[ExI] physics

[Anders Sandberg]

On 2016-04-25 17:38, John Clark wrote:
> On Mon, Apr 25, 2016  William Flynn Wallace <foozler83 at gmail.com 
> <mailto:foozler83 at gmail.com>>wrote:
>
>     ​ > ​
>     In the standard general relativity view there is a patch of
>     spacetime there, with its own properties (curvature).
>
>
> ​If "real" means something that's invariant then curved spacetime is 
> not real because it looks different for different observers.

Nope. I was using curvature here to denote the metric tensor (or, if you 
want to go deep, the Riemann tensor). Tensor equations are 
observer-invariant. So while you might measure using a different 
coordinate system from me and get different numbers, there is a simple 
rescaling between our results. In particular, the line element ds^2=g_ij 
dx_1 dx_2 will have the same length for us no matter how we move and 
reparametrize. Geodesic curves for one observer are geodesics for all 
others.

In general relativity spacetime is pretty much ontologically primary: it 
is all about some Riemannian manifold, described by its metric. How 
observers see things is irrelevant: the structure is still the same 
manifold and metric.

-- 
Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.extropy.org/pipermail/extropy-chat/attachments/20160425/23dac6ea/attachment.html>