How is Earth’s gravity generated?

Gravity is the warping of spacetime by mass or energy. A mass like the Earth warps spacetime so that the shortest path, the “path of least resistance,” for inertial movement is towards the Earth’s center.

Using instruments like the Hubble Space Telescope, it’s possible to literally see the warping of spacetime by very massive objects like galaxies and huge conglomerations of dark matter. When you’re looking at a very distant object and there’s a large mass along your line of site, it warps spacetime to produce a visual effect known as gravitational lensing. Here’s a schematic diagram showing how it works.

Here’s a real-world example of gravitational lensing, an “Einstein cross” — a single quasar is distorted by a massive object along our line of sight so it appears to be a cross of four quasars.

The arcs in this image are disc-shaped galaxies that appear warped by dark matter along our line of sight.

Here are some examples of Einstein rings, a lensing effect similar to Einstein crosses, where a massive object warps the galaxy behind it into a ring shape.

Finally, here’s a simulation of how the Milky Way disc would be lensed gravitationally if a black hole passed between us and it.


The really weird thing about gravity is that it warps time, not just space. In fact, at the scale of the earth, the time warping is more noticeable than the space warping. GPS satellites and the like need to account for the fact that they’re moving through time at a slightly different rate than objects on the ground. Pretty cool!

Another approach is called loop quantum gravity. The idea here is that spacetime is made of a tiny discrete network of loops. All of the fields and forces, including gravity, emerge from knots, links and kinks in the network.

Loop-variable quantum spacetime

There’s some indication that string theory and loop quantum gravity are different mathematical expressions of a single underlying theory. As with string theory, probing spacetime experimentally at the tiny scale of loop quantum gravity is very far out reach technologically, so for now, it too remains speculation, though it’s well-motivated mathematically. Only time will tell what the true story is.

Original question on Quora