We seem to understand how it behaves, but scientists are now finding out how mysterious this force really is

Reported by Kulvinder Singh Chadha

General relativity predicted expansion Einstein’s formula for general relativity predicted the expansion of the universe.

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.

Gravity in orbit above Earth is still 90 per cent what it is at the surface

That’s also how gravitational lenses work. While attempting to observe the most distant galaxies in the universe, astronomers make use of the gravitational force of clusters of galaxies, which are some of the most massive structures in the cosmos. Their huge mass warps space so much that light from more distant galaxies can become magnified, in exactly the same fashion as a microscope lens magnifies small objects. The galaxy clusters, in essence, become a great cosmic lens.

But Einstein wasn’t the first to think about gravity. Many others, from Galileo to Robert Hooke, had a hand in developing our early understanding of the force that keeps our feet on the ground, but it was Isaac Newton who made the biggest conceptual leap, his ‘eureka’ moment being the story of the apple falling on his head as he rested peacefully beneath an apple tree – though historians suspect this is probably not true. Using his new theory of gravitation, which described how every object in the universe is attracting every other object, he set about describing how gravity is the key to explaining how the planets orbit the Sun. Newton explained how the force of gravity was proportional to the masses of the gravitating bodies, which is also what Einstein showed: the more massive an object, the stronger its gravity. He also showed that the force of gravity was inversely proportional to the distance between objects. If you move twice as far away from an object, its gravity feels four times weaker.