Antimatter is made of antiparticles, which have the same mass as ordinary particles but opposite electric charge and other quantum properties. The positron is the antiparticle of the electron, and the antiproton is the antiparticle of the proton. Antimatter matters because it tests the deepest symmetries in physics and shows how mass can be converted into energy.
It also appears in medical imaging, cosmic ray physics, and experiments at particle accelerators.
When a particle meets its matching antiparticle, the pair can annihilate and transform their rest mass into other particles, often high energy gamma rays. For an electron and positron at rest, annihilation usually produces two gamma ray photons traveling in opposite directions to conserve momentum. Antimatter is made in high energy collisions or some radioactive decays, but it is difficult to store because it annihilates when it touches ordinary matter.
One of the biggest unsolved questions in physics is why the observable universe contains far more matter than antimatter.
Key Facts
- An antiparticle has the same mass as its particle partner but opposite electric charge.
- Electron charge = -e, positron charge = +e.
- Rest energy is E = mc^2.
- Electron positron annihilation at rest: e- + e+ -> gamma + gamma.
- Energy of each photon in electron positron annihilation at rest is 511 keV.
- Photon energy is E = hf = hc/lambda.
Vocabulary
- Antimatter
- Antimatter is matter made of antiparticles, such as positrons and antiprotons.
- Antiparticle
- An antiparticle is a partner particle with the same mass as the original particle but opposite charge and related quantum numbers.
- Annihilation
- Annihilation is the process in which a particle and its antiparticle interact and convert into other particles or energy.
- Gamma ray
- A gamma ray is a very high energy photon produced in nuclear processes, particle interactions, and some annihilation events.
- Matter antimatter asymmetry
- Matter antimatter asymmetry is the observed fact that the universe contains much more matter than antimatter.
Common Mistakes to Avoid
- Thinking antimatter has negative mass, which is wrong because antiparticles have the same positive mass as their particle partners.
- Assuming annihilation means energy is created from nothing, which is wrong because mass energy is converted according to E = mc^2 and total energy is conserved.
- Drawing only one photon from electron positron annihilation at rest, which is wrong because two photons are needed to conserve momentum.
- Treating antimatter as science fiction only, which is wrong because positrons are used in PET scans and antiparticles are produced in particle accelerators.
Practice Questions
- 1 An electron and a positron annihilate at rest. What is the total energy released in MeV, given that each has rest energy 0.511 MeV?
- 2 A photon from electron positron annihilation has energy 511 keV. Convert this energy to joules using 1 eV = 1.60 x 10^-19 J.
- 3 Explain why ordinary containers cannot directly hold antimatter and describe one method scientists use to confine charged antiparticles.