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. Scientists are left with more questions regarding this strange 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). This is a fascinating premise based on an actual 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. Even though the particle was not observed, it was obvious that Dirac’s equations were correct. (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. They produce only a small amount of antimatter (the P is for Positron) but not an entire 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. NASA has studied the use of antimatter explosions and normal matter explosions as a way to propel starships over vast distances. (The designs are purely hypothetical).
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 antimatter is already here: it’s made at CERN in tiny, expensive samples. 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. These steps required years of calculations, funding and inventive engineering.
“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.
Shortly, Perez said that “nobody believed this” about antigravity rising.
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? The ball fell downwards towards the center of the Earth, like it was 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. The physics community can now return to their drawing board to continue to explore the mystery of the universe, and ask why there is no antimatter.
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