This is the third in a series of articles exploring the birth of quantum physics.
Light is a paradox. It is associated with wisdom and knowledge, with the divine. The Enlightenment offered the light of reason as the guiding path to truth. We have evolved to identify visual patterns with great precision – to distinguish the foliage of the tiger or the shadows of an enemy warrior. Many cultures identify the sun as a god-like entity, provider of light and heat. Without the sun, after all, we wouldn’t be here.
Yet the nature of light is a mystery. Of course, we learned a lot about light and its properties. Quantum physics has been key on this path, changing the way we describe light. But the light is weird. We cannot touch it like we touch air or water. It is a thing which is not a thing, or at least which is not made of the matter which we associate with things.
If we go back to 17e century, one could follow Isaac Newton’s disagreements with Christiaan Huygens on the nature of light. Newton would claim that light is made up of tiny, indivisible atoms, while Huygens would argue that light is a wave that propagates on a medium that permeates all of space: ether. They were both right, and they were both wrong. If light is made of particles, what particles are they? And if it’s a wave propagating through space, what is this weird ether?
We now know that we can think of light both ways – as a particle and as a wave. But during the 19e century, the particle theory of light was largely forgotten, because the wave theory was so successful, and one thing could not be two things. In the early 1800s, Thomas Young, who also helped decipher the Rosetta Stone, performed some beautiful experiments showing how light diffracted as it passed through small slits, just as waves of water were known to do it. The light would travel through the slit and the waves would interfere with each other creating bright and dark fringes. Atoms couldn’t do that.
But then, what was the ether? All the great physicists of the 19e century, of which James Clerk Maxwell, who developed the beautiful theory of electromagnetism, believed that the ether was there, even if it escaped us. After all, no decent wave could propagate through empty space. But this ether was pretty weird. It was perfectly transparent, so we could see distant stars. It had no mass, so it wouldn’t create friction or interfere with planetary orbits. It was however very rigid, to allow the propagation of ultra-fast light waves. Pretty magical, right? Maxwell had shown that if an electric charge oscillated up and down, it would generate an electromagnetic wave. It was about the electric and magnetic fields bound together, one priming the other as they traveled through space. And more surprisingly, this electromagnetic wave would propagate at the speed of light, 186,282 miles per second. You blink and the light circles the Earth seven and a half times.
Maxwell concluded that light is an electromagnetic wave. The distance between two consecutive peaks is a wavelength. Red light has a longer wavelength than violet light. But the speed of any color in empty space is always the same. Why is it about 186,000 miles per second? Nobody knows. The speed of light is one of nature’s constants, numbers we measure that describe how things behave.
Stable as a wave, hard as a bullet
A crisis began in 1887 when Albert Michelson and Edward Morley performed an experiment to demonstrate the existence of ether. They couldn’t prove anything. Their experiment failed to show that light travels through an ether. It was chaos. Theoretical physicists had strange ideas, claiming that the experiment failed because the device had shrunk in the direction of motion. Anything was better than accepting that light could actually travel through empty space.
And then came Albert Einstein. In 1905, the 26-year-old patent officer wrote two articles that completely changed the way we imagine light and all of reality. (Not too shabby.) Let’s start with the second article, on the special theory of relativity.
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Einstein showed that if we consider that the speed of light is the fastest speed in nature, and that this speed is always the same even if the light source moves, then two observers moving one by relative to each other at a constant speed and making an observation must correct their distance and time measurements when comparing their results. Thus, if one is in a moving train while the other is in a station, the time intervals of the measurements they perform on the same phenomenon will be different. Einstein provided the two with a way to compare their results in a way that allowed them to agree. The corrections showed that light could and should propagate in a vacuum. He didn’t need an ether.
Einstein’s other article explained what is called the photoelectric effect, which was measured in the laboratory in the 19e century but remained a total mystery. What happens if the light is projected onto a metal plate? It depends on the light. Not on its luminosity, but on its color – or more exactly, its wavelength. Yellow or red light does nothing. But shine a blue or purple light on the plate, and the plate actually acquires an electrical charge. (Hence the term photoelectric.) How could light electrify a piece of metal? Maxwell’s wave theory of light, so good at so many things, could not explain this.
The young Einstein, daring and visionary, came up with a scandalous idea. Light can be a wave, of course. But it can also be made up of particles. Depending on the circumstances or the type of experience, one or the other of the descriptions prevails. For the photoelectric effect, we could imagine small “balls” of light hitting the electrons on the metal plate and expelling them like billiard balls flying off a table. Having lost electrons, the metal now holds an excess positive charge. It’s so simple. Einstein even provided a formula for the energy of flying electrons and equated it with the energy of incoming light balls, or photons. The photon energy is E = hc/L, where c is the speed of light, L its wavelength and h Planck’s constant. The formula tells us that smaller wavelengths mean more energy – more kick for photons.
Einstein won the Nobel Prize for this idea. He basically suggested what we now call the wave-particle duality of light, showing that light can be both a particle and a wave and will manifest differently depending on circumstances. Photons – our light balls – are the quanta of light, the smallest packets of light possible. Einstein thus introduced quantum physics into the theory of light, showing that both behaviors are possible.
I imagine Newton and Huygens are both smiling in heaven. These are the photons that Bohr used in his model of the atom, which we discussed last week. Light is both a particle and a wave, and it’s the fastest thing in the cosmos. It carries with it the secrets of reality in ways that we cannot fully comprehend. But understanding its duality was an important step for our perplexed minds.
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