In trying to write up the latest breakthroughs in particle physics, I realized I’d skipped over some fundamentals. Like, for instance… how do we even know what light is?
Or since I’m not convinced it won’t goddamn change again, how did we get to this wave-particle duality mess?
Today’s Moment of Science… Let there be a brief history of light.
The ancient Greek philosopher Empedocles postulated that everything was composed of four kinds of stuff: earth, air, water, and fire. Warhammers, marijuana, and armpits? We all came from the same garbage heap. It wasn’t a bad shot at chemistry a few decades before the conception of the atom. He suggested that a small fire was placed in the eye by the goddess Aphrodite, allowing us to see by the power of pointing friggin’ laser beams out of our friggin’ heads. Well, not quite. Since we don’t have night vision (yet), he thought our sight depended on interaction with some other source of light, like the giant glowing orb in the sky.
In 55BCE, the Roman natural philosopher Lucretius dug into atomic theory. Slight paraphrase, but he said “light’s an atom-thingamabober and it moves fucking fast.”
Things really started moving when Ibn al-Haytham, a polymath born a bit over a millennium ago in present-day Iraq, started running experiments to test his ideas. He sorted out how vision really worked along with some new and more complete ideas on reflection and refraction. He also suggested light moved at a zippy yet finite speed, and even wrote an early prototype for the scientific method.
Science was happening all around the world though, and the internet was super slow until, give or take, late last century. Some things needed to be “discovered” a few times before they caught on.
By the late 1600s, two titans were toiling away on theories that we’re still working with now: Christiaan Huygens and Isaac Newton.
Huygens, a Dutch astronomer and physicist, sorted out the laws of elastic collision and discovered one of the moons of Saturn, Titan. In 1690, he published his Treatise on Light. Amongst many other things, he proposed that light had finite speed and, much like sound, it traveled as a wave through a medium known as “aether.”
It’s not that Huygen’s work wasn’t impressive. And when Isaac Newton published Opticks in 1704, it probably wasn’t a drastically more convincing case than Huygens. It’s just that Isaac Newton had already fucking invented calculus. So when he was all “I sorted out the laws of motion, how the goddamn tides work, fucking gravity, oh, and uuuhhh light’s a particle,” people were like “sure, light’s made of particles, whatever this genius says is probably right.”
About a century later, another genius polymath named Thomas Young came along and went all “waves, motherfuckers,” with an early version of the double slit experiment.
It’s actually simple enough that you can perform it at home with a box, sunlight, and idk like four other things. Allow light through two small slits to see what shows up on the other side, and intuitively, what would you expect to see?
If we picture waves of water going through two slits, as the wavefronts propagate through and crash into each other, they form a familiar pattern of alternating constructive and destructive interference. We’d see areas with a higher wave and areas where the waves have canceled each other out crash into the back screen.
And since light behaves like a wave, it makes the same interference pattern. Which was a good enough answer for Thomas Young’s time. It’s a wave, end of story.
Around the dawn of the twentieth century, there was one final twist. It was discovered that when you blast a metallic surface with light, electrons will be ejected. This is known as the photoelectric effect. Light above a certain frequency was needed to cause electron emission; you could shine a low frequency light forever at high intensity, and it still wouldn’t chip off an electron. Furthermore, the velocity at which electrons were yeeted from the metal plate was independent of light intensity. These little quirks were incompatible with classical physics.
From this, it was Einstein who suggested that maybe light was traveling in discrete units. Though the term had been kicked around by other scientists with regards to electrons, matter, and energy, Einstein was the first to deem these singular packets of light “Lichtquanta.” A quanta of light, or a photon.
In a sane universe, it would be one or the other. But we live in this universe, so it’s gotta be a particle, and it’s gotta be a wave, at least until the next revolution in physics.
This has been your Moment of Science, turning down the photons for the night.
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