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Excellent questions about antimatter and annihilation, iwanttomakeachange and blazed420x.

When annihilation happens, other particles are produced: for example with an electron-positron collision, two photons are produced. It’s true photons aren’t really considered to be matter and don’t have mass. But here’s the thing: mass and matter aren’t conserved here. (“Cannot be created or destroyed” = conserved.) In real walking-around life, matter is conserved, meaning if you put a pound of something in a bucket and close it, it will never have more or less than a pound in it unless something gets in or out. But with subatomic particles, mass is not conserved. Energy and momentum are conserved, but not mass. Energy is the kinetic energy plus the rest mass energy. (Rest mass energy: E=mc², which we bet you’ve heard of.)

If annihilation of larger particles happens (say proton-antiproton), then other particles as well as photons might be produced. Particle collisions are like car accidents, except where if two cars collide a tractor trailer, two bicycles, and some light might be produced. (Get it?)

Matter plus antimatter particles annihilate because they are made of quarks and antiquarks respectively, and those annihilate. As for why that happens, the Clear Science staff isn’t up to explaining it. Maybe we’ll put that question up on the bulletin board here at the Clear Science labs. 

Antimatter is used practically in medical imaging. A PET scan stands for Positron Emission Tomography, and as we know a positron is a particle of antimatter. During a PET scan, a molecule very much like glucose (sugar) called FDG or Fludeoxyglucose (¹⁸F) is put into the body. This molecule is like glucose, so it goes where ever glucose would go. However, it has a fluorine-18 isotope in it, which emits positrons.

So positrons leave the FDG, but positrons are antimatter and annihilate anytime they encounter an electron (which is matter). This happens, and gamma photons are produced. These gamma photons can be detected outside the body. So, the annihilation event between matter and antimatter can be used to map where the FDG goes and how much of it there is.

We talked about the antiparticles, which form antimatter atoms when combined in the same way as regular particles and regular atoms. Hydrogen atoms are one electron orbiting one proton. Antihydrogen is one positron orbiting one antiproton.

Antihydrogen was produced at CERN in 1995. This was done by making antiprotons using a particle accelerator and shooting them into xenon clusters (a bunch of xenon atoms). Only a very small number of antihydrogen atoms can be made this way.

Theoretically, there would be a lot of antimatter in the universe, and therefore a lot of antihydrogen floating out there in space. This could result in higher antimatter atoms (helium, lithium, etc), and even antimatter stars and planets. This appears not to be the case, or at least it cannot be detected if it is. 

Natalie asked about antimatter, which is made of antiparticles. All the stuff you know (matter) is made of particles like protons, neutrons, and electrons. (You might need a refresher, so let Professor Venus Flytrap take care of that if you get a chance.) (The Clear Science staff love that guy.)

Turns out all these particle have opposites, and those are antiparticles. You know a proton is positive, but an antiproton is negative. The opposite of an electron is a positron. These particles are real, and can be observed in high-energy physics experiments, like with a particle accelerator.

Particle and antiparticles will annihilate if they collide. Note that that happens not just because of the opposite charge: neutrons and antineutrons are both non-charged, but they still annihilate if they come together. (They’re made of quarks and antiquarks, and those annihilate when joined.)