The ALPHA-g experiment: Exploring gravity’s impact on antimatter

The two pillars of modern physics are Einstein’s theory of and the standard model of particle physics. The first describes the motion of bodies on the cosmic scale, whereas the latter describes the motion of bodies on the subatomic scale. However, the presence of phenomena such as and dark energy hints at an incomplete understanding.

Additionally, the full integration of and gravity remains an elusive challenge, as does the absence of a fully realized quantum theory of gravity. This pursuit forms the backdrop for the Antihydrogen Laser Physics Apparatus (ALPHA)-g experiment (here, the g stands for gravity). , conducted at CERN, is an attempt to explore the behavior of antimatter under the influence of gravity.

In September 2023, researchers confirmed that antimatter “falls down” (e. g. , it is affected by gravity) in the same way as ordinary matter.

In this article, we explore the ins and outs of this experiment and what the findings imply for the future of physics. What is antimatter? Everything around us is made of matter, which itself is made of fundamental particles like electrons and . The standard model of particle physics provides a comprehensive framework for understanding and describing these building blocks of matter.

the counterpart of regular matter, consisting of . For every particle in the standard model, there is a corresponding antiparticle with the same mass but opposite charge. The electron has the positron, the proton has the antiproton, and so forth.

It is believed that matter and antimatter particles are inherently created in pairs, and their collision results in annihilation, along with a huge energy release and, in some cases, the creation of new, different particles. Scientists believe that matter and antimatter were created in equal amounts in the initial moments after the . However, as our observable universe is predominantly made of matter, there’s a puzzling asymmetry that scientists are still striving to understand.

Antimatter particles have been produced and studied, such as in high-energy or as a by-product of some nuclear reactions. However, it remains a challenge to create and manipulate quantities large enough for practical applications due to its annihilative nature when it comes into contact with matter. The equivalence principle Before we dive deep into the exact nature of the ALPHA-g experiment and its findings, we need to understand an important concept called the .

Proposed by Einstein in his , the equivalence principle establishes a connection between inertial and gravitational mass. Mass is an inherent property of any object. The inertial mass measures an object’s resistance to acceleration, while the gravitational mass determines the strength of its gravitational interaction.

The equivalence principle posits that the effects of gravity experienced locally (in a sufficiently small region) in spacetime are indistinguishable from those of acceleration. In simple terms, the force you experience when standing on a massive body (like the Earth) is the same as the force you would experience if you were in a spaceship traveling at an acceleration equal to the acceleration due to gravity on that body (on Earth this is 9. 8 m/s ).

The equivalence principle lays the groundwork for understanding gravity not merely as a force exerted between masses (as suggested by Newton) but as a caused by the presence of mass and energy. There are various forms of the equivalence principle in use. For this research, the focus is on the weak equivalence principle (WEP).

According to this, all objects fall at the same rate in a gravitational field, regardless of their mass or composition. In a vacuum devoid of other forces, such as air resistance, WEP dictates that if a feather and a bowling ball were dropped from the same height, they would reach the ground simultaneously. The ALPHA-g experiment Now we’ve established that all matter “falls down” the same way.

The next question becomes, “Does do the same thing?” And this is precisely what the ALPHA-g experiment sought to determine. Conducted at CERN, this project focuses on dissecting the behavior of antihydrogen, the antimatter counterpart to hydrogen. The researchers began by generating antiprotons and positrons via controlled particle collisions.

To create the antihydrogen atoms, an electric field was employed to capture and manipulate antiprotons and positrons, facilitating the formation of the antihydrogen atoms. Following this, the antihydrogen atoms were trapped using a combination of magnetic and electric fields. The magnetic trap surrounds the electric field to ensure the confinement of antimatter.

Next, the researchers tested the response of the antihydrogen atoms to gravity. In each trial, the researchers released about 100 antihydrogen atoms. However, the relatively hot and fast-moving nature of the antihydrogen atoms made it hard to assess the effect of gravity.

To counter potential confounding factors like stray , the researchers used magnetic fields that trap antihydrogen atoms to slightly manipulate their trajectories upon release. This manipulation allowed for controlled observations and comparisons. The experiment involved multiple trials, varying the direction and strength of the extra force applied during release.

This iterative process helped the researchers discern the effects of gravity on antihydrogen amidst the complexities of the experimental setup. The results are in Drum roll, please. .

. antimatter is affected by gravity in just the same way as ordinary matter. This result aligns with the predictions of WEP.

The data gathered from the experiment demonstrated a remarkable agreement with simulations where antihydrogen experiences standard gravity. The results indicated that antihydrogen undergoes a approximately 75 percent as strong as that experienced by ordinary matter. Further, the precision of the experiment allowed for a margin of error of around 20 percent, providing a statistically robust confirmation of theoretical expectations.

This level of agreement boosts confidence in the reliability of the results. Lead researcher Jeffrey Hangst from Aarhus University in Denmark emphasized the importance of empirical evidence and keeping an open mind. He told that 99.

9 percent of physicists would have predicted the same result. Even though most physicists would have predicted this result, there are some who have suggested alternate theories. In 2012, French cosmologist Gabriel Chardin proposed that antimatter might experience antigravity, meaning that it would rise “up,” not down on Earth.

The hypothesis suggested that the universe might contain equal amounts of matter and antimatter, with antimatter subject to repulsion antigravity. The concept put forth by Chardin and a colleague aimed to address the gaps in knowledge related to and dark matter. The proposed scenario envisioned matter and antimatter as subject to opposite forces, leading to their separation.

In this model, matter would clump together to form galaxies, while antimatter would spread thinly between galaxies, acting akin to dark energy. The presence of areas of empty space around galaxies would prevent matter-antimatter annihilations, mimicking the behavior of dark matter. However, the results from the ALPHA-g experiment rule out this antigravity hypothesis.

The future of physics This discovery marks a crucial juncture in physics, as it robustly establishes the effect of antimatter under the influence of gravity and validates the predictions of WEP. Beyond this, however, the results do not offer an explanation for the in the universe, which is critical for understanding the evolution of the universe and the dominance of dark matter and dark energy. The ALPHA-g experiment raises new questions about the fundamental forces acting on matter and antimatter.

As physicists navigate this uncharted terrain, the quest for a quantum theory of gravity or a “theory of everything” persists. The findings of the study are published in . .