The findings of a recent experiment serve as a sobering reminder for those who continue to believe that antimatter levitates rather than collides with gravity like regular matter.
Antihydrogen, an anti-proton, and an antielectron, or positron, is being studied by physicists who have demonstrated definitively that gravity drags it downward and does not push it higher.
Antigravity doesn’t exist, at least not for antimatter.
A group from the Antihydrogen Laser Physics Apparatus (ALPHA) partnership at the European Center for Nuclear Research (CERN) in Geneva, Switzerland, has reported the experimental findings in the Sept. 28 issue of the journal Nature.
The team calculates an antimatter gravitational acceleration of 1 g, or 9.8 meters per second per second (32 feet per second per second), which is comparable to the acceleration of regular matter on Earth. It was discovered to be closer to normal gravity by roughly 25% (one standard deviation).
Joel Fajans, a professor of physics at UC Berkeley, and colleague Jonathan Wurtele, a theoretician, first proposed the experiment more than ten years ago. “It surely accelerates downwards, and it’s within about one standard deviation of accelerating at the normal rate,” said Wurtele. The truth is that there is no such thing as a free lunch, and antimatter cannot be used to levitate.
Most physicists won’t be surprised by the outcome. Despite being developed before antimatter was identified in 1932, Albert Einstein’s theory of general relativity considers all matter equally, suggesting that antimatter and matter are equally susceptible to gravitational forces. Every particle of normal matter, including protons, neutrons, and electrons, has an antiparticle that has the opposite electrical charge and will utterly annihilate its normal matter counterpart if it comes into contact with it.
“The opposite result would have had big implications; it would be inconsistent with the weak equivalence principle of Einstein’s general theory of relativity,” said Wurtele, a professor of physics at UC Berkeley. The force of gravity acting on neutral antimatter has never before been directly measured until this experiment. It represents a further advancement in the study of neutral antimatter.
Fajans pointed out that no physical theory truly states that antimatter should be attracted to gravity. hypothetically impossible perpetual motion machines could hypothetically be made, according to certain physicists, if it were possible.
However, the possibility that antimatter and matter might be affected by gravity in different ways seemed intriguing since it might be able to explain some cosmic mysteries. For instance, it might have contributed to the early universe’s spatial separation of matter and antimatter, which would account for the universe’s sparse antimatter presence. According to most theories, the Big Bang that created the universe should have produced an equal amount of matter and antimatter.
Gravity is incredibly weak
Fajans asserts that a large number of indirect experiments, all of which strongly imply that antimatter gravitates regularly, have been conducted.
Why not conduct the obvious experiment and drop some antimatter, emulating the leaning tower of Pisa experiment, one could wonder. Galileo purportedly dropped a lead ball and a wooden ball from the top of the tower and demonstrated that they both arrived at the ground simultaneously. This experiment, however, was apocryphal and never took place.
The gravitational pull is incredibly small in comparison to electrical forces, which is the main issue, continued Fajans. Because any stray electric field will deflect the particle considerably more than gravity will, it has thus far been impossible to directly measure gravity with a drop-style experiment using a charged particle, such as a naked positron.
In actuality, the gravitational force is the least powerful of the four recognized natural forces. Because it hypothetically influences all matter over vast distances, it governs the evolution of the universe. However the impact is negligible for a microscopic antimatter particle. An antiproton experiences a force from a 1 volt/meter electrical field that is 40 trillion times greater than the gravitational pull of the planet Earth.
Wurtele was presented with a fresh strategy by the CERN ALPHA collaboration. Wurtele stressed to Fajans that since antihydrogen is charge neutral, it would not be impacted by electric fields and that they should investigate the prospect of a gravity measurement in 2011. By 2010, the ALPHA team was capturing sizable amounts of antihydrogen atoms.
After initially rejecting the concept, Fajans was finally persuaded to give it some serious consideration and run some simulations that indicated Wurtele’s theories had some validity. After getting engaged, postdoctoral researcher Andrey Zhmoginov and UC Berkeley lecturer Andrew Charman recognized that a review of earlier data could offer very rough limits on antimatter’s gravitational interactions with Earth.
With assistance from their ALPHA colleagues, this resulted in a report that stated that antihydrogen only experiences an acceleration from Earth’s gravity of no more than 100 times greater than that of conventional matter.
Despite the lackluster beginning, the ALPHA team was persuaded to design an experiment to obtain a more accurate measurement. The cooperation started building ALPHA-g in 2016, and it performed its first measurements in the summer and fall of 2022.
The gravitational constant for antimatter, according to the findings published in Nature, is 0.75 0.13 0.16 g, or, if you combine the statistical and systematic errors, 0.75 0.29 g, which is within error bars of 1 g. These findings are based on simulations and a statistical analysis of what the team observed last year. The researchers came to the conclusion that the likelihood of antimatter being attracted to gravity is so minuscule as to be useless.
According to Fajans and Wurtele, at least 12 UC Berkeley student physics majors, many of whom were from underrepresented groups in physics, took part in setting up and performing the experiment.
Many Berkeley undergrads have found it to be a wonderful opportunity, Fajans added. “Our students learn a lot from these entertaining experiments,”
A balance
According to Wurtele and Fajans’ design for ALPHA-g, 100 antihydrogen atoms could be held in a magnetic container that was 25 centimeters long at a time. Only antihydrogen atoms with a temperature of less than 0.5 Kelvin, or half a degree above absolute zero, may be contained by ALPHA.
The antiatoms are traveling at an average speed of 100 meters per second and bounce off the powerful magnetic fields at the bottle’s ends hundreds of times per second even at this extremely low temperature. The 10,000 Gauss magnetic fields that are compressed at either end of the bottle reject the magnetic dipole moment of an antihydrogen atom.
The atoms traveling downstairs will accelerate due to gravity if the bottle is vertical, while the atoms moving upward would decelerate. Atoms going downward will typically have more energy when the magnetic fields at both ends are balanced, or identical. As a result, they have a higher chance of escaping through the magnetic mirror and striking the container, where they will instantly disintegrate and release three to five pions. To ascertain if the antiatom escaped uphill or downward, the pions are detected.
Fajans compared the experiment to a typical balance used to compare objects with substantially comparable weights. Similar to how a regular balance shows the difference between 1 kilogram and 1.001 kilos, the magnetic balance makes the comparatively small gravity force obvious in the presence of much stronger magnetic fields.
Then, the mirror magnetic fields are gradually reduced, allowing all of the atoms to eventually escape. More antiatoms—about 80% of them—should escape out the bottom than the top if antimatter behaved like regular matter.
The fact that the antiatoms have various energies can be overlooked because of the balance, according to Fajans. The antiatoms with the lowest energies escape last, but all of them are still susceptible to balance and are affected more by gravity.
The experimental setup also enables ALPHA to alter the magnetic properties of the bottom mirror, making it stronger or weaker than the top mirror. This alteration provides each antiatom with an energy boost that can counteract or overcome the effects of gravity, allowing an equal or higher number of antiatoms to exit from the top than the bottom.
Since we can demonstrate to ourselves that we can control the experiment in a predictable way, this essentially offers us a powerful experimental knob that enables us to feel the experiment was successful, according to Fajans.
Due to the large number of unknowns, statistical analysis of the results was required: The number of antihydrogen atoms that could have been trapped, the number of annihilations that could have been detected, the absence of other magnetic fields that might have affected the antiatom trajectories, and the accuracy with which the magnetic field in the bottle had been measured were all unknowns to the researchers.
According to Wurtele, “ALPHA’s computer code simulating the experiment could be slightly off because we don’t know the precise initial conditions of the antihydrogen atoms, it could be off because our magnetic fields aren’t accurate, and it could be off for some unknown unknown.” However, the control offered by altering the balancing knob allows us to investigate the amount of any differences, giving us faith that our conclusion is accurate.
The physicists at UC Berkeley are optimistic that future upgrades to ALPHA-g and the computer codes will increase the instrument’s sensitivity by a factor of 100.
Although the project’s origins were at Berkeley, as Fajans noted, “this achievement represents a collaborative effort. ALPHA was built for spectroscopy of antihydrogen, not gravitational measurements of these antiatoms. The notion that Jonathan and I put forth was totally at odds with everything ALPHA had in mind, and without our labor and years of solitary development, the research probably would not have taken place.
Additionally, the experiment is a crucial test of general relativity, which has so far passed all other tests, despite the fact that the null result could be written off as uninteresting.
“Physicists in our department would all agree that this conclusion is not the slightest bit shocking if you asked them in the hallways. The truth is that,” Wurtele remarked. However, the majority of them will also assert that the trial was necessary because nothing is ever certain. An experimental science is physics. You don’t want to be so foolish that you skip an experiment that could reveal novel physics because you assumed you already knew the solution and it turns out to be something else.
