Lets say I have a cubic piece of antimatter, perhaps 1 in on each side that I really didn’t want anymore, so I want to drop this off in deep space while I journey to another star system. Assuming this doesn’t violate any littering laws, or copyright laws, what happens to it. And I am using the type of spaceship that doesn’t leave a trail of particals behind, so just assuming it’s only what’s in deep space.
So would the cube be blown apart by the force of the first stray atom combining with one on the cube? Would it just apparently move randomly as the reaction by a stray atom would be too small to destroy the cube, but would act to propel it? How long between particals hitting it?
In deep space, it’ll probably only hit one atom at a time: Densities in deep space are on the order of one atom (mostly hydrogen) per cubic meter. And the energy of one pair of atoms annihilating isn’t going to be anywhere near enough to blow apart a macroscopic object of reasonable strength. If anyone’s watching and close enough, though, it will provide a lightshow distinctively characteristic of a lump of antimatter interacting with the interstellar medium.
For the rate of reactions, it’d depend on how fast the object is moving (I’m assuming that, being dropped from a spaceship, it’s probably moving significantly faster than the average speed of ISM particles). On average, it’ll interact with one particle whenever it sweeps out a volume of one cubic meter. Since it has a cross-section of 1 square inch (0.00064516 square meters), it’ll need to travel 1550 meters per collision. So, if it were travelling at 1.55 kilometers per second, it’d have one collision per second. More generally, for every kps of speed it has, it’ll have .645 collisions per second.
Wouldn’t it tend to have most impacts on the side facing its direction of travel, and thus slow down with each impact until it’s stationary in regard to the local interstellar matter?
Huh? perhaps it could have been better phrased as “the average velocity of the local interstellar medium,” but interstellar matter is subject to the same laws as everything else, and if the antimatter tends to interact with the interstellar matter, the interstellar matter will tend to do so as well and so tend to attain velocities similar to the surrounding material.
All he’s saying is that the random hydrogen atoms are whizzing around, even if the cube isn’t, so in the end the cube will interact with atoms no matter how “still” it is. So the calculation is roughly correct, it seems.
If it’s losing one atom per second or less, then it would take a long-ass time for a one-inch cube to vaporize completely. Hell, most physical objects lose a lot more than one atom per second, I’d bet, and they’re not withering away at any rapid pace.
Just the quick, and hopefully obvious, point that antimatter is ipso facto no less stable than regular matter. On an Anti-Earth, antiwater would freeze at 0 C and boil at 100 C (32 and 212 F of course). In isolation, plain antihydrogen would burn in antioxygen to produce antiwater; anti-tritium would break down in 11 years as does normal tritium; free antineutrons would (and do) last 1000 seconds just like neutrons; free antiprotons would last a googleplex of years before breaking down. It’s only the overwhelming commonness of normal matter versus antimatter that makes it react in a MAM annihilation reaction as it does.
Also, wouldn’t the OP need to describe the density of the cube instead of just it’s volume? Would a 1 in cube of anti-lithium take fewer hits of hydrogen atoms to destroy it than a 1 in cube of anti-lead?
At least two photons, for the simplest reactions (like electron-positron). For the more complicated reactions, like a proton and antiproton, you’d get a mess of mesons, which would in turn decay into other things. A proton and an antiproton specifically would probably go into three pions, either three neutral ones, or a positive, a neutral, and a negative. The neutral pions would eventually each decay into two photons, each positive pion would decay into an anti-muon and a mu neutrino, and each negative would decay into a muon and a mu antineutrino. The muons would in turn decay (if they didn’t run back into each other first) into an electron, a mu neutrino, and an electron antineutrino (or the antiparticles of those, for the antimuon). The electron and positron might get free, or might end up annihilating into two more photons. So all told, a proton and an antiproton would end up decaying into either six photons; two photons, six assorted [anti]neutrinoes, and an electron and positron, or four photons and six assorted neutrinoes.
As for where the resulting stable particles go, they go either away from the block off into deep space (perhaps to be eventually detected by a telescope), or they hit something else in the block and get absorbed (antimatter absorbs light in exactly the same way that normal matter does; the photon is its own antiparticle).
Also, as the cube of antimatter gets smaller and smaller, would this begin to have an effect on the time it takes for it to interact with the hydrogen atoms? So annihilation would occur over ever increasing time spans.
