If a tree falls in a forest...
If a tree falls in a forest, does it make a sound even when nobody is there to hear it?
Yes, it makes a sound independent of anyone listening. The sound waves are predicted by the laws of nature, as sufficiently detailed physical models of crashing would demonstrate.
Nothing in physics (not even in quantum physics) needs a human observer to be there objectively. If it were so, we could not infer today anything about the remote past before observers existed. But it is done routinely in paleontology and astrophysics, and with success.
Everything we perceive has a basis in objective reality, though our interpretation of it may be subjective. The task of physics and the other natural sciences is the art of extracting from our perceptions an objective description that tells what is common between the perceptions of different subjects.
For example, we visually perceive from the outside world the close things big, the far away things tiny, but physics corrects for this in terms of perspective, and draws a more objective picture where sizes do not depend on the distance from the observer. This is more objective because it gives the same picture for all observers and for the same observer at all places and times.
What we call an optical illusion is something where the reconstruction produced by our brain from the raw senses is inconsistent with the actual situation (but seems ''natural'' in terms of the usual circumstances in which we interpret a picture).
All this holds similarly for illusions of sound (and also for illusions of forces). In the case of a falling tree, the air starts vibrating in a frequency pattern that any healthy human but also any tape recording (postprocessed by an appropriate classifier algorithm) would recognize as the familiar sounds made by a falling tree.
This i s sound, objectively described: ''Sound is a mechanical wave that is an oscillation of pressure transmitted through some medium (like air or water), composed of frequencies which are within the range of hearing.'' (quoted from wikipedia)
Any such sound has the potential to be observed by a sensor (e.g., heard by a human being, or recorded by a tape recorder). But what happens in the sensor depends on the particular observer (human or microphone), and is in this sense subjective.
Arnold Neumaier (Arnold.Neumaier@univie.ac.at) A theoretical physics FAQ