Antimatter has intrigued and confounded physicists for almost a century, and the effect of gravity on antimatter has been a point of disagreement. New research may have settled the debate by finding that antihydrogen atoms, the antimatter counterpart of hydrogen, are affected by gravity in the same way as their matter equivalents, ruling out the existence of repulsive ‘antigravity.’
In the seventeenth century, Isaac Newton proposed his gravitation theory after watching an apple fall from a tree and questioning why it fell straight down rather than sideways or upwards. Centuries later, Albert Einstein came up with his general theory of relativity, which remains the most successful, and tested, description of gravity. However, antimatter was unknown to Einstein.
In 1928, British physicist Paul Dirac theorized that for every particle, there exists a corresponding antiparticle, predicting the existence of the positron – or antielectron – which was first observed in 1932. Since then, there’s been a lot of speculation about the interaction between gravity and antimatter, with some arguing that antimatter is repelled by gravity and others that it’s attracted.
A new study by the Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at The European Organization for Nuclear Research (CERN) may have settled the argument, finding that atoms of antihydrogen, the antimatter counterpart of hydrogen, fall to Earth in the same way as their matter equivalents.
“In physics, you don’t really know something until you observe it,” said Jeffrey Hangst, a corresponding author of the study. “This is the first direct experiment to actually observe a gravitational effect on the motion of antimatter. It’s a milestone in the study of antimatter, which still mystifies us due to its apparent absence in the Universe.”
The ALPHA experiment is concerned with making, capturing and studying atoms of antihydrogen in a trapping device. Antihydrogen atoms are electrically neutral and stable particles of antimatter, making them ideal for studying the gravitational behavior of antimatter. Antihydrogen is created by combining the two component antiparticles, antiprotons and positrons. An antiproton is a subatomic particle with the same mass as a proton but with a negative electric charge.
The ALPHA team recently built a vertical apparatus called ALPHA-g, where the ‘g’ denotes the local acceleration of gravity, which, for matter, is 32.2 ft/sec2 (9.81 m/sec2). The ALPHA-g makes it possible to measure the vertical positions at which antihydrogen atoms meet up with their corresponding matter – a process called annihilation – once the trap’s magnetic field is switched off, allowing the atoms to escape.
The researchers trapped groups of about 100 antihydrogen atoms, one group at a time. They then slowly released the atoms over a period of 20 seconds by gradually reducing the current in the top and bottom trap magnets. Computer simulations predicted that 20% of the atoms would exit through the top of the trap and 80% through the bottom, a difference caused by the downward effect of gravity. Averaging the results of seven release trials, the researchers found that the fractions of anti-atoms exiting through the top and bottom aligned with the simulations. That is, antihydrogen atoms fell in the same way that hydrogen atoms would under 1 g, or normal, gravity.
Using the ALPHA-g apparatus, the 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 landed at the same time, showing that gravity causes objects with different masses to fall with the same acceleration.
While the researchers say their findings rule out the existence of repulsive ‘antigravity,’ the current study only marks the beginning of detailed, direct investigations into the gravitational nature of antimatter.
“It has taken us 30 years to learn how to make this anti-atom, to hold on to it, and to control it well enough that we could actually drop it in a way that it would be sensitive to the force of gravity,” Hangst said. “The next step is to measure the acceleration as precisely as we can. We want to test whether matter and antimatter do indeed fall in the same way.”
The study was published in the journal Nature, and in the below video, produced by CERN, Jeffrey Hangst explains how ALPHA-g works, the reasons for, and the results of, the antimatter gravity experiments.
Do antimatter and matter behave in the same way under gravity ?