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Nongeometric approach to gravity impossible? 
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#19
Feb2612, 10:53 AM

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#20
Feb2612, 11:48 AM

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Harmonic coordinates mean near the singularities as you mentioned. Does this mean inside the event horizon (say 10 light years across) or most inner part of it near the center (near planck scale)? Whatever, in quantum gravity which goes beyond the planck scale, it can address the issues inside the event horizon or just near the singularities? This is because it is not possible to address black holes in flat spacetime. Then would quantum gravity of spin2 field in flat spacetime be able to address black holes whose event horizon (say 10 light years across) is still much below the planck scale size? How? Note it is only near the singularity that planck scale physics address. Hope you get what I'm asking or I'd have to rewords this again. Thanks. 


#21
Feb2612, 01:49 PM

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Gravity is described by a second rank tensor, hence the spin 2 designation. Electromagnetism is described by a first rank tensor.



#22
Feb2612, 04:55 PM

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#23
Feb2612, 05:03 PM

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For an object to fall it takes two objects, both fall towards each other and the more mass is involved the faster it will be. 


#24
Feb2612, 08:18 PM

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#25
Feb2612, 11:11 PM

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#26
Feb2712, 01:16 PM

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http://www.physicsforums.com/showthread.php?t=14258 


#27
Feb2712, 07:09 PM

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if you were at a midpoint between Jupiter and the Earth (and not falling) then the rate that Jupiter falls towards the Earth would be the same as the rate that a canonball falls towards the Earth. What is different is the rate that the Earth accelerates towards Jupiter or the cannonball. So even a massless object could fall towards the Earth at the same rate as any other object, but the Earth would not accelerate towards the massless object. 


#28
Feb2712, 07:48 PM

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Midpoint is also incorrect  you mean center of mass. Also, in GR, the mass dependence of 'free fall' has another component  gravitational radiation, which is a nonlinear phenomenon. 


#29
Feb2812, 03:49 AM

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Precisely GR is formulated in a way that doen't assure energy conservation, or as Hilbert put it: General relativity has improper energy theorems instead of proper energy theorems. We have a FAQ in the cosmology subforum that deals with this and asserts that energy is not conserved in the cosmological models based on GR. That would lead me to think that according to GR it is not so sure that gravity cannot be shielded, in a way it could (see "relative energy of a black hole" thread, especially peterdonis posts). It would seem the gravitational energy is shielded, in the sense that it is not a source of curvature (meaning it is not part of the stressenergy tensor as explained by peter donis in the abovementioned thread). 


#30
Feb2812, 04:39 AM

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I need to know something about this spin2 field in flat spacetime = curved spacetime.
1. What is the reason for the Equivalence Principle in this Spin2 field in Flat Spacetime Field Theory of Gravitation? 2. A larger ball with more mass should suppose to fall faster because the gravitational field between the earth and the object is more (in this spin2 flat spacetime version). Yet Galileo showed they fell at the same rate. Is there a version of asympototic freedom in flat spacetime where larger mass would have fewer emitted gravitons to match the smaller sized object hence they falling at the same rate? 


#31
Feb2812, 10:44 AM

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So in that case what do you think your measurements would actually prove? 


#32
Feb2812, 11:04 AM

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"Asking for consistent self interactions leads essentially uniquely to GR and full general coordinate invariance [4, 5, 6, 7, 8, 9, 10] (see also chapter 13 of [2], which shows how helicity 2 implies the equivalence principle)." http://arxiv.org/abs/1007.0435 "As argued by Weinberg [38], the equivalence principle can be recovered as the spintwo case of his low energy theorem. ... In other words, massless particles of spintwo must couple in the same way to all particles at low energies." 


#33
Feb2812, 01:58 PM

P: 381

Thanks. I need to understand 2 basic things:
1. Say in the future gravitons were finally detected. Does it mean spin2 fields actually existed and they occur in the backdrop of flat spacetime. Or do gravitons imply spacetime curvature is a priori? But in what sense is there spacetime curvature and at the same time gravitons existing when the two are more of a dual much like photons and electromagnetic wave (these are dual descriptions)? Unless you mean detection of gravitons don't tell us whether spacetime is really curved or dynamics just occuring in flat spacetime by spin2 fields? 2. Can Loop Quantum Gravity be formulated as spin2 field in flat spacetime? Or does LQG stay valid only if spacetime is actually curved? 


#34
Feb2812, 02:21 PM

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Within string theory, gravitons are only approximate degrees of freedom, and strings are more primary. So in the string theory picture, curved spacetime is a coherent state of strings on flat spacetime. In the AdS/CFT picture, strings and space are both emergent, and neither are primary. 


#35
Feb2812, 05:04 PM

P: 3,967

Put it another way. In the rest frame of the Earth the acceleration of a falling object is proportional to G(M+m) where M is the mass of the Earth and m is the mass of the falling object. It is easy to see that if m goes to zero, that the acceleration does not go to zero. 


#36
Feb2812, 05:08 PM

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