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Re: What Causes Gravity?

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Posted by Zephram Cochrane/">Zephram Cochrane on November 15, 1999 02:35:15 UTC

: Gravity is not the geometry of space-time, that is merely our mathematical way of describing it.

The gravitational field or gmn is both what I refer to as the geometry of space-time and it is our mathematical way of describing it.

:General relativity is not an explanation of gravity, it is a description only, and it is also an incomplete description.

It absolutely is an explanation of macroscopic gravity. Not mearly an incomplete description. It is an incomplete explanation in that it doesn't explain micro/quantum gravity.

:General relativity describes the macroscopic effects of gravity through two fundamental postulates, those being the constancy of the speed of light and the Equivilance Principal. To do this you need geometry, since light "wants" to travel in straight lines; hence one ends up with general relativity.

Light doesn't "want" anything. It follows geodesics because space-time has geometry(a nonzero metric tensor). That is the macroscopic explanation.

:The "geometry" of gravity exists only in our description of it,

Not geometry of gravity. The gravitational field gmn IS what I'm referring to as the geometry of the space-time. And it does not mearly exist in description. It actually exists. Should humans never have been born to describe it the metric would still be non-zero.

:it is merely a simple consequence of the way light (and all objects under uniform motion) likes to travel in straight lines,

This is anthropomorphizing a bit much. They "like" nothing. In the absence of force they follow geodesics, because there is geometry to the space-time.

:but the force of gravity causes that motion to bend and not be straight.

More accurately the geometry causes the motion to be geodesic whether or not that is straight.

:That is what the geometry describes, and it is what we use to predict the motion of something in a gravitational field. Also, maybe you have not heard, but General relativity and its "geometry" is a classical theory, meaning it does not take into account quantum mechanics and does not and can not explain where gravity comes from on a sub-nuclear scale.

No kidding! I never said it did.

:Also, M-theory is at this point HIGHLY speculative,

Not really.

:and is not well formed

The creators not a theory is what is informed, and the creators are very well informed.

:and also lacks the observational evidence to support it,

You mean to support its uniqueness. It's predictions are consistent with all the observational data we have. The only problem is it has few predictions that are unique to the theory, and these can only be tested at higher energies than we can yet attain.

:and may always lack the evidence because it requires too much energy to confirm. You shouldn't be so quick to beleive everything you read.

You shouldn't be to quik to dismiss what is understood.

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