Intuition gets the bullet

Much of the world as revealed by science runs counter to what we would redict from intuition. Scientific understanding therefore requires a little effort, yet it remains an easily acquired taste - and a deeply satisfying one.

Commonsense observation tells us that we live on a flat stationary earth under a huge blue dome of sky, and every morning the sun rises at one side of the dome and slowly moves across it to disappear at the opposite side at sunset.

But science tells us that earth is a sphere that spins on its own axis and also rotates annually, in a large elliptical orbit around the stationary sun.

The daily journey of the sun across the sky reflects our spinning earth, not a moving sun. Also, although the earth spins at more than 1,000 kilometres per hour, we do not notice this, because the speed is steady and we, and everything around us, move along with the spinning earth.

We can all easily appreciate that the earth is round, because we have seen photographs of our planet taken from space. But hundreds of years ago, scientific minds deduced the shape of the earth by analysing simple observations.

It was noticed, for example, that the first glimpse one got of a ship approaching from afar was of the tips of its masts jutting above the horizon. Then, gradually, more mast became visible, then some of the hull and, soon, the whole ship. Also, as long ago as 350BC, Aristotle noted that, as you travel north or south, new stars appear over the horizon ahead and visible ones disappear beneath the horizon behind. The logical deduction was that the earth's surface was curved, not flat.

The laws of motion in physics lead to many conclusions that run counter to intuition. Many would feel, for example, that if you want to bomb a target from a plane, you should wait until you are directly overhead, then release the bomb.

If you do you will miss, because the bomb continues to move forward as it drops and hits the ground some distance beyond the target.

It would also be useful to know something of the law of conservation of energy were you ever to feel moved to celebrate by shooting bullets straight up into the air. This law predicts that your bullets will return pretty quickly to earth, travelling at the same speed at which they left your rifle, but now whizzing in your direction.

Now imagine you are in the middle of a desert and have two bullets, one loaded in a pistol, the other free in the hand that holds the pistol. Extend your arm horizontally and fire the pistol so the bullet flies off parallel to the ground.

At the instant you pull the trigger, drop the free bullet to the ground. Which of the two bullets will hit the ground first? Your intuition says the free-drop bullet, but both hit the ground at the same time. The rate of fall to earth of both bullets under gravity is the same.

Science also tells us things about the big and the small that our intuition, tempered in the scale of the everyday world, finds difficult to grasp. Consider a molecule whose incredibly small size can be illustrated as follows. Take a glass of water, which contains innumerable water molecules, and empty it into the sea.

Wait for a period to allow the molecules from your glass of water to mix evenly with the water in all the oceans of the world. Now go to any sea in the world and fill your glass. This glass of water will contain many of the same molecules that you originally poured into the sea. This means a glass of water contains many more molecules than all the seas in the world contain glassfuls of water.

The case of Clever Hans illustrates how careful we must be in interpreting the ways of the world. A German horse living at the start of the 20th century, Hans was trained by his master to solve arithmetical problems. He would tap out the answers with his hoof. Clever Hans could even take problems from strangers, as long as his trainer was close at hand, and tap out the correct answer.

It seemed Hans had a brain to rival the human brain. Puzzled scientists repeatedly studied Clever Hans. Eventually, in 1911, a Prof Pfungst showed that Hans was not doing arithmetic but picking up cues from his trainer. Hans could interpret gestures such as a slight nodding or holding of breath when the correct count was reached.

This sensitivity to body language is, of course, impressive in its own right. The case of Hans is now known in the behavioural sciences as the "Clever Hans error".

Science shows that the world is not as it seems. To appreciate this we must exercise the little grey cells, as Poirot would put it. The understanding that results from the effort is deeply pleasurable.

• William Reville is a senior lecturer in biochemistry and director of microscopy at UCC.