Does Antimatter Fall Up?

New submitter Doug Otto sends word that researchers working on the ALPHA experiment at CERN are trying to figure out whether antimatter interacts with gravity in the same way that normal matter does. The ALPHA experiment wasn't designed to test for this, but they realized part of it — an antihydrogen trap — is suitable to collect some data. Their preliminary results: uncertain, but they can't rule it out. From the article: "Antihydrogen provides a particularly useful means of testing gravitational effects on antimatter, as it's electrically neutral. Gravity is by far the weakest force in nature, so it's very easy for its effects to be swamped by other interactions. Even with neutral particles or atoms, the antimatter must be moving slowly enough to perform measurements. And slow rates of motion increase the likelihood of encountering matter particles, leading to mutual annihilation and an end to the experiment. However, it's a challenge to maintain any antihydrogen long enough to perform meaningful experiments on it, regardless of its speed. ... The authors of the current study realized that [antiatoms trapped in ALPHA] eventually escaped or were released from this magnetic trap. At that point, they were momentarily in free-fall, experiencing no force other than gravity. The detectors on the outside of ALPHA could then determine if the antihydrogen was rising or falling under gravity's influence, and whether the magnitude of the force was equivalent to the effect on matter."

Comments

Re:The answer is...

[lgw]

It's easy to assume answers, but measurements separate science from philosophy.

Background info

[AdamHaun]

The Usenet Physics FAQ has some background information on the theory behind this question. It's 14 years old but still worth reading. One interesting bit:

Based on what we currently know, we would expect that the only significant force acting on a piece of falling antimatter is gravity; by the equivalence principle, this should make antimatter fall with the same acceleration as ordinary matter. However, some theories predict new, as yet unseen forces: these forces would make antimatter fall differently than matter. But in these theories, antimatter always falls slightly faster than matter; antimatter never falls up. This is because the only force that would treat matter and antimatter differently would be a vector force (mediated by the hypothetical gravivector boson). Vector forces (like electromagnetism) repel likes and attract opposites, so a gravivector force would pull antimatter down toward the matter-dominated Earth, while giving matter a slight upward push.

Re:Maybe our universe is a 'matter bubble'

[dkf]

It would seem that antimatter could only fall up, if there was some way to distinguish gravitational and inertial mass. From my experience of how electrons and positrons were accelerated at SLAC, their inertial mass was identical. The only difference between them was their charge.

This is why it is important to conduct the experiment to see if the gravitational and inertial mass of antimatter are the same. Sure, we know that they're the same thing for ordinary matter and that antimatter and matter have the same inertial mass, but the effect hasn't been properly studied for antimatter (because that's a furiously difficult experiment). It could be that gravitational and inertial mass are the same for AM; that would be the most likely expected case, and we wouldn't learn that much about new physics if that's true. But we haven't checked, and so we must do so to make sure. After all, if they were different that would be a really important fact about the universe that we are currently unaware of. (It would be far more important than finding the Higgs boson.)

Let the experiment be done. Let us find out if the universe is even stranger than we thought it was. It's this sort of thing that a fundamental physics lab should study.