Matter

<p>In the <a href="https://en.wikipedia.org/wiki/Classical_physics" title="Classical physics">classical physics</a> observed in everyday life, <b>matter</b> can be defined as any substance that has <a href="https://en.wikipedia.org/wiki/Mass" title="Mass">mass</a> and takes up space, or as any substance made up of <a href="https://en.wikipedia.org/wiki/Atoms" class="mw-redirect" title="Atoms">atoms</a>, thus excluding other energy phenomena or waves such as <a href="https://en.wikipedia.org/wiki/Light" title="Light">light</a> or <a href="https://en.wikipedia.org/wiki/Sound" title="Sound">sound</a>. Observable <a href="https://en.wikipedia.org/wiki/Physical_body" title="Physical body">physical objects</a> are said to be composed of matter.<sup id="cite_ref-Penrose_1-0" class="reference"><a href="https://en.wikipedia.org/wiki/Matter#cite_note-Penrose-1">[1]</a></sup><sup id="cite_ref-mcgrawhill_2-0" class="reference"><a href="https://en.wikipedia.org/wiki/Matter#cite_note-mcgrawhill-2">[2]</a></sup> More generally, in (<a href="https://en.wikipedia.org/wiki/Modern_physics" title="Modern physics">modern</a>) <a href="https://en.wikipedia.org/wiki/Physics" title="Physics">physics</a>, matter is not a fundamental concept because a universal definition of it is elusive: <a href="https://en.wikipedia.org/wiki/Elementary_particle" title="Elementary particle">elementary constituents</a> may not take up space per se, and individually-<a href="https://en.wikipedia.org/wiki/Massless_particle" title="Massless particle">massless</a> <a href="https://en.wikipedia.org/wiki/Particle_physics" title="Particle physics">particles</a> may be composed to form objects that have mass (even when <a href="https://en.wikipedia.org/wiki/Rest_mass" class="mw-redirect" title="Rest mass">at rest</a>).

All the everyday objects that we can bump into, touch or squeeze are composed of <a href="https://en.wikipedia.org/wiki/Atom" title="Atom">atoms</a>. This ordinary atomic matter is in turn made up of interacting <a href="https://en.wikipedia.org/wiki/Subatomic_particle" title="Subatomic particle">subatomic particles</a>—usually a <a href="https://en.wikipedia.org/wiki/Atomic_nucleus" title="Atomic nucleus">nucleus</a> of <a href="https://en.wikipedia.org/wiki/Proton" title="Proton">protons</a> and <a href="https://en.wikipedia.org/wiki/Neutron" title="Neutron">neutrons</a>, and a cloud of orbiting <a href="https://en.wikipedia.org/wiki/Electron" title="Electron">electrons</a>.<sup id="cite_ref-Davies2_3-0" class="reference"><a href="https://en.wikipedia.org/wiki/Matter#cite_note-Davies2-3">[3]</a></sup><sup id="cite_ref-Hooft_4-0" class="reference"><a href="https://en.wikipedia.org/wiki/Matter#cite_note-Hooft-4">[4]</a></sup> Typically, science considers these composite particles matter because they have both <a href="https://en.wikipedia.org/wiki/Rest_mass" class="mw-redirect" title="Rest mass">rest mass</a> and volume. By contrast, <a href="https://en.wikipedia.org/wiki/Massless_particle" title="Massless particle">massless particles</a>, such as <a href="https://en.wikipedia.org/wiki/Photon" title="Photon">photons</a>, are not considered matter, because they have neither rest mass nor volume. However, not all particles with rest mass have a classical volume, since fundamental particles such as <a href="https://en.wikipedia.org/wiki/Quark" title="Quark">quarks</a> and <a href="https://en.wikipedia.org/wiki/Lepton" title="Lepton">leptons</a> (sometimes equated with matter) are considered "point particles" with no effective size or volume. Nevertheless, quarks and leptons together make up "ordinary matter", and their interactions contribute to the effective volume of the composite particles that make up ordinary matter.

Matter exists in <i><a href="https://en.wikipedia.org/wiki/State_of_matter" title="State of matter">states</a></i> (or <i><a href="https://en.wikipedia.org/wiki/Phase%28matter%29" title="Phase (matter)">phases</a></i>): the classical <a href="https://en.wikipedia.org/wiki/Solid" title="Solid">solid</a>, <a href="https://en.wikipedia.org/wiki/Liquid" title="Liquid">liquid</a>, and <a href="https://en.wikipedia.org/wiki/Gas" title="Gas">gas</a>; as well as the more exotic <a href="https://en.wikipedia.org/wiki/Plasma%28physics%29" title="Plasma (physics)">plasma</a>, <a href="https://en.wikipedia.org/wiki/Bose%E2%80%93Einstein_condensate" title="Bose–Einstein condensate">Bose–Einstein condensates</a>, <a href="https://en.wikipedia.org/wiki/Fermionic_condensate" title="Fermionic condensate">fermionic condensates</a>, and <a href="https://en.wikipedia.org/wiki/Quark%E2%80%93gluon_plasma" title="Quark–gluon plasma">quark–gluon plasma</a>.<sup id="cite_ref-RHIC_5-0" class="reference"><a href="https://en.wikipedia.org/wiki/Matter#cite_note-RHIC-5">[5]</a></sup>

For much of the history of the <a href="https://en.wikipedia.org/wiki/Natural_science" title="Natural science">natural sciences</a> people have contemplated the exact nature of matter. The idea that matter was built of discrete building blocks, the so-called <i>particulate theory of matter</i>, was first put forward by the Greek philosophers <a href="https://en.wikipedia.org/wiki/Leucippus" title="Leucippus">Leucippus</a> (₄₉₀ BC) and <a href="https://en.wikipedia.org/wiki/Democritus" title="Democritus">Democritus</a> (₄₇₀–380 BC).<sup id="cite_ref-6" class="reference"><a href="https://en.wikipedia.org/wiki/Matter#cite_note-6">[6]</a></sup></p>