Is antimatter the key to understanding more about our universe and propelling future spacecraft between the stars? All About Space investigates how close we are to finding out more about this exotic matter
Written by Gemma Lavender
Imagine a mirror held up to the universe, one that reflects matter on the scale of particles. Just like a normal mirror, the image would be reversed. Particles like protons with positive charge would suddenly look to be negatively charged, while electrons that spin in quantum fashion one way would appear to spin the other way. While the universe doesn’t really have a mirror, particles of matter do have mirror images of themselves, known as antimatter.
“[Matter and antimatter] are equal and opposite, that’s the theory so far,” says antimatter researcher and spokesperson for CERN’s Antihydrogen Laser Physics Apparatus (ALPHA), Jeffrey Hangst. “The antiparticle equivalent – antiprotons, antineutrons and positrons – are just like their matter counterparts but they have an opposite charge in the case of the charged particles and when they meet they annihilate.”
So when the two clash, they do so explosively. Just as Einstein’s famous equation E=mc2 describes the equivalence of mass and energy, when a particle and an antiparticle come into contact with each other, they utterly annihilate in a flash – there one moment, gone the next – converting all their mass directly into energy. It’s for this reason that, if antimatter could be harnessed, we would have an impressive energy source on our hands. The trouble is, there’s just not that much antimatter about.
That’s the major problem with antimatter, especially when its far more common counterpart – common matter – is found lurking everywhere. Antimatter can be created then destroyed in such a short space of time that experts do not have much time to hold antimatter down long enough for us to question what about its existence makes it special. As a result, there are not only gaps in our knowledge when it comes to this shy matter itself, but also in our theories of how the cosmos came into existence. “The names matter and antimatter are a bit arbitrary,” adds Hangst. “We believe that if you built a universe out of antimatter it would behave in the same way, so we don’t know why nature chose one over the other.”
Without a doubt, the Big Bang is a widely supported theory and it tells us that matter and antimatter should have been created equally at the beginning of time around 13.8 billion years ago. They should have annihilated each other leaving nothing behind, but we exist today in a universe with plentiful matter and scarcely a drop of antimatter. Thanks to our natural curiosity for exploring things and taking things apart to reveal the fundamental building blocks of matter, one thing remains unclear; what happened to the antimatter that once existed?
“[The study of antimatter] is motivated by the fact that we believe that matter and antimatter should have been produced in equal quantities at the [time of the] Big Bang and as far as we can observe so far the universe just contains matter, so we don’t really know what happened,” explains Hangst. “None of the theories that we have, or the so-called standard model [of particle physics] tell us what happened to the antimatter. That’s one of the biggest unsolved questions in physics – why is there a universe at all?”
While the Big Bang is an extraordinarily successful theory, as it currently stands, we shouldn’t exist since the matter from which we are built from should have been annihilated away. The University of California’s Professor Joel Fajans, who has recently enlisted the help of the ALPHA experiment to investigate if antimatter and matter are affected differently by gravity, echoes Hangst’s and other scientists’ thoughts that maybe there has been an error in our understanding of how much matter and antimatter was produced when the universe began. “I wish I knew why the amounts produced are not equal,” he tells All About Space. “Understanding antimatter is important to our very existence.”
And so, that’s what experts have been trying to do ever since antimatter was first proposed by physicist Paul Dirac in 1931; study the ying to matter’s yang in the hope of locking down something substantial, providing the answers to the mysteries of space that have eluded us for so long. A year after Dirac’s proposition the first antiparticle – the positron – was discovered, followed by the antiproton and antineutron two decades later.
But in our attempts to delve into ways of pinning down antimatter, scientists have hit a few snags. Creating it artificially is one thing, but making enough of it and keeping it within our grasp for long enough is quite another. “First you have to produce [antimatter], it can’t exist naturally [in significant quantities] in a matter universe so there are lots of very difficult technologies that you have to master to produce it and then to hold on to it,” explains Hangst. “It needs to be held in a vacuum, a very, very good vacuum.”
While the likes of Hangst and other scientists all over the world have been trying to get this down to a tee, Hangst insists that we still have much to figure out. “We are still learning how to efficiently produce it and handle it in a matter universe and even if you master these techniques, you are typically dealing with small quantities. It is not like you can buy a bottle of antihydrogen and make it [many] atoms at a time, so even after all that technology you are still left with very little of the substance.”
CERN has been able to produce thousands of atoms of the simplest antiatom, antihydrogen, at a time, yet capturing it has proven to be problematic. “We’ve only managed to trap one atom at a time,” explains Hangst. “We are really talking about a very, very rare substance.” Nevertheless, ALPHA has been able to trap some antiatoms for as long as 1,000 seconds – holding them still long enough for scientists to study them before they annihilate.
As a result of the painstaking methods used to make antiatoms, antimatter is deemed to be the most expensive material to produce with NASA suggesting that it would cost around $62.5 trillion (£41 trillion) to produce just one gram of antihydrogen. To date, only a few tens of nanograms have been created at particle accelerators like the Large Hadron Collider at CERN.
That’s why as things stand antimatter is never going to be a power source of the future; there’s just not enough of it to do anything with. “It takes more energy to make it than you would get back, so that’s a loser – that’s not what you want from an energy source,” states Hangst. “If some antimatter flew by and you could get a hold of it, that would be an energy source, but as far as producing it on Earth, that’s just not even close.”
Sadly, that means we may have to forget about antimatter-powered starships, for the moment at least. However, particle accelerators are not the only places where we can find antimatter. In 2011, 160 nanograms of antiprotons were discovered trapped in the Van Allen radiation belts above Earth, with similar amounts expected to exist in the magnetically organised radiation belts of other planets, including up to 260 nanograms around Saturn. Yet this is still a very tiny amount – add it all up and it still doesn’t even come to a gram.
In the same year, astronomers announced that the Fermi Space Telescope, which observes the universe in gamma rays, had detected antimatter not coming from space, but streaming into space from above thunderstorms in Earth’s atmosphere. Fermi detected high-energy gamma rays at just the right energy to indicate they were created when an antimatter particle annihilated a matter particle.
“Thunderstorm electric fields accelerate electrons to high energies,” explains Professor Joseph Dwyer of the Florida Institute of Technology. “These electrons make gamma rays, which then pair-produce electrons and positrons, which are the antimatter version of the electron. The positrons may play an important role in the electrical properties of thunderstorms – it has been quite surprising how common positron production is in our atmosphere and how the positrons can actually be important for understanding thunderstorms and lightning.”
Fermi was actually expecting to see gamma rays from matter-antimatter annihilation near the centre of the galaxy, as was its European counterpart, the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL), which discovered a lopsided cloud of positrons in the galactic centre where annihilation is taking place and producing gamma rays with energies of around 511,000 electronvolts. Meanwhile the state-of-the-art Alpha Magnetic Spectrometer (AMS) on board the International Space Station is searching for antimatter in cosmic rays.
So it seems we are really starting to make headway in our quest to solve the mysteries of matter and antimatter and ultimately the grand mystery of why the matter-dominated universe as we know it exists at all.
“There’s lots going on,” says Hangst. “It’s a very interesting time to be working in this field because we are getting more and more capabilities. At CERN we are studying antimatter to see if it behaves in the same way as matter; that’s a long-term project. We’re also looking for matter/antimatter asymmetry – does antimatter somehow behave different to what the laws of physics describe for matter? We’d like to study the spectrum of antihydrogen and compare that with what we have measured in hydrogen, or look at how antimatter behaves in a gravitational field. So those are the two big things: is antihydrogen quantum mechanically the same as hydrogen and does it fall up or down in gravity?”
The answers might unlock the secrets of the expanding universe and we could be extremely close to doing just that.