A landmark experiment at CERN may help explain why antimatter appears to have lost out in the early universe. If you drop antimatter, does it fall down or rise up? In a unique laboratory experiment, researchers have now observed the downward trajectory of a single antihydrogen atom, providing a clear answer:Antimatter falls downward.
This image shows antihydrogen atoms falling and annihilating inside a magnetic trap that is part of CERN's ALPHA-g experiment, designed to measure the effect of gravity on antimatter. Image source: National Science Foundation
While confirming the gravitational attraction of antimatter and regular matter, this discovery also rules out gravitational repulsion as the reason why antimatter is largely absent in the observable universe.
Researchers from the International Antihydrogen Laser Physics Facility (ALPHA) collaboration at CERN in Switzerland published their findings today in the journal Nature.
"The success of the ALPHA collaboration demonstrates the importance of teamwork across continents and scientific communities," said Vyacheslav "Slava" Lukin, program director in the National Science Foundation's Division of Physics. "Understanding the properties of antimatter not only helps us understand how the universe was formed, but also enables unprecedented innovations - such as positron emission tomography (PET), which saves many lives by applying our knowledge of antimatter to detect cancer tumors in the body."
The Antihydrogen Laser Physics Facility (ALPHA) collaboration is an international group at CERN using antihydrogen atoms to understand fundamental symmetries between matter and antimatter. Researchers have announced groundbreaking results from an experiment aimed at understanding the effects of gravity on antimatter. Image source: National Science Foundation
The elusive mutable twin of matter
With the exception of the antimatter-fueled warp drives and photon torpedoes imagined in Star Trek, antimatter is entirely real but mysteriously scarce.
"Einstein's general theory of relativity states that antimatter should behave exactly like matter," said Jonathan Wurtele, a plasma physicist at the University of California, Berkeley, and a member of the ALPHA collaboration. "Many indirect measurements show that gravity interacts with antimatter as expected." "But until today's results, no one had actually done direct observations to rule out the possibility that antihydrogen was moving up in the gravitational field instead of down."
Our bodies, the Earth, and most everything else in the universe that scientists know about is overwhelmingly made of regular matter composed of protons, neutrons, and electrons, such as atoms of oxygen, carbon, iron, and other elements in the periodic table.
Antimatter, on the other hand, is the twin brother of ordinary matter, albeit with some opposite properties. For example, antiprotons have a negative charge, while protons have a positive charge. Antielectrons (also called positrons) are positively charged, while electrons are negatively charged.
Kevin M. Jones is a project manager in the Physics Division of the National Science Foundation and the William Edward McElfresh Professor Emeritus of Physics at Williams College. He briefly introduced what antimatter is and the overall value of studying antimatter. Source: National Science Foundation
Perhaps most challenging for experimenters, however, "antimatter explodes as soon as it comes into contact with matter," said Joel Fajans, a plasma physicist at the University of California, Berkeley, and a member of the ALPHA collaboration.
The combined mass of matter and antimatter is completely converted into energy in a reaction so intense that scientists call it annihilation.
For a given mass, this annihilation is the most intensive form of energy release known to us. However, the amount of antimatter used in the ALPHA experiment is so small that only sensitive detectors can sense the energy generated by antimatter/matter annihilation. So we have to manipulate antimatter very carefully or we lose it.
Conceptual image of antihydrogen atoms in the magnetic trap of the ALPHA-g device. When the magnetic field strength at the top and bottom of the trap weakens, antihydrogen atoms escape, contact the trap walls and annihilate. Most of the annihilation occurs below the chamber, suggesting that gravity is pulling the antihydrogen atoms downward. The rotating magnetic field lines in the animation represent the invisible effects of magnetic fields on antihydrogen atoms. In actual experiments, the magnetic field does not rotate. Source: Keyi "Onyx" Li/National Science Foundation
Throwing "anti-matter bombs""
"Broadly speaking, we are making antimatter, and we are doing a Leaning Tower of Pisa-type experiment," Votel said. He was referring to a simpler intellectual ancestor of their experiments—Galileo's 16th-century experiment (perhaps allegorically) that demonstrated that two objects of similar size but different masses dropped at the same time had the same gravitational acceleration. "We put antimatter in motion and see if it goes up or down."
In the ALPHA experiment, antihydrogen gas is contained in a tall cylindrical vacuum chamber with a variable magnetic trap called ALPHA-g. The scientists reduced the strength of the magnetic fields at the top and bottom of the trap until the antihydrogen atoms were able to escape and the relatively weak gravitational effects became apparent.
As each antihydrogen atom escapes the magnetic trap, it hits the cavity wall above or below the trap and is annihilated, allowing scientists to detect and count it.
The researchers repeated the experiment a dozen times, varying the magnetic field strength at the top and bottom of the trap to eliminate possible errors. They observed that when the weakened magnetic field was precisely balanced at the top and bottom, about 80% of the antihydrogen atoms were annihilated beneath the trap - a result consistent with how ordinary hydrogen clouds behave under the same conditions.
Therefore, gravity causes the antihydrogen atoms to fall downward.
The matter/antimatter mystery
Although there aren't many sources of antimatter - such as the positrons emitted when potassium decays, and even antimatter in bananas - scientists don't see much antimatter in the universe. However, the laws of physics predict that antimatter should exist in roughly the same amount as ordinary matter. Scientists call this conundrum the rebirth problem.
One possible explanation is that antimatter was gravitational repulsed by ordinary matter during the Big Bang, but the new findings suggest that this theory no longer seems credible.
"We have ruled out the possibility that antimatter is gravitational repulsed rather than attracted," Votel said. "This does not mean there is no difference in the gravitational pull experienced by antimatter," he added. Only more precise measurements can prove this.
Researchers in the ALPHA collaboration will continue to explore the nature of antihydrogen. In addition to improving measurements of gravitational effects, they are using spectroscopy to study how antihydrogen interacts with electromagnetic radiation.
It would be revolutionary if antihydrogen was different from hydrogen in some way, because the physical laws of quantum mechanics and gravity both say antihydrogen should behave the same way. However, you will only know if you do the experiment.