How does Earth's rotation affect gravity?

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Devika Mahadevan

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How does Earth's rotation affect gravity?
« on: 11/05/2010 10:30:02 »
Devika Mahadevan  asked the Naked Scientists:
Dear Chris,

I have been recently reading (Hawkings and Greene) and watching youtube about general relativity. As far as I understand, grativity is the distortion in space time because of the acceleration of mass through space time. And from the logic, gravity to us (standing on the earth) is indistinguishable from acceleration of the earth. So I have a couple of questions (if indeed I have the understanding right in the first place)

i. Is one person at one end of the earth feeling the acceleration and the earth pushing up against him and the other person at the opposite end of the earth feeling the pull of the distortion caused by the earth's movement through space time similar to a whirlpool? And how is the rotation of the earth affecting the space time distortion, or the gravity felt by the persons at opposite ends of the earth?

2. Isn't the earth moving at a constant 87,000 miles an hour? So what does it mean it's accelerating?

Would really appreciate a response.

Thank you so much,

What do you think?
« Last Edit: 11/05/2010 10:30:02 by _system »


Offline JP

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How does Earth's rotation affect gravity?
« Reply #1 on: 11/05/2010 10:54:21 »
Hi Devika,

I think you're misunderstanding a point about general relativity and how it deals with gravity and acceleration.  What Einstein started from when formulating general relativity was the observation that if you're in an elevator, you wouldn't be able to tell if the elevator without any windows, you wouldn't be able to tell the difference between being at rest on the earth's surface and accelerating in empty space at 9.8 m/s2.  In other words, the acceleration of an object caused by a gravitational field is physically indistinguishable from acceleration caused by something else. 

To come back to your original point, if you have matter or energy present somewhere, it will distort space-time regardless of whether it's accelerating or not.  If the earth was sitting in deep space, neither orbiting anything nor rotating, you could still stand on it and feel gravity, even thought it isn't accelerating with respect to you.