The Strange World of Antimatter

The early twentieth century was an exciting time to be in physics. The staid and predictable world of Newton’s physics had just been overthrown by two new ideas — relativity and quantum mechanics. Physicists were busy sketching out the new theories, discovering new laws of reality, and getting famous doing it. As experiments probing the new world of physics revealed bizarre and almost unbelievable phenomena, the universe was revealed to be far stranger than anyone could have imagined.

Cloud chambers, like shown in this image, reveal the presence of subatomic particles. The lines that appear in the chamber show the path the particles took. Credit to CERN.
Cloud chambers, like shown in this image, reveal the presence of subatomic particles. The lines that appear in the chamber show the path the particles took. Credit to CERN.

Around 1912 scientists began to find streams of energetic particles bombarding the Earth from outer space. Physicists soon started to investigate these mysterious particles with the help of cloud chambers, a device which used strong magnetic fields and a fine mist to reveal the path, mass and electric charge of fast moving sub-atomic particles. The scientists using cloud chambers would occasionally find traces that couldn’t be explained by the handful of particles known at the time, but the experimental apparatus just wasn’t accurate enough to provide real evidence of something new. That would change in 1931, when Carl Anderson, working at the California Institute of Technology, built an improved cloud chamber. The evidence of a new particle could no longer be ignored, and after spending a year trying to definitively prove his discovery, Anderson announced the discovery of the positive electron in 1932.

The discovery was strange, as electrons were well known to hold a negative charge, a fact the electrical grids then rapidly spreading across the world depended on. To try and understand what was happening Anderson turned to theories of quantum mechanics and relativity. The recently discovered quantum physics could describe the behaviour of subatomic particles, but only when they were moving slowly. To describe particles moving close to the speed of light, as particles do in a particle accelerator or in deep space, quantum theory needed to be combined with Einstein’s theory of relativity.

Paul Dirac, an eccentric British physicist, discovered an equation that did just this, providing the first theoretical insight into the behaviour of highly energetic particles. One consequence of this equation was the prediction that each particle should have some kind of “negative” particle associated with it. At first Dirac thought this could be explained with existing particles — since he knew of the electron and proton, two particles with exactly opposite electric charge, Dirac proposed that the electron and proton were negatives of each other. Problems soon started to crop up with this idea, however, particularly relating to the huge difference in mass of the electron and proton.

Anderson’s discovery of the positive electron, or positron, fitted the job perfectly, and the demonstration that positrons and electrons were negatives of each other provided an experimental validation of Dirac’s theory. Dirac and Anderson would win Nobel Prizes for their work, both being awarded the prize at the extraordinarily young age of 31. It is a tribute to Dirac’s particular genius that the positive electron was the first particle to be predicted in theory before being experimentally detected. What Dirac predicted, and Andersen discovered, is now known as antimatter, and antiparticle equivalents have been discovered for almost every particle known to science.

The track of a positron in Anderson’s cloud chamber.
The track of a positron in Anderson’s cloud chamber.

Matter is the stuff that makes up the world around us. Although there are many different materials, a close look at these reveals that everything is built up from molecules and atoms. Looking even more closely we find that those atoms themselves are made up of three different types of particles — electrons, neutrons and protons. These three particles make up almost everything that surrounds us. Over the last century scientists have discovered many more particles; however, since these particles are unstable and break apart after fractions of a second, they are only found in particle accelerators or in deep space, where they form as byproducts of various astrophysical processes.

Particles differ from each other in several ways, but two of the most important properties are mass and electric charge. In the early 1930s, when Anderson was examining cloud chambers, only a handful of particles were known. These were the electron (very low mass and a negative electric charge), the proton (heavier mass, and an exactly opposite positive charge), the alpha particle (later understood to be the nucleus of a helium atom, made of two protons and two neutrons, and with twice the electric charge of a proton), and the photon (a particle of light, with zero mass and no electric charge).

Dirac’s theory predicted the presence of an antimatter partner for each particle. These anti-particles would have a kind of symmetry with particles — each would have exactly the same mass, but an opposite electrical charge. In all other respects the particles should behave in exactly the same way, in theory allowing the existence of antimatter planets, stars and galaxies. The positron fitted this theory perfectly, turning out to be identical to the electron in every way except for electric charge. The antiproton was discovered twenty years later in 1955, and scientists have since discovered many other antiparticles.

Dirac’s theory went further than just predicting the existence of matter and antimatter. He also predicted that matter and antimatter should annihilate each other in a burst of energy if they were ever to meet, and that pairs of particles and antiparticles should spontaneously appear if large amounts of energy were concentrated in one place. Evidence that this really happens was discovered 1933, when positrons and electrons were observed to appear and then quickly disappear through annihilation.

Antimatter is naturally extremely rare on Earth. Small amounts of antimatter reach the Earth in the form of cosmic rays, either from the Sun or from other sources deeper in space. Scientists have also recently discovered evidence of antimatter production during thunderstorms — though these antiparticles don’t survive for long.

A handful of labs around the world now regularly produce small quantities of antiparticles, using various techniques involving high energy particle accelerators or powerful lasers. Storing antiparticles for long enough to study them remains a challenge — normal matter based containers can’t be used due to the problems of annihilation, so physicists have developed methods of trapping the particles in magnetic fields. Even so, these antiparticles are still hot and energetic, and scientists are actively looking for methods of cooling them down to temperatures where more complicated physics can occur.

If scientists do manage to cool antiparticles down to close to room temperature then we should see antiprotons and positrons start to form anti-atoms, opening the way for scientists to start probing the world of anti-chemistry. Theoretically, thanks to the symmetry of matter and antimatter, anti-chemistry should be similar to the familar chemistry that rules our world. Some work has been done in this direction, with anti-atoms of the simplest element, hydrogen, already being produced in labs.

Bursts of energy recorded as antihydrogen atoms annihilate upon contact with matter. Credit to CERN.
Bursts of energy recorded as antihydrogen atoms annihilate upon contact with matter. Credit to CERN.

Soon after the discovery of the positron scientists realised that the symmetry of matter and antimatter, and the rarity of antimatter in the known universe, raised a problem. If matter and antimatter are symmetrical, then equal amounts of matter and antimatter should have been produced in the big bang. Since matter so clearly dominates the world today, what happened to all the antimatter?

One obvious early solution to this problem was the suggestion that some regions of the universe are made out of matter, and others are made out of antimatter. Perhaps there are entire galaxies in distant regions of the universe that are entirely composed of anti-stars. Although this was an attractive idea, there is one big flaw with it. If it were true, there should be regions in the universe where matter and antimatter border. Since the two types of particle annihilate, we should see huge amounts of energy coming from these boundaries. As no sign of this has been seen, the assumption has to be that the entire universe is dominated by matter.

Perhaps instead the symmetry between matter and antimatter is not completely perfect, and some process in the early universe favoured the creation of matter over antimatter? Physicists tend to like symmetry in theories, and questions like this were historically considered only with great reluctance. Nevertheless, throughout the second half of the twentieth century many examples were found of broken symmetry in physical theories.

Many of these examples come from the weak force, one of the two fundamental forces that are only relevant in the subatomic world (the third relevant force is electromagnetism, but this also has effects that extend into the larger scale world we are more familar with). In 1964 an example of symmetry breaking was found in particles called Kaons, an unusual particle that can transform into its own antiparticle. What was strange was that this process happens faster in one direction than the other — while the Kaon transforms into the anti-Kaon at one rate, the anti-Kaon transforms into the Kaon at another.

Since that intial discovery of a violation of symmetry between matter and antimatter a handful of other particles have been shown to break symmetry. So far, however, theory indicates that these symmetry violations are not enough to explain the amount of matter we see in the universe, and further experimental evidence is required to settle the issue. Until that evidence is found, the mystery of why the universe is dominated by matter, and not antimatter, will persist.

All Rights Reserved for William A. Isaacs

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