We can distinguish three different realms of matter, three levels on the quantum ladder. The first is the atomic realm, which includes the world of atoms, their interactions, and the structures that are formed by them, such as molecules, liquids and solids, and gases and plasmas. This realm includes all the phenomena of atomic physics, chemistry, and, in a certain sense, biology. The energy exchanges taking place in this realm are of a relatively low order. If these exchanges are below one electron volt, such as in the collisions between molecules of the air in a room, then atoms and molecules can be regarded as elementary particles. That is, they have "conditional elementarity" because they keep their identity and do not change in any collisions or in other processes at these low energy exchanges. If one goes to higher energy exchanges, say \(10^4\) electron volts, then atoms and molecules will decompose into nuclei and electrons; at this level, the latter particles must be considered as elementary. We find examples of structures and processes of this first rung of the quantum ladder on Earth, on planets, and on the surfaces of stars.

The next rung is the nuclear realm. Here the energy exchanges are much higher, on the order of millions of electron volts. As long as we are dealing with phenomena in the atomic realm, such amounts 'of energy are unavailable, and most nuclei are inert: they do not change. However, if one applies energies of millions of electron volts, nuclear reactions, fission and fusion, and the processes of radioactivity occur; our elementary particles then are protons, neutrons, and electrons. In addition, nuclear processes produce neutrinos, particles that have no detectable mass or charge. In the universe, energies at this level are available in the centers of stars and in star explosions. Indeed, the energy radiated by the stars is produced by nuclear reactions. The natural radioactivity we find on Earth is the long-lived remnant of the time when now-earthly matter was expelled into space by a major stellar explosion.

The third rung of the quantum ladder is the subnuclear realm. Here we are dealing with energy exchanges of many billions of electron volts. We encounter excited nucleons, new types of particles such as mesons, heavy electrons, quarks, and gluons, and also antimatter in large quantities. The gluons are the quanta, or smallest units, of the force (the strong force) that keeps the quarks together. As long as we are dealing with the atomic or nuclear realm, these new types of particles do not occur and the nucleons remain inert. But at subnuclear energy levels, the nucleons and mesons appear to be composed of quarks, so that the quarks and gluons figure as elementary particles.

21. The primary topic of the passage is which of the following?

(A) The interaction of the realms on the quantum ladder.
(B) Atomic structures found on Earth, on other planets, and on the surfaces of stars
(C) Levels of energy that are released in nuclear reactions on Earth and in stars
(D) Particles and processes found in the atomic, nuclear, and subnuclear realms
(E) New types of particles occurring in the atomic realm





22. According to the passage, radioactivity that occurs naturally on Earth is the result of

(A) the production of particles that have no detectable mass or electric charge
(B) high energy exchanges on the nuclear level that occurred in an ancient explosion in a star
(C) processes that occur in the center of the Sun, which emits radiation to the Earth
(D) phenomena in the atomic realm that cause atoms and molecules to decompose into nuclei and electrons
(E) high-voltage discharges of electricity that took place in the atmosphere of the Earth shortly after the Earth was fonned




23. The author organizes the passage by

(A) making distinctions between two groups of particles, those that are elementary and those that are composite
(B) explaining three methods of transferring energy to atoms and to the smaller particles that constitute atoms
(C) describing several levels of processes, increasing in energy, and corresponding sets of particles, generally decreasing in size
(D) putting forth an argument concerning energy levels and then conceding that several qualifications of that argument are necessary
(E) making several successive refinements of a definition of elementarity on the basis of several groups of experimental results




24. According to the passage, which of the following can be found in the atomic realm?

(A) More than one level of energy exchange
(B) Exactly one elementary particle
(C) Exactly three kinds of atomic structures
(D) Three levels on the quantum ladder
(E) No particles smaller than atoms




25. According to the author, gluons are not

(A) considered to be detectable
(B) produced in nuclear reactions
(C) encountered in subnuclear energy exchanges
(D) related to the strong force
(E) found to be conditionally elementary





26. At a higher energy level than the subnuclear level described, if such a higher level exists, it can be expected on the basis of the information in the passage that there would probably be

(A) excited nucleons
(B) elementary mesons
(C) a kind of particle without detectable mass or charge
(D) exchanges of energy on the order of millions of electron volts
(E) another set of elementary particles





27. The passage speaks of particles as having conditional elementarity if they

(A) remain unchanged at a given level of energy exchange
(B) cannot be decomposed into smaller constituents
(C) are mathematically simpler than some other set of particles
(D) release energy at a low level in collisions
(E) belong to the nuclear level on the quantum ladder