Jenni Barrett, of Cashmere High School, asks :-

I was told that stars twinkle because gravity bends the light. What else can gravity have an effect on?

David Wiltshire, an astrophysicist at the University of Canterbury, responded.

Actually, the twinkling of stars has nothing directly to do with gravity. The twinkling is caused by convection and turbulence in the atmosphere, due to warmer air rising through colder air, giving rapid changes in the amount the light is refracted - or bent in its path - by the air.

Gravity does bend the path of light, but even at the surface of our sun only by an incredibly small amount. The amount of bending is very much larger if you are dealing with an object with the mass of a black hole or a galaxy. Galaxies can act as gravitational lenses, producing multiple images of even more distant galaxies and quasars that happen to be on the same line of sight. Many of these have been photographed by the Hubble Space Telescope and can be viewed by following the links at http://vela.astro.ulg.ac.be/themes/extragal/gravlens/bibdat/engl/

To answer your question at a broader level, gravity is the one force of nature that acts on absolutely everything, although its effects are generally weak and unremarkable unless one is dealing with the strong fields caused by neutron stars, black holes and the like. Nevertheless, the Global Positioning System - which can give you a very accurate estimate of your position via a handset and a global network of satellites - would cease to function within a few hours if its software did not take into account gravitational time dilation. This is the effect that clocks at the surface of the earth run very slightly slower than clocks aboard the GPS satellites. So with modern technology even the more "unusual" aspects of gravity are significant enough to have an impact on our daily lives.