What is Antimatter?

antimatter1.jpgAntimatter is matter made up of antiparticles. To understand antimatter requires that you understand that all matter is made up of charged particles. In matter, particles (electrons) with a negative charge move around a particle (proton) with a positive charge. In the 1920s and 1930s, physicists realized that for every particle there is a twin particle with an opposite charge, an antiparticle. The first antiparticle to be discovered was an antielectron, which has a positive charge. Antielectrons are also called positrons. The positron is exactly the same as an electron except that it has a positive charge rather than a negative charge.

Every particle has an antiparticle with the same mass but the opposite electric charge. The proton has the negatively charged antiproton; the electron has the positively charged anti-electron, or positron. Neutral particles can have antiparticles, too. The neutron might have no charge, but quark, the smaller particles that make it up, do. Flipping the charge on a quark, creates antiquarks which make up an antineutron.

When a particle and antiparticle meet, they mutually annihilate each other and release their entire mass as energy.
An electron and a positron are mutually oblierated in a puff of light consisting of two photons sent out in opposite directions, each with an energy corresponding exactly to the mass of the electron (and positron). Similarly, matter and antimatter destroy each other, or annihilate, whenever they come into contact.

Who Discovered Antimatter?

In 1928, the British physicist Paul A. M. Dirac developed a theory for the motion of electrons in electric and magnetic fields. Dirac's theory and equations worked exceptionally well, describing many attributes of electron motion where his predecessors were unsuccessful. His theory and equations also led him to predict that the electron must have an "antiparticle," having the same mass but a positive electrical charge.

In 1932 Carl D. Anderson observed this new particle in an experiment and it was named the "positron". A cloud chamber picture taken by Anderson showed a particle entering from below and passing through a lead plate. The direction of the curvature of the path, caused by a magnetic field, indicated that the particle was positively charged, but it also had the same mass and other characteristics as an electron. This was the first known example of antimatter.

In 1955 researchers at the Univeristy of California used their particle accelerator to produce the first antiproton. The antiproton has the opposite charge of a proton, much like the relationship between an electron and an antielectron. Since there are both antielectrons and antiprotons, scientists theorized that it should be possible to create anti-atoms that have positrons circulating around antiprotons.

Why is Antimatter so Rare?antimatter2.jpg

Antimatter is very rare in a universe that is dominated by matter. Researchers are still trying to understand why there is so little antimatter in the universe. There is no real difference between particles and antiparticles in particle physics theories, and many scientists believe that at the time of the Big Bang, antiparticles and particles were created in almost equal numbers. What is not clear is why mostly only matter remains.

One theory is that particles outnumbered antiparticles in the Big Bang. According to some scientists, even if particles outnumbered antiparticles 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. Another theory is 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.

Can we Create Antimatter?

Researchers can create antimatter using giant particle accelerators. Recently the giant particle accelerators at CERN and the Fermilab have been able to create tiny quantities of antihydrogen. This was done by blasting a beam of high-energy protons into a target using particle accelerators, and creating a shower of subatomic particles. Then the researchers use powerful magnets to separate the antiprotons. The antiprotons were then slowed down and exposed to antielectrons that are naturally emitted from sodium-22. When the antielectrons orbit around the antiprotons, they make antihydrogen. In 1995 CERN created nine antihydrogen atoms. A few years later Fermilab was able to create one hundred atoms of antihydrogen.

In principle, anti-elements other than hydrogen could be produced as well, but the cost would be enormous. Producing even a few ounces of anti-atoms would cost billions of dollars. In 2004, it cost CERN $20 million to produce several trillionths of a gram of antimatter. To produce a single gram of antimatter would cost $100 quadrillion and the antimatter factory would need to run continuously for 100 billion years.

Is Antimatter Dangerous?

Penning Trap
Handling antimatter is extremely difficult, since any contact between matter and antimatter results in annihilation and creates an explosion. Putting antimatter in an ordinary container would be disastrous as it would explode when the antimatter touched the sides of the container. Scientists have created special antimatter traps that use magnetic fields to keep the antimatter particles in a vacuum away from any matter. Essentially this is a container with no walls. One such container that was invented, called TRAP, successfully created and held charged antiprotons for months.

Another type of antimatter trap, the Penning trap is being developed at Penn State University. The Penning traps uses 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.

Does Anitmatter have any Uses?

One current use of antimatter, is a Positron Emission Tomography (PET Scan) that doctors use to see how brains work. PET is also used in studies of electronic circuits. When an electron and positron meet, they annihilate, turning into energy which, at high energies, can rematerialize as new particles and antiparticles. This is what happens at machines such as the Large Electron Positron collider(LEP collider) at CERN. But, at low energies, the electron-positron annihilations can be used. The positrons used in PET 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 large scale energy sources.

Some researchers have theorized that antimatter may someday be used to power space ships. When antimatter annihilates normal matter, all the mass is converted to energy. The energy output per unit particle far exceeds the efficiency of chemical reactions such as burning hydrogen and oxygen that we use now to power rockets and the Space Shuttle. But, so far, high-energy antimatter particles are only produced in relatively large numbers at a few of the world's largest particle accelerators, and there is not nearly enough production to power a space ship.

Learn more on VIDEO
From Dr. Christopher Hill


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