Originally published on 16th March 2019
Most of us grew up with the idea of the warp core - the Star Trek universe's starship reactors which power starships through spacetime, driven by the explosive annihiliation of matter with antimatter.
The stuff of science fiction, yes – but both matter and antimatter are equally real (although not, as we shall see, in equal quantities). It's also true that they release enormous amounts of energy when they touch, annihilating each other as they do so, just like in the sci-fi classics.
Credit: Star Trek; Paramount Pictures
Particle physics is a bit of a nerdy obsession of mine. I studied metaphysics, so I’m fascinated by how physics challenges and informs what our best metaphysical theories need to account for.
So, what is antimatter?
For every type of particle of matter, there is a corresponding type of particle of antimatter. In most ways, antimatter particles are just like their matter counterparts. They share the same mass and the same properties in every respect, with the exception of their electric charges and magnetic properties, which are opposite. For example the antimatter version of the electron, which is negatively charged, is the positron.
Hydrogen and antiydrogen. A little rough, but you get the idea! Credit: author
It's intuitive to think that for each token particle of matter which existed at the time of the Big Bang, there was a corresponding token particle of antimatter. This is what theory predicts.
Yet if there had been, then all of the matter and antimatter should have annihilated each other, and nothing interesting would exist - no stars or galaxies, no planet Earth, no humans - just radiation.
But that isn't what happened. About one particle of matter per billion escaped annihilation and went on to form the reality we know today.
Almost all of the antimatter seems to have disappeared - matter has prevailed.
This is known as the baryon asymmetry problem, and remains one of the greatest mysteries in physics to this day.
So, how can this be?
Solution one - imbalance
The most likely reason, as it is seen at the moment, is that matter and antimatter where not created in equal quantities. At least, not quite. For every billion pairs of matter-antimatter, it seems that one extra particle of matter came into existence. Physicists don't know why this would be, although around the world experiments are happening to try to establish the cause of the asymmetry.
One idea is that matter might enjoy some property, or has more abundance of a property, that causes it to be favoured for existence ahead of antimatter.
Yet measurements at CERN have shown a genuine, almost perfect, symmetry with a very low experimental uncertainty. This makes it very difficult for physicists to continue looking at properties of the particles themselves as solutions to the problem.
Instead, the focus has shifted to how particles and antiparticles interact. Let's look at some promising areas of research.
Imbalanced magnetic field
Research led by the Arizona State University appeals to the cosmic magnetic field to solve the problem. Their experiments have shown that the universe might have a lopsided cosmic magnetic field, which could in turn have unbalanced the proportion of matter to antimatter during its creation.
Imbalance through energy released during annihilation
Rather than looking at properties of the particles themselves, researchers at Tevatron while it was operational have focussed rather on the interaction between particles and antiparticles. Their research has shown that different matter-antimatter pairs produced different types of energy. Electrons and positrons release photons, meanwhile protons and anti-protons yield an electron, an an anti-neutrino, a down anti-quark, a up quark, a bottom anti quark, and a bottom quark. It is postulated that this complexity could lead to the imbalance.
Imbalance caused by neutrino oscillation
Another promising idea involves neutrinos, tiny particles with the ability to oscillate between different types as they travel. Experiments like T2K in 2016 and more recent analyses have observed intriguing disparities in how neutrinos and their antimatter counterparts, antineutrinos, behave. For example, T2K detected that 32 muon neutrinos transformed into electron neutrinos, compared to just four antineutrinos undergoing the same change.
While these results were only statistically significant at a 95% confidence level back then, ongoing studies like NOvA have strengthened the case. NOvA's 2024 findings show improved precision in measuring neutrino oscillations and suggest a moderate likelihood that neutrinos and antineutrinos don't behave as mirror images of one another (they call this "charge-parity violation"). If confirmed, this could explain why matter came to dominate over antimatter after the Big Bang.
These findings make neutrinos one of the most promising pieces of the puzzle, although we’re still not at the level of certainty needed to declare this the solution.
Imbalance caused by initial particle oscillation
One idea is that "oscillating" particles in the first moments of the Big Bang were influenced by something, causing them to decay more often as matter than antimatter. What that "something" could be, though, is not known. These oscillations have been observed, but whatever might have influenced the bias has not.
Solution two - antigalaxies
Perhaps our perception that matter has prevailed over antimatter is false. On this view, there might be entire galaxies and superclusters comprised of antimatter.
Obviously if that were the case, they would exist where "ordinary", matter-comprised galaxies and superclusters are sparse, since they would annihilate on contact.
If this is true, then why are we not detecting huge bursts of radiation caused when matter meets antimatter? These days, it isn't considered likely that antimatter dominates any pocket of our universe. Nevertheless, some anomolous events resembling events involving antihelium have been obesrved, and it is not out of the question that they are signs of antimatter galaxies.
Wrapping up
Sadly, we won't be warping around the universe in starships powered by matter-antimatter reactors any time soon.
Apart from anything else, making just 1 gram of the stuff would cost more than a million billion dollars.
Advances in particle physics, however, bring us closer to unlocking antimatter's mysteries and its role in shaping the universe.
Want to learn more?
There is loads to read and watch, both online and offline on antimatter.
Symmetry Magazine is an extraordinary, free online resource - it is very easy to get absorbed, and I highly recommend it!
If you are interested in the deep forces shaping the universe, whilst published a while ago, Martin Rees' 1999 Just Six Numbers is an excellent and accessible read.
I have also enjoyed Frank Close's treatment of the subject in his 2009 book Antimatter.
