SURE, the big bang is cool, in a hot sort of way. The beginning of all things. Space, time, matter and energy bursting into existence from a pinprick of infinite temperature and density. Space racing away from itself faster than the speed of light. Maybe even the making of a multiverse.
But a second moment shortly afterwards doesn’t get half the press. Perhaps that is because it is when precisely nothing happened. Call it an anti-moment.
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It is when all the matter that suddenly and inexplicably came into being in the big bang equally suddenly and inexplicably failed to go out of being again. When it didn’t cease to be available to create stars, galaxies, planets, an unquantified quantity of questioning life and, on one world at least, some highly embarrassed physicists who predicted exactly that. “The fact that we are a world completely dominated by matter is completely un-understood,” says Chloé Malbrunot at particle physics lab CERN, near Geneva, Switzerland. “Theory says we shouldn’t be here.”
After decades trying to understand why we are here, we could now be nearing a breakthrough on multiple fronts. And the answer probably isn’t the one we first thought of. There is even a slim chance it could explain not only what happened after the big bang, but also great mysteries of our universe today, such as the nature of dark matter and dark energy. “We are just one experiment away from a revolution in our understanding,” says Jeffrey Hangst at CERN. “That’s what makes this so cool.”
At this story’s heart lies perhaps the most bizarre stuff in physics – antimatter. It was conceived by physicist Paul Dirac on the back of a theoretical envelope in 1928, only for others later to discover it was real. Antimatter represents a perplexing duplication of effort on nature’s part: a parallel world of stuff that looks just the same as normal matter, but which is oppositely charged and works like matter viewed in a mirror (see “What is antimatter?“).
But that isn’t the strangest thing. “The science-fiction part is that these two things can’t coexist,” says Hangst. Whenever an antiparticle meets a twin matter particle, they “annihilate” in a puff of light and energy.
“What makes antimatter different? Answering that is key to working out why we’re here”
This becomes a big problem when you wind back 13.8 billion years to the big bang. The standard model of particle physics, our best theory of matter and its workings, is underpinned by the quantum field theories that Dirac and many others developed. It depicts empty space as a roiling quantum vacuum of particles and antiparticles that pop up as pairs, and confidently predicts that the big bang created equal quantities of matter and antimatter. They would have indulged in cyclical orgies of annihilation and recreation until the cooling, expanding universe could no longer supply enough energy for this, at which point… not a lot happened. Certainly, the material universe of stars and galaxies and planets failed to materialise.
What is antimatter?
The most basic definition of an antimatter particle is that it is the same as a matter particle, except that it has the opposite charge. So the familiar electron, for example, with a negative charge -1, has an antimatter equivalent called a positron that has a charge of +1.
A complicating factor is that “charge” doesn’t just mean the familiar, everyday electric charge. Three fundamental forces are covered by particle physicists’ standard model: electromagnetism, and the strong and weak nuclear forces, which govern the interactions of the quarks within protons and neutrons and processes such as radioactive beta decay, respectively. Each of these forces has a charge associated with it, and antimatter particles have opposite values for these charges, too.
Not every particle has an antimatter equivalent, either. Particles called bosons transmit influences rather than respond to them, and these tend to be their own antiparticles. These include photons and the mass-giving Higgs boson. And to date, no one has been able to establish whether neutrinos, the most elusive of matter particles, and their partner antineutrinos are different, or the same thing.
But the fact that we exist to raise an eyebrow at this prediction is the only rebuke it needs. We are beings made of matter, living in a material world, while antimatter is reduced to an eternal bit-part player (see “Everyday antimatter“). One conservative solution is that the antimatter isn’t gone, it is just hiding, with far-flung regions of the universe made entirely of antimatter. The trouble is, you would be able to see the joins: long, thin seams of gamma-ray light produced by the annihilation of matter and antimatter wherever two opposing regions met. “We’ve never seen any signal like that from anywhere,” says Marco Gersabeck at the University of Manchester, UK, and CERN’s LHCb experiment.
More than half a century ago, the discovery of a phenomenon called CP violation gave a hint of a plausible alternative. The idea was that, since matter and antimatter were just mirror versions of one another, if you swapped particles for antiparticles in any process – and therefore broke charge, or “C” symmetry – while simultaneously looking at things in a mirror, breaking parity or “P” symmetry, the particles would behave in exactly the same way as if you had done neither of those things.
In 1964, investigations of particles known as kaons and their antiparticles showed that this wasn’t quite the case. Perfect CP symmetry – and the naive picture of matter and antimatter as perfect mirrors – didn’t hold. That raised many more questions. “What makes antimatter different? Are some types of antimatter more different than others, and why should this be? Are there other types of antimatter, corresponding to new types of matter we haven’t discovered yet?” asks Tara Shears at the University of Liverpool, UK, and LHCb. Answering those questions is the key to working out why we are here now.
Theorists later found that CP violation among kaons could be explained if three heavier, unknown particles were disrupting them. All three of these particles have since been found – the bottom, charm and top quarks – and their presence, along with CP violation, is now a mainstay of the standard model.
Those particles also provide new sources of CP violation. In the intervening half-century, we have measured all the main predicted sources, with LHCb measuring the last, among charm quarks, just last year. “Now the whole thing looks completely understood, and all is well,” says Gersabeck.
There is just one teensy problem. Patterns in the cosmic microwave background, the leftover radiation from the big bang, combined with calculations of the number of galaxies that must exist, tell us the early matter-antimatter imbalance we need to explain today’s matter domination. It is tiny – about one part in a billion. But the CP violation we have found so far doesn’t account even for a billionth of that.
Wishful thinking
Perhaps there are unknown sources of CP violation. In 2017, the LHCb experiment saw an unexpected hint of the effect among heavier versions of protons and neutrons. If the elusive particles known as neutrinos are their own antiparticles, that would also allow extremely rare processes to supply an extra source of asymmetry.
But to expect such small, unconfirmed effects to account for the current huge mismatch seems like wishful thinking. Here the antimatter problem meets a wider malaise in fundamental physics. “At the moment, we’re at quite a curious place because we’ve found all the particles of the standard model and it looks like the standard model can explain everything, including CP violation,” says Gersabeck. “But at the same time, we know it’s not right and there is other stuff out there.”
Stuff like dark matter, for example, the mysterious entity that makes up most of the gravitating matter in the universe. The Large Hadron Collider (LHC) at CERN was partly conceived to make heavy dark-matter particles in its high-energy collisions. But it has found diddly-squat besides the Higgs boson, the mass-giving particle discovered in 2012.
The machine is currently on sabbatical until 2021, undergoing an upgrade in the number of collisions it can produce. When it returns, the precise measurements of rare processes that LHCb specialises in will offer a chance to solve the antimatter and dark matter problems – not by manufacturing new particles, but by measuring their ghostly influence on already-known ones.
This is the physics equivalent of reaching up a hand to have a rummage around on a shelf you can’t actually see. “With some of these measurements, we can access mass scales for hypothetical particles two or three orders of magnitude beyond the reach of the LHC for producing them directly,” says Gersabeck. “It’s a very, very powerful search tool to cover a huge lot of ground.”
The hope is that these heavier particles could be sources of CP violation, in effect repeating the trick that solved the kaon problem. But there is no guarantee. “We’ve got a few hints here or there of things that might be going on, but nothing firm,” says Gersabeck.
A few kilometres due west of LHCb, Malbrunot, Hangst and others are betting on a different approach. Rather than searching for new physics at very high energies, says Hangst, “we decide to look really carefully at things we think we understand, and see if maybe we’ve overlooked something”.
Hidden down a side road on CERN’s main site, Building 393 seems an implausible portal to a parallel world of matter. Its grey, corrugated metal walls, roll-up lorry delivery bay and asphalt car park surroundings give it a shabby late 20th-century industrial estate chic. Only a sign above the entrance saying “ANTIMATTER FACTORY” gives the game away.
It expresses an aspiration only now becoming reality: to solve the antimatter mystery by making large quantities of whole anti-atoms. All the differences between matter and antimatter come about because they have opposite charges, so the idea is to cancel those differences by taking oppositely charged antimatter particles and making neutral atoms out of them. An atom of antihydrogen, the simplest imaginable anti-atom, should work exactly like a conventional hydrogen atom.
If it doesn’t, nature’s most profound symmetry is broken: CPT symmetry. This adds time reversal, or “T”, symmetry to the CP mix. If particles swap charges and their orientation in both space and time – if the universe is completely mirrored – then the laws of physics should work the same way. This assumption lies at the heart of relativity and the quantum field theories underlying the standard model. “If we find any difference, that would have dramatic impacts on physics,” says Malbrunot.
The problem is working with antiparticles, with their penchant for going up in smoke. To stand a chance, the Antimatter Factory is doing the opposite of what CERN is famous for: slowing particles down. A dedicated machine, the Antiproton Decelerator, is fed antiprotons and calms them so they can begin to create stable unions with positrons, and so form stable antihydrogen atoms.
A predecessor to the Antiproton Decelerator at CERN, known as LEAR, first manufactured antihydrogen atoms in 1995, but only in small quantities, and for very short times, and jiggling about too much to do precise measurements on them. Malbrunot’s experiment, ASACUSA, has been trying to solve these problems by making a de-excited antihydrogen beam that can be investigated by tickling it gently in flight with laser light. The collaboration reported the first tentative signs of success in 2014 – and just recently, more certain signs. “Right now, we are about to show we have succeeded in forming antihydrogen in this new way,” says Malbrunot.
Everyday antimatter
Antimatter exists in our world – you just have to be very alert to spot it.
Everyday antimatter generally takes the form of antimatter electrons, known as positrons, produced in radioactive beta decays. These positrons lead transient lives before annihilating with the first electron they encounter, producing energy in the form of gamma rays.
Like the occasional breaking of the most delicate of winds, we all emit the occasional positron, thanks largely to traces of radioactive potassium-40 in our tissues. A medium-sized banana produces one maybe every 75 minutes. A bag of Brazil nuts, or a worktop or bedrock made of granite, ups the ante considerably.
None of this is a danger to us. The energy released by each annihilation amounts to precisely two electron masses, or 1.022 megaelectronvolts – in the standard units of very small energies, considerably less than one millionth of the energy of a flying mosquito.
It follows that technologies to propel humanity further, such as antimatter drives or rockets, or blow us to kingdom come, such as antimatter bombs, aren’t exactly immediate prospects. The total energy of stable antimatter contained so far over decades of experiments isn’t enough even to boil the water for a cup of tea. Plus, you would need a vessel a couple of hundred metres across to hold it in place.
So what is antimatter good for? That is one of the favourite questions of Tara Shears at CERN’s LHCb experiment. “Antimatter is really esoteric, isn’t it?” she says. “You probably don’t expect antimatter to help diagnose cancer or help with heart problems, or have any practical benefit at all – but it does.” Positrons make up the P in a PET scan, which stands for positron emission tomography. Here the annihilation emissions of positrons in the radioactive tracer you swallow can illuminate all sorts of potential internal nasties. “That’s really useful,” says Shears.
Meanwhile, Hangst’s ALPHA experiment has stolen a march. Its approach is to cool antihydrogen atoms to within a whisker of absolute zero and hold them in suspended animation. Its best effort is trapping more than 1000 of them at once. “At every step of this, people said that this would never work,” says Hangst. “You would never make antihydrogen, if you made it, you would never trap it. If you trapped it, you would never have enough. And now we have all those things, but all that has taken about 30 years, and it’s only really worked in the last three.”
“If we find any difference between atoms and anti-atoms that would have dramatic impacts on physics”
In 2018, the ALPHA collaboration published its first comparative measurement of the frequency of a “hyperfine” transition between two positron states in antihydrogen, showing agreement with the hydrogen value to a . That might sound like conclusive evidence of nothing doing, but the hydrogen transition has been measured to 1000 times better precision, meaning there is still plenty of room for discrepancy.
Just last week, the team published a measurement of the even tinier “Lamb shift” in antihydrogen. This effect is caused by energy fluctuations in the quantum vacuum, and should be very sensitive to any signs of unknown physics. Again, there was no measurable difference compared with hydrogen – but any definitive statement would require much more sensitive measurements.
In late 2018, however, the Antiproton Decelerator was switched off to be hooked up to a new machine, the Extra Low Energy Antiproton ring, or ELENA. Long-term, ELENA will enable antiprotons to be slowed even more, increasing by between 10 and 100 times the number that experiments can play with. For Hangst, it was a blow. “They shut us down at the worst possible time,” he says. “We were just getting really mature with all of this.”
The beefed-up machine should be turned on again early in 2021. With it will come not just better measurements of antihydrogen, but also the answer to an even bigger question – one that could render all the previous discussion redundant. Does antimatter fall down, or up?
Again, there are strong suppositions. “A huge majority of theoretical physicists, perhaps too huge to be right, believes antimatter falls the same way as matter,” says CERN theorist Dragan Hajduković. If it turns out that it instead falls up, everything is to play for. “It would really turn everything on its head from the instant of the big bang,” says Hangst.
For a start, it means that anyone aiming to explain matter’s dominance in our universe through the breaking of fundamental symmetries might be chasing shadows. If matter and antimatter fall in opposite directions, they probably also repel each other gravitationally. In that case, they could have chased each other away to opposite ends of the universe before having a chance to annihilate each other. The option that antimatter is just hiding, now lurking beyond the horizon of our observable universe, is back on the table. “You could potentially imagine that matter and antimatter early on have completely separated because of some antigravity,” says Malbrunot.
“If it turns out that antimatter falls up, everything we know about the universe would be to play for”
The electromagnetic force is far more potent than gravity on small scales, so to measure how antimatter responds to gravity, we need anti-atoms to be electrically neutral. As soon as Hangst and his team made their antihydrogen-trapping breakthrough in 2018, they activated plans to essentially tip their horizontal trapping apparatus at right angles to function as a gravity detector, dubbed ALPHA-G. “I’ve worked harder during that stretch in my life than any other time,” says Hangst. “We started in May and worked seven-day weeks until the middle of November.” But at that point, with just a couple of weeks more needed to make their first measurements, the valve supplying the antiprotons was turned off. “It was so frustrating, you have no idea.”
Hangst isn’t the only one gagging for the antiprotons to come back on stream, due for early 2021. Another detector that Malbrunot is working on, AEGIS, and a third experiment, GBAR, are also gearing up to confirm whether antimatter falls up or down, and also answer the much more fiddly question of whether matter and antimatter feel the same strength of gravitational force.
Any deviation from expectations would have huge repercussions. Hajduković has shown how the existence of two opposing gravitational charges would allow the quantum vacuum itself to become a source of attractive and repulsive gravitational effects. That might account for both why there seems to be a lot of gravitating stuff out there that we can’t see – dark matter – and a mysterious force that seems to be speeding up the expansion of empty space, which we call dark energy.
Black holes sucking in matter could then also be white holes spewing out antimatter. Hajduković says the possible new source of antimatter could explain strange excesses of high-energy positrons and antiprotons observed in cosmic rays, as well as very-high-energy neutrinos seen coming from our galaxy’s centre by the IceCube detector in Antarctica back in 2014.
Existence of two opposing gravitational poles might even lend weight to the idea that the big bang wasn’t a beginning, but the latest in a series of cycling matter and antimatter universes. “Antimatter gravity experiments might be much more than a measurement of the gravitational acceleration of antimatter,” says Hajduković. “They might open a window towards a new physics and a new model of the universe.”
Or perhaps not. When the upgraded LHC and ELENA machines switch on next year, their antimatter investigations could bring a radical new understanding of how our universe works, and how we come to be in it. Or it could be, well, an anti-moment, one that sends us back to the drawing board in our quest to make sense of reality.
Hangst is philosophical. “In one direction, you win the Nobel prize, in the other, people say, ‘OK, well, we told you so’,” says Hangst. “These are very challenging experiments, and it’s rewarding to succeed no matter what answer you get.”
