The Mystery of Antimatter

Particles and Antiparticles

Particles and Antiparticles (credit: Anybody, cc by-sa 3.0)

Despite being prevalent in science fiction stories for generation, antimatter is real, and rooted firmly in modern scientific theory. Its existence was confirmed in 1955 by scientists at the University of California, Berkeley, for which they were awarded the Nobel prize in physics just four years later. When exposed to regular matter, antimatter annihilates, releasing energy that can in principle be harnessed to do work–power a lightbulb, or a city. While potential applications for harnessing such energy abound, none have ever fully materialized. The reasons why reveal one of the largest unsolved problems in physics today.

Antimatter refers to particles very similar to the ordinary matter with which we are all familiar. Such anti-particles have the same mass as their normal-particle counterparts, but are their opposites in some respects. For example, an anti-electron, commonly called a positron, has the same mass as a normal electron but has a positive charge opposite to that of the electron’s normal electric charge. Antimatter is routinely produced in particle accelerators and in collision of high-energy particles in the earth’s atmosphere. The mystery of antimatter lies in its marked absence. As best we can tell, the universe around us is composed almost entirely of regular matter, with no antimatter in sight. To understand why we should expect to find antimatter at all, it helps to think back to the beginning of the universe and the big bang.

The Big Bang Theory was developed in the early part of the 20th century to explain observations by Slipher, Lemaitre, and Hubble that the visible galaxies are moving away from earth in all directions. Two interpretations are possible: Either the earth is at the center of a cosmic explosion of galaxies, or space itself is everywhere expanding. The latter explanation is of course much more tenable, and has been the consensus view ever since. The effect is like that of blowing up a balloon or stretching a rubber band. Two marks on the surface of the rubber will move apart, not because of relative motion of the dots along the rubber, but because the space in which they exist is stretching apart.

Naturally, upon observing the continual expansion of space, one is forced to imagine what things looked like a million or a billion years ago. Things then must have been much closer together, more dense, and hotter. In fact, it’s relatively simple to figure out when everything must have been at one point … a little over 13.8 billion years ago by modern estimates. Around that time, as the theory goes, all of space and matter (and time) exploded out from that point, cooling as it expanded. Quantum fluctuations of photons in this plasma continually created pairs of particles, electrons and positrons, particles and antiparticles. These particles were created, and annihilated almost immediately. As the plasma continued to expand and cool, some of the particles and antiparticles were created but did not annihilate, and the particles eventually agglomerated to form the atoms, stars, planets, and everything we see around us today.

But what of the anti-particles? Where did they go? We know the processes that created all the matter we see, and we know that an equal amount of antimatter must have been simultaneously created. But where is it? This is one of the largest unanswered questions in modern cosmology, and many scientists are hard at work trying to solve the puzzle. One of them is Hui Chen, who lead a team at Lawrence Livermore in 2008 to produce more antimatter than had ever before been produced in a lab. She and her team used high-energy lasers to illuminate gold targets, many times thicker than previous targets. The laser photons ionize the gold atoms, creating high-energy electrons which traverse the gold target, losing energy as they go. Some of this energy transforms (via Einstein’s famous mass-energy equality) into positrons and electrons. Optimization of the process allowed Chen and her team to create far more antimatter particles than ever before, opening up the possibility of new research into the enigmatic asymmetry of matter and antimatter in the universe.

Will we find other planets, solar systems, and galaxies made of antimatter? NASA is betting we just might. If so, it would rank as one of the amazing discoveries of mankind. If not, then perhaps there is new physics awaiting us that we don’t yet understand, and that would be an equally exciting discovery.

Eric Sorte

Eric Sorte earned his PhD in physics from the University of Utah in 2011, where he developed new experimental evidence for fundamental theories of nuclear spin systems. With the intent to contribute to alternative energy solutions, he then came to Washington University in St. Louis, where he worked to further the understanding of metal hydrides as hydrogen storage systems. Sorte joined Georgetown University as a postdoctoral researcher in 2014, where he specializes in applying novel spectroscopic methods to understand electrocatalysis in electrochemical cells and batteries.