Recently we travelled to The Dark Side to learn about mysterious dark matter and dark energy, today we turn to another curious substance in our universe – antimatter. Antimatter’s father is widely regarded as Paul Dirac, who in 1928 predicted the possibility of anti-electrons. Dirac was working on an equation that would combine quantum theory and special relativity – a theory to describe the behaviour of an electron (a quantum particle) moving at a speed close to the speed of light. This equation was the relativistic version of Schrodinger’s wave equation and it had two solutions. Just as the square root of a number can have two solutions, a positive and a negative (square root of 9 is 3 and -3), the solution to Dirac’s equation has one for an electron with positive charge and one for an electron with negative charge. From the theory Dirac then made a bold move and postulated that for every particle there is exists a corresponding antiparticle with an opposite charge to its partner. The existence of Dirac’s antiparticles would indeed be verified at particle accelerators in years to come.
Particles have various quantum numbers that characterise their nature. The three that shall concern us today will be charge, baryon number and lepton number. Time for some terminology and book-keeping. Baryons are quark-based particles and are susceptible to the strong force, protons and neutrons are therefore baryons because they are made of three quarks. Baryons, funnily enough, have a baryon number of +1. Other particles which are not susceptible to the strong force and are not quark based are known as leptons, things like electrons and neutrons. These leptons have a lepton number of +1. Bit silly doesn’t it seem giving baryons and leptons a corresponding number of +1, what else would they have? Well that’s the whole point of the anti-particles, they have the number with an opposite sign! So an anti-proton has a baryon number of -1, an anti-neutron has a baryon number of -1 and an anti-electron, properly known as a positron, has a lepton number of -1. Finally any lepton has a baryon number of 0 and visa versa. Then of course, last but not least, we have charge. If a proton has a positive charge of +1 the anti-proton has a negative charge of -1. All of this was theorised and the experimental results at particle accelerator sites went on to confirm the existence of such entities matching the descriptions.
Collisions between particles and antiparticles lead to annihilations and give off energy (in the form of photons) proportional to the total mass of the particles in accordance with Einstein’s equation E=mc^2. Antiparticles are also created in regions of the universe where high-energy particle collisions take place, such as high-energy cosmic rays colliding with the Earth’s atmosphere. This antimatter can then be detected in the traces of its products after it has annihilated with matter – for example gamma rays which are themselves photons of the highest observed range of photon energy.
Now looking around us at everyday objects, the Earth, planets in the solar system, stars… why is it that matter seems to vastly outweigh antimatter? Since, all known processes for creating massive particles create both in equal quantities. The asymmetry of matter and antimatter in the universe is another great mystery but we should be very thankful for it. If an equal amount had existed shortly after the Big Bang all matter would have annihilated with its antimatter counterparts leaving nothing but energy behind. In fact it is estimated that in the early universe there was only one extra matter particle for every billion-matter antimatter pairs! Whatever hypothetical process that took place in the early universe that caused the matter-antimatter asymmetry is termed, by cosmologists, Baryogenesis (confusingly not solely to do with baryons). The difference in the abundance of matter and antimatter in the early universe left behind the matter remnants we see today, which allowed the formation of the universe as we know it… thank you Baryogenesis.
Anti-matter is a classic go-to weapon of science fiction stories due to its annihilating power and high energy potential. Recent examples include Angels & Demons where Professor Langdon tries to save Vatican City from an antimatter bomb and Star Trek’s starship the Enterprise which uses the energy released from matter-antimatter annihilation as propulsion to achieve faster-than-light travel. Though the substance anti-matter exists in the real world the feasibility of harnessing its powers is best left to the realm of sci-fi. Think about, in order to harness its power and manipulate it as a weapon, we would have to, as a first step, be able to store it. By what means could we achieve this?! Any container we put it in, it would annihilate with the matter that constitutes the walls of the container itself! Antimatter is a tricksy beast, storing it would be like trying to grasp a fistful of air.
Fun facts to round off, antimatter is the most expensive stuff on the planet, weighing in at a whopping $62.5 trillion a gram! However as obvious from the storage problem, a gram has not been produced. The most that has been created and stored at a time is about a billion antiprotons, but that’s only a meek one millionth of a billionth of a gram. If you fancy getting up close with some antimatter however, the answer is most likely sitting in your kitchen. Go grab a banana! Bananas contain potassium-40 which as it decays, gives off a positron (an “anti-electron”) at a rate of about one every 75 minutes. Too bad it collides with the nearest electron pretty much instantly…