# Why can't gravity repel things?

Gravity is the result of the curvature of space, so could the curvature actually send objects away from the source?

• Gravity is spin-2, and even-spin forces are attractive for like charges. However, gravity can be repulsive even for sources with everywhere positive energy densities if the principal stresses are negative and high enough in magnitude. An example of this is dark energy; cf. also the strong energy condition. Dec 15, 2015 at 4:34

## 3 Answers

Apart from the field-theoretical standpoint presented by Stan, one can repel objects in a sense, when taking orbital mechanics into account.

The slingshot maneuver extracts angular momentum and energy of an orbiting mass by the use of gravity. The trick here is that the probe's velocity just gets redirected in the planet's reference frame. But this results in an acceleration in the solar rest-frame that the probe essentially tries to leave.

As you were asking about curvature, I don't know how and if centrifugal potentials that play a role here are described by general relativity.

• I'm going to give you a bounty for good answer Dec 17, 2015 at 23:59
• @Jamie Bounties are nice, but there is greater value in the community when there are more people with a high level of reputation. The reputation gives you more ability to participate in other ways (reviewing edits, downvoting, etc) which further helps the site in general. If people with low reputation give away all their points with bounties, it slows them down. I'm not saying don't do it, but it will be helpful to you and the community if you hold onto what you can for now (if you plan on sticking around). Dec 18, 2015 at 17:23
• @Jamie: additional to what duzzy said: What exactly are you interested in? I can easily expand on my answer as this is 1st-2nd semester stuff we're talking about. You don't have to give away your rep for that. Dec 18, 2015 at 20:42
• Well, this answered my question and I already have a lot of points. Plus I can't remove a bounty. Dec 19, 2015 at 0:51
• @Jamie: Ah ok. However, are you interested in more detailed math-stuff? The "picture-math" can be found on wikipedia. The "math-math" I could provide.. somewhere between the upcoming holidays Dec 19, 2015 at 1:33

I'm not sure how accepted this is in the physics community, but a book by Andrew Thomas called Hidden in Plain Sight posits a theory that gravity may be repulsive within black holes (i.e. on scales smaller than the Schwartzchild radius for a given mass).

It's an interesting theory, and it avoids the need for a singularity, but to my knowledge it's completely untestable and therefore not really science.

• If this is a well-defined theory, what is it? Dec 17, 2015 at 20:27
• I'm not sure that I understand your question? The theory is outlined in the book, I'm not educated enough to be able to explain it and it's been a while since I've read the book. I just thought that the person who asked the original question might be interested... Dec 18, 2015 at 11:45
• I mean that if it's just a reference, this seems more like a comment than an answer (though I realize that right now you can't comment). That's beside the crank flags this puts out, being apparently a pop-sci book with nothing in the literature. Dec 18, 2015 at 23:51
• Bingo - can't comment yet. I've looked around in the literature but haven't found anything as such (though that's probably more a reflection on my searching skills). I have managed to find out that Andrew Thomas has an actual PhD in physics, but other than that I can't find anyone other than physics forums posts about his works, some of which are positive and some negative. Dec 21, 2015 at 8:23

As far as our current understanding of gravity, no. The common analogy is a rubber sheet with marbles. The sheet can only be pulled downwards, so you cannot have a repelling force.

To pull the sheet upwards would require something like inverse gravity, which is not yet known to current physics.

Of course dark energy seems to look like this, but as the origin of dark energy is currently completely unknown, it is not useful to speculate on this at this point in time.

• This answer treats the analogy way more seriously than it deserves, particularly since the upwards/downwards directions in it are not physical (absolutely nothing will change if it's "pulled upwards"). Furthermore, $\Lambda$-like violation of the strong energy condition is a very generic prediction of a scalar field coupled to gravity (at an extremum of field potential), and scalar fields do exist in the standard model. I don't think it's appropriate to dismiss those considerations. Dec 15, 2015 at 22:16