Wednesday, April 8, 2015

Making and storing anti-matter

Fictitious antimatter trap from movie Angels & Demons
If we really wish to fathom the mysteries of antimatter, we must first get to grips with the stuff itself. Easier said than done. How on earth do you pin down a substance that vanishes the moment it touches anything?

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.

Sure, we're a long way away from making half-a-million pounds of it, but this is the lightest, most stable form of antimatter we can make.

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.

Two CERN experiments, ATRAP and ALPHA, are grappling with that question. Their aim is to make antihydrogen - the simplest anti-atom possible, just an antiproton and a positron bound together - in sufficient quantity and for long enough to compare the spectrum of light it emits with that of regular hydrogen. Even the slightest difference between the two would shake up the standard model.

The experiments require a near-perfect vacuum, as encountering a mere atom of air would spell the end for any antiparticle, and there must be some way of trapping the antiparticles: not in a conventional container, but using electric and magnetic fields.

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.

Antimatter rocket
ATRAP and ATHENA, ALPHA's forerunner, did successfully isolate antihydrogen in 2002, bringing together antiprotons from a particle accelerator and positrons from a sodium radioactive source in a magnetic trap. Unfortunately, such success is fleeting: magnetic traps work just fine for charged particles such as antiprotons and positrons, but antihydrogen is neutral, so it can slip right through the containing field lines.

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.

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