...and there was no one to hear it fall, would it make a noise? I doubt that many people are vitally interested in answering that question - or even asking it - but I am. The question has been kicking around long enough to suggest I am not alone in finding it worth thinking about. For the benefit of those few who do, myself included, I shall forge on. Everyone else can stop reading now.

As is my habit, before attempting to answer this question I'd like to strip it to bare essentials. Forget the tree, the forest, the noise. At its essential core the question asks "can an event (e.g., the sound of a tree falling) occur unobserved?" This is actually a very profound question.

To get at the answer, first we must ask another question: "How do we know something happened?" Two ways: either we have observed the event directly, or we have indirect evidence - for example, a report by a reliable witness - that it has occurred. The reliable witness need not be a person. It may be a photograph, or a footprint, or, in general, direct observation of some event from which we can infer that some other event must have occurred to cause it.  

But if an event is not observed, there are no witnesses, reliable or otherwise. There is simply no evidence of its having occurred. No one and no thing in the entire universe has any knowledge of it. As far as we and the rest of the universe are concerned the event did not occur. There was no change in the state of the universe. Nothing happened.

And that's our answer: no, an event cannot occur unobserved. To say that something has happened is exactly equivalent to saying that something has been observed (though not necessarily by any one particular person or thing). The observation and the event are actually inseparable. There is nothing to distinguish between them. They are, in fact, one and the same.

This insight has many fruitful ramifications, philosophical and scientific, but that's a subject for many other essays. There is, however, a related question which we can profitably consider here. In it's popular form it would be, I suppose, "if it looks like a duck, quacks like a duck and walks like a duck, can we assume it is a duck?" Stripped to its essentials the question is "can there be a real difference too small to observe?"

There may be microscopic differences between things, invisible to naked eye, or there may be differences deemed too small to be significant, but what we are concerned with here are hypothetical differences that are simply not observable by any means. The situation is the same as with the unobserved events. Where no difference is or can be observed (directly or indirectly) there is no difference. Or, in other words, yes, it's a duck. (Note: we do have to make allowances for the unavoidable uncertainty of observation, but that's another story).

This insight, likewise, has far-reaching ramifications in science and philosophy. In addition, it points to the identity between events and differences. The observation of a difference between one thing and another is, after all, an event. Indeed, that's what events are: observations of differences between succeeding states of the universe.

If you are still with me, we are now ready to consider (in a future essay) how these insights about observations, events and differences together with the logic of the mechanism of cause and effect illuminate the mysteries of quantum mechanics. To think it all started with a tree falling in a forest...


Paul Wyszkowski