Antimatter has intrigued and puzzled physicists for nearly a century, and the effect of gravity on it has been a point of disagreement. New research may resolve this debate by finding that antihydrogen atoms (the antimatter counterpart of hydrogen) are affected by gravity in the same way as their matter counterparts, ruling out the possibility of a repulsive "antigravity."
In the 17th century, Isaac Newton came up with his theory of gravity after watching an apple fall from a tree. Centuries later, Albert Einstein proposed his general theory of relativity, which remains the most successful and testable description of gravity. However, antimatter was unknown to Einstein.
In 1928, British physicist Paul Dirac proposed a theory that every particle has a corresponding antiparticle, and predicted the existence of positrons (or antielectrons). Since then, there has been much speculation about the interaction between gravity and antimatter, with some suggesting that antimatter is repelled by gravity and others suggesting that it is attracted by gravity.
A new study from CERN's Antihydrogen Laser Physics Facility (ALPHA) collaboration may have settled that debate, finding that antihydrogen atoms (the antimatter counterpart of hydrogen) fall to Earth in the same way as their matter counterparts.
Jeffrey Hangst, the corresponding author of the study, said: "In physics, you can only truly understand something by observing it. This is the first experiment to directly observe the influence of gravity on the movement of antimatter. This is a milestone in the study of antimatter, which still mysteries us because of its apparent absence in the universe."
The ALPHA experiment involves creating, trapping and studying antihydrogen atoms in a trap. Antihydrogen atoms are electrically neutral and stable antimatter particles, making them ideal for studying the gravitational behavior of antimatter. Antihydrogen is composed of two antiparticles, antiprotons and positrons. An antiproton is a subatomic particle with the same mass as a proton but with a negative charge.
The ALPHA team recently built a vertical instrument called ALPHA-g, where the "g" represents the local acceleration of gravity, which for matter is 32.2 feet per second (9.81 meters per second). ALPHA-g can measure the vertical position of antihydrogen atoms as they meet their corresponding matter - a process known as annihilation - and the atoms escape once the trap's magnetic field is turned off.
The researchers captured a group of about 100 antihydrogen atoms at a time. They then slowly released the atoms over 20 seconds by gradually reducing the current in the top and bottom trap magnets. Computer simulations predicted that 20 percent of the atoms would be released from the top of the trap and 80 percent from the bottom, a difference caused by the downward effect of gravity. The researchers averaged the results from seven release experiments and found that the ratio of antiatoms flowing from the top to the bottom was consistent with the simulations. That is, antihydrogen atoms fall in the same way that hydrogen atoms fall under 1 gram (i.e. normal gravity).
Using the ALPHA-g instrument, researchers effectively recreated Galileo's famous gravity experiment. According to legend, the Italian scientist dropped iron balls of different weights from the top of the Leaning Tower of Pisa, and they all hit the ground at the same time, demonstrating that gravity caused objects of different masses to fall with the same acceleration.
The researchers say their findings rule out the possibility of a repulsive "antigravity," but the current study marks just the beginning of a detailed, direct study of the gravitational properties of antimatter.
"It took us 30 years to learn how to make this antiatom, how to grab it, and how to control it well enough that we could actually drop it in a way that made it sensitive to gravity," Hangst said. "The next step is to measure the acceleration as precisely as possible. We want to test whether matter and antimatter actually fall in the same way."
The research was published in the journal Nature. In the following video produced by CERN, Jeffrey Hangst explains how ALPHA-g works, the causes and results of the antimatter gravity experiment.