Download A Beginner`s Guide to Antimatter

A Beginner's Guide to Antimatter
It may be the ultimate fuel for space travel, but right now antimatter is fleeting, difficult to work
with, and measured in atoms not pounds!
by Ron Koczor
What do you think of when you hear the word "antimatter?" Something exotic, something unreal?
Something about your Chief Engineer not being able to keep the containment fields up during battle?
Well, to a few scientists and university researchers, antimatter may just be the future of human space
travel. When it comes to packing a punch, antimatter/matter reactions can't be beat. When a particle and
its antiparticle meet, they annihilate each other and their entire mass is converted into pure energy.
Many physics textbooks describe matter as something "that takes up space and has mass." Every
physical object that you've ever seen consists of matter. So if everything you know is made of matter,
then what's antimatter? Let's go back to the 1930s to find an answer.
In 1928, the British physicist Paul A.M. Dirac (1902-1984) formulated a theory for the motion of electrons
in electric and magnetic fields. Such theories had been formulated before, but what was unique about
Dirac's was that his included the effects of Einstein's Special Theory of Relativity. Dirac's equations
worked exceptionally well, describing many attributes of electron motion that previous equations could
not.
But his theory also led to a surprising prediction that the electron must have an "antiparticle," having the
same mass but a positive electrical charge (the opposite of a normal electron's negative charge). In 1932
Carl Anderson observed this new particle experimentally and it was named the "positron." This was the
first known example of antimatter. In 1955 the antiproton was produced at the Berkeley Bevatron, and in
1995 scientists created the first anti-hydrogen atom at the CERN research facility in Europe by combining
the anti-proton with a positron (the normal hydrogen atom consists of one proton and one electron). But
when these antihydrogen atoms are produced, they are travelling at nearly the speed of light and don't
last too long (40 nanoseconds is typical).
Dirac's equations predicted that all of the fundamental particles in nature must have a corresponding
"antiparticle." In each case, the masses of the particle and antiparticle are identical, and other properties
are nearly identical. But in all cases, the mathematical signs of some property are reversed. Antiprotons,
for example, have the same mass as a proton but the opposite electric charge. Since Dirac's time, scores
of these particle-antiparticle pairings have been observed. Even particles that have no electrical charge,
such as the neutron, have antiparticles. These have other properties with a sign (such as magnetic
moment) that can be reversed.
Interestingly, there is no real difference between particles and antiparticles in particle physics theories.
They are equivalent. Most theoreticians believe that at the time of the Big Bang antiparticles and particles
were created in almost equal numbers. But why, then, is antimatter so rare today?
The tentative answer (and it is tentative, since this question is a topic of on-going research) is in the word
almost. Present theory suggests that if particles outnumbered antiparticles in the Big Bang by as little as
one part in 100 million, then the present universe could be explained by those extra particles that were
not annihilated by an antiparticle counterpart. Other theories suggest that even if identical amounts of
antimatter and matter were created in the Big Bang, the physics of antimatter and matter are slightly
different. This hypothesized difference would favour residual matter after all original antimatter had been
annihilated.
So that's what antimatter is. Are we sure that there is no antimatter left in the
universe?
Dr. Charles Meegan, an astrophysicist at the Marshall Space Flight Centre,
noted that orbiting gamma-ray observatories have measured the sky in the
range of energies that would have detected the telltale signature of antimatter
annihilation.
"None of the instruments flown to date have uncovered evidence for vast
amounts of antimatter in the universe," says Meegan.
Lab. for Energetic
Particle Science
A Penning Trap
Penning traps use a
combination of low
temperatures and
electromagnetic
fields to store
antimatter. While
the traps can only
store incredibly
small quantities, the
traps will help in
developing the
technologies
needed for
advanced
propulsion
concepts.
There is evidence that very energetic reactions are taking place in isolated
spots -- in the cores of some galaxies and quasars, for example -- that create
antimatter which then annihilates. But this is not thought to be residual
antimatter left over from the Big Bang.
On Earth all antimatter that exists is counted
in individual atoms. Low energy positrons
are routinely used in a medical imaging
technique called Positron Emission
Tomography as well as studies of important
materials used in electronics circuits. These
positrons are the result of the natural decay
of radioactive isotopes. While useful in
medical and materials research
applications, there are not enough of these
anti-electrons to provide a useful form of
rocket fuel. High-energy antimatter particles
are only produced in relatively large
Laboratory for Energetic Particle
numbers at a few of the world's largest
Science
particle accelerators. The current worldwide
production rate of antimatter is on the order
This artist's concept of an
of 1 to 10 nanograms (billionths of a gram!)
antimatter-powered rocket ship
per year.
looks like a big space-borne
linear accelerator.
How can antimatter help human exploration
of space? The answer lies in Einstein's
famous equation E=mc2. When antimatter annihilates normal matter,
all the mass is converted to energy. The energy output per unit particle
vastly exceeds the efficiency of chemical reactions such as burning hydrogen and oxygen in the Space
Shuttle main engines. So, could an antimatter future lie ahead for space travel?...