If a tree falls in a forest and no one is around to hear it, does it make a sound?
Philosopher George Berkeley asked this famous question over three hundred years ago in 1710,
and people are still debating it. The question is deceptively simple, but it is not so simple
to answer it. Why? because it is vague and the answer is not straightforward.
Where is the forest located. How is sound defined?
Does it mean vibrations of air molecules that enter your ear? What if you are deaf? would
you be able to see or imagine the sound by watching the tree fall.
What do the words “no one” mean? Does it mean human being? What about animals? Aren’t
they also ones?
Let’s define this question, and answer once and for all from a scientific perspective, whether
this lonely tree makes a sound? The answer to this question is going to require us to
invoke, not just philosophy, and figure out who we are as human beings,
but also, believe it or not, biology and general relativity.
That’s coming up right now...
So the question is: If a tree falls in a forest and no one is around to hear it, does it make
a sound? First let’s tackle the question of what is a sound?
Three things are needed to make a sound. A vibration, a medium through which the vibration
can propagate, and a receiver to measure or acknowledge that a sound was made.
What we think of as sounds are really waves – waves of high and low air pressure called
compression and refraction. The amplitude of the wave determines how loud the sound
is. And the frequency of the waves determines the pitch that we hear. The shorter the wavelength
the higher the pitch.
These waves of pressure enter our ear canal, which vibrate our eardrum which transfers
this vibration to 3 tiny bones in the middle of our ear, which in turn transfer the vibrations
to fluids inside the cochlea.
And eventually these vibrations result in an electrical signal carried by the auditory
nerve to the auditory complex of the temporal lobe of the brain. It is here that the electrical
signals are processed and interpreted. This allows us to become conscious of the sound
and enables us to hear things.
Note that the medium does not have to be air, sound waves can be carried through water or
metal or just about any kind of medium. In fact, sound travels through water and most
solids better than it does through the air, because water and metal are less compressible
so the energy of the sound does not dissipate as quickly.
The frequency of sound that our brains can interpret however is limited to between 20Hz
and 20,000 Hz. One Hertz just means one cycle per second. The higher the frequency, the higher
the pitch.
Let’s take a tree that falls in the woods and no one is around. “One” usually means
a person. But is sound only a sound when a person hears it? What if a dog hears it? If
no person is around, the sound can still be perceived by animals.
Should the word “one” really mean only a person. This may be a philosophical question,
but if we define the word “one” meaning someone or something that is conscious enough
to perceive the sound, then I think we have to bring all conscious beings into consideration.
There may be levels of consciousness. Is a dog any less conscious than a human. They
and other animals hear sounds using the same mechanism as us. In many cases their hearing
is actually more acute than ours. Dogs for instance, can hear sounds more than 45,000 Hz.
Is it not the height of human arrogance to think that the sound only exists when a human
hears it? There are billions of creatures other than humans on earth that can perceive
sounds. If they perceive, they are conscious to some degree.
What if an advanced extraterrestrial being from another planet, who is perhaps much more advanced
and more conscious than humans, defined “one” as being only from their species – would
they believe that the sound did not occur if lowly humans heard the sound, and one of
their kind didn’t? My contention is that any animal with a hearing apparatus that can
receive the sound and is capable of processing the sound, can hear the sound.
What if we take all the animals out of the forest, so that there is no possibility of
any human or animal hearing the sound. What then?
Well now there are still thousands and possibly millions of insects in the forest that can
definitely hear sounds. A fly for example, can hears sounds in the range of 50,000 Hz - which
is even higher than dogs.
Flies may not be as intelligent as humans or dogs, but they are animals with ability
to hear sounds, and will definitely hear the tree fall in the woods.
What if we take all the creatures from the animal kingdom out of the picture, including
all the insects in the forest. How about now? Is there a sound?
Well, there are more than just insects in the area. There are other trees. In a 2014 study
published in the California Academy of Sciences, it was shown that plants can detect and
respond to sounds. It’s been theorized that sounds could trigger a response via mechanoreceptors,
or fine hair-like structures in plants.
In addition, there are trillions upon trillions of microscopic life forms still present – bacteria,
viruses, and other creatures in ponds and puddles of water, and in ther air. Can bacteria
perceive sounds? There is probably no way to directly know this. But in a 2016 study
by Shaobin Gu et al, experimental results showed that E. Coli bacteria exposed to sound
waves had a faster specific growth rate compared to the control group. In other words, sounds
affected bacteria. Is this not evidence for a kind of “hearing” of sounds by bacteria?
E. Coli bacteria are present in massive quantities in forests. Even with no animals around,
something can still hear the tree fall.
Let’s go one step further, let’s take all the microscopic creatures out of the picture,
and let’s take all other plants out of the picture. In fact. Let’s take all living
things out of the forest. Maybe it is lonely tree in the middle of a desert with no other
life forms. Note that this is hypothetical and not realistic, because even a lonely in
a desert would be surrounded by living things in the sand and air.
But for the sake of argument, we will take every single living thing out of the forest.
Then what happens if the tree falls? Well, if the tree falls, air is still present and
sand is still present. When it falls, air is disturbed, and sand is disturbed to some
degree. If there are no living things nearby to perceive the sound, did the tree create
a sound?
Well, it depends on how far the sound propagates. A large tree would likely create extremely
low frequency sound waves, which can travel very large distances. In fact, sonic waves
with a frequency below 20 Hz, which are imperceptible to the human ear, can be heard by elephants from
over 15 miles away. If the tree was located somewhere in Africa or in Asia, an
elephant might very well be within an 15 mile radius and could hear the sound.
But let’s take all elephants within an 15 mile radius out of the picture. What then? Well,
the sand was disturbed, the air was disturbed.
It turns out that if the air is disturbed, the micro disturbance of the air, particularly
low frequency sound in the hot air that would be present from a tree falling in a desert
could propagate for over 50 miles, particularly if there are no structures to absorb the sound,
as would be expected in a desert. Sound waves from nuclear blasts and volcanoes have been known to circumnavigate
the earth several times over. There are sure to be life forms within that radius that could
sense to some degree this pressure disturbance. There are small insects the size of fruit
flies that may be able to feel the small disturbances in the air.
There are microscopic viruses and bacteria in the air whose flight path may have been
changed by the disturbance in the air. Is this not a form of “hearing” or perception
of the sound.
But let’s go further and eliminate all living things within a 100 mile radius of the tree,
then what? Well if the tree falls it creates air vibrations that no animal or living thing
can perceive within a 100 mile radius. But wait, what about the tree itself? Just because
a tree falls, it does not mean that the tree is dead. If it was a living tree, it can survive
for quite some time even after falling, and even after its main has been limb completely severed.
The leaves of the tree will be disturbed. And given what I said earlier about plants being
able to perceive sounds, the tree itself will “hear” its own fall. Thus, it will make
a sound that can be perceived by itself.
But let’s go even further and say that this was a dry dead tree in a desert with absolutely
no living thing nearby for over a 100 mile radius. Then what? Is there any possibility
that it made a sound?
Well, it did make a sound, but no living thing was there to hear it. But a change in the
universe occurred. The potential energy of an upright tree was converted to kinetic energy.
And this kinetic energy was converted to disturbance in the air and the ground, and the sound hitting
the ground could propagate through the earth. How far this sound propagates depends on the
density of the ground, the temperature and other factors. If it was an extremely dense ground
that it was sitting on, this sound could propagate quite far, but it would probably be less than
100 miles, so no one, no animal, and no life form would be there to hear it.
Here we get into some semantics, if the question is interpreted as “did” the tree make
a sound vs. “does” the tree make a sound. It definitely did make a sound. But nobody
was there at the time to perceive the sound.
However, if a scientist is allowed on to the scene and is asked to find out whether the tree made a
sound. He would be able to ascertain from examining the changes in the sand, the changes
on the tree, and nearby structures, that indeed the tree must have fallen, and made
a vibration which created a sound because air was present, and the ground was present.
So I say the tree does make a sound because instrument can ascertain after the fact that
the tree unequivocally must have made a sound.
What if we take all the air away from the forest? Will the tree make a sound then? Well
yes, the sound will not propagate through the air, but now it will propagate through
the medium of the ground, as soon as the tree hits the ground. Sound merely needs a medium
to propagate. This medium does not have to be air.
Let’s go further and eliminate the air and the ground from the vicinity of the
tree. Let’s make the tree free floating in space, with no air and no ground. And we
let it fall. Well, if it’s in space, there is no up and down, so technically the tree
can not fall in space.
But for the sake of argument let’s just change the direction that the tree is pointing,
from the perspective that we may be viewing it in space, and lets say we apply a force
such that it changes the tree’s position in space relative to how we are looking at
it.
Well, in space if we apply a force to the tree to make it change its position, it will
continue to rotate because there is nothing to stop this rotation. There is no friction
in space.
The energy we impart to push the tree will be converted to the rotational or kinetic energy of the
tree, so the tree will continue to move. So to make this appear more like falling instead
of spinning, let’s say we applied a force to not only begin the rotation, but a force
to stop the rotation as well. What then? Did it make a sound?
Well, since there is no medium such as air or ground to carry the sound, the kind of
sound that we humans or animals or living things on earth may recognize, does not get
made. So, it made no known sounds that we are aware of.
But did it make a different kind of sound? And I will use the word “sound” in quotation
marks here.
Did the tree make any difference in the universe that could be perceived by an instrument?
I would have to say that it did in the sense that the tree has mass. Mass as we know always
bends space time according to general relativity. By moving the mass, we have made a change in the gravitational bending
of space that surrounds this tree. And we have added energy to the tree by increasing
its kinetic energy, which also affects space time. This is a difference the tree has made
in the universe. If we had sensitive enough instruments, we could detect this difference
in the gravitational disturbance made by the tree.
This instrument would allow us to “hear” the tree change its trajectory
even in space. But then again a tree in space, like I said could not really fall, so we are probably
just wallowing in semantics at this point.
But the main point is if a tree falls in a real forest, it has made a sound. And this
sound has made a difference in the environment. It has changed the universe in some small
way. That change can be perceived, or has the potential to be perceived by someone or
something, sometime, somewhere, somehow.
No tree can fall without changing the universe in some way. And any instrument or being that
can detect this change in the universe, will be
able to hear the tree fall.