Matter is usually classified into three classical
states, with plasma sometimes
added as a fourth state. From top to bottom:
quartz (solid), water (liquid),
nitrogen dioxide (gas), and a plasma globe
(plasma).
Matter is a poorly defined term in science
(see definitions below). The term
often refers to a substance (often a particle)
that has rest mass. Matter is
also used loosely as a general term for the
substance that makes up all
observable physical objects.
All objects we see with the naked eye are
composed of atoms. This atomic matter
is in turn made up of interacting subatomic
particles—usually a nucleus of
protons and neutrons, and a cloud of orbiting
electrons. Typically,
science considers these composite particles
matter because they have both rest
mass and volume. By contrast, massless particles,
such as photons, 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 quarks and leptons (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 commonly exists in four states (or
phases): solid, liquid and gas, and
plasma. However, advances in experimental
techniques have revealed other
previously theoretical phases, 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. 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).
Albert Einstein showed that ultimately all
matter is capable of being
converted to energy (known as mass-energy
equivalence) by the famous formula E =
mc2, 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. An example is that positrons and electrons
(matter) 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.
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 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
inside closed systems, and it can also be
created from energy, within such
systems. 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.
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 when there was no reason to distinguish
mass and matter. As such,
there is no single universally-agreed scientific
meaning of the word "matter."
Scientifically, the term "mass" is well-defined,
but "matter" is not. 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, in both physics and chemistry,
matter exhibits both wave-like
and particle-like properties, the so-called
wave–particle duality.
Definition
Common definition
The DNA molecule is an example of matter under
the "atoms and molecules"
definition.
The common definition of matter is anything
that has both mass and volume (occupies
space). For example, a car would be said to
be made of matter, as it
occupies space, and has mass.
The observation that matter occupies space
goes back to antiquity. However, an
explanation for why matter occupies space
is recent, and is argued to be a
result of the Pauli exclusion principle. Two
particular examples where
the exclusion principle clearly relates matter
to the occupation of space are
white dwarf stars and neutron stars, discussed
further below.
Relativity
In the context of relativity, mass is not
an additive quantity, in the sense
that one can add the rest masses of particles
in a system to get the total rest
mass of the system. Thus, in relativity usually
a more general view is that
it is not the sum of rest masses, but the
energy–momentum tensor that quantifies
the amount of matter. This tensor gives the
rest mass for the entire system. "Matter"
therefore is sometimes considered as anything
that contributes to the energy–momentum
of a system, that is, anything that is not
purely gravity. This view is
commonly held in fields that deal with general
relativity such as cosmology. But
in this view, light and other types of insubstantial
energy may be part of
matter.
The reason for this is that in this definition,
electromagnetic radiation (such
as light) as well as the energy of electromagnetic
fields contributes to the
mass of systems, and therefore appears to
add matter to them. For example, light
radiation (or thermal radiation) trapped inside
a box would contribute to the
mass of the box, as would any kind of energy
inside the box, including the
kinetic energy of particles held by the box.
Nevertheless, isolated individual
particles of light (photons) and the isolated
kinetic energy of massive
particles, are normally not considered to
be matter.
A difference between matter and mass therefore
may seem to arise when single
particles are examined. In such cases, the
mass of single photons is zero. For
particles with rest mass, such as leptons
and quarks, isolation of the particle
in a frame where it is not moving, removes
its kinetic energy.
A source of definition difficulty in relativity
arises from two definitions of
mass in common use, one of which is formally
equivalent to total energy (and is
thus observer-dependent), and the other of
which is referred to as rest mass or
invariant mass and is independent of the observer.
Only the latter type of mass
is loosely equated with matter (since it can
be weighed). However, energies
which contribute to the first type of mass
may be weighed also in special
circumstances, such as when trapped in a system
with no net momentum (as in the
box example above). Thus, a photon with no
mass may add mass to a system in
which it is trapped. Since such mass is measured
as part of ordinary matter in
complex systems, the "matter" status of "massless
particles" becomes unclear in
such systems. These problems contribute to
the lack of a rigorous definition of
matter in science, although mass is easier
to define as the total stress-energy
above (this is also what is weighed on a scale,
and what is the source of
gravity).
Atoms definition
A definition of "matter" based on its physical
and chemical structure is: matter
is made up of atoms. As an example, deoxyribonucleic
acid molecules (DNA)
are matter under this definition because they
are made of atoms. This definition
can extend to include charged atoms and molecules,
so as to include plasmas (gases
of ions) and electrolytes (ionic solutions),
which are not obviously included in
the atoms definition. Alternatively, one can
adopt the protons, neutrons, and
electrons definition.
Protons, neutrons and electrons definition
A definition of "matter" more fine-scale than
the atoms and molecules definition
is: matter is made up of what atoms and molecules
are made of, meaning anything
made of positively charged protons, neutral
neutrons, and negatively charged
electrons. This definition goes beyond atoms
and molecules, however, to
include substances made from these building
blocks that are not simply atoms or
molecules, for example white dwarf matter—typically,
carbon and oxygen nuclei in
a sea of degenerate electrons. At a microscopic
level, the constituent "particles"
of matter such as protons, neutrons, and electrons
obey the laws of quantum
mechanics and exhibit wave–particle duality.
At an even deeper level, protons
and neutrons are made up of quarks and the
force fields (gluons) that bind them
together (see Quarks and leptons definition
below).
Quarks and leptons definition
Under the "quarks and leptons" definition,
the elementary and composite
particles made of the quarks (in purple) and
leptons (in green) would be matter—while
the gauge bosons (in red) would not be matter.
However, interaction energy
inherent to composite particles (for example,
gluons involved in neutrons and
protons) contribute to the mass of ordinary
matter.
As seen in the above discussion, many early
definitions of what can be called
ordinary matter were based upon its structure
or building blocks. On the scale
of elementary particles, a definition that
follows this tradition can be stated
as: ordinary matter is everything that is
composed of elementary fermions,
namely quarks and leptons. The connection
between these formulations
follows.
Leptons (the most famous being the electron),
and quarks (of which baryons, such
as protons and neutrons, are made) combine
to form atoms, which in turn form
molecules. Because atoms and molecules are
said to be matter, it is natural to
phrase the definition as: ordinary matter
is anything that is made of the same
things that atoms and molecules are made of.
(However, notice that one also can
make from these building blocks matter that
is not atoms or molecules.) Then,
because electrons are leptons, and protons,
and neutrons are made of quarks,
this definition in turn leads to the definition
of matter as being quarks and
leptons, which are the two types of elementary
fermions. Carithers and Grannis
state: Ordinary matter is composed entirely
of first-generation particles,
namely the [up] and [down] quarks, plus the
electron and its neutrino. (Higher
generations particles quickly decay into first-generation
particles, and thus
are not commonly encountered. )
This definition of ordinary matter is more
subtle than it first appears. All the
particles that make up ordinary matter (leptons
and quarks) are elementary
fermions, while all the force carriers are
elementary bosons. The W and Z
bosons that mediate the weak force are not
made of quarks or leptons, and so are
not ordinary matter, even if they have mass.
In other words, mass is not
something that is exclusive to ordinary matter.
The quark–lepton definition of ordinary
matter, however, identifies not only the
elementary building blocks of matter, but
also includes composites made from the
constituents (atoms and molecules, for example).
Such composites contain an
interaction energy that holds the constituents
together, and may constitute the
bulk of the mass of the composite. As an example,
to a great extent, the mass of
an atom is simply the sum of the masses of
its constituent protons, neutrons and
electrons. However, digging deeper, the protons
and neutrons are made up of
quarks bound together by gluon fields (see
dynamics of quantum chromodynamics)
and these gluons fields contribute significantly
to the mass of hadrons. In
other words, most of what composes the "mass"
of ordinary matter is due to the
binding energy of quarks within protons and
neutrons. For example, the sum
of the mass of the three quarks in a nucleon
is approximately 12.5 MeV/c2, which
is low compared to the mass of a nucleon (approximately
938 MeV/c2). The
bottom line is that most of the mass of everyday
objects comes from the
interaction energy of its elementary components.
Smaller building blocks issue
The Standard Model groups matter particles
into three generations, where each
generation consists of two quarks and two
leptons. The first generation is the
up and down quarks, the electron and the electron
neutrino; the second includes
the charm and strange quarks, the muon and
the muon neutrino; the third
generation consists of the top and bottom
quarks and the tau and tau neutrino.
The most natural explanation for this would
be that quarks and leptons of higher
generations are excited states of the first
generations. If this turns out to be
the case, it would imply that quarks and leptons
are composite particles, rather
than elementary particles.
Structure
In particle physics, fermions are particles
that obey Fermi–Dirac statistics.
Fermions can be elementary, like the electron—or
composite, like the proton and
neutron. In the Standard Model, there are
two types of elementary fermions:
quarks and leptons, which are discussed next.
Quarks
Quarks are particles of spin-1⁄2, implying
that they are fermions. They carry an
electric charge of −1⁄3 e (down-type quarks)
or +2⁄3 e (up-type quarks). For
comparison, an electron has a charge of −1
e. They also carry colour charge,
which is the equivalent of the electric charge
for the strong interaction.
Quarks also undergo radioactive decay, meaning
that they are subject to the weak
interaction. Quarks are massive particles,
and therefore are also subject to
gravity.
Quark properties name symbol spin electric
charge
(e) mass
(MeV/c2) mass comparable to antiparticle antiparticle
symbol
up-type quarks
up u 1⁄2 +2⁄3 1.5 to 3.3 ~ 5 electrons
antiup u
charm c 1⁄2 +2⁄3 1160 to 1340 ~ 1 proton
anticharm c
top t 1⁄2 +2⁄3 169,100 to 173,300 ~ 180
protons or
~ 1 tungsten atom antitop t
down-type quarks
down d 1⁄2 −1⁄3 3.5 to 6.0 ~ 10 electrons
antidown d
strange s 1⁄2 −1⁄3 70 to 130 ~ 200 electrons
antistrange s
bottom b 1⁄2 −1⁄3 4130 to 4370 ~ 5 protons
antibottom b
Quark structure of a proton: 2 up quarks and
1 down quark.
Baryonic matter
Baryons are strongly interacting fermions,
and so are subject to Fermi-Dirac
statistics. Amongst the baryons are the protons
and neutrons, which occur in
atomic nuclei, but many other unstable baryons
exist as well. The term baryon
usually refers to triquarks—particles made
of three quarks. "Exotic" baryons
made of four quarks and one antiquark are
known as the pentaquarks, but their
existence is not generally accepted.
Baryonic matter is the part of the universe
that is made of baryons (including
all atoms). This part of the universe does
not include dark energy, dark matter,
black holes or various forms of degenerate
matter, such as compose white dwarf
stars and neutron stars. Microwave light seen
by Wilkinson Microwave Anisotropy
Probe (WMAP), suggests that only about 4.6%
of that part of the universe within
range of the best telescopes (that is, matter
that may be visible because light
could reach us from it), is made of baryonic
matter. About 23% is dark matter,
and about 72% is dark energy.
A comparison between the white dwarf IK Pegasi
B (center), its A-class companion
IK Pegasi A (left) and the Sun (right). This
white dwarf has a surface
temperature of 35,500 K.
Degenerate matter
In physics, degenerate matter refers to the
ground state of a gas of fermions at
a temperature near absolute zero. The Pauli
exclusion principle requires
that only two fermions can occupy a quantum
state, one spin-up and the other
spin-down. Hence, at zero temperature, the
fermions fill up sufficient levels to
accommodate all the available fermions—and
in the case of many fermions, the
maximum kinetic energy (called the Fermi energy)
and the pressure of the gas
becomes very large, and depends on the number
of fermions rather than the
temperature, unlike normal states of matter.
Degenerate matter is thought to occur during
the evolution of heavy stars.
The demonstration by Subrahmanyan Chandrasekhar
that white dwarf stars have a
maximum allowed mass because of the exclusion
principle caused a revolution in
the theory of star evolution.
Degenerate matter includes the part of the
universe that is made up of neutron
stars and white dwarfs.
Strange matter
Strange matter is a particular form of quark
matter, usually thought of as a
liquid of up, down, and strange quarks. It
is contrasted with nuclear matter,
which is a liquid of neutrons and protons
(which themselves are built out of up
and down quarks), and with non-strange quark
matter, which is a quark liquid
that contains only up and down quarks. At
high enough density, strange matter is
expected to be color superconducting. Strange
matter is hypothesized to occur in
the core of neutron stars, or, more speculatively,
as isolated droplets that may
vary in size from femtometers (strangelets)
to kilometers (quark stars).
Two meanings of the term "strange matter"
In particle physics and astrophysics, the
term is used in two ways, one broader
and the other more specific.
The broader meaning is just quark matter that
contains three flavors of quarks:
up, down, and strange. In this definition,
there is a critical pressure and an
associated critical density, and when nuclear
matter (made of protons and
neutrons) is compressed beyond this density,
the protons and neutrons dissociate
into quarks, yielding quark matter (probably
strange matter).
The narrower meaning is quark matter that
is more stable than nuclear matter.
The idea that this could happen is the "strange
matter hypothesis" of Bodmer
and Witten. In this definition, the critical
pressure is zero: the true
ground state of matter is always quark matter.
The nuclei that we see in the
matter around us, which are droplets of nuclear
matter, are actually metastable,
and given enough time (or the right external
stimulus) would decay into droplets
of strange matter, i.e. strangelets.
Leptons
Leptons are particles of spin-1⁄2, meaning
that they are fermions. They carry an
electric charge of −1 e (charged leptons)
or 0 e (neutrinos). Unlike quarks,
leptons do not carry colour charge, meaning
that they do not experience the
strong interaction. Leptons also undergo radioactive
decay, meaning that they
are subject to the weak interaction. Leptons
are massive particles, therefore
are subject to gravity.
Lepton properties name symbol spin electric
charge
(e) mass
(MeV/c2) mass comparable to antiparticle antiparticle
symbol
charged leptons
electron e− 1⁄2 −1 0.5110 1 electron
antielectron e+
muon μ− 1⁄2 −1 105.7 ~ 200 electrons
antimuon μ+
tau τ− 1⁄2 −1 1,777 ~ 2 protons antitau
τ+
neutrinos
electron neutrino ν
e 1⁄2 0 < 0.000460 < 1⁄1000 electron electron
antineutrino ν
e
muon neutrino ν
μ 1⁄2 0 < 0.19 < 1⁄2 electron muon antineutrino
ν
μ
tau neutrino ν
τ 1⁄2 0 < 18.2 < 40 electrons tau antineutrino
ν
τ
Phases
Phase diagram for a typical substance at a
fixed volume. Vertical axis is Pressure,
horizontal axis is Temperature. The green
line marks the freezing point (above
the green line is solid, below it is liquid)
and the blue line the boiling point
(above it is liquid and below it is gas).
So, for example, at higher T, a higher
P is necessary to maintain the substance in
liquid phase. At the triple point
the three phases; liquid, gas and solid; can
coexist. Above the critical point
there is no detectable difference between
the phases. The dotted line shows the
anomalous behavior of water: ice melts at
constant temperature with increasing
pressure.
In bulk[disambiguation needed], matter can
exist in several different forms, or
states of aggregation, known as phases, depending
on ambient pressure,
temperature and volume. A phase is a form
of matter that has a relatively
uniform chemical composition and physical
properties (such as density, specific
heat, refractive index, and so forth). These
phases include the three familiar
ones (solids, liquids, and gases), as well
as more exotic states of matter (such
as plasmas, superfluids, supersolids, Bose–Einstein
condensates, ...). A fluid
may be a liquid, gas or plasma. There are
also paramagnetic and ferromagnetic
phases of magnetic materials. As conditions
change, matter may change from one
phase into another. These phenomena are called
phase transitions, and are
studied in the field of thermodynamics. In
nanomaterials, the vastly increased
ratio of surface area to volume results in
matter that can exhibit properties
entirely different from those of bulk material,
and not well described by any
bulk phase (see nanomaterials for more details).
Phases are sometimes called states of matter,
but this term can lead to
confusion with thermodynamic states. For example,
two gases maintained at
different pressures are in different thermodynamic
states (different pressures),
but in the same phase (both are gases).
Antimatter
Antimatter
List of unsolved problems in physics Baryon
asymmetry. Why is there far more
matter than antimatter in the observable universe?
In particle physics and quantum chemistry,
antimatter is matter that is composed
of the antiparticles of those that constitute
ordinary matter. If a particle and
its antiparticle come into contact with each
other, the two annihilate; that is,
they may both be converted into other particles
with equal energy in accordance
with Einstein's equation E = mc2. These new
particles may be high-energy photons
(gamma rays) or other particle–antiparticle
pairs. The resulting particles are
endowed with an amount of kinetic energy equal
to the difference between the
rest mass of the products of the annihilation
and the rest mass of the original
particle-antiparticle pair, which is often
quite large.
Antimatter is not found naturally on Earth,
except very briefly and in
vanishingly small quantities (as the result
of radioactive decay, lightning or
cosmic rays). This is because antimatter that
came to exist on Earth outside the
confines of a suitable physics laboratory
would almost instantly meet the
ordinary matter that Earth is made of, and
be annihilated. Antiparticles and
some stable antimatter (such as antihydrogen)
can be made in tiny amounts, but
not in enough quantity to do more than test
a few of its theoretical properties.
There is considerable speculation both in
science and science fiction as to why
the observable universe is apparently almost
entirely matter, and whether other
places are almost entirely antimatter instead.
In the early universe, it is
thought that matter and antimatter were equally
represented, and the
disappearance of antimatter requires an asymmetry
in physical laws called the
charge parity (or CP symmetry) violation.
CP symmetry violation can be obtained
from the Standard Model, but at this time
the apparent asymmetry of matter
and antimatter in the visible universe is
one of the great unsolved problems in
physics. Possible processes by which it came
about are explored in more detail
under baryogenesis.
Other types of matter
Pie chart showing the fractions of energy
in the universe contributed by
different sources. Ordinary matter is divided
into luminous matter (the stars
and luminous gases and 0.005% radiation) and
nonluminous matter (intergalactic
gas and about 0.1% neutrinos and 0.04% supermassive
black holes). Ordinary
matter is uncommon. Modeled after Ostriker
and Steinhardt. For more
information, see NASA.
Ordinary matter, in the quarks and leptons
definition, constitutes about 4% of
the energy of the observable universe. The
remaining energy is theorized to be
due to exotic forms, of which 23% is dark
matter and 73% is dark energy.
Galaxy rotation curve for the Milky Way. Vertical
axis is speed of rotation
about the galactic center. Horizontal axis
is distance from the galactic center.
The sun is marked with a yellow ball. The
observed curve of speed of rotation is
blue. The predicted curve based upon stellar
mass and gas in the Milky Way is
red. The difference is due to dark matter
or perhaps a modification of the law
of gravity. Scatter in observations is indicated
roughly by gray
bars.
Dark matter
In astrophysics and cosmology, dark matter
is matter of unknown composition that
does not emit or reflect enough electromagnetic
radiation to be observed
directly, but whose presence can be inferred
from gravitational effects on
visible matter. Observational evidence of
the early universe and the big
bang theory require that this matter have
energy and mass, but is not composed
of either elementary fermions (as above) OR
gauge bosons. The commonly accepted
view is that most of the dark-matter is non-baryonic
in nature. As such, it
is composed of particles as yet unobserved
in the laboratory. Perhaps they are
supersymmetric particles, which are not Standard
Model particles, but relics
formed at very high energies in the early
phase of the universe and still
floating about.
Dark energy
In cosmology, dark energy is the name given
to the antigravitating influence
that is accelerating the rate of expansion
of the universe. It is known not to
be composed of known particles like protons,
neutrons or electrons, nor of the
particles of dark matter, because these all
gravitate.
Fully 70% of the matter density in the universe
appears to be in the form of
dark energy. Twenty-six percent is dark matter.
Only 4% is ordinary matter. So
less than 1 part in 20 is made out of matter
we have observed experimentally or
described in the standard model of particle
physics. Of the other 96%, apart
from the properties just mentioned, we know
absolutely nothing.
— Lee Smolin: The Trouble with Physics,
p. 16
Exotic matter
Exotic matter is a hypothetical concept of
particle physics. It covers any
material that violates one or more classical
conditions or is not made of known
baryonic particles. Such materials would possess
qualities like negative mass or
being repelled rather than attracted by gravity.
Historical development
Origins
The pre-Socratics were among the first recorded
speculators about the underlying
nature of the visible world. Thales (c. 624
BC–c. 546 BC) regarded water as the
fundamental material of the world. Anaximander
(c. 610 BC–c. 546 BC) posited
that the basic material was wholly characterless
or limitless: the Infinite (apeiron).
Anaximenes (flourished 585 BC, d. 528 BC)
posited that the basic stuff was
pneuma or air. Heraclitus (c. 535–c. 475
BC) seems to say the basic element is
fire, though perhaps he means that all is
change. Empedocles (c. 490–430 BC)
spoke of four elements of which everything
was made: earth, water, air, and fire.
Meanwhile, Parmenides argued that change does
not exist, and Democritus argued
that everything is composed of minuscule,
inert bodies of all shapes called
atoms, a philosophy called atomism. All of
these notions had deep philosophical
problems.
Aristotle (384 BC – 322 BC) was the first
to put the conception on a sound
philosophical basis, which he did in his natural
philosophy, especially in
Physics book I. He adopted as reasonable suppositions
the four Empedoclean
elements, but added a fifth, aether. Nevertheless
these elements are not basic
in Aristotle's mind. Rather they, like everything
else in the visible world, are
composed of the basic principles matter and
form.
The word Aristotle uses for matter, ὑλη
(hyle or hule), can be literally
translated as wood or timber, that is, "raw
material" for building. Indeed,
Aristotle's conception of matter is intrinsically
linked to something being made
or composed. In other words, in contrast to
the early modern conception of
matter as simply occupying space, matter for
Aristotle is definitionally linked
to process or change: matter is what underlies
a change of substance.
For example, a horse eats grass: the horse
changes the grass into itself; the
grass as such does not persist in the horse,
but some aspect of it—its matter—does.
The matter is not specifically described (e.g.,
as atoms), but consists of
whatever persists in the change of substance
from grass to horse. Matter in this
understanding does not exist independently
(i.e., as a substance), but exists
interdependently (i.e., as a "principle")
with form and only insofar as it
underlies change. It can be helpful to conceive
of the relationship of matter
and form as very similar to that between parts
and whole. For Aristotle, matter
as such can only receive actuality from form;
it has no activity or actuality in
itself, similar to the way that parts as such
only have their existence in a
whole (otherwise they would be independent
wholes).
Early modernity
René Descartes (1596–1650) originated the
modern conception of matter. He was
primarily a geometer. Instead of, like Aristotle,
deducing the existence of
matter from the physical reality of change,
Descartes arbitrarily postulated
matter to be an abstract, mathematical substance
that occupies space:
So, extension in length, breadth, and depth,
constitutes the nature of bodily
substance; and thought constitutes the nature
of thinking substance. And
everything else attributable to body presupposes
extension, and is only a mode
of extended
— René Descartes, Principles of Philosophy
For Descartes, matter has only the property
of extension, so its only activity
aside from locomotion is to exclude other
bodies: this is the mechanical
philosophy. Descartes makes an absolute distinction
between mind, which he
defines as unextended, thinking substance,
and matter, which he defines as
unthinking, extended substance. They are independent
things. In contrast,
Aristotle defines matter and the formal/forming
principle as complementary
principles that together compose one independent
thing (substance). In short,
Aristotle defines matter (roughly speaking)
as what things are actually made of
(with a potential independent existence),
but Descartes elevates matter to an
actual independent thing in itself.
The continuity and difference between Descartes'
and Aristotle's conceptions is
noteworthy. In both conceptions, matter is
passive or inert. In the respective
conceptions matter has different relationships
to intelligence. For Aristotle,
matter and intelligence (form) exist together
in an interdependent relationship,
whereas for Descartes, matter and intelligence
(mind) are definitionally opposed,
independent substances.
Descartes' justification for restricting the
inherent qualities of matter to
extension is its permanence, but his real
criterion is not permanence (which
equally applied to color and resistance),
but his desire to use geometry to
explain all material properties. Like Descartes,
Hobbes, Boyle, and Locke
argued that the inherent properties of bodies
were limited to extension, and
that so-called secondary qualities, like color,
were only products of human
perception.
Isaac Newton (1643–1727) inherited Descartes'
mechanical conception of matter.
In the third of his "Rules of Reasoning in
Philosophy," Newton lists the
universal qualities of matter as "extension,
hardness, impenetrability, mobility,
and inertia." Similarly in Optics he conjectures
that God created matter as
"solid, massy, hard, impenetrable, movable
particles," which were "...even so
very hard as never to wear or break in pieces."
The "primary" properties of
matter were amenable to mathematical description,
unlike "secondary" qualities
such as color or taste. Like Descartes, Newton
rejected the essential nature of
secondary qualities.
Newton developed Descartes' notion of matter
by restoring to matter intrinsic
properties in addition to extension (at least
on a limited basis), such as mass.
Newton's use of gravitational force, which
worked "at a distance," effectively
repudiated Descartes' mechanics, in which
interactions happened exclusively by
contact.
Though Newton's gravity would seem to be a
power of bodies, Newton himself did
not admit it to be an essential property of
matter. Carrying the logic forward
more consistently, Joseph Priestley argued
that corporeal properties transcend
contact mechanics: chemical properties require
the capacity for attraction.
He argued matter has other inherent powers
besides the so-called primary
qualities of Descartes, et al.
Since Priestley's time, there has been a massive
expansion in knowledge of the
constituents of the material world (viz.,
molecules, atoms, subatomic particles),
but there has been no further development
in the definition of matter. Rather
the question has been set aside. Noam Chomsky
summarizes the situation that has
prevailed since that time:
What is the concept of body that finally emerged?[...]
The answer is that there
is no clear and definite conception of body.[...]
Rather, the material world is
whatever we discover it to be, with whatever
properties it must be assumed to
have for the purposes of explanatory theory.
Any intelligible theory that offers
genuine explanations and that can be assimilated
to the core notions of physics
becomes part of the theory of the material
world, part of our account of body.
If we have such a theory in some domain, we
seek to assimilate it to the core
notions of physics, perhaps modifying these
notions as we carry out this
enterprise.
— Noam Chomsky, 'Language and problems of
knowledge: the Managua lectures, p.
144
So matter is whatever physics studies and
the object of study of physics is
matter: there is no independent general definition
of matter, apart from its
fitting into the methodology of measurement
and controlled experimentation. In
sum, the boundaries between what constitutes
matter and everything else remains
as vague as the demarcation problem of delimiting
science from everything else.
Late nineteenth and early twentieth centuries
In the 19th century, following the development
of the periodic table, and of
atomic theory, atoms were seen as being the
fundamental constituents of matter;
atoms formed molecules and compounds.
The common definition in terms of occupying
space and having mass is in contrast
with most physical and chemical definitions
of matter, which rely instead upon
its structure and upon attributes not necessarily
related to volume and mass. At
the turn of the nineteenth century, the knowledge
of matter began a rapid
evolution.
Aspects of the Newtonian view still held sway.
James Clerk Maxwell discussed
matter in his work Matter and Motion. He carefully
separates "matter" from
space and time, and defines it in terms of
the object referred to in Newton's
first law of motion.
However, the Newtonian picture was not the
whole story. In the 19th century, the
term "matter" was actively discussed by a
host of scientists and philosophers,
and a brief outline can be found in Levere.
[further explanation needed] A
textbook discussion from 1870 suggests matter
is what is made up of atoms:
Three divisions of matter are recognized in
science: masses, molecules and atoms.
A Mass of matter is any portion of matter
appreciable by the senses.
A Molecule is the smallest particle of matter
into which a body can be divided
without losing its identity.
An Atom is a still smaller particle produced
by division of a molecule.
Rather than simply having the attributes of
mass and occupying space, matter was
held to have chemical and electrical properties.
The famous physicist J. J.
Thomson wrote about the "constitution of matter"
and was concerned with the
possible connection between matter and electrical
charge.
Later developments
There is an entire literature concerning the
"structure of matter", ranging from
the "electrical structure" in the early 20th
century, to the more recent "quark
structure of matter", introduced today with
the remark: Understanding the quark
structure of matter has been one of the most
important advances in contemporary
physics. [further explanation needed] In this
connection, physicists speak of
matter fields, and speak of particles as "quantum
excitations of a mode of the
matter field". And here is a quote from de
Sabbata and Gasperini: "With
the word "matter" we denote, in this context,
the sources of the interactions,
that is spinor fields (like quarks and leptons),
which are believed to be the
fundamental components of matter, or scalar
fields, like the Higgs particles,
which are used to introduced mass in a gauge
theory (and that, however, could be
composed of more fundamental fermion fields)."
[further explanation needed]
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.
In the late 19th century with the discovery
of the electron, and in the early 20th
century, with the discovery of the atomic
nucleus, and the birth of particle
physics, matter was seen as made up of electrons,
protons and neutrons
interacting to form atoms. Today, we know
that even protons and neutrons are not
indivisible, they can be divided into quarks,
while electrons are part of a
particle family called leptons. Both quarks
and leptons are elementary particles,
and are currently seen as being the fundamental
constituents of matter.
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.
Summary
The term "matter" is used throughout physics
in a bewildering variety of
contexts: for example, one refers to "condensed
matter physics", "elementary
matter",[87] "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.
