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.
Understanding Physics: Antimatter
Antiparticles are not simply particles with a reversed charge label. Each particle carries a set of conserved quantum properties. These include electric charge, lepton number, baryon number, and some forms of spin-related behaviour.
An antiparticle has the matching opposite values where an opposite value is possible. This is why an antineutron is still electrically neutral, yet it is different from a neutron. Its baryon number is opposite.
Some particles have no distinct antiparticle at all. The photon is its own antiparticle because it has no electric charge or baryon number. Students should separate the idea of charge from the wider idea of particle identity.
High energy can create particle pairs. In pair production, energy becomes a particle and its antiparticle together. Conservation laws control the process from start to finish.
Charge must total the same before and after an event. Momentum must be conserved too. A single energetic photon moving through empty space cannot usually produce an electron positron pair, because there is nothing else to take the needed recoil momentum.
Near an atomic nucleus, the nucleus can recoil slightly. This makes pair production possible.
The energy supplied must at least equal twice the rest energy of an electron. Any energy above this minimum becomes motion of the new particles or recoil of the nucleus.
Positron emission tomography, often called PET, uses this physics in a precise way. A patient receives a substance containing a radioactive isotope that emits positrons. The substance is chosen to collect in a tissue of interest, such as active brain tissue or a tumour.
Each positron travels a short distance through the body before meeting an electron. Detectors around the patient record two gamma ray signals arriving at nearly the same time. A computer uses many such paired detections to estimate the line along which each event occurred.
Combining thousands of lines produces an image of where the radioactive substance was concentrated. The image is indirect, so its detail depends on detector timing, patient motion, and how far positrons travel before annihilation.
Making antimatter is possible, but collecting it is difficult. Charged antiparticles can be held away from container walls using electric and magnetic fields in a device called a particle trap. They must be kept in a near vacuum, since even a thin gas would cause collisions and loss.
Neutral antimatter is harder to confine because electric fields do not push on it. Experiments cool neutral antihydrogen to very low temperatures and use carefully shaped magnetic fields to trap it. Researchers then compare its light spectrum and gravity response with ordinary hydrogen.
Tiny differences could help explain why the early universe did not leave equal amounts of matter and antimatter. When learning this topic, keep track of which conservation laws are exact and which symmetries may have small violations.
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.