Gravity test: Antimatter falls down, but where did it all go?

From Star Trek to PET scans, antimatter has thrilled and worried humankind. Now, scientists have resolved a key mystery.

An artist's conceptual rendering of antihydrogen atoms falling out the bottom of the magnetic trap of the ALPHA-g apparatus, a tall cylindrical vacuum chamber used in an antimatter experiment by the international Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at the European Center for Nuclear Research (CERN) in Geneva, Switzerland as seen in this undated handout image. As the antihydrogen atoms escape, they touch the chamber walls and annihilate. Most of the annihilations occur beneath the chamber, showing that gravity is pulling the antihydrogen down. Keyi "Onyx" Li/U.S. National Science Foundation/Handout via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES
An artist's conceptual rendering of antihydrogen atoms falling out the bottom of the magnetic trap of the ALPHA-g apparatus, a tall cylindrical vacuum chamber used at CERN to test how antimatter reacts to gravity [Keyi 'Onyx' Li/US National Science Foundation via Reuters]

One of the great mysteries of physics has been solved: How does antimatter … fall?

It’s not a question that keeps most people awake at night, but some physicists have been waiting for its answer for years.

Scientists at CERN, the world’s largest particle accelerator in Switzerland, announced Wednesday that a pathbreaking experiment had confirmed that antimatter falls down with gravity — just like everything else. But that only leaves scientists asking more questions about this curious material.

What is antimatter?

In Star Trek, antimatter powered the warp drive of the Starship Enterprise into the 23rd century (and was used in a few of its torpedoes). It’s a tantalising premise, based on a real phenomenon.

In 1928, British mathematician Paul Dirac saw antimatter in a math equation. He was working out parts of quantum mechanics when he realised that an electron – one of the fundamental particles of the universe – could be its own opposite.

In other words, there could be negative electrons (matter) and positive electrons – or positrons (antimatter). In fact, it wasn’t just the possibility: Dirac concluded that antimatter had to be there. The math was clear, even if the particle had not yet been observed. (The Dirac equation is engraved on his tomb)

Until it was, just two years later, when antimatter was discovered in nature in the trails of cosmic rays sensed during a balloon mission. It’s been studied ever since.

Today, doctors use antiparticles in medicine: in PET-scan machines that look through our skin for cancers or heart function. These produce a part of antimatter – the P stands for Positron – but not the whole anti-atom.

It’s not a bad thing that whole anti-atoms can’t be found, because when antimatter meets normal matter – the stuff that makes us and the world around us – the two explode with the most powerful energy release scientists know of. The explosion is so packed with energy that NASA studied using antimatter and matter explosions to power starships to cover enormous distances (the designs remain purely theoretical).

Missing in nature

But it’s the lack of antimatter that remains one of the great unresolved mysteries in physics: If the standard model of physics is correct, the same amount of antimatter as matter should have appeared in those first hot moments after the Big Bang.

The two opposites, if created in equal measure, would have collided, annihilating one another almost instantly, leaving nothing but a white sky full of bristling energy, and no leftover matter at all.

Yet here we are, 14 billion years later, made of matter. Since Dirac, physicists have been scratching their heads, wondering where the antimatter went, or had it ever been there to begin with?

But there is some antimatter right here on Earth: It’s been made, in infinitesimally tiny samples at incredible expense, at CERN. For over a decade, scientists there have been assembling antimatter “atoms” piece by piece and trapping them in hi-tech magnetic bottles.

They want to know how they work, why they aren’t found in nature, and why the universe seems to have “chosen” the matter we’re familiar with.

But mostly, they wanted to drop it, to see if it fell upwards. Because if it did, it would have thrown physics into a total crisis. Gravity would have had a loophole.

The gravity test

Knowing how objects fall has always fascinated scientists because it’s how humans can see a fundamental and invisible law of nature at play.

The ALPHA experiment at CERN has made just a tiny amount, a hundred-millionths of a gram of antihydrogen, so physicists could perform basic experiments on it. They used CERN’s famous particle accelerator to generate antiprotons. They used radioactive isotopes to produce positrons, similar to how they are made for PET scans.

Then, they learned how to combine them into antimatter “atoms”, trap those in magnetic fields, slow them down, hold them so they wouldn’t annihilate at the edges of their containers, and finally stand those containers upright, rather than horizontally, to test how they react to gravity. Each of these steps took years, new calculations, steady funding, and ingenious engineering solutions.

“We want to test that every property that we know that matter has, antimatter has, or maybe not,” Rebecca Suarez, an experimental physicist at Uppsala University in Sweden who was not involved in the project, explained. “Because any small details there could explain what happened with antimatter.”

Most physicists assumed antimatter would not ‘fall’ upward, but they couldn’t say so until it was proven.

Patrice Perez, spokesperson of an antimatter experiment at CERN called GBAR, tried to summarise the problem in an interview with Al Jazeera in July. If antimatter fell upwards, opposite to gravity, he said, “one of the cornerstones of [Albert Einstein’s theory of] general relativity would be wrong, the equivalence principle [which says] if you drop any object on earth, it should fall at the same rate.”

“If we would find something different, it would be a complete revolution. We would not know what to do… It would mean we don’t understand physics, we don’t understand nature at all.”

Perez has worked on experiments to capture and stabilise antimatter for decades in staid and serious physics labs, but the question of whether it could fall upward, or whether it could be fuel for spaceships, made him laugh.

In short, he said about antigravity falling up, “nobody believes this”.

After almost two decades of work, the scientists leading the experiment tipped a few dozen antimatter “atoms” into a hi-tech vertical tube to test the question.

The result?  It fell, downward towards the centre of the earth, just like a ball.

Jeffrey Hangst is a physicist and spokesperson for the ALPHA experiment. Announcing the result, he held two apples, one red for matter, the other black for antimatter, as a visual aid (the black apple was not made of antimatter; if it were, the explosion in his hand would have destroyed part of Switzerland and France).

“So far as we can tell, they fall in the same way as regular matter,” he said happily.

Physics has been saved from crisis — for now. Physicists can get back to the drawing boards, to plumb the mysteries of the universe and keep asking why it didn’t make any antimatter.

Source: Al Jazeera