Eli5: How can some particles have no mass
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It’s actually weirder that any particles have innate mass. Let me explain.
Remember that Einstein proved that E=MC²? What this means is that mass and energy are equivalent. Mass can be turned into energy, and energy can turn into mass. They’re actually the same thing.
Well soon after that the first nuclear fission was accomplished. Researchers noticed that when an atomic nucleus is broken apart, the mass of the parts it breaks into can actually be more than the mass of the of the original atom. What happened, where did the mass come from? Well, mass is energy. They termed this energy ‘binding energy’ – the energy required to bind the neutrons and protons together, which becomes mass.
Fast forward some time, and it is discovered that protons and neutrons aren’t fundamental particles. They’re made of smaller particles, called quarks. These too have binding energy – and it’s really high compared to the mass of protons and neutrons. It accounts for 99% of their mass. So quarks have very, very little innate mass, and almost all the mass in the universe actually comes from the energy holding all the quarks together.
This is pretty wild! What even are particles, if all the mass in the universe is actually energy? Well, a new understanding of physics emerged that helps explain this. Rather than thinking of particles as little billiard balls, suppose that they are excitations of fields. An electron is just little wave riding around in the universal electromagnetic field, as is a photon. All the other particles are excitations of their respective fields. The math works out, and this is a very successful theory of physics.
But wait a minute, if all the mass comes from binding energy, and all the particles are just waves in fields, why do any of them have any innate mass? Why do electrons and quarks have an innate mass, and why do they not move at the speed of light like photons? Physicists proposed that there was an additional undiscovered field which these particles were interacting with. It isn’t surprising that some particles would interact with some fields and not others, because lots of particles we know about do just that. So maybe these particles with mass are interacting with this field, and that constant energetic interaction is what gives them inertia and mass and prevents them moving at the speed of light.
If the field exists, it should have an associated particle, a boson. And since one of the primary advocates for the field theory was a physicist called Peter Higgs, the particle was referred to as the ‘Higgs Boson’ in the 1960s. It wouldn’t be until 2012 that the Higgs Boson was experimentally documented, more or less confirming the theory. You may have heard of this at the time as the ‘god particle’ or ‘mass mechanism’ which proved the standard model of physics. That’s a bit overhyped, it just kind of solves this one question, but it was still very important.
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