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 often ripple with energy. When those ripples are in a very specific wavelength range (i.e. size), human beings can see them as flashes of light. The ripples are called photons, and the whole possible range of their wavelength is called the electromagnetic spectrum.
The range that we can see forms the colours of the rainbow, with the smallest visible wavelength forming the colour violet and the largest forming red. Every colour that is not in the rainbow, like brown, comes from mixing the rainbow’s ‘original’ colours.
On each side is ultraviolet and infrared, invisible to us, but not to some other species.
The molecular structure of many materials, like steel, wood, and granite is impenetrable to most photons in the visible light range. Some wavelengths get absorbed by them while others bounce back like ripples from a stone in a pond, and the objects appear opaque. Exactly which wavelengths they repel determines their colour.
This stone reflects photons in the red wavelength, so it appears red.
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.
Ice lets most photons slip through. Image source: Darren Jackson
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 photon ripples 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.
At night we see the faint ripples from other stars, having travelled for lifetimes across the electromagnetic field like an electron along a wire or a jump along a rope, before ending their journey on our retinas and telescopes.
These stars have shown a very small property of the electromagnetic spectrum has had unexpected but truly profound consequences for us. 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.
It has revealed that the universe is gargantuan in size, with the Earth being like a single speck of dust surrounding an average star, in an unremarkable part of an ordinary galaxy. From this information, we predict that the universe is full of hundreds of trillions of planets, representing a range of possibilities beyond the wildest dreams of our greatest minds.
Without the change in redshift, our position in the cosmos would forever be a mystery. Every star would be an indeterminable distance from any other, and travelling to one would be like travelling towards a light at the end of a dark tunnel, but without knowing how far we have to go.
The part of the electromagnetic spectrum that we can see is determined by the cells in our retinas, and it has been tuned to this frequency by our evolutionary history.
For instance, primates like us are fairly unique in being able to see the red, which is part of the spectrum. It is 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.
Some species can see a larger range of the spectrum than we can, and some peculiar species, like the 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.
Mantis shrimp may have evolved to see a much greater range of colour through sexual selection. This shrimp may be many times more colourful than what we can see. Image credit: George Grall, National Aquarium
But even the mantis shrimp can see only a tiny fraction of the whole.
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 a range of new colours.
This is because materials that reflect photons in the visible range may let different ranges shine through. If we decreased the visible wavelengths into the x-ray range, many materials that were opaque would become transparent. Muscle would become clear and everyone would look like walking skeletons.
If we tuned into gamma rays, walls and even the ground would start to become glassy and transparent. 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, particularly because they can be encoded with information.
Let’s take a look at each of them.
The wavelengths of electromagnetic radiation
When we discovered that we could artificially generate photons, one of the first things we did was revolutionise war.
This is because of radio waves, the largest possible wavelength in the spectrum. They can reflect off the underside of the atmosphere, meaning that we can use them to send wireless signals to someone that’s not within line of sight.
In 1914, at the start of World War 1, radios were bulky and had an effective 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 (4,800 km). The result was a boom in commercial broadcasting, beginning with entertainment for returned soldiers.
Over time 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 it is what connects our smartphones to the internet.
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.
Modern radios can read and extract the signal from both types.
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 straight 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 to 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.
A man using an infrared TV remote while radio waves fly around him. The technology that the electromagnetic spectrum allows would once have been considered magic.
This will have an intriguing 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 in infrared.
It turns out that all matter that contains heat naturally emits a small number of photons. 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 cameras have not tuned their colours to the same temperature range, but you can see the change in colour when the temperature does.
A Formula 1 car, seen through an infrared camera.
Before and after of an ice bucket challenge, seen in infrared.
At high temperatures, the type 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 UV and infrared. When you sort the wavelength from smallest to largest, you create the rainbow. When multiple wavelengths are combined, you get mixed colours like purple, silver, 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, they’re all clustered around the visible range. No species has evolved can that see in radio, gamma rays or X-rays.
This is because of our Sun. 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.
The wavelengths of light emitted by the Sun, and the wavelengths let through by the atmosphere. The visible spectrum is what lights most of the Earth. Image source: Wikipedia, Solar 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 might find that no other planet in the universe can ever be as vibrant and colourful to unaided human eyes as Earth.
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 result in 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.
Minerals reflecting light in UV.
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.
X-rays travel through soft materials but are stopped by denser materials like bone, which lets us image them.
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).
The ghostly world of gamma radiation. It travels through most materials, including concrete up to 10 centimetres.
Gamma rays will often crash into the electrons that surround atoms, which will be flung away. The atoms will go on to steal electrons from other atoms, irreversibly changing the structure of molecules. This is a big problem for living things, particularly if their DNA gets hit. With just a brief exposure they will 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 is dealing with gamma rays. Radiation shields with current technology are far too heavy, and this problem may be the most significant engineering problem preventing us from becoming a space travelling civilisation.
Future spaceships may use advanced magnetic fields to keep gamma rays at bay. Image source: Gizmodo