A tree which falls in the forest, it makes a sound, even if nobody is around to hear it.
Fact

If a tree falls in the forest, it makes a sound, even if nobody is around to hear 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 soundSound 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.[1][2]

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.[3]

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).[4]

  • 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.

Mechanical and Electromagnetic Waves.

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?

We can define sound in physiology and psychology as the reception and perception of mechanical “sound” waves by the brain, but in physics sound is not dependent upon perception.

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).”[5]

So sound is officially defined by its physics, and the auditory sensation evoked by the oscillation in a medium as it perceived is secondary.

In simple terms, we can say then, that a sound is made whether or not it is heard. (See the human senses and sensory memory for more details.)

If a tree falls in the forest, and there’s nobody around to hear, does it make a sound?

Yes, according to physics, a tree makes a sound when it falls.

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.



Conclusion

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.


Citations

  1. Sound” Wikipedia.org
  2. Quantum Theory: If a tree falls in the forest…” Blog.oup.com
  3. In Space, Can Anyone Hear You Scream?
  4. Categories of Waves” Physicsclassroom.com
  5. ANSI/ASA S1.1-2013” Wikipedia.org


"If a Tree Falls in a Forest and No One Can Hear it, it Makes a Sound" is tagged with: Light, Music Theory, Perception, Quantum Mechanics, Senses, Sound


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phil on

“However we frame the question, from a physical or psychological standpoint, we can definitively answer it in the affirmative.” No, Sound is not the same as a sound wave. It’s is better to call it a pressure wave. If a tree falls it creates a pressure wave but that only becomes ‘sound’ if it is registered by ears, nerves and a brain.

Thomas DeMichele
Thomas DeMichele on

Interesting take. Thank you. You are right, if we define sound as that which involves sensing the pressure wave, then there is never sound unless an entity which can receive the pressure wave receives it. In both ways we can use science to answer the question and don’t have to speculate. Think that is the real “answer” to the somewhat semantic overarching question.

Nick on

The truth is that sound is a human sense if their is no sound sensing organ and/or recording equipment the waves will not be converted to sound as sound is a secondary sense

Thomas DeMichele
Thomas DeMichele on

Well, this is partly semantic.

What we call sound is our auditory senses picking up on vibrations in molecules. The molecules will vibrate, and any being with the right sensory organs will be able to hear it.

That is what we mean.

If you say that sound is only the reception of vibrations, then you can maybe make a case here, but it is a somewhat semantic and philosophical stance.

A tree falls in a forrest, the molecules in the air vibrate in a wave like pattern, and thus there is “a sound.”

http://factmyth.com/mechanical-waves-versus-electromagnetic-waves/

See also: http://factmyth.com/factoids/there-are-only-5-senses/

#1: Mechanical waves like sound are pressure traveling from a source, through a solid, liquid, gas, or plasma, medium, distorting that medium’s molecules.

#2: Hearing (audioception) is a capacity to recognize auditory data from vibrations and is related to audial memory.

#3: We “hear” “sound,” but “sound” still exists even if we can’t hear it.

David Tait on
Doesn't beleive this myth.

Watermelon in Easter Hay
Frank Zappa

Central Scrutinizer:

This is the CENTRAL SCRUTINIZER . . . Joe has just worked himself into an imaginary frenzy during the fade-out of his imaginary song . . . He begins to feel depressed now. He knows the end is near. He has realized at last that imaginary guitar notes and imaginary vocals exist only in the imagination of The Imaginer . . . and . . . ultimately, who gives a F anyway? . . . So . . . So . . . Excuse me . . . Ha ha ha! Mm-mh . . . So . . . Ha ha ha . . . Ha ha ha! Who gives a F anyway? So he goes back to his ugly little room and quietly dreams his last imaginary guitar solo

So is Frank really saying that sound exists only in the imagination of the imaginer?

Yes, sound is heard as a purely internal thing (it is only inside our heads), it is the brain decoding complex pressure wave data into something we can make sense of.

For example can a deaf person hear?..NO! Of course not….Why?

Because their hearing or ability to decode pressure waves into what WE call sound is either broken damaged or non existant.

Can you hear all the music news and chat pumped out by all of the radio stations that are broadcasting radio waves through your body? NO!… Why?

Because human brains do not have built in radio receivers that’s why.

Can you hear all of the mobile phone conversations passing through your body?…NO!…Why?

Because the human brain does not have the microwave receivers etc required to receive and decipher the data that’s why.

So If a tree falls in the forest and there is no one there to hear it does it make a sound…NO!…Why?

Because sound exists only in the imagination of the imaginer!

The falling tree however does make a series of highly complex pressure waves, not SOUND pressure waves… as these do not exist…(well only in the imagination of the imaginer!)

The word “soundwaves” IS the human description for the INTERPRETATION of pressure waves, Not sound waves (as they do not exist) only pressure waves exist.

It takes at least one ear and an associated and capable brain to DECODE the incoming DATA from a series of pressure waves into something that is meaningful to us/them/birds/animals..etc

For example do insects with antenna hear? Or do they just feel vibrations? Are vibrations sound?..NO!

The answer to the question is in the question itself which of course is very clever and actually quite profound (requiring the student to be capable of removing themselves from their own reality so as to understand THE existential) And the operative words being “And there was no one there to hear it”

Get it?

Erin Georgen on

Can’t argue with someone using Zappa as a citation. 😀

I can and will however make an argument for the way we present our conclusion on this page (by making the argument that philosophically you may have an interesting point, but physically speaking the pressure waves made could be imagined as sound to the receiver, after sensory processing, if there was one there to receive the waves… and thus “sound” was made from this perspective).

That is of course the general point we are making on this page… certainly room to have the metaphysical debate surrounding the facts though. That is welcome (as are Zappa quotes).

Darren Delozier on

The waves only become sound through the mechanics of the ear, without the ear or a substitute i.e a microphone, it’s just air waves moving. Radio waves are all around use all the time, but you don’t hear music coming out of thin air, you need a radio to convert them. Same way with sound and the ear.

Thomas DeMichele
Thomas DeMichele on

I would say that if “sound” is only sound once it reaches our ear, then “if no one can hear it, then it does not make a sound… necessarily, by our definition.”

If however a sound is something that a human can hear if they are around to hear it, then it does make sound.

So I guess part of this is going to be based on our definition.

Thanks for the comment!