Antimatter in the gravity test

ALPHA-g

Physicists have constructed a magnetic trap in which antihydrogen’s response to gravity can be studied. © US National Science Foundation

Does antimatter fall down or up in a gravity field? So far, the answer to this has not been clear; physicists could only assume that antimatter is also attracted by gravity and falls downwards. Now a team at the CERN research center has succeeded for the first time in observing antihydrogen atoms falling and directly measuring the effect of gravity on antimatter. Their ALPHA-g experiment confirms that antimatter is also attracted by gravity. The anti-atoms released from a magnetic trap behaved similarly to normal hydrogen atoms in the same situation: most of them sank downwards. However, the margin of error in the measurements is still relatively large. It cannot therefore be ruled out that gravity acts on antimatter in the same way, but with slightly different intensity. Further measurements must now clarify whether this is the case.

According to the Standard Model of physics, for every type of particle of normal matter there is also a counterpart in the form of antimatter. The antiparticles are similar to their material counterparts such as images and mirror images, but have the opposite charge, for example in the case of the negatively charged electron and its positively charged antiparticle, the positron. When antimatter meets normal matter, annihilation occurs - both particles annihilate each other and release energy. However, it was previously unclear how antimatter behaves in relation to gravity: is it attracted like normal matter? Or does it follow some kind of anti-gravity and move in the exact opposite direction? Albert Einstein's general theory of relativity treats all forms of matter equally, so his weak equivalence principle, which describes the interaction of masses with gravity, should also apply to antimatter. However, antimatter had not yet been discovered in 1915; it was only detected in 1932.

Although theoretical models and indirect conclusions from experiments have long suggested that antimatter also reacts “normally” to gravity, this has not yet been directly proven experimentally. “One might ask why they didn’t just do the obvious and drop a piece of antimatter – similar to the lead and wood drop experiment attributed to Galileo,” says co-author Joel Fajans of the University of California at Berkeley and a member of ALPHA -Collaboration at CERN in Geneva. “But the problem is that the gravitational force is so incredibly weak compared to electromagnetic forces.” An electric field of just one volt per meter exerts a force on a charged antimatter particle that is 40 trillion times stronger than Earth's gravity. “So far, it has been impossible to directly measure the gravitational reaction of a positron, for example, with a falling experiment because any electric field in its environment would deflect the antiparticle far more than gravity,” explains Fajans.

Antihydrogen in the drop test

The problem is, among other things, that for a long time it was only possible to create individual charged antimatter particles such as anti-protons or positrons. In order to prevent the antimatter from coming into contact with normal matter and being wiped out, the particles must also be kept suspended in an electromagnetic trap. But this means they are under the influence of electromagnetic fields - and this distorts every measurement of the gravitational reaction. But a breakthrough achieved by physicists from the ALPHA collaboration at the CERN research center near Geneva in 2010 offered new perspectives: the team succeeded for the first time in combining anti-protons and positrons to form neutral antihydrogen and keeping it in a magnetic trap. Based on this, the physicists then began to plan and construct an experiment that would allow the reaction of antihydrogen to gravity to be directly measured for the first time.

In 2022, the ALPHA-g experiment was finished and the ALPHA collaboration team began measurements. The system consists of a cylindrical magnetic trap around 25 centimeters high, in which around 100 antihydrogen atoms are trapped at once. “The experimental strategy is conceptually simple: you capture and collect antihydrogen atoms, then open the barriers at the top and bottom of the vertical trap and test the influence of gravity on their movement as they escape the trap and extinguish on the apparatus wall to be read,” explain the physicists. The trapped anti-atoms are cooled down considerably, but still move around in the trap at around 100 meters per second. As soon as the magnetic barriers at the ends become weaker and then fall away completely, they gradually escape on their own. If they are now attracted by Earth's gravity, then a large part of the antihydrogen would have to come out at the bottom of the trap. If, on the other hand, gravity has a repulsive effect on antimatter, the anti-atoms would have to be driven upwards out of the trap and extinguish themselves when they come into contact with the wall. The detectors mounted above and below count how many annihilations occur above and below.

(Video: P. Traczyk and M. Brice/ CERN)

Antimatter is also attracted by gravity

The results of these measurements are now available. According to this, around 80 percent of the antihydrogen atoms escaped downwards from the magnetic trap and were extinguished there upon contact with matter. The antimatter behaved almost exactly as a cloud of normal hydrogen atoms would in this trap. Specifically, the physicists determined that the antihydrogen is attracted to the Earth's gravity by 0.75 ± 0.29 g. This means that the measurement result with its margin of error is in the range of the normal earth's gravity of 1 g, as the team explains. “We have thus ruled out the possibility that antimatter is repelled by the gravitational force,” says senior author and ALPHA collaboration member Jonathan Wurtele from the University of California at Berkeley. “The opposite result would have had enormous implications and would contradict the weak equivalence principle of Einstein’s general theory of relativity.”

The ALPHA collaboration team has succeeded for the first time in directly measuring the effect of gravity on antimatter and in clarifying the fundamental question of how antimatter responds to gravity. “If you were to ask the physicists here, they would probably all say that this result doesn’t surprise them in the least,” says Wurtele. “But most of them would also say that this experiment had to be done because you can never be sure. Physics is an experimental science.” In addition, the measurement results are still relatively inaccurate. It is therefore not impossible that antimatter is also attracted by gravity, but perhaps to a different extent. The ALPHA collaboration team hopes to be able to clarify this question with the help of more precise follow-up measurements. “This experiment is just the first step in the development of neutral antimatter research,” says Wurtele.

Source: ALPHA Collaboration, Nature, doi: 10.1038/s41586-023-06527-1

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