Matter is the substance that constitutes everything visible in the universe. Antimatter is its cosmic counterpart, a mirror image of ordinary matter where every particle has an associated antiparticle. Antimatter is structurally identical to matter, but possesses opposite fundamental properties. The difference in these properties dictates how they interact and why our universe looks the way it does. Their behavior is governed by a subtle lack of perfect symmetry in the laws of physics.
Fundamental Differences in Particle Properties
The most direct way to distinguish a particle from its antiparticle is by its electric charge. An antiparticle possesses the opposite charge of its matter counterpart. For instance, the electron carries a negative charge, while its antiparticle, the positron, carries an equal magnitude of positive charge. Similarly, the proton has a positive charge, and its twin, the antiproton, has a negative charge.
This relationship of opposite charge holds true for all charged particles and antiparticles. In all other intrinsic properties, the two are identical. A particle and its antiparticle share the same mass and possess the same quantum mechanical property known as spin.
Even particles with no electric charge have antiparticles, although the difference is more subtle. For example, the neutron has an antineutron, and the neutrino has an antineutrino. These neutral antiparticles differ from their matter twins in other quantum numbers, such as baryon number or lepton number, which are also reversed. The only exception is the photon, the particle of light, which is its own antiparticle because it has no charge or other distinguishing quantum numbers to reverse.
The Annihilation Reaction
The most dramatic difference between matter and antimatter is the result of their physical contact: annihilation. When a particle collides with its corresponding antiparticle, they instantly destroy each other. Their entire mass is converted into energy, following Einstein’s mass-energy equivalence formula.
The energy released is typically in the form of high-energy photons, specifically gamma rays. For a collision between an electron and a positron, the annihilation produces two gamma rays, each carrying 0.511 Mega-electron Volts of energy. These two gamma rays travel in opposite directions, a requirement to conserve both energy and linear momentum.
This energy release is far beyond that of any chemical reaction or nuclear fission process. Scientists exploit this specific interaction to detect antimatter, as the resulting gamma ray signature is unique. In medicine, this principle is used in Positron Emission Tomography (PET) scans, where detectors map the gamma rays produced by the annihilation of introduced positron-emitting substances.
Why Matter Dominates the Universe
The universe we observe is composed of matter, a situation known as the Baryon Asymmetry Problem. Since matter and antimatter are always created in pairs from energy, the Big Bang should have produced equal amounts of both. If perfect symmetry had held, the early universe would have been a sea of particles and antiparticles that completely annihilated each other, leaving behind only radiation.
The existence of matter means that a small imbalance must have occurred, allowing about one particle of matter to survive for every billion particle-antiparticle pairs that annihilated. The theoretical mechanism that allowed this excess of matter to survive is known as Charge-Parity (CP) violation. CP symmetry suggests that the laws of physics should remain the same if a particle is swapped with its antiparticle and its spatial coordinates are mirrored.
Experiments have shown that CP symmetry is broken in certain particle interactions, specifically those governed by the weak nuclear force. This violation means that matter and antimatter behave differently, decaying at different rates or into different products. While the CP violation observed in the Standard Model is not large enough to explain the entire observed cosmic imbalance, it provides the necessary framework for a matter-dominated universe. This phenomenon indicates that new physics, beyond the Standard Model, must be at play to fully account for the survival of matter.