Antimatter particles obey gravity

What are the implications for our understanding of physics?
29 September 2023

Interview with 

Ben Allanach, University of Cambridge




Antimatter particles obey gravity: that's the conclusion from a study this week testing whether antimatter - the mirror image material of the matter we're all made from - "falls" upwards, or downwards. Although very much the preserve of the sciFi arena, antimatter is a very real entity - we use it all the time in medical imaging, for instance. In traditional matter, the protons in the nuclei of atoms are positively charged and the electrons that surround them are negatively charged. Antimatter particles are the reverse with positive anti-electrons and negative anti-protons. And should matter and antimatter meet, they cancel each other out in a flash of energy. But what scientists did not know was whether it was just the charges that work in reverse, or whether gravity might have the opposite effect too: would the antimatter equivalent of Isaac Newton's famous apple actually float upwards out of the tree? Well, now we do know thanks to a team working at CERN who have done the experiment by watching how atoms of anti-hydrogen respond. Isaac Newton and his apple tree were not available for comment, but another Cambridge scientist, Ben Allanach who works on this branch of physics, agreed to talk through what the CERN team have found...

Ben - Antimatter is actually quite similar to matter, but it's got opposite electric charge. So in an atom for example, you have electrons buzzing around the outside. They have negative electric charge, but antimatter is an antimatter version of every particle with opposite electric charge. So the idea was if you have these kinds of particles, can you measure their reaction to gravity? And does it have the same reaction to gravity? The theory says that it should do. But of course you need to know experimentally whether it does or not.

Chris - This is not just the preserve of science fiction. There really is antimatter. We can make it, we can measure it.

Ben - Antimatter exists in the universe. It comes in terms of cosmic rays every day. So from the universe, it's being made, in the atmosphere and so on and it comes through us. It's not around generally. We are made of matter, not antimatter. And that's one of the puzzles that we'd like to solve. Why are we made of matter and not antimatter? All the theories say in the early universe, you had both actually. And mostly they cancelled each other out. When they meet, they just annihilate into radiation like photons or other particles which are not matter or antimatter. We know there's the potential for it to exist. And the question is where is it? But we also need to know how it behaves? And the theory says it should behave in exactly the same way as matter in terms of its gravitational force. But of course you want to measure and make sure there were some theories that it would repel. Gravity attracts things normally, and some people theorise that antimatter would have a repellent force under gravity instead. So you want to test that.

Chris - So if Isaac Newton had an antimatter apple, it wouldn't land on his head, it would go skywards.

Ben - Exactly. It would go floating off into space. Yeah.

Chris - How did they test it?

Ben - So they have a machine at CERN called the alpha experiment. It's basically a long tube, maybe 10 metres long or so, and they fill it with antihydrogen. So that's one of these anti electrons and an antiproton. Quite difficult to make actually, and keep it steady. If you saw Angels and Demons, it was the antimatter in that. But anyway, so they make this anti hydrogen and then you put it in this tube and see whether it sinks or floats basically. And you put a magnetic field, which makes it go up or down and you try and measure the effect of the gravitational force on top of that. So they're trying to do some clever maths and work out what the effect of the gravity on top was. Is it pulling them up or is it pulling them down? Does it agree with the normal gravitational acceleration that you get, which is roughly 10 metres per seconds per second.

Chris - And is that the case? Does the anti hydrogen obey gravity and fall downwards like normal hydrogen would?

Ben - That's exactly what they found. It falls down. The uncertainties are quite large, so it looks like it's got more or less the same force, but you can only tell within 25% whether it's the same as ordinary matter. But basically, yeah, they're coming out the bottom of the tube and not going out the top. So indeed it falls towards the Earth as expected.

Chris - What are the implications of this and why does this matter to the average person walking down the street?

Ben - One of the theories of why we don't see antimatter around in the universe today was that actually it repels matter. And so you'd get clumps of separated anti-matter and separated matter in large patches of the universe. That's one of the theories that this experiment has debunked, that no longer works because it all attracts itself. That's one of the things that's interesting. In terms of applications, knowing how anti-matter interacts gravitationally, you could try and use that in industrial applications. Gravity's used a lot. <laugh>. I actually find gravity a real pain. If I put something somewhere, I want it to stay there, but it seems to fall towards the Earth. And so now we know that antimatter does the same thing. And there may have been some ways of floating things in space, for example, if it behaves in a different way.

Chris - So one step closer to a warp drive, maybe.

Ben - Exactly. We're all Trekkies at heart. Yes.


Add a comment