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The question of whether or not a tree that falls in the forest makes a sound - if there is nothing or no one around to hear it - comes up frequently at my house.

So, my question is: is there any way to "prove" or "dis-prove" this using physics? If it can be proven what is the answer?

My idea is yes, of course it makes a sound even if there is nothing to sense it! However, my parents seem to think that if there is nothing to "take in" the sound waves, there is no sounds.

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closed as off-topic by Jim, JamalS, John Rennie, ACuriousMind, Danu Nov 27 '14 at 15:41

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It depends on your definition of "a sound". If a sound is not a sound unless it is perceived as a sound (that is, processed in the auditory system of a sentient being), then the answer is "no". If a sound is a coherent disturbance in the pressure distribution of the air, and this disturbance propagates through the medium "at the speed of sound", then the answer is "yes".

The fall of the tree causes vibrations: the vibrating tree / branches / ground interact with the air (their movement results in a change in momentum of the air molecules that hit the surface - if the surface is moving towards the air, the pressure increases; and if it's moving away, it decreases). This mechanism is independent of an observer, and thus when a tree falls, sound (definition 2) is created.

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Put a microphone & recorder near the falling tree. No one was around to hear the tree fall. Yet you can play the recording and provide evidence of sound.

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  • $\begingroup$ Then you would be hearing the tree fall. Just not instantly. $\endgroup$ – paul Nov 20 '14 at 4:06
  • $\begingroup$ That was my original argument with my family. However, you can't have anything to "hear" it either :) $\endgroup$ – L.B. Nov 20 '14 at 16:00
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if there is nothing or no one around to hear it

Given that trees/forests don't exist in isolation we would have a very difficult time finding a tree that falls with absolutely no creature capable of perceiving sound within a reasonable distance.

Go find a forest completely devoid of birds, rodents, snakes, insects, fish etc. Don't forget a 2+km buffer zone of purely botanical life. That's going to be one very unhealthy forest, at least until everything living nearby realizes that there's unoccupied territory to move into.

End result: Trees will fall. Something will hear it. Therefore they make a sound.

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  • $\begingroup$ In northern Canada (below the tree line though) you can easily find 2km stretches of forest in the winter that are completely devoid of animal life. I've seen video from helicopters flying over these areas, it's really eerie. I wouldn't want to be lost in those areas even though it's pretty safe from bears and wolves, etc $\endgroup$ – Jim Nov 20 '14 at 14:30
  • $\begingroup$ Perhaps the nuclear site at/near Chernobyl. $\endgroup$ – L.B. Nov 20 '14 at 16:00
  • $\begingroup$ @Jim completely devoid of animal life? You did an exhaustive biological survey? No arctic lemmings or mice living in tunnels under the snow? $\endgroup$ – paul Nov 23 '14 at 1:30
  • $\begingroup$ @L.B. There's LOTS living in the Chernobyl exclusion zone. $\endgroup$ – paul Nov 23 '14 at 1:31
  • $\begingroup$ @paul I thought there was some form of a dead zone close to the site. $\endgroup$ – L.B. Nov 23 '14 at 16:48

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