What does an antihydrogen (1 anti-proton in the nucleus and 1 positron orbiting the anti-proton) atom do in a gravitational field? Does it fall down or does it fall up? If it falls down, does the anti-hydrogen have the same weight as the hydrogen atom? Does the “Weak Equivalence Principle” hold for antihydrogen atoms?
No one knows the answers to these questions but fortunately there are multiple experiments at CERN studying the behavior of anti-matter. We should expect to have answers in the next decade. This is very exiting.
Before I proceed to the information regarding the CERN anti-matter experiments I should mention the theoretical expectations. Einstein’s General Relativity Theory says that gravitational force should work identically on matter and anti-matter. Similarly, the Standard Model of particle physics says that matter and antimatter are identical except for their electrical charge. I am glad that experimental physicists do not take these statements for granted. The experiments testing the gravitational interaction of anti-matter are extremely important. We might be in for a surprise.
CERN anti-matter experiments
In the movie “Angels and Demons” there is a mention of the anti-matter (antihydrogen) so people are generally aware that CERN can generate anti-atoms but people do not always realize the difficulty of performing experiments with anti-atoms.
The first antihydrogen atom was created at the LEAR facility of CERN in 1995. Seven years later, in 2002, ATHENA and ATRAP experiments produced large numbers of antihydrogen atoms using CERN’s Antiproton Decelerator (AD). Today, the AD serves five experiments that are studying antimatter in different ways: AEgIS, ALPHA, ASACUSA, ATRAP and BASE.
“ALPHA – ATHENA’s successor – is specifically designed to trap antihydrogen particles for longer than its predecessors, so they can be studied in finer detail than ever before. The ALPHA collaboration has already measured the electric charge of an antiatom to a much higher precision than before. The ASACUSA collaboration, which also has high-precision studies of antihydrogen in its sights, has demonstrated the first-ever production of a beam of antiatoms.
Earlier this year  further advances were made when the Baryon Antibaryon Symmetry Experiment (BASE) reported the most precise comparison of the charge-to-mass ratio of the proton to that of its antimatter equivalent, the antiproton. The study, which took 13,000 measurements over a 35-day period, showed that protons and antiprotons have identical mass-to-charge ratios.
The AEgIS experiment, which has just started operation this year , is designed specifically to measure the gravitational interaction of antimatter. Another, future experiment, GBAR, will make similar investigations.
These recent successes mark a growth in antimatter research that CERN’s AD can no longer keep up with, as more and more low-energy antiprotons are needed for experiments. An upgrade to the AD, called ELENA, will become operational in 2017. This is where GBAR will be installed.
ELENA will decelerate the antiprotons from the AD still further, allowing many more to be trapped by the experiments. With the additional ability to serve four experiments almost simultaneously, ELENA will usher in a new era in the investigation of the relationship between matter and antimatter in the universe.”
The ALPHA experiment at CERN is designed to make precise comparisons of hydrogen and antihydrogen. Here’s a statement from the ALPHA experiment at CERN (2013)
“Physicists have long wondered if the gravitational interaction between antimatter and matter might be different than that between matter and itself. Do atoms made of antimatter, like antihydrogen, fall at a different rate to those made of matter, or might they even fall up — antigravity? There are many arguments that make the case that the interaction must be the same, but no-one has ever observed what an anti-atom does in a gravitational field – until now.
Today, the ALPHA Collaboration has published results in Nature Communications placing the first experimental limits on the ratio of the graviational and inertial masses of antihydrogen (the ratio is very close to one for hydrogen). We observed the times and positions at which 434 trapped antihydrogen atoms escaped our magnetic trap, and searched for the influence of a gravitational force. Based on our data, we can exclude the possibility that the gravitiational mass of antihydrogen is more than 110 times its inertial mass, or that it falls upwards with a gravitational mass more than 65 times its inertial mass.
Our results far from settle the question of antimatter gravity. But they open the way towards higher-precision measurements in the future, using the same technique, but more, and colder trapped antihydrogen atoms, and a better understanding of the systematic effects in our apparatus.”
“The goal of the AEgIS experiment is to measure the gravitational acceleration of antihydrogen with 1% accuracy and test the Weak Equivalence Principle which lies at the base of General Relativity. Most of the equipment has been installed with the exception of the gravity module which is still the subject of research and development. We have performed a test experiment with a miniature copy of the gravity module. The results have shown that a micrometric shift is observable, and thus establish the feasibility of the proposed detection method. Obviously, the short term plan is to produce antihydrogen atoms. This requires improvements in antiproton cooling system, in the positron system and the optimization of positronium production. The goal is to produce an antihydrogen beam in 2015–2016 and to perform a ﬁrst rough measurement of the gravitational acceleration shortly thereafter.”