The Astrophysical Reason Why We See & Hear at Specific Frequencies — Part 1
We need information about our surroundings to survive.
For such a task, evolution has brought about two intricately complex sensory systems in the human body — the eyes and the ears.
Our eyes absorb electromagnetic energy — or light — to help us ‘see’. Our ears understand pressure energy in the air — or sound — to help us ‘hear’.
Both light and sound are oscillatory phenomena — light arises due to the oscillations of the electromagnetic field, and sound arises due to pressure waves in the air. As such, there are a range of frequencies these phenomena can carry from point A to point B.
But, our sensory systems are privy to only a specific band of these frequencies. ‘Visible’ light spans from 430 THz to 750 THz, and ‘audible’ sound ranges from 20 Hz to 20 kHz.
Note 1 -> 1 THz = 10^(12) Hz. That’s 12 zeros after 10.
Note 2 -> The conventional method to represent the visible spectrum is wavelength. 750 THz is equivalent to 400 nm (blue color), while 430 THz is equivalent to 700 nm (red color).
Note 3 -> Don’t compare the frequency ranges of light and sound. Both are entirely different types of waves. It will be akin to comparing apples to oranges.
This obviously begs the question — Why did our senses evolve to process such specific bands of frequencies?
Let’s understand eyesight in this article. We will talk about sound in the next part.
Let’s open our history books. Not human history, but the history of our solar system.
The tale begins with a cloud gas weighing approximately 2 × 10^(35) kgs. Due to its heavy mass and other external factors, this gas cloud started to collapse under its own weight. To conserve angular momentum, it started to rotate about the central axis as well.
As the cloud continued to collapse, the heat and pressure at the central point started to increase rapidly. Eventually, this combined effect triggered nuclear fusion at the core.
Thus, our Sun is born.
A large amount of mass was concentrated in the center, while the rest flattened out to form a disk. Parts of this disk later becomes planets and other solar system bodies. Fast forward 5 billion years, and we have a stable star converting 600 billions kgs of hydrogen into helium.
How does this relate to our question? Well, here’s how.
The temperature of the Sun’s surface is approximately 5500 C.
This point is crucial.
The equations of quantum mechanics tell us about a relationship between the temperature of an object, and the wavelength at which it emits the most number of photons (or the most intense amount of light).
Once we plug in the temperature, we observe that the Sun emits the most number of photons around 530 nm — that’s the color of green! The number tapers off as we move in either directions from 530 nm.
I believe you are slowly reaching the climax of this tale.
Because the Sun emits a large number of photons in the ‘visible’ region, life — especially plants — on Earth evolved to utilize that energy for important biochemical reactions. As animals — including us — started to walk the lands, they had to develop organs to understand this particular region of the electromagnetic spectrum.
This is why we can ‘see’ using light only between 400 nm and 700 nm.
To summarize, our ability to see specific wavelengths of light is a direct consequence of the conditions under which our Sun and the solar system formed.
If the initial conditions of the gas cloud was slightly different, the temperature of the subsequent star’s surface would differ. It would emit a large number of photons in a completely different region of light spectrum.
If a planet with potential for habitability did arise around this star, life there would ‘see’ its surroundings using a separate part of light than we do.
Personally, this is what I adore about astrophysics. It teaches me to observe life’s minutest of details from a cosmic perspective.
In the next part, we’ll explore how similar principles apply to the frequencies of sound we can hear.
Stay tuned!
Hardik Medhi
