Weighing antimatter

In Science, vox populi on 30 July, 2011 at 3:51 pm

Science fiction creators love antimatter. Nature, not so much. Hoping to shed some light on just why nature prefer’s matter over antimatter, scientists have weighed, or rather measured the mass of the anitproton, to an unprecedented level of accuracy — roughly one part per billion, according to a new paper published in the journal Nature.

“Imagine measuring the weight of the Eiffel tower, ” said Masaki Hori, a project leader in the Japanese-European ASACUSA (Atomic Spectroscopy And Collisions Using Slow Antiprotons) experiment at CERN which conducted the measurements. “The accuracy we’ve achieved here is roughly equivalent to making that measurement to within less that the weight of a sparrow perched on the top. Next time it will be a feather.”

Protons constitute half the world around us. That includes half the human body. This abundance of protons leads to the logical conclusion that the mass of a proton should be measurable to a greater accuracy than that of antiprotons. Naturally occurring protons have primarily been found in cosmic rays. Although they are stable particles, they do not exist for long because any collision with the abundant proton particles annihilates both particles is a burst of energy and secondary particles, which is probably why it is such a popular source of energy in sci-fi space travel.

“Antimatter has tremendous energy density,” Dr. Georges Schmidt told a far out propulsion conference in 1999, noting that matter-antimatter annihilation release more energy per unit mass of any reaction known to physics at that time.

To measure the mass of antiprotons, scientists first capture them inside helium atoms where they can be “tickled” using a laser beam. The laser frequency is adjusted until it causes the antiprotons to make a quantum jump within the atoms. The mass of the antiproton particles can then be calculated based on the frequency of the laser that caused the jump.

This method of measuring mass is not perfect, however.

Much like people, atoms are not stationary. They jiggle around. Some moving towards the laser beam while others move away. When they move, they experience slightly different frequencies. Think of it like an approaching siren or train whistle. As the vehicle or train approaches you then passes you, the pitch of the siren of the whistle seem to change.

To counter this inherent imprecision, the ASACUSA scientist used two laser beams which allowed them to measure the mass of an antiproton with a very high degree of accuracy.

“This is a very satisfying result,” Hori stated. “It means that our measurement of the antiproton’s mass relative to the electron is now almost as accurate as that of the proton.”

If you’re wondering what’s so important about measuring the mass of a particle that isn’t very common in nature and is destroyed when it collides with matter?

For starters: any difference between the mass of protons and antiprotons would indicate that the laws of nature work differently for matter and antimatter. In other words, different masses would mean we need different physics, and that would turn not just human science on its ear but also science fiction.

This new, more precise measurement is just one of the antimatter advances that have been made this year. In June, another CERN experiment trapped antimatter atoms for more than 16 minutes.

“We can keep the antihydrogen atoms trapped for 1000 seconds,” explained Jeffery Hangst, spokesperson for the ALPHA experiment. “This is long enough to begin to study them — even with the small number that we can catch so far.”

New sources of antimatter, which scientists theorize would have been created in amounts equal to matter at the Big Bang, have also been discovered. Scientists have created antimatter using particle accelerators, however, in January 2011, researchers with NASA’s Fermi Gamma-ray Burst Monitor (GBM) team announced that beams of antimatter had been detected above thunderstorms.

So while we are not quite able to venture into space using science fiction’s antimatter drives, we are taking baby steps in that direction. And who’s to say others haven’t beaten us there? Perhaps that’s where all the antimatter is being collected.


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