The laws of physics are called laws because they always work. So, based on what we know about mechanical waves and sound, a tree will always make a sound when it falls regardless of who is around to perceive it.
If a Tree Falls in a Forest and Nobody is Around to Hear it, Does it Make a Sound?
According to physics, if a tree falls and no one is around to hear it, it still makes a sound. Sound is a mechanical wave of pressure and displacement through a medium such as air or water. We don’t have to perceive a sound to know the laws of physics are in play.
An Argument Against the above: With the above said, if one were to define sound as the reception of a sound wave, then it could be argued that no sound occurred based on the fact that no one was around to receive it. In this respect, one might argue the answer to the question is dependent on how we define “sound.”Physics – Mechanics: Sound and Sound Waves (2 of 47) If a Tree Falls in the Forest…?
FUN FACTS: Speaking loosely, caveats aside, in space no one can hear you scream. This is because sound waves can’t travel through an “empty” vacuum. However, you can hear yourself scream in space, as the sound waves can still travel through your body (it is a medium). Meanwhile, if a tree falls in a forest it most certainly makes a sound, even if no one is around to hear it, because the tree necessarily displaces air molecules as it falls and that creates what we call “sound.” With that said, if a tree was floating free-form in space and it rotated or drifted toward earth (or whatever one would consider “falling” in space), it would not make a sound, as there would be no molecules displaced by the tree’s motion. #BecausePhysics.
What are Mechanical Waves? Mechanical Waves Versus Electromagnetic Waves and Mediums
There are a few different ways to classify waves, but for our purposes, we just need to consider the difference between mechanical waves (which have to travel through a medium and can’t travel through an empty vacuum) and electromagnetic waves (which don’t need a medium and can travel through a vacuum).
- A mechanical wave is a wave that travels through something (“a medium“, like air molecules, water molecules, or any other type of matter in any form that is “elastic or viscous enough”).
- Sound is a mechanical wave, caused by changes in pressure, which displaces and vibrates the molecules of a given medium causing what we call audible sound.
- Light isn’t a mechanical wave; it is an electromagnetic wave, caused by the vibration of charged particles, that can travel forever in a single direction without a medium. When light reflects off of objects, it causes visible light.
- Both waves have a large spectrum of wavelengths, only a tiny portion of which humans can perceive with our senses.
TIP: If we are looking at the behavior of waves we can consider classic waves versus quantum waves (see quantum field theory). We can also discuss how waves travel through different parts of a medium (see Longitudinal versus Transverse Waves versus Surface Waves). For our purposes, we don’t need to consider these.
What is Sound?
Sound is defined by ANSI/ASA S1.1-2013, published by the American National Standards Institute (ANSI), as: “(a) Oscillation in pressure, stress, particle displacement, particle velocity, etc., propagated in a medium with internal forces (e.g., elastic or viscous), or the superposition of such propagated oscillation. (b) Auditory sensation evoked by the oscillation described in (a).”
So sound is officially defined by its physics, and the auditory sensation evoked by the oscillation in a medium as it perceived is secondary.
How to Solve the “If a Tree Falls in the Forest…” Riddle With Science
Given the definition of sound being like a mechanical wave through a medium (and not an auditory perception), we can say that unless the tree falls in an otherwise empty vacuum (and thus there is no medium to displace), “a sound” will be made.
If we instead decide to interpret sound as our ability to perceive audible sounds, then we can say “a tree makes a sound only if something is around to hear it.”
If we consider that different animals can hear different frequencies, then we can say “a tree makes a sound only if something that can hear the audible range of its effects is around to hear it”
However we frame the question, from a physical or psychological standpoint, we can definitively answer it in the affirmative.
The question is meant to be philosophical, and still works in this respect. The concept is a valid question; how can we know something is real unless we can perceive it?What is Sound?
TIP: You can nitpick the tree falling in space concept. The tree is itself a medium, so if a tree broke in half in space (somehow) sound would happen in the medium to the tree itself. It just wouldn’t travel through space.
TIP: Aristotle described “metaphysics” (a branch of philosophy) as “the book that comes after physics,” in other words, a look at the questions physics can’t answer (or can’t answer yet). In this case, what was once metaphysics is now physics.