On The Hearing Of Falling Trees in Forests

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Never let it be said that I don’t tackle the big questions.

Today’s big question is that familiar old philosophical chestnut (yes, I know, at least three of these adjectives are redundant) : “If a tree falls in a forest and nobody is there to hear it, does it make a sound?

Why now you ask? For some reason it came up over a pre-Christmas dinner with some mates, and a fairly loud and prolonged argument ensued between two of us in particular. Fortunately the general volume in the restaurant was approximately that of a 747 at take-off, so no harm done. What is remarkable is that the bulk of the argument was had prior to any significant alcohol being consumed. Go figure.

So, what’s the answer? I’m firmly in the negative, while my friend (who, by the way, is a physics teacher), argued for the affirmative.

My argument is as follows:

1. The problem actually has a clue in it: it refers to someone ‘hearing’ the tree fall.

2. The process of hearing is done in the brain, after the sensory input is converted into electrical signals. That is what we call ‘sound’.

3. What is the sensory input? Simply sound pressure waves (as per Physics 101), which you can picture as ripples on a pond. It is these sound pressure waves which stimulate the various bits of our ears, causing the brain to register a ‘sound’.

4. Therefore, if there is no organ or device to convert the pressure waves into a perceived ‘sound’, then the falling tree cannot be heard.

Using a recording device doesn’t help either. In that case, the pressure waves are converted by transducers into patterns on tape or disc or memory of some kind, which we still can’t ‘hear’. In order to hear what is recorded, the device must convert those recorded patterns back into pressure waves, and for this we use some nice electronics, and critically, speakers. How do speakers work? They feature a membrane which bounces back and forth to disturb the air in precise ways, replicating the original sound pressure waves.

There are also thought experiments we can do to confirm this view.

For example, let’s remove the medium – the air. Does the tree make a noise now? No, even though we may be standing right next to it, we will ‘hear’ nothing. Why isn’t the tree making a noise now?

Let’s imagine a race of aliens who visit us, who have never evolved the ability to sense sound pressure waves, just as we have never evolved the ability to see in the infra-red part of the spectrum. Will they ‘hear’ the falling tree? Maybe they have developed some other sort of sense organ which is sensitive to changes in the visual field, resulting in a flight response. They may even ‘see’ the sound pressure waves as a colour, but it’s not necessarily perceived as sound.

So, all you budding philosophers, you may now proceed to shoot me down in flames.



8 thoughts on “On The Hearing Of Falling Trees in Forests

    luxinvestor said:
    January 16, 2012 at 9:42 am

    Oh, this is going to start a great deal of trouble on my facebook page. Thank you! 🙂

    ɹǝɯɐןq (@blamer) said:
    January 16, 2012 at 4:10 pm

    So the paradox/question becomes:

    Is it “sound” if these vibrations never vibrate an eardrum…?

      rationalbrain said:
      January 16, 2012 at 5:42 pm

      On the assumption that the vibration of the eardrum also passes on a signal to a functioning brain, then yes, perception of ‘sound’ requires an eardrum or similar. However, this opens the door on things like the cochlear implant. At the moment, this device works via the hearing apparatus in the head, however work has begun on the next generation which are wired directly into the relevant nerve systems into the brain. In this case, we would still perceive sound, but raises an interesting variation. If we wire ourselves so that the vibrations generate an electrical signal which stimulates colour perception (for argument’s sake), is that still classed as ‘hearing’? Perhaps the louder the sound, the higher the frequency of colour. This is analogous to the blind ‘seeing’ words using braille. Is it still seeing?

    Dan Rea said:
    January 18, 2012 at 11:15 am

    As far as I know it goes like this:
    If a tree falls in the forest and no one is around to hear it does it make a >(sound)<?
    Not 'noise'.
    Sound is not nesecarially of the perceptions- in most dictionaries you'll find sound described as a sensation as well as the physical vibration. So in this case, yes, it does make a vibration. Even without an atmosphere a sound would be transmitted through the ground.
    Also, the wording of the question could also be interpreted as 'if nobody hears the tree falling, does it (the tree) make a noise. If you want to be particularly pedantic about things (and we obviously really really do) you could say the tree makes noises during decomposition AFTER the tree has fallen.
    The words 'somebody' and 'nobody' generally refer to people/persons- which generally means human beings. Even by your reasoning this means that small chipmunks and adorable squirrels could hear/perceive the noise sensation of the tree falling.

      rationalbrain said:
      January 18, 2012 at 11:41 am

      Hi Dan,
      Just for fun, let’s follow this through.
      I don’t differentiate between sound and noise. Noise is simply some arrangement of sound, so that distinction is irrelevant. I agree that the falling tree will make a vibration – through the air and the ground. But the key issue is whether it’s ‘heard’. Unless that vibration is of the right frequency range, and there is a medium to get it to the ear, and a transducer converts it into electrical signals which the brain can interpret, no hearing can take place. Just ask someone who is profoundly deaf and goes to a dance party. No hearing is done, but plenty of vibrations.
      Re the ‘nobody/somebody’ thing, I don’t think I implied that this was limited to people. Sure, if a squirrel is there, they’ll hear it.
      My point is, something needs to get the vibrations converted into a suitable signal for the brain to interpret – then and only then can the sound be said to be ‘heard’. The critical thing is the definition of ‘to hear’. Since we’re quoting dictionaries, Webster’s says ‘to perceive or apprehend by the ear’ – this covers both the medium (by ear), and interpretation by the brain (to perceive).

    Vlad said:
    December 12, 2012 at 9:08 pm

    By following the definition I randomly picked out from an online dictionary (seriously now, since everything depends on a consensus of interpretation, must people truly argue such pointless questions?) i quote thus:
    “sound – mechanical vibrations transmitted through an elastic medium, traveling in air at a speed of approximately 1087 feet (331 meters) per second at sea level.”

    In response to the question “If a tree falls in a forest and nobody is there to hear it, does it make a sound?“ then according to that definition, YES, it does definitely make a sound. Elastic medium include air, the ground, the tree itself, a few squished bugs and an unlucky squirrel and maybe some unlucky guy’s car while he was out hunting a few miles over.

    It’s fun to argue philosophy, but is there any point at all in doing so when different frames of reference are used?

    I suggest to you that in the context of the previously mentioned definition, the only situation in which the tree falling in the forest would not make a sound, is if our universe was a simulation that only rendered existence based on observation, and there was nothing at all observing the tree. This would then result in the question of whether or not non-human, non-animal life-forms or non-life-forms themselves have the capacity of cognition. If they do, we are either in an unlimited simulation or not in a simulation at all.

    If they don’t, then we are in a simulation where cognition can only be attributed to sentient life-forms, and wouldn’t that be a interesting debate to have with a believer in universal sentience (such as a God, Gaea, etc.)

      rationalbrain responded:
      December 12, 2012 at 9:45 pm

      Agree it depends on the definition.
      It’s just more fun to argue about the definition to challenge peoples’ thinking.
      If no entity could perceive the sound then is it really sound, or just pressure waves in the medium?
      IMHO, and despite the popular common usage of the word, sound is the perception of those pressure waves by the brain’s apparatus. The pressure waves can do all sorts of things – move air, alter the distribution of particles on magnetic tape, form a nice pattern when digitised.
      But all those manifestations remain just that until perceived by the brain.

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