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Understanding the gravity of the situation: ALPHA scientists prove that antimatter falls at the same rate as matter


Francis Robicheaux
The ALPHA project has recently published results from an antimatter experiment that shows antimatter and matter fall at the same rate due to gravity. Dr. Francis Robicheaux of Purdue University Physics and Astronomy is a member of the ALPHA team who helped develop the computer programs used to interpret the results. These programs predict the motion of anti-hydrogen motions inside the vacuum of the experiment.  Photo by Brian Powell.



 Early on in science classes all over the world, students drop diverse things to learn how gravity works and learn how outside forces might affect the rate of fall.  But the science of falling goes well beyond the bowling ball and feather experiment. Einstein’s theory of relativity is still being studied, applied and proven a multitude of ways today. For instance, the Antihydrogen Laser Physics Apparatus (ALPHA) project based at CERN published in Nature today about the science of antimatter with the goal of discovering if gravity affects matter and antimatter in the same way.  Spoiler alert: it does!

Scientists with the ALPHA program, including Purdue University Prof. Francis Robicheaux, work with trapped antihydrogen atoms which they create and, by precise comparisons of hydrogen and antihydrogen, hope to study the fundamental symmetries between matter and antimatter. They have, for the first time, determined that antimatter falls at the same rate as matter and ruled out the theory of repulsive antigravity.  In this experiment, experimental and theoretical physicists learn how gravity affects the motion of antimatter.

“My experimental colleagues in ALPHA have performed wonders in this and other measurements on antihydrogen,” explains Robicheaux. “These experiments are incredibly difficult. ALPHA is the only experiment that has been able to measure properties of antihydrogen. Not once but many times.”

Robicheaux is a professor with the Purdue College of Science Department of Physics and Astronomy and has been a contributor to ALPHA for almost twenty years. Robicheaux’s involvement with this experiment and publication was to help develop computer programs that simulate the motion of the antimatter hydrogen atoms in the vacuum of the trap. These programs help interpret the results of the team’s measurements. Due to the difficulty of this experiment, the programs were also used to guide how the experiment should be performed to get the maximum effect.

“In the early days of ALPHA, I was asked to join to help with the simulation of the physics processes in the experiment,” he says. “When the collaboration was starting, it was decided there was a need for someone to specialize in simulating what was going on in the apparatus. The focus at that time was on simulating what would be needed to trap antihydrogen so experiments could be performed on it. Recently, I mostly simulate what is needed to make certain measurements possible.”


So how does it work?


Just how did they conduct this experiment? P. Traczyk and M. Brice of CERN produced this video to illustrate how the antimatter is created and tested.


 “Usually, antimatter is charged which means electric and magnetic forces are so large the effect from gravity can't be measured or the antimatter is moving so fast that gravity has a negligible effect,” explains Robicheaux. “In our experiment, we used cold antimatter hydrogen atoms to measure the acceleration from gravity on pure antimatter. Within our uncertainties, we did find that gravity pulled antimatter with the expected strength.”

According to the NSF, the ALPHA collaboration researchers will continue to probe the nature of antihydrogen in search of some contrarian aspect. In addition to refining their measurement of the effect of gravity, they are also studying how antihydrogen interacts with electromagnetic radiation through spectroscopy.

"The success of the ALPHA collaboration is a testament to the importance of teamwork across continents and scientific communities," says Vyacheslav "Slava" Lukin, a program director in NSF's Division of Physics. "Understanding the nature of antimatter can help us not only understand how our universe came to be but can enable new 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."


Bridging the gap from Purdue to CERN

Robicheaux’s role in ALPHA involves developing computer programs to aid in successful measurements and modeling. He has built a variety of programs for the team over the years. He uses highly sophisticated computers at Purdue University to perfect calculations. Most of the calculations were run using the Purdue supercomputer at the Rosen Center for Advanced Computing and, more recently, a smaller unix supercomputer constructed by Chris Orr and College of Science IT for use by the Department of Physics and Astronomy.

“I've been building a variety of programs over the past 19 years,” Robicheaux says. “Typically, the collaboration identifies a need and I try to develop a program to address it. Sometimes the need is to understand what type of equipment would be needed to successfully measure something. Other times, the need is to predict or to understand a measurement.”

For this discovery, he developed sophisticated computer programs that simulate motion. His programs helped the team understand and anticipate the motion of anti-hydrogen inside a vacuum.  

“The antimatter hydrogen would annihilate if it comes into contact with matter,” explains Robicheaux. “This is one of the things that makes the experiment challenging. We hold the anti-hydrogen using a specially shaped magnetic field inside an incredibly high vacuum. So, understanding the motion of the anti-hydrogen inside the vacuum is important for understanding what measurements can be done and what results should be expected.”

Robicheaux can perform most of his work with ALPHA at Purdue University and often has undergraduates helping perform this work. But, to stay connected with the project, he travels to CERN bi-annually.  This helps him stay integrated with the needs of the collaboration.

“Our current understanding of the universe guarantees that antimatter behaves exactly as matter does, except for the electric charge, in all processes,” he says. “This is a testable question and is what ALPHA is all about. The reason we study antimatter hydrogen is that the matter version of hydrogen is extremely well understood. So, if there was a tiny difference between hydrogen and anti-hydrogen, we would know that it was due to the failure of our fundamental understanding of the universe and not because we don't understand how some particular process works.”



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About the Department of Physics and Astronomy at Purdue University

Purdue Department of Physics and Astronomy has a rich and long history dating back to 1904. Our faculty and students are exploring nature at all length scales, from the subatomic to the macroscopic and everything in between. With an excellent and diverse community of faculty, postdocs, and students who are pushing new scientific frontiers, we offer a dynamic learning environment, an inclusive research community, and an engaging network of scholars.  

Physics and Astronomy is one of the seven departments within the Purdue University College of Science. World-class research is performed in astrophysics, atomic and molecular optics, accelerator mass spectrometry, biophysics, condensed matter physics, quantum information science, particle and nuclear physics. Our state-of-the-art facilities are in the Physics Building, but our researchers also engage in interdisciplinary work at Discovery Park District at Purdue, particularly the Birck Nanotechnology Center and the Bindley Bioscience Center. We also participate in global research including at the Large Hadron Collider at CERN, many national laboratories (such as Argonne National Laboratory, Brookhaven National Laboratory, Fermilab, Oak Ridge National Laboratory, the Stanford Linear Accelerator, etc.), the James Webb Space Telescope, and several observatories around the world. 



Francis Robicheaux, Purdue University Department of Physics and Astronomy

Jason Stoughton, National Science Foundation Public Affairs, U.S. National Science Foundation

Written by Cheryl Pierce, Communications Specialist

Last Updated: Sep 28, 2023 9:20 AM

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