Re: What causes gravity?

Hi Marcos

The important question here, as you correctly pointed out, is where does 
the energy come from that allows things to accelerate in a gravitational 
field? The short answer is space itself, or more precisely, curved 
spacetime. 

In the 1600's when Isaac newton first proposed an equation that described 
Gravity space and time where perceived as separate things. You had 3 
dimensional space; height, length and breadth or (x,y,z) and you had time 
t. The two things where separate and thought to be totally unchanging. 
This was the concept of absolute space and absolute time. In our normal 
everyday lives this seems to be true and it works as a good approximation 
for most things. Several very expensive space probes have successfully 
flown around the solar system using this approximation.

In the late 1800's a chap called Robert Maxwell developed a set of 
equations that described how light behaved as a wave. These are the 
Maxwells Equations of electromagnetism and accurately describe how light 
or any other electromagnetic waves behaves. The problem is that Maxwell's 
equations do not work in a Universe as described by Newton. They fall 
apart. Given that Newton's equations work, and Maxwell's, it was obvious 
that one of them was not a full description of what was happening in the 
real Universe.

It was also found out by experiment, by Michelson and Morley, that the 
speed of light (an electromagnetic wave) was constant regardless of the 
speed of the thing emitting the light. This led to someone called Lorentz 
proposing a modification of Newton's idea of an absolute Universe. 
According to Lorentz's equations how you measure time and the three 
spatial dimensions (x,y,z) depends on your  speed (velocity) relative to 
the thing you are measuring. 

This led to one of the profound concepts in modern Physics. Albert 
Einstein proposed that the only thing that is constant in the Universe is 
the Speed of Light. Space and Time could be measured differently between 
different observers. That is, for two people moving at different speeds 
relative to each other, the length of a second is different and the length 
of a ruler is different. This idea is called the Special Theory of  
Relativity. It is called the Special Theory as it only deals with objects 
moving at constant velocity (or speed if you will).

The next major development in this story was the development, by 
Minkowski, that space and time where not separate things affecting each 
other but part of a larger whole, called Spacetime. Spacetime is 4 
dimensional and is what we normally call space, everything around us. This 
in turn led Einstein to propose his General Theory of Relativity.

In General Relativity spacetime is treated as a problem in geometry. Using 
the assumption that acceleration can not be distinguished from the effects 
of Gravity and that spacetime is the same to all observors you get some 
startling results. What Einstein found is that if you have `flat ' 
spacetime and place a mass (A star or planet) in it then spacetime becomes 
curved, or warped. This curvature of spacetime has energy stored in it and 
is what we see as gravity. The standard analogy for this is to take a 
large stretched-out rubber sheet and place a heavy ball on it. The ball 
causes the rubber to bend into a well. Imagine a small ball placed onto 
the sheet. Where the sheet is less stretched there is less `potential' 
energy to extract. As you move down into the well there is more potential 
energy stored and this is converted into kinetic energy causing an object 
to accelerate towards the mass in the well. This is what we experience as 
Gravity. Further away from a mass there is less curvature of spacetime and 
less potential energy. A mass moving near the well will start to 
accelerate towards a mass as it converts the potential energy of curved 
spacetime into kinetic energy. 

For a very good description of this I would recommend getting Kip S. 
Thornes book, "Black Holes and Timewarps, Einstein's Outrageous Legacy". 
There are also some excellent on-line resources you can see as well. 
Suggest you have a browse of,

Ned Wight's cosmology 
tutorial
John Baez has many pages on 
Relativity, tutorials and the FAQ's for several physics groups on usenet.
Read 
Einstein's own words on Gravity.
Eric Baird's home page also lists HTML versions of some popular
books on Gravity and relativity. 

Finally. If you need clarification or further details on any of this, 
please mail me at my 
personal address

yours 

Dave Barlow