Thursday, November 17, 2011

Antimatter of fact

Readers who were paying attention in their maths classes may recall that quadratic equations often have two solutions, one positive and one negative. 
So when, in 1928, a British physicist called Paul Dirac solved such an equation relating to the electron, the fact that one answer described the opposite of that particle might have been brushed aside as a curiosity. 
But it wasn’t. Instead, Dirac interpreted it as antimatter—and, four years later, it turned up in a real experiment.
Since then antimatter—first, anti-electrons, known as positrons, and then antiversions of all other particles of matter—has become a staple of both real science and the fictional sort. 
What has not been available for study until recently, however, is entire anti-atoms. 
A handful have been made in various laboratories, and even held on to for a few seconds. 
But none has hung around long enough to be examined in detail because, famously, antimatter and matter annihilate each other on contact. 
But that has now changed, with the preservation of several hundred such atoms for several minutes by Jeffrey Hangst and his colleagues at CERN, the main European particle-physics laboratory near Geneva.
The reason this is important is that Dirac’s equation is misleading. 
Antimatter cannot be the perfect opposite of matter, otherwise neither would exist at all. 
If they truly were perfect opposites, equal amounts of the two would have been made in the Big Bang, and they would have annihilated each other long since, leaving only light and other forms of electromagnetic radiation to fill the universe. 
That galaxies, stars and planets—and physicists to ponder such things—exist therefore means there is a subtle asymmetry between matter and antimatter, and that nature somehow favours the former. 
Two such asymmetries have indeed been found. 
But neither is big enough to explain why so much matter has survived. 
Being able to look at entire anti-atoms might give some further clue.
Last November the ALPHA collaboration at CERN, which Dr Hangst leads, managed to put positrons into orbit around 38 antiprotons—thus creating anti-hydrogen atoms—and then held on to them in a magnetic trap for a few tenths of a second. 
Now, as they report in Nature Physics, the researchers have used their device to preserve anti-hydrogen for 16 minutes (aeons in atomic-physics terms). 
This gives the anti-atoms plenty of time to settle into their ground state, the most stable condition a particle or atom can attain. 
As a result, Dr Hangst and his colleagues can look in a leisurely manner for novel ways that antimatter might differ from the common-or-garden variety.
Their first experiment will involve nudging the trapped anti-atoms with microwaves. 
If the frequency of these microwaves is just right, they will flip an anti-atom’s spin. 
That reverses the polarity of the atom’s magnetic field and ejects it from the trap.
The frequency needed to do this can then be compared with that which flips the spin of an ordinary hydrogen atom.
If the two turn out to be different, it will point towards the nature of the mysterious cosmic asymmetry
Besides being of huge interest (it would, after all, be a legitimate answer to the question “why are we here?”), such a result would also have a pleasing symmetry of its own. 
The original discovery of antimatter was a nice example of theory predicting an undiscovered fact. 
This would be a fact that repaid the compliment by predicting an undiscovered theory.

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