What really is a particle? How can massless particles exist? How can it still be a particle if it doesn’t have any mass?

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What really is a particle? How can massless particles exist? How can it still be a particle if it doesn’t have any mass?

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This is a good question that points out the silliness of common use terms of scientific definitions. Particles seem to not fit because in everyday language we call all kinds of things particles that aren’t really particles and because through history as a people when we were learning about light and electric and magnetic things we didn’t know if they were waves or particles and so now everything is both. Even though now we know light waves are waves that ACT like a particle but light is not a particle, but it can act like a particle so we often call it a particle when making basic models to teach but it is not by definition a particle because it has no matter and anything without matter cannot be a particle.

By definition, matter is anything that has mass and volume and by definition a particle is a piece of matter. The science lessons break some serious science laws when making your basic model for introducing the topic of matter and that is ok because it is just a model but it causes misconceptions down the line when done certain ways. I see it all the time the readings say, “Matter is everything around you!” and it is not. There are all sorts of energies around you in the form of electric and magnetic fields and these fields do not have mass. We sometimes call light (electromagnetic energy) a particle because it often behaves like a particle BUT it really is not a particle. A particle is an even more basic science definition. We can almost ALWAYS be more specific then when using the word particle. Particle is generally a filler word for more other words like “molecule” or “atom” before a student has learned the words molecule or atom you can use particle to fill in. Either way, those molecules have mass because semantically the definition of a particle in science breaks down to molecule or atom, both which have mass OR by definition matter HAS to have mass and volume because that is the definition of matter and most “particles” refer to molecules and atoms which happen to have mass.

That is by far not an ELI5 question. Most people would struggle with it as an ELI25.

I would recommend the PBS Spacetime show on Youtube, they have series of videos on this topic, and are good at explaining it as concisely as it is possible to explain quantum physics.

But to give it a shot, particles are vibrations in quantum fields, which are currently the actual fundamental building blocks of reality, and by default should NOT have any mass. Being massless is actually a particle’s natural state. Most elementary particles gain mass due to the interaction of their fields with the Higgs Field (of Higgs Boson fame), while composite particles (hadrons) gain most of their mass from the energy of their gluon bonds (because E=mc^2).

Why some particles interact with the higgs field and some don’t is literally a PhD topic.

A particle is a bounded peice of energy which has broken off from its source event and become its own thing. It’s like a wavy moving droplet of water but instead of water, it’s energy like light or electricity.

Mass is a way particles can interact, if one doesn’t generate a gravity signal by how its energy moves it will not have any detectable mass. Like seeing does it ripple fast and small enough to match up with the particles we use to measure, or not?

A particle is a particle by its bounds, once it’s cut off or ejected from a high energy event is when it becomes a particle. How exactly that happens determines whether it has mass we can measure, sometimes it happens and sometimes it doesn’t.

I believe there are some misconceptions in your question:

1. Massless usually refers to it having no rest-mass i.e. not having any mass when the object is at rest. If an object however possesses kinetic energy or really any momentum, then according to Einsteins equivalence principle it also has mass. Since light has an energy of E=hf you can also assign a mass to it since mass and energy are two sides of the same coin.

2. As to what really is a particle? Well thats a bit more of a complicated question. Particles used to be defined by their charge, energy, shape, position and momentum, however that point of view is a bit outdated. At small scales particles tend to lose some well defined properties such as their position and momentum. For example: An electron has a well defined energy value around a nucleus, their position and momentum however are not well defined. Only once a physical process imposes certain boundary conditions on this system (for example an observation), the electron has a well defined position (or momentum; it cannot have both at the same time with arbitrary precision: see Heisenbergs uncertainty principle). There are numerous interpretations as to why particles behave that way, however since all of those make the same predictions and cannot be distinguished from one another experimentally we assume all of them to be equally correct.

Now as to what particles really are: According to our best model of standard physics (which is incomplete btw), each particle corresponds to a certain symmetry of our physical reality. Im still in my bachelors and havent had the honour to study the standard model in detail yet, so I unfortunately dont understand that part fully neither. Keywords for this are Gauge Symmetry and Poincare Invariance.