[thedoc (OP)]

Connor Raven asked the Naked Scientists:
   
A quick question that I heard while watching QI that i thought would be interesting to put on the forum to see people's responses and views.

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

Regards
Connor Raven


What do you think?

[RD]

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

The answer can only be "No" if you have an anthropocentric definition of the word "sound",
 i.e. sound = " the sensation a person perceives via their ears from vibrating* air ".

[ * vibrating in the range between 20Hz and 20KHz , and above a certain threshold ]

[alancalverd]

Poor - indeed misleading - definition of sound. Other species are able to detect pressure waves in air or water over a much wider spectrum and there's no clear discontinuity of structure or function between human ears and those of other mammals from whales to bats.

The idea that falling trees suddenly generated pressure waves when homo sapiens evolved is bizarre, to say the least. We have evidence from such events as Tunguska (where an explosion flattened several acres of trees) that the laws of physics apply when there are no human observers present. The only logical alternative is that the entire world, including these words, is an illusion. 

[Bill S]

Surely there are at least two definitions of “sound”

1.  Vibrations that propagate through a medium.

2.  Sensations perceived by creatures having auditory receptors, which are interpreted in a specific way by the creatures’ brains.

If we are talking about definition 2, which was probably the case when the question was originally asked; then RD is right: no auditory receptors – no sound.

If we opt for definition 1, the “sound” is, as far as we can tell, always part of the scenario.

Concluding that an answer based on definition 2 implies that the laws of physics don’t apply when there are no human observers present, does seem a bit of a “leap”, but it is a science forum rather than a philosophical one. 

BTW, Connor, what was the answer on QI?

[RD]

... what was the answer on QI?

SF on QI : "there is no right answer" ...

[alancalverd]

Surely there are at least two definitions of “sound”

1.  Vibrations that propagate through a medium.

2.  Sensations perceived by creatures having auditory receptors, which are interpreted in a specific way by the creatures’ brains.

The question was whether the tree made a sound. In order for it not to do so under definition 2 it would have to know that there was nobody around to hear it. Consider a deaf person clapping: he knows whether or not there is anyone else around to hear it but this doesn't alter the physics of what he does, and he knows it makes a sound. So you have to modify definition 2 to "perceivable" rather than perceived, and it is then indistinguishable from definition 1. 

[eternity]

Why does everything always have to boil down to how it is perceived by or effects  humans?

If a squirrel heard it, yes, it made a sound.