# How long and where could an antimatter object survive in space?

Are there any areas of space where an antimatter object, the size of a small house could survive without coming into contact with any or extremely minimal amounts of matter?

I had assumed in all areas of space the object would come into contact with enough dust and gas to make it explode/ disappear eventually, when researching I have had contradictory information, some saying it is possible in the galactic voids or some areas of interstellar space and others say it would not last long in all areas of space over cosmological time spans.

Where in the universe would be the best places for an antimatter object to reside and how long could it survive there?

• @JohnO that is what I was hoping for, that the area of space would have so few collisions that it would be comparable to a PET scan causing minimal damage. – user69935 Mar 19 at 19:50
• How old is this object, if it needs to exist since the big bag that changes answers. – John Mar 20 at 1:43
• @JohnO : that is completely and utterly wrong. Annihilation releases so much energy, that it won't just slowly sizzle. A bowling ball's worth of antimatter will release orders of magnitude more energy than a nuclear bomb. In a nuclear bomb only a few grams worth of matter is converted to energy. Compare it to a bowling ball of antimatter, the whole of it being converted into energy as soon as it gets in contact with ordinary matter. – vsz Mar 20 at 5:39
• @vsz Only the outer layer can annihilate. And while it's alot of energy, a single atom's worth or even a million won't blow the thing up into an aerosolized vapor that then can readily annihilate simultaneously. It will sit there sizzling. It's going to ruin your hardwood floor, it may even burn the house down. It's not going to nuke off on you though. The whole of it can't be converted into energy at once, the parts in the middle are shielded by the parts on the outside. This is plainly obvious. M/AM weapons would have to be clever to mix the two parts quickly. – John O Mar 20 at 6:44
• The effect will be slow, unless you think that somehow matter will spontaneously blip into existence inside the antimatter bowling ball to annihilate with it. These particles do not weigh as much as a bowling ball, and the kinetic energy pushes them away from the only thing with comparable density. The effect will be slow enough that it's not a bomb going off. You don't even understand the answer you linked to. The question it answers is "if you could put a gram of matter and a gram of antimatter together in an instant"... – John O Mar 20 at 13:53

## A cosmic void.

Your best bet would be in a cosmic void. While not entirely empty - they do contain small numbers of galaxies and clouds of gas - they are substantially rarefied compared to your average patch of the universe. Although I've been unable to find concrete numbers for the low-density gas that would permeate a void, we do have estimates for the mean galactic number density, which is roughly $$\mathcal{N}\sim0.004\text{ Mpc}^{-3}$$, in units of galaxies per cubic megaparsec. If each of these galaxies is the mass of the Milky Way, they contribute a number density of $$n\sim0.25\text{ m}^{-3}$$ - assuming the galaxies are largely composed of neutral hydrogen. It seems not unreasonable enough to assume that these galaxies are a decent proxy for the mean void density, and so we can safely assume a particle number density substantially lower than the numbers L.Dutch mentions - by many orders of magnitude! So let's say you have a cube of hydrogen atoms of side length $$l$$ with a cross-sectional area $$l^2$$ of one square meter and the density of water $$\rho$$, moving at close to the speed of light relative to the gas (which is the worst-case scenario, as it implies a higher collision rate and therefore a higher annihilation rate). Over a time $$\tau$$, it sweeps up $$N_a=\tau vl^2n$$ atoms of the void. It is composed of $$N_c=\rho l^3/m_H$$ atoms itself, and the cube is entirely disintegrated when $$N_a=N_c$$, or $$\tau\approx\frac{\rho l^3}{m_Hnvl^2}\approx2.5\times10^{14}\text{ years}$$ If you just want one millimeter shaved away, that should take three orders of magnitude less time - about $$\sim10^{11}\text{ years}$$. Over a time $$\Delta t$$, the cube will pass through $$N=\Delta t v l^2n$$ atoms, releasing $$E=N\cdot 2m_Hc^2$$ energy. The power emitted is then $$P=\frac{E}{\Delta t}=vl^2n\cdot 2m_Hc^2\approx 22.5\text{ milliwatts}$$ which is surprisingly little. Of course, the whole thing is kinda moot because any void is finite. After all, the Boötes void, for instance, is only 330 million light-years across, and so, traveling at the speed of light, the cube will only spend 330 million years inside, shaving off about 1.3 micrometers in the process. (You can of course increase this time significantly by decreasing your speed!) Your mileage may vary, of course, depending on both the size of your object and whether I actually did the calculations right.

## Notes

This is really just a toy model to get you an order-of-magnitude idea of how slowly annihilation will occur. A one cubic meter cube of hydrogen is an unrealistic object and would of course dissipate quickly, regardless of its environment - it's almost certainly not self-gravitating. Therefore, we can't really model the effects the annihilation would have on it. However, a realistic object would indeed see some more complex effects. For instance:

• Some of the emitted byproducts of annihilation (e.g. gamma rays) would be directed inwards, and would potentially ionize or (more likely) knock loose additional atoms of the object.
• The object would likely be slowed down by the drag of traveling through this sort of medium (even such a rarefied one).
• At the (again, unrealistic) relativistic speeds our toy model is moving at, there would indeed be special relativistic effects.

But again, this is just a toy model designed to point out that a cosmic void is an excellent place to set down your antimatter object.

• ah I just read this link and added it as a comment to L Dutches calculations. – user69935 Mar 19 at 19:26
• I mean I just read it before reading your answer but thanks for pointing it out, how long do you think the object could last with only millimetres taken off its surface area? – user69935 Mar 19 at 19:37
• @RandySavage I've added in that calculation. – HDE 226868 Mar 19 at 19:54
• oh wow that's a long time, brilliant, thank you. – user69935 Mar 19 at 20:00
• @JohnO My calculations are telling me it should radiate at about 23 milliwatts. – HDE 226868 Mar 19 at 20:21

How empty is the vacuum of space?

even the deep vacuum of intergalactic space is not devoid of matter, as it contains a few hydrogen atoms per cubic meter

The density of matter in the interstellar medium can vary considerably: the average is around $$10^6 m^{-3}$$, but cold molecular clouds can hold $$10^8–10^{12} m^{-3}$$

Considering that a mole of matter is made by $$10^{23}$$ atoms, to be fully annihilated that mole would need to cross a volume between $$10^{13}$$ and $$10^{17}\ m^3$$, if it was made of hydrogen.

$$10^{13} m^3$$ is the volume of a cube with side as large as the solar system, while $$10^{17} m^3$$ is the volume of a cube with side as large as the distance between the Sun and Tau Ceti (a bit more than 11 light years).

Though humongously large, those volumes are small when compared to the size of the universe, even if you scale up the mass to account for more than a mole.

How fast the object could last would depend on the velocity with which it is traveling. If it was traveling at 1000 m/s and had a cross section of 1 square meter, to swipe a volume of $$10^{13} m^3$$ would take $$10^{10} s$$, about 10 centuries.

If your criteria for "surviving as an object" is that at leas one atom of the original object is present, then the above is the estimate of the life span. If you are more strict and the first annihilation destroys the "object" as such, then the lifetime is much shorter, as the first impact will do.

• We need to keep in mind that the object needs to survive as an object, not just disintegrated antimatter dust. – Alexander Mar 19 at 18:53
• A small amount of collisions is acceptable but only enough that the object would barely be damaged, not to change or complicate the question if it was a living thing it wouldn't be enough to burn off its skin or even more damage. – user69935 Mar 19 at 19:09
• @L.Dutch-Reinstate Monica this other wiki link says galactic voids have 1/10th of the average density, do you think I can x10 the survival rate of your calacualtions of interstellar medium? en.wikipedia.org/wiki/Void_(astronomy) – user69935 Mar 19 at 19:24

There is a hypothesis that the Universe was formed with equal amounts of matter and antimatter. We know there is a lot of matter in constant expansion after the Big Bang. So, we have two options:

a) Some unbalance happened among them, and antimatter is rare. Far away of gravitational field of galactic groups the antimatter can remain for an undefined amount of time.

b) The Big Bang expansion split somewhat and there is an unreachable 'anti-universe'. In this scenario, antimatter remained and will remain like regular matter in our universe.

The wiki article Baryon asymmetry explains both.

• How does this answer the OP's question? – L.Dutch - Reinstate Monica Mar 19 at 18:45
• @L.Dutch-ReinstateMonica indeed, need add something else. Thanks. – Rodolfo Penteado Mar 19 at 19:06
• That wiki article has its formatting broken, how odd. – The Square-Cube Law Mar 19 at 19:17
• @RodolfoPenteado I was actually reading about plasma cosmology last night, and the double layer splitting antimatter and matter into their own regions, it is an interesting idea having an antimatter area of space but I would prefer it to be our known universe so my spaceship doesn't get destroyed approaching the object. – user69935 Mar 19 at 19:33