Is there gravitational attraction between matter and antimatter?

Is there gravitational attraction between matter and antimatter?

– Frank

This is a question that probes the fundamental interactions that govern our universe and teeters on the edge of current physics research. The short answer is yes – based on our understanding of gravity, there would be gravitational interaction between matter and antimatter.

But the long answer is a bit more complex. Essentially, we don’t yet know how these interactions would happen, because most of the time they’re on the quantum scale.

To help explain, we called up Shanette De La Motte, who is completing her PhD in particle physics at the University of Adelaide.

De La Motte explores physics using data from Japan’s SuperKEKB electron-positron collider, which smashes particles and antiparticles together. We first asked her, what is antimatter?

“When we think of antimatter, we think of ideas out of sci-fi,” she says. “Like, it’s really rare, it glows, and if you combine it with regular matter you get an atom cloud… very Star Trek kind of ideas.”

But antimatter is actually almost identical to regular matter – except with a change in quantum number.

“The easiest example of a quantum number that changes is electric charge,” explains De La Motte. “In the model of the atom, we know that there are negative electrons, positive protons and neutral neutrons. And we can come up with antimatter versions of these, so an antimatter electron would be a positron, an antimatter proton is an antiproton, and an antimatter neutron is an antineutron.”

Protons and neutrons are made up of even smaller particles called quarks, and they have their electric charge flipped, too.

We actually create antimatter on Earth on a daily basis – for example, in a Positron Emission Tomography (PET) scan, the patient is injected with a radionuclide that decays into other particles including positrons. These annihilate with electrons in your body to give particles of light, which are used to study the organ or tissue.we even know that antiparticles exist?

Four pet scan images of the brain, with different brain regions in different colours
These are computer-generated images called PET (positron emission tomography) of the brain. It consists of injecting a radioactive analogue of glucose, FDG (fluorodeoxyglucose) into the bloodstream; the 3D images of tracer concentration within the brain are then constructed by computer analysis. The more metabolically active areas will retain more FDG, and consequently retain more radiation (orange colour). Credit: wenht / Getty Images

“If you’ve had a PET scan, you’ve had radioactivity and antimatter existing in your body – and you survived to tell the tale,” De La Motte jokes.

But how do antiparticles interact with the fundamental forces of the universe?

“Antimatter is identical to regular matter, except for the flipped charge,” she says. “Any force that a regular particle interacts with, its matching antiparticle will do the same thing.”

Which brings us to the question at hand. Physicists also theorise that particles and antiparticles can interact with each other gravitationally, because gravity acts between any two objects with mass.

“If the original particle has mass, it could interact via gravity with its antiparticle,” says De La Motte. “We don’t know the exact mechanism – by how much energy, by how much force. We don’t know numbers. We just have a vague idea that it should.”

Black and white view of particle track
Cloud chamber photograph of the first positron ever observed. The thick horizontal line is a lead plate. Credit: Anderson, Carl D / Wikimedia Commons

We do know, however, that we can rule out gravitational repulsion.

“There are some people who’ve theorised that antimatter particles might have negative mass, so if you had a negative mass, then you would be repulsive under the gravitational force,” says De La Motte.

But this is highly unlikely, given that antiparticles have identical masses – in both amount and sign – to their particles.

It’s worth noting that there is nothing special about the antimatter and matter gravitational interaction. Gravity treats every pair of objects with mass the same.

“Hydrogen and anti-hydrogen could interact gravitationally, but it would be in the same way as if it were hydrogen-hydrogen gases,” says De La Motte.

But it does all depends on the scale.

“A soccer ball-sized amount of antimatter and matter would annihilate each other via the electromagnetic force instead of zooming towards each other with a noticeable gravitational attraction,” she explains.

“The only scale for which the gravitational force would dominate would be universal – such as an antimatter planet orbiting a star made of antimatter.”

In practice, if matter is going to interact with antimatter, it’s on the quantum scale.

Problem is, our theory of gravity is a macroscopic one, developed on a scale ranging from the size of humans to the size of the cosmos. But the kinds of particles we’re talking about here are tiny – smaller than the nucleus of a cell – in the quantum world where gravity tends to be overwhelmed by the other three forces.

In fact, physicists still don’t fully understand how gravity works for any subatomic particles.

“We have this great theory for large scale – general relativity – and this great theory for small scale – quantum field theory – but we can’t make them unite,” says De La Motte. “It’s an unanswered challenge. There are people who look into quantum gravity, but it’s still something that’s many, many years away.”

We do, however, have some theories. Physicists have suggested, for instance, that a hypothetical particle called a graviton could mediate the force of gravitational interaction.

“One [particle] will emit [a graviton] to tell the other particle, ‘hey, I’m massive, I’m here, we should be attracted to each other’,” says De La Motte. “But we haven’t really been able to nut out the maths as of yet.”

If a graviton was found, it would add another missing piece to the puzzle of the Standard Model – the framework that explains all the building blocks of matter and how they interact.

This, in turn, would be another step towards a Theory of Everything.

“This is a work of future scientists,” says De La Motte. “It’s going to be some time before we get to Year 12 [physics] questions that say, ‘I have an electron, and it’s next to a positron. How far did it move because of its gravitational attraction?’”

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There’s no such thing as a stupid science question, but sometimes the answers can be tricky to find.

This summer we’ve partnered with ACM for the Summer of science: Ask us anything! Send us your curliest chemistry conundrum, perplexing physics problem or any science question at all and we’ll get our journalists onto the case.

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