Scientists have observed antimatter free-falling due to gravity for the first time

Lars Martin, TRIUMF Support Physicist, making adjustments within the ALPHA-g detector. (TRIUMF)

If Sir Isaac Newton was watching an apple made from antimatter fall from a tree, would it go up or down?

For the first time, an international team of scientists can answer that question, after they directly tested how antimatter acts under gravity, a question which has been a subject of endless speculation among the scientific community.

What they found was that antimatter is affected by gravity in the same way that matter is.

“As it turns out, an anti-apple would also fall down,” Scott Menary, professor emeritus at York University, said in a press release.

This confirmation, described in a paper published Wednesday in the peer-reviewed journal Nature, only brings up further questions, Menary said.

“Such as, does antimatter fall in exactly the same way as matter or are there subtle differences in how it behaves that we haven’t discovered.”

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This is the first major result to come out of use of the new ALPHA-g apparatus, a specialized particle-trapping device which was funded through the Canada Foundation for Innovation. It is housed at the world’s largest particle physics laboratory on the Franco-Swiss border near Geneva, Switzerland, operated by the intergovernmental organization of CERN.

“The result is a technical tour de force given the difficulty of measuring the effect of gravity – a force much weaker than most people realize – on just a small collection of antihydrogen atoms,” Menary said.

Finally seeing how antimatter acts in gravity is a step forward into understanding one of the biggest mysteries in the physics world, researchers say: the mirror world that is antimatter itself.

The world that we live in is composed entirely of matter – molecules, atoms, electrons and other subatomic particles. But according to physics, when matter is created, something called antimatter is created also.

Antimatter is composed of particles with the opposite charge of the corresponding particles that make up matter. All subatomic particles have an anti-twin (antiquarks, antineutrons, etc.), and if regular particles come into contact with their anti-twin, they annihilate, producing energy in the process.

Scientists have been studying these particles for decades. The first antimatter particle to be observed, the positron or the antielectron, was discovered in 1933 after being theorized in 1931. At CERN, they can synthesize and isolate antiprotons by smashing protons together with nuclei and separating resulting antiprotons out with magnetic fields.

One of the questions that scientists wrestle with is that if our universe is composed of matter that got its start in the Big Bang around 13.8 billion years ago, theoretically an equal amount of antimatter should’ve been created – but where did it go?

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“Right now, we don’t have an explanation about where all the antimatter in the universe is,” Robert Thompson, physics professor at the University of Calgary and principal investigator of the ALPHA-g Canada Foundation for Innovation Project, said in the release.

“To find a solution for this conundrum, what we do is test the elements of physics of antimatter to see if we can find an inconsistency. In this case, we tested to see if the gravitational characteristics of antihydrogen mirror those of hydrogen, which is significant because it’s never been done before.”

HOW THE EXPERIMENT WORKS

If scientists have been capturing and synthesizing antimatter particles for decades, why hasn’t the impact of gravity on them been tested in an experiment before?

Because gravity isn’t strong enough to exert an observable force on charged particles. Although gravity acts on everything in the universe, the electrostatic force between two charged electrons makes the impact of gravity negligible and not something we can really measure. The same is true for positrons. You need a neutral atom created by charged particles that balance each other out to start being able to see gravity have an effect, according to scientists.

This is where the Antihydrogen Laser Physics Apparatus (ALPHA) experiment comes in.

ALPHA works by scientists taking positrons and antiprotons and shooting them up into a vertical device called the ALPHA-g, where the particles have the opportunity to combine and form antihydrogen atoms. Most of these particles will escape and be annihilated due to colliding with matter – but an “atom trap” in the centre of the ALPHA-g can capture a few antihydrogen atoms and keep them away from matter using magnetic fields.

For each point that scientists measured in this particular experiment, they ran the machine 50 times to accumulate enough antihydrogen atoms.

Once they had enough particles, it was time to release them from the trap and watch gravity take over.

The ALPHA-g opens the atom trap by releasing the barriers on the top and the bottom. When normal hydrogen atoms were run through the ALPHA-g, around 80 per cent exited the trap by the bottom due to the pull of gravity.

What researchers found in this experiment is that antihydrogen particles did the same thing.

It may seem like common sense that gravity would act the same on antimatter, as this falls in line with Einstein’s theory of general relativity, a cornerstone of physics, but although it was the leading theory, it wasn’t a guarantee. Numerous studies have theorized what the universe could look like if antimatter had a vastly different reaction to gravity than matter does, including scenarios in which antimatter serves as a negative gravitational mass.

Knowing that antimatter does react similarly to gravity is the first step in digging into the exact mechanisms involved – and what they might mean in unravelling the curious lack of antimatter in our universe compared to the amount anticipated by the laws of physics, according to researchers.

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“We know there’s a problem somewhere in quantum mechanics and gravity,” Timothy Friesen, assistant professor at the University of Calgary and major contributor to the Nature paper, said in another press release.

“We just don’t know what it is. There has been a lot of speculation on what happens if you drop antimatter, though it’s never been tested before now because it’s so hard to produce and gravity is very weak.”

The ALPHA-g was built by Canadian scientists within the facilities of TRIUMF, Canada’s particle accelerator centre, before being shipped to CERN to be installed.

Running the experiment and measuring the free-fall of the antiatoms involved researchers from numerous Canadian institutions as well as researchers and institutions from Europe, the U.K., the U.S., Israel and Brazil.

"This milestone is a culmination of nearly 20 years of dedication and teamwork. The contributions of the members of ALPHA-Canada were critical to our success,” Makoto C. Fujiwara, senior scientist at TRIUMF and ALPHA-Canada spokesperson, said in the release. "ALPHA-Canada is a pan-Canadian collaboration made up of a diverse group of students, postdoctoral scholars, academics and staff members, each who played a vital role in this project."