The force of gravity on antimatter has never been directly measured but a growing number of physicists believe that such an experiment is within their grasp. Today, a group attempting to design an experiment called AEGIS (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) outline their plans to measure this force.

In some ways it’s an ambitious plan. The team wants to build AEGIS at CERN, the European particle physics laboratory near Geneva, where the building blocks of antihydrogen, low energy antiprotons and positrons, are in relatively good supply.

The idea is to fire a beam of antihydogen atoms at a target and see how much they are deflected by gravity.

That’s easier said than done. Creating a beam of this stuff turns out to be remarkably tricky. The problem is that it’s easy enough to trap antiprotons and positrons in electromagnetic fields. It’s even fairly straightforwad to put them together so that they form antihydrogen. The problem is that antihydrogen is neutral and simply falls out of the trap. So some way has to be found to collect and trap these antiatoms.

I know what you’re thinking: why not do the experiment with antiprotons or positrons instead.

People have tried but it’s been impossible to completely remove any residual electromagnetic fields from such experiments. These are many orders of magnitude stronger than gravity and so even the smallest trace of them deflects charged particles by an amount that overwhelms the effect of gravity. That’s why neutral antihydrogen is so important.

Why bother? There are several flavours of general relativity that allow antimatter to experience an opposite gravitational force compared to ordinary matter. Finding evidence for this (or ruling it out) will have important consequences for some serious cosmological conundrums such as why we see so little antimatter around and the value of the cosmological constant.

If these guys get the go-ahead, it’ll be a few years before we hear back from them, but it’ll be worth the wait.

Ref: arxiv.org/abs/0805.4727: Formation Of A Cold Antihydrogen Beam in AEGIS For Gravity Measurements