ScienceWise - Summer 2013

The editor's corner

Don’t “believe” in science

Mention anything vaguely mystical to a scientist and chances are they’ll be skeptical. What’s your evidence they’ll probably ask? Because whilst one could argue that science may not explain every aspect of human experience, the scientific method of hypothesis built on observation is without a doubt the most logical and accurate description of the world humans have ever come up with. But that skeptical demand for evidence sometimes fails when the tables are turned. Let me give you an example.

A few years ago I attended a scientific conference and as is quite common at such meetings, there was a little light entertainment laid on for the evening. In this particular case it was a gentleman presenting a talk on fire walking - you know where people take off their socks and shoes and walk over red hot coals. It was all very entertaining. He even showed us a short film which included various people offering mystical explanations for what was happening all of which met with a predictable echo of laughter and scorn from the audience.

The presenter then proceeded to explain that real reason the walkers didn’t get burnt was that sweat on their feet was creating a Leidenfrost effect. That’s where a liquid boils frantically in contact with a hot surface and the little cloud of vapour keeps the drop hovering. Like when you sprinkle water in a really hot pan and the drops dance around.

The audience applauded loudly and there was a general air of smugness that science had once again carried the day. The trouble is, just like the mystical explanations, this explanation is was also completely and utterly wrong!

A room full of very clever physicists were taking an explanation on faith because it seemed to agree with their world view. But where was all the critical thinking that those big brains were so easily capable of?

0.1ml of water on a uniform surface can support it’s own weight by the Leidenfrost effect, yes. But a 70kg human on a contact area of perhaps 5cm2 would require a steam pressure of 10 atmospheres - That’s the working pressure of a marine boiler!

The real reason the walkers don’t get burnt is because although the embers are very hot, they have a very low heat capacity whilst a big lump of meat such as a human foot has quite a high heat capacity. Add to that the low thermal conductivity of both and what’s happening is that each time the foot touches a coal it sucks out enough heat to cool the surface of the ember right down without heating itself up enough to sustain significant damage in the process. Because the charcoal has such poor thermal conductivity the hot core of the ember can’t transfer energy to the surface quickly enough to replace the lost heat. Any contribution from the Leidenfrost effect is negligible in comparison.

Anyone in that room would have known that if they’d stopped to think for a moment. But they didn’t. And that worries me a lot because science should be proved not believed. That’s kind of the whole point of science, you’re supposed to question it!

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