Antigravity
WHEN SPACE IS THROWN UPWARDS
It's the force that throws objects up, and if found it could be the holy grail for space travel among the stars
It’s the force that throws objects up, and if found it could be the holy grail for space travel among the stars
Straddling the Franco-Swiss border near Geneva lies the European Organization for Nuclear Research, more popularly known as CERN.
This laboratory houses the accelerators, detectors and equipment comprising a number of different particle physics experiments, including the famed Large Hadron Collider (LHC) responsible for discovering a particle suspected to be the Higgs boson, the so-called ‘God particle’, in 2012.
CERN’s particle accelerators are designed to study the tiniest constituents of matter: the fundamental particles that form everything from stars and planets to your afternoon cup of tea.
This type of matter is known to scientists as ‘baryonic matter’. The vast majority of what we see throughout the visible cosmos is formed of baryonic matter, comprising particles including protons, neutrons and electrons – the latter are not technically baryons, but are usually considered baryonic matter by most astronomers.
However, this is not the only particle world to exist. Every ‘ordinary’ particle we know of is accompanied by an antiparticle – a mirror image that has the same mass as its ordinary counterpart but opposing properties – charge, spin, quantum numbers, magnetic moment. The electron, for example, has a charge of -1, and is partnered by the positron, which has a charge of +1. The proton partners the negatively charged antiproton, the neutron the antineutron and so on.
This exotic type of matter, known as antimatter, was first predicted to exist in the 1920s by British physicist Paul Dirac, and was discovered experimentally shortly after. Matter and antimatter particles are always produced in pairs, albeit spectacularly unfriendly ones. If the two meet and collide they destroy one another in a process known as annihilation, producing a burst of energy as both are wiped from existence.
Matter and antimatter should therefore be created in equal measure, and we would expect the Big Bang to have filled the universe with the same amount of each. These particles should have found their way to an opposing particle and annihilated over time, leaving an empty universe behind.
Controversially, we instead see a matter-filled universe and catch glimpses of antimatter only fleetingly during thunderstorms, natural radioactive decay and within phenomena such as cosmic rays. This implies that there may not be perfect symmetry between the processes at work within the matter and antimatter worlds, and that the laws of nature may not be mirrored between the two equal, yet opposing forces.
“Differences between the behaviour of matter and antimatter are embedded in our conventional theory, which is known as the Standard Model,” says Professor Chris Parkes of the University of Manchester in the UK and leader of the university’s involvement in the LHCb experiment at CERN’s LHC. In fact, “the Nobel Prize was awarded in 2008 to the theoretical work that embedded this matterantimatter asymmetry into the foundations of the Standard Model.”
“Differences between the behaviour of matter and antimatter are embedded in our conventional theory” Chris Parkes
To explore this asymmetry, scientists turned to the fundamental forces of nature. There are four such forces: strong, weak, gravitational and electromagnetic. The strong force holds nuclei together, gravity is an attractive force between masses, electromagnetism governs charge and magnetism and the weak force facilitates decay.
This final force, the weak force, may play a part in some of the known asymmetries of matter and antimatter. Interactions between particles and the weak force appear to occasionally violate something known as ‘CP-symmetry’, in which particles with differing charges and parities are not affected equally – ‘handedness’ is a good analogy, with some particles being ‘right-handed’ and others ‘lefthanded’. As matter and antimatter particles have largely opposite properties by definition, they thus behave differently in the weak force domain on rare occasion, resulting in a different number of interactions occurring for matter than antimatter over time.
This may have something to do with the surplus of matter we observe in the universe, but as with all open scientific questions, we are unsure of the full picture. Cosmologists suggest the Big Bang may simply have created more matter than antimatter for some reason, and it remains possible that while we see matter, all the antimatter is simply hiding elsewhere in the universe – or multiverse.
Antimatter-matter unbalance aside, we have a lot to learn about the exotic world of antimatter, and scientists are now exploring how another fundamental force interacts with antimatter: gravity.
“The gravitational force on antimatter has never been directly measured,” says Daniel Kaplan of the Illinois Institute of Technology (IIT). “For all we know, antimatter might even fall up. This possibility is known as ‘antigravity’: gravitational
repulsion between matter and antimatter.” Gravity is an attractive force exerted by objects with mass. It causes massive bodies throughout the cosmos to draw in material from their surroundings and clump together, gives all objects on Earth an associated weight and keeps our feet firmly anchored to the ground.
“The established theory of gravity is Einstein’s theory of general relativity,” explains Kaplan.
“This theory is in excellent agreement with all available evidence. However, there are reasons to believe it is wrong. For example, it is a ‘classical’ theory based on the assumptions that space and time are continuous, and that velocity, force and energy can have any value. Physicists believe these assumptions are incorrect, and that quantum theories represent the true nature of reality.”
While general relativity is well accepted and has been studied in depth for many years, aligning the theory with quantum mechanics, something that is at the forefront of current research, has proved difficult. Reconciling the two may be aided by matter-antimatter measurements to complement the largely matter-matter measurements that physicists are currently working with.
Some scientists propose that gravity may affect matter and antimatter differently, given the particles’ mirrored properties. Perhaps if you created a positron and set it free it would float rather than sink, feeling gravity as a repulsive force. However, this is an extremely controversial idea.
“There is no reason to assume that antimatter would fall upwards,” says Holger Müller of the University of California, Berkeley, and a member of CERN’s ALPHA collaboration. “Unlike electricity, gravity doesn’t have ‘charges’ that could be positive or negative. There is not a shred of evidence for antigravity, and quite a lot of evidence against it.”
Svante Jonsell of Stockholm University, Sweden, agrees. “Almost no one would bet on antigravity, from either a theoretical or experimental standpoint,” he says. “If there was a difference [in the effect of gravity on matter and antimatter], that would really say that antimatter has some as-yet-unknown property that’s different from matter, which would be a sensational discovery,” he continues. “But we know there are things about antimatter we don’t understand – most importantly why we don’t see as much antimatter as matter around us – so there are good reasons to probe its properties. And, of course, you cannot really know for sure until you have looked.”
However, it may not be a simple case of balloon and anvil – it may be one of anvil and an everso-slightly lighter anvil. “While it’s crazy to argue
“The gravitational force on antimatter has never been measured… For all we know, antimatter might even fall up” Daniel Kaplan
that antimatter might ‘fall up’, you can make a serious case for looking for subtle changes in the acceleration of free fall,” explains Müller. “Antimatter might fall down, but with a very slightly different acceleration compared to normal matter. Even this is hard to argue, but not crazy.”
Even antigravity sceptics such as Müller admit that there is serious science to be done in this area, and several experiments are working to better understand gravity’s relationship with antimatter. Many of CERN’s collaborations and projects – a swarm of particle-physics acronyms including AEGIS, ATRAP, GBAR, BASE, ASACUSA and the aforementioned ALPHA – deal with antimatter and plan to precisely study its properties – and in some cases, free fall – in coming years. Several have already managed to create and trap antimatter. ALPHA has managed to create, trap and probe the gravitational acceleration of antihydrogen, and has plans to do so much more accurately.
“Measuring the acceleration of antimatter in the gravitational field of Earth is a crucial test of general relativity,” explains Kaplan. “Only in recent years has it become feasible. ALPHA published the first limit in 2013, establishing that the gravitational acceleration of antihydrogen is no greater than
110 per cent, and no less than around 65 per cent that of matter. This clearly leaves the question of antigravity entirely open.”
A problem with antimatter experiments is that the electromagnetic force overwhelms and distorts the effects of gravity for non-neutral particles.
Storing antiparticles, even neutral ones, is also incredibly difficult: the walls of any container or apparatus are necessarily formed of matter, which leads to quick annihilation. But there are glimmers of hope: CERN’s Baryon Antibaryon Symmetry Experiment (BASE) has managed to somewhat circumvent this issue by using adequate magnets.
Accurately measuring antigravity, or any effect in that arena, requires far more precise measurements and advanced equipment. With this in mind, CERN is adding to its apparatus and will continue to further its antihydrogen research in coming years. Kaplan and IIT physicists are also developing the Muonium Antimatter Gravity Experiment (MAGE), which although still a work in progress would be capable of measuring minute differences in gravitational deflection on the order of mere picometres – 1 trillionth of a metre.
While most scientists believe that the difference between the gravitational acceleration of matter and antimatter is likely to be tiny, any difference at all would be hugely significant.
“Even a small discrepancy – for example, the gravitational acceleration of antimatter being larger or smaller than that of matter by as little as one part per million – would change our understanding of cosmology in important ways,” says Kaplan. “Although most likely there would be no practical applications, antigravity would have enormous implications for the nature and evolution of the universe, possibly even eliminating the theoretical need for dark energy and dark matter.”
“There is not a shred of evidence for antigravity, and quite a lot of evidence against it” Holger Müller