It keeps us on the ground, creates tides and holds planets in their orbits around stars and satellites revolving around Earth – and I maybe it did or didn’t cause that apple to fall on Isaac Newton’s head. We’re talking about gravity, of course. But amazingly, while we understand how gravity works, exactly how it is produced is still a source of great debate. Our current best theory of gravity is Albert Einstein’s general theory of relativity, which explains how mass bends space, and it’s these bends in space that we experience as gravity. It’s a bit like rolling down a hill; the steeper the slope, the faster we roll. So the more massive an object is, the more steeply it bends space. Another way to imagine it is if we have a rubber sheet held between two people, their hands holding each of the corners keeping the sheet taut. This rubber sheet is our space-time. Place a marble on the sheet and it creates a small dip. Put a bowling ball on the sheet and it will cause the sheet to warp and bend much more severely than the marble – indeed, the marble will probably start to run down the dip in the sheet towards the bowling ball. The only way the marble could escape is if it were moving fast enough. This is how the planets stay in orbit around the Sun rather than falling towards it and burning up: they are moving fast enough in their orbits.
That’s also how gravitational lenses work. While attempting to observe the most distant galaxies in the universe, astronomers make use of
