ANTIMATTER AND ITS DISAPPEARING ACT

Envision a scenario where the collision of two particles would result in their mutual annihilation. Now consider this: matter has a trusty counterpart called antimatter; when the two meet, sparks fly and you’re left with pure energy.

MATTER AS WE KNOW IT


Figure 2: galaxy cluster Abell 2744 (Pandora’s Box) [source]

Before diving headfirst into antimatter, matter’s evil twin, it’s worth zeroing in on what makes them identical and what sets them apart. Matter is everything tangible that makes up the universe. For centuries, thinkers have pondered over what makes matter, well, matter.

A brief rundown on mankind’s take on the topic, thus far: If we break matter down to a single unit, we’d be left with atoms. Atoms are composed of three particles – electrons, protons and neutrons. Subatomic particles have three distinct qualities to them – mass, charge and spin.

These intrinsic properties spearheaded the discovery of antimatter since antiparticles have the same mass and spin as particles but the opposite charge. With that out of the way, it does raise a question: if the universe is made up of matter, where does antimatter fit in? In theory, the universe is composed of both matter and antimatter, however, the universe has a bit of an antimatter scarcity.

THE BIG BANG THAT WASN’T

Figure 3: A galactic cloud of antimatter [source]

If we turn the time dial all the way back to when the universe as we know it came into existence, we’d land upon the Big Bang. A common misconception is that the Bing Bang was an explosion in space when in actuality, it was the expansion of space itself. However, explosion enthusiasts need not despair. The Big Bang was indeed accompanied by an explosion, albeit for a completely different reason.

Theoretically, the Big Bang ought to have created equal parts of matter and antimatter. This presents a slight problem – when matter and antimatter meet, they annihilate, leaving pure energy in their wake. In other words, nothing should exist. Yet, here we are. This is the Matter-Antimatter Asymmetry, and can be explained by alluding to the presence of one extra particle for every billion particle-antiparticle explosive units.

The massive amount of energy produced by the explosion of the particle-antiparticle units can be detected across the universe today, in the form of cosmic microwave background radiation, attesting to its occurrence. Additionally, the imbalance of particles during the Big Bang correlates to the amount of matter currently observed in the universe. The reasoning behind the imbalance, however, is still open to discovery.

One could throw all this aside and theorise that the antimatter exists in some far, isolated nook of the universe. Although it’s possible, the trail of energy this would leave behind would be massive, due to the occasional bumps with matter, as made evident by the pockets of antimatter found in our galaxy.

DOWN TO THE NITTY GRITTY


Figure 4: The Standard Model of Particle Physics. [source]

There was a time when atoms were considered indivisible and then electrons, protons and neutrons were discovered. Over time, subatomic particles have become much more comprehensive. Elementary particles are essentially Lego blocks that piece together the Universe, and the Standard Model functions as a user manual for these building blocks. The three intrinsic properties (mass, charge, spin) apply across all elementary particles and help classify them.

The 17 elementary particles, currently theorised, are segregated across two categories: fermions and bosons. Matter and antimatter are composed of fermions, while bosons are carriers of the fundamental forces of nature (and the Higgs field). Fermions are further classified into leptons and quarks, and they each come in six different flavours.

Thus, atoms are composed of leptons (in the electron flavour) and quarks (in the up and down flavours). The three subatomic particles – electrons, protons and neutrons, seamlessly fit into this model. Electronsare elementary particles as they can no longer be broken down and are one of six leptonflavours. Protons and neutrons, however, are composite particles, as they can each be broken down into quarks. That’s not to say that bosons (force-carriers) aren’t present in atoms. Gluons, the carriers of strong nuclear force, hold quarks together.

ANTIMATTER AND ITS ACCIDENTAL DISCOVERY

Antimatter fits cohesively into the Standard Model, since antimatter functions just like matter, right down to the elementary particles. In essence, the building blocks of the universe have two forms to them – matter and antimatter. The blocks build upon each other in their fundamental forms as particles and antiparticles, stitching together all the tangible aspects of the Universe.

To further demonstrate this, antiquarks form antiprotons and antineutrons. These can join to form nuclei, or in the presence of positrons (antielectrons), form atoms (like antihydrogen) and molecules (like antiwater).

Figure 5: Paul Dirac (top) [source] and Carl Anderson (bottom) [source]

Paul Dirac, a British physicist, stumbled about positrons (antielectrons) when he came up with the Dirac equation, which explains the behaviour of electrons travelling near the speed of light. Years later, positrons would be experimentally discovered by an American physicist, Carl Anderson, which would substantiate Dirac’s predictions and pave the way for the discovery of other antiparticles, like antiprotons and antineutrons. Paul Dirac even went on to speculate that there might exist an entire mirror universe of antimatter. 

WHY DOES ANTIMATTER MATTER?

Antimatter isn’t just a product of textbooks or science fiction. Particles with the capacity to annihilate us are just around the corner. They’re in the cosmic rays, thunderstorms and even our bodies. They’re just in incredibly small quantities. For instance, potassium-40 is a naturally occurring isotope present in all things that contain potassium (like bananas or humans). When the isotope decays, it emits an antielectron. In bananas, this would be one positron emitted about every 75 minutes. In humans, it would be much less.

Figure 6: PET scan machine (top) [source] and PET scan image of a human brain (bottom) [source]

Antimatter has its uses as well. PET scans use positrons to produce images of the body. Certain isotopes, much like potassium-40 in bananas, are injected into the blood where they meet electrons and annihilate. The energy thus produced help construct images.

Figure 7: AMS-02 attached to the ISS (top) [source] and Antiproton Decelerator at CERN (bottom) [source]

The study of antimatter has only accelerated over time. Scientists have graduated from observing subatomic antiparticles to creating antihydrogen and antihelium atoms in laboratories. The International Space Station is currently housing the AMS-02, a cosmic ray detector, which uses a magnetic field 4000 times stronger than that of the Earth to better understand antimatter. CERN houses an Antiproton Decelerator and is attempting to determine the effect of gravity on antimatter, among other things. Antimatter is a fundamental facet of the universe, that for some reason, has all but disappeared. The closer we get to determining what makes it tick, the better we get at understanding the world around us.

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