I know gravity is the result of mass. Gravity is nothing but the attractive force on a body due to its mass. I have a question that may be silly. In magnetism we have attraction and repulsion. Earth is a big magnet, so I'm assuming that like Earth, other celestial objects also are magnetic. I'm bringing in the same concept of magnetism here into gravity. Since planets can attract each other they should repel something. What is that something that planets repel?

Pardon me if I'm wrong anywhere.

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    $\begingroup$ This was actually considered by Johannes Kepler around year 1600, before gravity was considered. Magnetic poles of the Sun and the planets would explain the elliptic shape of their orbit, attracting and repelling each other along the orbit. He didn't manage to make that add up, though. $\endgroup$ – LocalFluff Dec 15 '14 at 13:15
  • $\begingroup$ This question appears to be off-topic because it is not about Astronomy. It is actually about a fundamental understanding of a physical force. Could migrate to Physics. $\endgroup$ – Rory Alsop Dec 16 '14 at 10:50
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    $\begingroup$ Astrophysics is considered on topic in Astronomy. The question is about gravitation and magnetism between planetary bodies - I think it qualifies here. $\endgroup$ – Mitch Goshorn Dec 16 '14 at 16:47
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    $\begingroup$ I'm voting to close this question as off-topic because it is not about Astronomy. It is actually about a fundamental understanding of a physical force. Could migrate to Physics $\endgroup$ – J. Chomel Feb 19 '18 at 6:46
  • $\begingroup$ A question about experimental tests of gravity based on astronomical observations might be on topic, but as written, I agree with @J.Chomel $\endgroup$ – uhoh Feb 20 '18 at 12:38

Gravity does not have polarity, it only attracts.

An analogy with magnetic or electric fields is appealing (because they are all field forces, decay with the square of the distance, etc...) but science is not made of analogies, it is made of observations. And no one has observed gravity repulsion.

Gravity is in fact much much weaker than the electric force (the factor has 42 zeros), and the only reason we feel gravity is because there is no repulsion, so all those little tiny mass pulls add up to something sensible. Electrical forces, albeit much stronger, usually cancel each other out due to an equilibrium of positive and negative charges.

This is of course, as far as we have seen in nature.

There is however, a very interesting speculation about what should happen with anti-matter. Would it repel "normal" matter?

In the physics page there is a more in deep answer using the results of quantum.

Recommended reading: Feynman Lectures: Theory of Gravitation

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    $\begingroup$ Anti-matter is nothing more exotic than the anti-particles of "ordinary" particles. They have opposite charge, baryon and lepton numbers but the same mass. They therefore experience gravity in the same way. You must be defining anti-matter in some unconventional way. $\endgroup$ – Rob Jeffries Feb 21 '15 at 15:40
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    $\begingroup$ In GTR, gravitational repulsion is equivalent to violation of the strong energy condition, which does actually happen on cosmological scales, but is irrelevant in the solar system. In regards to antimatter, Rob Jeffries is of course completely correct theoretically (if antimatter repels normal matter, pretty much everything in fundamental physics is wrong), though of course we haven't actually had enough antimatter to check. $\endgroup$ – Stan Liou Feb 21 '15 at 16:08
  • $\begingroup$ @StanLiou I second or third what you and Rob said. Weren't there small-scale tests of antimatter and its gravitational behavior? Very small-scale, though. $\endgroup$ – HDE 226868 Feb 21 '15 at 17:21
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    $\begingroup$ @HDE226868 If you conceptually split mass into three categories: inertial, passive gravitational (how it falls in grav field), and active gravitational (what grav field it produces), then for antimatter we can only test the equivalence of the first two (and there were some antihydrogen tests for it), but we can't make enough antimatter for tests involving the third. ... However, having it be different is certainly very bizarre and would kill any notion that gravity is mediated by a long-range spin-2 field (of which GTR is a particular case). $\endgroup$ – Stan Liou Feb 21 '15 at 22:41

The current best theory to describe the gravitation is the General Relativity. In it, gravity is not a force, but the change of the curvature of the spacetime as the result of the matter (energy) density in it. Essentially, it has 2 equation systems:

  • how the matter (energy) density affects the curvature of the spacetime
  • how things are moving in this curved spacetime

This can result anything, even a repelling gravitational force. For example, parallel laser beams going into opposite direction repel eachother. The problem is that we would need 40-60 orders higher energy densities to have a measurable effect, so it remains only a theoretical calculation.

In the "common" situations, i.e. if we have planets,

  • being significantly smaller as a black hole
  • and going with a speed significantly smaller as the speed of light,

the Newtonian gravity is a very good and very simple approximation. In it, gravity is always attracting and so is it.

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  • $\begingroup$ Have you got a source for the "parallel laser beams repel" thing? $\endgroup$ – Steve Linton May 20 '19 at 6:37

As tomassch said, the analogy of gravity and electromagnetism is not valid by current observations. However, some systems, depending on how they are arranged, can have gravitational interactions that may launch other objects at extremely high speeds. This is the case with three-body systems and gravitational assist.

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