Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 09/04/2013 12:30:01

Title: Are gravitational mass and space-time inseparable?
Post by: thedoc on 09/04/2013 12:30:01
Sal Napoli  asked the Naked Scientists:
I would like to comment on a question posed by listener, Bastiaan Bargmann   and answered by scientist,  Andrew Pontzen In the Naked Astronomy Podcast of February 25, 2013. - What is the speed of gravity?
The question was: What would happen if the sun were suddenly to disappear?  Andrew stated that the question was unanswerable because the causal theory of gravity, although presuming that information cannot travel faster than the speed of light, actually cannot be answered because of a built in idea of energy momentum conservation.
I agree that the question cannot be answered.  The reason is that; the question presupposes a stipulation that is impossible.  That is, a distinction between a large gravitational mass and its influence upon space-time.  I believe that the gravitational mass AND its influence on space-time is an inseparable whole.  It would be like asking the question, "What is the sound of one hand clapping?"
If you instantaneously cause the sun to vanish, you are also, of necessity causing its influence on space-time to vanish.  Consequently, if the sun AND its space-time distortion/effect were to suddenly vanish, then instantaneously, the earth would move off along the tangent to its previous orbit.

Sal Napoli, Florham Park, New Jersey, U.S.A.

What do you think?
Title: Re: Are gravitational mass and space-time inseparable?
Post by: yor_on on 10/04/2013 00:09:34
It's answerable, presuming we make some stipulations first. We define gravity as able to propagate, and we decide it does so at 'c'. Then we have defined it such as gravity becomes a net, in our ordinary four dimensions, 'distance(s) + time'. That net exist at all times but the information about it's constituents/properties/field are defined by mass, and mass have 'relative motion'. That makes 'gravity' a dynamically changing property, updated at 'c'.

So, using this, if the sun wen amiss, it would take Earth the same time it takes a ray to propagate from the sun, to notice the gravitational potential to change.
Title: Re: Are gravitational mass and space-time inseparable?
Post by: yor_on on 10/04/2013 00:15:30
What you suggest about it instantly being perceivable I don't expect to be possible macroscopically, although you can find entanglements to meet that requirement. But that is microscopically, using a different logic than what we find macroscopically.