Gravity’s effect on matter’s elusive twin is revealed

Down goes antimatter!

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There is still more to discover about the gravitational makeup of the universe, as evidenced by the emerging ideas of dark matter and energy. Our understanding may be lacking due to singularities in the general theory of relativity and the absence of a quantum theory of gravity. Therefore, investigating gravity in unusual physical systems makes sense.

Einstein was unaware of antimatter in 1915. The positron was discovered in 1932, while Dirac’s theory was published in 1928. Since then, there has been a lot of discussion regarding gravity and antimatter. Although some authors have thought about the cosmological consequences if antimatter were to be repelled by matter, the theoretical consensus is that the Earth must attract any laboratory mass.

Landmark CERN experiment answers the longstanding question of whether antimatter and matter are gravitationally attracted or repelled by observing the downward path taken by individual antihydrogen atoms.

Their research also contributes to solving one of science’s most unanswered mysteries: why is there so little antimatter in the visible universe?

University of California, Berkeley, plasma physicist and ALPHA collaboration member Jonathan Wurtele said, “Einstein’s theory of general relativity says antimatter should behave the same as matter. Many indirect measurements indicate that gravity interacts with antimatter as expected. Still, until the result today, nobody had performed a direct observation that could rule out, for example, antihydrogen moving upwards as opposed to downwards in a gravitational field.”

Beyond the imagined photon torpedoes and antimatter-fueled warp drives of “Star Trek,” antimatter is entirely real but enigmatically scarce. Most of what is known as baryonic matter—which makes up our bodies, the Earth, and nearly everything else in the universe—is present in all these objects. That is the material, such as oxygen, carbon, iron, and other well-known elements on the periodic table, that is primarily composed of regular protons and neutrons.

However, depending on the specific type of particle, antimatter can exhibit some opposite characteristics from conventional matter. Protons have a positive charge, while antiprotons have a negative charge. Antielectrons, also referred to as positrons, are positive.

ALPHA collaboration member and UC Berkeley plasma physicist Joel Fajans said, “As soon as antimatter touches matter, it blows up.. The combined mass of the matter and antimatter is transformed entirely into energy in a reaction so energetic that “explosion” doesn’t do it justice. Scientists call it an “annihilation.”

“For a given mass, such annihilations are the densest form of energy release we know of.”

But don’t worry, Scientists used a very small amount of antimatter in experiments the energy created by antimatter/matter annihilations is invisible to humans. Still, scientists had to manipulate the antimatter carefully, or we would lose it.

The expansive ALPHA experiment, which has been in development for nearly 20 years, is located outside of Geneva in a structure known as the “Antimatter Factory,” where antihydrogen is made from scratch by fusing antiprotons and positrons in a complex procedure. The CERN particle accelerator complex produces and supplies the antiprotons to the ALPHA experiment. The positrons are obtained from a synthetic sodium radioactive isotope that produces them. Plasmas, the fourth state of matter (solid, liquid, and gas are the other three), are created when both antiparticles are cooled to just below room temperature.

The gravity experiment will ultimately be carried out in a vertical ALPHA-g chamber when the antiproton and positron plasmas have been channeled there.

Most of the experimental time is spent doing plasma manipulations to get these antiparticle plasmas into the shape and temperature required. Hence, scientists had to cool them to about 10 degrees Kelvin — 10 degrees above absolute zero — and create them into needle-like shapes. After attaining the proper conditions, the plasmas are mixed with exacting precision inside the ALPHA-g chamber to create the precious antihydrogen atoms.

Wurtele said, “Broadly speaking, we’re making antimatter and doing a Leaning Tower of Pisa kind of experiment. We’re letting the antimatter go and seeing if it goes up or down.”

A variable magnetic trap keeps the antihydrogen inside the cylindrical vacuum chamber of ALPHA-g. This keeps the antimatter inside the chamber from reaching the walls and annihilating. The scientists weaken the trap’s top and bottom magnetic fields until the antihydrogen atoms can escape and the relatively weak gravity is revealed. Each antihydrogen atom that escapes the magnetic trap annihilates when it strikes the chamber walls above or below the trap, which the researchers can see and count.

Scientists repeated the experiment several times. Each time, they varied the magnetic field strength of the top and bottom of the trap to rule out possible errors. About 80% of the antihydrogen atoms were found to annihilate beneath the trap when the weak magnetic fields were precisely balanced at the top and bottom. This result is consistent with how a cloud of ordinary hydrogen would behave under identical circumstances.

As a result, the antihydrogen was falling due to gravity.

Fajans said, “Previously, many indirect observations strongly suggest that antimatter gravitates normally. But none of those observations were as incontrovertible as following the trajectories of antimatter under the influence of gravity; they all relied on much more subtle phenomena.”

You’ve probably noticed that no antimatter beings are wandering around obliterating everything in massive explosions. We don’t observe much antimatter anywhere in the cosmos, despite a few extremely small sources, such as naturally occurring positrons emitted by potassium, even inside bananas. That goes against the laws of physics, which state that antimatter should coexist with conventional matter in a roughly equal amount.

Tim Tharp, an ALPHA collaboration member and plasma physicist at Marquette University, said, “For the most part, we live in a universe of matter. And one big question is ‘why?’ There isn’t a lot of antimatter around, and we don’t understand why the universe developed this preference.”

This riddle is referred to as the baryogenesis dilemma by scientists. Antimatter may have been gravitationally rejected by ordinary matter during the Big Bang, sending it off to an unobservable universe region, which could account for its scarcity. That theory doesn’t hold up anymore.

An artist’s conceptual rendering of antihydrogen atoms contained within the magnetic trap of the ALPHA-g apparatus. As the field strength at the top and bottom of the magnetic trap is reduced, the antihydrogen atoms escape, touch the chamber walls and annihilate. Most of the annihilations occur beneath the chamber, showing that gravity is pulling the antihydrogen down. The rotating magnetic field lines in the animation represent the invisible influence of the magnetic field on the antihydrogen. The magnetic field does not rotate in the actual experiment.
Credit: Keyi “Onyx” Li/U.S. National Science Foundation

Wurtele said, “We’ve ruled out antimatter being repelled by the gravitational force instead of attracted. But that doesn’t mean there isn’t a difference in the gravitational force on antimatter that has not yet been measured. The force of gravity on antimatter may be weaker or stronger than regular matter, which requires a more precise measurement to determine.”

Vyacheslav “Slava” Lukin, a program director in NSF‘s Division of Physics, said“The success of the ALPHA collaboration is a testament to the importance of teamwork across continents and scientific communities. Understanding the nature of antimatter can help us not only understand how our universe came to be but can enable innovations never before thought possible — like positron emission tomography scans, which have saved many lives by applying our knowledge of antimatter to detect cancerous tumors in the body.”

Journal Reference:

  1. Anderson, E.K., Baker, C.J., Bertsche, W. et al. Observation of the effect of gravity on the motion of antimatter. Nature 621, 716–722 (2023). DOI: 10.1038/s41586-023-06527-1

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