A day in the life of a photon

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It’s Lumiere weekend, you find yourself gazing in awe at one of the 37 light installations illuminating the city. You pause to ponder for a short while and consider how amazing physical interactions between the world and our perfectly evolved eyes are, such that they allow us to see lights.

As you mull over your thoughts, you begin to wonder what exactly is light? How on Earth does it work? Luckily, that’s precisely what I’m about to shed a little light on.

We have all learnt in school that light is a wave, specifically an electromagnetic wave. It travels in straight lines bouncing around reflecting and refracting on different surfaces, carrying energy along with it. This is all true of course, and this theory was widely accepted by the end of the 20th century, after hundreds of years of research into different models of light.

That was until the ever so slightly eccentric Einstein published a paper on the photoelectric effect, a physical phenomenon that seemed to contradict the wave model of light. From his experiments, he found that when you shine light on a piece of metal, electrons (small charged sub-atomic particles) are released. But they appeared to be released by little packets of energy, not by a constant wavy bundle of energy as expected. Building on recent work by Max Plank, Einstein theorised that light must be in the form of these little packets. Later, scientists named these little packets photons.

This discovery was profound. A photon is an elementary particle and the basic unit that makes up all light.  However, it turns out that 20th century scientists were not wrong. Light exists as a particle, in the form of a photon, at the same time as it exists as a wave. A wave-particle duality, if you will.

WTF! Surely something cannot be two things at once. To be fair, it doesn’t actually make much sense. These ideas were the beginning of quantum physics, a famously mysterious branch of physics that Einstein himself struggled to accept the ideas of: “the most incomprehensible thing about the world is that it is comprehensible”. Richard Feynman (an all-round very clever man) put it even better: “I think I can safely say that nobody really understands quantum mechanics”.

I think I can safely say that nobody really understands quantum mechanics

richard Feynman

Although physicists don’t quite understand why light behaves as both particles and waves, they do have a good grip on how it behaves. So, what does a day in the life of one of these photons look like?

7:30am – time to wake up, exciting day ahead!

Atoms can exist in excited states. This means electrons are orbiting the nucleus in higher energy levels. If an excited electron jumps down to an inner orbital they release a tiny packet of energy, our photon! This is how photons are produced. There are funky instances when this mechanism happens to create light. For example, in a flame, the heat causes a process called incandescence, where atoms become excited and emit photons, as we just discussed.

The Lumiere installations will use low energy LEDs wherever possible, a choice made to make the event more energy-efficient and minimise the effect on the environment, yay! The materials inside LEDs become excited as electricity passes through, then emit photons.

Sonoluminescence is the emission of photons from bubbles that implode in a liquid after the atoms are excited by sound. Funky indeed.

1:30pm – what a day! This day is lasting an eternity!

Could light die out? Can photons decay? Scientists are suspecting probably not, or not any time soon anyway. By studying ancient light originating from the Big Bang, physicists have found that very few, if any, photons have decayed since the start of the universe. According to calculations from the Max Planck Institute for Nuclear Physics in Germany, this means photons live for at least 1018 years. That’s one billion, billion years! Talk about a long day!

photons live for at least 1018 years … Talk about a long day!

11:00pm – bedtime! I hope I’m absorbed by sweet dreams!

What happens to the energy photons are carrying? If light hits an atom, the packets of energy are absorbed by the atom. This is how electrons gain energy to become excited in the first place. The energy contained in the photons is transferred to the electron and now the electron has the required energy needed to jump up a level. That’s surely too much energy to sleep!

The study of light, or optics, is an interesting branch of modern physics. With lots still to learn about the quantum properties of light, there’s almost definitely more profound discoveries still to come. For now though, we can sit back, appreciate the pretty lights of Lumiere, and be amazed by the physics behind them.

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