What is antimatter? What happens if matter and antimatter interact? How was antimatter discovered? Why don’t we usually come across antimatter in our daily lives? All these questions and many more come to one’s mind when thinking about antimatter. But, first things first! Let us first define and understand what antimatter is.

Antimatter is the opposite of matter, literally. For every sub-atomic particle, such as an electron, a proton, a neutron, etc. there exists an antiparticle such as an anti-electron, anti-proton, and anti-neutron. The antiparticle will have the same mass as the particle, but it will differ in the sign of its charge and other quantum numbers. Some of these quantum numbers are the lepton number – one for the electron and each of the other five members of the lepton family, and the baryon number – one-third for each of the six quarks that make up the baryon family. Antiparticles are expected to interact with other antiparticles in exactly the same way ordinary particles interact with each other. The laws of physics are (almost) symmetric when it comes to matter and antimatter. In fact, a Universe made up of antimatter would be indistinguishable from ours!

Going into a little bit more detail, the anti-electron is called a positron (for positive electron) and it is positively charged with a lepton number of minus 1. The positron has exactly the same mass as the proton. The proton is a positively charged sub-atomic particle made up of three quarks giving it a baryon number of one. Its antiparticle, the antiproton, has the same mass as the proton but it is negatively charged with a baryon number of minus one (minus one-third contributed from each of the three antiquarks that make up the antiproton). Neutral sub-atomic particles, such as neutrons, are interesting. The antineutron has the same mass and zero charge as the neutron, but it will have a baryon number of minus one (again, minus one-third coming from each of the three antiquarks that make up the antineutron).

So what happens when matter and antimatter interact? The answer is fireworks! When a positron interacts with an electron they both annihilate to produce two X-ray energy photons! So, we are in luck that antimatter is so scarce in the Universe nowadays. Otherwise, we would have been burned by X-rays and gamma-rays every time matter and antimatter interacted. As a matter of fact, if you were to meet your anti-self then both of you would annihilate and release the equivalent energy of roughly 2500 megatons, almost one-third the total energy of the world’s arsenal of nuclear weapons! Antimatter could sure make for mighty powerful spaceship engines IF you can find a way to produce macroscopic amounts of antimatter AND a way to store it in isolation! Some modern particle accelerators regularly produce antiprotons for use in high-energy physics experiments.

Now that we know what antimatter is, let us see how physicists discovered it. Back in the year 1928, Paul Dirac, one of the founding fathers of the new science of quantum mechanics, was trying to solve an equation that included the effects of the theory of special relativity to describe the behavior of electrons in the microscopic world. To his surprise, the equation admitted solutions that corresponded to electrons with negative energy going back in time! Any insecure student of physics would have blushed with embarrassment and redid the math. However, Dirac was sure of his math. Instead, he reinterpreted his problematic solution to denote an “anti-electron” with positive energy going forward in time. Four years later, an experimental physicist by the name of Carl Anderson proved Dirac right by actually observing the anti-electron, the positron. The discovery of the positron earned Anderson the Nobel Prize in 1936. Dirac had already won the Nobel in 1933 for his contributions to atomic physics. Are you wondering where Anderson’s positrons came from? The positrons that Anderson discovered originated in atmospheric showers of sub-atomic particles that result when very high-energy cosmic rays (mostly protons) interact with atoms in Earth’s atmosphere.

The story of the prediction and subsequent discovery of the positron is instructive and shows how big discoveries in physics are often made. The device used by Anderson is called a cloud chamber and was a standard instrument used in nuclear physics labs at the time. A particle going through a cloud chamber would leave a sort of a trail of bubbles in its wake. Applying a known magnetic field would curve the particle according to its charge.

Credit: Insane Curiosity

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