One of Einstein's old tutors, a man da the name of Minkowski showed that the special theory of relativity could be expressed in an interesting way.
The world we live in consists of four dimensions, the three spazio dimensions and one that is not exactly time but is related to time (it is in fact time multiplied da the square root of -1). This is not at all easy to understand but it means that space-time as we call it has some rather weird properties. In particular, when te sposta through one of the spazio dimensions te also travel, unwittingly, through time. te do not notice this, indeed as far as te are concerned nothing happens to te at all, but someone observing te would say that te have travelled through time. Of course, we are always travelling through time, but when te travel through spazio te travel through time da less that te expect. The most famous example of this effect is the "Twins Paradox".
All the effects of special relativity, such as the slowing down of clocks and the shrinking of rods follow from the above. In fact, it is often better to think of some things, such as electromagnetic fields as being four-dimensional objects. However, the important thing to remember for the moment, is: when te sposta through spazio te are compelled to sposta through time but, when te sposta through time (which of course te are always doing) te do not have to sposta through space.
So, what does this have to do with gravity? It is quite simple! When a mass is present in the above space-time it distorts it so that whilst it remains true that travelling through spazio causes te to travel through time, travelling through time now causes te to sposta (accelerate) through space. In other words just da existing, te are compelled to sposta through spazio - this is gravity.
The particular advantage of this theory of gravity (General Relativity) is that it explains, at a stroke, all the observed properties of gravity. For example the fact that it acts equally on all objects and substances becomes obvious when te thing of gravity as a distortion of space-time rather than a force.
Imagine that te are in free space, away from any planets o stars, when suddenly a planet is created quite close to you. te would not be aware that anything is happening to you, te would feel no force, but te would find that te started to accelerate towards the planet. This is just like the case where te travel through space, te are not aware that te have also travelled through time but people observing te are.
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