In case you have not been following the news lately, NASA and Stanford University’s joint effort – Gravity Probe B after 17-odd months of launch and collecting data has proved one of the two not-as-yet-proven aspect’s of Einstein’s Theory of Relativity.
Simply put (and it will be simple as I only have a fairly basic grasp on Physics – it was one of my favorite subjects in School, but that was many moons ago ), Isaac Newton believed (as shown by NASA’s image below) that both time and space were fixed entities and gravity was the force which acted between the object. So, in Newton’s world a perfect gyroscope orbiting the Earth would remain forever fixed to relative space.
Einstein on the other hand more than 90 years ago said both space and time are relative entities which are interwoven into a “fabric” that he called space-time. In Einstein’s world an object like Earth distorts the fabric of time and space (as shown in NASA’s image on the right) and gravity is not a force as Isaac Newton discovered, but rather is a consequence of the distortion of both space and time. The bigger the object, the greater this distortion. It is both matter and energy that distort space-time, by curving it around themselves.
So in Einstein’s world the spin of the same perfect gyroscope orbiting Earth will precess i.e. change to the direction of the axis on which it is rotating in relation to relative space.
I liked the analogy used by the Guardian to describe this time-space:
Just as a bowling ball placed on a trampoline stretches the fabric and causes it to sag, so planets and stars warp space-time – a phenomenon known as the geodetic effect. A marble moving along the trampoline will be drawn inexorably towards the ball.
Thus the planets orbiting the Sun are not being pulled by the Sun; they are following the curved space-time deformation caused by the Sun. The reason the planets never fall into the Sun is because of the speed at which they are traveling.
The second as-yet unproven part part of the theory is called frame-dragging and states that as the object such as Earth in the aforementioned example, spins, it drags the the fabric of the universe behind it. This is still not proved and will be another few months until enough data is collected.