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Re: Mass And Gravitational Force

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Posted by yelmalio/">yelmalio on January 31, 2000 10:32:55 UTC

How is Mass related to Gravitational force. Instead of branding it saying it exist so be it. I would like to reason this out.

OK. The reason why we keep saying Relativity id the answer is becuase it takes many months and a lot of Maths to see why its the answer. A full treatment of this answer would take about 300+ pages of reasoning. I'll try and keep it shorter but this means somehting will be lost in translation.

Here goes. The problem facing physics in the mid 1880's was a simple one, what is light and what is Gravity? At the time it was thought that light was a wave propagating (moving) in a medium called the Ether. Two chaps called Michelson and Morley (read this up to satisfy yourself the ideas good) set out to find the properties of the Ether (or Aether). After 5 years of experiment they found no Ether but had found the speed of light was a constant.

Then a chap called James Clerk Maxwell put forward a theory called Electromagnetism that showed Electric and Magnetic fields where a special case of an Electromagnetic field. One result of this was that the speed of an EM field in a given substance only depends on two constants - it's a constant. It also said that light was an EM field. The other effect of EM theory was the development of radio, telegraphs, TV, The Internet and all modern communications systems.

That was all well and good but Maxwells theory did not work with the tried and trusted Newtonian Gravitational theory. Its a problem with Galilean Transforms, Maxwells eqns when transformed result in infinities and strange answers with no meaning.

This meant that either,

1. Newton was wrong 2. Maxwell was wrong 3. Some thing else was going on that no one understood.

As 1. and 2. where known to be right option 3 was the answer.

A chap called Lorentz modified the Galilean transorms so that time was no longer absolute. This allowed Maxwells equations to be transformed and work inside Newtonian theory. Though it was more a case of making it work and then explaining WHY it worked.

Then along came Einstein. He proposed in the special theory of relativity,

1. The speed of light is constant in a given inertial frame of reference. (already proven) 2. There exist global spacetime frames with respect to which unaccelerated objects move in straight lines at constant velocity

(Read for more details)

This led to some surprising results. It shows why the Lorentz transform works and shows that time and space are interlinked as spacetime. A practical application of this is the famous E=Mc^2 which was used to ultimately make Atomic Bombs.

Special Relativity only works for non-accelerating frames and does not explain gravity. What Einstein wanted was a theory to explain gravity and include accelerating systems, or frames. This led him to the General Theory of Relativity which says that,

1. Gravity and Acceleration are the same,

This is very, very important.

The reasoning goes like this. If you are in a gravitational field you are accelereting. The velocity of Light is a constant, light follows the shortest distance between two points. If light travels through a gravitational field, which it routinely does, it must accelerate. But the only way light can accelerate while appearing to maintain a constant speed is if the shortest distance between two points is NOT a straight line but a curve.

So Mass causes Gravity which is an acceleration caused by the curvature of spacetime. That is gravity is a result of curved space time. The curvature is caused by a mass. The more mass, the greater the curvature.

As Mathman says, this only explains WHAT is seen and not WHY mass causes spacetime to curve.

See link below for a better description of the history of Relativity. If you have any questions, please mail me.

  • History of Relativity.

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