<P> These quarks and leptons interact through four fundamental forces: gravity, electromagnetism, weak interactions, and strong interactions . The Standard Model of particle physics is currently the best explanation for all of physics, but despite decades of efforts, gravity cannot yet be accounted for at the quantum level; it is only described by classical physics (see quantum gravity and graviton). Interactions between quarks and leptons are the result of an exchange of force - carrying particles (such as photons) between quarks and leptons . The force - carrying particles are not themselves building blocks . As one consequence, mass and energy (which cannot be created or destroyed) cannot always be related to matter (which can be created out of non-matter particles such as photons, or even out of pure energy, such as kinetic energy). Force carriers are usually not considered matter: the carriers of the electric force (photons) possess energy (see Planck relation) and the carriers of the weak force (W and Z bosons) are massive, but neither are considered matter either . However, while these particles are not considered matter, they do contribute to the total mass of atoms, subatomic particles, and all systems that contain them . </P> <P> The modern conception of matter has been refined many times in history, in light of the improvement in knowledge of just what the basic building blocks are, and in how they interact . The term "matter" is used throughout physics in a bewildering variety of contexts: for example, one refers to "condensed matter physics", "elementary matter", "partonic" matter, "dark" matter, "anti" - matter, "strange" matter, and "nuclear" matter . In discussions of matter and antimatter, normal matter has been referred to by Alfvén as koinomatter (Gk . common matter). It is fair to say that in physics, there is no broad consensus as to a general definition of matter, and the term "matter" usually is used in conjunction with a specifying modifier . </P> <P> The history of the concept of matter is a history of the fundamental length scales used to define matter . Different building blocks apply depending upon whether one defines matter on an atomic or elementary particle level . One may use a definition that matter is atoms, or that matter is hadrons, or that matter is leptons and quarks depending upon the scale at which one wishes to define matter . </P> <P> These quarks and leptons interact through four fundamental forces: gravity, electromagnetism, weak interactions, and strong interactions . The Standard Model of particle physics is currently the best explanation for all of physics, but despite decades of efforts, gravity cannot yet be accounted for at the quantum level; it is only described by classical physics (see quantum gravity and graviton). </P>

Building blocks of all matter are known as
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