If a Tree Falls in the Forest...

If a tree falls in the forest, and no one is there to hear it, does it make a sound?

This age-old, 'unanswerable' question has a remarkably simple solution. First, let us agree on a definition of sound. Webster's dictionary defines a sound as 'a tone or noise that is heard.' I shall modify this definition only to clarify it, stating that a sound is a disturbance in the air or some other medium, as interpreted by a living being. So for a sound to exist, a living being must be present.

But what does 'hearing' a sound mean? If you are deep in thought, and someone says something to you, you will turn around and say, 'Sorry, I didn't hear you.' Let us define hearing a sound to mean receiving and reacting to a sound. But a rock will shudder in the wake of an explosion. Surely it does not hear it. Let us then say that hearing a sound implies reacting intelligently to that sound. But how intelligent does the reaction need to be? If you hear a loud noise and turn toward it, you haven't put much strain on your brain, but you have reacted in a remotely intelligent way.

But do you have to react visibly in order to hear the sound? I hear the hum of my hard drive from time to time, but I do not acknowledge it, or change my typing speed, or react in any externally visible way. Therefore a reaction to a sound need not be visible. So what of the rock that sits beside the falling tree? Does it have an invisible but intelligent reaction? Of course not, you say. Rocks have never been seen to display intelligent behaviour. Therefore it is logical to conclude that an entity that has been observed to display intelligent reactions to certain stimuli, and is capable of reacting to a disturbance in the air (or other medium), can be said to hear a sound.

Next, let us agree on a definition of a forest. In the real world, a forest consists of trees, smaller plants, and small animals, all of which are necessary for the healthy existence of the forest. Even if no human is present, these animals will hear the sound of the falling tree, and react intelligently to it, even if just to turn toward it.

But let us consider an abstract, idealized forest that consists merely of trees. Can the trees hear the sound? Can they react 'intelligently' to anything? If you expose a tree to a large enough disturbance in the air, its growth pattern might be affected slightly; if another tree falls close by, the disturbance carried through the ground may affect root growth. But is that an intelligent reaction? Can plants react intelligently to anything? A number of years ago, scientists performed a study of the effects of a certain species of tent caterpillar on a certain species of tree. The caterpillars invaded one side of the forest and started eating the leaves. The trees reacted by secreting a natural insecticide. As the scientists moved through the forest, they noticed something startling: trees that had not yet been infected were already secreting the insecticide. In fact, by the time the plague had spread halfway through the forest, trees on the other edge had already protected themselves. The trees had somehow communicated the fact that the caterpillars were invading them, perhaps using chemical signals.

Communication is an undeniably intelligent activity. And if trees are capable of such intelligent behaviour, shouldn't they be able to hear the fall of one of their brothers?

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