14.4 Antimatter

Introduction

In 1928, physicist Paul Dirac formulated a revolutionary equation that combined quantum mechanics and special relativity to describe the behavior of electrons. A startling consequence of this equation was that it predicted two possible energy solutions for a particle: one positive and one negative.

$E=\pm \sqrt{m^2{c}^4+p^2{c}^2}$

While the positive energy solution described the electron as we know it, the negative solution implied the existence of a new type of particle: an "anti-electron." Dirac theorized that for every particle, there must exist a corresponding antiparticle with the same mass and spin, but with an opposite electric charge. This prediction was confirmed in 1932 when Carl Anderson discovered the positron (the anti-electron) in cosmic ray interactions, opening the door to the study of antimatter.

Properties of Particles and Antiparticles

An antiparticle is the mirror image of its corresponding particle. The primary distinction is the reversal of certain quantum numbers, most notably the electric charge.

• Matter: The ordinary substance of the universe, made of particles like electrons, protons, and neutrons.
• Antimatter: Composed of antiparticles, such as positrons, antiprotons, and antineutrons.

The table below compares the properties of some fundamental particles and their antiparticles:

(Note: While the neutron is neutral, its constituent quarks have charges, which are opposite in the antineutron. This gives the antineutron an opposite magnetic moment.)

Annihilation

When a particle collides with its antiparticle, they annihilate each other. Their entire mass is converted into a burst of pure energy, typically in the form of high-energy photons (gamma rays), in accordance with Einstein's equation $E=m{c}^2$.

Pair Production

Pair production is the process in which a high-energy photon (gamma ray) interacts with the electric field of a nucleus and is converted into a particle and its corresponding antiparticle (such as an electron and a positron).

Applications of Antimatter

Despite its rarity, antimatter has significant practical applications, particularly in medicine and scientific research.

1. Medical Use:

   • Positron Emission Tomography (PET) Scan: This is a powerful diagnostic imaging technique. A patient is given a radiotracer that emits positrons. When these positrons encounter electrons in the body's cells, they annihilate, producing pairs of gamma rays that travel in opposite directions. Detectors outside the body pinpoint the location of these annihilations, creating a detailed 3D image of metabolic activity.
   • Cancer Therapy: Researchers are exploring the use of antiprotons for radiation therapy. When an antiproton annihilates with a proton in a tumor cell, it releases a localized burst of energy, which can destroy the cancer cell while minimizing damage to surrounding healthy tissue.

2. Scientific Research:

   • Particle accelerators like those at CERN regularly produce antiparticles to study the fundamental laws of physics. Scientists have successfully created and trapped simple anti-atoms, such as antihydrogen (an antiproton orbited by a positron), to test whether antimatter obeys the same physical laws as matter.

The Matter-Antimatter Asymmetry

One of the greatest unsolved mysteries in physics is why the observable universe is composed almost entirely of matter. According to the Big Bang theory, matter and antimatter should have been created in equal amounts. If that were the case, they would have annihilated each other, leaving behind a universe filled only with radiation. The fact that we exist implies a slight imbalance, or asymmetry, in the early universe that favored matter. Understanding the origin of this asymmetry is a major goal of modern cosmology and particle physics.

Cost of Antimatter

Producing antimatter is an incredibly energy-intensive and inefficient process. It requires smashing particles together in large accelerators to create a tiny number of antiparticles. This makes it the most expensive substance ever made, with an estimated cost of 62.5 trillion dollars per gram.