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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

When an object falls under gravity, is it really feeling a force, or is it just moving along a curved path through space?

What you are describing are two complimentary descriptions of the same process. You may either find it mathematically profitable to focus on the geometric properties of spacetime and the shapes of the 'straightest lines', or you can use the Newtonian description of forces. A falling body moves along a curved path because a gravitational force is present which changes its speed and direction at each instant, producing an acceleration. In relativity, a massive body warps spacetime in its vicinity in a precise way that depends on its mass, so that straight world lines in 4-dimensions become curved trajectories in 3-dimensional space which we see as ellipses or other 'conic sections'.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.