Gravity is a phenomenon through which all objects with mass attract each other. The force is proportional to the mass of the objects (ie. double the mass produces double the force), and inversely proportional to the square of the distance (ie. double the distance, produces four times less force).
Charged particles up to the size of small grains, may be more strongly influenced by electromagnetic forces than gravity. Gravity is always an attractive force, whereas electromagnetic forces may be attractive or repulsive. (See "Electromagnetic force" for a comparison between the electromagnetic and gravitational forces)
The physics of the influence of gravity and electromagnetic forces on a particle, is called "gravito-electrodynamics".
Hannes Alfvén wrote:
- "Gravitation is, of course, one of the dominating forces in astrophysics . However, as electromagnetic forces are stronger by a factor of 1039, gravitation is important only when electromagnetic forces neutralize each other, as is the case for large bodies. In our solar system, gravitational forces do not seem to be of primary importance in producing high energy phenomena" [.. For example .]
- "A plasma cloud approaching the Earth will already be stopped in the magnetosphere, or in any case, in the upper ionosphere, where gas clouds will also be stopped. The result will be a heating of the upper atmosphere which makes it expand and stop an additional infall of more distant clouds.
- "Low density plasma clouds approaching the Sun will be stopped very far away by the solar wind. A neutral gas cloud falling towards the Sun is likely to be stopped when it has reached the critical velocity. This occurs when the cloud is still very far from the photosphere. In fact, its kinetic energy will be only of the order of 10 eV when this occurs."