With hundreds of billions of galaxies each with hundreds of billions of stars, the universe contains unimaginable amounts of matter. But the universe contains something else as well. In 1928, right around the time the work of Henrietta Leavitt, Edwin Hubble, and others was revealing the extent and age of the universe, physicist Paul Dirac inadvertently discovered something altogether unexpected while attempting to describe electrons moving at relativistic speeds: a mirror image of matter known as antimatter.
Four years later Carl D. Anderson experimentally confirmed the existence of these antielectrons or “positrons” — so named because they possess a positive charge, unlike their counterpart electrons, but are otherwise identical. When a particle and its antiparticle meet, however, they mutually annihilate each other, transforming their mass fully into energy. It soon became obvious that with sufficient energy, the opposite process, pair production of particles and antiparticles, was inevitable. This led to a new question: What happened to the antimatter that must have been formed along with matter when the universe came into being at the moment of the Big Bang? Where did the antimatter go?
This Big Bang theory, first postulated by Belgian astronomer and priest Georges Lemaître, defines the alternative scenarios. Either all particles were annihilated with their corresponding antiparticles in the first instants of the universe, in which case the universe would be empty except for huge numbers of photons; or there are equal amounts of matter and antimatter, perhaps clumped in regions of the universe, safeguarded from mutual annihilation by the vast distances separating them.
The problem is that there are no traces of this primordial antimatter in the observable universe, even though it should be as prevalent as the matter that forms the multitude of luminous stars and galaxies. This fact is far from trivial. Astronomers make careful observations to determine the composition of stellar objects, which can only be studied using the light they emit. The difficulty is that light cannot be used to determine whether the objects from which it was emitted are composed of matter or antimatter. Why? Because the quantum field theories used to describe matter and antimatter predict that particles and antiparticles are identical in terms of the energy they emit. Hence any atoms formed from particles will also look identical to the antiatoms formed from antiparticles. For instance, a hydrogen atom and its antiparticle, antihydrogen, should have identical energy levels and thus light particles (or photons) of the same energies, making them indistinguishable in terms of the spectra that they produce that astronomers can observe.
It is only when matter and antimatter meet that their natures can be determined — albeit retrospectively. When matter and antimatter meet, they are mutually annihilated and new particles and antiparticles are formed. Since hydrogen (and, perhaps, also antihydrogen) forms the bulk of visible matter in the universe, it makes sense to look for proton-antiproton and electron-positron annihilations (since each hydrogen atom is simply a single proton orbited by a single electron). But the former produces a hopelessly complex mix of elementary particles — including pions, kaons, muons, electrons and neutrinos — as well as their antipartners. What is more, the photons resulting from these annihilations do not have any specific characteristics that would allow us to distinguish them from photons formed through myriad other (and more mundane) processes.
Fortunately, positron-electron annihilation produces a striking signal: two photons with a specific energy level (511 keV). X-ray telescope satellites have made it possible to search for these high-energy photons. Using such satellites, researchers discovered an unexpected strong source of positrons at the center of our own galaxy, most likely generated by the decay of radioisotopes produced in supernovae. No such signal has been seen anywhere else. Along with other evidence — especially the absence of boundary structures in the cosmic microwave background (a kind of signature of the Big Bang) and the delicate balance between the different nuclei produced in primordial nucleosynthesis shortly after the Big Bang — the existence of such matter-antimatter interactions at the center of our galaxy strongly suggests that antimatter must have disappeared at an extremely early stage of the universe.
Such a disappearance is highly unexpected. With only a very small number of exceptions, the known laws of physics are symmetric with respect to matter and antimatter, meaning that the properties of particles (mass, charge, lifetime, magnetic moment, coupling strengths, decays, etc.) are identical to those of their antiparticles. The symmetries underlying this equality — P for parity or mirror symmetry, C for charge symmetry, and T for time reversal symmetry — correspond to conservation laws, as was derived by physicist Emmy Noether. One exception is the decay of very short-lived particles such as neutral kaons or neutral B mesons. This has led some physicists to speculate whether such asymmetries could offer a clue about the asymmetry between matter and antimatter.
In 1967 Russian nuclear physicist Andrei Sakharov attempted to provide an explanation of what is called baryon asymmetry by seizing upon a discovery made three years earlier by James Cronin and Val Fitch. Cronin and Fitch showed that subatomic particles called neutral kaons and their antiparticles do not decay exactly the same way, violating the expected principle of charge symmetry — which holds that the laws of physics should operate the same way on particles of opposite charge — and parity symmetry — which holds that the laws of physics should operate the same way for mirror images of the same particles. When acting in concert, these two forms of symmetry are together known as CP symmetry. This “CP violation,” discovered by Cronin and Fitch, together with the assumptions that the universe is not in thermal equilibrium and that protons can decay — these three assumptions are now known as the Sakharov conditions — provides a conceptual framework in which to consider the formation of a fundamental asymmetry between matter and antimatter. Essentially, the breaking of the symmetry allows a possible oscillation between a matter-dominated universe and an antimatter-dominated universe to occur, and the expansion of the universe freezes out a (temporary!) imbalance between them. Unfortunately, the known symmetry violations are too small to make sense of the asymmetry between matter and antimatter, resulting in the postulation of further hitherto unobserved symmetry-breaking in known or yet to be discovered particles. Moreover, no proton decays have been observed to date, in spite of decade-long efforts.
It thus makes sense to contemplate other possible explanations of the asymmetry between matter and antimatter. One has to do with the breaking of a more fundamental symmetry called CPT (which contains the concept of time invariance in addition to charge and space symmetries like those discussed above). Another concerns the universality of couplings between particles (and antiparticles) and physical interactions such as gravity. In the absence of clear guidance about where to look for the origin of baryon asymmetry, an exploration on multiple fronts may be the most promising approach. Hopefully, such an approach can help us to unearth, study, and ultimately understand the underlying causes of our lopsided universe — thus explaining why the universe is not filled only with light.
Experimental searches for such symmetry violations have a long history. The most fascinating of these currently being carried out are at a dedicated facility at CERN in Geneva, Switzerland. These experiments use antiprotons, positrons, and antihydrogen atoms. (Measurements with positrons, which do not require CERN’s heavy infrastructure, are being carried out in a number of other laboratories worldwide as well.) Working with these particles, which are relatively long-lived compared to other elementary particles, has several advantages, for instance, that highly sensitive measurements allow researchers to compensate for external influences or to track what happens at ultra-low temperatures (such as with antihydrogen atoms).
While work with charged particles and antiparticles has been going on for a decade or so (and is still getting more and more precise), research using systems made purely or partly of antimatter, such as antihydrogen atoms, is quite new. These systems behave like exotic atoms, but like other atoms, they are amenable to being sensitively probed or manipulated by highly precise lasers, which can stabilize or cool them or measure their properties. Though building such exotic atoms is challenging, it has become routine. (The first non-relativistic antihydrogen atoms were formed about 15 years ago.) Still, holding on to them for subsequent study is an art only mastered by two groups of researchers thus far. In 2010, researchers in the ALPHA experiment at CERN successfully trapped antihydrogen atoms; just last year, the same group of researchers successfully carried out antihydrogen spectroscopy, probing antihydrogen atoms with lasers. Similarly, the laser tools used to stabilize positronium — meta-stable atoms composed of one positron and one electron — in an excited state have been developed only in the last years by a small number of experimenters. Much work is being done to improve the reach of these tools so that precision measurements of these atomic systems can become routine.
The long term prospects for these research programs are contingent on overcoming numerous challenges. The most significant of these is reaching ever lower temperatures, since the precision (or even feasibility) of a measurement is determined by the velocity of the system being investigated — and the velocity of the particles is determined by their temperature. Velocities of less than meters per second or even of millimeters per second are needed to eliminate systematic interference so that we can measure the weakest force of all: gravity. Advances made in the last decades in atomic and laser physics are greatly useful in this race to the bottom. Still newer technologies are needed, however, if we are to study antimatter systems more effectively — to observe higher rates of production of positronium atoms and the formation of beams of meta-stable antihydrogen atoms or to cool these antiparticles below the limits of current laser-cooling capabilities. To develop these technologies will require creativity and a willingness to learn from other fields of science as well as to risk failure.
It is difficult to chart beforehand what progress might look like under these circumstances. And while there has been continuous progress in the past few decades, the field has advanced slowly. It thus requires patience as well as vision on the part of the experimenters and those helping them reach their goals. Such focused work may yield the results needed to discover this searched-for asymmetry between matter and antimatter and explain why the universe does not behave as we expected based on what we already know. Of course, it could also be that, despite these efforts, no such asymmetry will ever be discovered and we will have not advanced beyond Sakharov’s attempt fifty years ago. Until we reach that point, however, we should keep looking for the tiny nothing that caused the universe to fill up with only matter.
- What kinds of future experiments and technologies might help us figure out what happened to antimatter after the Big Bang?
- What might the existence of asymmetries in physics suggest about the nature of reality?
- How can we deal with the possibility that in spite of all our efforts, we might ultimately never manage to find the underlying cause of the absence of antimatter?