Our galaxy’s antimatter fountain

We’ve long ago learned how to create antimatter positrons, which have moved from the sci-fi world to the high-tech marketplace. Positrons can create exquisite non-invasive images of the body: They’re the “P” in medical PET scans.

We’ve long ago learned how to create antimatter positrons, which have moved from the sci-fi world to the high-tech marketplace. Positrons can create exquisite non-invasive images of the body: They’re the “P” in medical PET scans.

In 1928, the shy, brilliant physicist Paul Dirac predicted the existence of antimatter. When it was actually discovered seven years later, Dirac should have become a household name. But his yearning to avoid publicity – he almost turned down the Nobel Prize – discouraged media attention, and he’s known today only among science geeks.

Antimatter has the same appearance and behavior as ordinary matter. An antimatter sun would look just like a normal one, and even spectroscopic analysis couldn’t tell them apart. But let an antimatter object touch anything made of conventional matter, and both vanish in a violent flash.

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Unlike most exotic particles and objects that live in the “weird section” of the modern cosmological zoo, antimatter is stone-simple to understand: It’s merely ordinary matter with all electrical charges reversed. An anti-atom’s nucleus is negative instead of positive. And it’s orbited by positive instead of negative electrons that are therefore called positrons.

We’ve long ago learned how to create antimatter positrons, which have moved from the sci-fi world to the high-tech marketplace. Positrons can create exquisite non-invasive images of the body: They’re the “P” in medical PET scans.

Just because they’re logically simple doesn’t rob them of mystery. Every version of the Big Bang Theory says that equal amounts of matter and antimatter should have been created 13.7 billion years ago. Yet somehow we find ourselves in a matter-dominated universe. What happened to all the potential anti-planets, anti-oceans and antipasto? Currently the best explanation is that, contrary to long-held theory that says nature has no preference for one over the other, a tiny bias in particle/antiparticle events has been observed, and matter seems to be slightly favored.

A good thing, too. A universe with much antimatter would be a dangerous place. No explosion is more powerful than when matter and antimatter meet. It’s a 100-percent E=mc² conversion of the masses of both objects. If a one-gram pencil eraser touched an anti-eraser, it would release two trillion trillion ergs of energy: enough to light every bulb in the US for ten days. Ounce for ounce, antimatter is 143 times more energy-potent than the Sun’s fusion furnace or an exploding H-bomb.

Although a peek into the night sky offers no way to know whether a particular star is made of matter or antimatter, there are good reasons to believe that ours is a matter neighborhood in a matter galaxy. The explosive contact between matter and antimatter produces gamma rays with a distinctive energy signature of 511,000 electron volts. Thus, if any antimatter fringe material contacted ordinary particles, they’d produce unique energy haloes, and these were not observed – at least, not until 1997. That’s when the Compton Gamma Ray observatory discovered positronic geysers right here in our own galaxy.

This frenzied energy from the Milky Way’s center extends for 3,500 light-years. The violent gamma-ray emissions are definitely caused by matter/antimatter collisions. But what could be the source of these positrons?

Whatever it is, it’s doing an efficient job, because ten million trillion trillion trillion positrons are encountering matter and being converted to energy each second. In the Milky Way’s central bulge, about 200 billion tons of positrons were annihilated in the time it took to read this sentence. In that time interval, some 500 trillion tons of matter are involved in the process, day in and day out. Where is this antimatter factory?

What could possibly be responsible? There is no source of activity visible to our telescopes or detectors. Since positrons in the vacuum of space live for millions of years, they might be coming from an old supernova, or lots of them – maybe from a long-ago era when our bulge was continually exploding like fireworks. Or perhaps it originates from some kind of black-hole antimatter production line. Other “usual suspects” that have been searched for and eliminated include pulsars, quasars and cannibalized satellite galaxies. In each case, there seems no way for so many positrons to get created, let alone be transported so far from their birthplace.

The problem is not merely one of genesis, but how 15 billion tons of positrons per second are being hurled like water spray thousands of light-years above the galactic plane. The most intriguing idea concocted by theorists (some would say more like a wishful hope) is that dark matter, whose nature is still mysterious, might be generating the antimatter through some as-yet-unknown interaction.

Translation: No one has a clue. Too bad Paul Dirac’s not here to enjoy it…shyly.

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