Matter is the substance of which all objects are made. Matter has many forms, all of which have a property called inertia. Inertia is an object’s resistance to a change in its state of motion. An object that is at rest tends to remain at rest due to inertia. A moving object tends to maintain its speed and direction of movement.
Mass is a measure of inertia. An object with a relatively large mass has a higher resistance to a change in its state of motion than does an object with a relatively small mass. Thus, more force is needed to move a more massive object than to move a less massive object under the same conditions. Mass is often defined as the amount of the matter of which an object is made.
Mass is also the source of gravitation, a force of attraction between two objects. Weight is a term used in science and technology for the gravitational force between a planet or another large object and a relatively small object. At any given distance from the center of a planet, the weight of the small object is proportional to its mass: The greater the mass, the greater the weight. But as an object moves away from the planet, the gravitational force decreases. Thus, for example, a space probe launched from the earth loses weight as it flies away, though its mass remains the same.
In commercial and everyday use, the term weight is understood to mean mass. Thus, people weigh themselves to measure the amount of matter that makes up their bodies. This use of weight can cause confusion. In the United States, the scale would be marked in pounds. But the pound is used as a unit of force in science and technology. In other countries, the markings would indicate kilograms. The kilogram is the metric unit of mass.
Matter can be changed into energy and energy into matter. The mass of an object represents the energy required to create its matter. Whenever energy is taken from something, such as when an electric battery is discharged, its mass decreases. Similarly, if energy is put into a battery by charging it, its mass increases. For the amounts of energy used in everyday life, however, the changes in mass are too small to measure.
Structure of matter
Matter has structure at many levels, from groupings of galaxies so vast that light rays take hundreds of millions of years to cross them to particles so small that scientists describe them as pointlike. The solid objects that we use in everyday life consist of molecules and crystals. These structures, in turn, consist of atoms that are linked together. An atom is made up of particles called protons, neutrons, and electrons. Protons and neutrons, which carry most of the atom’s mass, are composed of pointlike units known as quarks. Each proton or neutron consists of three quarks. Massless particles called gluons hold the quarks together. Scientists have not yet determined whether quarks can be broken down into smaller bits. Electrons are also considered to be pointlike. Particles smaller than an atom are called subatomic particles.
The diameter of an atom ranges from about 0.1 to 0.5 nanometer. A nanometer is a billionth of a meter, or 1/25,400,000 inch. Protons and neutrons make up the atom’s nucleus, which is about ten thousand times smaller than the atom. Electrons whirl around the nucleus.
Differences in electric charge hold the atom together. Protons have a positive charge, and neutrons are electrically neutral, so the nucleus as a whole is positively charged. Electrons are negatively charged. Because opposite charges attract, an electric force tends to keep the electrons in place.
Electrons whirl around the nucleus in layers called electron shells. The electrons in the outermost shells are not tightly bound to the nucleus. As a result, some outer electrons can be shared by two atoms in a chemical bond, a linking of atoms. Molecules consist of atoms bonded in this way. Outer electrons can also jump from one atom to another, producing positive and negative ions (charged atoms). Ions can bond to form crystals. For example, solid sodium chloride—common table salt—is a crystal consisting of positive sodium ions and negative chloride ions.
States of matter
Since ancient times, matter has been known to exist in three states, also called phases: (1) solids, (2) liquids, and (3) gases. In the 1900’s, scientists identified four additional states: (1) plasmas, (2) superconductors, (3) superfluids, and (4) Bose-Einstein condensates. In 2000, scientists announced that they had created quark-gluon plasmas. Matter can change from one state to another. When this occurs, the appearance of the matter may change drastically. However, the chemical composition remains the same, except in plasmas and quark-gluon plasmas. These form at such high temperatures that any chemical bonds are destroyed.
Solids, liquids, and gases occur at familiar temperatures and pressures. For example, ice is solid water. At normal atmospheric pressure—the pressure at sea level—ice melts at a temperature of 32 °F (0 °C), forming liquid water. When heat raises the temperature of the water to 212 °F (100 °C), the water boils, producing steam, a gas. The other five states of matter are found at very high or very low temperatures.
Solids
tend to retain their shape, and they resist compression (reduction in the amount of space they occupy). Most solids consist of small crystals packed together.
Liquids
assume the shape of their container and can flow freely. Like solids, liquids resist compression. The atoms or molecules of a liquid are in contact with one another but are not linked, so they can move freely past one another.
Gases
expand to fill their containers, and they can be compressed fairly easily. The atoms or molecules of a gas are not in contact with one another and are always moving violently. In gases at familiar pressures and temperatures, each atom or molecule collides with others millions or billions of times per second.
Plasmas
are gaslike substances in which some or all atoms have lost at least one electron, leaving a mixture of free electrons and positively charged ions. Plasmas form at temperatures of tens of thousands of degrees or higher, or through the action of an electric current. The sun and most other stars consist mainly of plasma. On earth, plasmas are found in lightning discharges and in fluorescent lights and neon signs.
Superfluids
are liquids that flow without resistance. As a result, they will pass through the pores of a container. Scientists have found this phase only in helium, one of the few substances that remain a liquid within a few degrees of absolute zero: –459.67 °F or –273.15 °C. At absolute zero, atoms and molecules would have the least possible energy.
Superconductors
are solids in which electrons move freely. Once started, an electric current in a superconductor can flow forever without the help of an external power supply. Many metals are superconductors at temperatures within a few degrees of absolute zero. Other substances superconduct at temperatures more than 100 Celsius degrees (180 Fahrenheit degrees) above absolute zero.
Bose-Einstein condensates,
also known as BEC’s, are clusters of millions of atoms that merge under conditions of extreme cold. BEC’s can form in some gases when they are cooled to within a few billionths of a Celsius degree of absolute zero. When a BEC forms, millions of atoms stop moving in different directions at different speeds and instead act as a single atom. The condensates are named after physicists Satyendra Nath Bose of India and Albert Einstein of Germany, who proposed the possibility of BEC’s in the 1920’s. The U.S. National Aeronautics and Space Administration (NASA) designed its space-based Cold Atom Laboratory (CAL) to allow scientists to study BEC’s in an ultracold environment. NASA launched the CAL to the International Space Station in 2018.
Quark-gluon plasmas
consist of quarks moving freely in a “sea” of gluons. This state of matter dominated the universe during its first 10 one-millionths of a second. At that time, the temperature was more than 1 million million °C. Today, quark-gluon plasmas have only a fleeting existence in nature.
Scientists create quark-gluon plasmas by colliding heavy atomic nuclei in a machine called a particle accelerator. The machine accelerates the nuclei to almost the speed of light. Quark-gluon plasmas occur when collisions compress nuclei to a fraction of their normal volume.
Unusual forms of matter
Scientists have discovered an unusual form of matter called antimatter. Dark matter, which may be fundamentally different from ordinary matter, apparently also exists. Physicists do not know what it is made of, however.
Antimatter.
Physicists can convert energy into matter with particle accelerators. When subatomic particles collide at high speeds, they create new particles. Whenever particles of matter are created, an equal number of particles of antimatter are also made. Antimatter particles are equal in mass to the equivalent particles of matter but opposite in electric charge or certain other properties. For example, positrons, which are positively charged, are the antimatter equivalents of electrons. If a matter particle meets an equivalent antimatter particle, the two particles destroy each other. Both particles are converted into energy.
Antimatter particles are rare and last only until they encounter matter and are destroyed. There appear to be no large concentrations of antimatter anywhere in the universe. On earth, they are mainly a laboratory curiosity. Medical technology, however, makes use of positrons in a technique called positron emission tomography (PET). PET images reveal the chemical activity of areas of the brain and other body tissues.
Dark matter.
More than 99 percent of the visible universe is made up of the two lightest kinds of atoms, hydrogen and helium. It appears, however, that most of the matter in the universe is invisible dark matter. Scientists have detected dark matter only through the influence of its gravitational force on the motions of visible matter. Many scientists believe that dark matter is composed of undiscovered particles.
Matter and fields
Scientists believe that the smallest particles of matter—the pointlike particles—have no size at all. They believe these particles simply serve as “centers of force” that attract or repel one another through fields of force. For example, the positively charged quarks that make up a proton attract an electron by means of an electromagnetic field. Fields of force consist of energy particles, or “bundles of energy.” An electromagnetic field, for instance, is composed of energy particles known as photons. One charged matter particle attracts or repels another by emitting a photon, which the other absorbs.
Thus, what appears to be solid matter is actually made up of little bundles of field energy. The rapid movement of particles gives matter its solidity in the same way in which the rapid rotation of fan blades makes the blades and the spaces between them seem to be one solid object.
