What is antimatter ? Where does antimatter come from ? What role does antimatter play in our understanding of the universe ? How will antimatter serve mankind ? These are just some of the questions that this paper will address in hopes of providing the reader with an introduction to what I have always found to be a very difficult yet fascinating concept.
In choosing antimatter as the topic for this paper, I expected to be reading about the cosmos, of antimatter's natural place within the more familiar framework of solar systems and galaxies. It was quite a surprise to find instead that nearly all of the available literature on antimatter focused on the world of very specialized physicists, the particle accelerator. While I thought that perhaps this paper was not appropriate for an astronomy course, I remembered what Stephen Hawking tells us in A Brief History of Time, that scientists had to turn the "search for an understanding of the universe from the theory of the extraordinarily vast to the theory of the extraordinarily tiny"!

Matter


I will begin with a brief description of matter, something we all should be somewhat familiar with, as we and everything we touch is made of the stuff. As far back as 450 B.C., the Greek philosophers Democritus and Leucippus believed "that all of nature was made of tiny, imperceptible but indivisible particles which they called atoms". Our current understanding of matter is a direct result of the ongoing study of these particles, which we can now describe in far more detail than could the ancient philosophers. Just how do we describe these particles ? By looking at the characteristics of these elementary building blocks of matter, we can then describe antimatter in terms of these very same characteristics.
Matter as we see it is typically in molecular form, that is, we see matter as it exists in various combinations of molecules. When we see water, we actually see the combination of molecules of hydrogen and oxygen. Molecules are combinations of atoms, and as we learned in high school, atoms themselves can be described as combinations of electrons, protons, and neutrons. Electrons belong to a family of particles known as leptons, and science thus far considers these particles to be absolutely elementary, that is, they cannot be described as a combination of yet smaller pieces. Protons and neutrons on the other hand are made up of smaller pieces known as quarks which, like electrons, are viewed as elementary particles.
Some common characteristics which scientists then use to describe these elementary building blocks of matter are as follows...
Some less common characteristics of particles (color, charm, strangeness, parity) are more difficult to describe and do not help in explaining the nature of antimatter. As such, I will leave them as mysterious properties, something my research into this topic has led me to be more comfortable with. As we can describe matter as that which is formed by various combinations of elementary particles, we can then describe antimatter in terms of combinations of elementary antiparticles.

What Is Antimatter


Let's begin with the electron and the antielectron, also known as the positron. The electron is a particle with very small mass (~ 9 x 10-28 [JL1]gram, 1/1836 the mass of a proton), an electric charge of -1 (~ 1.6 x 10-19 coulomb), a spin motion, an thus a magnetic moment. An antielectron is exactly the same as an electron, except that it's electric charge is a +1 . (actually, the parity of the antielectron is also reversed, but it is beyond my limited understanding to say more).
A proton is the combination of three separate quarks, two up quarks and one down quark. An up quark has an electric charge of +2/3, and the down quark has an electric charge of -1/3, so the proton then has a resulting electric charge of +1 (2/3 + 2/3 - 1/3). An antiproton would then be the combination of 2 antiup quarks each having an electric charge of -2/3 and 1 antidown quark having a +1/3 charge. This combination would give the antiproton a charge of -1 (- 2/3 - 2/3 + 1/3) exactly the opposite of the proton. Again, the parity would be reversed.
A neutron is a combination of 2 down quarks and 1 up quark, thus accounting for its lack of electric charge (- 1/3 - 1/3 + 2/3 = 0). An antineutron then would be the combination of 2 antidown quarks and 1 antiup quark, again, having no net electric charge (+ 1/3 + 1/3 - 2/3 = 0). How then does one tell the difference between a neutron and an antineutron ? By the difference in parity. The reason for this is that the magnetic field produced by the rotation of the down quarks is stronger than the magnetic field produced by the up quark, resulting in a net magnetic moment of ~ -2/3 of the magnetic moment of a proton. An antineutron would then have the opposite magnetic moment (the antineutron's north magnetic pole would be in the location of the neutron's south magnetic pole and vice versa).
Having established an understanding of antimatter in terms of the elementary particles, it is easy to imagine that these would then combine to form antiatoms, which could then form antimolecules, etc. Where a hydrogen atom consists of a proton with an orbiting electron, antihydrogen would then be an antiproton with an orbiting antielectron.

Who Would Have Thunk It ?


While science fiction writers have long enjoyed antimatter as a springboard for the imagination, the very first clues to its existence came from a young physicist, Paul Dirac in December of 1929. In the early part of the century there were all kinds of new notions introduced to explain the physical forces of nature, including...
Schrodinger and Heisenberg had developed a quantum theory of physics which was complete except that it only dealt with particles moving at low velocities, and did not take into account a particle's spin. It was Paul Dirac who finally developed a set of equations which described the relativistic quantum behavior of a spinning electron. It was an odd result of these equations which led to Dirac's prediction of antimatter. According to Sir Isaac Newton's laws of physics, particles always had a positive value of energy. Dirac's equations led to solutions where particles could have either positive or negative energy. While this seemed contrary to common sense, Dirac could not ignore this part of his work, because his equations were successful in explaining experimental observations that otherwise had baffled scientists. Dirac proposed in a second paper (in 1931) that his equations showed that for each and every particle, there must exist an equal but opposite (in electric charge) mirror image. Paul Dirac was predicting antimatter. In 1933, Dirac & Schrodinger were awarded the Nobel Prize.

Antielectron Discovered in 1932.


In 1932, Carl Anderson was a young physicist working with Robert Millikan at California's Institute of Technology, studying cosmic rays (which were discovered in 1911 by Victor Hess). At the time, these cosmic rays were the highest energy source known, carrying energies up to 15 million electron volts (Mev). Cosmic rays are simply the nuclei of atoms traveling through space at nearly the speed of light. As most of these nuclei are those of hydrogen, they are simply protons. As these protons enter earth's atmosphere, they collide with ordinary atoms and break them apart into all sorts of subatomic particles. These particles collide with other atoms, and so on, and so on as they descend through the atmosphere, resulting in a cascade of particles by the time they reach earth's surface. It was these subatomic particles that Anderson wanted to study. To do so, he had built a cloud chamber to act as an early form of particle detector, a chamber of air saturated with water vapor and surrounded by a very strong magnetic field. As particles traveled through the chamber, the magnetic field would cause them to bend according to their charge and mass, and by photographing the trails these particles left the scientists were able to capture on film their journey through the chamber. Anderson was not aware of Dirac's predictions of antimatter, so when he found trails which were identical to those of electrons in exactly the opposite direction, he was quite baffled. He did claim to have discovered new particles resulting from cosmic rays, which were equal in mass to the electron with the exact but opposite electric charge. After a year of studying this phenomenon, colleagues connected his discovery to Paul Dirac's predictions. Antimatter had been discovered. In 1936, Anderson was awarded the Nobel Prize.

1932 - 1955, Hold Everything.


Following the discovery of the positron, one might expect that scientists could then quickly move on to discover the antiproton and antineutron. Actually, it was 23 years before the next antiparticle was discovered. To understand why, we need to take a quick look at Einstein's most famous equation, e=mc2. This equation defines a relationship between energy and mass, and as mass is one the characteristics that matter and antimatter share, this equation holds true for both. As Anderson's discovery showed, where there was enough energy, matter and antimatter could be created. The source of energy in those observations were the cosmic rays, with energies as high as 15 Mev. Because the proton and neutron had mass that was some 1836 times that of the electron, the energy-mass of these particles is much higher, ~ 938 Mev. The best way to turn energy into matter is to cause particles to collide at very high velocities, and smashing very low mass electrons would never create the energy needed. As such, in order to create such energies, two protons would have to be collided. Knowing the energy-mass of a proton, and understanding that an antiproton could only be created as part of a particle pair (a proton and an antiproton together), led physicists to estimate that a kinetic energy of nearly 6 billion electron volts would be necessary. In 1932, and not until 1954, there was no known source of such high energy.

Searching For The Antiproton


In 1954, a team of scientists at the University of California at Berkeley were working on a machine that could create the energies believed necessary to create an antiproton. This machine was known as the Bevatron, "Bev" representing billions of electron volts. By using electrical fields to begin a proton beam moving, and then sending in precisely tuned radio waves, energies of billions of electron volts became possible. The difficult part now would be detecting the antiproton, as it would be accompanied by a wide array of subatomic particles resulting from such high energy collisions.
Up until then, all physicists had to do to detect particles was to set up a stack of photographic plates behind the collision, and trace the paths that the particles would create as they came spraying out. The thickness of the path and the angle of curvature would identify the mass and charge of the particles. In order to be absolutely sure before announcing the discovery of the antiproton, the team looked for alternative methods. In the end, the team had built a maze that was so complex and precise in its design that only an antiproton could possibly make it all the way through. This provided the team with a fool proof method of detection, and their announcement of discovery would not be questioned. The maze itself was a fascinating piece of machinery. To be identified as an antiproton, a particle would have to...

It took the team 18 months to set up the maze and on October 4, 1955 they were ready. The Bevatron was started up, and within 15 minutes the detectors had registered that an antiproton had indeed been created and had passed though the maze exactly as they had expected. In 1959, two members of this team, Owen Chamberlain & Emilio Segre, were awarded the Nobel Prize.

Antineutrons


The discovery of antiprotons lead to the relatively quick discovery of antineutrons. The antineutron would have the same mass as the antiproton, so the energy needed was available in 1955. The difficult part would again be detecting these particles, and this would be even more difficult than for the antiproton since the neutron and antineutron would have no electrical charge. Nonetheless, as the neutron and antineutron would have opposite magnetic moments, a new team of physicists at Berkeley was able to detect the antineutron only 1 year after the antiproton's discovery.

Antimatter Here on Earth


Since the discovery of antiprotons, physicists have found them to be very useful particles for research purposes due to the fact that they have the opposite charge of protons. Until antiprotons were discovered, all particle collisions were based on smashing protons into a fixed target, typically a metal such as copper or tungsten. All of the initial energy for the collision had to come from accelerating the proton beam. Now that scientists have antiprotons in their bag of tricks, they can use them to create head on collisions with protons using the very same accelerators. The antiprotons, with their opposite electrical charge, circle within the accelerator in the opposite direction of the protons. This enables the physicists to stage collisions with far more energy than that of the proton hitting a fixed target.
When Paul Dirac predicted the existence of antimatter particles, he also predicted that an annihilation of matter and antimatter would occur should the two ever meet. In fact, this has been observed, when a particle meets its mirror image antiparticle, all of the mass is converted into energy. While this does lead to some exciting possibilities in using antimatter as an energy source, it also creates some basic problems, such as 'how do you store antimatter ?'. To solve this problem, scientists had to create ordinary matter storage devices which would not allow the antimatter within them to come into contact with the containers themselves, in other words the antiparticles would have to be 'suspended' in the container. Such devices have been developed, such as the Penning trap, which use combinations of electrical and magnetic fields to suspend the antiprotons or antielectrons. Another type of trap is the Paul trap, which can hold both positively and negatively charged antimatter simultaneously. This might lead to the combination of antiprotons and positrons to form antihydrogen.
Another problem with antiproton technology is simply the cost to produce antiprotons; current technology produces very few antiprotons at a very high cost. When a proton beam is collided with a fixed target, the most efficient colliders can only create 4 to 5 antiprotons for every 100 protons in the source beam. Due to difficulties in focusing the spray of particles from the collision, only 10% to 30% of antiprotons created can be captured. Of these antiprotons, perhaps only 1% can be collected into a steady antiproton beam which can then be cooled enough to start thinking about storing the antiprotons. It's not hard to see that an awful lot of matter and energy are required to create and collect very small amounts of antimatter.
Besides using antiprotons for high energy physics experiments, antimatter is also being used today in the field of medical technology. Antielectrons are used in PET (positron emission tomography) scanners, and antiprotons are also used similarly in APR (antiprotonic radiography) devices. Antimatter may also be used by doctors to attack tumors directly, or to cauterize internal lesions.
Getting back to 'the universe', antimatter may one day provide the answer to one of the biggest problems in space exploration, rocket fuel technology. Without too much background in rocket science, one can still quickly see why antimatter would be such an attractive material for a fuel. Propulsion is based on the Sir Isaac Newton's basic law that for every action there is an equal and opposite reaction. A rocket fuel's performance can be described by it's 'specific impulse', which is a measure of how much thrust force it can produce per pound of fuel per second. The way in which this is then translated into a rocket's capabilities (maximum velocity and distance traveled) has to do with the mass ratio of the rocket, its mass without fuel divided by its mass with fuel. Antimatter as a fuel is so much more efficient than today's fuels that only 10 milligrams of antimatter could produce the same propulsion as 200 metric tons of chemical fuel.

Research into the use of antimatter for rocket propulsion began in the early 1950's by a German scientist Eugene Sanger, and at that time the only antimatter to have been discovered was the positron. Sanger's thoughts were that the annihilation of electrons with positrons would produce gamma rays, and if only these could be directed out the rear of the rocket, they could provide thrust. Sanger never did find a material which would direct the gamma rays, the photons simply penetrated all known surfaces in whichever direction they might be moving. More recent understanding of the complex annihilation process of protons with antiprotons makes them far more suitable for use as a rocket fuel. In this annihilation, short-lived particles called pions are created, and as charged particles, these can be directed by strong magnetic fields. These particles could be used then to heat a liquid into a high pressure gas, which could then be used to propel the rocket.

Antimatter in The Cosmos


The laws governing nature as we know them seem to be full of symmetry and conservation. In every particle collision experiment here on earth, matter and antimatter are always produced simultaneously as pairs of particles. Because of this, one might expect that matter and antimatter would be created together. If this conversion of energy into mass is typical of all such conversions, then why do we see such a preponderance of matter in the universe ? Where is the antimatter ? Why don't we see antigalaxies made of antistars and antiplanets ? While experimentation and theory would suggest this type of a universe, observations do not.

Antimatter is found as cosmic rays come into contact with ordinary matter, resulting in the creation of electrons and positrons, as was seen in the early 1930's. These antiparticles are insignificant though when compared to the amount of matter we see. By sending balloons and eventually satellites into space, equipped with very sensitive particle detectors, scientists have found a source of antimatter known as positronium (an electron circling a positron). This is seen by way of the 'signature' event of the eventual annihilation of both particles into pure energy, in this case 2 gamma rays with 511 Kev of energy. The source of this positronium is located near the center of our galaxy, and is believed to be either the remnants of a supernova explosion, or possibly a cloud of gas at the edge (event horizon) of a black hole. In any case, what is odd about it is that it is still there to be detected (so there must be a very large amount of raw energy as the source), and that it varies in brightness much like a variable star. What is puzzling about it is that we don't see such events scattered throughout the universe in the same way we see matter.

Conclusion

Research in astronomy that leads to a better understanding of just what happened during the big bang may lead to the answers to these questions. Until then, the greatest mystery regarding antimatter will not focus on it's nature (which is already very well understood), but instead it will focus on what exactly led to the creation of so much matter, and so little antimatter.

BIBLIOGRAPHY

  1. Stephen Hawking, A Brief History Of Time, 1988
  2. Robert L. Forward, Ph.D. & Joel Davis, Mirror Matter - Pioneering Antimatter Physics, 1988
  3. Martin Gardner, The Ambidextrous Universe, 1964
  4. Internet web site http://fnnews.fnal.gov/, Fermi National Accelerator Laboratory