# Matter being repelled by gravity

Let's say that, in a universe modelled after Patema Inverted, there are two "flavors" of matter: one which is attracted by gravity (so when dropped on Earth it would fall down), and one which is instead repelled (so when dropped on Earth it would fall up). For the rest the interactions are the same.

Would we ever be able to notice such repulsive substance in our present world?

• You're describing the movie "Upside Down"(2012) Dec 23, 2016 at 7:33
• No, I am referring, as stated, to the anime Patema Inverted. And the only difference being the interaction with gravity, no other difference (in Upside Down long interaction leads to combusion)
– L.Dutch
Dec 23, 2016 at 8:12
• You may want to ask about dark energy on physics.stackexchange.com Dec 23, 2016 at 8:58
• We wouldn't notice it, the solar system and planet was formed by gravity anything repelled by gravity would be far far away Dec 23, 2016 at 10:34
• The antigravitational effects of particles with negative mass would be very much like what you describe (with some additional complications). One way you could detect negative mass particles is if there happen to be traversable wormholes, which are mathematically impossible without this form of exotic matter. Though I'm not at all sure how the stuff might work optically.... Dec 23, 2016 at 13:10

# Gravity repellent material will be far away.

Without attempting to math it out, the presence of two materials, one attracted and one repelled by the dominant force at work on macro scales, will result in two separate halves of our universe expanding away from each other.

Lets call this gravity-repelling matter 'nega-matter'. Assume the universe was created in the big bang as a mass of hot expanding matter and nega-matter. Wherever there is a more dense lump of matter in the initial soup of creation, that point would at the same time attract more matter and repel more nega-matter than other points in the expanding universe. As more and more matter coalesced into the matter part, the power of repulsion would be greater and greater on the nega-matter.

You did not specify if the nega-matter has its own force that attracts it to itself, which is relevant for the final shape of your universe. If nega-matter is self-attracting, then it will also try to coalesce while being pushed away from regular matter. From a 3-d space perpective, there will eventually be a plane where one side is an expanding matter universe (like ours) and the other an expanding nega-matter universe.

If nega-matter is not self-attracting, then our universe will expand as it does today, with a spherical 'shell' of nega-matter being pushed outwards just behind the event horizon of the big bang.

In conclusion, either way there will be no nega-matter anywhere near us for us to observe. In the second case, the existence may be inferred from the cosmic background radiation somehow, in the first case, it seems unlikely that we would be able to recognize the nature of an expanding mirror nega-universe.

• Pretty much my thoughts too. If the substance is repelled from Earth, you're not going to find it on Earth - etc. +1 Dec 23, 2016 at 15:23

Unless this antigravitational matter has its own special photons, photons still have positive (relativistic) mass and equal gravitational and inertial masses, even when emitted by systems made of antigravitational matter. This would be detectable on a large scale, because gravitational lensing around such systems would be inverted. These systems would produce gravitational blueshifting instead of gravitational redshifting, which could be detected by looking at emission spectra.

• If anti-gravitational matter emits photons with ordinary "positive mass charge", it would than gain negative mass each time it reaches lower energy state - which makes no sense, as gaining negative "mass charge" means increasing energy, as long as this matter gravitationally pulls on itself. I'm not a pro in physics, but as far as I can tell, matter with negative mass either emits "antiG photons", or none at all. Dec 23, 2016 at 14:20
• That would be true if this antigravity matter had negative mass-energy. However, if that were the case it wouldn't be able to interact with normal matter. The OP references Patema Inverted, where characters are able to eat antigravity food and gain sustenance from it. This implies positive mass-energy, negative gravitational mass and positive inertial mass, as negative energy chemistry is incompatible with positive energy chemistry. Ultimately, the concepts are fundamentally incompatible, so there's bound to be inconsistencies somewhere. Dec 23, 2016 at 17:46

Too little data for a meaningful answer

Something being repelled by gravity isn't enough to tell how it would behave. Gravity is the weakest of the four basic forces (by a huuuuge margin).

If you could for example replace the hydrogen in ordinary water with "anti-gravity hydrogen", it would still stay bonded to the oxygen. The resulting water would weight less on a scale (14g/mole instead of 18g/mole), but it would still be subject to gravity (or rather the oxygen in it would, with the anti-hydrogen counteracting to reduce the apparent weight, while the electric forces keep this water together).

Depending on the stuffs other physcial properties it could stay bound/mixed with normal matter, or it could separate. The process highly depends on all its interaction properties, so without defining if it has charge, which basic forces it responds to: Nothing definitive can be said.

• though I appreciate this idea, and I am fine with you that there might be too little data, I then question how could the anti-gravity hydrogen get bounded to oxygen under @kingledion explanation.
– L.Dutch
Jan 5, 2017 at 6:09
• @L.Dutch It could primoridally bind to normal hgydrogen to form H2, right after the big bang when the universe is cool enough (~300k years), long before any large scale separation could even take place. If it would ultimately separate or not then again depends on the particular properties, such as the ratio of anti-g to normal-g. The hydrogen was also just meant as a simple example to demonstrate how the anti-gravity property can be overruled by other basic forces. Too make a meaningful prediction, these properties need to be defined. Jan 5, 2017 at 14:56

## We already do (w/ caveats)

According to gravity calculations, matter with negative mass exerts a repulsive force on matter with positive mass. Conversely, matter with positive mass exerts an attractive force on matter with negative mass (Source). Matter with negative mass has applications for manipulating space, building flying cars, allowing FTL travel and so forth (Source).

This kind of exotic matter has been put forward as a candidate for "dark matter" (Source), which is invisible to all known forms of radiation and detected only by its gravitational effect on other matter. That is, we see it because its gravity bends light around it in lieu of bouncing off it in a manner incongruous with the gravity of the matter we can see.

Caveat: dark matter (and "dark energy") is a hypothesis to explain this odd behavior of gravity and has not been confirmed to exist, hence "dark." Alternative explanations such as "dark fluid" don't include it and simply redefine how gravity works at larger scales (Source).