You could think of a Marvel Comics tale or the first book in the Southern Reach Trilogy when you hear the term “annihilation.” You may also consider the word’s dramatic common definition: devastation to the brink of extinction.
An annihilation has a distinct meaning for particle physicists
When a particle of matter collides with its antimatter counterpart, annihilation occurs. It is transition, not destruction, that occurs.
A fresh pair of particles develops on the opposite side of the annihilation process. They may retain their previous identities, transition into a totally new particle-antiparticle pair, or completely transform into photons, which carry electromagnetic energy.
The idea that energy cannot be generated or destroyed governs this reaction (and all reactions). This includes energy in the form of any mass-bearing substance. According to Flip Tanedo, a theoretical particle physicist at the University of California, Riverside, “we’re happy with E=mc2, so mass is energy.”
However, mass isn’t the sole source of energy. There’s also kinetic energy, which is tied to motion, and radiant energy, which is electromagnetically stored as photons.
When two particles annihilate, the energy of whatever they form must be equal to that of the original pair.
Tanedo explains this by imagining two large, hefty particles: a bowling ball and an anti-bowling ball. He explains, “They’re simply sitting around because they’re so hefty.” “However, if these two bowling balls collide and transform into two billiard balls, those billiard balls must be travelling very quickly [and hence have a lot of kinetic energy] to compensate for the mass difference.”
Quantum leaps are quantum jumps in time
Our exploration of annihilation started in 1928, when British scientist Paul Dirac predicted the presence of antimatter for the first time.
Dirac was working on an equation to explain the motion of an electron when he came upon an interesting mathematical quirk: His formula correctly described a particle with all of the characteristics of an electron. However, if all of those qualities were multiplied by -1, it would indicate the presence of a particle that behaved precisely like an electron but reflected all of its quantum features.
Four years later, Carl Anderson, an American professor, discovered the positron, which Dirac had anticipated. Anderson was observing the high-energy cosmic rays that regularly assault Earth from space using a cloud chamber—a gadget that enabled him to view ionizing subatomic particles by sight. Anderson saw a strange particle scurrying through the cloud chamber. It possessed the same characteristics as an electron, but because of the way it curled in a magnetic field, it was assumed to have a positive charge.
Physicists discovered a year later, in 1933, that when a positron originated from cosmic rays in a cloud chamber, it was always followed by an electron spiraling in the opposite direction from the same spot. Electromagnetic radiation is converted into a particle-antiparticle pair in this process, nicknamed “pair creation.”
Physicists rapidly started looking for a way to transform electrons and positrons into energy in the other direction. Several groups had witnessed this mechanism experimentally by the mid-1930s: we had annihilation.
When energy transforms into matter, the equal quantity of antimatter arises, as far as humans can tell. There should be an equal quantity of matter and antimatter in the cosmos, or merely a sea of photons left over after matter-antimatter annihilation.
Our cosmos, however, ended up with an imbalance that overwhelmingly favors matter for reasons that physicists don’t understand.
“If the cosmos contains one gram of matter, there should be one gram of antimatter,” Tanedo adds, based on what we know about physics principles. “However, it is not what we observe; instead, we see a lot of matter and practically no antimatter. There’s a strange imbalance.”
Annihilation and you
Asking a particle physicist what makes annihilation interesting is akin to asking them to wax lyrical about covalent bonds or elastic collisions. Tanedo comments, “It’s sort of a strange question.” “This is simply one form of response, and there’s nothing unique about it compared to others.”
Annihilation occurs often. Unless you’ve been living under a mile of rock, you’ve been assaulted by cosmic rays similar to the ones Carl Anderson witnessed when he first spotted the positron. Energy is converted into equal quantities of matter and antimatter in these and other high-energy astronomical phenomena. In our matter-filled world, however, the lonely antiparticles quickly discover their counterpart matter particles and annihilate into photons.
But destruction isn’t limited to outer space. On Earth, positrons are infrequently produced through radioactive decay of uncommon potassium atoms present in all plant and animal tissue, particularly potassium-rich banana tissue.
Antimatter can even be created in the lab, according to scientists. However, producing it is costly and energy-intensive; in more than half a century, scientists at European accelerator facilities such as CERN and DESY, as well as the US Department of Energy’s Fermi National Accelerator Laboratory, have produced fewer than a few hundred nanograms of the substance. Accelerators like CERN’s Large Electron-Positron Collider and Fermilab’s Tevatron ran matter and antimatter beams through each other to release energy through annihilation. The energy from those annihilations was transformed into matter, resulting in the discovery of previously unknown particles.
Important considerations
Despite how mundane annihilation may seem to particle physicists and astrophysicists, it is nonetheless vital in their study.
Photons, for example, operate as their own antiparticles. Scientists are investigating neutrinos to see whether this is the case with them as well. The discovery that neutrinos may destroy other neutrinos might provide scientists with a crucial answer to the enigma of the universe’s matter-antimatter imbalance.
Astrophysicists believe that annihilation might aid us in indirectly detecting dark matter particles that have only been detected due to gravity’s attraction. If dark matter particles and antiparticles annihilate, their energy may be converted into a form that is more readily visible.
“We live in an ocean of dark matter, so maybe we should be hunting for its destruction products,” Tanedo, a dark-matter expert, adds.
The Fermi Gamma-ray Space Telescope discovered unexpected dispersed gamma-ray radiation from the Milky Way’s core in 2009. The scientists who saw the additional radiation first assumed it was the result of dark-matter annihilation—and the math supported their hypothesis.
However, astrophysicists have yet to find evidence to back up this theory. Another source of energy, such as pulsars or a black hole, might be the source of the energy. The Alpha Magnetic Spectrometer, a newer detector, has discovered some positrons of unknown origin, but there is no conclusive proof that these are the product of dark-matter annihilation.
Even so, dark-matter annihilation might be taking place somewhere, and physicists are keeping a close check on it. “The mystery has remained entertaining,” Tanedo adds.