What's the Matter with Antimatter?

May 29, 2000 -- 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 NASA 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.

Right: An artist's concept of a robotic antimatter-powered probe sailing past planets in an imaginary nearby solar system. Credit: Laboratory for Energetic Particle Science at Pennsylvania State University.

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.

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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.

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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 traveling at nearly the speed of light and don't last too long (40 nanoseconds is typical).

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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.

see caption Right: A Penning trap is tested at Penn State University. 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. Credit: Laboratory for Energetic Particle Science at Pennsylvania State University.

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 favor 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 Center, 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.

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.

Above: Astronomers have discovered evidence for antimatter near the center of our Milky Way galaxy by observing photons with an energy of 511 keV -- the energy created when a positron and an electron collide and annihilate. This image shows contours of 511 keV radiation detected by NASA's Compton Gamma Ray Observatory overlaid on an optical picture of the Galactic center. The vertical structure is a jet of mutually-annihilating electrons and positrons. [more information from Northwestern University and NASA HQ] Image Credit: Ron Murphy (Naval Research Laboratory) .

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 numbers at a few of the world's largest particle accelerators. The current worldwide production rate of antimatter is on the order of 1 to 10 nanograms (billionths of a gram!) per year.

see ca[topm Right: This artist's concept of an antimatter-powered rocket ship looks like a big space-borne linear accelerator. Credit: Laboratory for Energetic Particle Science at Pennsylvania State University.

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.

In Part 2 of this story, coming soon, we'll explain why some rocket scientists think that antimatter is the ultimate fuel and why nature may not cooperate!

What is matter? Our ideas about matter have changed drastically in the past 2500 years!

600 BC - Thales of Miletus noticed that rubbing amber with fur caused it to attract small bits of hair and other light objects. He suggested that a mysterious force came from the amber.

460 BC - Democritus, from Greece, developed the concept of dividing matter into smaller and smaller pieces until you could divide it no more. He called these smallest pieces atoms. Aristotle considered the idea of atoms to be worthless.

1687 AD - Sir Isaac Newton used arguments based upon the theory of atoms to explain the gas laws.

1803 - John Dalton, English chemist, formulated his theory that all matter consists of atoms, that chemical reactions result from the union and separation of these atoms and that atoms have characteristic properties.

1897 - English physicist, J.J. Thompson discovered the electron and proposed a model of matter similar to "raisins in a pudding."

graphic showing production of particles and anti-particles 1911 - Ernest Rutherford bombarded gold foil with helium nuclei (alpha particles) and noticed that most go through the gold unchanged. But some deflect into all directions and some even bounce back in the direction they came. He concluded that matter is mostly empty space and consists of a small positively charged center surrounded by negatively charged particles.

1912 - Neils Bohr suggested that the electrons orbiting the nucleus of atoms can only have certain discrete energies and that each element had different electron energies.

1924 - Louis deBroglie, from France, suggested that matter, like light, consisted of waves not particles.

1925 - Austrian physicist Erwin Schrodinger formed a model of a complete atom as interacting waves. The particles became like vibrations on a violin string, only they were closed in circles.

Shrodinger's Cat (icon) 1926 - German physicist Werner Heisenberg formulated his "uncertainty principle" which says that you cannot know the position and momentum of a particle simultaneously. The better you know one, the worse you know the other. Atoms were now visualized as a nucleus surrounded by a cloud of electron waves.

1960 - Murray Gel-Mann, American physicist, proposed that protons and other basic particles in the atom consist of even more fundamental particles he called quarks.

The art in this sidebar is by Duane Hilton.

Web Links

Scientists consider ways to give an extra kick to spaceships- an overview of antimatter and space transportation from highway2space.com

Reaching for the stars - Scientists examine using antimatter and fusion to propel future spacecraft. A Science@NASA story from April 12, 1999.

ANTIMATTER CLOUDS AND FOUNTAIN DISCOVERED IN THE MILKY WAY -- 1997 NASA Press Release

What's the Matter with 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!

What's the Matter with
Antimatter?