Matter and Energy

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The entire observable universe, of which the Earth is a very tiny part, contains matter in the form of stars, planets, and other objects scattered in space, such as particles of dust, molecules, protons, and electrons. In addition to containing matter, space also is filled with energy, part of it in the form of microwave radiation.


Matter itself has energy, called "rest energy." What distinguishes matter-energy from other forms of energy is that all matter has inertia and is subject to the force of gravity when at rest as well as when in motion. Inertia measures the resistance of an object to being accelerated by a force, and the inertia of an object at rest is proportional to its mass.

According to a law of physics first formulated by Isaac Newton and later modified by Einstein in his general theory or relativity, any object with mass can be accelerated by applying a force to it. Physicists use the term "acceleration" not only to describe the speeding up or slowing down of an object but also for changing its direction. A car going around a curve at constant speed is accelerating because its direction changes.

If you flick a small plastic ball on a table top with your finger, thereby exerting a small force on the ball, you will see it move rapidly from its resting position. But if you do the same with a steel ball of the same size, the same flick of the finger (the same force) will produce noticeably less motion. The steel ball has greater mass and therefore greater resistance to being accelerated. The ratio of the accelerations of two objects experiencing the same force is equal to the ratio of their masses.

The basic unit of mass is the kilogram, which is the mass of a standard platinum cylinder located in the city of Paris. A kilogram has a weight of 2.2 pounds. The basic unit of energy is the joule, which is equal to the kinetic energy that a one-kilogram object has when it is moving at a speed of 1.41 meters per second or the amount of potential energy the object has when lifted to a height of 0.102 meters.


Matter on earth commonly takes the form of solids, liquids, or gases, but may also be in the form of plasmas, which are "ionized" gases, that is, gases in which some of the atoms of the gas have lost one or more of their electrons. These electrons move within the plasma.

Gases, liquids, and solids have different physical properties. A gas fills its container, so that if a certain amount of gas is transferred from a small container into a large one, the gas will expand to fill the new container. If there is a hole in the top of a container filled with gas, the gas will escape. A liquid keeps the same volume when transferred from one container to another, but takes the shape of the new container. On Earth, a liquid has a flat, horizontal surface, If there is a hole in its container below that surface, the liquid will spill out. A solid keeps both its shape and its volume when transferred from one container to another.

Solids, liquids, and gases all change their volumes when the temperature is changed. All gases and nearly all solids and liquids tend to expand when their temperature is raised. When heat is applied to a solid, its temperature normally goes up, but at a certain temperature it can change its state (phase) to a liquid while the temperature remains constant. Similarly, as heat is applied to a liquid, its temperature normally rises until a certain temperature, when it changes its state to a gas. On still further heating, the gas will expand if the container has a movable piston to let it expand; otherwise, the pressure of the gas will increase. Eventually, if enough heat is applied, the gas can become partially ionized, that is, it can turn into a plasma.

The sun is a partially ionized gas or plasma. It has no container to hold it together. Instead, the enormous gravitational forces in the sun do the job. Even on Earth, the atmosphere (a gas) does not escape to outer space because of gravity. On the surface of the moon, gravity is only one-sixth as strong as on the surface of the earth. This is not strong enough to hold gases on the moon, so the moon has no atmosphere.

According to Albert Einstein's theory of relativity, no object with mass can travel as fast as the speed of light in empty space (in vacuum). So another definition of matter is anything that is subject to gravity and is either at rest or traveling slower than the speed of light in vacuum. The speed of light in vacuum, denoted by the special symbol c, is a constant speed (186,000 miles per second, or 300,000 kilometers per second) and is the speed in vacuum of any quantum (packet) of energy that has no mass. (At this speed, light can travel seven times around the world in a little under a second.) Light, and anything else that travels at the speed of light in a vacuum, is not considered to be matter. However, all things that travel at the speed of light, including light itself, possess kinetic energy.

The theory of quantum mechanics says light has some properties of a wave (for example, a wavelength), but its energy is concentrated in little packets called photons. These quanta, or particles of light, do not have mass and always travel at the speed of light in vacuum. Light travels slower in a material medium (such as glass) than in a vacuum because the photons get absorbed and reemitted by the atoms of the medium, thereby slowing down the progress of the light wave.

Forces, such as the gravitational force, are transmitted by means of quantities that scientists call "fields." All matter is influenced by forces carried by gravitational fields. Electrically charged particles are influenced by forces carried by electromagnetic fields. The gravitational and electromagnetic fields are not matter, but they have energy.

As far as is known, ordinary matter is made of tiny building blocks called elementary particles. For example, an atom is made up of a nucleus surrounded by one or more electrons. As far as scientists have been able to determine, the electrons are elementary particles, not made of anything simpler. However, an atomic nucleus is not elementary, but is a composite particle made up of simpler particles called protons and neutrons. (The lightest nucleus is the nucleus of ordinary hydrogen, which consists of only a single proton.) Today, physicists believe that even protons and neutrons are not elementary but are composite particles made up of still simpler building blocks called quarks.

At the present time, quarks are believed to be elementary particles. All the particles in an atom, whether elementary or not, are particles of matter and possess mass. Electrons, protons, and neutrons can also exist outside of atoms.

In addition to ordinary matter, scientists have evidence for the existence in the universe of "dark matter." Some of the dark matter is ordinary matter, such as dust in outer space and planets going around other stars. Astronomers cannot see ordinary dark matter because any light coming from such matter is too faint to be observed in telescopes. However, most of the dark matter in the universe is believed not to be ordinary matter. At the present time it is not known what this mysterious dark matter is, or what it is made of. Scientists know that this dark matter exists because it exerts a gravitational force on stars (which are made of ordinary matter), causing the stars to move faster than they otherwise would. According to present estimates, there is perhaps five times as much dark matter in the universe as ordinary matter.


Gravity is a force that acts not only on matter but on anything, such as light, that possesses energy. A gravitational force cannot speed up light or slow it down, but it can accelerate light by changing its direction. Light from stars directly behind the sun can be seen on earth during an eclipse of the sun because the sun's gravity bends some starlight around it. An observation of this effect was first made in 1919 during a solar eclipse, and the amount of bending observed was in agreement with the predictions of general relativity.

Gravity can also give energy to light or take energy away from light. If light from a laser is directed down from the top of a building to the ground, the light will gain a small amount of energy by "falling" in the gravitational field. Scientists can measure this slight energy gain as an increase in the frequency (decrease in the wavelength) of the light. Conversely, if the laser light shines upward, the light loses energy, and its frequency slightly decreases.

According to the theory of relativity, all matter has a kind of energy, called rest energy, denoted by the symbol E. If an object at rest has a mass denoted by m, its rest energy Eis given by Einstein's famous formula E = mc2. Because the speed of light cis such a large number, a small amount of matter contains a large amount of rest energy.

It has been noted that a one-kilogram object moving at 1.41 meters per second has a kinetic energy of 1 joule. The rest energy of the object is 90 million billion times as great. In fact, the rest energy of ordinary objects is so large that some people dream of unlocking that energy and converting it into more useful forms of energy, such as kinetic energy and heat. The laws of physics do allow matter to be converted into energy and energy into matter. However, at present, no way is known to convert the rest energy of matter entirely into energy except by "annihilation" in a collision with a form of matter known as antimatter.


Earth and the sun, and, as far as is known, the stars and planets in the rest of the visible universe, are made of ordinary matter. However, according to a theory first proposed by Paul Dirac in 1928, for every kind of particle of ordinary matter that exists in nature, there can exist an antiparticle made of antimatter. Some antiparticles have been discovered: for example, the antiparticle of the electron, called the positron, was discovered in 1932 in cosmic rays falling on earth and have also been created in experiments performed in the laboratory. Antimatter is very similar in some of its properties to ordinary matter, while other properties are quite different. For example, an electron is a particle of ordinary matter with negative electric charge. However, although the positron has a mass equal to that of an electron, it has positive electric charge. The mysterious dark matter of the universe is not the antimatter of ordinary matter.

According to general relativity, both matter and antimatter are attracted by gravitational forces. However, as yet no experiment has succeeded in showing that antimatter falls under Earth's gravity. The reason is that only small particles of antimatter, such as antiprotons, antineutrons, and positrons, have been created in the laboratory. The electric and magnetic forces acting on these particles are much stronger than the gravitational forces and mask the effects of gravity.

The most spectacular difference between a particle and an antiparticle is that, as the result of a collision, the particle and antiparticle can both annihilate into pure energy. For example, if an electron and a positron collide, they may destroy each other (annihilate) into radiation. This is an example in which the rest energy of matter and antimatter is converted entirely into another form of energy. Conversely, under some conditions the kinetic energy of rapidly moving particles can be converted into new particles of matter, usually together with particles of antimatter. Because antimatter is rare in the universe, nobody has to worry about our earth colliding with enough antimatter to annihilate the earth, although some particles on earth are annihilated by antiparticles from outer space.

It is because of the rarity of antimatter that we cannot use annihilation of matter as a source of kinetic energy, heat, light, and other forms of energy. Of course, scientists can create antimatter, but they have to supply the energy to create it. When the created antimatter annihilates, the scientists get back only the energy that they put in. It is actually much worse than that, because creation of antimatter is a very inefficient process, and most of the input energy is wasted. Furthermore, it is very difficult to store antimatter. It cannot be stored in any container made of matter, as it will annihilate with the walls of the container. Antimatter has to be contained by electromagnetic forces in a vacuum.


Although it is impractical to convert the rest energy of matter entirely into other forms of energy, nevertheless, a small fraction of rest energy is converted in chemical and nuclear reactions. For example, if hydrogen is burned in oxygen (a chemical reaction), the product is water plus heat and light. A scientist can describe this process by saying that burning converts chemical energy into heat and light. However, the process can be looked at in another way. If careful measurements are made, it is found that the mass of the water is slightly less than the sum of the masses of the original hydrogen and oxygen. So it can also be said that "the burning process" converts a small amount of rest energy of the hydrogen and oxygen into heat and light.

Normally, electrons in an atom are "bound" in the atom by the attractive electrical forces between the electrons and the atomic nucleus. A certain amount of energy must be applied to an atom to release an electron from the atom, thereby ionizing it. This amount of energy is called the "binding energy" of the electron in the atom. The binding energy of the electrons in hydrogen and oxygen is slightly different from the binding energy of the same electrons when the oxygen and hydrogen are combined in water. A change in binding energy causes a change in rest energy of the same amount. This difference in binding energy (or rest energy) is the source of the heat and light when hydrogen is burned.

Just as electrons are bound in atoms, so are protons and neutrons bound in atomic nuclei, but the binding energy of the protons and neutrons is far greater. Consequently, changes in the binding energy in nuclei are a much greater source of heat and light than changes in the binding energy of electrons. More than a million times as much matter can be converted into energy in a nuclear reaction as in a chemical reaction, and even in such a nuclear reaction only about 0.1 percent of the matter is converted into energy.

Most of the energy of the sun comes from changes in binding energy when hydrogen is converted into helium in nuclear reactions. When very light nuclei, such as hydrogen nuclei, are combined to produce nuclei having less total mass than the very light nuclei, energy is released. The process is called "nuclear fusion." The energy released in the sun in nuclear fusion is what causes the sun to shine.

When very heavy nuclei, such as those of uranium and plutonium, are split into lighter nuclei having less total mass than the very heavy nuclei, energy is released. The process is called "nuclear fission." In either nuclear fission or nuclear fusion, much of the converted rest energy emerges as kinetic energy, heat, and light.

The explosion of an atom bomb (a uranium or a plutonium bomb) and the operation of a nuclear reactor are cases of energy released in nuclear fission, the first in a very fast process and the second in a slower, controlled way. In a nuclear reactor, a small fraction of the rest energy of the uranium or plutonium is converted into heat. The heat is then used to turn water into steam, which drives a turbine attached to an electric generator in order to generate electricity (electrical energy). In a hydrogen bomb, most of the energy released comes from nuclear fusion. Scientists have tried for almost fifty years to build a fusion reactor. Although scientists have been able to generate a small amount of heat by controlled fusion, they have not succeeded in generating large amounts of heat from controlled fusion in a profitable way.

Don Lichtenberg

See also:Einstein, Albert; Hydrogen; Molecular Energy; Nuclear Energy; Nuclear Fission; Nuclear Fusion; Thermodynamics; Units of Energy.


Clark, J. O. W. (1994). Matter and Energy: Physics in Action. New York: Oxford University Press.

Einstein, A. (1961). Relativity, tr. R. Lawson. New York: Three Rivers Press.

Schrödinger, E. (1953). "What Is Matter?" Scientific American 189:52-57.