Antimatter embraces Earth, falling downward like normal matter: Study reveals gravity's effect on matter's elusive twin

One dose of cold reality for those who continue to hope that antimatter levitates instead of falling in a gravitational field like conventional matter is provided by the findings of a recent experiment.

Researchers looking at antihydrogen, which is an antiproton coupled with a positron or antielectron, have definitely demonstrated that gravity pushes the antihydrogen downward rather than upward.

Antigravity does not exist, at least not for antimatter.

A group from the Antihydrogen Laser Physics Apparatus (ALPHA) cooperation at the European Center for Nuclear Research (CERN) in Geneva, Switzerland, has published the experimental results in the journal Nature on September 28.

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 that of Earth's regular matter. To be more exact, it was discovered to be within one standard deviation, or around 25%, of normal gravity.

The experiment was initially proposed more than ten years ago by UC Berkeley physics professor Joel Fajans and his colleague, theorist Jonathan Wurtele. "It surely accelerates downwards, and it's within about one standard deviation of accelerating at the normal rate," said Fajans. "The fact remains that antimatter will not enable us to levitate, and there is no free lunch."

Most physicists won't be surprised by the outcome. Despite being developed before to the discovery of antimatter in 1932, Albert Einstein's theory of general relativity handles all matter in the same way, suggesting that matter and antimatter react to gravitational forces in the same way. Every regular matter particle, including protons, neutrons, and electrons, has an anti-particle that has the opposite electrical charge. These anti-particles totally destroy their normal matter counterparts when they come into contact with them.

According to Wurtele, a physics professor at UC Berkeley, "the opposite result would have had big implications; it would be inconsistent with the weak equivalency principle of Einstein's general theory of relativity." For the first time, the force of gravity on neutral antimatter has been directly measured thanks to this experiment. It's a further advancement in the study of neutral antimatter."

Fajans pointed out that no scientific theory genuinely suggests that antimatter should be attracted to gravity. It is technically impossible to develop a perpetual motion machine, according to some physicists, even if it were possible.

However, the possibility that matter and antimatter may experience gravity in different ways was intriguing, since it could provide an explanation for many cosmic mysteries. For instance, it may have resulted in the early universe's spatial separation of matter and antimatter, which would account for the sparse presence of antimatter in our surrounding cosmos. According to most theories, the Big Bang that created the universe should have produced an equal amount of matter and antimatter.

But gravity is so feeble.

Though these investigations have been very delicate, Fajans claims that a large number of indirect experiments clearly show that antimatter gravitates naturally.

Why not carry out the obvious experiment—a sort of leaning tower of Pisa experiment—and drop a chunk of antimatter? He mentioned the mythical experiment in which Galileo is claimed to have dropped a wooden ball and a lead ball from the top of the tower and shown that they both reached the ground simultaneously.

Fajans continued, "The true issue is that gravitational forces are incredibly weak in comparison to electrical forces." Because any stray electric field will deflect the particle far more than gravity will, it has not been able to directly detect gravity using a drop-style experiment using a charged particle, such as a bare positron.

Among the four recognized forces of physics, the gravitational force is really the weakest. It controls the universe's development since, in theory, it affects all matter over enormous distances. However, this has very little effect on a miniscule amount of antimatter. An antiproton experiences a force from a 1 volt/meter electrical field that is around 40 trillion times greater than the force of gravity from Earth.

A fresh strategy was proposed to Wurtele by the ALPHA collaboration at CERN. Since antihydrogen is charge neutral, electric forces cannot impact it, hence in 2011 Wurtele demanded to Fajans that they investigate the potential of a gravity measurement. By 2010, the ALPHA team was successfully trapping large amounts of antihydrogen atoms.

After rejecting the concept for several months, Fajans was ultimately convinced to give it some thought and run some simulations that seemed to validate Wurtele's theories. After becoming involved, postdoctoral researcher Andrey Zhmoginov and lecturer Andrew Charman of UC Berkeley found that extremely rough limitations on the gravitational interactions of antimatter with Earth may be obtained by analyzing past data retrospectively.

This resulted in a report, with assistance from their ALPHA colleagues, that found antihydrogen accelerates, either up or down, just around 100 times as much as conventional matter does as a result of Earth's gravity.

Despite this lackluster beginning, the ALPHA team was persuaded to construct an experiment in order to obtain a more accurate measurement. ALPHA-g, a new experiment that the cooperation started building in 2016, carried out its first observations in the summer and fall of 2022.

Based on statistical analysis and simulations of the team's observations from the previous year, the results published in Nature estimate the gravitational constant for antimatter to be 0.75 ± 0.13 ± 0.16 g. When the systematic and statistical errors are taken into account, the value comes out to be 0.75 ± 0.29 g, which is within error bars of 1 g. The group came to the absurd conclusion that there is very little possibility that gravity will repel antimatter.

According to Fajans and Wurtele, at least twelve student physics majors from UC Berkeley took part in the experiment's setup and operation; several of them were from underrepresented racial and ethnic groups.

According to Fajans, "many Berkeley undergraduates have found it to be a great opportunity." They're enjoyable experiments that teach our pupils a lot.

A harmony

Wurtele and Fajans' planned ALPHA-g design called for the confinement of around 100 antihydrogen atoms at a time within a 25-centimeter-long magnetic bottle. 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 even at this incredibly low temperature, and they are bouncing hundreds of times per second off the intense magnetic fields at the ends of the container. (The constricted 10,000 Gauss magnetic fields at either end of the bottle reject the magnetic dipole moment of an antihydrogen atom.)

The atoms traveling downstairs will accelerate owing to gravity if the bottle is tilted vertically, whereas the atoms travelling upward would decelerate. When the magnetic fields at both ends are equal, or balanced, the energy of the atoms flowing downhill will be higher overall. As a result, there is a greater chance that they will break free through the magnetic mirror and strike the container, where they will explode in a burst of light and produce three to five pions. To ascertain whether the antiatom fled downhill or upward, the pions are identified.

Fajans compared the experiment to a typical balance that is used to evaluate weights that are quite comparable. In the same manner that a standard balance shows the difference between 1 kilogram and 1.001 kilos, a magnetic balance shows the comparatively little gravity force in the face of significantly stronger magnetic fields.

After then, the mirror magnetic fields are gradually reduced until all of the atoms have left. Antiatoms should escape from the bottom more often than the top, almost 80% of the time if antimatter behaves like regular matter.

"We can overlook the fact that the antiatoms have varying energies because of the balancing," Fajans stated. The antiatoms with the lowest energy are the last to depart, but they are still affected by gravity and the balance.

Additionally, because of the experimental configuration, ALPHA may alter the strength of the bottom magnetic mirror relative to the top mirror. This provides an energy boost to each antiatom, negating or surpassing the effects of gravity and enabling an equal or higher amount of antiatoms to exit the top than the bottom.

Because we can demonstrate to ourselves that we can control the experiment in a predictable way, "this gives us a powerful experimental knob that allows us, basically, to believe the experiment actually worked," explained Fajans.

The numerous unknowns required a statistical analysis of the results: The number of antihydrogen atoms the researchers had trapped, their ability to identify every annihilation, their ability to rule out the possibility of unidentified magnetic fields influencing the antiatom trajectories, and their accuracy in measuring the magnetic field inside the bottle were all uncertain.

"We don't know the exact initial conditions of the antihydrogen atoms, so ALPHA's computer code simulating the experiment could be slightly off; it could also be off because our magnetic fields are off, and it could be off for some other unknown," Wurtele added. "However, we can investigate the extent of any discrepancies thanks to the control the balance knob offers, ensuring that our result is accurate."

The scientists at UC Berkeley expect that future modifications to the computer programs and ALPHA-g will increase the instrument's sensitivity by a factor of 100.

"Although the project originated at Berkeley, this result is a team effort," Fajans stated. "ALPHA was designed for antihydrogen spectroscopy, not gravitational measurements of these antiatoms." Without the labor of Jonathan and myself and years of solitary development, the research probably would not have taken place. Our suggestion was totally at odds with all of ALPHA's intentions."

Furthermore, even if the null result can be written off as uninteresting, the experiment represents a crucial test of general relativity, which has so far passed all previous tests.

The physicists in this department will all tell you that this conclusion is not in the slightest surprising if you question them as you go down the hallways. It's the truth, Wurtele stated. "However, the majority of them will also assert that the experiment was necessary since uncertainty never exists. The science of physics is experimental. Being foolish enough to forego an experiment that may uncover novel physics because you believed you already knew the solution and it turned out to be something else is not what you want to be."

Josh Clover, Haley Calderon, Mike Davis, Jason Dones, Huws Landsberger, and Nicolas Kalem are among the undergraduate participants. James McGrievy, Dana Zimmer, Larry Zhao, Ethan Ward, Sara Saib, Shawn Shin, and Dalila Robledo.