It is reasonably easy to accept that space can be curved – after all, we all know that a disk has a curved edge, but how can time be 'curved'? The secret lurks in the way that space and time is combined into space-time. Normally, a space-time diagram is drawn with a straight horizontal spatial axis and a straight vertical time axis. Just bend the two straight axes a little and we have curved space-time.

It's not quite that simple, but the effects of gravity can be viewed as no more than an arrow of spatial movement and an arrow of time movement that are both slightly bent. The bending is accomplished by the energy locked up in every massive object. It has been stated that 'mass tells space-time how to curve and curved space-time tells mass how to move'.

Summary

Have we got an answer to the question 'what is gravity?' Not quite. At the most fundamental level, no one really knows. We can say that it is an effect that keeps planets and moons in their orbits. We can say that it is a force that makes one mass attract another mass or we can say that it is what controls the movement of matter through curved space and time. Lately, we can also say that it is (perhaps) a manifestation of 'quantum gravity' - but that's another story…

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