The Mesmerizing Physics of Light
We humans have an intimate, evolutionary relationship with light. But what is light, really?
Light is so fundamental to our way of processing the world around us that we tend to take it for granted. But what is it? And why should any non-scientist care?
We have different ways of explaining light that most people are familiar with: Light is color, light is energy, light is a wave. It’s all of those things at once, and more.
As you read this, your eyes are probably already glazing over from flashbacks to high school physics class. You shrug your shoulders. So what?
Here’s one reason I want to take a moment to marvel: We humans have an intimate relationship with light.
What Is Light?
Light, in a certain sense, helped make us human. Perhaps that’s why we’re constantly “humanizing” light by investing colors with all kinds of emotional attributes.
We don’t have great vision compared to other animals or insects, and that underwrites our evolutionary fear of the dark.
To overcome our shortcomings, we’ve managed to maximize what we can do with light. That’s true in the context of our caveman ancestors mastering fire and warding off potential predators. It’s also true in the use of lasers, computers, and medical applications of infrared radiation — technology possible because of breakthroughs in quantum physics.
But what is light? The truth is, we’re still figuring it out.
Light: A Wave and a Particle
Light is part of the electromagnetic spectrum. As the term suggests, light is a wave with electric and magnetic properties. James Clerk Maxwell developed the theory of electromagnetism in 1865 — an achievement called the “second great unification in physics” after Isaac Newton’s contributions.
Maxwell found that light was situated on the spectrum between low frequency waves like radio and microwaves that have longer wavelengths on one end, and the higher frequency, shorter wavelength X-rays and gamma rays at the other.
The light visible to us is only part of the spectrum — we know about infrared, a lower frequency wave that’s closer to microwaves, and the higher frequency ultraviolet waves that approach X-rays.
The one significant difference is that the human eye can see visible waves. We can’t see radio waves, even though their wave lengths are as huge as buildings.
Why can we see light waves? Part of the answer is that we see how light affects the matter it hits.
All waves move energy from one place to the other, and light waves are no exception.
Matter is made up of atoms, which have electrons with different energy levels. When light waves hit something, they are raising the energy level of the atoms. Matter absorbs part of the wavelength as a photon, then rebalances its energy by shooting off two photons that have a lower energy than the one absorbed.
What Are Photons?
Light is a wave. But a light wave can also be described as a stream of massless particles moving at the speed of light. A photon is the smallest “quantum” of energy.
That “quantum” is the origin of the term “Quantum Mechanics,” the field of physics that deals with how tiny things like atoms and photons behave.
How can light be both a particle and a wave? How can a particle be massless? The quantum world operates differently than the large scale “common sense” world dealt with in classical physics. But atoms and photons don’t exist separately from this larger world, and while we can observe them behaving one way at the quantum level, as we zoom out from atoms to molecules to the everyday objects around us, their behavior changes to the classical properties of physics.
As celebrated physicist Richard P. Feynman put it back in 1964, light and electrons behave in their own unique way:
Now we know how the electrons and light behave. But what can I call it? If I say they behave like particles I give the wrong impression; also if I say they behave like waves. They behave in their own inimitable way, which technically could be called a quantum mechanical way. They behave in a way that is like nothing that you have seen before. Your experience with things that you have seen before is incomplete. The behavior of things on a very tiny scale is simply different. An atom does not behave like a weight hanging on a spring and oscillating. Nor does it behave like a miniature representation of the solar system with little planets going around in orbits. Nor does it appear to be somewhat like a cloud or fog of some sort surrounding the nucleus. It behaves like nothing you have seen before.
How Can a Light Particle Be Massless?
A photon has no mass, but it has many other properties like speed, momentum, and position that come together to describe the particle’s physical reality. As Dr. Christopher Baird, Physics professor at West Texas A&M University notes, “the presence of mass does not confer on an object any extra degree of physical reality, even though mass is the property that we are most familiar with in everyday life.”
“We are so familiar with mass in everyday life that we may be tempted to say, ‘an object with no mass does not really exist’,” Baird explains. “But this statement is false.”
Mass is one of those “common sense” classical properties we experience in everyday life, but it’s important to remember that mass is ultimately just one more form of energy.
But in physics, momentum is mass times velocity. How can something without mass have any momentum to impact anything else through its motion?
Light photons transmit energy at the speed of light (just under 300,000,000 meters per second). When light photons hit something, their momentum does impact an object. The effect is often too small to notice, unless the object under observation is also very small.
Light doesn’t slow down like objects in classical physics — it always travels at the speed of light. Light loses momentum by lowering its frequency — as opposed to lowering its speed like things that have mass.
The energy it loses when it transitions to a lower state is transmitted over to what it collides with. Heat is the most common byproduct of this transfer. Some materials re-emit the light back out as a little light (such is the case with glow-in-the-dark stickers). The photosynthesis process in plants is the transfer of light energy into chemical energy that changes the bonds between atoms.
What Is Darkness?
Darkness travels at the speed of light. Or rather, because darkness is nothing but the absence of light, it is the byproduct of light that stops arriving. It’s what comes right after the light ceases.
If the light is moving at the speed of light, then its departure — signified by the darkness — is also the speed of light.
This is why the hypothetical question, “How soon would we know if the sun went out?” is answered by the time it takes for the light from the sun to travel to the Earth: 8 minutes and 19 seconds.
The speed of light is a universal unit of measurement, not only one that calculates the speed of light. In fact, all other massless particles-waves travel at the same speed in a vacuum.
Gravitational waves move at the same speed as light from the sun, meaning that if the sun “went out,” not only would Earth meet the darkness 8 minutes and 19 seconds later, our planet would also be sent hurtling out of the solar system at the same moment because the gravity of the sun would untether us from its orbit.
But where does the light go when we turn off the lights?
Why can’t we keep light in a closed container after the light source is extinguished?
Without mass, there’s nothing for us to keep. Light has no atoms. And because light is always traveling, it is constantly being absorbed by the atoms of objects that do have mass. The absorption of light is the destruction of a photon that converts its energy into another form.
Light Helped Make Us Human
Human evolution, especially the evolution of the human brain, is an expression of our relationship with light just as much as the photosynthesis of plants.
Our eyes — exposed extensions of our brains via the optic nerves — are such complex structures that religious opponents of Darwin’s theory of evolution often point to them as evidence of divine creation. How could gradual evolution have created the retina, optic nerves and other complicated components of the eye to make vision possible when none of those structures by themselves allow us to see?
The reality, of course, is that humans don’t have the best eyes among living creatures. One would imagine a divine creator doing a little better job for humans than for cats, spiders, and bees. No such luck.
But humans are resourceful. We used our inferior vision to evolve as tool-users and develop scientific ways to “see” further with technology and culture. Every adaptation is vital to survival, and the way our eyes evolved are part of our survival story.
What’s more, the variation in the animal kingdom afforded by evolution allows humans, in all our resourcefulness, to compare eyes in different stages of evolutionary development across species. Studying other eyes gives us a greater appreciation of our own evolution because we can see ourselves in the animals with lesser developed eyes.
If you think about it, Darwin’s theory of evolution is its own kind of evolutionary revolution for humanity, since it provides a key to unlock our own origins in a way that’s more profound than religion is able to provide.
Why Do We See the Colors We Do?
In the electromagnetic spectrum, visible light is just one segment of the light waves, and humans see only a sliver of those compared to some animals.
Why can’t we see other wavelengths? First of all, we are watery, carbon-based lifeforms, and we operate on both a physiological and a chemical level. As we’re functioning on the biological level, we’re also functioning on a quantum level.
When we look at an object, our eyes are taking in light — the combination of electrical and magnetic fields traveling at the speed of light in a specific wavelength. That wave of blue or orange travels to the retina and causes chemical changes in the cells of our eye cones and that signal travels along our optic nerves into our brains.
To think of this on the quantum level, the atomic makeup of our bodies — especially our carbon and water-heavy eyes — reacts to photons in the visible light range.
We can see visible light at wavelengths that range from about 400 to 700 nanometers — the size of bacteria. Although longer wavelengths like radio waves are a lot bigger, they don’t hit our eyes “hard” enough to set off the chemical reactions involved in vision processing. (That’s why humans can’t see infrared light.)
The shorter wavelengths of ultraviolet light, by contrast, carry too much energy, so they shoot through us, agitate our atoms and upset our chemistry, causing damage instead. That’s why X-rays and gamma radiation are so deadly to organic life. (It’s also why ultraviolet light causes sunburn.)
What Are We Seeing When We See Colors?
Photons are some of the tiniest particles known to physics, but they’re also visible to the human eye.
In fact, they are the only things we really do see.
When we look at a red ball, we’re not actually seeing the ball. We’re seeing the photons that have bounced off of the ball and entered our eyes. The photons, arranged in the shape of the ball and traveling at the speed of light, hit our retinas and the optic nerve sends the message into the brain.
Colors Exist Outside of the Human Brain.
We didn’t invent colors. Color is a property of light based on the length and frequency of its wave — its position on the electromagnetic spectrum.
Human eyes have color receptors for red, green, and blue. And all the other colors like orange and yellow are funneled through one of those receptors to the brain and interpreted there. But we don’t make them up.
This is one of the most confusing aspects of light for human beings, who have created all kinds of cultural associations with colors throughout history. Blue might be a symbol of purity or of sadness. Red might be romantic passion or rage. We invest colors with human attributes as if they were just figments of our imaginations. We invest light with a lot of psychological baggage.
One human might interpret a color differently than another human, but colors exist independently of our brains.
Maxwell’s electromagnetic spectrum has resolved the philosophical puzzle about whether you see the same red as I do. We are being hit by waves, or streams of particles, that share the same frequency. Whether the cones in our eyes interpret them as the same is a matter of our physical anatomy — an entirely knowable object of scientific inquiry, as colorblind diagnoses show.
Red, closer to 700 nanometers, has a longer wavelength than the 400-nanometer violet, and that is true even if you are blind.
Regardless of how our cones transmit the data to the brain, or what psychological reaction results, my red is your red.
In fact, our red is the universe’s red and would exist without human witnesses at all.
And that’s worth marveling over.