# Gravitation force- Attraction and repulsion

Gravitation force is always attractive.

Now assume(not a practical one): I took our Earth in my hand and started shaking up and down. This will create a disturbance in space-time warp and it moves like a wave outward direction. So when the crest part of this disturbance comes in touch with another mass, it should carry that mass with it and start moving away from Earth. It was visible because I used a very massive object like Earth. Though it is not visible it should be true with low mass objects too? So is gravitation field is always attractive?

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– twistor59 Dec 23 '12 at 15:06
Why would the wave carry the mass with it? A wave of water doesn't carry particles with it, it merely makes the particles bob up and down in the one position. – Mew Dec 23 '12 at 15:17
@Chris : If suddenly Earth disappear, what would happen? How would you explain the movement of Earth? – Inquisitive Dec 23 '12 at 15:24
@sree: It's ambiguous what would happen, gravitationally, if the Earth would suddenly disappear. The laws of physics forbid this, and it would create a momentary discontinuity in the metric tensor that would break the differentiability of spacetime, which is one of the assumptions required to assure that the theory is predictable. If you did this smoothly but rapidly, somehow, I'm sure that you'd have gravitational waves, but there are lots of conserved quantities that you'd have to non-conserve in order ot make the earth disappear. – Jerry Schirmer Dec 23 '12 at 16:24
@Chris, regarding your first comment -- clearly you haven't surfed much ;) – DilithiumMatrix Dec 23 '12 at 22:16

That's a very interesting question.

The short of it is that Gravity is always attractive.

When you imagine a gravitational wave (GW) going by, its not like the gravity is pushing and pulling on the object, its like it pulling-less, and pulling-more. By analogy its a lot like the tides. There is a tidal bulge on the side of the earth opposite the moon, not because the moon is pushing that part away from it, but because its just being pulled less than the earth itself.

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Gravitational waves (or electromagnetic waves) are not like water waves, in that they have only transverse modes and not longitudinal ones. This is related to the fact that these waves travel at the speed of light.

Contrary to zhermes' answer, gravitational waves do not push or pull objects in their directon of motion. Instead they stretch or compress along axes perpendicular to their motion. Here is Wikipedia's illustration of what a gravitational wave coming out of the screen might do:

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That was my first reaction too, but the first few sentences of the introduction to this reference seem to suggest that the waves impart momentum. – twistor59 Dec 24 '12 at 11:14
Ah wait, according to MTW p961, after the plane wave has passed, the particles retain some net motion (which is in the direction towards each other), this must be what is meant by the impartation of constant momentum in that reference. – twistor59 Dec 24 '12 at 11:38

## protected by Qmechanic♦Mar 18 '13 at 22:57

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