
The Question
(Submitted April 07, 2006)
General relativity explains gravitation without using the force of gravity
by postulating that mass curves spacetime. Why would a curved spacetime
accelerate me towards the earth?
The Answer
Thanks for your excellent question. This is not really an easy
question to answer in a few sentences. We'll give you a brief answer
here and then recommend you to better websites for more details. As you
already know, matter tells spacetime how to curve and curved spacetime,
then, tells matter how to move. The idea of acceleration and how it
relates to gravity comes from Einstein's Equivalence Principle, which
states that you cannot tell the difference between a gravitational
field and an equivalent uniform acceleration. So that objects in free
fall under gravity all accelerate by the same amount, or in other words,
they move the same way as if there was no gravity (ie weightlessness).
So what seems to be the result of gravity is really the result of
curved spacetime. Since spacetime is curved around massive objects,
the Earth is merely traveling along the shortest path in curved spacetime,
which has the same appearance as if the Sun were pulling on the Earth
due to gravity. Einstein used a simple gedanken experiment using light
to illustrate his ideas. We recommend the following websites to follow
this gedanken experiment and understand General Relativity better:
http://www.astronomynotes.com/relativity/s3.htm
http://en.wikipedia.org/wiki/General_relativity
Hope this helps,
Georgia & Mike
For "Ask an Astrophysicist"
