Many questions remain unanswered when it comes to matter, but the biggest one is: Why is there any matter to begin with?
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Back in September, researchers at CERN managed to create and capture a sample of antihydrogen (the antimatter version of hydrogen). They held the sample in a magnetic field so precarious that any slight misalignment would cause it to annihilate against the walls of its container. And then they dropped it.
The ALPHA-g experiment was designed to answer the question of just how ‘anti’ antimatter really is. Since antimatter was first proposed in the 1920s, we’ve learned to produce it in experiments and seen evidence for it in high-energy astrophysical environments in space. And we’ve seen that any contact between a particle of antimatter and its regular-matter counterpart results in annihilation into high-energy radiation.
Despite its violent tendencies, antimatter has generally shown itself to be far less outlandish than its reputation suggests. As far as we can tell, an anti-electron (a positron) is exactly like an electron, except it has the opposite charge (+1 instead of -1), and is opposite in ‘parity’ (like a mirror reflection).
Like other versions of antimatter, the mass of a positron exactly matches that of its regular-matter counterpart. But until ALPHA-g, physicists had yet to experimentally confirm that antimatter’s mass acts the same as that of ordinary matter.
Could antimatter have some kind of anti-gravity, too? Does antihydrogen (a positron bound to an anti-proton) fall up, instead of down, when dropped?
Alas, the ALPHA-g experiment showed that antimatter does, in fact, fall down. As far as gravity is concerned, antimatter is, really, just matter. But that might lead one to ask: what is matter, really?
What counts as matter in physics depends on the context of the question. The simplest definition of matter is anything that has a rest mass: a mass that is inherent to the particle and exists when it’s at rest (as opposed to an ‘effective’ mass, which depends on its motion). Atoms, molecules, liquids, solids, gases – all of these are matter, as are the protons, neutrons and electrons that make them up.
But what about massless elementary particles, like photons, gluons or the hypothetical graviton? Even though we classify them as particles, they wouldn’t count as matter in this context.
Weirdly, however, things that ar
