Eli5: How can some particles have no mass
In: 42
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.
It’s the other way around, actually.
When particle models were originally formulated, there was a big problem. According to vanilla quantum mechanics, all particles are massless. They measurably aren’t.
That’s why an additional effect had to be bolted onto the Standard Model, later dubbed the Higgs mechanism. A way to give massless particles mass. That’s why discovery of the Higgs Boson was such a big deal in 2012.
All mass comes from “trapped” energy. That’s why less than 1% of a proton’s mass is actually the quarks, the rest is the energy of the bonds between them. Why the mass of two separate atoms after fission is lower than before.
The Higgs mechanism works as a “drag” on particles, like they were moving through something viscous, and that creates a similar effect of “trapping” energy and gives them mass.
To find a satisfying answer here we need to unpack two words we throw around like they have some intuitive meaning, but they don’t. “Particle” and “mass”.
So for one “particle” doesn’t mean a practice as a dust particle. If we are talking about fundamental particles the particle is just the collective name we give these little things with the lack of a better word. So these things behave in a strange way as without interacting with anything they are literally waves (waves of probability) and once interacting they resemble more classical objects. Like how light is an EM wave but when it scatters on an electron we can treat it as a litte pool ball and we call that a photon. Or rather we call the smaller indivisible amount of energy some frequency of light can have.
The other thing “mass” has a more tangible definition. We will need special relativity for this. In SR we can look at moving objects in a 4D space. So we still have a good old 3D coordinate system but we add a 0th time like component to it. So locations turn into events. In this 4-vector formalism we can reintroduce (if thats a word) quantities like momentum as a 4-vector. We basically construct how physical quantities look in our new 4D spacetime. So lets grab 4-momentum and calculate its lenght (of course lenght here is a bit abstract). Well we directly calculate the length squared and that gives us m²c².
Ok, so we introduce a quantity called 4-momentum that describes how this move in spacetime and its lenght is mass (/c). Ok so lets look at what 4-momentum is for a path that light would take! (So for the object that would move on a light-path, these paths are called null-geodesics.) We will get exactly 0 for the lenght of that vector which was our definition of mass. So objects that move on paths that light would take have 0 mass based on our definition of mass. And this has some profound consequences like how massless things can only travel on light-paths and they have no rest frame.
And the beauty of SR is that we can start form very fundamental ideas and just derive all of this. After a bit of side-talk you can formulate this definition for mass and as it turns out this is really what we would want to call mass.
We could also go with a bottom up approach and look at how things are and aren’t coupled to the Higgs field and the reasoning is the same as for charge. A particle is coupled to the EM filed so it can excite it and how much it’s coupled and what way can be described by a signed scalars we call charge. For mass is even simpler its just a scalar value we can assign to particle. Now the “real” thing here is the interactions between the fields and particles giving particle values like mass and charge is our description of the interaction.
But still I think the SR definition of mass is just less painful to go with than trying to unpack the standard model.
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