These posts make more sense when read in order.
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| Supersonic shockwaves are made visible in this image. We can’t see them, but we hear these mechanical waves as sound. J.T. Heineck/NASA. |
If the doors of perception were cleansed every thing would appear to man as it is, infinite.
For man has closed himself up, till he sees all things thro’ narrow chinks of his cavern.
—William Blake’s poem The Marriage of Heaven and Hell, c. 1794
What is light? We use it to see, but light is not in the real world, at least not the way it appears to us. Light is just as tangible as x-rays and it differs from microwaves only in its wavelength. Light, x-rays, and microwaves are all just electromagnetic radiation. They’re parts of a continuum that for convenience we arbitrarily divide and give names like infrared, gamma rays, and radio waves. Light is just a tiny portion of that spectrum. Actually it’s less than one ten-billionth of the spectrum. The only difference between light and radio waves is the frequency (or intensity) of their wavelengths. We just call radiation with light’s small section of wavelengths “light”, because it’s the part that animals can see. Lengthen or shorten the wavelengths outside what animals can see and we call it something else, but it’s still all electromagnetic radiation.
Sometimes we describe electromagnetic radiation as being waves and sometimes as particles, but it’s both...or something else entirely. Waves and particles are just two ways of looking at it. Sound waves in water or air are mechanical waves. These can also be thought of as particles, with the smallest particle of sound being a phonon. Such waves are caused by disturbances or vibrations. They propagate by molecules bumping into the one next to it. Nothing physical moves along with these waves, other than energy, like in a falling line of dominoes. This is why sound can’t pass through the vacuum of outer space; the molecules are too far apart to pass along energy if they bump together. Gravitational waves are similar, but are distortions in the fabric of spacetime itself, so they can pass through space’s vacuum.
Electromagnetic waves, on the other hand, are fields of electricity and magnetism. On their own, both electricity and magnetism can be static. Static electricity is what makes your hair stand on end, while refrigerator magnets have static magnetism. Refrigerator magnets, by the way, never wear out unless you drop or hit them, knocking their atoms out of alignment. By combining changing magnetic fields with changing electric fields, you produce electromagnetic fields.
Electromagnetic waves are also called radiation and can be described by three related characteristics—wavelength, frequency, and energy. If you increase the energy, the wavelength shortens and the frequency quickens. Reduce the frequency and the wavelength lengthens and the energy decreases.
If you think of waves out in the ocean rolling along, their wavelength is the distance from the crest of one wave to the crest of the next wave. On the electromagnetic spectrum, the shortest wavelengths are much smaller than an atom. These are gamma rays, which are produced by neutron stars, pulsars, supernova, nuclear explosions, lightning, and radioactive decay. They are very energetic with extremely high frequencies. We don’t know how long the longest wavelengths are because we can’t measure them, but we do know some measure 18.6 million miles—78 times the distance between earth and the moon. The longest waves we normally use are radio waves that measure from the width of a baseball to about a third of a mile (0.5 km). The waves inside your microwave oven and emanating from your cell phone are just a bit longer than your foot.
The light we’re able to see is just a tiny sliver of this spectrum. From violet to red the wavelengths are only about 1.7 to 2.8 hundred-thousandths of an inch (420 to 700 billionths of a meter)—about the size of a virus. Put another way, visible light has a frequency between 420 THz and 700 THz, where THz means terahertz, or a trillion cycles per second. The sliding scale of frequencies denote the different colors we see. Change the frequency and you get the different colors of the rainbow, from violet at the lower frequency of 420 THz to red at the more energetic frequency of 700 THz. Our eyes are blind to everything else, just as our ears can’t hear dog whistles or elephant rumbles, although we do feel infrared light as heat.
When we look at an object—say a picture of the Cheshire Cat, for example—what we see is the light that bounces off the cat. His brown color is something of an illusion caused by some wavelengths of light being reflected. The wavelength of brown is somewhere around 600 nanometers—it’s actually mostly red with a little bit of yellow and blue mixed in. If you shrink or expand the wavelengths of energy reflecting from him, his color changes until it is outside our visual range. If you decrease it, down through all the colors into ultraviolet—which we can’t see—the Cheshire Cat disappears, except for what remains of the white around his grin. If you keep going, it’s only radio waves reflecting off the cat. Lengthen the wavelength back down through the visible light spectrum until it becomes infrared and he disappears again.
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| Cheshire cat. © John Richard Stephens, 2024. |
Obviously there’s a lot going on that we can’t see, since only 0.0035% of the electromagnetic spectrum is visible to us. We are constantly bombarded by electromagnetic waves. There’s TV, radio, satellite signals, weather radar, cell phone, and Wi-Fi signals. There are garage door openers, remote controlled toys, and medical devices. And there’s cosmic rays and radioactivity—small amounts of which come up at us out of the ground from radium. The electromagnetic spectrum is regulated by governments that license bits of it for hundreds, if not thousands, of different uses.
Some of these waves, like x-rays can pass right through us, although some are stopped by denser objects, like our bones. X-rays are not small enough to go through lead, but other things can, like cosmic rays and neutrinos. This is because nothing is as solid as we think. In fact, the subatomic particles that we and everything else is made of are so tiny and so spread apart that we’re mostly nothing. Atoms have a bit of matter in their nucleus which is circled by much smaller bits of negative energy. They’re more than 99.9999999% (feel free to add more nines) empty space, consequently, so are we. We’re barely here at all. In fact if you took all of humanity and removed the empty space from our atoms, we—roughly 7.5 billion people—would take up about as much space as a sugar cube. We’ll take a closer look at this later.
The Imperceptible World
As with us, how other organisms see the world depends on their senses. Their perceptions are very different from ours, which makes it difficult to imagine what their views of the world are like. They usually can see, hear, and smell things we can’t. Some have unusual senses that we’re still trying to understand or identify. For example, fruit flies (Drosophila melanogaster) sense changes in humidity through a small sac on their antennae; desert toads sense drops in barometric pressure, signaling the approach of rain, which they need in order to reproduce; and the Japanese sea catfish (Plotosus lineatus) have a pH sense they use to detect the slight rise in acidity around the burrows of bristle worms, which they like to eat. But, like us, they all have a limited view of the world. Often it’s much more limited and specialized than ours.
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| A rattlesnake in a tube so it could be fitted with a transmitter. U.S. Forest Service. |
Many insects see ultraviolet light. Many flowers take advantage of that by having ultraviolet markings on their petals—invisible to us—that act as a runway for bees to land on, directing them to the nectar they’re after. Pit vipers, such as rattlesnakes, have a pit near their eyes that senses infrared radiation (heat) from their prey so they can hunt at night.
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| A tarsier. Meldy Tamenge, CC BY-SA 4.0. |
Then there are the sounds of silence...for us anyway. Everything is vibrating. The vibrations are all around us and emanate from us, but we can’t hear most of it. Other creatures can though. A number of animals can hear high-pitched ultrasound, but so far the only ones we know who communicate with it are dolphins, whales, cats, some bats and rodents, and tarsiers—the cute huge-eyed primates of Southeast Asia. People used to think tarsiers were yawning, until they discovered the tarsiers were actually screaming. Elephants use low-frequency infrasonic sounds to communicate with each other over distances of many miles, as do whales. Peacocks also make infrasonic sounds when they’re courting peahens.
There’s a lot going on that we’re oblivious to.
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