Unless you take classes in modern physics, the concepts presented here might seem quite foreign.
Indeed, discussions between people that have been exposed to the concepts of Relativity and
those who have not can get very heated very quickly. Making the statement that gravity is not an
attractive force between objects can easily start these debates, and perhaps this is a good thing
because it inspires people to learn more about this wonderful universe in which we live. We have
learned a lot in the last 300 years. It's a good time to be alive.
There is no doubt that Sir Isaac Newton was a brilliant person. His work describing the universe
helped our civilization progress for hundreds of years. However, a more accurate model of the
universe is now available and is utilized in many aspects of our society.
With what is now known, it is necessary when using the word "gravity" to make some important
For example, one should realize that an object's unforced path is determined merely by the
geometry of that object's local spacetime. An object will follow its geodesic through the local
spacetime - regardless of its curvature - without any forces present*. There are no forces required,
and none exist or are measurable, in keeping the planets orbiting around the Sun. They are merely
following each of their geodesics in their regions of curved spacetime.
[*note: This is not strictly true in our universe because in curved spacetime there is always a
continuous gradient in the magnitude of that curvature, and so an object that has a non-zero length
(in the direction of that curvature) will experience an apparent stretching force due to resistance to
that gradient. This, for example, is what tore apart the Shoemaker-Levy 9 comet as it approached
Jupiter in 1994.]
Also, if one uses the phrase "the force of gravity", it's important to realize that the force applied to
someone's body when falling through the Earth's atmosphere, or sitting on the ground, or floating in
a swimming pool, is what is keeping that person from truly free falling (i.e. following their geodesic),
and but for that, no forces would be applied to that person.
References and resources
Relativity, The Special and the General Theory
Gravity, An Introduction to Einstein's General Relativity
James B. Hartle
Spacetime and Geometry, An Introduction to General Relativity
Sean M. Carroll
(Click on the Special & General Relativity link, and note that you must register before you are able
to search these forums. However, this is an excellent place to get answers to questions by some
extremely knowledgeable people.)
A good site on the history of General Relativity
Feel free to contact me.
Website revision history
October 6, 2007 - First upload of updated webpages.