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?
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