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Q & A: Gravity proven attractive?

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Most recent answer: 04/09/2013
Q:
My physics teacher had told me that there is no proof that gravitational force is attractive . Is it true? As it doesn't have any proof, why can't we say that it is repulsive?
- Varun (age 15)
India
A:
It's hard to figure out what your teacher meant. We have lots of experience with gravity, all of which shows that when you describe it as a force, it's always an attractive force. That's what comes out of Newton's description or Einstein's.

 Is it possible that your teacher was referring to a cosmological effect, the accelerating expansion of the universe? That is likely to be driven by some dark energy whose density stays fixed as the universe expands. That drives space to expand more rapidly, pulling things apart, unlike the familiar case of objects whose mass stays the same regardless of how much space they occupy. But maybe your teacher was referring to something else. Perhaps you can ask him or her to clarify?

Mike W.

(published on 04/09/2013)

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