Drifter said:
This, and the OP's original question, have more to do with definitions than anything else. If sound is defined as airwaves within a specific frequency range then, yes, a tree falling makes sound and we can prove it to the extent we can prove anything else. If the definition of sound requires an element of hearing, then it would depend on how we defined hearing. But, based on the definition, we could still come up with a definite yes or no.
As to the original questions "Is a lack of evidence somthing exists, evidence it doesnt? I think thats ridiculous but it keeps becoming an argument. Maybe becouse it touchs apon personal beliefs? Any thoughts on this?":
Actually, the falling tree question isn't so much about the definition of sound as it is about whether reality exists independent of perception. The way the thought experiment is phrased obfuscates that a bit, unfortunately. Maybe a better one: If a photon passes through a screen with only two small holes in it, and no one measures which hole it went through, did it still definitely pass through one of them but not the other? If a cat in a box is alive or dead depending on which hole the photon went through, is the cat definitely alive or definitely dead if the box remains unopened...? ^.^The question being driven at is really one of
counterfactual definiteness. When we come upon a fallen tree in a forest, far from where anyone could have heard or otherwise witnessed the event of its falling, we still infer that such an event took place just as it would have if we (or anyone) had been there to observe it. That inference cannot be proven however, it is merely an inference… and that's the point. ^.^ Asking about which day of the week the tree fell on might be akin to asking about which hole the photon passed through… can you be sure it definitely fell on one particular day and not the others (we'll assume it didn't fall exactly at midnight or anything ^^; )?
As for the OP: I think it depends on the extent that the evidence was
expected and yet is still absent. For example, Einstein's theory of General Relativity predicted that the gravity of our Sun ought to bend starlight, changing the apparent position of stars nearby it. If we looked and
didn't see that deflection, it would have blown the theory almost entirely… the only hope would be measurement error or some unforeseen circumstance somehow negating the lensing effect. The theory says the light has to bend, so we'd better see it when we look for it.
For a fuzzier example: while the overall lack of evidence cannot constitute proof that the Loch Ness Monster isn't real, it is reasonable to conclude that its existence is unlikely because one would expect far more evidence to materialize if Nessie were present. The absence of evidence is indeed evidence of absence, but in this case it is not conclusive.
Also, as a general note: "proof" is a very slippery concept, yet seems to come up often. Strictly speaking, scientific theories cannot be proven. Ever. Mathematical theorems can be
proven to follow from a set of axioms (read: assumptions), but they can never be known to be absolutely True, because the axioms they hinge on can't be proven. I am not sure when others refer to "proof" if they mean it more loosely than that (perhaps short for "proof beyond reasonable doubt"?), or to what extent they mistakenly believe in the literal provability of things…