So the presence of a constant gravitational force can make things unstable, which generally means that there are unbalanced forces and things will shuffle around until balance is restored. If a solid object becomes unstable it flips over or falls down, and any liquid or gas surrounding it will just flow around to make room for the movement. But what happens when the thing that is unstable isn’t a single solid object like a balloon, but the fluid itself?
Strike a match, light the wick of the candle, and a fountain of bright, hot gas is switched on. Candle flames have cast a warm glow over scribes, conspirators, schoolchildren, and lovers for centuries. Wax is a soft, unassuming fuel, and that makes its transformation all the more surprising. But each one of these familiar yellow flames is a compact and powerful furnace, fierce enough to smash apart molecules and forge tiny diamonds. And each one is sculpted by gravity.
As you light the wick, the heat of the match melts both the wax in the wick and also the wax close to it, and the first transformation is to liquid. Paraffin waxes are hydrocarbons, long chain molecules with a carbon backbone that’s twenty to thirty atoms long. The heat doesn’t just give them the energy to slither over each other like a pile of snakes (which is what liquid wax would look like if you could see the molecules). Some will get enough energy to escape completely, drifting out and away from the wick. A column of hot gaseous fuel forms—so hot that it pushes hard on the surrounding air, taking up a huge amount of space for a relatively small number of molecules. The molecules are the same, so the gravitational pull on them is the same in total. But now they’re taking up more space, so the gravitational pull for every cubic inch has gone down.
Just like a bubbly snotty snail in the ocean, this hot gas must rise because there is cool dense air trying to slide underneath it. The hot air is pushed up an invisible chimney, mixing with oxygen along the way. Even before you’ve moved the match away from the candle, the fuel is breaking apart and burning in the oxygen, making the gas even hotter. These are the blue parts of the flame, and they reach a staggering 2,600°F. The fountain that you’ve started intensifies, as the hot air is pushed upward ever more quickly. It’s fed from beneath because the wick is just a long thin sponge, soaking up other wax molecules that have been melted by the furnace.
But the fuel doesn’t burn perfectly. If it did, the flame would stay blue and candles would be useless as light sources. As the long chain molecules are snapped and bullied by the heat, some of the detritus remains unburned because there isn’t enough oxygen to go around. Soot, tiny specks of carbon, is carried upward by the flow and heated. This is the source of the comforting yellow light that glows as the soot reaches 1,800°F. The light of a candle is only a byproduct of the fierce heat, and this light is just the glow of a miniature hot coal in a fire. These tiny carbon particles are so hot that spare energy in the form of light is pouring off them, and out into the surroundings. It’s been discovered that the maelstrom of a candle doesn’t just produce soot in the form of graphite (the stuff we think of as black carbon). It also produces tiny amounts of the more exotic structures that can be formed when carbon atoms join together: buckyballs, carbon nanotubes, and specks of diamond. It’s been estimated that the average candle flame produces 1.5 million nanodiamonds each second.
A candle is the perfect example of what happens when a fluid needs to rearrange itself to satisfy the pull of gravity. Hot burning fuel rises very quickly as cool air pushes underneath, forming a continuous convection current. If you blow out the candle, the column of gaseous fuel will keep streaming upward above the candle for a few seconds, and if you lower a match down from above, you’ll see the flame jump to the wick as the column is re-lit.??
Convection currents like this help move energy around and share it out wherever a fluid is heated from below. They are why fish-tank heaters, underfloor heating, and saucepans on the stove are all so effective—none of those things would work nearly as well without gravity. When we say “heat rises,” that’s not quite true. It’s more that “cooler fluid sinks as it wins the gravitational battle.” But no one thanks you for pointing that out.
Buoyancy doesn’t just matter for hot air balloons, snails, and romantic candlelit dinners. The oceans, the vast engines of our planet, take their marching orders from gravity just like everything else. The depths are not still. Water that hasn’t seen sunlight for centuries is flowing across and around the planet, on a long, slow journey back to the daylight. But before looking down into the depths, look up. Next time you see a tiny moving glint high in the sky on a clear day, a passenger jet at cruising altitude, allow yourself to appreciate just how high up it is: about 6 miles. Then imagine yourself standing at the deepest part of the ocean floor, the bottom of the Marianas Trench. The ocean surface would be the same distance above you as that plane is.?? Even the average depth of the oceans is 2.5 miles, a little under half the distance to that plane. The ocean covers 70 per cent of the surface of the Earth. There is a lot of water out there.
And hidden in those dark depths is a familiar pattern. The same mechanism causing raisins to dance in lemonade is also driving the vast oceans of the Earth on their slow journey around the planet. The scale is different and the consequences are more important, but the principle is exactly the same. The blue of our blue planet is in motion.