Physicists at the University of California, Riverside, have set forward lab experiments to determine if antimatter behaves differently in gravity than baryonic matter. This could provide an explanation why the Universe doesn’t seem to have much antimatter and why it’s expanding at an ever-increasing rate.
The researchers are trying to measure the free fall of positronium, a bound state between a positron and an electron. Positrons are the antimatter equivalent of electrons, and have identical mass, but a positive charge. When a positron and electron collide, they annihilate and produce two gamma rays.
David Cassidy and Allen Mills separated the positron from the electron in positonium in order to have enough time to measure the effect of gravity on it. The unstable system has to resist annihilation long enough for this to happen.
Lasers were used to excite positronium into a Rydberg state, rendering the atom very weakly bound. This stopped them from destroying each other for a while, allowing enough time for lab experiments to be performed.
At the Rydberg level, the positronium’s lifetime increases by a factor of 10 to 100. They plan on using a technique that imparts high angular momentum to Rydberg atoms, allowing for a lifetime of about 10 milliseconds, an increase of 10,000.
Once they have attained this, they will use the positronium to make a beam and look at its deflection as a function of light. If antimatter and matter don’t behave in the same way, it would come as a surprise as this is what has been assumed. Since there hasn’t been much antimatter discovered, scientists have been trying to find explanations why this is the case.