In the first moments of the universe, matter overpowered antimatter, its mirror opposite. We may soon find out why.
All cultures have wondered about their origins. Modern physics posits that space and time were born in the Big Bang, some 13.6bn years ago. Science gives no definite answer for why that event occurred, but observations with powerful telescopes and experiments at places like Cern, the particle physics laboratory near Geneva, give us a good understanding of what happened next. We know that matter is, in effect, the result of energy being converted into substantial forms. We know how the energy in the heat of the Big Bang created the basic seeds of matter, and how over the eons these particles have formed galaxies of stars, including our own Milky Way and solar system.
Here on Earth, clusters of septillions of atoms are able to think, and although they are not yet able to comprehend what makes them—us—conscious, they can look out in wonder at the universe, and build machines that can revisit our origins in the Big Bang. Out of this has come an astonishing discovery. Matter is not the Big Bang’s only child. It was born with a long-lost twin: antimatter.
Matter and antimatter are the yin and yang of reality. When an infant on the seashore digs a hole in wet, hard, flat sand to build a sandcastle, the castle is a metaphor for matter and the hole for antimatter. When the energy of the Big Bang congealed into the fundamental particles of matter, an imprint in the form of metaphorical holes, their antimatter siblings, was also formed.
While all the evidence points towards this being how matter was born, it raises a paradox. The transmutation of radiant energy into matter and antimatter, which occurred in those first instants of time, is not a one-way voyage. When played in reverse order, the meeting of any material substance with its antimatter doppelgänger leads to mutual annihilation; the sandcastle refills the hole, perfectly. When antimatter destroys matter, the energy that was previously trapped within them is liberated as radiation. In the dense cauldron of the infant universe, such collisions would have been very common, and the newborn material would not have survived long. Yet the universe has survived, and appears to be made of matter, such as the familiar stuff which makes air, rocks and living things, and not antimatter.
Antimatter is real. Scientists have made a few thousand atoms of anti-hydrogen, although none of them lasted very long before being annihilated by their surroundings. If you were to see a lump of antimatter, you wouldn’t know it; to all outward appearances it looks no different to ordinary stuff. However, touching some would be lethal, as atoms in our hands would be destroyed completely, and anything of us that remained would be irradiated with the resulting gamma rays. Were there large clumps of antimatter in the cosmos, any interstellar material that hit them would lead to mutual destruction, leaving behind these tell-tale gamma rays. No such signals have been seen, which suggests that antimatter galaxies do not exist.
The vanishing of antimatter is the greatest disappearing act in history. The material universe that survives today contains the remnants of a great annihilation between antimatter and matter, which was one of the first events after the Big Bang. The intense radiation that ensued—a feebler replay of the original Big Bang—has cooled for billions of years, and today forms the ubiquitous microwave background radiation, at a temperature just three degrees above absolute zero, or minus 270 degrees Celsius. Astronomers have measured its temperature, and, by knowing how fast the universe is expanding, can play back history on their computers. This confirms that around 13.6bn years ago the universe was indeed so hot that matter and antimatter would have formed from the radiant energy. Our observations and experiments are all consistent with this theory, but we remain unsure on one thing: how did some matter survive the great annihilation?
Everything that we can see, from the world around us to the galaxies, appears to be the debris of an even grander creation. Did some mutation occur in the immediate aftermath of the Big Bang whereby the metaphorical material sandcastle no longer quite fit into antimatter’s hole? Is the apparent duality between matter and antimatter an illusion? This is currently one of the biggest questions in cosmology. Physicists are excited that clues are beginning to emerge from experiments at Cern’s particle accelerator, the Large Hadron Collider (LHC).
To begin to understand antimatter, let’s first take a voyage into ordinary matter, such as ourselves. Our personal characteristics are coded in DNA. These are miniature helical spirals made of complex molecules, which in turn are made of atoms, the smallest pieces of an element—such as carbon or hydrogen or oxygen—that can exist and still retain the characteristics of that element.
Antimatter also consists of molecules and atoms, which at first sight are no different from ordinary ones. Atoms of anti-carbon would make anti-diamond as beautiful and hard as the diamond we know. Anti-soot would be as black as soot, and the full stops in an anti-magazine the same as those you see here. If we could enlarge the dots of this magazine to be 100 metres across, we would be able to see the individual atoms of carbon within. Were we to do the same to an anti-magazine, we would find that atoms of anti-carbon are indistinguishable from those of carbon: even at the basic level of atoms, matter and antimatter look the same.
Atoms are very small, but they are not the tiniest things, and to see the difference between matter and antimatter, we must enter the atom. Each atom contains a labyrinthine inner structure. At the centre is a dense compact nucleus, which accounts for all but a trifle of the atom’s mass.
While enlargement of our ink-dot to 100 metres is sufficient to see an atom, you would need to enlarge it to the size of the Earth if you wanted to see the atomic nucleus. The same is true for anti-dots and anti-atoms. But the constituents of atoms and anti-atoms are different.
The first clues to the existence of this weird counterpoint came not from experiment, but from beautiful mathematical patterns discovered by the English mathematician Paul Dirac in 1931. Dirac was attempting to marry Einstein’s theory of special relativity with the ephemeral world of uncertainty that rules within atoms. As crotchets, minims and semiquavers on a stave are mere symbols until interpreted by a maestro, so can arid equations miraculously reveal harmony in nature. Dirac’s equation leads to an astonishing insight: it is impossible for nature to work with only the basic seeds of matter that we know. To every variety of subatomic particle, nature is forced also to admit a negative image, a mirror opposite, which follows the same strict laws as conventional particles. The simplest example, and in 1932 the first to be discovered, is the positron, which is the positively charged antiparticle of the ubiquitous negatively charged electron.
Thousands of metres above our heads, high-energy torrents of subatomic particles from outer space are crashing into the upper atmosphere. It was in these “cosmic rays” that the positron was first sighted. The positron was not an extraterrestrial invader but had been created in the atmosphere by cosmic radiation itself, when energy, released in violent collisions, is transformed into new particles of matter and antimatter, most commonly electrons and positrons.
The energy released in some forms of radioactivity also can produce positrons. This is what happens in the heart of the sun, which, in the course of converting its hydrogen fuel into helium, courtesy of fusion, emits positrons. Collisions with electrons within the sun annihilate these positrons, giving rise to gamma rays—light with very high energy—which are scattered so much as they rise to the surface that it takes them a thousand centuries to get there. By that time they have lost most of their energy and they emerge as sunlight, visible to our eyes. The rays of the Sun are in part the result of positrons annihilated 100,000 years ago.
Here on Earth, many unstable nuclear isotopes can produce positrons as easily as they can produce electrons. The major practical difference between the two possibilities lies in what happens next.
An electron may flow as electric current or join in the dance of planetary electrons in neighbouring atoms, later to initiate chemical reactions and countless other adventures in the future of the universe. A positron by contrast finds itself surrounded by matter containing hordes of negatively charged electrons. Unless some specially designed combinations of electric and magnetic fields steer it away from its material surroundings, the positron and an electron in its vicinity mutually annihilate in a flash of light. This has become the key to the practical use of positrons, notably in medical diagnostics.
If a patient ingests some liquid containing traces of radioactive atoms that emit positrons, the subsequent annihilation of those positrons within the patient’s body can be a life-saving diagnostic. By surrounding the patient’s head with a halo of cameras, which record the result of the positron annihilation, images of the brain can be built up. This technique is known as positron emission tomography, or PET. The particular isotopes of interest tend to be rather short-lived, but can be made in small, customised particle accelerators, which are housed in or near to the medical centres. Thus Dirac’s arcane prediction of antimatter is today being used to save lives.
Large particle accelerators, such as the LHC, can make intense beams of high-energy particles, whose collisions with targets of material simulate those of the cosmic rays. Such experiments have produced not just positrons, but also antiprotons and antineutrons—the antimatter counterparts of conventional atomic nuclei. As the familiar particles—electrons, protons and neutrons—combine to build atoms and matter, these contrary versions can make structures that at first sight appear to be the same as normal matter, but are fundamentally different.
Inside atoms we find swirling electric currents, powerful magnetic fields, and electrical forces that attract some things and repel others. Within atoms of antimatter, identical currents, fields and forces are present, but with their polarities reversed: north poles become south; positive charges become negative. Such a swapping of charges turns what we know as matter into what we call antimatter. Whereas atoms consist of lightweight electrons, negatively charged, whirling remotely around a compact massive central nucleus of positive charge, anti-atoms have their nucleus negatively charged, surrounded by positrons.
There seems to be no reason for nature to prefer one choice—matter—rather than the other—antimatter. Dirac summarised this enigma on receiving his Nobel Prize in 1933: “We must regard it rather as an accident that the Earth (and presumably the whole solar system) contains a preponderance of negative electrons and positive protons. It is quite possible that for some of the stars it is the other way about.”
If we were to look into the night sky at those stars, some made of matter, others of antimatter, there would be no way of distinguishing them. However, we can infer that they are made of matter, albeit indirectly.
When stars explode, their bits and pieces are ejected into space which, if trapped by the magnetic arms of our planet, crash into the upper atmosphere as cosmic rays. Had an anti-star exploded and permeated the cosmos with anti-elements then these also would be present in the cosmic rays, but no anti-elements have turned up so far. Searches for antimatter in the rays are being made by experiments in a balloon that floats to the edge of the Earth’s atmosphere above the South Pole; and out in space, sensitive particle detectors in a satellite are taking readings. However, no antimatter has been detected there, in contrast to the abundance of individual positrons and antiprotons, which are created by the collisions in the atmosphere.
Perhaps these anti-elements, ejected by anti-stars, have been destroyed en route? While this is possible, there is no evidence for it. There would be distinctive gamma ray bursts coming from the annihilation of positrons by electrons in the interstellar medium, and the annihilation of anti-protons also would give themselves away.
All of the evidence suggests that everything hereabouts is made of matter. However, there is still a lot of unexplored space where antimatter could dominate. As the universe expanded and cooled after the Big Bang, could matter and antimatter have become separated into large independent domains?
Although possible, no completely satisfactory model of such a universe has yet been developed. Most physicists suspect that there is some subtle difference between the way that matter and antimatter behave, and that this enabled some matter to survive the great annihilation.
Hints that matter and antimatter are not simply yin and yang emerged during the 1990s at the forerunner of the LHC, the Large Electron Positron collider (LEP), where magnets steered beams of electrons and positrons along a tunnel. The positrons sped around the 27-kilometre ring beneath Swiss vineyards and magnetic fields steered electrons and positrons on the same circular paths but in opposite directions. A small hollow tube in the centre of the magnets was home to a vacuum better than that in outer space, lest the circulating current of positrons collide with and be destroyed by a stray atom of air. At four points, small pulses of electric and magnetic forces deflected the beams slightly so that their paths crossed. Occasionally a positron and an electron made a direct hit, leading to their mutual annihilation in a flash of energy.
That was the key moment. In a small region of space the conditions were similar to those in the universe within a microsecond after the Big Bang. By seeing what forms of particle and antiparticle emerged from this simulation, scientists learned how energy was first converted into substance in the real Big Bang of the early universe.
Highly complex pieces of electronics recorded the emergence of these primeval pieces of matter and antimatter. They confirmed that the basic particles of matter and antimatter formed in matching pairs.
Experiments have shown that quarks are the basic seeds of matter as we know it. There are also exotic forms of matter, containing what are known as strange, charm or bottom quarks, which rarely exist independently, except under very special conditions, such as briefly during or just after the Big Bang. They are unstable and their decays produce the stable forms from which our mature universe is made. The LEP produced and studied these. The results showed that although the initial production of matter and antimatter balances, when the exotic forms decay, the progeny of the antimatter versions do not precisely balance those of their matter siblings. This is a proof that matter and antimatter can have subtle differences, but it does not seem to explain the large-scale dominance of stable matter today.
Somewhere in the first moments of the universe, earlier than the billionth of a second that was studied by experiments at the LEP, an imbalance between matter and antimatter must have arisen. The LHC at Cern can simulate conditions within a mere trillionth of a second after the Big Bang. If this includes the instant when antimatter disappeared, the LHC should reveal how.
These are early days for the LHC, if not for the universe, but tantalising results are beginning to emerge. As data accumulate, the experiments at Cern will reveal sharper images of the processes at work in the immediate aftermath of the Big Bang. Why the Big Bang happened is likely to remain an enigma. Why the universe managed to survive, and evolve, may soon be answered.