Matter is generally considered to be a substance (often a particle) that has rest mass and (usually) also volume. The volume is determined by the three-dimensional space it occupies, while the mass is defined by the usual ways that mass is measured (see the article on mass). Matter is also a general term for the substance of which all observable physical objects consist.

Typically, matter includes atoms and other particles that have rest mass (not all particles have rest mass). However, not all of the particles with rest mass have a classical volume, and fundamental particles such as quarks and leptons (which are sometimes equated with matter) are considered in physics to be "point particles" without any effective size or volume. This challenges the first definition above. 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. The composite particles such as atoms, atomic nuclei, and nucleons, all have both rest mass and volume. By contrast, massless particles such as photons are not considered to be matter, and these have neither rest mass or volume.

Matter should not be confused with mass, as the two are not quite the same in modern physics. For example, mass is a conserved quantity which means that its value is unchanging through time, within closed systems. However, matter (unlike mass) is not conserved in such systems, although this is not obvious in ordinary conditions on Earth, where matter is approximately conserved. Still, special relativity shows that matter may disappear by conversion into energy, even from closed systems, and it can also be created from energy, within such systems. This transformation has been observed in practice. In high-energy accelerator experiments, for example, matter particles such as electrons, positrons, and even protons and neutrons can be produced from various types of non-material energy such as kinetic energy and potential energy. However, because mass (like energy) can neither be created nor destroyed, the quantity of mass and the quantity of energy remain the same during a transformation of matter (which represents a certain amount of energy) into non-material (i.e., non-matter) energy. This is also true in the reverse transformation of energy into matter.

Albert Einstein showed that ultimately all matter is capable of being converted to energy, by the formula:

where E is the energy of a piece of matter of mass m, times c2 the speed of light squared. As the speed of light is 299,792,458 metres per second (186,282 mi/s), a relatively small amount of matter may be converted to a large amount of energy. This equation therefore represents the equivalence of mass and energy, while at the same time it may be used to represent the transformation of matter into non-material energy. An example is positrons and electrons (matter) which may transform into photons (non-matter). However, although matter may be created or destroyed in such processes, neither the quantity of mass or energy change during the process.

Different fields of science use the term matter in different and sometimes incompatible ways. Some of these ways are based on loose historical meanings, from a time where there was no reason to distinquish mass and matter. Sometimes in the field of physics "matter" is simply equated with particles that exhibit rest mass (i.e., that cannot travel at the speed of light), such as quarks and leptons. However, there is no single universally-agreed scientific meaning of the word "matter." Scientifically, the term "mass" is well-defined, but the term "matter" is not. For this reason, none of the uses of the word "matter" in this article should be considered definitive.

For much of the history of the natural sciences people have contemplated the exact nature of matter. The idea that matter was built of discrete building blocks, the so-called particulate theory of matter, was first put forward by the Greek philosophers Leucippus (~490 BC) and Democritus (~470–380 BC). All objects we see with the naked eye are composed atoms and this atomic matter is in turn made up of interacting subatomic particles, usually a nucleus of protons and of neutrons, and a cloud of orbiting electrons.

Matter is commonly said to exist in four states (or phases): solid, liquid, gas and plasma. However, advances in experimental techniques have realized other phases, previously only theoretical constructs, such as Bose–Einstein condensates and fermionic condensates. A focus on an elementary-particle view of matter also leads to new phases of matter, such as the quark–gluon plasma.

In physics and chemistry, matter exhibits both wave-like and particle-like properties, the so-called wave–particle duality.

In the realm of cosmology, extensions of the term matter are invoked to include dark matter and dark energy, concepts introduced to explain some odd phenomena of the observable universe, such as the galactic rotation curve. These exotic forms of "matter" do not refer to matter as "building blocks", but rather to currently poorly understood forms of mass and energy.

Read more about Matter:  Structure, Phases, Antimatter, Other Types of Matter

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