“The vastness of the heavens stretches my imagination — stuck on this carousel my little eye can catch one — million — year — old light. A vast pattern — of which I am a part… What is the pattern, or the meaning, or the why? “ — Richard Feynman
Light. One of nature's wonders, that we take for granted because we see it every day. Literally, we not only see it — but we see nothing but it.
Seeing is defined as interacting with the electromagnetic waves, from the radiation from the sun rays, the photons hitting our detector eyes, there are many words for the same. Rays are scattered, different wavelengths give us different colors, everything we see is light by definition.
If we define light broadly as every radiation on the electromagnetic spectrum, then there is more to light that we catch our vision. Our eyes are a set of detectors, that can detect electromagnetic waves, but only in a specific range of wavelengths.
Other wavelengths can be “seen” by detectors in telescopes. Observing the Universe in various wavelengths can make us see what our human vision would not be capable of, even if we could see through the incredible distances through the galaxy and across the Universe.
So, light is a wave, but light is also a particle, the photon, hitting our eye detectors, the larger the photon stream, the more intense the light.
Every particle is also a wave, this is the particle-wave duality. Just like the electron, it behaves as a wave, when propagating through space, but can be detected as individual particles when it is measured.
Both of those definitions are really just two sides of the same case, but the fundamental building blocks, including light, is neither point-like spherical particles nor waves, those are both mere shadows of something we don’t understand completely yet.
A more complete definition in modern physics is defining the fundamental blocks for matter and energy as quantum fields, with one type of field for each particle. Hence, there is an electromagnetic field, an electron field, an up quark field, and so on, for each fundamental particle. More about that later.
Check out my mindmap of the many definitions of light and the electromagnetic force below. I will zoom in on the separate definitions in the separate sections below.
Let’s break it up to take a look at the many definitions of the concept of light, starting with the electromagnetic wave and the electromagnetic spectrum.
Light as an electromagnetic wave
The electromagnetic wave is also one of the four fundamental forces: Electromagnetic force. The force is a wave propagating through space at the speed of light.
We can relate to the concept of waves as the waves propagating through water. When a wave moves through water, it’s not the water itself that propagates in the direction of the wave, the water molecules move mostly up and down, while the waves passes through them. Likewise, the electromagnetic wave is a force field, that oscillates in strength, while it passes through space.
There are deviations from the water wave analogy. The EM wave does not require a matter medium to move through, it can move through the vacuum in the empty space. This is where it moves with the known “speed of light” of 300,000 km/s. When light moves through a different medium, such as water, air, or glass, it moves slower than through vacuum. Still, 200,000 km/s through glass is a very nice speed.
Another difference is that an EM wave consists of two waves moving perpendicular to each other, the electric field and the magnetic field propagating in the same direction. Those two waves follow each other, so when the electric field is largest, the magnetic field tops as well, perpendicular to its counterpart.
Hence, electricity and magnetism are part of the same force, which is also the same force as the light that we see. It’s all connected.
Our vision and light detection
When we observe with our eyes, we detect the light reflected from surfaces. Our eyes can detect electromagnetic waves that are reflected, but only if the wave has a certain wavelength between approximately 400 and 750 nm. We call light with shorter wavelength for UV (Ultraviolet) light, X-rays, and the shortest wavelengths for gamma and cosmic rays, while longer wavelengths are IR (infrared), microwaves, and radio waves.
There is nothing universally special about the electromagnetic wavelengths of the light we can see as compared to the rest of the electromagnetic spectrum. The colors that we see in the optical wavelengths are not a universal property, they are emergent properties from our brain and how it interprets the received signals.
Our eyes are adapted to the most common wavelengths of light from the Sun. If our Sun was an enormous blue star, our vision would probably be in the range of UV-radiation, and if it was a red dwarf, we might be seeing the infrared. Of course, then this would just be our visible normal, and our brain could just as well have been adapted to see colors in those wavelengths.
… and the rest of the electromagnetic spectrum
The shortest wavelengths in the spectrum are the cosmic rays and the gamma rays, which are deadly for our cells because they carry enough energy to break up the structure of molecules and cells.
X-rays follow. We can’t see x-rays unless we are Superman, but they can go through the soft tissue of our bodies without being stopped, while bones will absorb them, which is why it is possible to see our skeleton with x-rays.
UV-light has a wavelength just below our visible spectrum and carries less energy than x-rays, but more energy than optical light. Luckily, most UV-light from the Sun gets absorbed by the ozone layer in the stratosphere, otherwise, we would get a deadly dose of this high energy level radiation.
Wavelengths slightly longer than the visible light are infrared and can be felt as heat. Our body also radiates heat as IR blackbody radiation, and someone with IR vision would be able to see us light up in the dark.
Some animals can see UV or IR light, while we have yet to see a chicken with x-ray vision or a horse that sees our radio and internet signals. The world would surely be a brighter place if we could see those. And maybe a bit more confusing.
The intensity of the light is proportional to the amplitude squared of either the electric of the magnetic field. The intensity can loosely be defined as the “number” of photons passing through a given area in a given time. Because yes, light is an electromagnetic wave, but it’s also photon particles.
Light as particles
“Is he a dot, or is he a speck?
When he’s underwater does he get wet?
Or does the water get him instead?
Nobody knows, Particle man”
— Particle man, song by They Might Be Giants.
A particle is a quantum of matter or energy, and the photon is defined as the smallest possible quantum of electromagnetic radiation, where a quantum means a quantity or amount. Unlike in our macro-world, a particle can only have discrete levels of energy and this is what quantum mechanics is all about.
A particle has a probability distribution, where it would be possible to detect it. In the microworld, the particle exists in a superposition of all of those positions, until measured to a specific location. We might only be seeing a small part of the Universe because we don’t see all the other states of the particles. Everything we observe or interact with is “collapsed” to one state of many.
There exist two kinds of fundamental particles: Bosons and Fermions. The Bosons are the force carriers of the four fundamental forces, where the electromagnetic force is one of them (the other three are the strong and the weak force and gravity, although we have yet to be able to detect a graviton particle). So in that sense, particles are not only the solid matter in the sense that we know in our everyday life, but all the possible interactions between matter are also due to fundamental particles.
The photon is the force carrier of the electromagnetic force. A photon is the smallest package of energy that can be carried, and the energy is directly proportional to the frequency of the vibration of the photon, E = hf, where h is Planck's constant, and f is the frequency.
Whenever two charged particles are attracted to or repelled by each other by the electromagnetic force, it can be thought of as the particles “playing tennis” with a virtual photon, constantly exchanging energy. The positively charged nucleus with protons and the negatively charged electrons in an atom are held together by electromagnetic force, hence you could think of it as a virtual photon being exchanged back and forth in the “empty space” in the atom.
Light as quantum fields FTW!
A modern way of looking at the particle and wave duality is to imagine the fabric of space consisting of endless particle fields. There is a field for every type of fundamental particle and the most common examples are the electron fields and the electromagnetic fields interaction.
When an electron absorbs a photon and gets excited (it just won the tennis game!), we think of it as an energy transfer because the photon carries energy. But thinking of the universe as consisting of particle fields, it’s the electron field interacting with the electromagnetic field, where the EM field loses one excitation quantum, while the electron field gains one.
What we think of as particles are then excitations and vibrations in the fields. Good vibes!
Our experience of the Universe is defined by light
The electromagnetic force is not only what defines our vision, but it also defines our other senses. When we touch something, it’s once again the electromagnetic force working.
The electromagnetic force is the bond that keeps atoms together by attracting negatively charged electrons and positive nucleus. It’s also what keeps the molecules together, usually by the positively charged nucleus sharing a cloud of electrons. It’s the reason that we don’t fall through the chair we sit on and through the ground, we stand on. The electromagnetic bonds between molecules in solid keeps them together and makes the solid impenetrable.
The sound that reaches our ears are vibrations through air and through our bones, and in the end, those vibrations are also only possible through the electromagnetic force, interacting and pushing other particles.
Smell and taste are chemical reactions that get interpreted by our brain as such, and in the end, those are also due to the electromagnetic force.
The electromagnetic force is how we experience the physical Universe through our senses, not only our vision but all of our senses.
General properties of light
“What if I could ride a beam of light across the Universe?” — Albert Einstein
Light is not only fundamental for us and for how we see and define the Universe, but it also has properties that seem to define the Universe.
This is something that Einstein spend a lot of time thinking about before he came up with his special theory of relativity in 1905.
Light speed is constant and it doesn’t matter how fast you travel yourself. Unlike the speed of anything else, the speed of light is not relative to the observer! If you ride a virtual car at 200,000 km/s (you’re on fire, baby!) and turn on the lights, the light will still travel with the speed of light at 300,000 km/s ahead of you, not just 100,000 km/s, which would be the difference between you and the speed limit of the Universe. Instead, spacetime will be distorted, so time slows down considerably as you yourself accelerate near the speed of light and space will be shortened.
Lightspeed is always constant, and not relative to the speed of the observer, space and time are malleable. Light is the master ruler of the Universe.
The faster you go, the slower time appears to move relative to your surroundings. At the speed of light, there is no time component, time stops. If you could experience the Universe as a massless light beam, you would experience everything from the Big Bang to the end of the Universe in one single moment. Time is stretched out only by our mass and the gravitational potential. It’s impossible to move with the speed of light having mass, because it would require an endless amount of energy to accelerate mass to the speed limit.
Lucky for us, it gives us at least a little time to experience the beauty of the Universe, given by light itself.