Anti-hydrogen is the antimatter equivalent of the hydrogen atom. The simplest atom in our Universe, hydrogen is usually made of a single proton and a single electron. Hydrogen is also one of the most abundant elements in our Universe by a large margin, but its antimatter counterpart has been rather difficult to study. Anti-hydrogen was recently in the news, as the folks at CERN have only recently succeeded in measuring the spectrum of light that a positron (anti-electron) produces when it is bound to an antiproton. So far as we can tell, anti-hydrogen makes exactly the same spectrum as regular old hydrogen which is good news for the current model of particle physics.
Anti-matter’s main feature is that it will rather catastrophically annihilate if it comes into contact with regular matter. These matter/antimatter annihilations are cases for Einstein’s E=mc^2 equivalence, as the energy that is produced by the destruction of the antiparticle and the particle itself is equivalent to the mass of the two particles, multiplied by c, the speed of light, squared. If an anti-hydrogen came into being in the room that you’re standing in, the first thing we would expect it to do is annihilate as soon as it runs into an air molecule. (I must note that an atom of anti-hydrogen spontaneously coming into being in your living room is extremely unlikely, or the ATLAS team would have a much easier time trying to coax anti-hydrogen atoms together, instead of doing the painstaking work to collect atoms together.)
A positron, our antimatter equivalent of the electron, when it annihilates with an electron, creates gamma radiation. This is about as clean an annihilation as you’re going to get -- the two particles convert their mass directly into light, with no intermediary cascade of other particles. Gamma radiation is generally bad for humans in large doses, but a single positron/electron annihilation event only produces two gamma ray photons. That’s it. And the photons go in opposite directions, so you’re only ever going to get hit by one of them.
Proton/antiproton annihilation tends to be a bit messier, with more of a cascade of intermediary, unstable particles. In the end, these events also produce high energy light particles, though they’re not usually gamma rays.
To get a handle on how much of a radiation dose you could potentially have from a single atom of antihydrogen, I’m assuming that the entire atom annihilates into light, with no leftover particles like neutrinos (which are a common byproduct of proton/antiproton collisions). This means that we’d be left with the maximal amount of energy as gamma radiation. If you convert the entirety of a hydrogen atom into energy, you get 1.5 x 10^-10 Joules. That looks like a very small number, and it is; but gamma radiation is usually bad for you, right? Let’s calculate the dose.
Sieverts are used to calculate radiation doses, and they’re measured in Joules/kilogram. If you are a person, on average, you are made of 80 kg of material. If you divide our tiny amount of joules by 80 kilograms, you get a radiation dose of 1.87 x 10^-12 Sievert. A small number here is good, because a single Sievert of radiation exposure is usually considered to be not something you should do every day. It's actually the career limit for cumulative exposure for a female astronaut at age 25. A milliSievert (0.001 Sievert) per year is the recommended threshold to stay underneath if you’re a member of the general public.
However, our number is so low that it is below the Banana Equivalent Dose, which is the amount of extra radiation your body endures in the time immediately after eating a banana, and is a comically small amount of radiation. The Banana Equivalent Dose is 9.8 x 10^-8 Sieverts; our atom of antihydrogen is 1,800 times less of a dose than eating a banana.
To give you a sense of how little banana we’re working with, I weighed a banana I had: 137 grams. Dividing that by 1800 gives us 0.07 grams of banana. My kitchen scale doesn’t work on precision that small, so here’s my analogy; eating a piece of banana about the size of the last digit of your little finger is 26 times more of a radiation dose that you’d be exposed to from a single atom of antihydrogen annihilating in your room.
You wouldn’t notice.
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