Re: [Phys-l] definition of gravity

[John Denker <jsd@av8n.com>]

On 11/10/2011 09:29 AM, William Maddox asked about:

> A) Spacetime curvature means real physical curvature.

Answer:  Yes.

The amount and especially the direction of curvature is 
often non-obvious to the untrained eye, but it is perfectly 
real and quantitative physics.  It is not some hand-wavy 
analogy.  It is not an approximation.  It is curvature.

> B) This means additional dimensions are needed (center of circle not on
> circle)

Answer:  No.  Extra dimensions are not "needed".  You can
measure the intrinsic curvature of a surface without leaving
the surface.

For the purpose of making /embedding diagrams/ you need
one extra dimension ... but embedding diagrams are
themselves not needed, and are not entirely faithful
representations of the physics.

> 1. Yes now that string theorists have made it safe to talk about more than
> 3+1 dimensions.

A simple "yes" suffices.  Strings are not needed.  The idea 
of spacetime curvature predates cosmic strings by many decades.