Tuesday, June 7, 2011

Antimatter matters ... the hole truth(?)

Follow-up to my January post Antimatter matters ... the talented folks at CERN, who previously brought us the Large Hardon Collider, have learned to store antimatter effectively indefinitely.

(What is antimatter? Take two particles of identical mass and equal but opposite electrical charge. By convention the rarer type of the mirror-image pair is dubbed the antiparticle. When a fundamental particle, like an electron, meets its antiparticle, a positron, the pair transforms into electromagnetic energy. Proton/antiproton encounters also transform into energy -- but since protons and antiprotons are not fundamental particles [they're composed of quarks and antiquarks, which are], the transformation is a multistep process. The bottom line: you can't store antimatter in a regular-matter box.)

Back to the news ... "Ephemeral Antimatter Trapped for Amazingly Long 16 Minutes," reports Livescience.com. What makes this story newsworthy is that the antimatter being stored is atoms of antihydrogen. Positrons and antiprotons are electrically charged; suspending them in a vacuum -- so that they don't encounter any normal matter -- is no different in principle than storing electrons or protons in a vacuum. That is a trick routinely managed in particle accelerators worldwide, using magnets to interact with the particles' electrical charges.

Combine a positron with an antiproton (or an electron with a proton) into a simple atom and the electrical charges offset each other. A storage container for antihydrogen must interact with something other than electrical charge, because the atom, being neutral, doesn't have any overall electrical charge.

Interact with what, then? Tiny spinning electrical charges -- and they don't come tinier than charged subatomic particles -- generate tiny magnetic fields. The two spinning particles that comprise an atom of hydrogen or antihydrogen are slightly separated, and that separation creates a magnetic dipole. The CERN antimatter trap interacts magnetically with the trapped antihydrogen atoms.

Per my previous antimatter post, creating and capturing even a single antimatter particle takes some doing (and beaucoup money). Hence, CERN is dealing with only a few antihydrogen atoms at a time. Less science-y articles -- like this, from a USA Today writer -- alas likened the latest achievement to the Dan Brown book Angels and Demons, a present-day novel that employs antimatter in bomb-equivalent quantities.  A wildly exaggerated production level for antimatter is not too egregious as literary license. This is: characters in Brown's novel imbued the antimatter with all manner of theological significance. It was pure (with extreme euphemism) poppycock.

 (I set my antimatter-intensive novel, InterstellarNet: New Order, well into the next century. I used magnetic confinement, as CERN ended up using, with new wrinkles I think will come along given a century of R&D advances. With much better justification, I: NO has more than enough antimatter for some really big explosions ...)

The big mystery about antimatter is why the Big Bang didn't create antimatter and familiar matter in equal amounts. If both forms of matter had been created in equal amounts, soon enough they would have obliterated one another. Happily, the universe did not turn out that way, which we know because ... we're here  to puzzle about it.

Or maybe antimatter is an illusion.

Paul Dirac, one of the visionaries in the early development of quantum mechanics, extended QM to incorporate special relativity. The extended theory is called quantum electrodynamics (QED -- gotta love the name). One of QED's predictions was the existence of antiparticles. In short: Dirac was a very bright fellow entitled to comment about the nature of antimatter.

He suggested that antiparticles are virtual. Suppose that perfectly ordinary electrons and protons (and other particles) have completely filled the lowest energy states in the universe: like water seeking its own level. Call those particle-filled states the "Dirac Sea." What we experience as the universe is, effectively, made from the higher-energy particles above the sea. Islands, as it were ...

Occasionally an energetic event -- say, a proton-proton collision at CERN -- knocks, say, an electron from its invisible-to-us-in-the-normal-course-of-affairs low-energy state within the Dirac Sea. What our instruments report as an electron/positron pair created out of the energy of the collision are (a) the knocked-loose electron and (b) the absence of that electron in the normally unseen low-energy state. A bit of froth in and above the sea ...

The absence of an electron from the sea exposes the opposite charge of some proton in the sea. The apparent movement of the positron is the movement of electrons as the vacancy moves (thus successively uncovering the positive charge of one hidden proton after the next). The eventual apparent recombination of electron and positron is merely an electron reoccupying the vacancy in the sea. Like any particle dropping from a higher energy level to a lower one, the electron emits a photon to carry off the difference in energy. That photon is what we conventionally describe as the energy released by particle/antiparticle mutual destruction.

If positively charged virtual particles rings a bell ... they should.  Electronics engineers deal all the time in electrons and "holes." Holes are the positive charge carriers -- quite virtual -- that move through a chunk of semiconductor. Holes are mobile "missing electrons." (Skeptical? The computer, iPad, or smartphone on which you're reading these words is packed with transistors designed on this principle.)

Suppose there is a Dirac Sea. Why don't we see it, only disruptions of it? I don't know. Of course we also don't see dark matter (unknown stuff inferred from the motion of stars in galaxies and galaxies in clusters) or dark energy (unknown stuff inferred from the accelerating expansion of the universe). I find the concept of a Dirac Sea less mysterious than the other two phenomena.

Hmm ... could the unseen particles of the invisible Dirac Sea be dark matter? The math to assess the possibility is beyond me, but on a purely Occam's Razor basis, I like it.

Thought-provoking stuff, this antimatter ...

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