A research group at the European Organization for Nuclear Research (CERN) has performed the first-ever high-precision measurement of the gravitational weight of antimatter, a rare substance that appears identical to ordinary matter in all properties except for electric charge. As described in a recent paper, antimatter experiences the same amount of gravitational attraction on Earth as ordinary matter, at least up to the precision limits available in ALPHA-g, the experiment that produced the results. The finding offers new insights into the physical nature of antimatter. See also: Antimatter; Earth's gravity field; Electric charge; Matter; Weight (physics)
The existence of antimatter was first posited by English theoretical physicist Paul Dirac in 1928. For each known elementary particle of ordinary matter, there is a corresponding antiparticle with opposite charge. For example, the antiparticle counterpart to the negatively charged, normal-matter electron is the positively charged positron. When antiparticles collide with particles, the materials mutually annihilate, releasing a large amount of energy. Antiparticles are produced naturally in various particle and nuclear decays, as well as artificially in particle accelerators. In either case, antimatter does not exist for long before encountering ordinary matter and subsequently annihilating, posing unique challenges in studying antimatter’s properties before these annihilations can occur. Many mysteries surround antimatter, including why less of it was created in the big bang, thus allowing for a universe composed of matter. Scientists have continued to probe antimatter’s properties, looking for intrinsic differences between antimatter and matter that could explain the latter's dominance. See also: Big bang theory; Electron; Elementary particle; Particle accelerator; Positron
The Antihydrogen Laser Physics Apparatus (ALPHA) collaboration at CERN relies on both natural and artificial means of antimatter generation to make atoms of antihydrogen, composed of an antiproton (with negative charge) orbited by a positron. Positrons are produced by natural beta-decay of sodium-22, whereas antiprotons are artificially produced from a proton beam colliding with a target. Researchers use magnetic fields to accumulate antiparticles into clouds and transport those clouds into a Penning-Malmberg magnetic trap to contain the antiparticles. Then, the two clouds are slowly introduced to one another in a process called mixing, where particle bonding occurs. The resulting antihydrogen is cooled and contained in an Ioffe-Pritchard magnetic trap, which uses magnetic field interactions with the magnetic moment of antihydrogen to temporarily prevent the anti-element from interacting with normal matter comprising the trap's hardware. See also: Antihydrogen; Beta particle; Electromagnetic field; Magnetism; Radioactivity
The research team used the ALPHA-g apparatus to determine the strength of gravity on contained antihydrogen atoms. First, the magnetic Ioffe-Pritchard trap containing antihydrogen was oriented vertically. By slowly decreasing the trapping magnetic field, the experimenters allowed a controlled amount of antimatter out of containment. When those antihydrogen atoms annihilated with surrounding matter, the experimenters could determine what portion of atoms escaped from the top or bottom of the trap and consequently determine the strength of gravity on the atoms. Previous computer modelling suggested that, if antimatter is affected by gravity in the same way as ordinary matter, roughly 80% of atoms would escape from the bottom of the trap, with the remaining 20% of atoms escaping from the top of the trap. The findings of the experiment were consistent with these values, suggesting that antimatter also experiences gravity as an attractive force to the same degree as matter. See also: Gravity
In this way, the experiment provides support for the weak equivalence principle, which states that gravity acts identically on all particles of equal mass. The weak equivalence principle follows from the general theory of relativity, a cornerstone of physics. The researchers were also able to ascertain the strength of the local acceleration of gravity for these antiparticles by adding an additional magnetic field to offset the effect of gravity. The resulting measurement of acceleration due to gravity for antimatter was consistent with the local acceleration due to gravity for matter—9.81 meters per second. Further experimentation will increase the precision of the measurement, continuing to test the weak equivalence principle and determine if, perhaps, any difference exists between the way in which matter and antimatter feel the force of gravity. See also: Mass; Relativity
Alpha, Beta, and Gamma Rays
Do Astronauts Need Sunscreen?
What Are Isotopes?
Hubble and the Bermuda Triangle of Space
Have We Found All the Elements?