Expert: Steve Nelson - 2/17/2010

Question
2 particles attract each other.Why?The general theory of relativity says "the curve of space-time is to blame".But how is the effeect explained simply?Of the gravitational force."curve of space-time"is a scientific slang.How is this easily understood?Thank You

Answer
It's not easily understood, that's the problem.  You can understand it as curvature of spacetime with a few years of study of the tensor math and metrics of spacetime, but it's just not simple.   So for my students, I came up with a simpler analogy.  Imagine that we're hurtling through the time dimension, and the presence of an object causes curvature of spacetime.  Refer to the attached picture.  Imagine that instead of a y-dimension, the y-dimension in the picture is time, so we're constantly flying past whatever objects are nearby in time.  And the x-dimension is one of the three spatial dimensions.  You see how what is a "straight line" as you go by in the time dimension is actually deflected towards the object in the center?  That would be an acceleration.  Imagine that we're constantly flying past object in the time dimension which is near you, providing an acceleration towards that object.

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It didn't attach the picture, so see here:  
http://www.zamandayolculuk.com/cetinbal/PU/p_curvature.jpg