Asked by: Sam Holland, by E-mail
Everything is affected by gravity, regardless of size – even particles of light. This makes gravity unique among all the fundamental forces of the Universe – and also causes theorists enormous problems. And that’s because gravity also acts on itself, causing runaway ‘feedback loops’ in their equations making them tricky to solve.
But gravity is also the weakest of the fundamental forces, and so in many everyday situations its effects are small enough to be ignored. For example, the shapes of solid objects as big as mountains are dictated principally by the chemical bonds between their constituent molecules, rather than their own gravity. In general, gravity only begins to dictate the overall shape of objects bigger than several hundred kilometres in size – such as small moons and asteroids.