New ALPHA-g Detector Poised to Search for Signs of Antigravity

Scientists at CERN are racing to fire up a detector that will explore the effects of gravity on antimatter—before the whole facility shuts down for two years

The ALPHA-g detector
Photo: Stu Shepherd/TRIUMF
The ALPHA-g detector, built at Canada’s particle accelerator facility, TRIUMF, is the first of its kind designed to measure the effect of gravity on antimatter.
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Deep underground, within the concrete walls of CERN, Switzerland’s world-famous particle accelerator, lies a 200-kilogram machine encased in a shield of argon and carbon dioxide gases. After years of careful design and assembly, the device is nearly ready to make its debut. While the detector looks futuristic, it’s actually quite similar in function to previous generations of detectors—with one exception: This one was crafted to measure the effects of gravity on antimatter.

Blueprints for this detector, dubbed ALPHA-g, were first drawn in 2013. In recent months, its creators have worked around the clock in Vancouver, Canada, to finish building it. Finally, in July, ALPHA-g was shipped via cargo plane to CERN, the only location in the world that can provide the amount of antimatter needed for these experiments.

Now, time is running short. Scientists are currently testing the device and must solve any technical issues before CERN shuts down for two years of maintenance. The ALPHA-g team is rushing to conduct its gravity experiments before the 12 November cutoff—but just a single misplaced wire could cause them to miss the deadline.

Understanding whether antimatter obeys the same laws of gravity as matter is an important step toward confirming whether decades of theory surrounding antimatter stand true. Antimatter is just like the regular matter that makes up the stars, planets, and every observable object in the universe, but it exhibits some opposing quantum properties (for example, whereas regular matter has negatively charged electrons, antimatter has positively charged ones, called positrons).

Something else is different about antimatter, though—something that has caused it to almost completely vanish from our universe. Scientists suspect that immediately following the Big Bang, an equal amount of matter and antimatter existed. Yet, in the universe today, there is almost none of the latter left. The mystery of where it all went is one of the biggest outstanding questions in physics.

Major experimental breakthroughs to create, trap, and analyze antimatter have occurred only in recent decades, beginning with the first experimental creation of nine antihydrogen atoms in 1995. However, when antimatter and regular matter collide, both particles are annihilated—so these first antihydrogen atoms existed for about 40 billionths of a second before, traveling at nearly the speed of light, they collided with ordinary matter and were annihilated. To facilitate the study of antimatter, scientists needed a way to keep the two substances separate.

It wasn’t until 2010, as part of the first ALPHA experiment, that physicists succeeded in using a magnetic field to trap the particles, suspending them within the chamber of an antimatter detector. Subsequent experiments have revealed that antimatter shares many of the same properties as regular matter; for example, it shares the same colors and charge-to-mass ratio.

But does it have the same gravitational properties as regular matter, as theory predicts? ALPHA-g, the third of its generation, is poised to explore this critical question. Whereas previous ALPHA detectors were oriented horizontally with narrow chambers, this new one stands vertically at 2.3 meters tall. Huge coils encircle its chamber, creating a magnetic field that will contain antihydrogen atoms as if they were trapped in a plastic bottle. But instead of a regular bottle, envision one with a lid at both the top and bottom. During experiments, the magnetic field must be manipulated precisely so that both the top and bottom lids “open” simultaneously. The ALPHA team will then observe whether, in the presence of Earth’s gravitational field, antihydrogen atoms fall down like regular matter—or move upward, defying gravity altogether.

This latter possibility is extremely unlikely. But, if such a phenomenon is observed, our current understanding of the universe, as outlined by Einstein’s theory of general relativity, will be completely upended.

“If we do see any difference, any tiny difference [between antihydrogen and regular hydrogen], we would have to completely rewrite the entire textbook,” says Makoto Fujiwara, the lead scientist on the team, based at Canada’s particle accelerator TRIUMF.

The ALPHA-g team aims to get repeated and more precise measurements of the gravitational effects of antimatter once CERN reopens in two years. But for now, the researchers hope to at least observe whether antimatter goes up or down before the facility shuts down for maintenance.

Fujiwara remains cautious about overstating the team’s ability to pull off this initial observational experiment in such a short time frame, noting that the experiments involve newly developed technology. In particular, the new magnetic field design is complex. Fujiwara says, “The control of the magnetic field is a major factor. If we don’t control it very well, or characterize it very well, we may be fooled by the results.”

One problem the team already encountered has to do with the machine’s ability to create an electric field to manipulate the positively and negatively charged subcomponents of antihydrogen to combine (with the help of a laser) within the detector.

Shortly after ALPHA-g was set up at CERN, one of the device’s 256 gold-plated tungsten wires broke, coiling up on itself and entangling other wires—ultimately shorting two electrodes that create the electric field. To fix this, ALPHA team members had to fly in from Vancouver, set up an adjacent sterile work area, and disassemble much of the detector in order to access the faulty wires. After one week of intense work, the team had the ALPHA-g back in operation.

“There are always problems that we don’t anticipate [and] which require immediate attention,” says Pierre Amaudruz, project manager. “This is what research in uncharted territory is all about. We have to try to resolve unforeseen issues constantly.”

The team hopes to iron out any final wrinkles during the testing phase and conduct the initial experiments—just observing the up or down movement of antihydrogen­—before CERN’s steady supply of antiprotons and positrons (subcomponents that make up antimatter) is cut off. If they pull it off, the momentous task will bring humanity closer than ever before to answering a fascinating question: Does antimatter fall down or up?

This post was updated on 16 October 2018.

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