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Theory and practice

October 23, 2009 Leave a comment

There’s this old idiom “Don’t throw out the baby with the bathwater”, which means, essentially, that if some idea is to be discarded because it’s become useless, you might not want to discard all of that idea’s features, which might still be useful, valid or fruitful. The bathwater is useless after the bath, but don’t forget that there are still precious things in it.

There are some theories which have turned out not to be valid in the situations we thought they were, and a prime example is the Bernoulli Effect in aircraft wing design. It was originally believed (and you can still see this in many textbooks, even today) that aerofoils work because of the Bernoulli Effect, which describes how air travelling above and below the wing’s surface creates a pressure differential which results in an upthrust to the wing, hence the possibility of flight.

Now, this was later discovered to be nonsense – experiment revealed that this is not how aerofoils work at all. But this doesn’t invalidate the existence of the Bernoulli Effect, which is still a real phenomenon in many other situations. And neither does it suddenly mean that aircraft are going to stop flying, or that their wings must all be redesigned.

We discovered that wings work, with this special shape that we observe in birds and fish, and we made our wings likewise, experimenting with different shapes until they seemed to be optimally efficient for actual flight. The theory came afterwards, and it just happened to be the wrong one – very probably many wrong ones. But that makes little functional difference to the wings themselves.

The right theory, we now know (or, to be strict, the theory that explains it all much better*) involves Newton’s Laws, and a few other fluid dynamics theories such as the Coanda Effect.

But the interesting thing is that, while engineers continue to enjoy heated debate about what the fundamental cause of Aerodynamic Lift is, aeroplanes still work, and birds – who have no interest in these causes, nor have any need of such information – still fly.

We’re always catching up with the universe, and this is where the humility of science should really show, and always does when we abandon a theory that we’ve discovered is wrong.

The fact that the universe does things whether we understand their causes or not is in direct opposition to the standard religious type of view that says that the universe should be the way we want it to be: the way that makes us feel special or important or loved. See this Laurence Krauss lecture for much more on this.

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