[snorkfort (OP)]

I know there have been other threads on this topic. But I think it is the most interesting question of all. Gravity and the other forces give order to the universe.

There were some interesting answers on the following thread, in particular Reply #6:
http://www.thenakedscientists.com/forum/index.php?topic=38879.0

So, what is your theory on why matter causes spacetime to bend? Give it your best shot.
Has there been any recent scientific progress on this question? Do you think we will ever know the answer?

[Blame]

Note the word spacetime rather than space. And a good point. We know gravity involves a slowing down of time but I have always wondered if space is really bent.

Einstein used gravity bending light as proof. This with the logic that light has no mass. 

Thing is light does have mass. Not admittedly rest mass but E=MC2 rearrange to M=E/C2. If you could put a perfect mirror coating inside  a bottle and fill it with photons that is how much the contents would mass.

So when starlight passes the sun (Einstein's proof) gravity is going to bend its path a little, and, conserving momentum the sun must get a bit of a shift too. Admittedly not enough to notice.

Anyway, I sort of suspect that the time dilation is the most fundamental characteristic of gravity. I don't think the speed of time and the speed of light can be separated as at the most basic level everything travels at the speed of light - ether directly, as a standing wave, or some combination.

So, without being able to explain why mass reduces the speed of light it might be possible to simplify the maths by assuming it does.  It does explain the pull of gravity. Given E=MC2 then E drops with C inside a gravitational field and that's where the potential energy comes from.

[geordief]

It is my understanding that  the movement of objects in the presence of mass-energy  can be modeled  when the co ordinate system of the (spacetime)  model is curved  according to the "mass-energy neighbourhood."

It is my further understanding that this mechanism , whilst accurate  is not understood in terms of "why it should be so". (what causes this to be so)

I also understand that it is  not generally accepted that "spacetime" , as a physical entity bends  - rather it is the spacetime model(an intellectual construct)  that is curved.

I would be very happy to be disabused of these  "understandings" by someone who is  competent  to do so.

[saspinski]

I think that "why" questions like that can be rearranged to:

Which are the independent postulates and definitions of physics, from which other consequences arises?
In the case of mass, it is a measure of inertia. Given that concept of mass, it is somewhat implied that a bunch of mass must bend spacetime (according to the concept of spacetime)? I don't think so. I can imagine an universe full of mass but without gravity. Sure there were then no reason for stars or any massive bodies to form, so it would be an universe formed only by hydrogen atoms and molecules, expanding forever.