Hola! I'm Alexa Guido, a young and curious woman passionate about science. Join me on an exciting journey to explore the wonders of the universe through the lens of physics.
Have you ever wondered if there is a mirror version of our world? A realm where everything is just like here, but with opposite properties? Well, in the fascinating world of particle physics, such a concept exists, and it is called antimatter! Antimatter is the counterpart of ordinary matter, composed of antiparticles that mirror the particles we know!
Antiparticles are subatomic particles with the same mass as one of the particles we ordinarily know but have the opposite electric charge and magnetic moment. Only particles satisfying certain conditions (for example, if they are electrically neutral) may be their own antiparticles. The only examples are photons, Z particles, gluons, Higgs particles, and gravitons… and possibly the three neutrinos. But every other particle has a distinct anti-particle, with the same mass but opposite electric charge!
The positron is the antiparticle of the negatively charged electron. Like the ordinary neutron, the antineutron has a net electric charge of zero, but its magnetic polarity is opposite to that of a similar neutron. Then the neutrino, an uncharged particle that travels very close to the speed of light, spins counterclockwise as viewed from behind, whereas the antineutrino spins clockwise as viewed from the same direction.
Moreover, when a particle meets its antiparticle, something extraordinary happens: they annihilate! (In most of the cases). This interaction releases a burst of pure energy, converting their entire mass into high-energy photons. This process is said to be one of the most efficient ways to release energy, which is why antimatter is often described as the ultimate energy source in science fiction. However, in reality, a particle and its anti-particle, which are immobile, can annihilate to make a particle and its antiparticle, and then photons, or just stay as other particles, or transition to phonos first, or…
How do we know about the existence of antimatter? Well, the physicist Paul Dirac predicted them by accident. In 1928 when he formulated the Dirac equation, a fundamental description of relativistic quantum mechanics. The equation suggested the existence of particles identical to electrons but with a positive charge. Dirac's equation appeared in his paper The Quantum Theory of the Electron, thus Dirac won the Nobel Prize in Physics in 1933.
But the equation posed a problem: just as the equation x2=4 can have two possible solutions (x=2 or x=-2), so Dirac's equation could have two solutions, one for an electron with positive energy, and one for an electron with negative energy. But classical physics (and common sense) dictated that the energy of a particle must always be a positive number.
Dirac interpreted the equation to mean that for every particle there exists a corresponding antiparticle, exactly matching the particle but with opposite charge. In his 1933 Nobel lecture, Dirac explained how he arrived at this conclusion and speculated on the existence of a completely new universe made out of antimatter! Finally, this theoretical prediction was confirmed in 1932 when Carl Anderson discovered the positron in cosmic rays[1].
If antimatter exists, why don’t we see entire galaxies made of it? This is one of the biggest mysteries in physics! The universe, as we observe it, seems to be made almost entirely of matter, with only traces of antimatter appearing in high-energy cosmic events.
Furthermore, one of the leading theories suggests that during the early moments of the universe, a tiny portion of matter – about one particle per billion – managed to survive in the annihilation of matter and antimatter. This phenomenon, called baryon asymmetry, is a crucial area of research in modern physics, in the past few decades, particle-physics experiments have shown that the laws of nature do not apply equally to matter and antimatter.
Physicists are keen to discover the reasons why. Researchers have observed spontaneous transformations between particles and their antiparticles, occurring millions of times per second before they decay. Some unknown entity intervening in this process in the early universe could have caused these "oscillating" particles to decay as matter more often than they decayed as antimatter.
[1] For a deeper understanding of cosmic rays visit “Have you heard about cosmic rays?”.
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