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Re: Mass And Gravitational Force, WHY?

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Posted by pmb on February 11, 2000 13:35:31 UTC

: Great, mass curves space-time and causes gravity, WHY?

Strictly speaking there are three things which are the source of gravity: stress, energy and momentum. These three things together form an mathematical entity called the stress-energy-nmomentum tensor. Why? Nobody knows why ... at least not in a general relativity sense. I don't know about string theory.

: Distance is measured differently to an object with velocity relative to a reference, why?

It's due to the way we measure distance. We assume the postulates of relativity are true. From that we deduce by logical reasoning (and, using a bit of math), that when we measure distances experimentallly then there will be a velocity dependance. The validity of the prediction thus rests on the validity of the postulates and experiments. Since the postulates are formed from observation (i.e. experiments already done) the stregth of the postulate rests on the ability to predict new phenomena.

re - It is because the laws of physics are frame covariant and the Lorenz speed is finite. is a bit off.

First - It is because the laws of physics are frame covariant (which is correct) means, in case you're unfamiliar with the language of relativity, that the laws of physics (usually expressed mathematically) are independant of the frame of the observer.

Second - and the Lorenz speed is finite - I would phrase "the speed of light is locally invariant" (i.e. independant of the frame of referance).

: Time dilation is always preasent with space shortneing, why?

This isn't true. One can have a non-Euclidean space in which there is no time dialtation.

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