earthquakes and seismology?

In recent weeks I have been making coffee with milk, not often, but enough to notice something slightly strange. While heating the milk in a small saucepan, I have accidentally tapped the side of the pan while the milk was in it. The tap, perhaps unsurprisingly, produced a ripple on the surface of the milk propagating away from the point of tapping. But what was surprising was that a very short time later, a second ripple was generated, this time from the other side of the pan propagating back towards the original wave.

The first ripple had not yet travelled across the milk surface before the second ripple had been generated and travelled back towards it. Something was causing a vibration on the other side of the pan before the first ripple had had a chance to get there. Was the pan acting like a type of bell which, as I tapped it, started to resonate all around its circumference?

Assuming that the vibration of the tap travels at the speed of sound through the metal of the pan, it would take about 50 μs for the vibration to travel half way around the circumference of the pan (diameter 14cm, with a speed of sound in steel ~ 4500 m/s). But then, if the pan were resonating, the resonance frequency would depend on the speed of sound in the milk filling the pan, which would increase as the milk was warmed. Would we see evidence for this if we video’d tapping the pan as we heated the milk?

coronal hole, Sun
Observing periodic changes to the luminosity of stars can indicate the elements within them. Image credit and copyright NASA/AIA

Rather than watching the liquid within, we could also learn about the interior of a cup of coffee by listening to it. The “hot chocolate effect” is the classic example of this. The effect occurs when hot chocolate powder is added to warm water or milk and stirred. Think about the pitch of a sound made by tapping gently on the base of your mug while you make a cup of hot chocolate. Initially, adding the powder and stirring it will introduce air bubbles into the liquid. As you stop stirring the hot chocolate but continue to tap the base of the cup the air bubbles leave the drink. The cup is acting as a resonator, so the sound that you hear (the resonance of the cup) is proportional to the speed of sound in the liquid in the cup. As the speed of sound in hot water containing lots of air bubbles is lower than the speed of sound in hot water without the air bubbles, the note that you hear increases in pitch as the bubbles leave the drink. You can read more about the hot chocolate effect in an (instant) coffee here.

It is here that we find the first connection between coffee and earthquakes. Seismologists have been listening to the vibrations of the Earth for years in order to learn more about its interior. By observing how, and how fast, waves travel through the earth, we can start to understand not only whether the inside is solid or liquid, but also what the earth is made from. This is similar to learning about the air bubbles in our hot chocolate by listening to the sound of the mug. More recently, the seismologists have shown the effect of the Covid-19 related “lockdowns” on reducing seismic noise. Something that does not have an obvious coffee cup analogy.

But seismology is not just confined to the Earth. Vibrations of a different kind have also been used recently to learn more about the interior of stars, although here it is a mix of seeing and ‘listening’. Generally, when the surface of an object vibrates, it leads to compressions and expansions of the medium within the object. This is the essence of what sound is. But in a star, these compressions and expansions also result in changes to the luminosity of the star. So, by looking carefully at the frequency of the variation in brightness of different stars, it should be possible to work out what is going on inside them. It is a branch of physics now known as “Astroseismology”. Recent astroseismology results from NASA’s Kepler satellite have been used to challenge theories about how stars form and evolve. It had been thought that as a star develops, the outer layers expand while the core gets smaller. The theories proposed that this would result in a certain change to the rotation speed of the core of the star. The astroseismology observations have revealed that, while the gist of the theory seems right, the core rotates between 10 and 100 times slower than the theories would predict. As one astroseismologist said “We hadn’t anticipated that our theory could be so wrong…. For me, finding that problem was the biggest achievement of the field in the last ten years.”.

We now use strain gauges in electronic measuring scales. They were originally invented for an entirely different purpose.

Seismology and astroseismology offer clear links between listening to your coffee cup and earthquakes (or star quakes). But there is one more earthquake related connection to the coffee cup and it could be noticed by any of us who want to improve our home brewing technique.

To brew better coffee, we need to measure the mass of the coffee beans that we are using. Typically we will use a set of electric scales for this. Inside the scales is a device, called a strain gauge, that shows a change in its electrical resistance as a result of the pressure on it (from a mass of coffee for example). The scales translate this change in the electrical resistance to a mass that is shown on the display. One of the inventors of the strain gauge however was not thinking about measuring the mass of coffee at all. His interest was in earthquakes and specifically, how to measure the effect of the stresses induced by earthquakes on elevated water tanks. In order to do that he needed a strain gauge which led to the devices that you can now find in your measuring scales.

Two links between your coffee cup and earthquakes or seismology. Are there more? Do let me know of the connections that you find, either in the comments below or on Twitter or Facebook.

Hear no evil… at the Inverness Coffee Roasters, Inverness

Coffee in Inverness

Inverness Coffee Roasting Company.

Hear no evil, see no evil, speak no evil, so the saying goes and so the monkey, that was sitting on the sofa at the Inverness Coffee Roasting company (and café) indicated. And while I would not like anything evil to be said on this website generally, today it will be taken to an extreme as this cafe-physics review will not say much at all. Not because the coffee was not good, my Americano was a lovely complex dark and very enjoyable brew. Nor because there weren’t things to write about, several avenues suggested themselves for a ‘cafe-physics’ type review. There were also plenty of things to enjoy nibbling on while sitting down in this warm and comfy environment taking in the surroundings. Chocolate from The Chocolate Place was clearly labelled (and so I could enjoy it confident that it was nut-free) with a good variety of interestingly flavoured chocolates. The chocolate/coffee combination always goes well and the salted dark chocolate indeed complemented the coffee. A variety of freshly roasted coffees were in jars ready for selling to the home-brewing crowd and I heard both people behind the counter discussing coffee tastes with different customers to ensure that they could properly recommend a coffee for each of them.

So why, if the coffee was good, the service friendly and the environment interesting am I not going to write much about Inverness Coffee Roasting Company? Well, largely because I had been on holiday and so the preoccupations of the days before would necessarily influence what I noticed about this little café. Although I could happily write about neolithic monuments and considerations about inter-generational solidarity in relation to the re-use of refuse heaps at Skara Brae (as building material) and our own use (or misuse) of refuse in our environmental behaviour today. And it could even fit into a cafe-physics review of this venue as a sign on the wall above the door invited everyone to join the Plastic Free Scotland movement. However, it is not really what a Bean Thinking cafe-physics review is about. The idea behind the cafe-physics reviews is that things are often connected in surprising and beautiful ways and we can generally only see the connections if we slow down and look for them.

hear no evil, see no evil, speak no evil

Monkey on a sofa, but there was much more to notice at the Inverness Coffee Roasting Company if you looked around you.

Therefore, while you may (or may not) prefer to read about my holiday considerations of all things thousands of years old, what I thought I’d do with this café review is suggest a few things that I noticed in the café, things that offered a variety of potential thought-trains and then see what you think, what you notice, what you see (or don’t see). Perhaps you will observe something in one of the photos that clicks into a thought train for you, perhaps you can look around you, wherever you are right now – and think about the connections you could make to things you sense there instead? But whichever you do, it is a great time to sit back with a coffee (or perhaps a tea), breathe in and take in your surroundings.

Back in Inverness, the first thing of course was the monkey. Eyes shielded with an arm, suggestive of those who would prefer not to see what is in front of them. Nancy MacLean in “Democracy in Chains” notes that a training in the humanities perhaps opens students to question their society more than other, more utilitarian, subjects may do. Is it hurt pride that makes me rebel against this idea? Can’t physicists question too?

Perhaps it is connected but a sign by the door, and an identical one by the sofa, was written on the glass front of a box of coffee beans: “in emergency break glass”. This suggested so many avenues for exploration to me, I wonder which occur to others?

Behind our seats, a lizard was painted on/engraved into a stone, suggestive of fossils, geology and how we collect evidence. But a second lizard suggested a different direction. An ornamental lizard was positioned as if climbing up the counter. How do lizards climb? What is it about their feat? What connects lizards to a coffee company? Above our heads and above the door, stereotypical of Scotland perhaps, loomed a deer head complete with antlers. But this one was different: it was made of coffee beans and string. How a bean based diet could replace a meat one? The nature of units and how it would not necessarily be sensible to measure the mass of a deer in units of coffee beans? The mind jumped. Jumping beans?

Deer head in beans

Bean there done that?
Gruesome ornament or vegan friendly?

Finally, the logo of the cafe which was featured on signs around the interior and exterior of the space: a flaming coffee bean. The Maillard process and the changes in coffee as it is roasted? The nature of heat/temperature and the manner in which we measure it? What we hear as fire burns, lightning bangs or on “the first crack” of roasting and what this tells us about our atmosphere, our planet and our coffee is made of?

Whatever you notice, please get in touch, either by Facebook, Twitter, leave a comment or send me an email. But one last thing on coffee thought trains that links to real trains and is perhaps reflective back onto what it means to pause and watch. We left Inverness the day after our visit to Inverness Coffee Roasting by train. Inverness station has a relatively steep (1:60) gradient for 20 miles on leaving the station. It is Scotland in autumn, it had been raining and leaves had been falling on the line. Five or ten minutes out of the station, our train to Kings Cross juddered and came to a halt. A signal failure apparently. As the driver re-started the train, it slipped backwards, and again. We weren’t able to get up the hill. And so we had to return into Inverness station. Once back at Inverness station, the guard came across the tannoy and suggested that the signallers had given us the go-ahead to ‘have another run’ at the hill to see if we could get up it this time. So we tried again, juddering and shaking to a stop a few hundred metres beyond where we had stopped before. Back to Inverness station it was. The ever hopeful guard came across the tannoy again: “Third time lucky, fingers crossed”. This time the train left the station faster, building up speed, moving along more quickly and powering out of the station. The carriage was silent, were we going to make it? Past the first point we stopped at, past the second, a bit further, the family behind clapped, the train continued then slowed down and shook, juddered and then sped up again. We were over the hill and on our way back to London.

A last consideration on the conservation of energy and its relation to coffee and thought trains? Or a metaphor for how we may not find those connections in that cafe come to us quickly but if we persist and keep noticing, we can go on a fascinating journey?

Do let me know your thoughts.

The Inverness Coffee Roasting Company can be found at 15 Chapel Street Inverness, IV1 1NA