# Repulsive gravity [duplicate]

IANAP, so feel free to berate me for thinking apocryphal thoughts! Just as magnetism has two charges, in which particles of like-charge repulse and particles of dissimilar charge attract, might gravity have two charges in which particles of like-charge attract and particles of dissimilar charge repulse?

In practice, the state of magnetism means that there is no system composed of many particles in which all particles attract. Rather, there is a net 0 charge if there are equal numbers of each particle type.

My silly theory regarding gravity would mean in practice that there would be two (or more) "clumps" (or universes) in existence, which are racing away from each other. So in our clump (universe) we see only attracting particles, because all the opposing particles have long since separated out and are racing away beyond the boundary of the observable universe. Just like the alien who lands in China and assumes that all humans have slanted eyes, we only observe the attracting particles (or "charge") and disregard the other, unobservable, "charge".

Is there any way to disprove this idea, or like string theory can I go one believing it as it can never be disproved?

Thanks.

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## marked as duplicate by Manishearth♦Jun 23 '13 at 11:42

Possible duplicate: physics.stackexchange.com/q/11542/2451 and links therein. – Qmechanic Aug 18 '12 at 18:56
To the abbreviation-impaired reader: IANAP in the beginning of the question(v1) apparently stands for I Am Not A Physicist. – Qmechanic Aug 18 '12 at 18:58

Let's start by assuming you believe General Relativity (on the grounds that it has given the right answers every time it's been tested).

There are various ways to get repulsive gravity in GR. The most obvious way is to have matter with a negative mass. See http://en.wikipedia.org/wiki/Negative_mass for a discussion of this. Such matter has never been observed, so I suppose we can't absolutely rule it out. However it seems unlikely. Your example of two "universes" racing away from each other would mean the universe is uneven on large scales, and the observations so far show the universe is basically the same in all directions. In addition you'd have to come up with a mechanism for the positive and negative matter to have separated in the first place.

There are ways to get repulsive gravity that don't require negative mass. For example there appears to be dark energy (http://en.wikipedia.org/wiki/Dark_energy) generating a gravitational repulsion. Also it's widely believed that at very early times the universe underwent a massive expansion, called Inflation (http://en.wikipedia.org/wiki/Inflation_(cosmology) ) due to repulsive gravity.

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Thank you. I'm not sure that the universe is so smooth on large scales. The CBM has a dipole anisotropy, and even the existence of the Great Attractor indicates large-scale unevenness. As for inflation, my silly idea actually could help explain it: when the two particles were still mixed up in the early universe, the universe expanded rapidly due to the repulsion. Only after the two particle "charges" were able to conglomerate and separate from one another did the blobs of conglomerate stop expanding rapidly. Our whole observable universe is just the blob that we wound up in. – dotancohen Mar 2 '12 at 12:55
The dipole anisotropy of the CMB is consistent with the earth moving relative to the rest frame of the CMB i.e. it's just a Doppler effect. I say "is consistent with" because of course you can't prove that it isn't some other effect that looks the same as a Doppler shift. For inflation to work it had to be vastly faster than could be driven by a mix of positive and negative matter. The problem is that matter would be diluted by inflation while the inflaton field is not diluted. – John Rennie Mar 2 '12 at 14:37

If you wish to pursue your idea in more depth, you may want to examine Sean Carroll's paper in which he speculates that within a larger multiverse (think of big fan-out tree of universes with differing physics) there exists a specific branch that is an anti-universe to ours, one that travels backward in time relative to our universe. His full paper describes the origin of these two-repulsed-in-time universes as an "empty de Sitter phase [in which a] small temperature [fluctuates] into a proto-inflationary configuration [that leads into a] conventional Big-Bang spacetime [with the] same thing happen[ing] in the far past, but with a reversed arrow of time." I know that doesn't sound much like your much simpler charge model, but the outcomes are strikingly similar: Two universes moving away from each other, in Carroll's case along the axis that we perceive as time. I do not know and will not attempt to speculate whether Carroll's idea also leads to gravity repulsion between the contents of those two inverse universes, or if that question even has meaning. (Well, I will offer this speculation: Reversed time certainly could lead to anti-gravity, so...)

And I just have to mention this: Your question caught my attention strongly because I recently drew up a table on how emergent behaviors in large collections of active entities -- that is, of flocks -- could be derived by using a richer variety of charges and charge behaviors than exists in real physics. I used different numbers of unique charges (1, 2, 3, etc.) with different attraction or repulsion values for their groupings (tuples). Your gravity idea was one of the first entries in my table! In my table I had it as a variant of the 2-charge electric force instead of the 1-charge gravity force (and BTW, the strong force is 3-charge). However, it never even occurred to me that a {2-charge, sames-attract, opposites-repel} model might have applicability in real physics! I think it's very cool that you came up with this speculative force symmetry and then applied it to produce a model that ends up surprisingly similar to Sean Carroll's far more mathematically detailed de Sitter based model.

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I checked with someone with high familiarity with the Sean Carroll model, and they did not see any obvious way that the regions-of-space approach model used in his paper could be rephrased as a gravitation dual-charge model. So, while I remain intrigued by your idea for its delightful and unexpected symmetry, please be warned that the reference I suggested would be likely a dead end for further study. Sorry! – Terry Bollinger Mar 3 '12 at 21:32

There is speculation whether antiparticles like antiprotons have different gravitational interactions than protons. One of the explanations of the predominance of baryons in our universe is similar to you speculation, that an antiuniverse got all the antibaryons.

The question of whether antimatter has also antigravity, or behaves with gravity differently than matter is to be studied with the AEGIS experiment at CERN.

The primary scientific goal of the AEgIS experiment is the direct measurement of the Earth’s gravitational acceleration g on antihydrogen. In the first phase of the experiment, a gravity measurement with 1% precision will be carried out by sending an antihydrogen beam through a classical Moire deflectometer coupled to a position sensitive detector.

This will represent the first direct measurement of a difference in a gravitational effect on an antimatter system. Until then the science fiction possibility you envisage might have a leg to stand on in physics, though I am not holding my breath.

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Thank you. I understand the term antiparticle to mean a particle with opposite electric charge from a "normal" particle. I don't think that this implies that the particle is also of opposite "gravitational charge". In fact aren't particles and antiparticles supposed to annihilate one another upon contact? Doesn't contact imply some attractive force between them? Or is that attractive force the different charges, which would overcome the supposed "repulsive gravity effect" just as the repulsion of like-electric charges overcome the attraction of gravity? – dotancohen Mar 2 '12 at 13:12
The structure of our understanding of elementary particles is such that the expectation is that the mass of the antiparticle has the same gravitational properites. Until this is measured it is is not a known fact, and that is why the experiment is attempted. – anna v Mar 2 '12 at 17:59