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Starts With A Bang

Ask Ethan: What’s so “anti” about antimatter?

There are lots of properties inherent to particles, and while everyone has an antiparticle, not everyone is matter or antimatter.

For every particle of matter that’s known to exist in the Universe, there’s an antimatter counterpart. Antimatter has many of the same properties as normal matter, including the types of interaction it undergoes, its mass, the magnitude of its electric charge, and so on. But there are a few fundamental differences as well. Yet two things are certain about matter-antimatter interactions: if you collide a matter particle with an antimatter counterpart, they both immediately annihilate away to pure energy, and if you undergo any interaction in the Universe that creates a matter particle, you must also create its antimatter counterpart. So what makes antimatter so “anti,” anyway? That’s what Robert Nagle wants to know, as he asks:

On a fundamental level, what is the difference between matter and its counterpart antimatter? Is there some sort of intrinsic property that causes a particle to be matter or antimatter? Is there some intrinsic property (like spin) that distinguishes quarks and antiquarks? What what puts the ‘anti’ in anti matter?

To understand the answer, we need to take a look at all the particles (and antiparticles) that exist.

The particles and antiparticles of the Standard Model obey all sorts of conservation laws, but there are fundamental differences between fermionic particles and antiparticles and bosonic ones. Image credit: E. Siegel / Beyond The Galaxy.

This is the Standard Model of elementary particles: the full suite of discovered particles in the known Universe. There are generally two classes of these particles, the bosons, which have integer spins (…, -2, -1, 0, +1, +2, …) and are neither matter nor antimatter, and the fermions, which have half-integer spins (…, -3/2, -1/2, +1/2, +3/2, …) and must either be “matter-type” or “antimatter-type” particles. For any particle you can think about creating, there are going to be a slew of inherent properties to it, defined by what we call quantum numbers. For an individual particle in isolation, this includes a number of traits you’re likely familiar with, as well as some that you may not be familiar with.

These possible configurations for an electron in a hydrogen atom are extraordinarily different from one another, yet all represent the same exact particle in a slightly different quantum state. Particles (and antiparticles) also have intrinsic quantum numbers that cannot be changed, and those numbers are key in defining whether a particle is matter, antimatter, or neither. Image credit: PoorLeno / Wikimedia Commons.

The easy ones are things like mass and electric charge. An electron, for example, has a rest mass of 9.11 × 10^–31 kg, and an electric charge of -1.6 × 10^–19 C. Electrons can also bind together with protons to produce a hydrogen atom, with a series of spectral lines and emission/absorption features based on the electromagnetic force between them. Electrons have a spin of either +1/2 or -1/2, a lepton number of +1, and a lepton family number of +1 for the first (electron) of the three (electron, mu, tau) lepton families. (We’re going to ignore numbers like weak isospin and weak hypercharge, for simplicity.)

Given these properties of an electron, we can ask ourselves what the antimatter counterpart of the electron would need to look like, based on the rules governing elementary particles.

In a simple hydrogen atom a single electron orbits a single proton. In an antihydrogen atom a single positron (anti-electron) orbits a single antiproton. Positrons and antiprotons are the antimatter counterparts of electrons and protons, respectively. Image credit: Lawrence Berkeley Labs.

The magnitudes of all the quantum numbers must remain the same. But for antiparticles, the signs of these quantum numbers must be reversed. For an anti-electron, that means it should have the following quantum numbers:

  • a rest mass of 9.11 × 10^–31 kg,
  • an electric charge of +1.6 × 10^–19 C,
  • a spin of (respectively) either -1/2 or +1/2,
  • a lepton number of -1,
  • and a lepton family number of -1 for the first (electron) lepton family.

And when you bind it together with an antiproton, it should produce exactly the same series of spectral lines and emission/absorption features that the electron/proton system produced.

Electron transitions in the hydrogen atom, along with the wavelengths of the resultant photons, showcase the effect of binding energy and the relationship between the electron and the proton in quantum physics. The spectral lines between positrons and antiprotons have been verified to be exactly the same. Image credit: Wikimedia Commons users Szdori and OrangeDog.

All of these facts have been verified experimentally. The particle matching this exact description of the anti-electron is the particle known as a positron! The reason why this is necessary comes when you consider how you make matter and antimatter: you typically make them from nothing. Which is to say, if you collide two particles together at a high enough energy, you can often create an extra “particle-antiparticle” pair out of the excess energy (from Einstein’s E = mc2), which conserves energy.

Whenever you collide a particle with its antiparticle, it can annihilate away into pure energy. This means if you collide any two particles at all with enough energy, you can create a matter-antimatter pair. Image credit: Andrew Deniszczyc, 2017.

But you don’t just need to conserve energy; there are a slew of quantum numbers you also have to conserve! And these include all of the following:

  • electric charge,
  • angular momentum (which combines “spin” and “orbital” angular momentum; for individual, unbound particles, that’s only “spin”),
  • lepton number,
  • baryon number,
  • lepton family number,
  • and color charge.

Of these intrinsic properties, there are two that define you as either “matter” or “antimatter,” and those are “baryon number” and “lepton number.”

In the early Universe, the full suite of particles and their antimatter particles were extraordinarily abundant, but as they Universe cooled, the majority annihilated away. All the conventional matter we have left over today is from the quarks and leptons, with positive baryon and lepton numbers, that outnumbered their antiquark and antilepton counterparts. (Only quarks and antiquarks are shown here.) Image credit: E. Siegel / Beyond The Galaxy.

If either of those numbers are positive, you’re matter. That’s why quarks (which each have baryon number of +1/3), electrons, muons, taus, and neutrinos (which each have lepton number of +1) are all matter, while antiquarks, positrons, anti-muons, anti-taus, and anti-neutrinos are all antimatter. These are all the fermions and antifermions, and every fermion is a matter particle while every antifermion is an antimatter particle.

The particles of the standard model, with masses (in MeV) in the upper right. The Fermions make up the left three columns; the bosons populate the right two columns. While all particles have a corresponding antiparticle, only the fermions can be matter or antimatter. Image credit: Wikimedia Commons user MissMJ, PBS NOVA, Fermilab, Office of Science, United States Department of Energy, Particle Data Group.

But there are also the bosons. There are gluons which have for their antiparticles the gluons of the opposite color combinations; there is the W+ which is the antiparticle of the W- (with opposite electric charge), and there are the Z0, the Higgs boson, and the photon, which are their own antiparticles. However, bosons are neither matter nor antimatter. Without a lepton number or baryon number, these particles may have electric charges, color charges, spins, etc., but no one can rightfully call themselves either “matter” or “antimatter” and their antiparticle counterpart the other one. In this case, bosons are simply bosons, and if they have no charges, then they’re simply their own antiparticles.

On all scales in the Universe, from our local neighborhood to the interstellar medium to individual galaxies to clusters to filaments and the great cosmic web, everything we observe appears to be made out of normal matter and not antimatter. This is an unexplained mystery. Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA).

So what puts the “anti” in antimatter? If you’re an individual particle, then your antiparticle is the same mass as you with all the opposite conserved quantum numbers: it’s the particle that’s capable of annihilating with you back to pure energy if ever the two of you meet. But if you want to be matter, you need to have either positive baryon or positive lepton number; if you want to be antimatter, you must have either negative baryon or negative lepton number. Beyond that, there’s no known fundamental reason for our Universe to have favored matter over antimatter; we still don’t know how that symmetry was broken. (Although we have ideas.) If things had turned out differently, we’d probably call whatever we were made of “matter” and its opposite “antimatter,” but who gets which name is completely arbitrary. As in all things, the Universe is biased towards the survivors.

Send in your Ask Ethan questions to startswithabang at gmail dot com!

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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