Why is Space-time Warped by Gravity?

EInstein's General Theory of Relativity is said to have re-defined the nature of gravity as warped space-time caused by mass. This is often graphically depicted as space time being concave in the presence of a mass as a waterbed would if a bowling ball was placed on it. But how does the General Theory explain why an object will be attracted "down the hill" of this warped space-time, ie. why aren't objects attracted "up" the warped space-time? Doesn't this still require Newton's principles that there is something about two objects themselves that attract each other?
- Dr. Rob Backstein (age 44)
Toronto, Canada

That's a very perceptive question. We've addressed it some here, https://van.physics.illinois.edu/qa/listing.php?id=175, especially in follow-up #7. The key point is that it's not the up-down-ness of those curved sheets that determines the geodesic paths, the things that play the role of straight lines. It's just the curvature. The curvature induced by any mass has nothing to do with the way somebody chooses to embed a picture of that curved space in a higher-dimensional flat space. Turn the picture sideways or upside down and the rules for the paths that things follow don't change at all. (See also http://van.physics.illinois.edu/QA/listing.php?id=21373.) 

So no Newtonian force is needed to give this gravitational behavior. Still, we haven't explained why it is that energy should curve spacetime, so there's an element of the theory that's put in, not derived.

Mike W.

(published on 01/15/2016)