In the standard model of particle physics, which describes the known elementary particles and their interactions, there are several particles that are considered to be truly massless. These particles do not experience gravitational forces and travel at the speed of light. Here are some examples:
Photon: The photon is the quantum particle of light and electromagnetic radiation. It has zero rest mass and carries the electromagnetic force. Photons do not experience gravitational forces.
Gluon: Gluons are the mediators of the strong nuclear force, which binds quarks together inside protons, neutrons, and other particles. Gluons are also massless and do not experience gravity.
Graviton (hypothetical): While not yet experimentally confirmed, the graviton is a theoretical particle that is predicted in some quantum gravity theories. If it exists, it would be the quantum particle associated with the gravitational force and would be massless.
It's important to note that the masslessness of these particles refers to their rest mass, which is the mass they would have when at rest. While they do not experience gravitational forces, they can still be influenced by gravitational fields due to their energy and momentum.
It's worth mentioning that the Higgs mechanism, proposed in the standard model, gives mass to certain particles. The W and Z bosons, for example, gain mass through interactions with the Higgs field. However, particles like photons and gluons do not interact with the Higgs field and remain massless.
It's also worth noting that the standard model of particle physics does not incorporate gravity into its framework. The nature of gravity and its compatibility with quantum mechanics is still an active area of research, and a more complete theory that unifies gravity with the other fundamental forces is a goal of theoretical physics.