Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

What Makes Up The Universe?

Everything in the universe, from elementary particles and atoms to people, houses, and planets, can be classified into one of two categories: fermions or bosons. The behavior of a particle or group of particles, such as an atom or a house, determines whether it is a fermion or boson. The distinction between these two categories is not noticeable on the large scale of people or houses, but it has profound implications in the world of atoms and elementary particles. Fundamental particles are classified according to whether they are fermions or bosons. Fundamental fermions combine to form atoms and other more unusual particles, while fundamental bosons carry forces between particles and give particles mass.

In 1925 Austrian-born American physicist Wolfgang Pauli formulated a rule of physics that helped define fermions. He suggested that no two electrons can have the same properties and locations. He proposed this exclusion principle to explain why all of the electrons in atoms have slightly different amounts of energy. In 1926 Italian-born American physicist Enrico Fermi and British physicist Paul Dirac developed equations that describe electron behavior, providing mathematical proof of the exclusion principle. Physicists call particles that obey the exclusion principle fermions in honor of Fermi. Protons, neutrons, and the quarks that comprise them are all examples of fermions.

Some particles, such as particles of light called photons, do not obey the exclusion principle. Two or more photons can have the exact same characteristics. In 1925 German-born American physicist Albert Einstein and Indian mathematician Satyendra Bose developed a set of equations describing the behavior of particles that do not obey the exclusion principle. Particles that obey the equations of Bose and Einstein are called bosons, in honor of Bose.

Classifying particles as either fermions or bosons is similar to classifying whole numbers as either odd or even. No number is both odd and even, yet every whole number is either odd or even. Similarly, particles are either fermions or bosons. Sums of odd and even numbers are either odd or even, depending on how many odd numbers were added. Adding two odd numbers together yields an even number, but adding a third odd number makes the sum odd again. Adding any number of even numbers yields an even sum. In a similar manner, adding an even number of fermions yields a boson, while adding an odd number of fermions results in a fermion. Adding any number of bosons yields a boson.

For example, a hydrogen atom contains two fermions: an electron and a proton. But the atom itself is a boson because it contains an even number of fermions. According to the exclusion principle, the electron inside the hydrogen atom cannot have the same properties as another electron nearby. However, the hydrogen atom itself, as a boson, does not follow the exclusion principle. Thus, one hydrogen atom can be identical to another hydrogen atom.

A particle composed of three fermions, on the other hand, is a fermion. An atom of heavy hydrogen, also called a deuteron, is a hydrogen atom with a neutron added to the nucleus. A deuteron contains three fermions: one proton, one electron, and one neutron. Since the deuteron contains an odd number of fermions, it too is a fermion. Just like its constituent particles, the deuteron must obey the exclusion principle. It cannot have the same properties as another deuteron atom.

The differences between fermions and bosons have important implications. If electrons did not obey the exclusion principle, all electrons in an atom could have the same energy and be identical. If all of the electrons in an atom were identical, different elements would not have such different properties. For example, metals conduct electricity better than plastics do because the arrangement of the electrons in their atoms and molecules differs. If electrons were bosons, their arrangements could be identical in these atoms, and devices that rely on the conduction of electricity, such as televisions and computers, would not work. Photons, on the other hand, are bosons, so a group of photons can all have identical properties. This characteristic allows the photons to form a coherent beam of identical particles called a laser.

The most fundamental particles that make up matter fall into the fermion category. These fermions cannot be split into anything smaller. The particles that carry the forces acting on matter and antimatter are bosons called force carriers. Force carriers are also fundamental particles, so they cannot be split into anything smaller. These bosons carry the four basic forces in the universe: the electromagnetic, the gravitational, the strong (force that holds the nuclei of atoms together), and the weak (force that causes atoms to radioactively decay). Scientists believe another type of fundamental boson, called the Higgs boson, gives matter and antimatter mass. Scientists have yet to discover definitive proof of the existence of the Higgs boson.