There is an invisible field that underlies the entire universe like a net stretching in every direction.
The field itself is invisible and intangible, but it will ripple with energy. When those ripples are in a very specific wavelength range (i.e. size), human beings can see them as light. The ripples are called photons, and the possible range of wavelengths of the photon is called the electromagnetic spectrum.
The specific range that is visible to us forms the colours of the rainbow – from the smallest ripples creating the colour violet to the largest ones creating red.
Photons tend to bounce off everyday objects like steel, wood, and granite like ripples being reflected from a boulder in a pond.
These objects appear to have colour they do because they absorb some ripples of certain wavelengths, but reflect others. We see the wavelengths that get reflected.
Other materials like water, glass, and air let photons in the visible range slip through and keep going on the other side. These objects appear transparent, and any photons that they do repel appear as a shimmer.
The Sun is the source of most of the photons on Earth. It can be imagined as a vibrating behemoth at the centre of the solar system, pushing endless tidal waves of photons towards the planets. In the daytime, we face the torrent head on. The photons bounce off almost every surface, and in doing so illuminate our world.
As a result, the underlying electromagnetic field dances like a pond in a downpour.
These stars have let us see a very small property of the electromagnetic spectrum that’s had unexpected but truly profound consequences.
We discovered that photons gradually decrease in wavelength as they travel very long distances in a process called ‘redshift’. Astronomers found that they could use redshift to estimate the distance between Earth and far off stars and galaxies.
From this data, we’ve been able to build 3D models of where the Earth and Sun sit within local star fields, where the star fields fit within the Milky Way galaxy, and where the Milky Way sits among billions of other galaxies in the universe. It’s a model that is being extended all the time, and it is perhaps the most profound project in the history of science, because it is also the most humbling.
This property of the electromagnetic spectrum has revealed that the universe is gargantuan in size. Our entire Earth is like a single speck of dust in cosmos that is vaster than the wildest dreams of our greatest minds just a few generations ago could have imagined.
Without the change in redshift, every star would be an indeterminable distance from any other, and our position in the cosmos would forever be a mystery.
The specific range of the electromagnetic spectrum that is visible to us is determined by the cells in our retinas, and it has been tuned to this frequency by our evolutionary history.
Primates like us are fairly unique in the animal kingdom in being able to see red. It’s hypothesised that our distant tree-dwelling ancestors developed this ‘extra’ range to more easily distinguish ripe fruits and berries from unripe ones. Many other animals, like dogs, struggle to distinguish red from green.
Just smaller than violet lies the ‘ultraviolet’ range, and larger than red is ‘infrared’. They are invisible to us, but some other species have a visible range that is far greater than ours.
Some species, like the peculiar Mantis Shrimp, can see far into the ‘invisible’ ultraviolet range. It is unknown if they see this range in the same colours as we do, or as a kaleidoscope of new colours that have never seen by a human being.
But even the mantis shrimp can see only a tiny fraction of the entire spectrum.
If we had the ability to turn a dial and see in any frequency we chose, our perspective of the world would change in an instant. We would see the structures of materials differently, and perhaps like the mantis shrimp, a range of new colours.
We mentioned earlier that some objects reflect photons in the visible range, so they appear solid to us. But those same objects may let other wavelengths shine through.
If we tuned into the x-ray range, many materials that were opaque would become transparent. Clothes, muscles, and tendons would become clear and everyone would look like walking skeletons.
If we turned the dial even further until we reached gamma rays, skeletons would become glassy and transparent, as would walls and even the ground. We would see metal support beams through buildings and engines working on the inside of moving cars.
These other areas of the electromagnetic spectrum are categorised into radio waves, microwaves, infrared, visible light, ultraviolet, x-ray, and gamma rays.
Through each of them naturally occur, we’ve also learned to create them through artificial means like through radio transmitters. Each one has different characteristics and uses. Let’s take a look at how we’ve managed to do it.
Many new technologies find applications in warfare, and radio was no exception.
This is because of a property of radio waves, which are the largest possible wavelength in the electromagnetic spectrum. They can reflect off the underside of the atmosphere, meaning that we can use them to send wireless messages to someone that’s not within our line of sight.
In 1914, at the start of World War 1, radios were bulky and had a range of about 4 miles, or 6.4 km. Still too heavy to be easily moved on land, they were used on planes to direct artillery barrages and on warships to deliver news and coordinate positions.
Pressure to make portable, lighter radios that could bring the communications advantages of radio onto the battlefield led to many developments. By the end of the war, radios were small and could receive signals over 3,000 miles, or 4,800 km. The result was a boom in commercial broadcasting, beginning with entertainment for returned soldiers.
The technology only became more advanced. We still use radio waves in the military and in commercial broadcasting but because of their ability to send wireless signals, it’s now the technology behind Wi-Fi, Bluetooth, GPS, and 4G/5G.
There are two methods of encoding a signal in a radio wave. Within the ‘amplitude’ of the wave (AM radio), or in the ‘frequency’ (FM radio). AM is easier to detect in difficult conditions, but FM is capable of higher signal quality.
These oscillating waves hit the electrons in a radio antenna which vibrate back and forth, creating an electrical signal. The signal is filtered, amplified, and sent to a speaker or computer.
It’s important to know that while radio waves can make electrons vibrate, they don’t have enough energy to separate them from their atoms or to break chemical bonds, and so are not damaging the antenna or living things.
Radio waves have become so ubiquitous in the modern world that the Earth is now saturated with them. They move through the air and through our bodies every day completely unseen.
This saturation has an interesting but unintentional long-term side effect. While many radio waves reflect off the underside of the atmosphere, sometimes they’ll break through and radiate out into space. As we speak, an ever-expanding bubble of radio waves is broadcasted inadvertently into the unknown. This growing bubble will travel across space for millennia, and may one day be the most lasting record of human civilisation.
With infrared, the electromagnetic spectrum gets weird.
When we discovered ways to detect the infrared part of the spectrum, we discovered that almost everything in the world glows.
It turns out that all matter that contains heat naturally emits a small number of photons. But at the typical temperatures that we get on Earth, objects tend to emit radiation in the infrared range which can be seen with special camera equipment.
These two GIFs have not tuned their colours to the same temperature range, but you can see the change in colours when the temperature does.
At high temperatures the wavelength of photons that things emit changes, and objects like a molten rock or a fireplace poker start to glow within the visible spectrum.
Infrared-sensitive cameras are also used in night vision devices. These devices work by amplifying visible light, but can also illuminate an area with an infrared torch when combined with an infrared camera. This creates a high-resolution image for the camera while being invisible for animals and unaided observers.
Visible light is a small band between infrared and UV. When you sort the wavelength from smallest to largest, you create the rainbow. When you mix colours, you get colours like purple, brown, and gold. When all of the visible wavelengths are combined, you get white light. The reason that we can see this range is a consequence of evolution, but also of physics. Although some animals can see into the ultraviolet range, almost all are centred around the visible range. No species has evolved can that see in radio, gamma rays or X-rays.
This may be because of our Sun and atmosphere. The bulk of the Sun’s light is emitted around the visible range, which is also the range which the atmosphere is most likely to let through, where many other wavelengths are at least partially absorbed. In the graph below, the yellow line represents the output by our sun, and the red represents what Earth’s atmosphere lets in to reach the surface.
Because of this, on Earth, visible light is consistently the brightest part of the spectrum.
Different planets will have different chemicals in their atmospheres, which will let different wavelengths of light in. A consequence of our evolutionary history may be that if humanity has a space-faring future, explorers may find that no other planet in the universe can ever be as vibrant and colourful to unaided human eyes as Earth.
Up to 10% of sunlight is in the ultraviolet range. Though it’s invisible to us, we are naturally exposed to it every day. It’s less intense than x-rays and gamma rays, although long, repeated exposure will cause sunburn and skin cancers.
This happens because UV light tends to break down large molecules, which kills our cells. It’s fatal to many types of small microorganisms, so one method of sterilising tools or water is to put them under a UV lamp, or just leave them out in the Sun for a while. It will also break down the large molecules in plastics and dyes, making them fade and crack with extended exposure.
Many flowers and corals reflect colourful patterns in UV light, and numerous insects, birds, and mammals have the ability to see them. When we shine a special UV lamp onto coral, some of the light they reflect is in the visible range and lets us approximate what these animals might see.
X-rays have short wavelengths, which means that they can be even more damaging to cells than UV light. It also means they can travel through soft materials like skin and muscles, but they are not small enough to travel through dense materials like bones. With proper shielding, they’re used in medicine to let us see internal problems like broken bones. X-rays also have many industrial uses like inspecting electronics, pipelines, welds, and ceramics.
Gamma rays are the most intense and dangerous part of the electromagnetic spectrum because their wavelength is incredibly small, at just one trillionth of a meter. Their energy is concentrated in such a small point that they have no problem travelling through the gaps between atoms and molecules (up to a distance of a few meters).
Gamma rays will often crash into the electrons that surround atoms, which will be flung away. These atoms will often steal electrons from other atoms to make up for the ones they lost, which will irreversibly change the structure of molecules. This is a big problem for living things, particularly if their DNA gets hit. With just a brief exposure they can develop cancer.
Gamma rays come from fusion within stars, nuclear explosions, and in supernova explosions. Once created, they will travel across the universe for millions of years and are only stopped when they encounter something dense, like a star or a planet. They are part of the reason why space is hostile to living creatures.
Planets that have magnetospheres, like the Earth, provide shelter for life just like harbours shelter boats in a storm.
But if human beings are ever going to travel through space, one of our major problems will be dealing with this most extreme end of the electromagnetic spectrum.
Radiation shields with current technology are far too heavy, and this may be the most significant engineering problem preventing us from becoming a space travelling civilisation.