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A new experiment has observed the fall of antimatter under gravity for the first time, confirming Einstein's prediction of general relativity in this regard. This experiment also provides a new way for the further study of antimatter and gravitational phenomena in the future.

(by Chen Qiang / tr. by Robert Taylor)

We all know that if we drop an apple, it will be attracted by the earth's gravity. But what if we drop an anti-apple?

An anti-apple refers to an apple made up of antimatter, which is made up of antiparticles, just as ordinary matter is made up of ordinary particles. For example, an antiproton and an antielectron (positron) can form an antihydrogen atom, just as a proton and an electron can form a hydrogen atom. When matter and antimatter meet, they annihilate and release a large amount of energy.

In theory, antimatter should behave like ordinary matter under the action of gravity. In other words, if we drop an anti-apple, it will also be attracted by the earth's gravity. This assumption is based on Einstein's general theory of relativity. The theory holds that gravity is not a force, but a manifestation of the bending of space-time, which bends with the mass and energy of objects and affects the motion of other objects in space-time. Therefore, gravity does not depend on the electric charge or other properties of an object, but only on its mass and energy.

In reality, however, it is not easy to observe the behavior of antimatter under gravity. At present, scientists can only produce a small number of antimatter particles in experiments, and they are so unstable that they are easy to come into contact with ordinary matter and annihilate. In addition, it is also very difficult to measure the influence of gravity on antimatter particles, because gravity is a very weak force, and other interference factors, such as temperature, electric field, magnetic field and so on, can easily mask the effect of gravity.

Because of these challenges, no one has done the gravitational fall of antimatter in the past. But now all that has changed.

Recently, scientists from the antihydrogen Laser Physics Institute (ALPHA) of the European Center for Nuclear Research (CERN) successfully carried out the gravitational fall of antihydrogen atoms, and the results were published in the journal Nature.

ALPHA scientists first used CERN's High-Speed Particle Collider to create antiprotons and then collected positrons from a radioactive source. The scientists then trapped the antiparticles together in a vacuum to keep them away from ordinary matter. Antiprotons and positrons attract each other to form neutral antihydrogen atoms, which is the antimatter version of hydrogen.

At ultra-low temperatures, antihydrogen atoms are driven into a vertical magnetic field trap called ALPHA-g by an electromagnetic field. This magnetic trap can create a free-falling environment for antihydrogen atoms. Here, scientists can systematically release antihydrogen atoms, measuring how many escape from the bottom or top of the trap and annihilate. Atoms must be at ultra-low temperatures, only a little higher than absolute zero, because heat, like an electric field, can easily mask the effects of gravity.

For ordinary hydrogen atoms, scientists have calculated that 80% of the atoms will fall, while 20% of the atoms will escape gravity and escape from the top of the trap. If Einstein's general theory of relativity is correct, then the result should be the same proportion for antihydrogen atoms.

And it is true. Within the error range of the experiment, scientists did not detect the gravitational difference between antihydrogen atoms and hydrogen atoms.

ALPHA scientists then carried out further research. They push antihydrogen atoms up or down faster by adjusting the magnetic force in the ALPHA-g trap to resist or help gravity. Here, scientists have once again detected that antihydrogen atoms and hydrogen atoms behave exactly the same.

The experiment helps explain the mystery of antimatter. According to current physical theories, the Big Bang should create the same amount of matter and antimatter. But if that's the case, then the universe should be empty: all matter and antimatter should annihilate each other. Today, however, the universe is made up almost entirely of ordinary matter, and antimatter is only occasionally exposed to radioactive decay or cosmic rays. Physicists believe that there must be some primitive imbalance that allows ordinary matter to dominate the universe, but exactly how and why remains a mystery.

One possible explanation is that antimatter was repelled by gravity during the Big Bang. However, the experiments done by ALPHA scientists today basically rule out this possibility. However, the experiment is not accurate enough to determine whether there is any difference in gravity between antimatter and ordinary matter, although there is no physical theory that there is such a difference.

To uncover the truth, we need to improve the accuracy of the experiment. Scientists at ALPHA say follow-up experiments are already planned and could start early next year. They plan to use lasers to further cool antihydrogen atoms, thus greatly improving the sensitivity of experimental equipment to gravitational effects. Of course, if they can really find any difference between antimatter and ordinary matter, then it is possible to unravel more mysteries about antimatter.

References:

Http://dx.doi.org/10.1038/s41586-023-06527-1

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