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Why is antimatter so rare?

Antimatter incredibly expensive to create, very hard to store, and cannot come in contact with regular matter without disappearing in a flash of gamma rays and neutrinos.

From the everyday to the extraordinary

Isaac Newton and Gottfried Wilhelm Leibniz gave rise to the field of classical mechanics, which described how bodies behaved when various kinds of forces were applied to them. While classical mechanics were good enough to understand the phenomena that most humans experienced on a daily basis for over two hundred years, it fell short when investigating the motions of planets, stars and galaxies, or the subatomic realm. Light has been pretty problematic to understand for science. One of the issues is that waves of light travel through the vacuum of space, but waves technically should be unable to propagate without a medium. In the seventeenth and eighteenth centuries, Scientists such as Robert Boyle, Christiaan Huygens and Isaac Newton all explained the phenomenon by suggesting that there was a pervasive medium throughout space, that allowed light to travel. This medium, invoked just to allow light to travel in vacuum, was dubbed “luminiferous ether”.

Einstein’s Theory of Relativity, proposed early in the 20th century is made up of two related components. These are special relativity which was proposed earlier, and general relativity, which also takes gravity into account. The theory of special relativity showed that time and space cannot be considered as separate entities, and are instead the spacetime continuum. The theory also removed the need for a luminiferous ether for the propagation of light.

While the understanding of how objects interact with each other when different forces are applied to them was good enough for say predicting the motion of planets, there were several problems when it came to studying motion at a subatomic level. The investigations of how particles and energies behave in the subatomic realm by researchers such as Max Planck, Niels Bohr, Werner Heisenberg, Wolfgang Pauli, Erwin Schrödinger, Satyendra Nath Bose, and Albert Einstein gave rise to the field of quantum mechanics. Quantum here, just means very small spaces. The Heisenberg Uncertainty Principle showed that you could either precisely measure the velocity or the position of a particle, say a photon, but never both. The Schrödinger equation allowed for the study of quantum mechanics using probabilities, as all the properties of particles could not be measured with accuracy.

The theory of relativity explained phenomena on a macro level, and the quantum mechanics did the same on a micro scale. Both had weird and wonderful implications that were proven through experiments through the 20th century. Einstein’s special theory of relativity indicated that time is not the same for everyone, that the speed of light is the same no matter how fast the observers are moving, and that there was a limit to how fast anything could possibly travel. Quantum mechanics implied that particles could jump through barriers they should not be able to pass, properties of particles gaining a value at the time of measurement from a range of probabilities, and the existence of several simultaneous, not parallel universes. However, special relativity had not been described in the context of quantum mechanics till Paul Dirac came along, leading to one of the weirdest and most wonderful implication of all – antimatter.

Dirac’s Equation

Within an atom, the proton has as a positive charge while the electron has a negative charge. A shortcoming of quantum mechanics at that time was that it could not explain the motions of subatomic particles at high speeds. Dirac wanted to explain the motion of electrons travelling at speeds close to the speed of light. The equation he came up with to do so, was sufficient enough for his purpose, but had far reaching consequences. Dirac’s equation was consistent with both the theory of relativity as well as quantum mechanics, and is in some ways similar to the Schrödinger equation. Dirac’s equation had an inherent “problem”. Just as quadratic equations have two solutions, or roots, Dirac’s equation also had two solutions. The equation implied that the electron had an antiparticle, with a positive charge instead of a negative charge, now known as a positron.

Carl David Anderson, while studying cosmic rays in a magnetic environment, found traces of a particle that was moving in the opposite direction of the electron, but was too light to be a proton. The new particle was called the positron, and Anderson won the 1936 Nobel Prize in Physics for the discovery. After the discovery, the results of previous studies showed that the positron had been experimentally observed before, but such observations were not followed up, or the particle was assumed to be a proton.

The Bevatron. Image: Berkeley Lab.

Particle accelerators

The idea that every particle had a counterpart with an opposite charge was known as charge symmetry. The Bevatron was a device designed to specifically test the validity of charge symmetry. It was a particle accelerator, that would use electromagnets to propel protons to speeds close to that of light, and then make them collide against stationary protons or neutrons. In 1954, the Bevatron became operational at the Berkeley Lab, operated by the University of California.

Just about a year later, the first antiprotons were observed at the Bevatron. A year after that, the first antineutrons were observed. There now was sufficient evidence for the existence of antiparticles for all three components of the atom. Observations of the antiproton, antineutron and the positron proved the that the notion of charge symmetry was indeed true. If atoms are the building blocks of matter, then antiatoms are the building blocks of antimatter. Now that all the particles that make up an atom were known to have antiparticles, the question was that there was an antiatom.

Two particle accelerators, the proton synchrotron at CERN and the alternating gradient synchrotron at the Brookhaven National Laboratory both detected the first antinuclei in 1965. Deuterium, a stable isotope of Hydrogen with one neutron and one proton. The atomic nuclei detected was an antideuterium. In 1978, researchers at CERN produced antiprotons from the proton synchrotron, and kept them going round in circles in a machine dubbed the Initial Cooling Experiment, or ICE for a period of 85 hours. It was the first time that anyone, anywhere in the world had stored antiprotons. In 1995, thirty years after the first antinuclei was observed, scientists managed to create the first antiatom in a lab environment. To create an antiatom, it was necessary to have low energy antiparticles, which required specialised equipment. The antiparticles created so far were “hot”, with energy levels too high to allow for the creation of antiatoms.

Hydrogen is the simplest atom in the periodic table, and is made up of a proton and an electron. It is the most abundant element in the universe, and comprises nearly 75 percent of the baryonic mass in the universe. Baryonic refers to matter, in other words, the stuff that is not anti-matter. The first antiatom created in a lab environment was the antihydrogen. This was done through an instrument at CERN known as the Low Energy Antiproton Ring. Currently, CERN uses the Antiproton Decelerator to produce antimatter that is of a low enough energy level to allow scientists to study them. In 2003, small quantities of antihelium was produced at Brookhaven National Laboratory. This remains the most complex antiatoms created by humans.

An antihydrogen annihilation event in the ATHENA experiment. The pink silicon microchips indicate four pairs of quarks and antiquarks, the yellow cubes show captured energy and the red line denotes the gamma rays produced by the annihilation. Credit: CERN

Antimatter traps

In 2011, CERN managed to store antimatter for a period of sixteen minutes, long enough to start experimenting on the substance, and unlocking its mysteries.

By now, you should have had a clear idea of why antimatter is so rare. It takes an incredible amount of energy to create antimatter, and to store it. Even after the laborious process, only a few antiatoms are created. A feasibility study by NASA to explore the use of antimatter for fuelling spacecraft, pegged the price of creating antimatter at $62.5 trillion a gram, a figure that estimates by CERN agree with. It takes 25 trillion kWh of electricity to make a single gram of antimatter. CERN has produced less than 10 nanograms of antimatter so far, and has the capability to create about one billionth of a gram every year. CERN would take 1 billion years to produce one gram of antimatter. Antimatter cannot be stored in a normal container, as matter and antimatter mutually annihilate each other on contact, usually producing gamma rays and neutrinos in the process. Researchers do not even know if they have created antimatter or not, and can only confirm the process of creation after detecting the annihilation event.

Antiparticles are stored by suspension in vacuum through electric fields, or if the antimatter has no charge, through superconducting magnets. A penning trap at CERN stored a single antiproton for 57 days, which is the world record for antiparticle storage. Researchers are developing magnetic bottles and optical traps using lasers to store antimatter.

An antimatter trap. Image: CERN.

Antimatter in nature

Conditions similar to that of particle accelerators are created in nature as well, including pulsars and the magnetic fields of celestial bodies. A supermassive black hole accelerates particles in a jet stream that interacts with the gas clouds of two colliding galaxies, Abell 3411 and Abell 3412 to create a natural particle accelerator. Naturally occurring antiparticles have been detected in the Van Allen Belts around Earth. If there are entire galaxies made up of antimatter, they would be indistinguishable from regular galaxies through our telescopes. If such galaxies exist, they would be from the early days of the big bang, near our horizon of visibility.

The radiation from unstable atomic nuclei also produce small quantities of antimatter in some cases. Human beings produce antimatter by eating, drinking and breathing. This is because of the potassium-40 in the body, and a person weighing 80 kg produces about 180 positrons every hour.

The big bang actually created equal amounts of matter and antimatter. Soon after though, most of the antimatter disappeared, leaving a small amount of matter behind, that we can observe. Theoretically, there should be the same amount of matter and antimatter in the universe. Why this is not the case, remains a mystery.

Aditya Madanapalle

Aditya Madanapalle

An avid reader of the magazine, who ended up working at Digit after studying journalism, game design and ancient runes. When not egging on arguments in the Digit forum, can be found playing with LEGO sets meant for 9 to 14-year-olds.