This September, a group of researchers at CERN finally, after years of engineering an extraordinarily precise experiment, managed to create and carefully capture a sample of antihydrogen (the antimatter version of hydrogen).
They held the sample in a magnetic field containment so precarious that any slight misalignment would cause it to immediately 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 – initially just as a rather creative way to balance out an equation that seemed to have an extra solution – we’ve learned to produce antimatter in experiments, and we’ve seen evidence for it in high-energy astrophysical environments in space.
As its defining feature, 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 popular reputation suggests. As far as we can tell, an anti-electron, which is called a positron, is exactly like an electron, except it has the opposite charge (+1 instead of -1), and is opposite in ‘parity,’ which means it is like a mirror reflection.
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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 – maybe – have some kind of anti-gravity, too? Does antihydrogen (a positron bound to an anti-proton) fall up when dropped, instead of down?
Alas, rather than revealing some dramatic new violation of gravity or a scrapping of some of the most crucial aspects of Einstein’s general relativity, 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?
The crux of the matter
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 is at rest (as opposed to an ‘effective’ mass, which depends on its motion).
Atoms, molecules, liquids, solids, gases – all of these are straightforwardly matter, as are the protons, neutrons, and electrons that make them up.
Other mass-having elementary particles, like quarks (which are components of protons and neutrons) and neutrinos (very light particles produced in nuclear reactions), count as matter too.
But what about massless elementary particles, like photons, gluons, or the still-hypothetical graviton? Even though we classify them as particles, they wouldn’t count as matter in this context.
Weirdly, things that are definitely not matter can contribute to the mass of something else. In Einstein’s general relativity, the gravitational effect of an object is due not just to its matter, but to its energy as well.
In principle, a hot bowling ball should weigh more than an otherwise identical cold one, because the heat energy adds to its gravitational effect.
Similarly, an atom or molecule is just a tiny bit heavier than its constituent particles due to the binding energy of the electromagnetic forces holding it together, even though the electromagnetic field can be said to be made of photons that don’t have rest masses and therefore aren’t considered matter.
When it comes to protons and neutrons, only a tiny fraction of their mass comes from the mass of their matter components (the quarks). The rest comes from various kinds of energy involved in holding the quarks together.
What could the matter be?
In another category, there’s what physicists refer to as ‘exotic matter’, which broadly includes any substance that is still hypothetical or has properties at odds with what we think of as the rules of the Universe.
Dark matter – a kind of invisible stuff that we think is made of an as-yet undiscovered particle – could be called exotic matter, but in a sense, it seems to be fairly ordinary as far as matter is concerned.
It acts just like regular matter, except without interacting with light. For some truly exotic matter, we could look to hypothetical substances like negative-mass matter, which probably doesn’t exist, but could potentially hold up stable wormholes if it did.
Whatever we call the matter we have, we seem to be very lucky that it exists at all. Our best existing theories of particle physics and the Big Bang seem to suggest that matter and antimatter should have come into existence in the cosmos in equal amounts.
That would have been very bad for all involved, because everything would have been annihilated completely into radiation, with nothing left over.
For reasons we can’t yet fully explain, there seems to have been a slight imbalance, allowing all the antimatter to annihilate away while leaving a bit of regular matter to become the stars, galaxies, planets, and people we see today.
Antimatter experiments like ALPHA-g might give us more insights into not just the nature of matter, but also why any of us exist at all. That’s worth dropping everything for.
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