How can a particle without mass be useful? person rather new to physics here, and I just started on a rather simple book on particle physics, but I was flustered to learn that there are particles without mass enforcing the fundamental forces (Like the gluon or the Higgs boson). There is some Greek term for it but I don't remember. I just can not comprehend this because my idea of a particle is something like a grain of sand floating in space: You can fiddle around with it and it can interact with other particles. But how can a particle exist without mass? Or how can it interact with other particles if it does not have mass? What even is a particle at this point?
I'm sorry if its a stupid question but this just bothers me.
 A: This is not a stupid question and you are not alone.  Many have struggled with the same concern when first introduced to the idea of a massless particle.  In physics today, we use the word mass in association with the properties of a particle that is at rest (not moving).  However, it is also true that mass is associated with energy through Einstein's famous equation: E = mc2.  This introduces the idea that as the energy of an object increases with its velocity that this will result in an increase in the overall energy content of the object.  This energy content can be thought of as being partly made up from the object's mass (when it is at rest) and partly from the mass associated with the energy required to provide motion to the object. There is no requirement that both of these conditions must contribute to the overall energy content.  For example, an object sitting on the table has only mass (from being at rest) and no additional mass (energy) associated with its motion (because it is at rest).  Likewise, it is possible for an object to get all of its mass from the energy of its motion with no contribution from an at rest state.  This is what is meant by a massless particle. Generally speaking, a massless particle cannot exist at rest, it must be moving to achieve a property of mass that comes from the energy of its motion.
A: As for the title question, massless particles are useful because despite being massless they still carry information. Individually it's a tiny amount but added up it can be any amount needed.
