Sunlight streams down on my kitchen table. Eight and a half minutes earlier, 93 million miles away, these photons left the surface of the sun. These photons were created deep below the sun’s surface, where hydrogen, crushed by the Sun’s incredible gravity, fused into helium. It takes four hydrogen atoms to make one of helium, and in each such fusion 0.032 atomic mass units of mass is converted into electromagnetic radiation – heat and light.
Ninety-three million miles they traveled at a speed of 186,000 miles per second. (I don’t need to look up these numbers. They have been in my head since I was 11 years old. Which also explains why I talk in miles.) Light behaves both as a stream of particles and as a wave. Were it not so, the sun’s energy could never travel through the vacuum of space – if light were just waves, like sound waves, they would stop dead when there is no longer something for them to pass through.
After such a long journey in a perfectly straight line (I know, I know – gravity bends the spacetime continuum, and who knows what a straight line really is in that context anyway?) at the final moment, just a few kilometres above my house, they hit atmosphere, and for the first time, change direction and speed. Some photons of certain wavelengths are scattered, contributing to the blueness of the sky and the red of the sunrise. Others hit water vapour or dust particles, and are absorbed – their journey cut short without warning or reason. Quadrillions of others hit pavement, rooftops, sidewalks and tennis courts.
Some of their energy is converted into heat and radiated back up into the air, creating temperature imbalances that cause breezes, evaporation, precipitation – in short, weather. Others hit plants and trees that, after billions of years of evolution, have developed the ability to convert this energy and power factories that create sugars and proteins, leaves and flowers, fruits and vegetables, grains and grasses, bone and flesh pulling this planet out of the realm of geology and into the land of the living.
Some photons bend through the upper atmosphere at just the right angle to strike my kitchen window. They change direction and velocity when they hit the denser pane of glass and change again as they exit the other side – 93 million miles and then zig-zag-zig. They hit the table and bounce off, this time at different wavelengths – the unique shades of light and dark brown that indicate this table is made from varnished wood. A tiny portion of these photons get refracted through the lenses of my eyes, and end their journey on my retinas, where they cause a message to shoot up my optic nerve and jangle millions of neurons and axons.
I do more than register that photons of a particular type have entered my visual system – I understand what I’m seeing. These photons, a tiny percent of a tiny percent of a tiny percent of the light exploding off the Sun every second – these particles whose journey was, to every possible means of understanding it, arbitrary – allow me to move around my house, spread out the paper, and wake up to the world.
This kind of narrative is not always at the front of my mind, but it is almost always at the back. It is why I began The Quantum List Project. Physics is everywhere. It has power and beauty. To call it massively useful would be an understatement.