Rabu, 31 Maret 2021

Scientists figure out how to put the brakes on antimatter atoms - CBC.ca

Antimatter atoms get annihilated whenever they contact matter — which makes up everything.  That makes them hard to study, which has been a problem, scientists say, because studying antimatter is key to understanding how the universe formed.

So the question has been, how can you manipulate antimatter atoms in order to study and measure them properly? 

A team of scientists say they have figured out a way to do that by slowing down antimatter atoms with blasts from a special Canadian-built laser. And they say that could make it possible to create antimatter molecules — larger particles more similar to the matter we encounter in the real world — in the lab.

"This is where it really gets exciting for us," said Makoto Fujiwara, a research scientist at TRIUMF, Canada's particle accelerator centre in Vancouver, B.C.  "You can really start doing things that are basically unimaginable previously,"

Fujiwara is a member of the international scientific collaboration known as ALPHA, which has created the Canadian-built laser they say could allow scientists to manipulate, study and measure antimatter like never before. The new technique would allow them to study its properties and behaviour in more detail, compare it to matter, and help answer some of the most fundamental questions in physics about the origin of the universe.

The collaboration, based at the underground lab of CERN, the European Organization for Nuclear Research, published the new research in the journal Nature Wednesday.

The group includes scientists from countries around the world, including Canadian researchers at the TRIUMF, University of British Columbia (UBC), Simon Fraser University, University of Victoria, British Columbia Institute of Technology, University of Calgary and York University in Toronto It receives funding from government agencies including the European Research Council and the National Research Council of Canada, and a few trusts and foundations.

What is antimatter?

According to our understanding of physics, for each particle of matter that exists, there is a corresponding particle of antimatter with the same mass, but opposite charge. For example, the "antiparticle" of an electron — an antielectron, usually called a positron — has a positive charge. 

Antimatter is produced in equal quantities with matter when energy is converted into mass. This happens in particle colliders such as a the Large Hadron Collider at CERN. It's also believed to have happened during the Big Bang at the beginning of the universe.

But there is no longer a significant amount of antimatter in the universe — a big puzzle for scientists. 

Scientists would like to be able to study antimatter to figure out how it's different from matter, as that might provide clues about why the universe's antimatter has apparently disappeared. But there's a problem — when antimatter and matter encounter each other, they both get annihilated, producing pure energy. (A huge amount — that's what powers the fictional warp drive in Star Trek).

Because our world is made of matter, working with antimatter is tricky. For a long time, scientists could produce antimatter atoms in the lab, but they'd last just millionths of a second before hitting the matter walls of their container and getting destroyed.

WATCH | Bob McDonald explains why those earlier antimatter experiments were a big deal

Bob McDonald explains why the antihydrogen experiment is a big deal 1:59

Then in 2010, the ALPHA collaboration developed a way to capture and hold antimatter atoms using an extremely powerful magnetic field generated by a superconducting magnet. That magnetic field could keep them away from the sides of their container, which is made of matter, for up to half an hour — giving scientists plenty of time to do measurements on anti-hydrogen that compare it to hydrogen.

Makoto Fujiwara's 'crazy dream'

There was a problem though. Much as images you take with your camera are blurry if the object you're photographing is moving too fast, it was hard to get precise measurements on hydrogen anti-atoms without being able to slow them down. But Fujiwara had an idea of how to do that.

"It's one of my crazy dreams I had a long time ago — that is, to manipulate and control the motion of antimatter atoms by laser light," he recalled.

He knew that regular atoms could be slowed down by "laser cooling" (atoms move more slowly at colder temperatures and stop moving at a temperature of 0 Kelvin or 0 K, equivalent to -273.15 C, called absolute zero). Atoms of each element are sensitive to specific colours of light. Hitting them with those specific colours under certain conditions can cause them to absorb light and slow down in the process.

In theory, hydrogen anti-atoms should respond to the same colours as regular hydrogen atoms (something the researchers ended up confirming in 2018.)

WATCH | An ALPHA Canada animation explains how the ALPHA experiment makes and traps hydrogen and takes one kind of measurement

ALPHA Canada animation explains its breakthrough experiment 3:25

So as soon as ALPHA succeeded in trapping antimatter atoms of hydrogen, Fujiwara proposed trying laser cooling on them.

His colleagues laughed, initially, he recalled, "because everybody knew that a laser would be so hard to build for this."

The colour they needed, represented in physics by its wavelength (for example, red has a wavelength of around 700 nanometres and blue has a wavelength of around 450 nanometres) had to be very precise. It needed a wavelength of exactly 121.6 nanometres . A laser of that colour had never been built before. The laser would also have to fit in a very confined space in a very complex experimental setup with lots of components.

Then, one day, Fujiwara ran into his colleague Takamasa Momose, a UBC chemistry professor, in the cafeteria at TRIUMF in Vancouver. He mentioned the problem, and Momose said he could make the laser.

The two worked together, and after nearly 10 years, they succeeded.

What you can do with ultra-slow antimatter atoms

Antihydrogen atoms are created and trapped at very cold temperatures, about 0.5 Kelvin or K (-272.65 C). But even at that temperature, they're moving at about 300 kilometres per hour. With laser cooling, the researcher managed to get them down to 0.01 K (-273.14) and a speed of 36 kilometres per hour.

"Almost you can catch up by running," said Fujiwara (that is, if you're Usain Bolt, who averaged 37.58 kilometres per hour in his record-breaking 100-metre sprint).

Makoto Fujiwara stands in front of ALPHA experiment apparatus at the European Organization for Nuclear Research (CERN) in Switzerland. The international collaboration equipped the apparatus with the special laser to slow down and cool antimatter atoms of hydrogen. (Maximilien Brice )

The team was able to measure the colours that represent the "fingerprint" of the cooled antihydrogen atoms. And those slow speeds, the measurement was four times sharper than the blurry measurements they had taken at faster speeds and higher temperatures.

Momose said that when the atoms move more slowly, it also allows them to bunch closer together — and perhaps even connect to form bigger particles of antimatter, which he said is his next goal.

"So far we have only antihydrogen atoms," he said. "But I think it's cool to make a molecule with antimatter."

Fujiwara also wants to measure the force of gravity on the antimatter atoms to see if it's the same as the force of gravity on matter. The force of gravity is very weak on something with as tiny a mass as an atom, and its signal typically gets drowned out by signals from other atomic movements. But because atoms stop moving at absolute zero, those other motions can be greatly reduced with extreme cooling.

Why it's a 'nice step forward'

Randolf Pohl is a professor of experimental atomic physics at the University of Mainz in Germany who was not involved in the study, but has worked with antimatter in the past. He has been following ALPHA's work, and said its latest results are "a nice step forward" toward precise measurements of antihydrogen's "fingerprint."

But he thinks the new technique will have an even bigger impact on measurements of gravitational acceleration on antimatter atoms:  "The big question is: will antimatter fall down to earth — will it be attracted to matter? Or could it be repelled by matter or fall upwards?"

He added that so far, no one expects a difference between matter and antimatter in its behaviour, but that theory still needs to be tested.

"Because there have been some occasions in the past where people measured something where nobody expected to see a discrepancy, and then suddenly a discrepancy showed up," he said. "And that changed our view of the world."

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2021-03-31 18:29:18Z
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