Hong Kong, 2014.
An enormous alien, emerging from deep beneath the sea, is threatening the safety of all humanity. As it approaches the shore, water streaming off its gargantuan hide, it raise an enormous claw and swipes at a passing oil tanker, snapping it in half like a twig. Reaching down into the wreckage, it picks up a handful of hapless sailors and throws them into its house-sized mouth.
In response to this disaster, humanity has built a giant humanoid robot, which stands 60 metres tall, is powered by a nuclear reactor, and can throw a punch with the force of a train. The robot marches out into the water, the deep bay only coming up to its waist. It strides through the water towards the alien, grabs it by the neck, and throws it into the nearest building.
But is that actually how things would go down?
In reality, giant creatures and robots fall victim to a quirk of physics known as the square-cube law. The key point of this law is that a body’s volume increases faster than its surface area, when scaled up by any degree. While the weight of a creature or robot is only dependant on its volume, the strength of the bones and steel depend on the cross-sectional surface area. This means that while an ant may be incredibly strong at normal size, having a high surface-area-to-volume ratio, the same ant scaled to human size would collapse under its own weight. Scaling a human more than twice the average height produces the same effect.
Additionally, the square-cube law also comes into effect with a little thing called respiration: the oxygen needed for a creature to survive increases with the volume, but its intake ability increases with the surface area of its lungs, meaning that past a certain size a creature will be unable to get enough air to survive. This is why larger land creatures (like dinosaurs and megafauna) existed in the past: the oxygen content of the atmosphere was once much higher.
So what would really happen during our giant-alien-vs-giant-robot fight?
The giant alien emerges from the ocean. No longer supported by its buoyancy in water, its spine begins to press into the rest of its body. It attempts to raise an enormous claw, but its muscles are unable to pull the weight of the hand out of the water. Floundering and unable to draw sufficient oxygen into its lungs, it begins to suffocate.
Meanwhile, enormous doors open, revealing humanity’s saviour. Our giant robot is only able to stand with the assistance of a mobile gantry. Detaching itself to fight the monster, it tries to walk, but breaks its leg as soon as weight it puts weight upon it. The iron giant falls into the water, sinking to the ocean floor, while the water pressure begins to crush its hull.
The citizens of Hong Kong look on in confusion as these two great beasts are defeated by the merciless laws of physics, and go back to their daily lives.