Starts With A Bang

Why Doesn’t Antimatter Anti-Gravitate? (Synopsis)

Neutral antimatter, like antihydrogen, could be isolated from matter for long enough to determine which direction it falls in a gravitational field. Image credit: NSF.

“If something doesn’t reach you on a personal level, let it go. It’s hard enough dealing with everything that does.” -Judi Culbertson

There are two types of electric charge: positive and negative. Like charges repel; opposite charges attract. In gravitation, though, there’s only one kind of gravitational charge, more commonly known as mass. And everything we know of has a positive mass. But since there’s a counterpart to matter — antimatter — isn’t it possible that antimatter would have negative gravitational charge, and fall “up” in a gravitational field?

If there were some type of matter that had negative gravitational charge, it would be repelled by the matter and energy that we are aware of. Image credit: Muu-karhu of Wikimedia Commons.

It sounds like it might be plausible, and that it’s something that should certainly be experimentally tested. Indeed, the direct measurements of gravitational acceleration of neutral antimatter are perhaps two orders of magnitude away from being able to definitively say that antimatter doesn’t do exactly that. But a combination of other theoretical and experimental results strongly disfavor anti-gravitation, not only for antimatter, but for any potential form of matter at all.

The ultramassive, merging dynamical galaxy cluster Abell 370, with gravitational mass (mostly dark matter) inferred in blue. It’s all attractive. Image credit: NASA, ESA, D. Harvey (Swiss Federal Institute of Technology), R. Massey (Durham University, UK), the Hubble SM4 ERO Team and ST-ECF.

Come get the full story as to why, courtesy of Sabine Hossenfelder!