ON 11 November last year, a small birthday party was held in an apparently unremarkable hangar onthe outskirts of Geneva, Switzerland. Nothing too fancy, just a few people gathered around a cake. The honourees were there. Well, sort of â they were still locked in the cage where they had spent their first year. But then again, there is no other way to treat a brood of antimatter particles.
The antimatter realm is so bizarre as to be almost unbelievable: a mirror world of particles that destroy themselves and normal matter whenever the two come into contact. But itâs real enough. Cosmic rays containing antiparticles constantly bombard Earth. A banana blurts out an anti-electron every hour or so. Thunderstorms .
Making and manipulating antimatter ourselves is a different kettle of fish. Hence that birthday party held at the particle physics centre CERN, . Thereâs a lot we would like to learn from these caged beasts and their ilk, not least this: do they fall up?
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Cards on the table, few physicists believe that such âantigravityâ effects exist â that if you released one of those antiprotons and somehow ensured it free passage through the hostile world of matter, it would magically float up. But the recalcitrant nature of antimatter means weâve never done the experiments, and until we do, we simply donât know. âProgress is often made by asking the questions we think we already know the answer to,â says of the Illinois Institute of Technology in Chicago.
The scepticism about all forms of antigravity dates back to the 1950s, when the physicist Hermann Bondi was pondering the implications of general relativity, Einsteinâs theory of how gravity arises from warping the fabric of the universe. Gravity is an odd sort of force, not least because it only ever works one way. With electromagnetism, say, there are positive and negative charges that attract and repel. With gravity, however, there are only positive masses that always attract.
Bondi showed what a bizarre world it would be if this were not the case, demonstrating how negative mass would end up pursuing positive mass across the universe (see diagram, above). This sort of ârunaway motionâ does not appear to exist â but we should be careful about what we draw from that, says of the Frankfurt Institute for Advanced Studies in Germany. âPeople who speak of the runaway problem often jump to conclusions from Bondiâs argument and conclude that anti-gravitation itself is inconsistent,â she says. âBut it merely requires a modification of general relativity.â
And hereâs the thing: general relativity is probably due a modification. The theory is incompatible with quantum mechanics, the other great pillar of modern physics, and if we are to find a way to make a unified description of the universe, that must change. Then everything is up for grabs.
So in a few labs around the world, the search for negative mass and its associated effects goes on (see âLosing weightâ). Antimatter is a particularly promising place to look. It is just like normal matter but with the opposite electric charge and a few other mirrored quantum properties. Thereâs no reason to think it has the opposite mass and anti-gravitates, and some good reasons to think it canât have.
But if antimatter did anti-gravitate, that might help with another of its central mysteries: where most of it is. Our theories say matter and antimatter should have been created in equal proportions in the big bang, and yet we live in a matter-dominated world.
The emptiest box
Explaining this glaring inconsistency has largely been a case of trying to find asymmetries in the processes of particle physics that favour normal matter. Such asymmetries do exist â but they are about a trillionth of the size needed to explain matterâs supremacy. âPeople have been trying to make it work â and it doesnât work,â says Kaplan.
Antigravity could provide a better explanation. A repulsive gravitational interaction could have driven matter and antimatter away from each other so they never had the chance to annihilate in the early universe. Since then, the ongoing expansion of the universe would have driven the twain ever farther apart â and the antimatter might eventually have created its own galaxies in other corners of the universe. âThen the missing antimatter would be hiding in plain sight,â says Kaplanâs colleague .
Add to that the technological possibilities that levitating matter away from Earthâs surface might bring, and even the US air force wants in â it has to antimatter researchers over the years. Unfortunately, doing the experiments turns out to be quite an ask.
The problems start with needing a home for antimatter that is almost entirely free of normal matter. That requires some of the emptiest boxes on Earth, containing just hundreds of gas molecules per litre (there are about 1022 in a typical litre of air). But even these boxes have sides. To stop the antimatter banging into them and instantly annihilating, you must slow it down by cooling it to within a few degrees of absolute zero and then catch it in a vortex of electromagnetic fields. Little by little, weâve been perfecting these arts, holding antimatter particles for seconds, minutes, days â and for a year, as celebrated at Novemberâs party.
That milestone was reached by CERNâs (BASE), one of six experiments competing to measure antimatterâs fundamental properties that are all housed in CERNâs . Inside, past a sign marked âAntimatter factoryâ, the most noticeable things are the bright yellow cranes, swinging around the vats of liquid nitrogen required for cooling. Somewhere below, a beam of particles from CERNâs Proton Synchrotron accelerator smashes into a block of metal, creating a plethora of particles. A system of magnets selects the antiprotons and funnels them into a ring of more magnets that keep them on course as they are decelerated for trapping.
Experiments have been running here since the 1990s, studying whether antimatter and matter particles truly are as close to identical as we think. In 2015, by measuring how antiprotons danced around in a magnetic enclosure known as a Penning trap, BASE produced the most precise measurement yet of their mass-to-charge ratio. They showed it was the same as a protonâs, to about 69 parts per trillion, four times more precise than the previous best value (). Last November, the neighbouring produced the most accurate measurement yet of the antiprotonâs mass, finding no evidence of a different value from the protonâs ().
The same value â but is the mass positive or negative? That is the multimillion dollar question, and it takes the experiments to a new level of fiddliness. Gravity is weak and easily overwhelmed by the electromagnetic force, so using charged particles such as antiprotons and controlling them with magnetic fields wonât do. You could try getting an antiproton in position and shutting off the magnets to see which way it falls, but the antimatterâs electrostatic interactions with its surroundings would overwhelm any gravitational push or pull it might feel.
A better bet is neutral atoms of antimatter, such as antihydrogen. Making these is no cakewalk, but they have a tiny electric polarity that makes it worth going the distance â their electrostatic interaction isnât strong enough to swamp gravity, but very strong magnetic fields will still hold them in place. CERNâs (ALPHA) experiment has been doing this since 2005, and now routinely traps and holds bunches of antihydrogen atoms for about 15 minutes. âJust the other day we trapped 350,â says Jeff Hangst, head of ALPHA.
Not up, just less down
In 2013, ALPHA published a proof of principle measurement, briefly collecting a cloud of 434 antiatoms, turning off the magnets and tracking their subsequent motion by where they annihilated. It was a crude test, and inconclusive â the final answer was compatible with the .
Work on a souped-up version that gives the particles more space to fall should start this year. âWeâre going to knock out a wall and build a vertical version of the experiment next door,â says Hangst. Getting the necessary accuracy wonât be easy, because the antiatoms ALPHA uses are relatively hot and so jiggle around, which clouds the issue. But large enough numbers of antiatoms should help us answer the central question. âUp or down â that should be possible,â says Hangst.
A further CERN experiment, , also aims to perform tests within a few years. Kaplan is planning experiments with muons, heavier cousins of the electron, and a team led by of University College London is planning to use positronium, an âatomâ consisting of an electron and its antimatter partner, a positron, orbiting one another.
Back at CERN, the , experiment intends to tackle the question using a single antihydrogen ion, a combination of one antiproton and two positrons. In theory, it should be easy to hold this charged speck in place with magnetic fields and cool it with lasers. The idea is then to knock off a positron using another laser, making the antiatom neutral. At this point it would cease to feel the effect of the trapping field and fall â up or down. GBARâs head, Patrice Perez, says they expect to make measurements sensitive to detect even a 1 per cent deviation from the gravity felt by normal matter.
Construction of the experiment wonât start until later this year, and requires new lasers and an extra . Hangst is confident of beating the upstart to the punch. âI view GBAR as a case of five miracles happen and then it works,â he says. One telling fact is that GBAR plans on using only one detector, below the trap. âWe really do not expect antimatter to fall up,â says Perez.
Even if it falls at all differently, however, that would still be hugely interesting. âIn all the descriptions I know, antimatter cannot antigravitate,â says of CERN. Whatâs more plausible, he thinks, is that there might be other forces that modify gravity whose effects cancel out on normal matter, but not on antimatter. In that case, antimatter might not fall up â just less down. âNow, thatâs not natural, but it is logically possible,â he says. Similar gravity-modifying effects might be produced if the graviton, a quantum particle proposed to carry the force of gravity, has a small mass, rather than being massless as is usually assumed.
Even so, we probably shouldnât be holding our breath for amazing self-levitating machines any time soon. A more immediately practicable way of using antimatter to beat gravity might be to harness the energy released when it annihilates. One firm, in Livermore, California, has been developing the idea with financial support from PayPal co-founder , among others.
Positron-fuelled rockets could power spacecraft much further and faster than is currently possible, according to Positron Dynamics co-founder Ryan Weed. âOur vision is to create technology that allows humanity to venture outside of our solar system,â he says. The companyâs involves harvesting positrons from radioactive sodium-22 and using these to start off a nuclear fusion reaction that generates thrust. Weed says the team is set to test the device in a lab and wants to test it in orbit in the next few years.
But experience makes Stefan Ulmer, the head of the BASE experiment, cheerfully sceptical of immediate progress. Antimatter wonât be easily tamed. âIn the whole history of the CERN Antimatter Deceleration Hall, weâve produced about enough to heat up a cup of water by about 5 °,â he says. Not even enough, in other words, to make a pot of tea to wash down that birthday cake.
Losing weight
Is it crazy to think something might have less than zero mass? It seems like it, but , a physicist at the Dresden Institute of Technology in Germany, is not so sure. âItâs a bit like walking on top of a mountain and seeing the ground beneath you,â he says. âYou think, âhey Iâm on the groundâ. But there are lower bits of ground that are not up a mountain.â
The question isnât necessarily the same as whether anything can anti-gravitate (see main story). There are two types of mass: gravitational mass quantifies how strongly an object feels the force of gravity, whereas inertial mass quantifies an objectâs resistance to acceleration. Experiment after experiment has shown that these two quantities always have the same value â a mysterious equivalence that lies at the heart of Einsteinâs description of gravity, general relativity.
Break the equivalence principle and you could have an object with normal gravitational mass, but a negative inertial mass. Such a body would fall normally in a gravitational field, but give it a push and it would accelerate towards whatever is pushing it, not away from it. Pair that with normal mass and you could create a system that self-accelerates. âMy motivation is to build something like a warp drive,â says Tajmar.
There are a few theoretical avenues for creating negative mass, says Tajmar. One comes from the US historian and physicist , who proposes that by cobbling together bits of general relativity you can make particle masses fluctuate, even into negative territory. Woodward this effect since the early 2000s.
Tajmar is working to test this too, as well as investigating another proposal. This hinges on a theory called Weber electrodynamics that is viewed with narrowed eyes by most theorists â a position Tajmarâs latest unpublished results seem to support. âWhat I can say, is that if it is there, the effect is very small,â he says. Plus, he thinks it could only be turned into a propulsion system inside a charged cage. âSo thatâs not very useful.â
This article appeared in print under the headline âUpwardly mobileâ
