# Gravity

Steve
Joined: 25 Feb 06
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### RE: RE: As for books, it

Message 25675 in response to message 25669

Quote:
Quote:

As for books, it depends on how many yards you want stride and and what depth! Cheers and welcome! Mike

A suggestion: "Subtle is the Lord..." by Abraham Pais.
It is a scientific biography of Einstein, a good textbook on relativity.
Tullio

Tullio, this book cam 4 days ago. So far I am enjoying it a bunch. I have to be near a google tool bar when I read it :) It has many terms I am unfamiliar with and I have to keep google searching words. It seems to be a good primer and maybe more as I get into it. Thanks.

debugas
Joined: 11 Nov 04
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### actually i dont understand

actually i dont understand why we should treat gravity as curvature of space-time ?
Why not simply look at it as some information on the flat space-time , the information that alters the interaction of matter ?

Actually i am more for a Life-game style of explanation. We have a bunch of information for every space-time cell and there are special rules that rule how this information is transmitted to the neighbouring space-time cells.

There is no forces, there are only rules that tell us how information is transmitted to neighbouring cells.

Then inflation of the universe can be explained as outbound manifestation of this information covering larger and larger 4D surface

debugas
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### there are NO forces, there

there are NO forces, there are only rules that tell us how information is transmitted to neighbouring cells.

let's take Maxwell eq for EM field for example:

rot E = -dB/dt
rot B = (1/c^2)*dE/dt+mu*j

div B=0
div E= ro/epsilon

what does it tell ?
it tells us that in a given cell of space the new value of B can be calculated from the neighbouring cells values of E (and also B to keep div B=0)

and also that new value of E can be calculated from the neighbouring values of B, j and ro ( here flow j=v*ro so new ro values too can be calculated from neighbouring cells values of B,E)

debugas
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### and what is catching me in

and what is catching me in Maxwell equations is that they do not depend on mass
and somehow we know that light is bended by gravitational mass so there should be some sort of Maxwell equations that include dependance of (E,B,j,r) info-quadruple at given cell on the gravitational field information at the neighbouring cells

MarkF
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### debugas: I GR theory

debugas:
I GR theory electro-magnetic fields need to be treated as an anti-symmetric tensor. The equivalent to your four equations involves the covariant derivatives of that tensor and contraction using the contravariant metric tensor or the 4-dimenstional Levi-Civita tensor.
[url=http://en.wikipedia.org/wiki/Maxwell's_equations]Maxwell[/url]
Derivative

debugas
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### ah thanx MarkF, well i am

ah thanx MarkF, well i am aware of the Maxwell equations expression in the tensor form via J current vector and EM field strength tensor F but my question is why
F is combined with stress energy tensor T from einstein equation via einstein tensor G (or more precisely via metric tensor g) ? Would not it be more simple to combine J and F with T directly , itroducing T in the maxwell equations directly and playing in the flat minkowsky space without calling for curved space-time?

I must say i never looked into it and einstein interpretation of the gravity as curved space-time is really elegant but nevertheless is it the necessary complication ?

MarkF
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### debugas: That the

debugas:
That the electro-magnetic E&B fields are really components of a four dimensional anti-symmetric tensor comes from SR and quantum electro-dynamics. If these theories where serious flawed it should have been detected long ago by particle physicist playing with their accelerators.

The form of Maxwellâ€™s equations used in GR is implied by general covariance. So if space is curved then the extended version of Maxwell's equations must be used. In fact the extended form must used if the coordinate system is curvilinear even if the space is flat (for example spherical coordinates).

There have been numerous attempts to reproduce the successes of GR using alternatives that keep a flat space, so far none of the competing theories have matched the combination of GR predictive abilities and simplicity of the equivalence principle.

The stress energy tensor for an electro-magnetic field also uses the metric tensors in combination with products of the F tensors. Again this arises from general covariance and the other statements of para 2 apply.