Although it sounds exotic, antimatter would look no different to matter if you came across a lump of it. Even individual atoms of matter and antimatter would be indistinguishable. It’s only inside the atoms that their true nature is evident.
Inside atoms of matter – the stuff that makes everything – are electrons whirling around a central nucleus. An atom of the simplest element, hydrogen, consists of a single electron and a nucleus made of a single proton. The electron carries negative electric charge while the proton is positive. Opposite charges attract, keeping the atom together.
An atom of antihydrogen is the same but the electric charges are reversed. A central, negatively charged ‘antiproton’ grips a positively charged ‘antielectron’, also known as a ‘positron’. Positive and negative attract just the same, so the electric and magnetic forces that build atoms into molecules, and therefore matter, should apply to antiatoms too.
When a particle meets its antiparticle twin, they mutually annihilate in a flash of energy. This annihilation isn’t just the stuff of science fiction. Some radioactive substances emit positrons naturally. In fact, the annihilation of positrons with electrons has been used in medical diagnosis for decades in the form of the PET (Positron Emission Tomography) scanners found in hospitals.
But why is there matter in the Universe, rather than nothing at all, when the laws of physics imply that the energy of the Big Bang should have congealed equally into matter and antimatter? They should have annihilated each other.
But was the theory correct? Well, it was put to the test in the 1990s by annihilating electrons and positrons in a particle accelerator. Accelerated to nearly the speed of light, they were collided head-on. The resulting flash of energy, in an area smaller than the size of a single nucleus, was akin to the conditions in the Universe just moments after its birth.
By recording the results of these ‘mini-Bangs’, the experiments confirmed that energy can change into counterbalanced particles and antiparticles. It reinforced the idea that matter and antimatter emerged in perfect balance. So where was the missing antimatter?
Solving this mystery requires antimatter atoms to study. If a positron happens to be caught by the electric forces of an antiproton, you have an atom of antihydrogen. This has no net electric charge, but it will respond to magnetic fields. But how do you keep a substance that destroys anything it touches?
First, you need a very good vacuum so that the antimatter doesn’t inadvertently bump into a stray atom in the air. Then you need to keep it away from the sides of your container as these are made of matter too. The solution is a ‘magnetic bottle’ that uses electric and magnetic fields to imprison the antimatter.
To study antihydrogen, however, you first need to make and store lots of atoms. The challenge is getting a positron and an antiproton near enough to each other that their electrical attraction has a chance to ensnare them and form an atom of antihydrogen before they’ve annihilated with ordinary matter.
This has been done at CERN, the European centre for nuclear research, by slowing the antiprotons in a machine called the AD (Antiproton Decelerator). Electric and magnetic forces then gather them together with positrons. Since 2009, ALPHA has trapped atoms in a magnetic bottle on a few hundred occasions.
In 2011, the ALPHA experiment at CERN managed to make atoms of antihydrogen, the antimatter equivalent of hydrogen, and store them for nearly 17 minutes. The following year, scientists changed the magnetic orientation of antiatoms by shining microwaves on them. It showed that more detailed measurement of their properties was possible.
In January 2014, scientists at CERN created a beam of antihydrogen and detected 80 antimatter atoms in the beam. It’s a step closer to unlocking the mysteries of antimatter, since large numbers of antihydrogen atoms are required for gathering sufficient data to answer the big questions.
What scientists will study is the atomic spectra – a pattern of coloured lines resembling a barcode. The behaviour of the positron in an atom of antihydrogen is predicted to be exactly the same as the electron in hydrogen, so their atomic ‘barcodes’ should be identical.
If there’s any difference in their atomic ‘barcodes’, we’ll have found a difference between matter and antimatter, although scientists will still have to figure out what it means. When they do, we may be closer to solving the enigma of the missing antimatter, and the question of why there’s something rather than nothing.
So far, scientists at CERN have managed to store a few hundred antimatter atoms. If they could make more, the possibilities are profound. Just one gramme of antimatter could be used to power a spacecraft all the way to Mars, or create a bomb the equivalent of the warhead dropped on Hiroshima.
Science may prevent such applications, however. Using current technology it would take 10 billion years to make a gramme, a billion bottles to store it and require at least as much energy as you’d get back. Perhaps the world is better off with a little antimatter safely stored.