Why firewalking is like mainframes

No-one really understands mainframes unless they’ve worked on them for any length of time. Cloud providers are always talking about modernizing mainframes when they really mean migrating data or applications off it. Distributed people don’t understand just how few people you need to run the equivalent of a hundred or more of their servers on one box. And non-IT people don’t really get what’s going on at all.

Firewalking is a lot like that because everyone thinks that if you walk on hot coals, you will burn your feet. Or they’ll feel confident in asserting that only through intense mind control can you walk on hot coals without feeling the pain. Or it may be that only an intense religious experience can preserve the integrity of the skin on your feet when you walk across coals that are over 500°C. Again, none of those ideas are correct.

In fact, firewalking is possible because of the laws of physics, in particular, what’s known as thermal effusivity. Thermal effusivity (also known as thermal inertia or thermal responsivity) is a measure of an object’s ability to exchange thermal energy (ie heat) with its surroundings. It is defined as the square root of the product of the material's thermal conductivity and its volumetric heat capacity.

Before we drill down into that, let’s just remind ourselves of the second law of thermodynamics. It states that in a natural thermodynamic process, the sum of the entropies of the interacting thermodynamic systems never decreases. An easier way to think of it is that when two bodies of different temperatures meet, the hotter body will cool down, and the cooler body will warm up – and never the other way round. Once they are both at the same temperature, no more changes occur.

So, that would seem to support the argument that the hot coals will heat up your very much cooler foot – which means firewalking is impossible without serious injury. And yet people seem to be doing it all the time! So that’s where thermal effusivity comes in. Let’s go back to the components of its definition.

The thermal conductivity of a material is a measure of its ability to conduct heat. Materials with low thermal conductivity have a much lower rate of heat transfer than materials – like metal – that have a high thermal conductivity and transfer heat very quickly.

The volumetric heat capacity of a material is the heat capacity of a sample of the substance divided by the volume of the sample. It can also be expressed as the specific heat capacity (heat capacity per unit of mass) times the density of the substance.

So, if the hot coals don’t conduct heat very well, the foot of the firewalker won’t burn – unless they stand still for a slightly longer period of time on the coals. That means the secret to successful firewalking is in the name – walking, and not standing still.

Other factors on the side of the firewalker rather than the hot coals are:

• The blood supply to their foot carries away some of the heat.
• Firewalkers keep moving, so they do not spend much time on one lot of embers before their foot moves off.

However, it’s not just if people stand still that they can get burned, it’s also if they run through the embers, their feet push deeper into those embers, which can lead to burning.

There are also two psychological issues to overcome. Firstly, your brain is reluctant to put your body in a dangerous situation, ie putting your feet on hot coals. The solution is to logically understand why it’s not as dangerous as it looks, and to just do it! Secondly, the prefrontal cortex of your brain will want to take control to make sure that you are not burning your feet. It will want to monitor information coming from the temperature sensing nerve cells in your feet. These thermoreceptors can be found all over your skin. While the prefrontal cortex is sensing what’s going on, it will slow down your walking pace – which as mentioned above can lead to burning of the feet. The solution is to sing a catchy tune to yourself, which keeps the prefrontal cortex occupied, or repeat a mantra like, “cool wet grass” as you walk along.

The reason that I am so interested in firewalking is because my younger daughter and I are walking through hot embers on 17 September for charity (homeless people and ex-service people). You can read more about it here. If you’d like to sponsor us to support the charity, you can find our Just Giving page here. Thank you.