What Is Antimatter? Definition and Examples
Atoms of matter and antimatter have the same mass, but the protons and electrons have opposite charges and the quantum numbers of subatomic particles are different.
Antimatter is a real substance and not just a science fiction topic. Antimatter is matter composed of antiparticles with the opposite electrical charge of ordinary particles and different quantum numbers.
A regular atom has a nucleus of positively-charged protons and neutrons that is surrounded by a cloud of negatively-charged electrons. An antimatter atom has a nucleus of negatively-charged antiprotons and neutral (yet different) neutrons surrounded by positively-charged antielectrons, which are called positrons. Matter and antimatter atoms and ions behave exactly the same as each other. Antimatter forms chemical bonds and presumably molecules, exactly the same as matter. If suddenly everything in the universe switched from matter to antimatter, we wouldn’t know the difference.
When matter and antimatter collide, the result is annihilation. The mass of the particles converts to energy, which is released as gamma photons, neutrinos, and other particles. The energy release is immense. For example, the energy released by reacting one kilogram of matter with one kilogram of antimatter would be 1.8×1017 Joules, which is just slightly less than the yield of the largest thermonuclear weapon ever detonated, the Tsar Bomba.
Examples of Antimatter
Three conditions regularly form antimatter: radioactive decay, extremely high temperatures, and high-energy particle collisions. Particle colliders have produced positrons, antiprotons, antineutrons, anti-nuclei, antihydrogen, and antihelium.
But, you can encounter antimatter without visiting a high energy physics facility. Bananas, the human body, and other natural sources of potassium-40 release positrons from β+ decay. These positrons react with electrons and release energy from the annihilation, but the reaction poses no health threat. Lightning also produces positrons, which react with matter to generate some gamma radiation. Cosmic rays contain positrons and some antiprotons. PET scans involve positrons. Solar flares may release antiprotons, which become trapped in the Van Allen radiation belt and can cause an aurora. Neutron stars and black holes produce positron-electron plasma.
Antimatter is more common than you might think. It occurs in thunderstorms, radioactive potassium decay in bananas (and humans), solar flares, PET scans, and black holes.
Uses of Antimatter
In addition to research, antimatter is used in nuclear medicine and may find use as a fuel or weapon.
Positron emission tomography (PET) uses radioactive isotopes that emit positrons. The positrons emit gamma rays when they annihilate electrons. A detector maps the gamma ray emission to form a three-dimensional image of the body. Antiprotons may also find use as a therapy to kill cancerous cells.
Antimatter might be a fuel for interplanetary and interstellar travel because antimatter-matter reactions have a higher thrust-to-weight ratio than other fuels. The difficulty is directing the thrust, since the annihilation products include gamma radiation (for electron-positron reactions) and pions (for proton-antiproton reactions). Magnets might be used to control the direction of charged particles, but the technology still has a long way to go before you can hitch a ride to Mars on an antimatter rocket.
Theoretically, antimatter can be used as a trigger for a nuclear weapon or a matter-antimatter reaction could be an explosive. The two drawbacks are the difficulty producing enough antimatter and storing it.
How Is Antimatter Stored?
You can’t store antimatter in an ordinary container because it would react and annihilate an equal amount of matter. Instead, scientists use a device called a Penning trap to hold antimatter. A Penning trap uses electric and magnetic fields to hold charged particles in place, but it can’t hold neutral antimatter atoms. Matter and antimatter atoms are held by atomic traps (based on electric or magnetic dipoles) and by lasers (magneto-optical traps and optical tweezers).
Asymmetry of Matter and Antimatter
The observable universe consists almost entirely of ordinary matter, with very little antimatter. In other words, it is asymmetrical with respect to matter and antimatter. Scientists believe the Big Bang produced equal amounts of matter and antimatter, so this asymmetry is a mystery. It’s possible the amount of matter and antimatter wasn’t homogeneous, so most of the matter and antimatter annihilated each other. If this happened, it produced a lot of energy and either a (relatively) small amount of ordinary matter survived or the universe consists of pockets of matter and antimatter. If the latter event occurred, we might find distant antimatter galaxies. Antimatter galaxies, if they exist, would be hard to detect because they would have the same chemical composition, absorption spectra, and emission spectra as regular galaxies. The key to finding them would be to look for annihilation events at the border between matter and antimatter.
History
Arthur Schuster coined the term “antimatter” in 1898 in letters to Nature. Schuster proposed the ideas of antiatoms and matter-antimatter annihilations. The scientific foundation for antimatter began with Paul Dirac. In 1928, Dirac wrote that the relativistic equivalent to the Schrödinger wave equation of the electron predicted antielectrons. In 1932, Carl D. Anderson discovered the antielectron, which he named the positron (for “positive electron”). Dirac shared the 1933 Nobel Prize in Physics with Erwin Schrödinger “for the discovery of new productive forms of atomic theory.” Anderson received the 1936 Nobel Prize in Physics for the discovery of the positron.
References
- Agakishiev, H.; et al. (STAR Collaboration) (2011). “Observation of the antimatter helium-4 nucleus”. Nature. 473 (7347): 353–356. doi:10.1038/nature10079
- Amoretti, M.; et al. (2002). “Production and detection of cold antihydrogen atoms”. Nature. 419 (6906): 456–459. doi:10.1038/nature01096
- Canetti, L.; et al. (2012). “Matter and Antimatter in the Universe”. New J. Phys. 14 (9): 095012. doi:10.1088/1367-2630/14/9/095012
- Dirac, Paul A. M. (1965). Physics Nobel Lectures. 12. Amsterdam-London-New York: Elsevier. pp. 320–325.
