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Simple question about warping space 
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#1
May1810, 03:30 AM

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Is it true that gravitating bodies actually warp the fabric of space towards them like in this picture? http://www.astronomynotes.com/evolutn/grwarp.gif



#2
May1810, 04:07 AM

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That's a nice analogy, but personally I'd be careful with words like "the fabric of space".
It is true, that light is bent in the direction of gravitational masses. In fact, if the mass is large enough (or more strictly speaking, the mass density) light can be bent so strongly that its orbit is bent into a circle or even more. In that case you have black hole. Usually, however, the effects are visible near stars. In that case, an object can lie behind a star, like in the lower picture you linked. However, for an observer at the tip of the arrow, the light seems to have originated from somewhere like the far upper corner of the sheet (just draw a tangent line to the last part of the light orbit). This effect has been measured for the sun, as one of the first experimental tests of GR (actually, this effect also exists in Newtonian gravity, but its a factor off which GR gets right) and since it has been seen in action numerous times in so called gravitational lensing, mostly with large clusters and gas clouds. 


#3
May1810, 04:15 AM

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Thanks but my question is still unanswered. Is space bent towards a gravitating mass? Or does no one know? What did Einstein think?
Thanks, Jake 


#4
May1810, 04:35 AM

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Simple question about warping space
I suggest you take a look at the gravitomagnetic field equations, which are a firstorder approximation to GR but good enough to give you an idea of what is going on. The effect is similar to the effect due to a magnetic field caused by a moving charge.
"Is space bent towards a mass?" is, I think, a strange question to ask. To properly examine the curvature, you have to take time into account as well. The result of the curvature is that straight lines in spacetime appear to be curved toward masses in space. 


#5
May1810, 04:47 AM

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Thanks, Jake 


#6
May1810, 04:52 AM

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#7
May1810, 04:59 AM

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Ok, gravitating bodies warp spacetime toward them. Is that correct then? What do you mean by "when isolated in space"?
Thanks, Jake 


#8
May1810, 05:50 AM

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well I think 'isolated in space' is means that body of certain mass is alone to be observe or in other words it is alone. You can imagine that it is easy to observe the effect when it is only one who shows some deformation in light's straight line path.
prakash0 


#9
May1810, 08:32 AM

P: 836

What I meant is that geodesics, which are the straightest possible paths in spacetime, sppear curved in space (i.e. not straight lines in space).
What does it mean that something warps spacetime towards it? 


#10
May1810, 09:35 PM

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#11
May1910, 02:00 AM

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#12
May1910, 09:24 AM

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If you think about space time as a baloon where the stretchiness of the ballon at a spot on its surface is determined by its mass/energy density, then the surface of the ballon will be dimpled. The rate of time and the spacial dimensions are all determined by the radius of the dimple. Motion across the surface of the baloon means that you will be moving through dimples in space time as well as causing a dimple to propagate over the surface.



#13
May1910, 09:44 AM

P: 836

I guess it works as a 2D analogy of a closed universe, but it doesn't help jaketodd, since inhabitants on the baloon surface cannot experimentally determine the direction of the curvature (positive if on the outside, negative if on the inside, but this is impossible for the 2dimensional inhabitants to determine). Nevertheless, the baloon analogy is exellent for demonstrating that asking in what direction spacetime curves is nonsense. We can see that on the balloon, spacetime is embedded in 4 dimensional space (2 spatial dimensions, 1 temporal dimension and a fourth dimension into which spacetime also curves). By analogy we can see that we would need a 5dimensional space in which to embed our 4dimensional spacetime for us to be able to ask in which direction spacetime curves, and even then it would be a question of definiton. 


#14
May1910, 04:59 PM

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#15
May2010, 02:49 AM

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#16
May2010, 05:39 AM

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#17
May2010, 05:40 AM

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http://www.physics.ucla.edu/demoweb/...spacetime.html 


#18
May2010, 05:53 AM

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