Matter
Matter is defined as any substance that has mass and occupies space, distinguishing it from energy and other abstract concepts in physics.[1] It forms the basis of all tangible objects in the universe, from subatomic particles to galaxies, and is the subject of study in fields like physics, chemistry, and materials science.[2] At its most fundamental level, matter is composed of atoms, which are the smallest units of elements that retain their chemical properties; each atom consists of a dense nucleus containing protons and neutrons, surrounded by a cloud of electrons.[3] Protons carry a positive charge equal in magnitude but opposite to that of electrons, while neutrons are electrically neutral, and the number of protons (atomic number) determines the element's identity.[4] Atoms can combine to form molecules through chemical bonds, leading to the diverse array of substances observed in nature, such as water (H₂O) or diamond (carbon atoms in a lattice).[5] Matter exists in multiple states or phases, primarily determined by temperature, pressure, and intermolecular forces: solids have fixed shapes and volumes due to strong particle bonds; liquids flow and take the shape of their container but maintain a fixed volume; gases expand to fill their container with neither fixed shape nor volume; and plasma, the most common state in the universe (e.g., in stars), consists of ionized particles with free electrons and nuclei.[6] These states can transition via processes like melting, boiling, or ionization, which are reversible physical changes without altering the substance's chemical composition.[7] Key physical properties of matter include mass (a measure of inertia), volume, density (mass per unit volume), color, odor, melting and boiling points, solubility, and conductivity, all of which can be observed or measured without changing the substance's identity.[8] In contrast, chemical properties describe how matter interacts with other substances, such as flammability, reactivity with acids, or oxidation potential, leading to irreversible chemical changes that form new substances.[7] These properties enable the classification of matter into elements (pure substances like oxygen), compounds (like salt, NaCl), and mixtures (homogeneous like air or heterogeneous like soil).[9] Conservation laws underpin matter's behavior: in classical chemistry, matter is neither created nor destroyed in chemical reactions (law of conservation of mass), though in nuclear reactions or via Einstein's E=mc², matter and energy are interconvertible.[10] Ordinary baryonic matter, made of protons and neutrons, constitutes about 5% of the universe's energy content, with the rest dominated by dark matter (∼27%) and dark energy (∼68%), though the nature of these remains an active area of research.[1]Definitions and Properties
Classical Definition
In classical physics, matter is defined as any substance that possesses mass and occupies a finite volume of space, making it detectable through sensory perception or physical interaction. This conceptualization emphasizes matter's tangible nature, distinguishing it from abstract or non-material entities. For instance, everyday objects such as a wooden chair (solid), a glass of water (liquid), or the air in a room (gas) exemplify matter in its common forms, each exhibiting measurable mass and spatial extent. Unlike energy, which classical physics treats as the capacity for work or motion without inherent mass or volume—such as the kinetic energy of a moving object or heat transfer—matter maintains its identity through interactions. Key properties include inertia, the resistance to changes in motion proportional to its mass, as articulated in Newton's first law of motion; impenetrability, whereby two portions of matter cannot coexist in the same spatial point simultaneously; and divisibility, allowing matter to be subdivided into smaller units down to atoms in early atomic models like those proposed by John Dalton. These attributes underpin the mechanical behavior of matter in classical frameworks.[11][12][13] Historically, the classical understanding traces back to Aristotle's hylomorphism, in which matter (hylē) represents pure potentiality—the underlying substratum capable of receiving form (morphē) to become actualized substances—without independent existence or qualities of its own. As described in Aristotle's Physics and Metaphysics, matter persists through change as the indeterminate principle that form shapes into specific entities, such as bronze as potential statue. This philosophical foundation influenced subsequent classical views, evolving toward empirical models in the Scientific Revolution. This intuitive, macroscopic perspective laid the groundwork for later refinements in quantum mechanics.[14]Particle Physics Definition
In particle physics, matter is understood through the lens of quantum field theory as being composed exclusively of fermions, which are elementary particles characterized by half-integer spin values such as \frac{1}{2} and that adhere to the Pauli exclusion principle, preventing two identical fermions from occupying the same quantum state simultaneously.[15] This principle, a cornerstone of quantum mechanics, ensures the stability and structure of matter by dictating how fermions interact and arrange in systems like atomic orbitals.[15] Fermions are categorized into two main families: quarks and leptons, both of which carry specific quantum numbers, including spin, charge, and, for quarks, color charge, that define their roles in the fundamental interactions.[16] Baryonic matter, which constitutes the ordinary matter observed in everyday phenomena, consists primarily of baryons—composite particles formed from three quarks bound together by the strong nuclear force.[17] Protons and neutrons, the key building blocks of atomic nuclei, exemplify these baryons: a proton comprises two up quarks and one down quark, while a neutron consists of one up quark and two down quarks, with their stability arising from the confinement of quarks within color-neutral combinations.[17] This three-quark structure distinguishes baryons from other hadrons and underpins the composition of all visible matter in the universe.[18] In the Standard Model of particle physics, the fundamental fermions are organized into three generations, but the first generation provides the essential constituents of stable baryonic matter.[16] The quarks in this generation are the up quark (with charge +\frac{2}{3}) and the down quark (with charge -\frac{1}{3}), while the leptons include the electron (charge -1) and the electron neutrino (neutral).[16] These particles, all fermions obeying the Pauli exclusion principle, form the protons, neutrons, and electrons that assemble into atoms.[18] In stark contrast, bosons—particles with integer spin, such as photons, gluons, and W/Z bosons—mediate the electromagnetic, strong, and weak forces but do not contribute to the material substance of matter itself.[15]Relativistic and Cosmological Perspectives
In the framework of special relativity, Albert Einstein established the mass-energy equivalence principle, expressed by the equation E = mc^2, where E is energy, m is rest mass, and c is the speed of light.[19] This relation demonstrates that matter possesses intrinsic energy equivalent to its mass, blurring the classical distinction between the two and allowing matter to convert into other forms of energy under certain conditions.[19] Extending to general relativity, matter's energy content, including its rest mass, contributes to the stress-energy tensor T_{\mu\nu}, which sources the curvature of spacetime via Einstein's field equations G_{\mu\nu} = \frac{8\pi G}{c^4} T_{\mu\nu}. The stress-energy tensor encapsulates the distribution of mass, energy, momentum, and stress within matter fields, dictating how they influence gravitational fields and geodesic motion. Consequently, concentrations of matter, such as stars or galaxies, curve spacetime, manifesting as the gravitational attraction observed in the universe. In cosmology, matter plays a central role in the universe's composition and evolution within the Lambda cold dark matter (ΛCDM) model. As of 2024, observations indicate that ordinary (baryonic) matter constitutes approximately 5% of the total energy density, while dark matter accounts for about 27%, yielding a total matter fraction of roughly 32%.[20] These proportions are derived from measurements of the cosmic microwave background and baryon acoustic oscillations, with the total matter density parameter \Omega_m \approx 0.315.[20] The remaining ~68% is attributed to dark energy, which drives the accelerated expansion.[20] The early universe transitioned through distinct eras dominated by radiation and matter following the Big Bang. During the radiation-dominated era, the energy density was governed by relativistic particles and photons, but matter domination began around 51,000 years after the Big Bang, when the matter density surpassed that of radiation at redshift z \approx 3400.[21] This shift marked a pivotal point in cosmic expansion, slowing the rate compared to the prior era and enabling the growth of large-scale structures through gravitational instability.[21]Composition and Structure
Atomic and Molecular Level
Matter at the atomic and molecular level consists of atoms, which serve as the fundamental building blocks of all ordinary matter. Each atom comprises a dense central nucleus containing protons—positively charged particles—and neutrons, which are electrically neutral and contribute to the atom's mass.[22] Surrounding the nucleus are electrons, negatively charged particles that occupy probabilistic orbitals, determining the atom's chemical behavior through their arrangement and interactions.[23] The number of protons defines the element, while the balance between protons and electrons maintains electrical neutrality in isolated atoms.[24] The periodic table organizes all known chemical elements based on increasing atomic number, which is the count of protons in the nucleus and uniquely identifies each element.[25] This classification reveals periodic trends in chemical properties, such as reactivity and valence, arising from the electron configurations in outer shells; for instance, elements in the same group exhibit similar bonding tendencies due to comparable numbers of valence electrons.[26] These patterns enable predictions of how elements combine to form compounds, underpinning chemistry's foundational principles.[25] Isotopes are variants of the same element with identical atomic numbers but differing numbers of neutrons, affecting atomic mass without altering chemical properties.[27] Stable isotopes, like carbon-12 with six protons and six neutrons, do not undergo radioactive decay and thus contribute to the long-term stability of matter in biological and geological systems.[28] In contrast, unstable isotopes such as carbon-14, with six protons and eight neutrons, decay over time, releasing radiation and playing roles in processes like radiometric dating, though they represent a minor fraction in natural matter.[27] At the molecular level, atoms combine through chemical bonds to form molecules, exhibiting emergent properties distinct from individual atoms. Covalent bonds involve the sharing of electron pairs between atoms, as in diatomic oxygen (O₂), fostering strong, directional connections in nonmetals.[29] Ionic bonds result from the electrostatic attraction between oppositely charged ions, typically formed by electron transfer from metals to nonmetals, yielding crystalline solids like sodium chloride (NaCl).[30] Metallic bonds feature delocalized electrons shared among metal atoms, enabling high electrical conductivity and malleability in substances like copper.[31] A key example is water (H₂O), where polar covalent bonds between oxygen and hydrogen atoms create a molecule with partial charges—oxygen slightly negative and hydrogens positive—due to oxygen's higher electronegativity, leading to unique properties like hydrogen bonding and solvent capabilities.[29]Subatomic Particles
Subatomic particles are the fundamental building blocks of atoms, consisting primarily of protons, neutrons, and electrons, which together determine the structure and properties of matter. These particles interact through fundamental forces to form stable atomic nuclei and electron clouds, enabling the formation of elements and compounds. Protons and neutrons reside in the nucleus, while electrons occupy orbitals around it, with their charges and masses dictating electromagnetic interactions and nuclear stability. The proton is a positively charged subatomic particle with a charge of +1e, where e is the elementary charge of approximately $1.602 \times 10^{-19} C, and a mass of approximately $1.67 \times 10^{-27} kg.[32] Protons define the atomic number of an element, which corresponds to the number of protons in the nucleus and thus determines the element's chemical identity and position in the periodic table.[22] Their positive charge repels other protons but is overcome by the strong nuclear force, allowing multiple protons to coexist in the nucleus. The neutron is an electrically neutral subatomic particle with a mass of approximately $1.67 \times 10^{-27} kg, slightly greater than that of the proton.[33] Neutrons contribute to the stability of the atomic nucleus by providing additional binding through the strong nuclear force, which counteracts the electromagnetic repulsion between protons without adding to the positive charge.[34] The number of neutrons can vary in isotopes of the same element, affecting nuclear stability and enabling phenomena like radioactive decay. The electron is a negatively charged subatomic particle with a charge of -1e and a mass of approximately $9.11 \times 10^{-31} kg, making it about 1/1836 the mass of a proton.[35] Electrons govern chemical bonding by occupying outer orbitals and participating in electromagnetic interactions, which dictate the reactivity of atoms and the conduction of electricity in materials.[22] Their arrangement in electron shells determines the valence and thus the chemical properties of elements. Within the nucleus, protons and neutrons—collectively known as nucleons—are bound together by the strong nuclear force, one of the four fundamental interactions, which acts at very short ranges (about 10^{-15} m) to overcome proton repulsion and maintain nuclear integrity.[34] The weak nuclear force, another fundamental interaction, plays a role in processes like beta decay, where a neutron transforms into a proton (or vice versa), emitting an electron or positron and altering the atomic number.[36] Protons and neutrons themselves are composite particles made up of more fundamental quarks, though their substructure is explored in greater detail elsewhere.[37]Fundamental Constituents
In the Standard Model of particle physics, all ordinary matter is composed of elementary fermions known as quarks and leptons.[16] These particles are the fundamental building blocks, with quarks participating in the strong nuclear force and leptons not.[16] There are twelve such fermions in total, organized into three generations or families, each containing two quarks and two leptons, with masses increasing across generations.[38] Quarks come in six flavors: up, down, charm, strange, top, and bottom.[38] The first generation includes the light up quark (mass approximately 2.2 MeV/c², electric charge +2/3) and down quark (mass approximately 4.7 MeV/c², charge -1/3), which are stable within composite particles and constitute the protons and neutrons of atomic nuclei.[39] The second generation features the charm quark (mass ~1.27 GeV/c², charge +2/3) and strange quark (mass ~94 MeV/c², charge -1/3), while the third includes the heavy top quark (mass ~173 GeV/c², charge +2/3) and bottom quark (mass ~4.18 GeV/c², charge -1/3); the latter two are short-lived, decaying rapidly due to their high masses.[39] Leptons also number six: the charged electron (mass 0.511 MeV/c², charge -1), muon (mass 105.7 MeV/c², charge -1), and tau (mass 1.777 GeV/c², charge -1), paired with their neutral counterparts—the electron neutrino, muon neutrino, and tau neutrino (with upper mass limits of <0.0008 MeV/c² for electron neutrino, <0.19 MeV/c² for muon neutrino, and <18 MeV/c² for tau neutrino, all at 90% CL).[40] Only the first-generation leptons (electron and electron neutrino) are stable and prevalent in ordinary matter, while the muon and tau decay into lighter particles on timescales of microseconds to femtoseconds.[38] Neutrinos interact only via the weak force and gravity, making them notoriously difficult to detect.[16] The three generations exhibit a pattern of increasing mass, with only the first generation appearing stably in everyday matter due to the instability of heavier particles.[16] Quarks, unlike leptons, carry "color charge" and are subject to color confinement: they cannot exist in isolation but are perpetually bound within color-neutral hadrons, such as baryons (e.g., protons, composed of three quarks) or mesons, through the exchange of gluons mediated by quantum chromodynamics (QCD).[41] This confinement arises from a linearly increasing potential between quarks, ensuring that attempts to separate them produce new quark-antiquark pairs instead.[41]| Generation | Quarks (Flavor, Approx. Mass in MeV/c², Charge) | Leptons (Type, Approx. Mass in MeV/c², Charge) |
|---|---|---|
| 1 | Up (2.2, +2/3); Down (4.7, -1/3) | Electron (0.511, -1); Electron Neutrino (<8×10^{-7}, 0) |
| 2 | Charm (1273, +2/3); Strange (94, -1/3) | Muon (105.7, -1); Muon Neutrino (<0.19, 0) |
| 3 | Top (172600, +2/3); Bottom (4183, -1/3) | Tau (1777, -1); Tau Neutrino (<18, 0) |