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.

Discussion Questions:

  1. What kinds of future experiments and technologies might help us figure out what happened to antimatter after the Big Bang?
  2. What might the existence of asymmetries in physics suggest about the nature of reality?
  3. 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?

 Discussion Summary

A number of insightful and thought-provoking comments were received by my article. Two salient points of were picked up by readers and triggered further reflection, allowing also to bring out their deep relationship:

On one hand, readers asked about the importance of symmetries in describing and understanding the universe, but also in forming our expectations as to how it should be. The central role of symmetries is indeed linked to our experience that there are regularities and continuities in our experience of the world as well as our subsequent expectation that the same rules govern the universe independently of our specific position in time or space.

The second important point raised by readers concerned the limits of knowledge and, indeed, of inquiry into the ultimate explanation of the universe we inhabit. These questions go well beyond what a practitioner of science encounters on a daily basis, but rightly bring into focus the underlying assumptions and teleology of science. We expect that the universe can be understood, from its invisible, smallest components to its largest structures. And so, based on past successes, the goal of an all-encompassing understanding appears at least plausible, if rather grand.

The topic of antimatter seems very well suited to exploring these two facets. In a way, the two aspects are complementary and underline the importance of humility in considering the state of knowledge: We should neither assume any privileged frame of reference nor despair (any more than have others in the past) of having not yet achieved that knowledge and understanding of nature towards which we work. As our knowledge increases, so too does our knowledge about our ignorance.

10 Responses

  1. Alfred MacDonald says:

    It seems as though the more we learn about modern physics and astronomy, the more we learn about our own ignorance — how much we will never, in some cases even in principle, understand about the universe.

    What does this say about the nature of this type of inquiry? Doesn’t this sort of undercut the idea that physics is our most reliable way to arrive at a complete account of nature? We won’t ever get there!

    • Michael Doser Michael Doser says:

      This was indeed one of the central points that I was trying to make: that humility towards our state of knowledge is appropriate. This progressively deeper understanding of the limits of our knowledge is something to be celebrated, in a certain manner, since while we see the limits of what is known in a stark light, we also see an ever expanding area in which our best models are applicable, and to some degree, even our ignorance has become mappable.

      Only a few decades ago, dark matter was an unquantified hypothesis, and dark energy utterly unknown. Now, thanks to precision measurements of the cosmic microwave background, among others, both have been provided with boundaries, and have been brought into the realm of experimental testability, at least in principle.

      I am an optimist: I believe that our knowledge about the universe will continue to expand, and equally, that new questions will arise. And far from causing me to despair, this too fills me with hope, as it means that human curiosity about the world will always have something to sink its teeth into.

      In this context, I can not help but quote Terry Pratchett, from his books Equal Rites:

      “Cutangle: While I’m still confused and uncertain, it’s on a much higher plane, d’you see, and at least I know I’m bewildered about the really fundamental and important facts of the universe.

      Treatle: I hadn’t looked at it like that, but you’re absolutely right. He’s really pushed back the boundaries of ignorance.

      They both savoured the strange warm glow of being much more ignorant than ordinary people, who were only ignorant of ordinary things.”

  2. Feynman Fan says:

    I don’t quite follow how antimatter relates to the Big Bang. Is it just the idea that if all matter has antimatter, then antimatter should have been created along with matter?

    • Michael Doser Michael Doser says:

      Indeed, it is the transformation of energy into mass that lies at the heart of the problem: this transformation always produces a particle and its antiparticle at the same time, for reasons of conservation of (generalized) charge. The energy of the Big Bang produced vast amounts of matter particles, but by that token, must have produced identical numbers of the corresponding antiparticles.

      This simultaneous pair production of a particle with its antiparticle has been observed vast numbers of times in the lab, and is indirectly tested every time particle collisions are investigated and analyzed in high energy physics experiments, as any asymmetry would destroy the agreement between predictions and experiment.

  3. CosmosGeek says:

    How do we know that the absence of evidence for all this antimatter isn’t evidence of absence? In other words, couldn’t our theory be wrong and this stuff just isn’t there?

    • Michael Doser Michael Doser says:

      There are two elements to this question: the observational absence of (primordial) antimatter in the universe, and the theoretical expectation of the formation of equal amounts of matter and antimatter.

      The latter is based on the observationally motivated assumption of the conservation of charge, and more generally on a theorem called CPT invariance, which predicts identical properties for particles and antiparticles. This theorem in turn is provable if one makes very general assumptions about Lorenz invariance (the laws of physics are the same for all observers, independent of their relative motion), locality (no instantaneous action at a distance) and causality. There are however theories that allow a violation of this symmetry, but experimental tests have excluded all but the tiniest violations.

      Antimatter is produced easily in the lab (and even, via radioactive decays of e.g. potassium, in humans), and has been studied carefully since about 50 years. We thus know relatively well what signatures the presence of antimatter at different times in the development of the universe would leave behind. These signatures could be different ratios of the elements formed during the first few minutes (baryosynthesis) than what standard (well-tested) nuclear physics predicts, domain-like structures in the cosmic microwave background, or the presence of high energy photons stemming from positron-electron annihilations at large-scale matter-antimatter domain boundaries, none of which are observed. From this, one can conclude the absence of primorial antimatter in the observable universe from very early on. Indirectly, the very large ratio of photons to other particles in the universe is also evidence of a very early annihilation process.

      Nevertheless, this absence of evidence for the presence of antimatter in the observable universe should not be construed a final evidence of absence, as at least one loop-hole could still be invoked: the presence of antimatter outside of the observable universe, which however is an untestable hypothesis.

  4. Luci says:

    You mention CERN. What exactly would experimental confirmation of this sort look like? How would (or how will) the experimenters know when they’ve found what they’re looking for? And how (or when) would they know when to stop looking?

    • Michael Doser Michael Doser says:

      Without theoretical guidance as to where a potential violation of the CPT symmetry between matter and antimatter could be found (neither the system, nor the magnitude), the current experimental approach is to cover as many systems as precisely as possible. Experimental searches for the breaking of this symmetry mainly focus on comparing the properties of a system made of matter with those of a system made of antimatter; confirmation of such a symmetry breaking relies on detecting a difference between them.

      A difference could for example turn up in the spectrum of light emitted by hydrogen and antihydrogen atoms (a relative measurement, where a difference would be glaring, as long as it is larger than the experimental uncertainty), or in the laser excitation spectrum of a system composed of a particle and of its antiparticle (positronium — consisting of an electron and a positron — or protonium — a system consisting of a proton and an antiproton) in comparison to the best theory of such systems, Quantum Electrodynamics. It might also show up in the way an antimatter system, such as an antihydrogen atom, falls, if it were to fall more or less rapidly than a system composed purely of matter.

      In all cases, we are still rather far away from the precision reached in identical systems made of matter, so in the absence of the detection of any difference, experimenters would continue improving their methods until they reach comparable sensitivities in both systems. As to whether they would continue then, or would stop trying to find a difference, depends on advances in other fields, on the experimenter’s ingenuity, and to a non-negligible extent, on their obstinacy and perseverence.

      A some point, however, in the absence of any confirmation, the search will be put on ice, regarded as one of those questions which are too difficult to tackle now, and perhaps will remain in the realm of philosophy, as has happened in other areas in the past.

  5. Cody Whittington says:

    Thank you, professor. Could you please unpack the concept of symmetry a bit further (for those of us non-scientists)? In particular, I’m trying to understand why some scientists think this concept might be helpful in connection with antimatter?

    • Michael Doser Michael Doser says:

      The theoretical physicist Emmy Noether uncovered a very deep link between conservation laws and symmetries, in that each conserved quantity is closely linked to an underlying symmetry. As an example, the fact that physical processes are the same at different times and locations (invariant under temporal or spatial translation) is linked to conservation of, in this case, energy and momentum.

      Symmetries can be broken, sometimes unexpectedly. Mirror symmetry (parity, P) holds for objects in common life: a stone dropped in front of a mirror will be describable in exactly the same way as its mirror image. However, for some particles, this symmetry does not hold: a neutrino with a right hand spin would be seen in a mirror as a neutrino with a left-handed spin, a particle that does not exist; parity is thus maximally broken, as was discovered in 1957, in the weak interaction.

      One can construct products of symmetries: for example, adding in time invariance (T) or charge symmetry (C), to P, and apply the product of such symmetries, for example CP, to the situation above. A CP transformation then not only produces a left-handed neutrino as the mirror image of the right-handed neutrino, but furthermore transforms the left-handed neutrino in the mirror into a left-handed antineutrino, a particle that does exist. CP is thus (reasonably good) symmetry, but is also slightly broken in the decays of some short-lived particles built up of a quark and an antiquark (K mesons being the system in which this symmetry breaking was discovered). The observed asymmetry between the decays of matter and antimatter seen in CP-non-conserving processes is however insufficient to explain the observed asymmetry in the universe.

      The product of all three symmetries C, P and T should however correspond to an unbroken symmetry, the so-called CPT symmetry. That this is the case can be derived from very fundamental assumptions about the laws of physics; furthermore, this symmetry predicts that particles and antiparticles have identical physical properties. Any breaking of this CPT symmetry (or a larger breaking of the CP symmetry, for example in systems that have not yet been studied) could have engendered a universe that starts out asymmetrically (with an excess of matter over antimatter), and where only the tiny excess of matter over antimatter survives the initial mutual annihilation.

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